Turbulence statistics and scalar transport in an open-channel flow

Abstract

In this study, direct numerical simulations (DNS) of a 2D fully-developed turbulent open-channel flow were performed. Numerical investigations were carried out for two Prandtl (Schmidt) numbers 1.0 and 5.0, with a Reynolds number of 200, based on the friction velocity and flow depth. The budgets of the Reynolds stresses and the effects of the free-surface turbulence on the scalar transport were mainly investigated. The results were that typical turbulence structures affected by the presence of the free surface appeared in the 5% region from the free surface. The effect of these free-surface turbulent structures on the fundamental turbulent quantities is not important because the free surface does not contribute to the turbulence generation in the open-channel flow without surface deformation. However, heat transfer across the free surface was enhanced by a large horizontal vortex and the flow depth scale was also affected by this free-surface turbulent structure. In the case of a scalar field with Pr = 5.0, besides this large horizontal vortex, a high filamentary fragment exists because the time scale of the fluid motion is so fast compared to the scalar diffusion time scale. Moreover, the high wavenumber fluctuation effect on the scalar transport is larger than the scalar field of Pr = 1.0. These indicate that the Reynolds analogy between momentum and scalar transport cannot be applied to high Prandtl or Schmidt number fluids. Consequently, for the accurate prediction of scalar quantities near free-surface turbulence models based on the unsuitable assumption should be modified in the high Prandtl (Schmidt) number fluids.

This article was chosen from selected Proceedings of the Eighth European Turbulence Conference (Advances in Turbulence VIII (Barcelona, 27-30 June 2000) (Barcelona: CIMNE) ed C Dopazo. ISBN: 84-89925-65-8).