Wall Layer of Plane Turbulent Wall Jets without Pressure Gradients

Abstract

Experiments with jet to mainstream velocity ratios, Uj/Ue from 1.5 to oo, were conducted in a two-dimensional wall-jet facility with jet slot heights of 0.056 and 0.155 in. over a span of 30 in. For cases with mainstream flow, the mainstream velocity Ue was #120 fps. Velocity profiles were computed from measured pressure and temperature profiles. Experimental friction coefficients were determined using Preston tubes. The experimental velocity profiles on the semilogarithmic plot of U vs logy + [where U = n/w., Y = u,y/v, Ur = (rw/p)] converge to a common line near the wall. This linear portion has a 15% lower slope than Coles' of the for boundary layers. At logF=2.3, U agree closely with Coles' equation. (In the foregoing definitions, u is local velocity, y is distance from the wall, v is kinematic viscosity, rw is wall shear stress, and p is density.) The data were analyzed assuming that the velocity profiles approached Coles' equation for the law of the wall. The corresponding friction velocities, wt, and hence the friction coefficients, were evaluated by demanding a systematic approach to the law of the wall, considering all velocity ratios. For large Um/Ue, the velocities deviated from Coles' equation nearly linearly with y. For low Um/Ue (Um/Ue<2.5) the velocity profiles asymptotically approach Coles' equation. For Umll7e near 1.0, a boundary-layer-type wake region is formed in the wall layer. It is believed that pressure measurement errors due to turbulence effects and Preston tube errors account for the fact that all the experimental data yielded a common semilogarithmic region of reduced slope (compared to Coles' equation) near the wall. The nearly linear correlation of the velocity deviations from Coles' law of the wall, as evaluated from