Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2006
Publication Date: 6/1/2007
Citation: Chan, H.C., Leu, J.M., Lai, C.J., Jia, Y. 2007. Turbulent flow over a channel with fluid-saturated porous bed. Journal of Hydraulic Engineering. V. 133, No. 6, 610-617.
Interpretive Summary: The present study presents the results obtained from a numerical simulation of two-dimensional turbulent open channel flow over and through a porous medium with porosity ranging from 0.6 to 0.8 and Darcy number ranging from 1.6x10-4 to 1.6x10-2. The flow within the porous medium is computed at the macroscopic level. The single domain approach is suitably applied to compute the flow field within the hybrid domain. Our simulated results remained consistent with the previously published microscopic and experimental data. Notably, the macroscopic model approach provides an efficient way to study the problem of flow over the porous medium. The simulated results show that the porous medium reduces the flow velocities inside the clear fluid region relative to an impermeable bed, making the integral constant B of the velocity logarithmic distributions lower than the traditional value B=5.25. The penetration of turbulence results in the extension of turbulent kinetic energy into the porous area. The distributions of the turbulent kinetic energy demonstrate that, for highly permeable materials, the extra production in the (k)i equation is particularly important. Moreover, the increase in Da and porosity significantly enhances the levels of turbulent shear stress within the upper part of the porous medium. The thickness of penetration quantifies the effect of the propagation of turbulent shear stress across the interface and its value remains proportional to Da and porosity. Our simulated findings concerning the turbulent flow over the porous bed and their observed consistency with the existing data indicate that the present model can successfully represent the penetration of turbulence into the upper part of a permeable medium.
Technical Abstract: The characteristics of fully developed turbulent flow in a hybrid domain channel, which consists of a clear fluid region and a porous bed, are examined numerically using a model based on the macroscopic Reynolds-averaged Navier–Stokes equations. By adopting the classical continuity interface conditions, the present model treats the hybrid domain problem with a single domain approach, and the simulated results are noted to coincide with the existing experimental data and microscopic data. The effects of porosity and Darcy number Da on the flow properties over and inside the porous bed are further investigated in the selected ranges of 0.6 to 0.8, and 1.6x10-4 to Da=1.6x10-2. It has been demonstrated that the presence of the porous bed causes the significant reduction of the flow velocities inside the clear fluid region relative to that of a smooth impermeable bed, and also reduces the magnitude of the integral constant B of the velocity logarithmic distributions from its traditional value 5.25. Moreover, turbulent shear stress within the upper part of the porous bed increases significantly with the porosity and Darcy number Da. The thickness of turbulence penetration remains proportional to the values of porosity and Darcy number Da.