Numerical investigation of velocity distribution of turbulent flow through vertically double-layered vegetation

Abstract

The velocity structures of flow through vertically double-layered vegetation (VDLV) as well as single-layered rigid vegetation (SLV) were investigated computationally with a three-dimensional (3D) Reynolds stress turbulence model, using the computational fluid dynamics (CFD) code FLUENT. The detailed velocity distribution was explored with a varying initial Froude number (Fr), with consideration of the steady subcritical flow conditions of an inland tsunami. In VDLV flows, the numerical model successfully captured the inflection point in the profiles of mean streamwise velocities in the mixing-layer region around the top of short submerged vegetation. An upward and downward movement of flow occurred at the positions located just behind the tall and short vegetation, respectively. Overall, higher streamwise velocities were observed in the upper vegetation layer due to high porosity, with Pr = 98% (sparse vegetation, where Pr is the porosity), as compared to those in the lower vegetation layer, which had comparatively low porosity, with Pr = 91% (dense vegetation). A rising trend of velocities was found as the flow passed through the vegetation region, followed by a clear sawtooth distribution, as compared to the regions just upstream and downstream of vegetation where the flow was almost uniform. In VDLV flows, a rising trend in the flow resistance was observed with the increase in the initial Froude number, i.e., Fr = 0.67, 0.70, and 0.73. However, the flow resistance in the case of SLV was relatively very low. The numerical results also show the flow structures within the vicinity of short and tall vegetation, which are difficult to attain through experimental measurements.