Numerical Modeling and Flow Visualization in the Gradient Freeze Configuration During Centrifugation
A visualization system was used for observation of convection in water in the gradient freeze configuration during centrifugation with a 1.6-m radius arm. Without centrifugation, the usual axisymmetric flow pattern with a toroidal vortex near the bottom of the test cell was observed. With centrifugation at low rotation rates, the flow was primarily rotational about the cell’s vertical axis. Centrifugation also modified the bottom toroidal vortex. As the rotation rate was increased, this vortex became asymmetric and was pushed toward the bottom. A flow pattern transition occurred at about 25 rpm with a flat bottom and at ~30 rpm with concave bottoms. There was a slight minimum in the horizontal velocity 3 mm above concave bottoms at about 35 rpm, and at ~20 rpm with a flat bottom.
Three-dimensional steady-state numerical modeling of the experiments was performed that included the centrifugal and Coriolis forces. Up to 20 rpm, the calculated flow patterns viewed in the tangential plane and a 3-mm horizontal plane agreed with those observed in the experiments. The numerical modeling predicted a minimum in mass flux in the vertical direction at about 10 rpm. The numerical modeling appeared to be invalid at rotation rates above 20 rpm, probably because the mesh was too coarse due to computer limitations.
The experimental velocities were insensitive to the curvature of the bottom, while the calculated velocities increased significantly with increasing concavity. This indicates that the bottom plastic surface was not isothermal as assumed in the theoretical model.
KeywordsFlow Pattern Rotation Rate Coriolis Force Tangential Plane Flat Bottom
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