Discussion of “Three-Dimensional CFD Modeling of Self-Forming Meandering Channel” by Nils Reidar B. Olsen

Abstract

The paper presented the computational simulation of a selfforming meandering channel from an initially straight channel with a three-dimensional computational fluid dynamics ~CFD! model. The simulated meandering channel wavelength and magnitude are closer to the experimental results ~Friedkin 1945! as compared to the simulated results with an enhanced 2D model ~Duan et al. 2001!. However, the discusser feels that the simulated results will be more convincing if the author explained in detail the approaches in calculating suspended sediment and bedload transport and plotted the simulated velocity vector field and bed topographic configurations shown in Fig. 7. In Eq. ~1!, the author set the suspended sediment diffusion coefficient equal to the eddy viscosity taken from the k‐« model. Studies have shown that the mass diffusion coefficient can be expressed as G5v t /s c , in which v t5eddy viscosity, and s c5turbulent Schmidt number, which represents the ratio of eddy viscosity to eddy diffusivity. A value of s c50.5 has been found suitable in previous calculations of pollutant spreading in an open channel ~Rastogi and Rodi 1978!. Ye and McCorquodale ~1997! have used s c50.15 in the simulation of pollutant dispersion with a depth-averaged 2D model. With respect to suspended sediments, the mass diffusion coefficient is equal to the eddy viscosity only when the Schmidt number equals 1.0, which is not true based on previous studies ~Rastogi and Rodi 1978; Ye and McCorquodale 1997!. In particular, the mass diffusion coefficient for suspended sediments in the vertical direction « z is much larger than that in the horizontal direction and relates to the fluid momentum diffusion ~van Rijn 1984! as follows: