Stratified wake of an accelerating hydrofoil

TL;DR

This study experimentally investigates how buoyancy influences the initial vortex development in the wake of an accelerating hydrofoil in stratified water, revealing vortex stretching and reduced drag during acceleration.

Contribution

It demonstrates the impact of stratification on vortex formation and wake development behind an accelerating hydrofoil, providing new insights into wake dynamics in stratified fluids.

Findings

Vortices are stretched along the streamwise direction in stratified fluids.

Stratified wakes exhibit lower momentum thickness in the late wake.

Drag on the hydrofoil is reduced during acceleration in stratified conditions.

Abstract

Wakes of towed and self-propelled bodies in stratified fluids are significantly different from non-stratified wakes. Long time effects of stratification on the development of the wakes of bluff bodies moving at constant speed are well known. In this experimental study we demonstrate how buoyancy affects the initial growth of vortices developing in the wake of a hydrofoil accelerating from rest. Particle image velocimetry measurements were applied to characterize the wake evolution behind a NACA 0015 hydrofoil accelerating in water and for low Reynolds number and relatively strong and stably stratified fluid (Re=5,000, Fr~O(1)). The analysis of velocity and vorticity fields, following vortex identification and an estimate of the circulation, reveal that the vortices in the stratified fluid case are stretched along the streamwise direction in the near wake. The momentum thickness profiles show lower momentum thickness values for the stratified late wake compared to the non-stratified wake, implying that the drag on an accelerating hydrofoil in a stratified medium is reduced at the acceleration stage. The findings may improve our ability to predict drag due to maneuvering of micro-air/watervehicles in stratified conditions.