Doctoral defense
Experimental studies of turbulent boundary layer separation and control
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| Defendant |
Main Advisor |
Extra Advisor |
Date |
| Kristian P. Angele |
Barbro Muhammad- Klingmann |
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2003-06-12 |
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| Opponent |
| Per-Åge Krogstad, NTNU, Trondheim
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| Evaluation committee |
William George, CTH
Carl Häggmark, Alfa Laval
Said Zahrai, ABB CR
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AbstractThe object of the present work is to experimentally study the case of a turbulent boundary layer subjected to an Adverse Pressure Gradient (APG) with separation and reattachment. This constitutes a good test case for advanced turbulence modeling. The work consists of design of a wind-tunnel setup, development of Particle Image Velocimetry (PIV) measurements and evaluation techniques for boundary layer flows, investigations of scaling of boundary layers with APG and separation and studies of the turbulence structure of the separating boundary layer with control by means of streamwise vortices. The accuracy of PIV is investigated in the near-wall region of a zero pressure-gradient turbulent boundary layer at high Reynolds number. It is shown that, by careful design of the experiment and correctly applied validation criteria, PIV is a serious alternative to conventional techniques for well-resolved accurate turbulence measurements. The results from peak-locking simulations constitute useful guide-lines for the effect on the turbulence statistics. Its symptoms are identified and criteria for when this needs to be considered are presented. Different velocity scalings are tested against the new data base on a separating APG boundary layer. It is shown that a velocity scale related to the local pressure gradient gives similarity not only for the mean velocity but also to some extent for the Reynolds shear-stress. Another velocity scale, which is claimed to be related to the maximum Reynolds shear-stress, gives the same degree of similarity which connects the two scalings. However, profile similarity achieved within an experiment is not universal and this flow is obviously governed by parameters which are still not accounted for. Turbulent boundary layer separation control by means of streamwise vortices is investigated. The instantaneous interaction between the vortices and the boundary layer and the change in the boundary layer and turbulence structure is presented. The vortices are growing with the boundary layer and the maximum vorticity is decreased as the circulation is conserved. The vortices are non-stationary and subjected to vortex stretching. The movements contribute to large levels of the Reynolds stresses. Initially non-equidistant vortices become and remain equidistant and are confined to the boundary layer. The amount of initial streamwise circulation was found to be a crucial parameter for successful separation control whereas the vortex generator position and size is of secondary importance. At symmetry planes the turbulence is relaxed to a near isotropic state and the turbulence kinetic energy is decreased compared to the case without vortices.
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