A Study of the Axisymmetric Turbulent Wake Generated by Co-flowing Incompressible Streams PDF Download
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Author: James Mackson III. McMichael
Publisher:
ISBN:
Category :
Languages : en
Pages : 388
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Author: James Mackson III. McMichael
Publisher:
ISBN:
Category :
Languages : en
Pages : 388
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Author: A. Demetriades
Publisher:
ISBN:
Category :
Languages : en
Pages : 92
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The mean flow field of the axisymmetric turbulent wake shed by a very slender body at Mach 3 has been mapped with the aid of the pitot and static pressure probes and the hot-wire anemometer. The experimental geometry was chosen so that no inviscid wake existed, and the measurements were carried out to about 100 diameters downstream of the base. The axial variation of the velocity and temperature defects appears to obey classical decay laws following a relaxation distance of at least 40 diameters from the transition region. By contrast, the radial profiles of these properties relax quickly to a self-preserving form; both the temperature and the velocity radial variations fit single-parameter Gaussian distributions quite well. The proper radial similarity variable is the radius divided by the transverse wake scale; equally good agreement obtains for a Howarth-Dorodnitzyn-transformed radius, although this issue is still unsettled because of the low Mach number used. The turbulent Reynolds and Prandtl numbers computed indirectly from the data agree well with similar results from low-speed turbulent flows. (Author).
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Category : Dissertations, Academic
Languages : en
Pages : 644
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Author:
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ISBN:
Category : Aeronautics
Languages : en
Pages : 892
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Author: Eduard Naudascher
Publisher:
ISBN:
Category : Turbulence
Languages : en
Pages : 56
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The development of any inhomogeneous free-turbulence flow in an incompressible fluid is predicted through the use of a general self-preservation hypothesis. The analytical solutions are verified by comparison with experimental data on mean-flow and turbulence characteristics of plane and axisymmetric jets in coaxial parallel flow with and without confinement, of an axisymmetric jet in a cross flow, and of the wake behind a self-propelled body. Emphasis is placed on the distinct turbulence structure of the various flows as revealed by the different axial variations of a shear parameter relating turbulence shear to mean velocity. The satisfactory predictions obtained by the new approach make it appear to have a more general application (e.g., to plumes and wall jets) than the examples chosen might imply. (Author).
Author: Stanley F. Birch
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Mathieu Grandemange
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ISBN:
Category :
Languages : en
Pages : 228
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An experimental study of the turbulent wake past different geometries is performed by increasing the complexity from axisymmetric bodies to road vehicles. Whatever the geometry is, two kinds of coherent wake motions are likely to be observed. First, at timescales of the order of 5D/U, D and U being the characteristic size and velocity of the flow respectively, the wake may generate periodic oscillations. These coherent motions are usually associated with the interaction of two facing shear layers of opposite vorticity. As the corresponding frequencies rely at first order on the distance between the shear layers, two distinct frequencies are reported when the afterbody has a cross-flow aspect ratio different than 1. These unsteady global modes seem to weaken when the Reynolds number and the complexity of the geometry increase. The second type of coherent motions corresponds to the development of stationary cross-flow instabilities. They are linked to the symmetry breaking modes observed in laminar regimes and their domains of appearance are defined from geometry considerations in the cases of parallelepiped bodies in ground proximity. These instabilities are responsible for strong asymmetries in the instantaneous flow and may generate bistable dynamics with a characteristic time scale of the order of 1000D/U. The study of these phenomena, combined with sensitivity analyses to small perturbations, places the diminution of the cross-flow asymmetries of the instantaneous wake as a relevant strategy for drag reduction. In particular, it is found that both local and global pressure gradients on the sides of the body are source of streamwise vortices increasing the drag. Parabolic dependences between the drag and the cross-flow forces are reported suggesting similarities with the mechanisms of induced drag that are well-known in aeronautics. Consequently, as they often generate significant wake asymmetries, the development of the cross-flow instabilities is identified as a drag contributor. On the contrary, the part of the drag related to the periodic global modes seems to be negligible especially for complex geometries at high Reynolds number.
Author:
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ISBN:
Category : Dissertation abstracts
Languages : en
Pages : 500
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Author: R. D. Sandberg
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ISBN:
Category :
Languages : en
Pages :
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Author: Peter B. V. Johansson
Publisher:
ISBN: 9789172911789
Category :
Languages : en
Pages : 22
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