with an integral length scale and r.m.s. velocity is analyzed in the absence of a mean flow as it impacts onto a fixed undulating surface of wavelength and amplitude a, with a low slope so that The boundary condition of zero normal velocity at the surface or ‘blocking’, causes irrotational fluctuations to be generated within a ‘source’ layer with thickness of order Very close to the surface there is a thin viscous layer

The curl of the gradients of Reynolds normal and shear stresses within cause the growth of a mean vorticity which induces a mean velocity of order within and a weaker recirculating velocity of order in a deeper layer with thickness of order outside The wavelength of the nonlinear motion is with downward motions over the crests and troughs. Secondly the effects of anisotropy in the impinging turbulence is studied over a sloping plane surface where principle axes of the anisotropy tensor lie at an angle to the surface. This leads to gradients of the Reynolds shear stresses normal to the surface and a mean motion up or down the slope of order Thus in the case of anisotropic free stream turbulence the mean motion has the same wavelength as that of the undulating surface. This effect also explains how in complex turbulent flows near rigid

surfaces the anisotropic eddy structure affects the mean flow, such as reattaching shear layers, and two phase flow in sloping pipe flow. A new physical explanation is proposed for this surprising phenomenon in which a homogeneous fluctuating velocity field induces a mean vorticity field through the action of irrotational fluctuations caused by blocking of eddies at the surface. Flow visualization experiments conducted in a mixing box with