Wake Growth and Collapse in Stratified Flow

Abstract

The objective of this research was to obtain data that could be used to predict the effect of stratification on the development of a momentumless wake. Experiments were performed in which a grid was oscillated in a stablystratified flow to produce the steady-state counterpart of the momentumless wake of a self-propelled vehicle. A pH-sensitive indicator was used to produce a neutrally-buoyant tracer to visualize wake development and subsequent vertical collapse. Measured velocity profiles in the simulated wake indicated that it was nearly momentumless, and vertical temperature surveys revealed the degree of mixing in the wake. The wake growth before and after collapse and the distance to collapse were correlated by using power laws and a theoretical analysis of wake collapse. The scaling relations established for predicting stratified flow wake dimensions revealed that the important parameters were the Froude number, defined as the product of the flow velocity and the Brunt-Vaisala period divided by the initial wake size, and the ratio of the time after wake generation to the Brunt-Vaisala period. Through the use of these parameters, two unique curves were obtained for estimating the horizontal width and vertical height of a wake in a stratified flow.