Effect of traversal jet injections on skin friction in a turbulent channel

Abstract

The effects of small spanwise inclined jets on the wall shear stress were investigated in a fully developed turbulent 2-D channel flow. Five small jets inclined at 45° in the spanwise direction are introduced only along the upper wall of a Perspex channel measuring 20mm by 300mm. Hot-wire measurements were carried out to measure both the shear stress and the velocity profiles on the upper wall at various positions downstream of the jet injections and for jet flow rates from about 1% to 5% of the that of the main channel flow. Local and spanwise averaged mean shear stress reductions of up to 50% and 17% respectively were measured under optimal conditions. The velocity profile development is tracked downstream at points of minimum shear stress. The results suggest that the spanwise forcing by the inclined jets is able to weaken the turbulence of the flow and increase coherence in the flow. Although different mechanisms seem to be responsible for the reduction in the mean shear stress, its reduction is likely to be largely the result of this increase coherence.