Experimental friction factors for turbulent flow with suction in a porous tube

Abstract

Measurements of pressure gradient have been obtained with air flowing in a porous tube of circular cross-section with fully-developed turbulent profile at the entrance and with uniform mass extraction through the wall; an empirical correlation for the axial gradients with suction is presented. A slight but insignificant gradient in the radial direction was detected. The experiments covered an inlet Reynolds number range of 11000–101000 with a ratio of the transverse velocity at the wall to the mean axial velocity at inlet from zero to about 0.027. The radial distributions of both the temporal-mean axial velocity and the absolute turbulent velocity fluctuation were measured at the tube exit and the velocity profile parameter has been correlated with a suction parameter. The form of the temporal-mean velocity profile is found to depend critically upon the suction rate, becoming more flat at modest rates of suction but more peaked at high rates. The relative turbulence level is found to increase with suction at all radii, save for some reduction in the region of the wall at very low rates of suction. Local and average values of the effective friction factor with suction have been computed and are presented in graphical form. At a fixed inlet Reynolds number average values increased markedly with suction but decreased along the tube. For a given local Reynolds number local values displayed a similar trend.