Sedimentation kleiner, schwerer Partikel durch eine Ebene, zweidimensionale mischschicht

Dreier T.M.

Theoretical model calculations of particle-laden shear layers (Martin and Meiburg [1994] and Raju and Meiburg [1994]) have shown that particles accumulate on the periphery of vortex structures and as a consequence the particle concentration is decreased in the centre of the eddies. The examination of this particle redistribution is the central issue of the present study.In order to investigate this redistribution phenomenon experimentally, an appropriate flow had to be produced. Fur this purpose an experimental facility which produced a natural mixing layer showing the typical two-dimensional vortex structures was built and tested.A new flow measurement technique was developed to measure simultaneously, but independently the velocity fields of the particle and fluid phases at the same location.Subsequently, an image-processing routine, called least-square-matching, was used in order to calculate the local, affine transformation parameters from the experimentally detected double frames of the particle and fluid phases. Additional routines for evaluation were made available for determining the velocity fields from these parameters. Furthermore, the least-square-matching allowed also to investigate the divergence (two-dimensionality) and the vorlicity in the mixing layer (in particular in the presence of sediment particles).The experiments verified for the first time the effect predicted by Martin and Meiburg 11994] and Raju and Meiburg [1994] proposing that the particles accumulate on the periphery of vortex structures leading to a decrease in the particle concentration in the centre of the eddies and then a settling in band-like structures along the mixing layer.in addition, the validity o( the Maxey and Riley [1993] equation, which describes the motion of the particles on condition that the particle phase is dilute, could be confirmed for a mixing layer outside the accumulative zones.