Shear-free turbulence near a wall

The mean shear has a major influence on near-wall turbulence but there are also other important physical processes at work in the turbulence/wall interaction. In order to isolate these, a shear-free boundary layer was studied experimentally. The desired flow conditions were realized by generating decaying grid turbulence with a uniform mean velocity and passing it over a wall moving with the stream speed. It is shown that the initial response of the turbulence field can be well described by the theory of Hunt & Graham (1978). Later, where this theory ceases to give an accurate description, terms of the Reynolds stress transport (RST) equations were measured or estimated by balancing the equations. An important finding is that two different length scales are associated with the near-wall damping of the Reynolds stresses. The wall-normal velocity component is damped over a region extending roughly one macroscale out from the wall. The pressure-strain redistribution that normally would result from the Reynolds stress anisotropy in this region was found to be completely inhibited by the near-wall in influence. In a thin region close to the wall the pressure-reflection effects were found to give a pressure-strain that has an effect opposite to the normally expected isotropization. This behaviour is not captured by current models.