Mean flow and turbulence measurements of a two-dimensional, incompressible, constant-pressure, flat-plate turbulent boundary layer beneath a nearly homogeneous, nearly isotropic (grid-generated) turbulent free stream are presented. 1600 < Ree < 5000. Thomas & Hancock (1977, appendix 1) have shown experimentally that for high Reynolds numbers the impermeable-surface condition .v = 0 on uniformly-convected grid-generated turbulence primarily affects the large scale motion; the condition affects a layer approximately 2Le in thickness, where Le is the free-stream length scale. It is shown in the present thesis that the ratio between free-stream and boundary layer length scales, Le/6, has a large effect, at least when Le/6 = 0[1]. This effect was largely obscured in previous work.A careful set of mean velocity profiles and skin-friction measurements have substantially clarified the quantitative effect on the mean flow. The law of the wall is unaffected while the defect law is dependent upon both the free-stream intensity, (u'/U)e (strictly u~/u, ), and Le/6. An empirically-derived correlation for the skin friction is given.A slightly heated boundary layer and a three-wire probe Finally, I wish to thank my wife, Jan, for her continued and enthusiastic support (sometimes amidst rampant gloom or frustration); she will be more than pleased to see this ended. DECLARATIONThe work reported by and included here as appendix 1 was equally supported by Dr N.H. Thomas and the present author. Had there not been joint interest in that work a similar experimental program would have been followed by the present author and incorporated in the main body of this thesis.The main data-reduction computer programs employed were taken from those generally available within the Department of Aeronautics. The more important modifications made to them are discussed in the text.The remainder of the work reported herein was conducted solely by the present author under the guidance of his supervisor. 3.2 Directory of mean velocity profile measurements;single-row-of-square-bars turbulence generators. Le/5995 z 0.7 3.34 Velocity-defect profiles, 7.6 cm grid, XLE = 2.06 m 3.35 Velocity-defect profiles, 15.2 cm grid, XLE = 2.06 m 3.36 Velocity-defect profiles, 15.2 cm grid, XLE = 2.06 m 3.37 Velocity-defect profiles, (u'/U)e z 0.025 xi -3.38 Velocity-defect profiles, (.u'/U)e z 0.040 3.39 Velocity-defect profiles, (.u'/U)e , z 0.052 3.40 Velocity-defect profiles, LēJ6995 1.93.41 Velocity-defect profiles, (u'/U)e n 0.059, Le/6995 z 1. The presence of free-stream turbulence in wind tunnels can lead to discrepancies between model and full-size tests (see for example, Milligan & Klein, 1933), in addition to those due to Reynolds number effects. Downstream of a reattachment point a new sub-boundary layer will develop with fluid from the reattached mixing layer approximating to a field of relatively unsheared "external" turbulence -1 - (Chandrsuda, 1975).For wavelengths of the free-stream turbulence (or ordered unsteadiness, or both) very much la...

Measurements have been made in the turbulent boundary layer on a flat plate in the presence of grid-generated free-stream, turbulence with a wide range of lengthscales. The data include conditionally sampled averages in which free-stream fluid was distinguished from boundary-layer fluid by heating the latter. Free-stream turbulence increases the standard deviation of the hot–cold interface as a proportion of the boundary-layer thickness, whilst the average position is mainly dependent upon the lengthscale. The shear correlation coefficient of the boundary-layer fluid decreases, and it is shown that the change in structure is directly related to the fluctuating-strain rate.Transport velocities representing the diffusion of turbulent kinetic energy and shear stress have opposite signs in the boundary-layer fluid to those in the free-stream fluid, and it is shown that they are also related to the fluctuating-strain rate.Complete balances of turbulent kinetic energy and shear stress have been evaluated, dissipation and pressure–strain redistribution having been deduced by difference. The dissipation length scale $L\tau = (-\overline{uv})^{\frac{3}{2}}/\epsilon $ is little affected by free-stream turbulence, whereas the corresponding parameter based on turbulent energy instead of shear stress is strongly affected.

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