Separated Flows Over Non-slender Delta Wings
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Wings with sweep are ubiquitous for engineering applications, and are observed in biology for swimmers and fliers alike. In particular, delta wings have wide ranging applicability arising from their low speed maneuvering characteristics, and favourable properties approaching the speed of sound. This thesis examines experimentally the flow separation over low-sweep angle or 'non-slender' delta wings, specifically focusing on the coherent vortices characteristic of such geometries. First, stereo particle image velocimetry and surface pressure are used to reconstruct the flow the over a delta wing at steady-state. Vortex tilting is revealed to play an important role in lift generation at angles of attack near maximum lift. Coherent structures absent from the literature are measured and described. Second, time-resolved planar particle image velocity, forces/moments and pressure are used to measure the dynamic-stall process during a streamwise gust. Separated flow near the wing apex is shown to reattach during the gust, resulting in lift enhancement. Strong favourable pressure gradients drive the flow to promote reattachment, even after the gust has terminated. Combining the steady-state and gust studies, the sensitivity of non-slender delta wings to gusts is shown to be dependent on the initial steady-state flow field. The steady-state flow structure remains coherent below maximum lift by means of vortex tilting, but fully separates along the span above maximum lift, causing bifurcation in gust response.