CFD for Predicting Shear Stress in a Curved Trapezoidal Channel

Abstract Continuing advances in computer speed and the availability of user friendly software has made computational fluid dynamics a cost effective compliment and at times alternative to physical modeling in the field of civil engineering hydraulics. Validation of the CFD models for open channel flow conditions remains limited due to the significant cost of obtaining data. Furthermore, once the flume study has been conducted, a numerical study is typically no longer required. This paper explores the ability of CFD to reproduce free surface flow in a trapezoidal channel in a bend. Laboratory data was collected at MIT during the late 1950's and early 1960's to obtain an understanding of shear stress distribution and flow patterns in bends. A portion of those studies was reproduced using CFD and a comparison made between observed and predicted values. The objective of the paper is to demonstrate that CFD can offer a cost effective alternative and compliment to physical modeling of smooth, rigid boundary conditions. The CFD software Fluent (1998) was used to model flow in a trapezoidal channel using two turbulence models, K-Epsilon and Reynolds Stress. Model results show that observed and predicted water surface elevations typically differ by less than 2.5 percent, and predicted shear stresses differ by less than 10 percent. Model results are also used to illustrate the limitations of the K-Epsilon model as well as conditions under which it produces very similar results to the significantly more expense Reynolds Stress model. Results of the study clearly demonstrate the strengths and weaknesses of using CFD as an alternative or compliment to physical modeling.