Experiments and simulations of low \textrm{Re} sphere wakes with and without stratification*

Bluff body wakes in both stratified and unstratified background have been studied extensively due to their geophysical and naval applications. A global map showing the dependence of near-wake structures behind a towed sphere on initial Reynolds number and Froude number, has been provided by Lin et al.\ (\textit{J. Fluid Mech.}, 240, 315\--354, 1992) and Chomaz et al.\ (\textit{J. Fluid Mech.}, 254, 1\--21, 1993). Here full-field measurements of the sphere wakes in both homogeneous and linearly stratified ambient are provided by simulations and experiments, at Reynolds number $\textrm{Re} \leq 1000$ and Froude number $\textrm{Fr} \geq 0.5$. In a homogeneous fluid, the structural transitions with increasing $\textrm{Re}$ are consistent with previous studies. The stratified results from simulations and experiments are in good agreement. Stratified wakes undergo similar transitions with decreasing \textrm{Fr} for \textrm{Re} in current range, except that similar transitions occur at larger \textrm{Re} as \textrm{Fr} decreases, thus making the wake structure at high \textrm{Re}, low \textrm{Fr} similar to that at low \textrm{Re}, high \textrm{Fr} .