Doctoral defense
Computational fluid-dynamics investigations of vortex generators for flow-separation control
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| Defendant |
Main Advisor |
Extra Advisor |
Date |
| Florian von Stillfried |
Arne Johansson |
Stefan Wallin |
2012-05-16 |
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| Opponent |
| Jonathan Morrison, Imperial College London
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| Evaluation committee |
Gunilla Efraimsson, KTH Aeronautics and Vehicle Engineering
Shia-Hui Peng, FOI
Johan Revstedt, LTH
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AbstractMany flow cases in fluid dynamics face undesirable flow separation due to adverse pressure gradients on wall boundaries. This occurs, for example, due to geometrical reasons as in a highly curved turbine-inlet duct or on flow-control surfaces such as wing trailing-edge flaps within a certain angle-of-attack range. Here, flow-control devices are often used in order to enhance the flow and delay or even totally eliminate flow separation. Flow control can e.g. be achieved by using passive or active vortex generators (VGs) for momentum mixing in the boundary layer of such flows. This thesis focusses on such passive and active VGs and their modelling for computational fluid dynamics investigations.
First, a statistical VG model approach for passive vane vortex genera- tors (VVGs), developed at the Royal Institute of Technology Stockholm and the Swedish Defence Research Agency, was evaluated and further improved by means of experimental data and three-dimensional fully-resolved computations. This statistical VVG model approach models those statistical vortex stresses that are generated at the VG by the detaching streamwise vortices. This is established by means of the Lamb-Oseen vortex model and the Prandtl lifting-line theory for the determination of the vortex strength. Moreover, this ansatz adds the additional vortex stresses to the turbulence of a Reynolds-stress transport model. Therefore, it removes the need to build fully-resolved three-dimensional geometries of VVGs in a computational fluid dynamics mesh. Usually, the generation of these fully-resolved geometries is rather costly in terms of preprocessing and computations. By applying VVG models, the costs are reduced to that of computations without VVGs. The original and an improved calibrated passive VVG model show sensitivity for parameter variations such as the modelled VVG geometry and the VVG model location on a flat plate in zero- and adverse-pressure-gradient flows, in a diffuser, and on an airfoil with its high-lift system extracted. It could be shown that the passive VVG model qualitatively and partly quantitatively describes correct trends and tendencies for these different applications.
In a second step, active vortex-generator jets (VGJs) are considered. They were experimentally investigated in a zero-pressure-gradient flat-plate flow at Technische Universität Braunschweig, Germany, and have been re-evaluated for our purposes and a parameterization of the generated vortices was conducted. Dependencies of the generated vortices and their characteristics on the VGJ setup parameters could be identified and quantified. These dependencies were used as a basis for the development of a new statistical VGJ model. This model uses the ansatz of the passive VVG model in terms of the vortex model, the additional vortex-stress tensor, and its summation to the Reynolds stress ten- sor. Yet, it does not use the Prandtl lifting-line theory for the determination of the circulation but an ansatz for the balance of the momentum impact that the VGJ has on the mean flow. This model is currently under development and first results have been evaluated against experimental and fully-resolved computational results of a flat plate without pressure gradient.
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