A two-dimensional separation bubble on a ®at plate is studied experimentally by means of hot-wire anemometry and ®ow visualization. Separation of the laminar boundary layer on the plate is caused by an adverse pressure gradient imposed by a curved wall opposite to the plate. The instability of, and transition process in, the separation bubble are focused on. The bubble is found to be highly susceptible to high-frequency two-dimensional instability waves, which are studied under both natural and forced conditions. A similar development of these instability waves in the separation bubble is found in both cases. The exponential growth of the twodimensional disturbances dominates the ®ow except for in the reattachment region, where large-scale three-dimensional structures appear. Some di¯culties associated with experimental investigations of boundary-layer separation-bubble ®ows are discussed.

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Rheological characterization of the solids phase in bio‐industry disc‐stack centrifuges is considered. Three discriminatory properties of the solids phase are investigated, namely, the particle settling velocity, the angle of repose, and the solids phase viscosity. The solids phase produced in disc‐stack centrifuges in the food or biopharmaceutical industry possesses both liquid‐like and solid‐like properties which can be quantified by means of these test methods. The resulting rheological characterization can be used for diagnostic purposes in the selection process of industrial centrifuges and for optimization of centrifuge solids handling in biomass processing of suspension systems.

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