Disturbances, lift and drag forces due to the translation of a horizontal circular cylinder in stratified water

Abstract

The uniform flow of a fluid with a narrow stratified layer past a horizontal circular cylinder is studied experimentally. This is done through a Galilean transformation of the problem to a situation where the cylinder moves, and the water is at rest. Measurements were made of the interfacial waves formed behind the cylinder towed horizontally at constant speeds. A specially designed stiff force measuring system with a resolution of 0.5 mN measured the drag and lift forces exerted upon the cylinder. When fluid buoyancy forces dominate, it is shown that the increased drag force and other pertinent properties of the problem are efficiently described in terms of a densimetric Froude number, and explicitly independent of the Reynolds number. Lee-waves were detected at all towing speeds less than the speed of long interfacial gravity waves. Maximum wave heights occurred at half of that speed. Vortex shedding was hampered for speeds less than 0.65 of the long interfacial wave speed. Recommended values of increased drag-coefficients are given. The importance of a finite thickness of the stratified layer is documented. The critical densimetric Froude number defining when stratification starts to be important becomes lower with increasing layer thickness. And, with the cylinder located in the stratified layer, the drag-force does not increase although internal waves of appreciable height develop. The situation modelled has its engineering counterpart in the flow past submerged tube bridges.