|Ph.D Thesis||Department of Aerospace Engineering|
|Supervisors:||Assoc. Prof. Iosilevskii Gil|
|Assoc. Prof. Rimon Arieli|
|Full Thesis text|
This study addresses the transition (from laminar to turbulent flow) over a pair of typical wing sections at Reynolds number of several ten thousands. The study was mainly experimental, and it was based on flow visualizations and quantitative volumetric measurements using hot-wire anemometry. Nonetheless, high fidelity numerical simulations were used, where possible, to provide additional data on the flow field. Flow visualizations used ink tracing in a water tunnel and temperature sensitive paint imaging in a wind tunnel. The latter provided a thermal footprint of the flow structures that developed in the vicinity of the wing surface.
The results suggest that at these Reynolds numbers the transition is a three-stage process evolving with increasing Reynolds number or the angle-of-attack. Its onset is characterized by essentially steady two-dimensional flow field, practically unaffected by the free shear-layer instabilities that break the wing’s wake into an ordered vortex street. This is stage 1. With increasing Reynolds number or the angle-of-attack, the shear-layer instabilities begin closer to the wing’s trailing edge, and eventually break up above the wing’s surface. This is stage 2. With further increase in Reynolds number or the angle-of-attack the flow turns three-dimensional and large flow structures that are ordered in a spanwise cellular pattern develop over the wing surface.
It is shown that classical linear stability analysis, based on experimentally measured velocity profiles over the wing’s suction side, predicts fairly well both the most unstable disturbance modes and their growth rate. It fails to predict the growth rates of low-frequency disturbances observed in the vicinity of the wing’s leading edge. This mismatch is, perhaps, the most conspicuous result of this study, since it suggests that the last stage of the transition process is governed by global flow mechanisms. Being so, local three-dimensional flow structures that develop over the wing’s suction side affect the wing’s spanwise circulation distribution which results in spanwise pressure gradient in the vicinity of the wing’s leading edge and vice versa.