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 la yer was studied experimentally. The desired flow conditions were reali zed by generating decaying grid turbulence with a uniform mean velocit y 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 describ ed by the theory of Hunt & Graham (1978). Later, where this theory cea ses to give an accurate description, terms of the Reynolds stress tran sport (RST) equations were measured or estimated by balancing the equa tions. An important finding is that two different length scales are as sociated with the near-wall damping of the Reynolds stresses. The wall -normal velocity component is damped over a region extending roughly o ne macroscale out from the wall. The pressure-strain redistribution th at normally would result from the Reynolds stress anisotropy in this r egion was found to be completely inhibited by the near-wall in influen ce. 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 t he normally expected isotropization. This behaviour is not captured by current models.