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UBC Theses and Dissertations

Modelling stalled airfoils Yeung, William Wai-Hung


The thesis deals with some new applications of the wake source model, a two-dimensional incompressible potential flow model used for bodies experiencing flow separation. The body contour is conformally mapped to a circle, for which the flow problem is solved using source singularities to create free streamlines simulating the separating shear layers. In common with other inviscid theories, it generally requires the pressure in the separated flow region, and the location of separation if boundary-layer controlled. Different mapping sequences and flow models have been constructed for the following five problems, 1. the trailing-edge stall for single element airfoils, 2. flat plates with separation bubbles, 3. separation bubbles upstream of spoilers with downstream wakes, 4. spoiler/slotted flap combinations, at which the spoiler inclination is arbitrary, and 5. two-element airfoils near (trailing-edge) stall. Predictions of pressure distribution are compared with wind tunnel measurements, and good agreement is found in cases 1 and 5. The initial shape of the separation streamlines also appears to be satisfactory. Results in cases 2 and 3 are promising although more work is needed to improve the bubble shapes and their pressure distributions. Partial success has been achieved on spoiler/ slotted flap configurations, depending on the spoiler inclination. For strong wake effect on the flap (e. g. δ = 90° ), the model predicts a very high suction peak over it. Whereas the experimental data resemble a stalled distribution even though flow visualization indicates the flap to be unstalled. This may be related to a limitation of the method, also noted in the separation-bubble problems, that it cannot specify a complete boundary condition on a free streamline. This discrepancy diminishes as the spoiler angle becomes smaller (e. g. δ = 30° ) in the cases of higher incidences so that the wake boundary tugs away from the flap sooner.

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