Numerical assessment of local forcing on the heat transfer in a turbulent channel flow

Abstract

The influence of local forcing on an incompressible turbulent channel flow is numerically investigated. The extensive information provided by the direct numerical simulations enables us to have a better understanding of the physical mechanism responsible for local heat transfer enhancement. Time-periodic blowing/suction is applied by means of thin spanwise slots located at the lower and upper walls. The molecular Prandtl number is 0.71 and the Reynolds number based on the wall friction velocity and the channel half-height, Reτ, is 394 for the unforced case. The normal perturbing velocity is varied sinusoidally in time at several perturbing frequencies between 0.16<f¯<1.6 or 0.011<f+<0.11 and at a fixed amplitude of Ao=0.2. A phase-averaging procedure is employed to discriminate between the coherent and incoherent fluctuating components. It is shown that coherent thermal fluctuations reach peak values near the forcing slot, then sharply decay and almost disappear in a short distance downstream. The incoherent thermal fluctuations also show peak values next the source; however, they decay downstream to resemble the incoherent fluctuations of the unperturbed channel. It was concluded that the forcing frequency of f¯=0.64 or f+=0.044 produced the largest local increase in the skin friction in the region 0.1<x/Lx<0.3 (where Lx is the channel length), followed by the highest augmentation of the Stanton number. It is found that augmentation of the wall shear stress fluctuations is the major cause of skin friction, wall heat flux, and Stanton number enhancement downstream from the local forcing source. On the other hand, local maxima of Reynolds shear stresses, wall-normal turbulent heat fluxes, and the incoherent component of streamwise vorticity fluctuations exhibited analogous behavior along the streamwise direction.