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Numerical simulation of flow in a laboratory tank using a z-coordinate numerical model Jaramillo, Sergio


A z-coordinate numerical model is employed to study the dynamic response of an upwelling favorable flow in a rotating laboratory tank with a submarine canyon. A vertically uniform body force is exerted on the fluid inside the tank over a time of 30 s, equivalent to the duration of an upwelling event in the real ocean. The results show a flow pattern that is in qualitative agreement with previous numerical, theoretical and field studies. The agreement between laboratory and numerical results is an improvement on previous results obtained using a terrain-following numerical model. However consistent differences at the upstream rim of the canyon exist. A number of processes are investigated to understand the reason for these differences. Results show that the model is extremely sensitive to changes in the bottom friction and interior mixing coefficients. Although the model can be "tuned" to obtain better agreement, this approach is not used because it could mask other important problems. Instead, results from an increased resolution run using distributed processors show that the observed differences are not related to errors in the bottom boundary layer parameterization. Non-hydrostatic effects are estimated, showing a package of standing internal waves that is generated at the upstream side of the canyon. These internal waves are dominated by a second baroclinic vertical mode that is well predicted by the linear theory. However the non-hydrostatic effects are not strong enough to influence the main characteristics of the flow in the canyon. Different advection schemes for the active tracers are compared, showing that non-linear schemes with a flux limiting algorithm produce the best results from a physical point of view. Flux limiters are designed to regulate the flow in areas with steep gradients of the advected quantity; the influence of these algorithms is investigated and it is shown that they constrain the stretching of isopycnals at the upstream rim and thus they account at least in part for the observed errors in this area. Comparisons between stretching and relative vorticity show that the model results are overwhelmed by frictional effects and that the stretching and compressing of isopycnals is poorly represented. Through these modeling steps, it is concluded that the vorticity observed in the numerical model is more related to flow separation than to stretching vorticity effects. This also contributes to the observed differences between the numerical and laboratory results.

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