NSF Award
2422860
Leading edge bubbles on airfoils are mostly created by laminar separation followed by transition to turbulence that results in reattachment. They may be long, but they are usually shallow. They are associated with low Reynolds numbers and are generally avoided by trip strips or vortex generators. Leading Edge Vortices (LEVs) have to be considered differently as they contribute substantially to lift on delta wings and swept-back thin wings at high Reynolds numbers and at high operational incidence angles. The liftoff of LEVs and their path over the wing has a major effect on trim, thus limiting the operational incidence of airplanes. The goal of the proposed study is to understand the interior structure of a large LEVs that is not encumbered by complex wing design, and to create a theoretical model capable of explaining some recently made observations. It is believed that the LEV structure could be explained by separating the flow into components, one being normal and another tangential to the leading edge. Both represent mixing layers that are susceptible to Kelvin-Helmholtz instability, but they differ with the normal component being perhaps absolutely unstable while the spanwise one may only be convectively unstable. If this is proven experimentally, it will provide a tool for flow control consisting of a small source of periodic excitation located near the apex. Theoretical criteria could then follow and account for the unsteadiness and lack of stationarity. A single steady jet can deflect the LEV either upward or sideways and generate large differences in pitch that is either positive or negative (nose-up or down). It therefore provides a paradigm shift in the control of tailless aircraft configurations.<br/><br/>A simple, flat plate cranked wing model with a sharp leading edge being beveled from the bottom surface would provide some fundamental information about this flow, but there is a lot to be understood to achieve the benefits. This configuration eliminated many secondary independent variables such as a leading-edge radius or outwash that muddied the results in the past, making the study less definitive. Since the amplification of the Kelvin-Helmholtz vortices is inviscid and the separation line is fixed by the sharp leading edge, the study is almost independent of Reynolds number and the wing does not require trip-strips. Periodic zero-mass-flux perturbations emanating from a point source will be introduced near the crank in addition to the steady jets that were already tested, and their growth along the span will be monitored because it should affect first the convective instability along the span. Measurements will include first phase locked time resolved 3D particle image velocimetry data that is complemented by the occasional use of hot-wires and flow visualization. Surface pressures with some time resolved pressure data should also be taken. These results could lead to theoretical approaches used to better understand and control turbulent mixing layers, jets, and wakes.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
