Daniel P. Brzozowski

Transient Separation Control Using Pulse-Combustion Actuation

The transitory response of the flow over a stalled, two-dimensional (NACA 4415) airfoil to pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale is investigated experimentally (Re = 570, 000). Actuation is effected by momentary [O(1 ms)] pulsed jets that are generated by a spanwise array of combustion-based actuators integrated into the center section of the airfoil. The flowfield in the cross-stream plane above the airfoil and in its near wake is computed from multiple high-resolution particle image velocity images that are obtained phase locked to the actuation waveform and allow for tracking of vorticity concentrations. The brief actuation pulse leads to a remarkably strong transitory change in the circulation about the entire airfoil that is manifested by a severing of the separated vorticity layer and the subsequent shedding of a large-scale clockwise vortex that forms the separated flow domain. The clockwise severed vorticity layer that follows behind this detached vortex has a distinct sharp streamwise edge that grows and rolls up as the layer is advected along the surface. It is shown that the shedding of the severed vortex and the accumulation of surface vorticity are accompanied by a transitory increase in the magnitude of the circulation about the airfoil that lasts 8—10 convective time scales. The attached vorticity layer ultimately lifts off the surface in a manner that is reminiscent of dynamic stall, and the flow separates again.

DYNAMIC FLIGHT MANEUVERING USING TRAPPED VORTICITY FLOW CONTROL

Closed-loop feedback control is used in a series of wind tunnel experiments to effect commanded 2-DOF maneuvers (pitch and plunge) of a free airfoil without moving control surfaces. Bi-directional changes in the pitching moment over a range of angles of attack are effected by controllable, nominally-symmetric trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge. Actuation is applied on both surfaces by hybrid actuators that are each comprised of a miniature [O(0.01c)] obstruction integrated with a synthetic jet actuator to manipulate and regulate the vorticity concentrations. In the present work, the model is trimmed using position and attitude feedback loops that are actuated by servo motors and a ball screw mechanism in the plunge axis. Once the model is trimmed, the position feedback loop in the plunge axis is opened and the plunge axis is controlled in force mode so to maintain the static trim force on the model, and alter its effective mass. Meanwhile the servomotor in the pitch axis is only used to alter the dynamic characteristics of the model in pitch, and to introduce disturbances. Attitude stabilization and position control of the model is achieved by closing the position loop through the flow control actuators using a model reference adaptive controller designed to maintain a specified level of tracking performance in the presence of disturbances, parametric uncertainties and unmodeled dynamics associated with the flow. The controller employs a neural network based adaptive element and adaptation laws derived by a Lyapunov-like stability analysis of the closed loop system.

A Closed-Loop Flight Control Experiment using Active Flow Control Actuators

Closed-loop pitch control on a moving 1-DOF wing model is investigated in wind tunnel experiments. The models attitude is controlled over a broad range of angles of attack when the baseline flow is fully attached using bi-directional pitching moment that is effected by flow-controlled trapped vorticity concentrations on the pressure and suction surfaces near the trailing edge. In the present work, the model is trimmed using a position feedback loop and a servomotor actuator. Once the model is trimmed, the position feedback loop is opened and the servomotor acts like an inner loop control to alter the dynamic characteristics and to introduce disturbances. Position control of the model is achieved by the flow control actuation using an arbitrary reference model based adaptive outer loop controller. The control architecture employs a neural network based adaptive element that permits adaptation to both parametric uncertainty and unmodeled dynamics.

Closed- Loop Aerodynamic Flow Control of a Free Airfoil

Transitory flow arising from the dynamic response of a free-moving airfoil model to commanded pitch and plunge maneuvers is investigated in wind tunnel experiments. The airfoil is mounted on a 2-DOF traverse and its trim and dynamic characteristics are controlled using position and attitude feedback loops that are actuated by servo motors. Commanded maneuvers are achieved without moving control surfaces using bi-directional changes in the pitching moment over a range of angles of attack that are effected by controllable, nominally-symmetric trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge. Actuation is applied on both surfaces by hybrid actuators that are each comprised of a miniature [O(0.01c)] obstruction integrated with a synthetic jet actuator to manipulate and regulate the vorticity concentrations. The present work focuses on the transitory response of the flow to step-modulated changes in the actuation input while the model’s position is maintained using the systems controller. Flow control effectiveness is demonstrated by the closed-loop response in plunge to a momentary force disturbance which is analogous to the free flight response to a sudden gust.

Aerodynamic performance of airfoils with tangential synthetic jet actuators close to the trailing edge

Aerodynamic properties of an airfoil can be modified by trapping concentrations of vorticity close to the trailing edge. Experimental work has shown that synthetic jet actuators can be used to manipulate and control this trapped vorticity. A time-resolved detailed model is used to simulate the action of tangential-blowing synthetic jet actuators mounted near the trailing edge of the airfoil at low angle of attack. This detailed model resolves the temporal and spatial scales involved in the synthetic jet dynamics. The detailed synthetic jet model along with the Computational Fluid Dynamics (CFD) computations in which they are embedded are validated against wind tunnel data acquired by Dr Ari Glezer’s group at Georgia Tech. Numerical results show the effects of the actuators on the vortical structure of the flow, as well as on the aerodynamic properties. The main effect of the actuation on the time averaged vorticity field is a bending of the separation shear layer from the actuator toward the airfoil surface, resulting in changes in the aerodynamic properties. Similar to the experimental observations, full actuation of the suction side actuator reduces the pitching moment and increases the lift force, while the pressure side actuator increases the pitching moment and reduces the lift force. The effectiveness of the actuator is measured by the change in the aerodynamic properties of the airfoil in particular the lift (�Cl) and moment (�Cm) coefficients. Computational results for the actuator effectiveness show very good agreement with the experimental values over the range of 2 ◦ to 10 ◦ . While the actuation modifies the global pressure distribution, the most pronounced effects are near the trailing edge in which a spike in the pressure coefficient ( Cp), consistent with experimental results, is observed.

A Tangential Synthetic Jet Model Based on Reynolds Stress Field for Flow Control Simulation of an Airfoil

Computational simulations of aerodynamic applications involving flow control with synthetic jets have become a very active research field in the Computational Fluid Dynamics (CFD) community over the last decade. The Computational study presented in this paper is part of the AVOCET (Adaptive VOrticity Control Enabled flighT) project and is intended to provide computational support for the design and evaluation of closed-loop flow control with synthetic jet actuators for small scale Unmanned Aerial Vehicles (UAVs). This computational work gives support and complements the experimental work providing detailed information of the dynamics of the controlled flow. For this purpose, two different synthetic jet models were implemented and validated: a detailed model and a new Ad-Hoc tangential synthetic jet model called Reynolds Stress Synthetic Jet (RSSJ) model. The RSSJ model is based on information from static simulation of a modified NACA 4415 airfoil at different angles of attack using a detailed synthetic jet model. Numerical results show the effects of the actuators on the time-averaged vortical structure of the flow, as well as on the aerodynamic properties. The main effect of the actuation on the time averaged vorticity field is a bending of the separation shear layer from the actuator toward the airfoil surface, resulting in changes in the aerodynamic properties. Computational results for the actuator effectiveness show very good agreement between the two models and the experimental results (over the range of 2 ◦ to 6 ◦ ). The modifications of the aerodynamic properties of the airfoil, brought on by the synthetic jet actuators, are related to the trapped vorticity and flow acceleration close to the trailing edge. The RSSJ model is designed to capture the synthetic jet time averaged behavior so that the high actuation frequencies are eliminated. This allows the time step to be increased by a factor of 5 and demonstrates that by only representing the Reynolds stress field close to the synthetic jet outlet, it is possible to capture the basic actuation effects on the flow field and particularly on the aerodynamic properties. The RSSJ was also tested in dynamic simulations, in which its capacity to capture the detailed model average performance was demonstrated.