Session AE: Flow Control I

Unsteady Aerodynamics Effected by Controlled Trapped Vorticity Concentrations

The transitory response of the flow-about a free-moving airfoil to time-dependent fluidic actuation that yields aerodynamic forces and moments in the absence of conventional control surfaces is investigated in wind tunnel experiments. Desired maneuvers are achieved using a 2-DOF feedback-controlled traverse that is programmed for trim and dynamic characteristics. Bi-directional changes in the pitching moment are effected by controllable, nominally-symmetric trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge. Actuation is independently applied on each surface by hybrid actuators that are each comprised of a miniature [O(0.01c)] obstruction integrated with synthetic jets which manipulate and regulate vorticity flux near the surface. Simultaneous measurements of the unsteady forces and moments and of the associated velocity field above and in the near wake of the airfoil are used to asses the coupling between the flow and vehicle dynamics with emphasis on control authority and optimal actuator placement and operating parameters. Flow control effectiveness is demonstrated by closed-loop response to a momentary force disturbance analogous to the response to a sudden gust in free flight.   

Low-Order Modeling of Airfoil Pitch Control Effected by Trapped Vorticity Concentrations

We describe construction of spanwise vorticity modes by proper orthogonal decomposition (POD) using data obtained by two- component PIV measurements in turbulent flow past a NACA 4415 airfoil undergoing time-periodic pitching motion due to synthetic-jet actuation near the trailing edge. Three- dimensional effects are characterized in terms of a ``mass deficit'' in the phase-averaged continuity equation. Such effects, in the actuated or unactuated flow, are significant in the near wake, are thought to arise from the three- dimensionality of the geometry of the actuators, and are accounted for in the model. The incorporation of forcing by the jets into two- and three-dimensional models, and the use of vorticity POD modes, are discussed in the context of low-order modeling for feedback control. The vorticity transport equation is used to obtain an ordinary differential equation (ODE) system by Galerkin projection, whose solution behavior is discussed.   

Unsteady Flow Simulation of a Controlled Airfoil

An airfoil moving with two degrees of freedom (pitching and plunging) is simulated with a closed-loop flow control system. The simulation of the unsteady airfoil is computed using a Delayed Detached Eddy Simulation (DDES), a hybrid non-zonal RANS-LES turbulence model based on DES. The control system controls the airfoil in two modes, first through direct application of forces and torques, and second, through the use of tangential synthetic jet actuators. The approach was designed for an investigation of flow control via synthetic jet actuators on a pitching and plunging airfoil in A. Glezer's wind tunnel at Georgia Tech. The software definition of the controller used for the wind tunnel facility, which includes a robust servomechanism Linear Quadratic Regulator (LQR) and a neural network based adaptive controller, is coupled to a CFD model, which includes a model for the synthetic jet actuators. The coupled CFD/controller model is used to simulate maneuvers of the airfoil as performed in the wind tunnel, and the coupled model is validated against experiment results. Both the results of the validation, and the characteristics of the controlled flows will be discussed.   

Aerodynamic Control of a Wing at Low Angles of Attack using Synthetic Jets and a Gurney Flap

Experimental tests were performed on a symmetric wing at low angles of attack to determine the effectiveness of pairing an array of synthetic jet actuators with a Gurney flap for active, aerodynamic flow control of the wing. Sectional lift and quarter-chord pitching moment data were acquired at $Re_c = 1.45 \times 10^{5}$ for different configurations of the Gurney flap and synthetic jet array. For configurations where significant aerodynamic control was observed, the flow physics in the vicinity of the flap and actuators were investigated with PIV. The net effect of the Gurney flap and synthetic jet actuator control scheme was an increase in the wing section lift and a corresponding decrease in the pitching moment. These effects were the result of an increase in the circulation of the wing section by a modification of the trailing edge flow with the synthetic jet control.   

Trailing Edge Flow Modification on a Wing by a Near-Trailing-Edge Gurney Flap

A Gurney flap is a small, perpendicular tab placed at or near the trailing edge of an airfoil. The effect of the tab is to augment the lift of the airfoil with no attendant increase in drag. A set of experiments were performed on a wing section at low-to- moderate angles of attack with a 2\% x/C Gurney flap located at x/C~=~0.95 on the lower surface of the airfoil. The configuration was tested at $Re_c = 1.45 \times 10^{5}$ to determine the sectional lift and quarter-chord pitching moment characteristics for the Gurney flap when not located exactly at the trailing edge. For this flap position, an increment in lift was still observed and was accompanied by an associated decrease in the pitching moment. Two-component velocity measurements were taken with PIV in the near wake region of the model to develop of an understanding of the flow physics in the wake of the Gurney flap. These measurements revealed that the closed recirculation region present in the lee of the flap at low angles of attack decreased in size as the angle of attack increased and eventually was eliminated completely when the angle of attack reached 12$^{\circ}$.   

Active Flow Control on Low-Aspect Ratio, Low-Reynolds Number Airfoils

Insect flight observations show high-lift mechanisms that rely on leading-edge vortex stabilization. These processes are intimately coupled to the flapping motion of the insect wing. In fixed wing applications, suitable for micro-air vehicles, active flow control may be capable of providing similar influence over vortex formation and stabilization. Steady and pulsed mass injection strategies are used to explore the open-loop response of both the evolution of the flow structures and the forces experienced by the wing. Flow structures will be quantitatively visualized using Defocused Digital Particle Image Velocimetry (DDPIV) and forces measured via a six-axis balance. Insect flight typically occurs at Reynolds numbers of 10$^{2}$ to 10$^{4}$, and aspect ratios near three. For this investigation, Reynolds numbers are approximately 10$^{3}$. The airfoil models are NACA 0012 profiles with aspect ratio two.   

Lift force time delays on 2D and 3D wings in unsteady flows

Active flow control (AFC) used for enhancing the maneuverability of wings is usually applied during conditions of steady external flow. However, when the external flow is unsteady or the wing is maneuvering, then at least two time delays become important; namely, the time delay of the lift to changes in external flow, $\tau _{f}$, and the time delay to changes in AFC actuation, $\tau _{a}$. These time delays were measured in wind tunnel experiments using two- and three-dimensional wings in an oscillating freestream and with variable duty cycle actuation. Dimensionless freestream oscillation frequencies from k = 0.01 to k = 0.2 with amplitudes of 5 percent of the mean speed were used to characterize the system. As a demonstration of the important role of the two time constants, AFC is used to damp lift force oscillations occurring in an unsteady freestream using a feed forward control system. The instantaneous velocity provides input to a control algorithm which adjusts the duty cycle of the AFC actuator to suppress lift fluctuations.   

Lift Enhancement through the Modification of the Three-Dimensional Wake Vortex Dynamics

Three-dimensional flow simulations are performed to understand the flow physics around low-aspect-ratio wings at low Reynolds numbers of 300 and 500. The aerodynamic characteristics of such wings and the dynamics and stability of the wake vortices are investigated. Of particular interest in the current research is the application of flow control to alter the vortex formation and evolution for lift enhancement at high angles of attack. Unlike separation control or circulation control, we modify the dynamics of the wake vortices to achieve lift increase. Steady downstream blowing at the trailing edge is found to be particularly effective. Such forcing allows for the tip vortices to be strengthened and generate stronger downward induced velocity upon the leading-edge vortices. Close roll-up of the leading-edge vortices results in the placement of the low-pressure core directly above the wing for lift enhancement, in some cases by double.   

Active flow control over a finite wing. Part 1: Experimental investigation

The effect of active flow control, via arrays of synthetic jet actuators, on the flow field around a finite wing was investigated experimentally and numerically. In the present abstract, the experimental component is discussed. To fully and properly implement flow control, a fundamental understanding of the interaction of the synthetic jets with the three-dimensional cross flow must be possessed. The experiments were conducted in a wind tunnel on a finite wing having a cross-sectional profile of NACA 4421at a wide range of angles of attack, Reynolds numbers, and several arrangements of synthetic jets. Stereoscopic PIV data were collected in conjunction with dynamic surface pressure. Using these data, the complex 3-D interactions were analyzed to form a cohesive understanding of the parameters that may impact the effectiveness of flow control on 3-D configurations.   

Active flow control over a finite wing. Part 2: Numerical investigation

In complement to the experimental investigations, numerical simulations are performed to study the effects of active flow control via arrays of synthetic jet actuators. The complexity of flow structures due to 3D interactions over varied range of angles of attack, sweep, Reynolds numbers, and arrangements of synthetic jets, create a need for adaptive flow simulations. In this work, solution-based mesh adaptivity is applied in order to effectively capture the flow features and relevant quantities by attaining local mesh resolution that matches the physical length scales of the flow structures in a non-uniform fashion. Furthermore, anisotropy in the flow field is also taken into account by adaptively constructing anisotropic elements, especially layered and graded elements in the boundary layers to resolve near-wall flow structures. Using these techniques the 3D interactions in the flow field are studied and compared with the experimental data to analyze the effectiveness of flow controls on 3D configurations.