Current Flow Past Large Concrete Piers:CFD Analysis Vs. Physical Model Tests

Abstract

During the design, construction and installation of a bridge pier over the Tacoma Narrows, the pier was constructed in place as a floating caisson, which was moored to hold it against a large current due to tidal changes in the Narrows. The design of the mooring system required the knowledge of the magnitude of current force due to ebb and tide. This current force on the caisson was determined via a physical model test as well as a numerical model based on the Computational Fluid Dynamics (CFD) method. The purpose of this paper is to compare the CFD computations with the model test data and show the similarity and disparity of the two results. A comparison between the CFD modeling and physical modeling results was made. The mean drag, lift and vertical force coefficients for the caisson are directly compared. Both the frequencies and magnitudes at which the force peaks appear are directly compared through FFT analysis. The frequency contents of the inline and transverse loads are compared in the frequency domain. The frequencies give a qualitative comparison of the extent of the vortices present in the CFD vs. model tests, since the frequencies are a direct function of the number of vortices present in the flow. The magnitudes of the peaks are indicative of the strength of the vortices present and showed if the CFD method was able to identify accurately the flow pattern past the caissons as well as highlighted any possible scaling effects of model tests.