A fully developed turbulent channel flow is subjected to a mean strain that approximates that in a spatially developing adverse-pressure-gradient (APG) boundary layer. This is done by applying uniform irrotational temporal deformations to the flow domain of a conventional direct numerical simulation channel code. The velocity difference between the inner and outer layer is also controlled by accelerating the walls in the streamwise plane, in order to duplicate the defining features of both the inner and outer regions of an APG boundary layer. Eventually, the flow reverses at the wall. We address basic physics and modelling issues, and create a database that makes detailed testing of turbulence models easy. As in the corresponding spatial layers, distinct inner-and outer-layer dynamics are observed: a decrease in turbulence intensity near the wall is accompanied by increased energy in the outer layer. The 'extra strain' effect associated with the diverging outer-layer streamlines is documented, particularly in the Reynolds-stress budgets.
IntroductionThis paper is a continuation of an earlier numerical study of three-dimensional wallbounded shear flows (Coleman, Kim & Spalart 2000, hereinafter referred to as CKS). In that work we examined the response of wall-bounded turbulence to the strain fields induced by streamwise and spanwise pressure gradients, as they introduce mean threedimensionality into an originally two-dimensional boundary layer. We found, among other things, that the turbulence is much more sensitive to the mean streamwise deceleration (∂U/∂x < 0) and/or wall-normal stretching (∂V /∂y > 0) than it is to mean skewing (∂W/∂x = ∂U/∂z). (The x-, y-and z-coordinates, respectively, indicate the streamwise, wall-normal, and spanwise directions, aligned with the upstream twodimensional flow.) This led us to consider the case presented here, where the strain field imitates a simpler two-dimensional non-equilibrium adverse-pressure gradient (APG) boundary layer. (We use 'non-equilibrium' as a synonym for 'perturbed' or 'non-stationary', to indicate a flow subjected to a relatively rapid change of the mean field and the ensuing finite-time-lag response of the turbulence.) Our aim is to better understand the physics of a prototypical perturbed wall-bounded shear layer, and ultimately improve turbulence models when applied to APG flows of engineering interest. As in the previous study, we employ direct numerical simulation (DNS) of a strained-channel flow (but at higher Reynolds number), with a focus upon the