A numerical study of strained three-dimensional wall-bounded turbulence

Channel flow, initially fully developed and two-dimensional, is subjected t o mean strains that emulate the effect of rapid changes of streamwise and s panwise pressure gradients in three-dimensional boundary layers, ducts, or diffusers. As in previous studies of homogeneous turbulence, this is done b y deforming the domain of a direct numerical simulation (DNS); here however the domain is periodic in only two directions and contains parallel walls. The velocity difference between the inner and outer layers is controlled b y accelerating the channel walls in their own plane, as in earlier studies of three-dimensional channel flows. By simultaneously moving the walls and straining the domain we duplicate both the inner and outer regions of the s patially developing case. The results are used to address basic physics and modelling issues. Flows subject to impulsive mean three-dimensionality wit h and without the mean deceleration of an adverse pressure gradient (APG) a re considered: strains imitating swept-wing and pure skewing (sideways turn ing) three-dimensional boundary layers are imposed. The APG influences the structure of the turbulence, measured for example by the ratio of shear str ess to kinetic energy, much more than does the pure skewing. For both defor mations, the evolution of the Reynolds stress is profoundly affected by cha nges to the velocity-pressure-gradient correlation Pi(ij). This term-which represents the finite time required for the mean strain to modify the shape and orientation of the turbulent motions-is primarily responsible for the difference (lag) in direction between the mean shear and the turbulent shea r stresses, a well-known feature of perturbed three-dimensional boundary la yers. Files containing the DNS database and model-testing software are avai lable from the authors for distribution, as tools for future closure-model testing.