Turbulence statistics and scalar transport in an open-channel flow

In this study, direct numerical simulations (DNS) of a 2D fully-developed t urbulent open-channel flow were performed. Numerical investigations were ca rried out for two Prandtl (Schmidt) numbers 1.0 and 5.0, with a Reynolds nu mber 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 t urbulence structures affected by the presence of the free surface appeared in the 5% region from the free surface. The effect of these free-surface tu rbulent 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 transfe r across the free surface was enhanced by a large horizontal vortex and the flow depth scale was also affected by this free-surface turbulent structur e. In the case of a scalar field with Pr = 5.0, besides this large horizont al vortex, a high filamentary fragment exists because the time scale of the fluid motion is so fast compared to the scalar diffusion time scale. Moreo ver, the high wavenumber fluctuation effect on the scalar transport is larg er than the scalar field of Pr = 1.0. These indicate that the Reynolds anal ogy between momentum and scalar transport cannot be applied to high Prandtl or Schmidt number fluids. Consequently, for the accurate prediction of sca lar quantities near free-surface turbulence models based on the unsuitable assumption should be modified in the high Prandtl (Schmidt) number fluids.