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In this study, the hydrodynamics and turbulence in a square tank stirred with the hydrofoil impeller Lightnin A310 is investigated using a large eddy particle image velocimetry approach. Particle image velocimetry data are used as the large scale part of a large eddy simulation and the small scales are modelled by direct calculation of the turbulent stress tensor using filtered particle image velocimetry data. The spatial distribution of the calculated dissipation rate showed agreement with previous work. The macro instabilities of the flow structure were investigated by means of spectral analysis. Low frequency phenomena separated from the mean flow were detected. The cause of these could partly be explained by the circulation time for the tank.

in Wiley InterScience (www.interscience.wiley.com).In this study, the spatial distribution and significance of macro instabilities in two cylindrical tanks of different size, and one square tank, stirred with standard six bladed Rushton impellers, are investigated. The instabilities of the flow structure are investigated by searching for significant frequencies using the Lomb algorithm on instantaneous velocity data collected using laser Doppler anemometry at various locations in the tanks. The investigation of the energy levels shows that the harmonics of the blade passage differ in both magnitude and spatial distribution for different tank sizes, geometries, and energy input. Several significant dimensionless frequencies, that is, frequency Strouhal numbers, are detected and identified both in relative magnitude and spatial range.