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Optical Methods for Data Processing in Heat and Fluid Flow by James Buick, John Cosgrove, Clive Greated

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Section 4 PIV

14

Spatio-temporal Reconstruction of the Unsteady Wake of Axi-symmetric Bluff Bodies via Time-recording DPIV

C Brücker

Introduction

The present study was undertaken with the objective of a more detailed quantitative analysis of the evolution of the flow field in the wake of axi-symmetric bluff bodies like spheroids or bubbles. Much data exist for the wake of nominally two-dimensional bodies like the wake of a cylinder. On the other hand, the wakes of axi-symmetric bodies exhibit grave differences in the shape and dynamic of the vortices being shed in comparison to the two-dimensional case. The wake structures are basically three-dimensional and unsteady which makes the measurement and interpretation difficult. Up to now, experiments providing detailed instantaneous flow field data in the wake do not exist. Therefore, our current day knowledge about the vortical structures contained in the wake is still based mainly on earlier flow visualization studies. A pair of attached vortices was observed by Nakamura (1) for sphere wakes in the range 210 < Re < 270. For higher Reynolds-numbers up to Re = 490, the flow visualization experiments revealed a shedding of hairpin-like vortex structures (2–4). The wake appears as a chain of hairpin-vortices with the heads pointing always to the same side. However, any flow pattern visualized with dye-injection technique always depends on the way, how, and where the dye is released in to the flow. Recent numerical simulations of the ...

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