An investigation on Effects of Bed Permeability on Characteristics of Turbulent Flow in Various Porosities

Abstract: The two main characteristics of a porous bed are the bed material diameter – representing its coarseness - and the porosity which represents its permeability. In this paper the effect of bed permeability on the average structure of flow turbulence is investigated. Flow is modeled using an innovativecontinuum approach based on Volume Averaged Navier-Stokes Equations in several different channel bed porosities. Results of four different simulations with various porosities are presented. Bed permeability can be represented by the permeability Reynolds number, Rek, which is the ratio of effective diameter of porosity and the length scale of eddies near the bed. The Reynolds permeability number (Rek) is the best expression for the bed permeability quantity. In small Rekthe bed acts as a solid/rigid boundary and in large Rek, the bed will behave as a high permeable boundary with negligible viscosity effects. Under these conditions, the turbulence eddies along the flow are rarely observable. The reasons can be due to: 1) the mechanism of free turbulence transfer through permeable layer and/or 2) a considerable decrease in the average shear stress due to no wall-blocking and low-viscosity effects. The dominant characteristic of turbulence near a high permeable bed is relatively large eddy structures, probably originating from so-called Kelvin-Helmholtz instabilities. Suchflow pattern with large vortexes leads to high momentum exchange between the free channel flow and the two-phase flow in porous media below permeable bed. This process also increases the friction between flow and the substrate and therefore will result in moving the Logarithmic-low region slightly downward. In addition, it is observed that the log-low cur near a high permeable bed is 4% steeper than in solid and rough bed.