COMPARISON OF TURBULENCE MODELS FOR STAGE-DISCHARGE RATING CURVE PREDICTION IN REACH-SCALE COMPOUND CHANNEL FLOWS USING TWO-DIMENSIONAL FINITE ELEMENT METHODS

Abstract

This paper attempts to assess the accuracy of constant eddy viscosity, Elder and k-ε turbulence models in the numerical simulation of reach-scale compound channel flows using two-dimensional (2D) finite element methods. Assessment was conducted using benchmark stage-discharge data collected from straight and meandering compound channel configurations at the UK Engineering and Physical Science Research Council (EPSRC) Flood Channel Facility. For mesh resolutions and topologies used in reach-scale studies, all models were found to be adequate predictors (<5% error in predicted flow depth) of the stage-discharge relationship at moderate overbank flows (Figs. 1 and 2)Fig. 1Main channel cross-sectional details of the EPSRC FCF at a distorted scale (a) series A straight channel (b) series B meandering channel.Fig. 2Finite element meshes developed for the: (a) EPSRC FCF series A channel with trapezoidal cross-section; (b) EPSRC FCF series B channel with trapezoidal cross-section.. However, at inbank and low overbank flows the Elder and k-ε turbulence models can reproduce stage-discharge points with much greater accuracy than the constant eddy viscosity model. Hence, for an unsteady simulation where low flows are relevant a constant eddy viscosity turbulence closure may prove problematic. In terms of computed lateral distributions of depth-averaged velocity for both channel configurations, at higher depths (Relative depth=0.666) all turbulence models predict the velocity with greater accuracy than at a lower depth (Relative depth=0.333). At this latter depth, all turbulence models predict the depth-averaged longitudinal velocity distribution with poor accuracy (>20% error). Also, sensitivity of the turbulence parameter calibration with respect to the predicted flow depth showed that the constant eddy viscosity model's performance can be highly dependent on the choice of turbulence parameter value.