15:40   Control of separated/unsteady flows I Chair: Weerachai Chaiworapuek The Effect of Forcing Amplitude on the Bursting of a Laminar Separation Bubble on a Pitching Airfoil Connor Toppings, Theodoros Michelis, Marios Kotsonis, Serhiy Yarusevych Abstract: The bursting of a laminar separation bubble subject to periodic forcing is studied experimentally on a NACA0018 airfoil at a chord Reynolds number of 70000 during a transient ramp increase in angle of attack past the static stall angle with a maximum non-dimensional pitch rate of 0.01. Forcing at a constant frequency, equal to the most amplified frequency in the separated shear layer at the initial angle of attack, is applied to the boundary layer by a spanwise-uniform alternating current dielectric-barrier-discharge plasma actuator at the leading edge of the airfoil. In the unforced flow, bursting of the laminar separation bubble begins before the maximum angle of attack is reached. Weak periodic forcing, although unable to prevent bursting, delays the onset of bursting past the end of the pitching motion. Periodic forcing enables a laminar separation bubble to persist for a longer period of time under a stronger adverse pressure gradient relative to the unforced flow. Although periodic forcing may delay bursting, the dynamics of the bursting process are similar between unforced and forced conditions, requiring a period of approximately 30 convective time units for the flow to settle to a massively separated state. The delay in the onset of bursting increases and becomes more random as the forcing amplitude increases, until the forcing amplitude is sufficient to prevent bursting entirely. Bursting is observed to be an irreversible process, with the flow always progressing to a state of massive separation after the initial cessation of reattachment. Control of transonic buffet with air-jet vortex generators Deepak Prem Ramaswamy, Eric Ibarra, Sven Scharnowski, Christian J. Kähler Abstract: Modern commercial aircraft increasingly operate in the transonic flow regime, where local flow velocities on the suction side of a supercritical wing can exceed the sonic limit and terminate in a shock wave. While the flow field is generally stable, it can become highly unsteady for certain combinations of angle of attack, Mach number (M), and Reynolds number (Re). This dynamic flow instability is commonly referred to as transonic buffet and manifests as a large-scale, self-sustained periodic oscillation of the shock wave, accompanied by significant variations in shock strength and periodic thickening and thinning of the downstream shock-induced separation. It results in pronounced oscillations in lift coefficients, induces strong fluctuating aerodynamic loads on the airframe accompanied by intense vibrations (buffeting), and imposes a substantial drag penalty. Consequently, the operational envelope of the aircraft is constrained, and both safety and performance are adversely affected. In this study, we aim to use a single row of spanwise-inclined AJVGs to attenuate transonic buffet over a rigid OAT15A wind tunnel model. The primary effects of jet injection pressure on control effectiveness will be examined across different flow regimes, ranging from pre-buffet and buffet onset to fully developed buffet and buffet offset conditions. Kirigami sheets for passive flow control Adrian Carleton, Yahya Modarres-Sadeghi Abstract: Kirigami designs consist of a pattern of slits, or a combination of slits and creases, which makes them different from origami designs that are solely based on creases. The ease of production and deployment (which is often done simply by applying external tension on a 2D sheet) has caused several recent applications for kirigami sheets in ``structural mechanics, materials, optics, electronics, robotics, and bioengineering". Despite this extensive use in other fields, kirigami designs have not been used for fluid mechanic applications. Here, we discuss the potentials of kirigami sheets as passive flow control devices by considering the behavior of kirigami sheets placed perpendicular to an incoming flow and observing their response to the flow forces and their corresponding wakes.

end %-->