Unsteady Topology and Control of a Turbulent Boundary Layer Separation over an Airfoil

Austin Ma
2020
The subject of interest for this thesis is the detachment of a turbulent boundary layer. Engineers are interested in techniques that delay or suppress flow separation entirely because the performance of many fluidic devices, such as airfoil and diffuser, are hindered by this flow phenomenon. The sensitivity of flow separation to numerous flow-related parameters, and unsteady nature of the flow phenomenon contribute to the complexity of the subject. As a result, it is difficult to predict for
more » ... occurrence of flow separation using numerical methods. A fundamental understanding of separated flow is required to advance development of flow control techniques and predictive models for flow detachment. An experimental approach is used to characterize the unsteady topology of a three-dimensional flow separation from the trailing edge of an airfoil. Near-wall streamlines of the time-averaged flow revealed a flow topology known as a stall cell, which feature a saddle point and a pair of counter-rotating foci at the sides of the airfoil. Inspection of the timeresolved measurements revealed similar, but smaller, stall cell structures within the separation front. The instantaneous structures are created because of a region of strong local flow in either downstream or upstream direction, such as high-speed streaks. The momentum of the high-speed streaks are converted into a rotational motion at the separation front. In the latter part of the project, the feasibility of separation control using piezoelectric actuators is briefly explored through a parametric investigation. It is shown that the separation front is shifted downstream when an array of actuators operate synchronously. This effect is improved with increasing frequency, and the separation front was shifted by 4% of the chord length at the maximum frequency tested. No change is observed when adjacent actuators operated 180 o out-of-phase. iii
doi:10.7939/r3-myva-c886 fatcat:kmmmkvbuazd5djbqon76locjbe