Numerical investigation on the suitability of a fourth-order nonlinear k-ω model for secondary current of second type in open-channels

This study evaluated a typical nonlinear fourth-order k-ω model and the associated roughness boundary condition for their suitability for simulating secondary current of the second kind in open-channels induced by roughness non-uniformity. Three test cases were simulated: square duct flow, open-channel flow with uniformly roughened bed and smooth side walls, and open-channel flow over roughness patches. The nonlinear model and the roughness boundary condition performed relatively well for the first two cases where the corner effect was dominant. For the third case, with alternate roughness patches, strong pressure gradient was generated at the edge between adjacent patches and the secondary current was greatly over-predicted. Two remedial options were tested: increasing the roughness height for smooth walls to a reasonable value and smoothing the roughness transition with a hyperbolic tangent function. We found the first option works better and the second only marginally improves the results.