Effect of channel cross-section on mixing length

Abstract

Establishment of longitudinal dispersion requires estimation of mixing length. Mixing length is also required for the measurement of flow in streams by the dilution method where sample is taken at one point. Available methods for estimation of mixing length are limited to rectangular channel section for constant velocity, depth of flow and transverse mixing coefficient. However, in practice these parameters are not constant and rather vary in the transverse direction. This paper deals with effect of channel cross-section of varying velocity, depth of flow and transverse mixing coefficient across the width on mixing length. Mixing lengths for twelve typical cross- sections of streams have been computed using Ahmad's finite volume scheme. Computed mixing lengths for constant mixing coefficients are more than mixing length for variable mixing coefficients. For more non-uniform section of a channel, the mixing length is found to be more for constant mixing coefficient. Relationship for mixing length for varying transverse mixing coefficient for plane source of finite length located at bank and middle of stream has been proposed. The proposed relationship for mixing length produces satisfactory results. © 2007 Taylor & Francis Group, LLC.