Boundary layer transition over rotating disks

This thesis summarizes results of a combined numerical and experimental study investigating the effects of surface roughness, and of the geometry of the ow domain (confinement) on the boundary{layer transition over rotating disks. Numerically, a three{dimensional enclosed cavity ow in a rotor{stator flow configuration is simulated. The effects induced by surface roughness of the rotor disk and the effects induced by the stator geometry enclosing the ow domain are investigated. The steady{state velocity pro les of the boundary{layer ow on the rotating disk are obtained, subjected to a linear stability analysis and compared to relevant data from the literature. Experimentally, the ow over rotating disks is studied for smooth disk surfaces and for disks with concentric grooves representing distributed roughness. The disks are mounted submerged inside a water {filled tank. Due to the surrounding perimeter wall of the tank and the liquid surface this arrangement resembles the classic rotor{stator flow configuration. Comprehensive measurements of the boundary{layer ow and its laminar{turbulent transition were performed by means of an hot{ lm anemometer. The computational results suggest that, for the rotor{stator ow investigated, the roughness{induced effects are very similar to the geometry{induced effects, both in nature and magnitude. This suggests that it may be di cult to distinguish between both effects in experiments where the ow domain is restricted. Nevertheless, in comparison to previous hot{ lm measurements employing the same experimental facility, the data of the current study have been significantly improved by means of introducing a new calibration technique. The new experimental data discussed here confirm recent theoretical results of our research group in that they corroborate that an increase in the roughness level can reduce the number of stationary vortices and also stabilize the Type{I (cross{ ow) instability mode. However, the detailed analysis of the experimental data, in comparison to the theoretically predicted magnitude of the roughness{induced and the geometry{induced effects, reveal that future studies would greatly bene t from the availability of a new air{based rotating{disk apparatus.