Abstract: This article presents a brief account of the life and work of Hans W. Liepmann, a distinguished fluid dynamicist, an outstanding teacher and leader, and the third Director of the Graduate Aeronautical Laboratories, California Institute of Technology.

Publication: Annual Review of Fluid Mechanics Vol.: 45ISSN: 0066-4189

ID: CaltechAUTHORS:20130502-133421110

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Abstract: This paper considers the lectures on turbulent shear layers and wakes presented and discussed at the Marseille meeting in 1961 and provides our perspective on progress in understanding the mechanics of these flows since that time. The initial discussion is based on the understanding in 1961 gained from prior work. Particular emphasis is then placed on the subsequent experimental revelation of the large-scale vortical structure (coherent structure) found to be essential to understanding the mechanics of the turbulent shear layer. Critical insight into the mechanics that determines the growth rate (the shear stress), for example, is provided by the Biot–Savart relationship. Conclusions are drawn from the experiments and some unresolved questions posed. This is followed by a discussion of plane wakes. Four regions of the plane wake are identified and experimental results on the large-scale structure are discussed. Again emphasis is placed on the vorticity and the vorticity fluxes that contribute directly to the derivative of the principal Reynolds stress. Results from numerical calculations offer new insights into the mechanics, especially through the vorticity and vorticity fluxes that could not previously be measured. For this case too, conclusions are drawn and outstanding questions posed.

Publication: Journal of Turbulence Vol.: 13 No.: 51 ISSN: 1468-5248

ID: CaltechAUTHORS:20130301-151223359

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Abstract: Hans Wolfgang Liepmann, a pioneering researcher and passionate educator in fluid mechanics, passed away at the age of 94. Liepmann, the Theodore von Kármán Professor of Aeronautics, Emeritus, at the California Institute of Technology (Caltech), passed away June 24 at his home in La Cañada Flintridge. Widely honored for his contributions to aeronautics, Liepmann came to Caltech in 1939 and was the third director of Caltech’s Graduate Aeronautical Laboratories (GALCIT), from 1972 to 1985.

Publication: Shock Waves Vol.: 19 No.: 5 ISSN: 0938-1287

ID: CaltechAUTHORS:20141201-123635773

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Abstract: We have studied the effects of controlled damping on the amplitude and frequency response profiles of an elastically mounted cylinder in cross-flow. The dimensionless damping parameter, b^*=2b/ρLDU, which is closely related to the traditional “mass-damping” parameter, m^*ζ, was varied over a wide range of values through the use of a variable magnetic eddy current damping system. For low damping and sufficiently high Reynolds number we observe the previously described large-amplitude, three-branch (initial, upper, lower) response profile, and for high damping or low Reynolds number we observe the small-amplitude, two-branch (initial and lower) response profile. However we find that, because of the influence of Reynolds number, the traditional labels of “high mass-damping” and “low mass-damping” are incomplete with regard to predicting a large or small-amplitude response profile. In our experiments, as damping is systematically increased, we observe a transition between these two profiles characterized by a gradual “erosion” and eventual disappearance of the large-amplitude section (upper branch) and the scaling down of the lower branch region. We find that jumps from the upper to the initial branch originate on the 2S/2P boundary in the Williamson–Roshko plane. Another new finding is a hysteresis between the lower branch and the desynchronized region, which only appears at low Reynolds numbers. We also explore changes in the frequency response profile, which are connected with the changes in the amplitude profile, for our upper branch cases. We observe that analogous to the three amplitude branches, there are three distinct branches for the frequency response.

Publication: Journal of Fluids and Structures Vol.: 22 No.: 6-7 ISSN: 0889-9746

ID: CaltechAUTHORS:20110630-142700268

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Abstract: The response of a freely oscillating circular cylinder (“free vibration”) in cross-flow has been studied experimentally using controlled magnetic eddy current to provide variable damping. In general, the nondimensional response amplitude, A*, and dominant frequency, ω*, depend on the Reynolds number, Re, and the nondimensional mass, m*, damping, b*, and elasticity, k*, of the system. The main objective of this study is to characterize the maximum amplitude that is achieved for a given system as cross-flow velocity is varied. We find that this maximum amplitude, A*_(max), occurs within a small range of values of k*_(eff) = ω*^2m* + k*. For values of Reynolds number in the range 525

ID: CaltechAUTHORS:20141201-122718180

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Abstract: Laser-induced fluorescence (LIF) diagnostics and highspeed, real-time digital image acquisition techniques are combined to map the composition field in a water mixing layer. A fluorescent dye, which is premixed with the lowspeed freestream fluid and dilutes by mixing with the highspeed fluid, is used to monitor the relative concentration of high-speed to low-speed fluid in the layer. The three digital LIF pictures shown here were obtained by imaging the laser-induced fluorescence originating from a collimated argon ion laser beam, extending across the transverse dimension of the shear layer, onto a 512–element linear photodiode array. Each picture represents 384 contiguous scans, each at 400 points across the layer, for a total of 153 600 point measurements of concentration. The vertical axis maps onto 40 mm of the transverse coordinate of the shear layer, and the horizontal axis is time increasing from right to left for a total flow real time of 307 msec. The pseudocolor assignment is linear in the mixture fraction (ξ) and is arranged as follows: red-unmixed fluid from the low-speed stream (ξ=0); blue-unmixed fluid from the high-speed stream (ξ=1); and the rest of the spectrum corresponds to intermediate compositions. Figures 1 and 2, a single vortex and pairing vortices, respectively, show the composition field before the mixing transition. The Reynolds number based on the local visual thickness of the layer and the velocity difference across the layer is Re=1750 with U_2/U_1=0.46 and U_1=13 cm/sec. Note the large excess of high-speed stream fluid in the cores of the structures.

ID: CaltechAUTHORS:20181119-155309490

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Abstract: Phenomena associated with flow-induced transverse oscillation of an elastically mounted body are considered. The use of a recently introduced parameter that combines the effect of mass and elasticity—effective elasticity—is exploited to demonstrate the predictive value of the new approach and to provide insights into solution branching, the maximum amplitude of vibration, and modeling.

Publication: Journal of Fluids and Structures Vol.: 15 No.: 3-4 ISSN: 0889-9746

ID: CaltechAUTHORS:20141201-122214519

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Abstract: Transverse oscillation of a dynamically supported circular cylinder in a flow at Re=100 has been numerically simulated using a high-resolution viscous-vortex method, for a range of dynamical parameters. At the limiting case with zero values of mass, damping and elastic force, the cylinder oscillates sinusoidally at amplitude A/D=0·47 and frequency fD/U_∞=0·156. For zero damping, the effects of mass and elasticity are combined into a new, “effective” dynamic parameter, which is different from the classic “reduced velocity”. Over a range of this parameter, the response exhibits oscillations at amplitudes up to 0·6 and frequencies between 0·15 and 0·2. From this response function, the classic response in terms of reduced velocity can be obtained for fixed values of the cylinder/fluid ratio m*. It displays “lock-in” at very high values of m*.

Publication: Journal of Fluids and Structures Vol.: 15 No.: 1 ISSN: 0889-9746

ID: CaltechAUTHORS:20141201-121315237

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Abstract: A central theme in the history of the turbulence problem is about the method of ‘closure’ in the models and ‘theories’ which have been proposed. Closure has invariably been by empirical calibration with experimental data. In this note we draw attention to a paper by Morris, Giridharan and Lilley, in which for the first time empiricism is obviated. For the turbulent mixing layer, this is accomplished by including in its description the mechanism for production of turbulent shear stress (i.e. turbulent momentum transfer), by large-scale instability waves. Some implications for the theory of turbulent shear flows are discussed.

Publication: Current Science Vol.: 79 No.: 6 ISSN: 0011-3891

ID: CaltechAUTHORS:ROScs00

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Abstract: Dave Belden's letter announcing the award was really a surprise, almost a shock. At first l wondered whether it was another example of a story which you may have heard and which, I believe, originated in the FSU. Two friends are at a grand reception sipping cocktails when one notices a man with his chest almost completely covered with medals. Says one to the other, "Do you have any idea what those medals are for?" and the other replies ... Well, you see that one at the top left? That one was a mistake: and the others followed automatically ... I humored myself out of that thought but not out of a feeling of guilt. You see, I suddenly felt terrible that I was not a member of the ASME. There had been opportunities but somehow l had let them go by. One reason is that I was concerned about another onslaught of communications, information and other paper that always results and require attention. Fortunately, ASME lost no time in relieving my guilt. In a few weeks I received a nice invitation and forms to fill out, and am I am Member No. 6143358. And sure enough, information has begun to roll in: a beautiful, glossy magazine, notice of various meetings, etc.

Publication: Journal of Applied Mechanics Vol.: 67 No.: 2 ISSN: 0021-8936

ID: CaltechAUTHORS:20141201-124222548

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Abstract: Measurements of the fluctuating liftoff height of turbulent jet flames were made using a linear photo-diode array. A wide range of experimental conditions was investigated, including varying the fuel, nozzle diameter and liftoff height. The amplitudes of the fluctuations in h were found to be of the order of the local large scale of the jet. There is a slight increase in normalized fluctuation level h′/h¯ with the mean liftoff height h¯ and there is some variation of h′/h¯ with fuel type. The dominant time scales of the fluctuations of h were found to be considerably longer than the local large-scale time of the turbulence defined as the local jet width divided by the centerline velocity, τ_δ ≡δ/U_(cl). By using fuels of different chemical times to vary τ_δ. the measured correlation time τ_(1/2) normalized by τ_δ was found to collapse with Richardson number ξ_h However, the Richardson numbers achieved are dependent largely on the chemical rates of the fuels. Also, experiments in which the nozzles were oriented horizontally showed no change in τ_(1/2).

Publication: Combustion Science and Technology Vol.: 155ISSN: 0010-2202

ID: CaltechAUTHORS:20141201-130213554

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Abstract: In this presentation, we briefly describe some of the research projects in GALCIT in which flow visualization played a central role.

ID: CaltechAUTHORS:20141201-131151732

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Abstract: Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.

Publication: Journal of Fluid Mechanics Vol.: 1994ISSN: 0022-1120

ID: CaltechAUTHORS:FRIjfm94

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Abstract: A research program to study the effects of density ratio and exit Mach number on the development of a compressible, axisymmetric jet has been undertaken, using different gases to change the density of the jet. However, each gas and Mach number combination requires a unique nozzle contour, and the manufacture of the planned number of nozzles by traditional methods would have been prohibitively expensive. A plating technique to inexpensively manufacture high precision nozzles was developed, and a prototype nozzle constructed. Data are presented which shows the flow quality to be as good as or better than the nozzles used by previous experimenters.

ID: CaltechAUTHORS:20170901-105248315

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Abstract: Data have been obtained in the 12-foot pressurized wind tunnel at NASA Ames Research Center on flow past smooth and rough circular cylinders at high Reynolds numbers. Steady and unsteady surface pressures were measured around and along the cylinders. Analysis of the results provides lift and drag coefficients and Strouhal numbers for cylinders of several roughness at Reynolds numbers up to 8 × 10^6.

Publication: Journal of Wind Engineering and Industrial Aerodynamics Vol.: 49 No.: 1-3 ISSN: 0167-6105

ID: CaltechAUTHORS:20130509-075105354

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Abstract: The variations of drag and base suction of circular cylinders and bluff plates over the range of Reynolds number from 10 to 10^7 is discussed, with emphasis on the importance of the separated shear layers. A model for flows with a wake splitter plate is proposed. Effects of “three dimensionality” are discussed.

Publication: Journal of Wind Engineering and Industrial Aerodynamics Vol.: 49 No.: 1-3 ISSN: 0167-6105

ID: CaltechAUTHORS:20141201-131826320

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Abstract: A general result valid for any compressible fluid is noted. It gives the maximum values of the flux densities of mass, momentum, and kinetic energy in steady and unsteady flows which are expanding isentropically from a reservoir.

Publication: Physics of Fluids A - Fluid Dynamics Vol.: 5 No.: 4 ISSN: 0899-8213

ID: CaltechAUTHORS:20120308-110759673

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Abstract: Increasing attention is being paid to the large scale structure of turbulence and to the so-called “coherent” vortical structures which have been disclosed and studied for a number of turbulent shear flows in laboratory experiments and in numerical simulations. The coherent structures develop from the instability waves which create the flow; they portray the genesis of the turbulence in the primary instability of the global vorticity distribution. This is not in the sense of classical laminar instability, whichinitiates transition to turbulence, but as thedriving instability in the fully developed turbulence. That instability provides the link to the amplitude of the turbulent motion, which in classical turbulent modelling must be calibrated empirically as a fundamental step or steps in the closure of the Reynolds averaged equations of motion. It also rationalizes the dependence on various parameters, such as compressibility, clarifies response to external disturbances, and suggests the possibility of “turbulence control”. The global instability is being incorporated into new models of turbulent shear flow development and the coherent structures form the basis for new, Lagrangian models of chemical mixing and reaction in these flows.

ID: CaltechAUTHORS:20170731-142554277

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Abstract: This research investigates the effects of spanwise and streamwise coherent structures in a turbulent mixing layer on the deflection of a thin light beam which is transmitting transversely through the mixing layer from the high-speed side to the low speed side. Both equal and unequal density mixing layers of varying pressures and velocities are studied, using a lateral effect detector to dynamically track the motion of a He-Ne laser beam. Beam deflections in the streamwise direction are found to be associated mainly with the spanwise coherent structures; at low Reynolds Numbers the beam deflection is directly related to the part of a spanwise structure through which the beam passes. Maximum deflections are associated with the trailing edge of the spanwise coherent structures. Spanwise deflections are caused mainly by the streamwise coherent structures and as such exhibit large variations across the span of the flow. With the development of the streamwise structures, spanwise deflections are found to exceed streamwise deflections. Mixing transition, as scaled using the momentum thickness of the high-speed side, is found to cause a peak in therms fluctuations of both the streamwise and spanwise deflections.

ID: CaltechAUTHORS:20141210-134738343

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Abstract: Flow visualization has important applications in engineering studies of complex flow fields and it is an indispensable tool in fluid-mechanics research. Some examples of the role which it has played in research discovery are discussed.

ID: CaltechAUTHORS:20141201-132549656

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Abstract: I have been tapped as the moderator for this panel, and I hope that everybody else is prepared to say a few things. I think that our job as panellists is to provoke a little discussion from the audience, and I would like to say a few words to get things started. First, to clarify the subject of the discussion a little bit, I think that what is implied in the title by "experiments" are laboratory experiments. I think that the direct numerical simulations are, in fact, also experiments, and that we should be talking about using direct numerical simulation in the same way that we use the laboratory, to produce flows that can be studied and brought into the research picture.

No.: 268
ID: CaltechAUTHORS:20141201-135146578

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Abstract: Photographs of a jet issuing from a wall into a crossflow display the four types of vortical structures which exist in the near field: namely, the jet shear layer vortices, the nascent far field vortex pair, the near-wall horseshoe vortices, and a system of vortices in the wake of the jet. It is shown that the wake vorticity is not “shed” from the jet but is formed from vorticity which originated in the wall boundary layer. The sources of vorticity for the other types of structures are also briefly discussed.

ID: CaltechAUTHORS:20170726-131022892

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Abstract: The term "mixing transition" denotes an increase in molecular mixedness observed in a shear How which has earlier experienced the conventional (momentum) transition from laminar flow. First defined by Konrad (1976), from measurements of concentration in a free shear layer, the transition has been described and measured by a number of other methods, in particular by flow visualization, by measurement of chemical reaction product and by hot-wire anemometry, in aqueous as well as gaseous flows. In this presentation we review some of the measurements and try to assess what insight they may give on several questions that occur. 1. What is the relation of the mixing transition to Reynolds number and to other events: the momentum transition; vortex pairing; development of streamwise vortex structure? 2. How much does interfacial area increase during the transition? 3. How fast docs this occur? 4. Docs chaotic advection play a role? The answers arc tentative and incomplete.

No.: 268
ID: CaltechAUTHORS:20141201-133432952

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Abstract: I have a rather provocative question to ask the speakers. What I am wondering is: When you apply the proper orthogonal decomposition - I know this is a procedure you use and it is probably standard - you remove the mean flow and you only look at the perturbations. Why do not you include the mean flow, because if you did you would suppress the cubic terms, and also you would be getting something that would be closer to what people call coherent structures, such as hairpin vortices.

No.: 357
ID: CaltechAUTHORS:20141201-140640640

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Abstract: This afternoon all of the models that we have heard fall in the same class; namely, local closures. First-order local closure (K-theory or eddy diffusivity) models the momentum fluxes as down-gradient of the mean momentum. The second-order local closure models the third moments as down-gradient of the local second moments, or local mean variables. There is another completely different class of modeling or class of closure, and that is non-local turbulence closure. I mentioned before about the transilient matrix that describes the mixing between different points separated a finite distance in space. One can parameterize this matrix in terms of mean flow state or mean flow instability. When you do that, you can then make forecasts of the mean field in a turbulent flow that takes into account this nonlocal mixing. That has been done. For the ocean, we found results as good as third-order local closure. For the atmosphere, results were as good as second-order local closure. We've used it in three-dimensional weather forecast models covering the whole United States. This is a new concept of non-local closure, which is different from all the other local closures. When would you want to consider using a non-local kind of closure? Well, if any of you are dealing with turbulent flow that has a spectrum of eddy sizes where your greatest energy is in the largest wavelengths, or if you are dealing with turbulent flow that has large structures in it that are causing non-local mixing, then you might want to consider a non-local turbulence closure.

No.: 357
ID: CaltechAUTHORS:20141201-142720632

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Abstract: Measurements have been made of the base pressure coefficients of long circular cylinders at low Reynolds numbers from 35 to 1100 in a cross flow. The variation of "base suction" (negative base pressure coefficient or - C_(pb)) with Reynolds number shows a remarkably good correspondence with the variation of the Strouhal number. It is also possible to relate the variation in suction with the physical modes of wake formation found in previous studies. It is shown that the level of base suction is affected in only a minor fashion by whether parallel or oblique shedding is present, and it is found that the data are independent of the base pressure hole sizes and cylinder diameters that were used. The base suction is essentially constant along the span of a cylinder (outside of regions extending about 20 diameters in from each end), and the results arc independent of cylinder aspect ratio (L/D) provided that one exceeds a critical value of L/D.

Publication: Zeitschrift für Flugwissenschaften und Weltraumforschung Vol.: 14 No.: 1 ISSN: 0342-068X

ID: CaltechAUTHORS:20141201-152316304

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Abstract: Professor Launder's position paper on phenomenological modelling is an impressive survey and valuable account of the status of second-moment closure, principally as applied to Reynolds Stresses. In this respect, it supplements and updates the monograph of Professor Rodi (1980), in which the emphasis is on the status of first-order closures, in particular the κ-ε model, as of 10 years ago. Between them the two works provide an excellent reference source containing the equations; the rationale for modelling decisions that are made; tables of the constants that have been selected; displays of flow computations and their comparison with experimental measurements for a varied number of flows; and extensive reference lists.

No.: 357
ID: CaltechAUTHORS:20141201-142123436

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Abstract: The growth rate and turbulent structure of the compressible, plane shear layer are investigated experimentally in a novel facility. In this facility, it is possible to flow similar or dissimilar gases of different densities and to select different Mach numbers for each stream. Ten combinations of gases and Mach numbers are studied in which the free-stream Mach numbers range from 0.2 to 4. Schlieren photography of 20-ns exposure time reveals very low spreading rates and large-scale structures. The growth of the turbulent region is defined by means of Pitot-pressure profiles measured at several streamwise locations. A compressibility-effect parameter is defined that correlates and unifies the experimental results. It is the Mach number in a coordinate system convecting with the velocity of the dominant waves and structures of the shear layer, called here the convective Mach number. It happens to have nearly the same value for each stream. In the current experiments, it ranges from 0 to 1.9. The correlations of the growth rate with convective Mach number fall approximately onto one curve when the growth rate is normalized by its incompressible value at the same velocity and density ratios. The normalized growth rate, which is unity for incompressible flow, decreases rapidly with increasing convective Mach number, reaching an asymptotic value of about 0.2 for supersonic convective Mach numbers.

Publication: Journal of Fluid Mechanics Vol.: 197ISSN: 0022-1120

ID: CaltechAUTHORS:PAPjfm88

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Abstract: When a body oscillates laterally (cross-flow) in a free stream, it can synchronize the vortex formation frequency with the body motion frequency. This fundamental “lock-in” regions is but one in a whole series of synchronization regions, which have been found in the present paper, in an amplitude-wavelength plane (defining the body trajectory) up to amplitudes of five diameters. In the fundamental region, it is shown that the acceleration of the cylinder each half cycle induces the roll-up of the two shear layers close to the body, and thereby the formation of four regions of vorticity each cycle. Below a critical wavelength, each half cycle sees the coalescence of a pair of like-sign vortices and the development of a Karman-type wake. However, beyond this wavelength the like-sign vortices convect away from each other, and each of them pairs with an opposite-sign vortex. The resulting wake comprises a system of vortex pairs which can convect away from the wake centerline. The process of pairing causes the transition between these modes to be sudden, and this explains the sharp change in the character of the cylinder forces observed by Bishop and Hassan, and also the jump in the phase of the lift force relative to body displacement. At precisely the critical wavelength, only two regions of vorticity are formed, and the resulting shed vorticity is more concentrated than at other wavelengths. We interpret this particular case as a condition of “resonant synchronization”, and it corresponds with the peak in the body forces observed in Bishop and Hassan's work.

Publication: Journal of Fluids and Structures Vol.: 2 No.: 4 ISSN: 0889-9746

ID: CaltechAUTHORS:20170726-124346409

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Abstract: Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Kármán-vortex-street frequency is observed. Beyond this region of decay, larger-scale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased. The far-wake structure does not depend directly on the scale or frequency of Kármán vortices shed from the cylinder; i.e. it does not result from amalgamation of shed vortices. The growth of this structure is due to hydrodynamic instability of the developing mean wake profile. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re [approximate] 6000), which do not initially shed Kármán-type vortices into the wake. Measured prominent frequencies in the far cylinder wake are in good agreement with those estimated by two-dimensional locally parallel inviscid linear stability theory, when streamwise growth of wake width is taken into account. Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed and a mechanism for its development is suggested based on a secondary parametric instability of the subharmonic type.

Publication: Journal of Fluid Mechanics Vol.: 190ISSN: 0022-1120

ID: CaltechAUTHORS:CIMjfm88

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Abstract: Visual spreading rates of turbulent shear layers with at least one stream supersonic were measured using Schlieren photography. The experiments were done at a variety of Mach number-gas combinations. The spreading rates are correlated with a compressibility-effect parameter called the convective Mach number. It is found that for supersonic values of the convective Mach number, the spreading rate is about one quarter that of an incompressible layer at the same velocity and density ratio. The results are compared with other experimental and theoretical results.

Publication: Sādhanā (Academy Proceedings in Engineering Sciences) Vol.: 12 No.: 1-2 ISSN: 0256-2499

ID: CaltechAUTHORS:20141210-135606485

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Abstract: An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge. It was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the “shedding” type instability of the entire bubble. A method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.

ID: CaltechAUTHORS:20170731-100953029

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Abstract: An experimental investigation of flow over an axisymmetric cavity shows that self-sustained, periodic oscillations of the cavity shear layer are associated with low cavity drag. In this low-drag mode the flow regulates itself to fix the mean-shear-layer stagnation point at the downstream corner. Above a critical value of the cavity width-to-depth ratio there is an abrupt and large increase of drag due to the onset of the ‘wake mode’ of instability. It is also shown by measurement of the momentum balance how the drag of the cavity is related to the state of the shear layer, as defined by the mean momentum transport $\rho\overline{u}\overline{v}$ and the Reynolds stress $\rho\overline{u^{\prime}v^{\prime}}$, and how these are related to the amplifying oscillations in the shear layer. The cavity shear layer is found to be different, in several respects, from a free shear layer.

Publication: Journal of Fluid Mechanics Vol.: 177ISSN: 0022-1120

ID: CaltechAUTHORS:GHAjfm87

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Abstract: The development of three-dimensional motions in a plane mixing layer was investigated experimentally. It is shown that superimposed on the primary, spanwise vortex structure there is a secondary, steamwise vortex structure. Three aspects of this secondary structure were studied. First, the spanwise vortex instability that generates the secondary structure was characterized by measurements of the critical Reynolds number and the spanwise wavelength at several flow conditions. While the critical Reynolds number was found to depend on the velocity ratio, density ratio and initial shear-layer-profile shape, the mean normalized wavelength is independent of these parameters. Secondly, flow visualization in water was used to obtain cross-sectional views of the secondary structure associated with the streamwise counter-rotating vortices. A model is proposed in which those vortices are part of a single vortex line winding back and forth between the high-speed side of a primary vortex and the low-speed side of the following one. Finally, the effect of the secondary structure on the spanwise concentration field was measured in a helium-nitrogen mixing layer. The spatial organization of the secondary structure produces a well-defined spanwise entrainment pattern in which fluid from each stream is preferentially entrained at different spanwise locations. These measurements show that the spanwise scale of the secondary structure increases with downstream distance.

Publication: Journal of Fluid Mechanics Vol.: 170ISSN: 0022-1120

ID: CaltechAUTHORS:BERjfm86

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Abstract: The phenomenon of cavity flow oscillation is investigated to determine the conditions for onset of periodic oscillations and to understand the relationship between the state of the shear layer and the cavity drag. Experiments have been performed in a water tunnel using a 4" axisymmetric cavity model instrumented with a strip heater on the nose cone and pressure taps in and around the cavity. A complete set of measurements of oscillation phase, amplitude amplification along the flow direction, distribution of shear stress and other momentum flux is obtained by means of a laser Doppler velocimeter. Drag measurements were made by integrating the mean pressure over the solid surfaces of the cavity. Results indicated exponential cavity drag dependence on the length of the cavity. A jump in the cavity drag coefficient is observed as the cavity flow shows a bluff body wake type behavior. An independent estimate of the drag, which is obtained by integration of shear and mean momentum transfer terms over the peripheral area of the cavity, confirms the exponential dependence of drag on the length of the cavity. Results, also reveal that the drag of the cavity in the non-oscillating mode is less than the case if the cavity were replaced by a solid surface. Natural and forced oscillations of the cavity shear layer spanning the gap are studied. The forced oscillations are introduced by a sinusoidally heated thin-film strip which excites the Tollmein-Schlichting waves in the boundary layer upstream of the gap. For a sufficiently large gap, self-sustained periodic oscillations are observed, while for smaller gaps, which do not oscillate naturally, periodic oscillations can be obtained by external forcing through the strip heater. In the latter case resonance is observed whenever the forcing frequency satisfies phase criterion ψ/2π = N, and amplitude exceeds certain threshold levels, but the phenomenon, is non-self-supporting. The drag of the cavity can be increased by one order of magnitude in the non-oscillating case through external forcing. For naturally occurring oscillations, it is possible for two waves to co-exist in the shear layer (natural and forced). Also, it is possible to completely eliminate mode switching by applying external forcing. For the first time a test is performed to cancel or dampen the amplitude of Kelvin-Helmholtz wave in the cavity shear layer. This is done through introducing an external perturbation with the same frequency of the natural component but having a different phase. Reduction by a factor of 2 is obtained in the amplitude of the oscillation.

No.: 85-72
ID: CaltechAUTHORS:20170726-123311179

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Abstract: This paper describes an experimental investigation of the shielding effects of various disks placed coaxially upstream of an axisymmetric, flat-faced cylinder. Remarkable decrease of the drag of such a system was observed for certain combinations of the basic geometric parameters, namely the diameter and gap ratios. For such optimum shielding the stream surface which separates from the disk reattaches smoothly onto the front edge of the cylinder, in what is close to a ‘free-streamline’ flow; alternatively, the flow may be viewed as a cavity flow. For the optimum as well as other geometries, flow pictures, pressure distributions and some LDV measurements were also obtained. From these, several flow regimes depending on the gap/diameter parameters were identified. Variations on the axisymmetric disk–cylinder configuration included a hemispherical frontbody, rounding of the front edge of the cylinder and a change from circular to square cross-section.

Publication: Journal of Fluid Mechanics Vol.: 156ISSN: 0022-1120

ID: CaltechAUTHORS:KOEjfm85

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Abstract: The effects of a periodic disturbance, applied to the one of the the free streams, on large-scale structure and mixing processes in chemically reacting shear layers and wakes were investigated over a range of Reynolds numbers above and below the mixing transition range. Two different methods were employed to measure the amount of chemical product and thus the extent of molecular-scale mixing. Absorption by reacted phenolphthalein provided cross-stream average product thickness and laser induced fluorescence intensity provided the product concentration distribution. These methods provided, in addition, effective flow visualization of the large-scale structures and of their response to the periodic forcing. The detailed effects of periodic forcing on distribution of mixing along a free shear layer are complex, but the predominant, overall effect is to increase mixing at low Reynolds number (in respect to the mixing transition) and to decrease it at high Reynolds number.

ID: CaltechAUTHORS:20141210-133626177

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Abstract: The broad problem being addressed in our research is to identify and describe the primary vortical ("coherent", "organized") large structures in various turbulent shear flows; to determine how they contribute to the mixing processes; and to make use of them in modelling and in possibly controlling or modifying those flows. Accumulating experimental evidence suggests that these primary vortical structures are different in different shear flows. Conclusions which follow from these views are that (i) there cannot be a universal turbulence model for these different flows; and (ii) existence of such structures implies the possibility of their manipulation or control and thus modification of the flow itself. These organized structures (and their response to excitation) are manifestations of instability response to natural or imposed disturbances and thus may be important in cooperation with other, physical processes, e.g. , acoustic coupling, rate controlled chemistry, etc., in problems like combustion chamber instability.

ID: CaltechAUTHORS:20141114-143226217

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Abstract: The turbulent mixing layer between two streams of different velocities continues to play a central role in research aimed at improved understanding of turbulent shear flows in general, At present, not all researchers are in agreement as to what various experiments imply about the structure of mixing layers at high Reynolds number. The views which are held differ on the question as to how and to what extent three dimensionality develops in these flows and whether the Characteristic spanwise organized large vortex structures (rollers) continue to be a dominant feature. The traditional view, as extended to the contemporary scene, is that ultimately (i.e., sufficiently far downstream or at sufficiently high Reynolds number) the flow will be completely disorganized. The view put forward by "eddy chasers" is that such vortex structures are primary elements, characteristic of the underlying mean vorticity field, which is particularly simple for the mixing layer, and that, as long as the velocity difference is maintained, there is a mechanism to regenerate these primary structures by what, for convenience, may be called a Kelvin-Helmholtz instability, The heart of the controversy then is whether, or to what extent, secondary and higher instabilities will ultimately break down, completely disorganize or prevent formation of organized primary structures. In a plane mixing layer, the primary structures would, ideally, be two dimensional, containing the basic single component of vorticity while secondary and higher modes of instability would introduce three dimensionality and the other two components of vorticity into the flow. An interesting question follows: to what extent do such secondary instabilities change the properties (e.g., the growth rate; the Reynolds stress) that the mixing layer would have in ideal two dimensional development? In this paper we examine several aspects of this question and discuss some recent relevant experiments.

No.: 136
ID: CaltechAUTHORS:20201007-081705336

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Abstract: The interaction of an unseparated supersonic turbulent boundary layer with a compression corner produces an extremely rapid rise in pressure at the corner, followed by a more gradual increase to the final pressure. In this paper, the flow in the corner region is analyzed by an integral method with the objective of predicting the initial pressure rise. Comparisons with experimental pressure rise data are presented for cases covering supersonic and hypersonic flows of practical interest. Also presented are some calculations and comparisons of downstream pressure distributions obtained by using the predicted corner rise as the first point in an existing inviscid method.

ID: CaltechAUTHORS:20141222-110906638

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Abstract: The objective of this study is to obtain better understanding of the flow over two tandemly positioned bluff bodies in close enough proximity to strongly interact with each other. This interaction is often beneficial in that the drag of the overall system is reduced. Prototypes for this problem come from tractor-trailer and cab-van combinations, and from various add-on devices designed to reduce their drag. The primary object of the present investigation is an axisymmetric configuration which seems to have first been studied by Saunders (1966). A disc of diameter d_1 is coaxially placed in front of a flat-faced cylinder of diameter d_2. For a given ratio d1/d2, there is a value of gap ratio, g*/d_2, for which the drag of the forebody system is a minimum. In the most optimum configuration, d_1/d_2 = 0.75, g*/d_2 = 0.375, and the corresponding forebody drag coefficient is 0.02, a remarkable reduction from the value of 0.75 for the cylinder alone. For each value of d_1/d_2, the minimum drag configuration, g*/d_2, appears to correspond to a minimum dissipation condition in which the separation stream surface just matches (joins tangentially onto) the rearbody. Support for this idea is furnished by comparison with some results derived from free-streamline theory and from flow visualization experiments. However, when g*/d_2 exceeds a critical value of about 0.5, the value of C_(Dmin) is almost an order of magnitude higher than for subcritical optimum gap ratios. The increase seems to be connected with the onset of cavity oscillations. For non-axisymmetric geometry (square cross-sections) the separation surface cannot exactly match the rearbody and the subcritical minimum values of drag are higher than for circular cross-sections.

ID: CaltechAUTHORS:20170731-102517069

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Abstract: The problem of turbulent now continues to be an outstanding one in technology and in physics. Of the nine Dryden research lectures so far, four have been on some aspect of the turbulence problem. At meetings such as this one the turbulence problem is always the subject of some sessions and lurks in the background of many others; for example, separated now, combustion, jet noise, chemical lasers, atmospheric problems, etc. It is continually the subject of conferences, workshops and reviews. In his time Hugh Dryden wrote several reviews of turbulent now. In reading some of them again, one statement particularly relevant to the present lecture caught my attention: "-it is necessary to separate the random processes from the nonrandom processes. It is not yet fully clear what the random elements are in turbulent now." Neither is it fully clear what the nonrandom, orderly elements are, but some of them are beginning to be recognized and described. Generally the picture one has had of turbulence is of chaos and disorder, implicit in the name. Although it was known that organized motion could exist, superimposed on the background of "turbulence," for example, vortex shedding from a circular cylinder up to Reynolds numbers of 10^7, such examples were regarded as special cases closely tied to their particular geometric origins and not characteristic of "well-developed" turbulence. It was known that large structures are important in the development of turbulent shear flows and that these ought to possess some definable features. But even when the concept of a characteristic "big eddy" was explored, it was usually in the context of a statistical quantity. The earliest and most decisive attempts to define the form of such large eddies were made by Townsend and his students. In recent years it has become increasingly evident that turbulent shear flows do contain structures or eddies whose description is more deterministic than had been thought, possessing identifiable characteristics, existing for significant lifetimes, and producing recognizable and important events. More accurate descriptions of their properties, how they fit into the complete description of a turbulent flow, to what extent are they central to its development, and how they can be reconciled with the apparent chaos and disorder, are problems which are becoming of interest to an increasing number of researchers. It is the purpose of this lecture to describe some of these new developments. The discussion will draw largely on experiences from our own laboratory; it is not intended to be a complete survey. Other discussions of these ideas can be found in various recent publications.

Publication: AIAA Journal Vol.: 14 No.: 10 ISSN: 0001-1452

ID: CaltechAUTHORS:20141201-155051947

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Abstract: Experimental results are presented for the effects of Mach number, Reynolds number, and corner angle on flare-induced separation of a supersonic, turbulent boundary layer. Measurements were obtained for upstream interaction distance ℓ_0 from the flare to the beginning of the interaction for Mach numbers 2≤M≤4.5, boundary-layer thickness Reynolds numbers 10^5 < R_δ < 10^6, and adiabatic wall conditions. Flares of angle α≤40° were attached to a hollow-cylinder model of 12 in. diam at either x_c= 14 or 18 in. downstream from the sharp leading edge. It was found that ℓ_0/δ_0 decreases with increasing Mach number and Reynolds number and increases with flare angle. For constant α, when ℓ_0/δ_0 is plotted vs the local skin-friction coefficient, C_(f0), the Mach number dependence disappears. From this observation, a simple correlation formula was obtained and used to compare results from other investigations, and also to correlate incipient separation data.

Publication: AIAA Journal Vol.: 14 No.: 7 ISSN: 0001-1452

ID: CaltechAUTHORS:20141201-160815389

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Abstract: For many years experimental research in turbulence was devoted to the measurement of various correlations and special functions which had evolved from the statistical theories and from engineering computing methods based on the hierarchy of Reynolds equations. A recent change in direction toward a more deterministic description of turbulent structure has been initiated by the discovery of large coherent structures in several turbulent shear flows. The new point of view suggests that with every shear flow (jet, boundary layer, mixing layer, etc.) is associated an identifiable, characteristic structure; the development of the flow is controlled by the interactions of these structures with each other. An understanding of their properties should give insight into actual physical processes in turbulent flows, such as entrainment, transport, mixing, noise production, gustiness, etc. and should lead to improved methods for analyzing and computing them. Experiments designed to study these properties are aided by recent developments in instrumentation technology such as computer-aided control of the experiments, but the venerable technique of flow visualization is still an indispensable aid.

ID: CaltechAUTHORS:20141222-105100548

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Abstract: Experimental results are presented for the effects of Mach number, Reynolds number, and corner angle on flare-induced separation of a supersonic, turbulent boundary layer. In particular, measurements were obtained for the variation with flare angle, α, of the ratio ℓ_0/δ_0 of the upstream interaction length to the boundary-layer thickness at the beginning of the interaction for Mach numbers 2≤M≤4.5, boundary-layer thickness Reynolds numbers 10^5 < R_δ < 10^6, and adiabatic wall conditions. The model consisted of a hollow cylinder of 12-in. diameter and 51-in. length. Flares of angle 9°≤α≤40° were attached to the cylinder model at either of two location, viz., at x_c= 14 or 28 in. downstream from the sharp leading edge. Measurements consisted chiefly of surface-pressure distributions. Profiles for the undisturbed (flare-off) boundary-layer were also obtained. By varying the several parameters upstream interaction lengths as large as ℓ_0/δ_0 = 30 were observed. It was found that ℓ_0/δ_0 decreases with increasing Mach number and Reynolds number and, of course, increases with flare angle. It was also found that, for constant α, when ℓ_0/δ_0 is plotted vs the local skin-friction coefficient, C_(f0), the Mach-number dependence disappears. From this observation, a simple correlation formula was obtained and used to compare results from other investigations, and also to correlate incipient separation data. The present results complement the incipient-separation data obtained previously by us in the next higher decade of Reynolds number and further confirm the trends established there. It was also found that, for large α, the separated region upstream of the flare has free-interaction characteristics similar to those of upstream-facing steps at high Reynolds number.

ID: CaltechAUTHORS:20141223-095148400

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Abstract: Plane turbulent mixing between two streams of different gases (especially nitrogen and helium) was studied in a novel apparatus. Spark shadow pictures showed that, for all ratios of densities in the two streams, the mixing layer is dominated by large coherent structures. High-speed movies showed that these convect at nearly constant speed, and increase their size and spacing discontinuously by amalgamation with neighbouring ones. The pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainment rates. Large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle; it is concluded that the strong effects, which are observed when one stream is supersonic, are due to compressibility effects, not density effects, as has been generally supposed.

Publication: Journal of Fluid Mechanics Vol.: 64 No.: 4 ISSN: 0022-1120

ID: CaltechAUTHORS:BROjfm74

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Abstract: Measurements of mass flow rate and mean density have been made in separated laminar boundary layers with large transverse density gradients. Two-dimensional shear layers were formed by exhausting a half-jet of one gas into a reservoir of another gas with a different molecular weight. Two freons with a density ratio of 1-98 and unusual properties which permitted the measurement of the mass flow rate with a single hot wire were used. A n analysis of the mass flow rate fluctuations showed that a negative density gradient (i.e. light gas flowing into heavy) increases the amplification rate of the instability oscillations and reduces the frequency and wave number. Opposite trends were observed when the density gradient was positive. These findings are in agreement with recent theoretical predictions.

Publication: Journal of Fluid Mechanics Vol.: 53 No.: 3 ISSN: 0022-1120

ID: CaltechAUTHORS:DAVjfm72

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Abstract: An experimental investigation of the steady, laminar near-wake flowfield of a two-dimensional, adiabatic, circular cylinder ·with surface mass transfer has been made at a freestream Mach number of 6.0. The pressure and mass- concentration fields associated with the transfer of argon, nitrogen, or helium into the near wake were studied for mass transfer from the forward stagnation region, and from the base. For sufficiently low mass transfer rates from the base, for which a recirculating zone exists, the entire near-wake flowfield correlates with the momentum flux, not the mass flux, of the injectant, and the mass-concentration field is determined by counter-current diffusion into the reversed flow. For mass addition from the forward stagnation region, the pressure field is undisturbed and the mass- concentration field is nearly uniform in the region of reversed flow. The axial decay of argon mass concentration in the intermediate wake, downstream of the neck, is explained with the aid of an integral solution in the incompressible plane, from which the location of the virtual origin for the asymptotic far-wake solution has been derived as one result.

Publication: AIAA Journal Vol.: 8 No.: 5 ISSN: 0001-1452

ID: CaltechAUTHORS:20141201-162815216

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Abstract: The flow within a shock wave is governed by the relaxation times of the molecular degrees of freedom. Advances in shock-tube design and instrumentation in recent years have made it possible to resolve all the relaxation times including the shortest, corresponding to the translational degrees of freedom. The shock tube thus becomes an important tool for critical experiments in the study of the range of applicability of the Navier-Stokes equations and similar approximations and of the character of solutions of the Boltzmann equation. Significant progress has recently been made in the understanding of the most obvious such problem, the flow within a shock in a monatomic gas. Theory and experiment are now in substantial agreement and the over-all process of energy exchange is understood. Progress has been made in problems connected with shock wave reflection from real walls, but a host of others remain to be studied including surface interaction effects. The extension of this type of shock-tube research to more complicated systems, reacting gases, gas mixtures, and the like has begun and some progress can be reported. Recent experimental progress is illustrated by a number of measurements made in the 6- and 17-in. shock tubes at the California Institute of Technology.

Publication: Physics of Fluids Vol.: 12 No.: 5 ISSN: 1070-6631

ID: CaltechAUTHORS:20130509-142125916

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Abstract: An experimental study was made of the conditions necessary to promote incipient separation of a turbulent boundary layer in two-dimensional supersonic flow over a compression corner. The aim was to extend Kuehn's earlier results to higher Reynolds numbers. Measurements were obtained for Mach numbers in the range 2 to 5 and at Reynolds numbers , based on the boundary-layer thickness, in the range 10^6 to 10^7, nearly two orders of magnitude greater than those reported earlier. The main result was that the trend with Reynolds number established by Kuehn for the pressure rise for incipient separation does not continue to the high Reynolds number values of the present experiments; in fact, it is reversed. Pressure distributions were also obtained for conditions with and without separation. For the latter case, the upstream influence was considerably less than one boundary-layer thickness end the initial part of the pressure rise was practically a jump, suggesting that the oblique shock has its origin deep in the boundary layer.

ID: CaltechAUTHORS:20170726-143226034

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Abstract: An experimental investigation of the steady, laminar near-wake flow field of a two-dimensional, adiabatic, circular cylinder with surface mass transfer has been made at a free-stream Mach number of 6.0. The pressure and mass-concentration fields associated with the transfer of argon, nitrogen or helium into the near wake were studied for mass transfer from the forward stagnation region, and from the base. For sufficiently low mass transfer rates from the base, for which a recirculating zone exists, the entire near-wake flow field correlates with the momentum flux, not the mass flux, of the injectant, and the mass-concentration field is determined by counter-current diffusion into the reversed flow. For mass addition from the forward stagnation region, the pressure field is undisturbed and the mass-concentration field is nearly uniform in the region of reversed flow. The axial decay of argon mass concentration in the intermediate wake, downstream of the neck, is explained with the aid of an integral solution in the incompressible plane, from which the location of the virtual origin for the asymptotic far-wake solution has been derived as one result.

ID: CaltechAUTHORS:20141210-132533513

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Abstract: In any real fluid motion there exists regions in space-time in which the fluid is far from thermodynamic equilibrium. The relative extent of these non-equilibrium regions is determined by the ratio of the molecular relaxation times and the corresponding length scales to the macroscopic time and space scales appropriate to the flow. Gas flow within such non-equilibrium regions is properly called "rarefied". In recent years the shock tube has become a rather efficient tool in the investigations of rarefied gas flows and I intend to illustrate progress in this use through a discussion of some very recent and typical investigations of the GALCIT group carried out under NASA sponsorship.

No.: 4/68
ID: CaltechAUTHORS:20170801-130148469

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Abstract: The flow within a shock wave is governed by the relaxation times of the molecular degrees of freedom. Advances in shock tube design and instrumentation have made it possible in recent years to resolve all the relaxation times including the shortest, corresponding to the translational degree of freedom. The shock tube thus becomes an important tool for critical experiments in the study of the range of applicability of the Navier-Stokes equations and similar approximations and of the character of solutions of the Boltzmann equation. Significant progress has been made recently in the understanding of the most obvious such problem, the flow within a shock in a monatomic gas. Theory and experiment are now in substantial agreement and the over-all process of energy exchange is understood. Problems connected with shock wave reflection from real walls have made progress but a host of problems remain to be studied including surface interaction effects. The extension of this type of shock tube research to more complicated systems, reacting gases, gas mixtures, and the like has begun and some progress can be reported. Recent experimental progress is illustrated by a number of measurements made at GALCIT in the 17- and 6-in. shock tubes. Sophistications in shock tube design and instrumentation will be discussed.

Publication: Bulletin of the American Physical Society Vol.: 13 No.: 5 ISSN: 0003-0503

ID: CaltechAUTHORS:20130510-081002532

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Abstract: The equations for both the boundary layer and the outer potential flow over a yawed cylinder can be resolved into equations for the crosswise and spanwise velocity components. These components of the boundary layer are evaluated using Sears’ method, and the separation point is found to be uninfluenced by the yaw angle. The potential-flow solutions for the spanwise and crosswise flows are added together to determine vortex patterns behind the cylinder. The approximate direct dependence of the Strouhal number upon the cosine of the yaw angle and/or the drag coefficient upon the square of the cosine, are verified. Experimental determinations of the Strouhal number and visualization of the flow pattern are consistent with the analysis.

Publication: Journal of Fluids Engineering Vol.: 89 No.: 4 ISSN: 0098-2202

ID: CaltechAUTHORS:20141210-143927153

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Abstract: An experimental study of the near-wakes of a family of flat-based cylinders of varying chord-thickness ratio, L/d, shows that the phenomena of vortex shedding and transition are similar to those for a cylinder of circular section. The Reynolds number for appearance of turbulence in the wake is correlated with L/d.

Publication: Physics of Fluids Vol.: 10 No.: 9 ISSN: 0031-9171

ID: CaltechAUTHORS:ROSpof67

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Abstract: It has been observed experimentally by Hama and discussed theoretically by Weinbaum that effects of the fast expansion and consequent lip shock at the shoulder of a supersonic base or downstream-facing step can be quite appreciable at high Mach number. Hama found that the lip shock can be much stronger than has been assumed. He also drew attention to characteristic humps or peaks in the pressure distribution on the reattachment surface; these, he showed, could be attributed to secondary waves directed toward the surface from the point of interaction of the lip shock with the main recompression shock. Scherberg and Smith have also drawn attention to the possible strong effects connected with a lip shock. In this note, we report some further observations of the occurrence of this phenomenon, and its elimination by a small modification at the shoulder to alleviate the fast expansion there.

Publication: AIAA Journal Vol.: 5 No.: 4 ISSN: 0001-1452

ID: CaltechAUTHORS:20141202-143708545

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Abstract: Supersonic flow over a downstream-facing step on the circumference of a large, ducted, axisymmetric body was used to study flow reattachment. Step heights h were 0.25, 1.00, and 1.68 in., compared to a body radius of 6 in. Freestream Mach numbers were in the range 2 to 4.5. Theturbulent boundary-layer thickness just ahead of the step varied from 0.14 to 0.19 in. (momentum thicknesses of about 0.01 in.). Surface pressure distributions throughout the region of separation and reattachment were measured, and points of reattachment were determined. Comparison of the shapes of the pressure distributions for various step heights shows that the initial (steepest) parts of the reattachment pressure rise, up to the point of reattachment, tend to become superimposed when plotted against x/h. Downstream reattachment the curves branch out, exhibiting a dependence on geometry and probably on initial shear layer profile. In the region of the initial pressure rise (near the end of the "dead air" region) dynamic pressures are low; the pressure rise there apparently is balanced by turbulent shear stress.

Publication: AIAA Journal Vol.: 4 No.: 6 ISSN: 0001-1452

ID: CaltechAUTHORS:20141202-142049273

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Abstract: Experimental measurements of test time were obtained in the GALCIT 17-in. shock tube using both air and argon for driven gases. One series of tests was conducted using a constant driver pressure (pure helium) for various initial pressures of the driven gases. Another series was conducted using air for the driven gas at various initial pressures holding the shock Mach number constant. The data are presented and compared to theoretical predictions computed from the theory in two recent papers by Mirels for the case of a laminar and turbulent wall boundary layer.

Publication: AIAA Journal Vol.: 2 No.: 1 ISSN: 0001-1452

ID: CaltechAUTHORS:20141202-141432548

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Abstract: The program of this symposium, which was held in Paris 26–29 June, 1962, included papers on new developments in the theory of flow of neutral and partly ionized gases at low densities, interactions between gas molecules and solid surfaces, high-speed and thermal molecular beams, measurements of drag and heat transfer in the transition and free molecule flow regime, and new experimental techniques. About 25 of the 60 papers presented are discussed in the following pages; the complete program is reproduced as Appendix I.

Publication: Progress in Aerospace Sciences Vol.: 5ISSN: 0376-0421

ID: CaltechAUTHORS:20170731-143548171

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Abstract: A shock tube for studying problems in rarefied gasdynamics is described. The motivation for operating at low density (to increase the length and time scales of certain interesting flows) and the effect of low density on the performance and design of the shock tube are discussed. In order to guarantee uniform and reproducible shock waves of moderate strength, the configuration of the tube is conventional. However, innovations are introduced (for example in the suspension, the pumping system, and the diaphragm loading and rupturing mechanism) to simplify the operation of the large facility. Care in the design of the tube as a vacuum system has resulted in a leak rate of less than 0.01 μ Hg per hour. A series of shakedown runs at relatively high pressures has shown, for example, that the reproducibility of a given shock Mach number is ±0.6%.

Publication: Review of Scientific Instruments Vol.: 33 No.: 6 ISSN: 0034-6748

ID: CaltechAUTHORS:20130509-141109412

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Abstract: The new GALCIT 17-in. low-pressure shock tube is equipped with a novel device which was designed to overcome some of the difficulties encountered in attempting to precisely select and control the bursting pressure of very thin diaphragms. This device is a cutter, shown in Figs. 1 and 2, which consists of two blades set at right angles in a cruciform configuration and positioned in the tube on the low-pressure side of the diaphragm.

Publication: Physics of Fluids Vol.: 4 No.: 11 ISSN: 1070-6631

ID: CaltechAUTHORS:20120907-153109132

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Abstract: Measurements on a large circular cylinder in a pressurized wind tunnel at Reynolds numbers from 10^6 to 10^7 reveal a high Reynolds number transition in which the drag coefficient increases from its low supercritical value to a value 0.7 at R = 3.5 × 10^6 and then becomes constant. Also, for R > 3.5 × 10^6, definite vortex shedding occurs, with Strouhal number 0.27.

Publication: Journal of Fluid Mechanics Vol.: 10 No.: 3 ISSN: 0022-1120

ID: CaltechAUTHORS:ROSjfm61

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Abstract: The severe decrease of flow duration in shock tubes operating at low pressures, previously reported by Duff, is confirmed by experiment and by an analysis of the effects of the laminar-boundary layer behind the shock wave. The latter leads to a shock tube similarity length parameter X, which depends on the tube pressure, diameter and shock Mach number, and to a flow duration parameter T. The theoretical relation T = T(X) is determined and compared with experimental results. From the theoretical result Tmax = 1, the maximum possible flow duration τm in a shock tube is determined; it increases linearly with the initial pressure and the square of the tube diameter and decreases strongly with shock Mach number.

Publication: Physics of Fluids Vol.: 3 No.: 6 ISSN: 0031-9171

ID: CaltechAUTHORS:ROSpof60

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Abstract: The experimental measurements of reference 1 show an effect of free-stream pressure on Reynolds Number relation for a vortex-shedding cylinder.

Publication: Journal of the Aerospace Sciences Vol.: 26 No.: 2 ISSN: 1936-9999

ID: CaltechAUTHORS:20141202-140924487

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Abstract: The flow in a rectangular cavity, or slot, in the floor or a wind tunnel is described by the results or pressure and velocity measurements. Pressure distributions on the cavity walls as well as measurements of friction are presented. The effects of varying depth-breadth ratio are shown.

ID: CaltechAUTHORS:20141114-111545203

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Abstract: A modification to Kirchhoff's free streamline introduces the parameter k = √(1- C_ps), which allows arbitrary base pressure and which must depend on the dynamics of the wake. For a cylinder of given cross-sectional shape, the drag, C_D, and the wake width, d', are functions of k only. These functions are used to relate C_D and the dimensionless shedding frequency , S = nd/U_ ∞ to another number, S* = nd' / U_s, which is based on wake parameters. It is found that S* = 0.16 for all cylinders. In another approach, k is evaluated by using Karman's solution for the vortex street.

Publication: Journal of the Aeronautical Sciences Vol.: 22 No.: 2 ISSN: 1936-9956

ID: CaltechAUTHORS:20141202-134209863

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Abstract: Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hot-wire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At R=40 to 150, called the stable range, regular vortex streets are formed and no turbulent motion is developed. The range R=150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. This transition first occurs in the range R = 150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations. In the stable range the vortices decay by viscous diffusion. In the irregular range the diffusion is turbulent and the wake becomes fully turbulent in 40 to 50 diameters downstream. It was found that in the stable range the vortex street has a periodic spanwise structure. The dependence of shedding frequency on velocity was successfully used to measure flow velocity. Measurements in the wake of a ring showed that an annular vortex street is developed.

ID: CaltechAUTHORS:ROSnacarpt1191

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Abstract: Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hotwire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At R = 40 to 150, called the stable range, regular vortex streets are formed and no turbulent motion is developed. The range R = 150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. This transition first occurs in the range R = 150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations. In the stable range the vortices decay by viscous diffusion. In the irregular range the diffusion is turbulent and the wake becomes fully turbulent in 40 to 50 diameters downstream. It was found that in the stable range the vortex street has a periodic spanwise structure. The dependence of shedding frequency on velocity was successfully used to measure flow velocity. Measurements in the wake of a ring showed that an annular vortex street is developed.

ID: CaltechAUTHORS:20141114-105935749

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Abstract: Measurements of the reflection characteristics of shock waves from a flat surface with a laminar and turbulent boundary layer are presented. The investigations were carried out at Mach numbers from about 1.3 to 1.5 and a Reynolds number of 0.9 x 10^4. THe difference in the shock-wave interaction with laminar and turbulent boundary layers, first found in transonic flow is confirmed and ,investigated in detail for supersonic flow. The relative upstream influence of a shock wave impinging on a given boundary layer has been measured for both laminar and turbulent layers. The upstream influence of a shock wave in the laminar layer is found to be of the order of 50 bounday-layer thicknesses as compared with about 5 in the turbulent case. Separation almost always occurs in the laminar boundary layer. The separation is restricted to a region of finite extent upstream of the the shock wave. In the turbulent case no separation was found. A model of the flow near the point of impingement of the shock wave on the boundary layer is given for both cases. The difference between impulse-type and step-type shock waves is discussed and their interactions with the boundary layer are compared. Some general considerations on the experimental production of shock waves from wedges and cones are presented, as well as a discussion of boundary layer in supersonic flow. A few exampies of reflection of shock waves from supersonic shear layers are also presented.

ID: CaltechAUTHORS:LIEnacarpt1100

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Abstract: The design of a small supersonic wind-tunnel test section (4 by 10 in.) incorporating a flexible nozzle is outlined. The flexible nozzle consists of a high-strength stepped steel plate. Two screw jacks provide an easy means of continuously changing the nozzle's shape according to the aerodynamic requirements. The boundary-layer compensation can also be varied during operation. Pressure surveys, together with schlieren and interferometric analysis of the test section, show the flow to be uniform over the operating range (M = 1.1 to 1.5).

Publication: Journal of the Aeronautical Sciences Vol.: 18 No.: 4 ISSN: 1936-9956

ID: CaltechAUTHORS:20141202-143708452

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