Water tunnel experiments of three-dimensional separation bubbles on a flat plate

Andreas Kremheller, Hermann Fasel

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Three-dimensional laminar separation bubbles generated on a flat plate by a three-dimensional displacement body were investigated in a water tunnel for a Reynolds number range of ReD = 2,000 to ReD = 30,000 based on displacement body thickness, D. The separation bubble was generated on a flat plate to exclude complex surface curvature effects. Boundary layer suction was applied on the surface of the displacement body in order to prevent flow separation from the displacement body. Several displacement bodies with different aspect ratios (Λ = 0.5 - 3) were used to allow for a variation of the spanwise extent of the imposed pressure gradient. The influence of the pressure gradient on the separation bubble was also investigated by changing the distance of the displacement body from the flat plate. Velocity vector field measurements using a Particle Image Velocimetry (PIV) system as well as dye flow visualizations were employed for investigating the physical mechanisms governing the dynamics of the three-dimensional separation bubble. With these techniques we were able to identify the reverse flow region in the bubble and obtained limiting streamline patterns. The topologies of the separation bubbles are documented for different Reynolds numbers and different pressure gradients. For a Reynolds number of ReD = 5,000 and an aspect ratio of Λ = 0.5 the separation bubble and the wake were found to be steady without any shedding. For higher Reynolds numbers the bubble was found to shed vortical structures which possibly resulted from an inviscid Kelvin-Helmholtz instability. These instabilities of the separated shear layer in combination with a slowly varying reverse flow magnitude were found to likely be the cause of a so-called bubble "breathing" which manifests itself as an intermittent change of the dimensions and intensity of the separation bubble. The streamline pattern of the three-dimensional separated region for a higher aspect ratio (Λ = 2) was found to be similar to that of low aspect ratio wings at large angles of attack (close to stall).

Original languageEnglish (US)
Title of host publication40th AIAA Fluid Dynamics Conference
PublisherAmerican Institute of Aeronautics and Astronautics Inc.
ISBN (Print)9781617389221
DOIs
StatePublished - 2010

Publication series

Name40th AIAA Fluid Dynamics Conference
Volume1

ASJC Scopus subject areas

  • Fluid Flow and Transfer Processes

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    Kremheller, A., & Fasel, H. (2010). Water tunnel experiments of three-dimensional separation bubbles on a flat plate. In 40th AIAA Fluid Dynamics Conference [2010-4738] (40th AIAA Fluid Dynamics Conference; Vol. 1). American Institute of Aeronautics and Astronautics Inc.. https://doi.org/10.2514/6.2010-4738