Stratified Propelled Wakes

This paper presents experimental results on the wake of a propelled bluff body towed at a constant horizontal speed in a linearly stratified fluid. Three regimes of the wake have been found, depending on the angle of attack and on the ratio of drag force to propeller thrust. Most of the experiments were obtained in a first regime where a strong momentum flux is created in the wake, which can be oriented backward or frontward depending on the ratio of drag force to thrust of the propeller. The velocity amplitude, wake width and Strouhal number of the wake can be predicted by defining a momentum thickness based on the drag coefficient of the bluff body and the thrust of the propeller. A second regime is obtained on a narrow band of towing velocities, with a relative width of 4%, in which the momentum flux is found to vanish. The wake is characterised by the velocity fluctuations; the scaling exponents of the velocity, vorticity and width of the wake are measured. A third regime is obtained for wakes with a small angle of attack, with a null momentum flux. The mean profile of the wake is found to be asymmetric and its amplitude and wake width are measured. Finally, the relevance of these results to the case of a real self-propelled bluff body is discussed. The presence of weak internal waves or of weak fluctuations of background velocity would lead to a wake in the regime with momentum flux, and would allow prediction of the amplitude, width and Strouhal number of the wake

Self-preservation in stratified momentum wakes

A general model is described for drag wakes in a linearly stratified fluid, based on the self-preservation of the flow. It is assumed that the buoyancy-controlled self-similar wake expands in the horizontal direction due to turbulent diffusion and in the vertical direction due to viscous diffusion. The mean characteristics of the wake (height, width and velocity defect) are analytically derived and show good agreement with existing data from experimental and numerical results. Moreover, the three regimes previously found in the literature that characterise different dynamical phases of the wake evolution are recovered, and two new regimes are found. The model allows for prediction of characteristic length and velocity scales at the high Reynolds numbers of large-scale applications of geophysical and naval origin

Empirical scaling of antisymmetric stratified wakes

Proceedings of the "Bluff Body Wakes and Vortex-Induced Vibrations - BBVIV-4"Initially turbulent wakes of a propelled cylinder at nonzero angles of yaw to the mean flow were measured in the horizontal centerplane plane up to approximately 100 buoyancy times, where vertical velocities are very small. The profiles of mean velocity were found to be antisymmetric throughout their lifetime, with both width and maximum velocity decaying at the same rate as previously studied momentum wakes. The maximum velocity of the profile is proportional to the angle of yaw, but the width is constant. Both the mean flow and fluctuating quantities show that the late wake is self-similar, with scaling laws that are consistent with previous work on propelled and drag wakes

A signature of a bluff body in the internal wave field?

The wakes of bluff bodies are known to last longer in a stratified fluid than in a non-stratified fluid [3]. However, it was recently found that these wakes show little dependence on the shape of the body, since they are self-preserved [4]. The long lasting wake in fact forgets the initial conditions. Another interesting feature of stratified wakes is that they emit internal waves, which propagate on large distances without being attenuated. In this presentation, we investigate the influence of the shape of a bluff body on the set of internal waves emitted in a linear stratified fluid. At low Froude numbers, the Lee waves are predominant, as in the case of a sphere[1]. They are shown in the following figure in the case of a cylinder, moving from right to left. However, their very complex structures seem to be dependent on the shape of the object, due to the modulation of the amplitude of the various modes by the shape of the bluff body. These results will be compared to the linear theory found by Lighthill [2]

Self-propelled wakes in a stratified fluid

In studying canonical problems for decaying turbulence in stratified flows, a good argument can be made for those having zero net momentum as being most broadly applicable, while the drag wake, represented by numerous studies, might be a more special case. Experiments were conducted on bodies at, or very close to zero-momentum in a stratified flow tank. Three classes of wake flows and scaling behaviour were observed, and will be described. Since submerged wakes are often accompanied by internal waves, the net zero-momentum vortex wake may not be the most relevant for practical applications

Momentumless and almost-momentumless wakes in a stratified fluid

Extensive measurements have been made on the wakes of towed spheres and other bodies for steady horizontal motion in a stratified fluid. It has been proposed, and largely verified, that these bluff body wakes can serve as canonical cases for the decay of initially turbulent motions in the presence of a stable background density gradient and applications have ranged from oceanography and climate modelling to prediction of signatures from undersea objects. While experimental data on momentumless wakes has been available for more than 25 years, it has not been in such quantitative detail as the towed body case and so the two have been difficult to compare directly. Here the similarity and scaling behaviour of mean and turbulence quantities are reported for slender and blunt bodies, at, or close to momentum balance. Results are then compared with literature values, and with the standard towed configuration, to identify those features that are general and those that are not