The streamwise spacing of adjacent coherent structures in stratified wakes

The longevity and surprising stability of late wakes emerging from an initially-turbulent state in a stably stratified ambient has been noted frequently in the literature. Here, the streamwise spacing is proposed and detailed as a diagnostic for the structure generated by a towed sphere. The wake Strouhal number and its evolution in time is reported over 2 orders of magnitude range in internal Froude number F∈[2,200]. Over the entire range of F, the decay rates adhere closely to a constant St∼Nt−1/3 power law that can be argued from simple geometric grounds. There is no tendency for values of St to aggregate at, or close to a constant value, such as St=0.175, as would be expected for a near-wake spiral shedding mode. The collective Nt−1/3 evolution is a statistical product of individual, nearest-neighbor interactions. Characteristic departures from the general rule in the lifetime of specific like-signed vortex pairs show pairing (or tripling) interactions with an asymmetry in the spacing distribution evo...