Abstract:
This thesis studies mixing and convection in a rectangular basin driven by a specified
heat flux at the surface. A numerical model is constructed for this purpose. The main
focus of the study is on the density and circulation structure resulting from the thermal
forcing.
In chapter two, a simple vertical one-dimensional model is developed to examine
the mixing processes under a given surface heat flux. In order to simulate strong vertical
mixing in the region where stratification is unstable, turbulent processes are modeled
by a convective overturning parameterization of eddy viscosity and diffusivity. The
results show that the density structure is strongly affected by the convective overturning
adjustment as surface cooling prevails, and the resulting density field is nearly depth
independent.
In chapter three, a more complicated two-dimensional model is constructed to
simulate mixing and circulation in a vertical rectangular basin with rigid boundaries.
The aspect ratio of the basin ranges from 1 to 0.001 and Rayleigh number from 104 to
2 x 1012. It is found that the circulation pattern is dominated by these two important
numbers. The roles of density overturning and density-momentum overturning mixing
are further investigated. The results show that the convective overturning not only
homogenizes the density field in the unstably stratified region but also contributes to
increase the circulation. A crude scale analysis of the system shows that the characteristics
of the density and momentum fields from the analysis agree well with the
numerical results.
