Monograph records
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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 18:27:33 +0000On Reflection of Shock Waves from Boundary Layers
https://resolver.caltech.edu/CaltechAUTHORS:LIEnacarpt1100
Authors: Liepmann, H. W.; Roshko, A.; Dhawan, S.
Year: 1952
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.https://authors.library.caltech.edu/records/2haej-rzc24On the Development of Turbulent Wakes From Vortex Streets
https://resolver.caltech.edu/CaltechAUTHORS:20141114-105935749
Authors: Roshko, Anatol
Year: 1953
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.https://authors.library.caltech.edu/records/jpc6y-qvc49On the Development of Turbulent Wakes from Vortex Streets
https://resolver.caltech.edu/CaltechAUTHORS:ROSnacarpt1191
Authors: Roshko, Anatol
Year: 1954
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.https://authors.library.caltech.edu/records/5kj7e-c4187Some Measurements of Flow in a Rectangular Cutout
https://resolver.caltech.edu/CaltechAUTHORS:20141114-111545203
Authors: Roshko, Anatol
Year: 1955
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.https://authors.library.caltech.edu/records/d4pqs-j5d92Incipient Separation of a Turbulent Boundary Layer at High Reynolds Number in Two-Dimensional Supersonic Flow over a Compression Corner
https://resolver.caltech.edu/CaltechAUTHORS:20170726-143226034
Authors: Thomke, G. J.; Roshko, A.
Year: 2017
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.https://authors.library.caltech.edu/records/qwwhg-vpe43Effect of Flow Oscillations on Cavity Drag and a Technique for Their Control
https://resolver.caltech.edu/CaltechAUTHORS:20170726-123311179
Authors: Gharib, M.; Roshko, A.; Sarohia, V.
Year: 2017
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.https://authors.library.caltech.edu/records/tgdfe-8b591