An experimental study of flow past a triangular cape in a linearly stratified fluid

Abstract

Data are presented from a series of laboratory investigations into the steady uniform channel flow of a stratified fluid past a triangular cape. The spatial and temporal developments of the flow in the wake of the cape are described for a range of the principal controlling dimensionless parameters Re and S (the Reynolds number and stratification parameter respectively). It is shown that the characteristics of the primary eddies generated at the cape tip are determined essentially by the scales of the secondary eddies formed between the lee side of the cape and the lateral wall. In turn, the scales of these secondary eddies are shown to depend upon the Re and S through the influence of these parameters upon the vigour of the vertical motions within the disturbance flow. When vertical motions are weak (for example, with high values of S and/or low intermediate value of Re), the primary and secondary eddies have relatively large horizontal scales: in consequence, strong mutual interactions occur between them, resulting in the formation of eddy pairs in the far wake. As the vigour of the vertical motions increases (for example, with low values of S and/or high values of Re), the size of the secondary eddies is reduced, and the mutual interactions between primary and secondary eddies are weakened. In such circumstances (and for cases in which the length scale of the cape is reduced), the frictional effects of the lateral side wall play a dominant role in the flow development. The mode of eddy shedding from the cape is also shown to depend upon both Re and S. In particular, the transition from the shedding of relatively large eddies to the appearance of small intense tip eddies, and the dependence of this transition upon both Re and S, are demonstrated. It is shown that this transition is accompanied by increases in the value of the Strouhal number from about 0.09 for the large eddies to values for the tip eddies which depen upon both Re and S. The spin-down process for the far-field eddies is shown to be dominated by the frictional influence of the upper and lower solid bounding surfaces of the fluid. Specifically, measurements of the temporal decay of the maximum vorticity in the primary eddies indicate that the spin-down periods of such features are much shorter than the viscous timescales D 2 v . Energy dissipation by internal wave generation is negligible in the system considered.