A study of momentum and zero-momentum flows in stratified fluids

The topic of the research conducted is closely related to the general fluid mechanics problem of formation and evolution of basic coherent vortex structures generated by various concentrated forcing (force, force doublet, force couple, etc.) in a viscous fluid under different environmental conditions (stratification, shear, etc.). The study is concentrated mostly on two aspects of this fundamental problem: (i) vortical flows generated in a stratified fluid by a moving concentrated source of momentum, and (ii) the role of background shear in the formation and evolution of vortical flows generated by a stationary concentrated source of momentum in a stratified fluid. Both aspects have not been addressed previously.; A moving jet with controllable momentum flux (force) and negligible mass flux is employed to model the action of a concentrated (“point”) momentum source in a stratified fluid. The structure of the resulting flow field is determined experimentally for three basic configurations: co-flow, counter-flow and cross-flow action of the source relative to the direction of its horizontal motion. In all three configurations, the source acts either impulsively or continuously. It is shown that, depending on the duration of force action, either one large dipolar eddy or a system of dipoles organized into a quasi-two-dimensional vortex street emerges in the flow. Critical conditions discriminating these two different flow regimes are found experimentally. The problem considered has no external length scale, and an internal scale is introduced from general principles. Using this scale, the proper Reynolds and Froude numbers are defined. The experimental results are explained using these parameters.; A general analysis of the equation that describes the flow in the far field behind a localized forcing submerged into a uniformly moving homogeneous fluid is present. The general solution of the problem is derived in terms of the Laguerre and Hermite polynomials in 3D and 2D cases respectively. In the case of zero-momentum flows (force doublet, quadruplet etc.) new integral relations are established between the intensity of forcing and the characteristics of the flow.; It is also demonstrated that background vertical shear either prevents the formation of dipoles or significantly reduces their lifetime. A flow regime diagram is derived experimentally for different values of the governing parameters. A theoretical model that predicts the critical conditions for the dipole formation as well as gives the estimates of their lifetime is proposed and verified experimentally.