DIRECT NUMERICAL-SIMULATION OF STRAINED 3-DIMENSIONAL WALL-BOUNDED FLOWS

Fully developed two-dimensional channel-flow turbulence is subjected t o mean strains that imitate those produced by pressure gradients in th ree-dimensional boundary layers. This is achieved by applying irrotati onal temporal deformations to the flow domain in a conventional channe l direct-numerical-simulation (DNS) code; straining deformations at an angle with respect to the initial flow direction generate a mean cros s flow and thus mean three-dimensionality. The velocity difference car ried by the near-wall region is further controlled by mean pressure gr adients (or by accelerating the walls in-plane), thus introducing anot her effect of pressure gradients in boundary layers. ''Numerical exper iments'' allow the effects of the inviscid skewing mechanism, adverse pressure gradient, and inner layer to be isolated; our primary interes t here is in the outer layer. We present five simulations. In-plane sk ewing decreases both the Reynolds shear stress and turbulent kinetic e nergy, whereas strains characteristic of two-dimensional adverse press ure gradients increase them. In all cases, the structure parameter a(1 ), the ratio of shear stress to energy, is diminished, which implies a reduction in the efficiency of the kinetic energy production by the m ean shear. (C) Elsevier Science Inc., 1996