Non-symmetric Holmboe waves

• Main Author: Haigh, Susan Patricia
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7225

When two flows of different velocity and density meet, a shear layer with a density gradient is formed. Under certain conditions this flow can be unstable. A statically stable stratified shear flow in which the density interface is much thinner than the shear layer thickness can be linearly unstable to two modes with equal growth rates and equal and opposite phase speeds. The superposition of these two modes is called a Holmboe instability. This instability is only possible when the flow is symmetric about the center of the shear layer. We examine the effect of breaking this symmetry by allowing the center of the density interface to be displaced with respect to the center of the shear layer. There are three major components to this study: linear stability analysis, nonlinear numerical simulations, and comparison with laboratory experiments. Linear stability analysis is used to examine the effect of the density interface offset on the overall stability of the flow. Both inviscid theory with piecewise linear background velocity and density profiles, and viscous theory with smooth background profiles are used. As in previous studies, it was found that the growth rate of one mode increases and that of the other mode decreases as the density interface displacement is increased. The precise behaviour depends on the relative thickness of the density interface with respect to the shear layer thickness. For inviscid theory with piecewise linear background profiles, we show that the initial perturbations must be two-dimensional. When the effects of viscosity and diffusion are included, however, it may be possible that the weaker mode is initially three-dimensional. Detailed analysis of the energy transfer in the linear regime indicates that when the background flow loses its symmetry, it is the mode with the larger growth rate that is primarily responsible for the extraction of energy from the mean flow. Two-dimensional numerical simulations are used to examine the nonlinear development of “non-symmetric” Holmboe instabilities. We start by perturbing the flow with the stronger mode predicted by linear theory. We then examine the response of the flow to weaker mode. Finally, we impose both modes. By comparing the development of these three flows we are able to study the interaction between the two modes and the effect of initial conditions on the development of instabilities. Although the initial development of instabilities depends on the initial conditions, this dependence weakens as the density interface offset is increased. Also, preliminary results indicate that long term behaviour of the perturbations are independent of initial conditions. Results of the numerical simulations are compared to both symmetric and nonsymmetric Holmboe instabilities that have been observed in laboratory experiments. Since we are unable to compute the flow at the high Prandtl number of the salt stratified experimental flows, we run the simulation for a range of increasing Prandtl numbers to determine how instabilities of thermally stratified flows (with low Prandtl number) and salt stratified flows differ. Results of these simulations indicate that differences between the experimental and numerical flows can be attributed to the thicker density interface and lower Prandtl number used in the numerical simulations. When the density interface is thicker and the Prandtl number lower, the waves or billows formed by the instabilities are not as sharply defined as in the experiments.