Experimental Investigations of Mixing-Processes in The Wake of A Circular Cylinder in Stratified Flows

摘要

The existence of oxygen-rich saltwater in the deeper basins of the Baltic Sea is mainly caused by sporadic inflow-events of salty and oxygen-rich saltwater from the North Sea into the Baltic Sea. These inflows take place over the narrow and shallow Drogden Sill into the first basin, the Arkona Sea. Actually different offshore wind farms are planned in this region, which opens a whole string of questions about the ecological influence of offshore wind farms on the mixing of both layers. To answer these questions, numerical simulations of the mixing processes in the wake of wind turbine bases have been carried out. For the evaluation and quantification of these mixing processes a laboratory-experiment with a simplified model of the natural configuration has been realized. For this purpose a new water-channel has been build. This channel allows to simulate the inflow of saltwater in a size-scale of 1:100 to reality by keeping the densimetric Froude-Number. The experimental configuration consists of a long circular cylinder with a diameter of 8 cm in a 10 cm thick saltwater-layer flowing under a stationary fresh-water layer of 30 cm thickness. Focus point of this investigation is the wake of the cylinder in the stratified flow and the mixing-processes in the shear-layer due to the influence of the cylinder. The stratified flow around the cylinder induces the typical Karman-vortex wake, horseshoe-vortices at the bottom and in the shear layer and Kelvin-Helmholtz-instabilities in the shear-layer. Nonintrusive optical measurements were taken with planar laser-induced fluorescence (PLIF) combined with two dimensional particle imaging velocimetry (PIV). The combination of both techniques allows the determination of instantaneous velocity components u and w from PIV-measurements, the salinity s from PLIF-experiments, their variations u', w', s' and the correlations of those like Reynolds-stress terms (u'u', u'w', w'w') and turbulent- or Reynolds-flux terms (w's', u's'). Especially the vertical Reynolds-flux w's' is the characteristic parameter to evaluate entrainment-velocity and entrainment-coefficient.