Effects of background rotation on a towed-sphere wake in a stably-stratified fluid

The wake of a towed sphere in a stable background density gradient can be considered a convenient model problem for studying the emergence and longevity of the coherent patches of alternate-signed vertical vorticity that comprise the late wake. In practical applications it is likely that the lifetime is such that rotation effects should also be considered, and strong rotation can also be used to deliberately disturb the inferred three-dimensional wake structure. Competing effects of stratification and rotation can be measured by the ratio of rotation to buoyancy timescales, fON, which varies from 0.05 to 0.22 in experiments conducted on the 14 m diameter rotating table at Coriolis. Wake anticyclones, with sense of rotation opposite to the background rotation, were spread out over a larger area, and were less strongly peaked than their cyclonic counterparts, with the magnitude of the asymmetry depending on fON. Deformation of anticyclones into highly elliptical shapes coincided with a broader range of wake vortex interactions than in the non-rotating case. At much longer times (hundreds of buoyancy periods, 5-10 rotation times), a recircularisation of elliptical anticyclones can occur. The observed asymmetries are consistent with existing data on homogeneous wake flows with rotation

Forced Stratified Turbulence: Successive Transitions with Reynolds Number

Numerical simulations are made for forced turbulence at a sequence of increasing values of Reynolds number, R, keeping fixed a strongly stable, volume-mean density stratification. At smaller values of R, the turbulent velocity is mainly horizontal, and the momentum balance is approximately cyclostrophic and hydrostatic. This is a regime dominated by so-called pancake vortices, with only a weak excitation of internal gravity waves and large values of the local Richardson number, Ri, everywhere. At higher values of R there are successive transitions to (a) overturning motions with local reversals in the density stratification and small or negative values of Ri; (b) growth of a horizontally uniform vertical shear flow component; and (c) growth of a large-scale vertical flow component. Throughout these transitions, pancake vortices continue to dominate the large-scale part of the turbulence, and the gravity wave component remains weak except at small scales.Comment: 8 pages, 5 figures (submitted to Phys. Rev. E

Acoustic Mapping Velocimetry (AMV) for in-situ bedload transport estimation

Despite the importance of sediment transport processes in large rivers, the measurement of sed-iment transport rate in the in-situ, especially bedload, is difficult, costly and time consuming using conven-tional methods. In this paper, a novel indirect bedload estimation methodology is presented that is based on the Acoustic Mapping Velocimetry (AMV). AMV is a combination of acoustic and imaging techniques that provides 2D bedform velocity maps. As such, it can only be used if bedload is represented by bedform migra-tion. This paper illustrates the applicability of the bedload estimation method using as test case a section of the Ohio River in the United States. Repeated measurements of the bathymetry provided by multi-beam echo sounder serve as input data for AMV. Cross-sectional distributions of bedload transport rates obtained with AMV are compared with the estimates provided by another non-intrusive technique, ISSDOTv2, developed by the US Army Corps of Engineers. The good agreement between the results with the two different methods is encouraging and suggests further field tests covering a wider range of hydro-morphological situations