Turbulent Flow in Rectangular Ducts

Abstract

The hydraulic radius of any conduit cross section is nothing else but the corresponding parameter of an equivalent two-dimensional channel; consequently, it cannot account for shape effects. Such correlating power is, however, implied whenever the resistance laws for circular pipes are used to predict skin friction resistance in noncircular conduits. Inherent in this procedure is the assumption that the same similarity relations that led to the derivation of the pipe laws are also satisfied in the general conduit. An experimental investigation of the turbulent mean flow in smooth rectangular air ducts yielded strong evidence in favor of a universal inner law of velocity distribution; however, no universal outer law correlation of the velocity data was possible. Further results indicated friction coefficients smaller than those for circular pipes and a trend toward greater uniformity with increasing Reynolds number of conventional nondimensional representations of both wall shear stress and velocity. The static pressures inside the flow cross sections were found to be higher than those at the periphery.