Memory effects in turbulence

It is shown that the effects of a turbulent flow are not strictly local and that memory effects play a significant role, so that momentum exchange processes are not determined by the local gradient of the mean velocity alone. This is demonstrated by experimental investigations of the wake flows of a hemisphere in a turbulent boundary layer and of a cylinder. Both cases involve a rapidly changing flow in the developing region. When the shear stress is expressed by the velocity gradient and a vorticity viscosity, the vorticity viscosity, when corrected for the memory effect, equals the value for the developed flow.