Unsteady flow around impacting bluff bodies |
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Authors: | T Leweke L Schouveiler MC Thompson K Hourigan |
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Institution: | aInstitut de Recherche sur les Phénomènes Hors Équilibre – CNRS, École Centrale, Aix-Marseille Université, 49 rue Frédéric Joliot-Curie, B.P. 146, F-13384 Marseille Cedex 13, France;bFluids Laboratory for Aeronautical and Industrial Research, Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia;cDivision of Biological Engineering, Monash University, Victoria 3800, Australia |
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Abstract: | The flow resulting from the collision without rebound of generic bluff bodies with a wall in a still viscous fluid is investigated both computationally and experimentally. Emphasis is on the case of a circular cylinder impact (two-dimensional geometry), but comparisons with the flow generated by the impact of a sphere (axisymmetric geometry) are included. For normal cylinder impacts, the two counter-rotating vortices forming behind the body during its motion continue their trajectory towards the wall after the collision, leading to the generation of opposite-signed secondary vorticity at the cylinder and wall surfaces. Secondary vortices forming from this vorticity at higher Reynolds numbers exhibit a short-wavelength three-dimensional instability. Comparison with the sphere impact reveals significant differences in the scales of the vortices after the collision, due to the additional vortex stretching acting in the axisymmetric geometry. This leads to a delay in the onset of three-dimensionality and to a different instability mechanism. Oblique cylinder impacts are also considered. For increasing impact angles, the wall effect is gradually reduced on one side of the cylinder, which favours the roll-up of the secondary vorticity and increases the rebound height of the vortex system. |
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Keywords: | Bluff bodies Wall impact Unsteady flow Axisymmetric versus two-dimensional flow 3D instability |
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