Direct numerical simulation of turbulent boundary layers with adverse pressure gradient

Abstract

Direct numerical simulations of the turbulent boundary layer subjected to an adverse pressure gradient were performed. The main emphasis of this work was placed on the alteration of the turbulent structures due to adverse pressure gradient. The spatially developing characteristics of the turbulence stresses in the non-equilibrium adverse pressure gradient flows were carefully examined. The instantaneous flow fields and vorticity fluctuations were analyzed to characterize the response of the outer turbulence to an adverse pressure gradient. The present results showed that the mean flows are greatly affected by an adverse pressure gradient, and the coherent structures in the outer layer of the adverse pressure gradient flows were more activated than those in the zero pressure gradient flow which may be attributed to increased turbulence intensities, shear stresses and pressure fluctuations in the adverse pressure gradient systems. Examination of the Reynolds stress budget revealed that the energy redistribution was enhanced in the outer layer of the adverse pressure gradient flows compared to the zero pressure gradient flow. The effects of adverse pressure gradient on the turbulent structures are investigated by carrying out direct numerical simulations of turbulent boundary layers subjected to adverse and zero pressure gradients. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. Two-point correlations of the velocity fluctuations were used to show that the spanwise spacing between near-wall streaks is affected significantly by a strong adverse pressure gradient. Flow visualizations of the flow with an adverse pressure gradient show that low-momentum regions are dominant in the middle of the boundary layer as well as in the log layer. Linear stochastic estimation was used to provide the statistical evidence for the presence of low-momentum region in the adverse pressure gradient flow and their stati...