Turbulence structure of a boundary layer beneath a turbulent free stream

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.