A loss of memory in stratified momentum wakes

Abstract

In this paper we compare the wakes of various bluff bodies in a stratified fluid at moderately high Froude numbers (F≡2UB/ND>8) and Reynolds numbers (Re≈5000). The size and amplitude of the long-lasting wakes clearly depend on the shape of the bluff body, the wake width being small for a streamlined object and large for an object with sharp edges. However, the wake width can be collapsed when it is normalized by an effective diameter based on the drag force, often called the momentum thickness. General laws for the wake width, the velocity defect, and the Strouhal number are thus deduced and fit the data well. Finally, the cross-fluctuations of the velocity and the turbulent kinetic energy are analyzed. Their amplitudes and widths are proportional to those of the mean profile. Thus, the wake remembers only the momentum flux given by the bluff body to the fluid and not any other aspects of its geometry.