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Seidel J.,U.S. Air force | Fagley C.,U.S. Air force | McLaughlin T.,U.S. Air force | McLaughlin T.,Aeronautics Research Center
33rd AIAA Applied Aerodynamics Conference

Fully coupled CFD-CSD simulations are used to characterize the dynamic aeroelastic behavior of a torsionally flexible, finite aspect ratio NACA0018 wing. The main aeroelastic instability, which is also known as stall flutter, is a cycle from increasing angle of attack, large lift, then flow separation and a reduction of the angle of attack due to the elastic forces in the wing. The same sequence of events happens for negative angles of attack. This dynamic instability is a very periodic limit cycle oscillation. A single blowing port was integrated in the wing to control the lift oscillation. Constant blowing at various amplitudes had significant impact, reducing the limit cycle amplitude. The data was used to develop a simple model of the damping added to the structural dynamics due to the forcing. The model showed that the response to forcing is highly nonlinear and that a optimum blowing strength exists. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. Source

Post M.L.,U.S. Air force | Cummings R.M.,U.S. Air force | McLaughlin T.E.,U.S. Air force | McLaughlin T.E.,Aeronautics Research Center
50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

The Aeronautical Engineering program at the U.S. Air Force Academy over the past decade has focused on integrating experimental and computational experiences in all Aerodynamic sequence courses. The required courses in the Aerodynamic Discipline include Aeronautical Fluid Dynamics, Computational Aerodynamics, and Advanced Aerodynamics. The experiences in these courses prepare cadets for capstone-like experiences in the Experimental/Computational Discipline of the required Aeronautical Laboratory and the elective Advanced Computation Aerodynamics. The paper highlights the experiences in each of these courses and summarizes how these contribute to success in the program. Source

Font G.I.,U.S. Air force | Font G.I.,Stanford University | Enloe C.L.,U.S. Air force | Enloe C.L.,Stanford University | And 8 more authors.
AIAA Journal

Atmospheric pressure dielectric barrier discharge plasma actuators are experimentally investigated. The temporal force characteristics and dielectric surface charging are determined using interferometry and split electrode techniques. The experiments are conducted at atmospheric pressure in diminishing levels of oxygen content to investigate the effects of oxygen ions. The results show that the force production is dominated by oxygen ions down to a level of 2-5% oxygen content. Temporal force measurements show that the plasma accelerates the air twice during the bias cycle for all oxygen levels, including pure nitrogen. Surface charging measurements show that, for oxygen content levels above 5%, a positive voltage region builds up on the dielectric downstream of the actuator. In the absence of oxygen, no such buildup is observed. The temporal force production characteristics in the pure nitrogen discharge appear to be greatly affected by the dielectric surface charging. Finally, at a 20% oxygen content level, the majority of the force is produced by the actuator while the exposed electrode is negative. When all of the oxygen is removed, the majority of the force is produced while the exposed electrode is positive. Source

Fagley C.,U.S. Air force | Porter C.,U.S. Air force | Porter C.,National Research Council Italy | McLaughlin T.,U.S. Air force | McLaughlin T.,Aeronautics Research Center
AIAA Journal

The asymmetric vortex regime of a von Kármán ogive with a fineness ratio of 3.5 is experimentally studied at a Reynolds number of 156,000. The wake of an axisymmetric bluff body is an ideal candidate for active feedback flow control because minute fluidic disturbances and geometry perturbations near the tip of the ogive get amplified through the flow's convective instability. The resulting disturbance interacts with the quasi-steady vortex location and produces a deterministic port or starboard asymmetric vortex state (i.e., side force). Accurate control or manipulation of this asymmetric vortex state holds the potential for increased maneuverability and stability characteristics of slender flight vehicles. For implementation of an active feedback flow-control system, plasma actuators at the tip of the ogive are used as the flow effector, and surface-mounted pressure sensors are used to estimate the vortex configuration in real time. A linear time invariant model developed from open-loop experimental tests and a proportional-integral control law are used to close the loop in the experimental setting. Closed-loop experimentation shows the ability to arbitrarily track a side force set point while also suppressing low-frequency fluctuations. Thus, the adopted model-based feedback flow-control approach is validated experimentally for a complex, three-dimensional flow. Copyright © 2014 by the American Institute of Aeronautics and Astronautics, Inc. Source

Porter C.,U.S. Air force | Seidel J.,U.S. Air force | Fagley C.,U.S. Air force | Farnsworth J.,U.S. Air force | And 2 more authors.
6th AIAA Flow Control Conference 2012

The flowfield around an axisymmetric forebody at a moderate angle of attack (40° < α < 60°) can produce a significant side force as the result of an asymmetric pressure distribution around the body. The asymmetry of the pressure distribution results from a steady, asymmetric vortex configuration around the body even though the body is axisymmetric. Unsteady laminar simulations were performed on a von Kármán tangent ogive forebody with a fineness ratio of 3.5, angle of attack of 50 degrees, and a diameter based Reynolds number of 220,000. As a first step towards feedback flow control of the asymmetric vortex state, open-loop disturbances similar to those produced by a Dielectric Barrier Discharge (DBD) plasma actuator near the tip of the model were simulated. The resulting side force from the open-loop simulations are compared to the unforced simulations. In the unforced case, a large side force was observed with maximum amplitudes similar to those observed in a companion experiment. However, the side force fluctuates between the port and starboard sides, in contrast to experimental observation where the side force is relatively steady. When forcing is turned on, the resultant asymmetric vortex state locks into one position where the magnitude of the side force is proportional to the strength of the applied forcing. These simulations, both forced and unforced, are used to develop a flow state database through Proper Orthogonal Decomposition (POD) for the development of reduced order models. It is shown that the second POD mode (including the mean) captures the asymmetry of the different vortex states tested. Source

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