Phillip Munday

On the lock-on of vortex shedding to oscillatory actuation around a circular cylinder

We numerically investigate the influence of sinusoidal flow control on the von Karman vortex shedding behind a circular cylinder in two-dimensional flow. Actuator location, direction, frequency, and amplitude are varied to examine their effects on the wake and the corresponding change in drag on the cylinder. We place focus on the conditions for which the cylinder wake locks onto the actuation frequency. The lock-on region is found to be consistent with stability horns observed in oscillator dynamics. Under certain conditions, the actuation reduces drag by elongating the wake structure to appear more streamlined than the wake without flow control. In other cases, the use of actuation led to less streamlined wakes, resulting in no significant drag reduction or for some instances in a drag increase. Purely steady and oscillatory actuation components are examined to highlight their individual influence on the lock-on and drag characteristics. We also note that low frequency oscillations are observed for case...

Drag Reduction Control for Flow over a Hump with Surface-Mounted Thermoacoustic Actuator

Abstract : Motivated by the recent development in fabricating graphene/carbon nanotube-based surface compliant loud speakers, the effectiveness of thermoacoustic actuators that locally introduce high-intensity acoustic waves for active flow control is examined by performing high-fidelity large eddy simulation for compressible flow over a wall-mounted hump at a Reynolds number of 0:5 x 1000000 and Mach number of 0.25. Based on performance characteristics of the grephene-based thermoacoustic actuators, high-frequency actuation around Helmholtz number of 3.0 is considered. We observe that the actuation is able to introduce small-scale perturbations to the shear layer in the separated ow and attenuate the formation of large scale spanwise vortices. This ow control technique elongates the recirculation zone and shifts the low-pressure region downstream of the hump. As a consequence, the drag on the hump is reduced by approximately 4.31% and 6.33% for two and three-dimensional simulation, respectively.

Nonlinear Lift on a Triangular Airfoil in Low-Reynolds-Number Compressible Flow

Numerical and experimental analyses of the aerodynamic performance of a triangular airfoil in low-Reynolds-number compressible flow are performed. This airfoil is one of the candidates for propeller blades on a possible future Martian air vehicle design. Based on past experimental studies conducted in the Mars Wind Tunnel at Tohoku University, this airfoil is known to exhibit nonlinear lift behavior. In the present study, direct numerical simulations of low-Reynolds-number compressible flow over a spanwise periodic triangular airfoil are conducted to identify the source of nonlinear lift. The numerical results reveal that the source of the nonlinear aerodynamic behavior is the lift enhancement provided by the large leading-edge vortex generated. For compressible low-Reynolds-number flow, the wake structure becomes elongated, causing the nonlinear lift enhancement to appear at higher angles of attack compared to the case of incompressible flow.

Laminar free shear layer modification using localized periodic heating

The application of local periodic heating for controlling a spatially developing shear layer downstream of a finite-thickness splitter plate is examined by numerically solving the two-dimensional Navier-Stokes equations. At the trailing edge of the plate, oscillatory heat flux boundary condition is prescribed as the thermal forcing input to the shear layer. The thermal forcing introduces low level of oscillatory surface vorticity flux and baroclinic vorticity at the actuation frequency in the vicinity of the trailing edge. The produced vortical perturbations can independently excite the fundamental instability that accounts for shear layer roll-up as well as the subharmonic instability that encourages the vortex pairing process farther downstream. We demonstrate that the nonlinear dynamics of a spatially developing shear layer can be modified by local oscillatory heat flux as a control input. We believe that this study provides a basic foundation for flow control using thermal-energy-deposition-based actuators such as thermophones and plasma actuators.

Effects of Wall-Normal and Angular Momentum Injections in Airfoil Separation Control

The objective of this computational study is to quantify the influence of wall-normal and angular momentum injections in suppressing laminar flow separation over a canonical airfoil. Open-loop control of fully separated, incompressible flow over a NACA 0012 airfoil at

Use of local periodic heating for separation control on a NACA 0012 airfoil

\alpha = 9^\circ

Separation control on NACA 0012 airfoil using momentum and wall-normal vorticity injection

and

The Role of Vorticity Injection in Separation Control

Re = 23,000

Turbulent Flow Modification With Thermoacoustic Waves for Separation Control

is examined with large-eddy simulations. This study independently introduces wall-normal momentum and angular momentum into the separated flow using swirling jets through model boundary conditions. The response of the flow field and the surface vorticity fluxes to various combinations of actuation inputs are examined in detail. It is observed that the addition of angular momentum input to wall-normal momentum injection enhances the suppression of flow separation. Lift enhancement and suppression of separation with the wall-normal and angular momentum inputs are characterized by modifying the standard definition of the coefficient of momentum. The effect of angular momentum is incorporated into the modified coefficient of momentum by introducing a characteristic swirling jet velocity based on the non-dimensional swirl number. With this single modified coefficient of momentum, we are able to categorize each controlled flow into separated, transitional, and attached flows.