MODELLING SHALLOW WATER FLOW AROUND PILE GROUPS.

Abstract

The velocity vector fields and far wake instabilities associated with flow through pile groups is shallow water were measured in physical models using particle tracking velocimetry. The piles were in a staggered arrangement with spacings of two, four and eight diameters. The flow structure showed similar large scale features in all cases. A stagnant near wake zone was formed with a length that increased as the pile spacing increased. Wake oscillation occurred downstream, with a remarbly constant Strouhal number of about 0.22 based on upstream velocity and pile group width. However, wake oscillation was intermittent for the eight-diameter spacing, suggesting that it might die away for greater spcings. The results could be broadly reproduced in a depth-averaged shallow water model by using suitably increased values of bed friction (Manning's n is used), a very simple modelling strategy. However, the n values required are about twice the magnitude that is calculated from straightforward drag representation based on single piles. Based on the results, a methodology for including effects due to pile groups in shallow water computer models of rivers and marine environments is suggested.