Generation and control of artificial large-scale motions for directed separation control
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Authors
Wylie, John, David Berry
Issue Date
2025-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Aeronautical engineering
Alternative Title
Abstract
An experimental study was conducted to test a method of active flow separation control for transitioning and turbulent boundary layers over flat plates and airfoils. The work explored the manipulation of turbulent coherent motions called large-scale motions (LSMs) to provide mixing potential and momentum injection for boundary layer re-energization and ultimately separation control. The LSMs were generated with finite span, rectangular synthetic jet (SJ) actuators to ensure a repeatable and identifiable set of artificial LSMs. The artificial LSMs were targeted with a hybrid control actuator, a jet-assisted, surface-mounted actuator (JASMA). The JASMA combined the passive downwash of a cantilevered, low aspect ratio circular cylinder, or pin, with the active unsteadiness of a synthetic jet. First, testing was conducted in a zero pressure gradient environment in a laminar boundary layer at Re_δ = 2.34×10^3 to demonstrate the nonlinear interaction between the artificial LSMs and the control actuator in an environment with low turbulent fluctuations, informing later higher Reynolds number experiments. Results were obtained using centerline two-component particle image velocimetry (PIV) for faster iteration of control parameters and using three-component stereoscopic PIV at several planes to construct flow volumes to better understand flow features and vortical interactions. The laminar boundary layer experiments demonstrated that the Reynolds stresses, TKE, and downwash were increased when the JASMA was used to control the SJ structures, relative to the SJ-only case. The SJ and JASMA interaction (SJ+JASMA) amplified small asymmetries in the flow field, creating a strong asymmetric flow field with a single streamwise vortex in the wake. Second, turbulent boundary layer experiments were conducted at Re_θ = 2.83 × 10^3 on a tripped, zero pressure gradient flat plate model. The interactions demonstrated a strong centerline increase in momentum due to the SJ+JASMA interaction with wake regions outboard from the pin horseshoe vortices. At a JASMA blowing ratio of 0.3, the streamwise momentum was lower than the static pin case, but when the blowing ratio was increased, the centerline momentum exceeded the SJ-only case. The SJ inputs were modified to generate periodic wave packets of ten actuation cycles to mimic naturally occurring LSMs. The pulsed inputs and control demonstrated markedly weaker vortical structures and integrated streamwise momentum and vertical velocity compared to the steady state due to the brevity of actuation. As the SJ ramped up, the weaker initial vortices in the artificial LSM contained lower streamwise momentum that increased as the SJ strength reached its maximum amplitude and thereafter. Third, the control experiments at the Air Force Research Laboratory (AFRL) were conducted on two different airfoil models at Re_c = 3.20 × 10^5 to explore both boundary layer re-energization and separation control with three scenarios with a nonzero pressure gradient. The control on an airfoil model with the NACA 63A210 profile demonstrated an increase in streamwise velocity starting at a streamwise distance of 5.5 pin diameters downstream of the JASMA. This was due primarily to the interaction between the SJ and the static pin, with little benefit due to the JASMA. Tests were also conducted on a different transonic airfoil geometry provided by the AFRL with small to moderate separated flow at the model TE depending on angle of attack. The results from the new airfoil model showed that the SJ+JASMA combination provided the strongest directed separation control compared to the SJ-only case as a benchmark. Control was most effective for a spanwise region behind the JASMA, as confirmed by PIV measurements off the model trailing edge. Exploring the spanwise velocity distributions and integrated effects, the SJ+JASMA case improved streamwise velocity most compared to the uncontrolled generation case (SJ-only) at the centerline, where the control parameters were refined in the parametric study. Similar wake deficits from the pin horseshoe vortices were seen outboard of the actuator centerline location. When the model pitch was increased, the moderate level of separated flow reached the JASMA location. Here, the effects of the control were poorer than the SJ-only case across the entirety of the span. Again, for the second airfoil model, the SJ+JASMA case amplified asymmetry in the domain, creating a more imbalanced result along the span.
Description
December2025
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY