  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.
