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Modelling a moving propeller system in a stratified fluid using OpenFOAM
Moving propeller systems can introduce significant disturbances in stratified environments by mixing the surrounding fluid. Restorative buoyancy forces subsequently act on this region (or 'patch') of mixed fluid, causing it to eventually collapse and spread laterally in order to recover the original stratification.
OpenFOAM's buoyantPimpleFoam is an example of a model that is capable of representing such stratified environments and buoyancy-driven phenomena, including mixed patch collapse. Moving meshes represent one possible approach to modelling the mixing action of propeller systems. However, the existing buoyantPimpleFoam model does not currently incorporate support for moving meshes.
This work describes a recent extension to buoyantPimpleFoam to support moving meshes. Its application considers a KCD-32 propeller system rotating at 120 rpm in a laboratory-scale wave tank. The fluid is stably-stratified and characterised by linearly-decreasing temperature with respect to depth. Fluid flows through the propeller at a speed of 0.01 m/s.
Mixing of the stratification is successfully achieved by the moving propeller system. At early times, the mixed region of fluid grows in both height and width as inertial forces dominate. At later times, buoyancy forces act to restore the stable stratification. The mixed patch begins to collapse vertically and spread out laterally.
The point at which mixed patch collapse occurs has been studied experimentally by Merritt (1972) and Lin & Pao (1979). Their empirical estimates for the temporal evolution of mixed patch height and width are compared with the numerical results presented herein. The results agree closely, both qualitatively and quantitatively, thereby representing a successful first step towards the validation of the extended model.