Browsing by Author "Ghommem, Mehdi"
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- Control of Limit Cycle Oscillations of a Two-Dimensional Aeroelastic SystemGhommem, Mehdi; Nayfeh, Ali H.; Hajj, Muhammad R. (Hindawi Publishing Corporation, 2010)Linear and nonlinear static feedback controls are implemented on a nonlinear aeroelastic system that consists of a rigid airfoil supported by nonlinear springs in the pitch and plunge directions and subjected to nonlinear aerodynamic loads. The normal form is used to investigate the Hopf bifurcation that occurs as the freestream velocity is increased and to analytically predict the amplitude and frequency of the ensuing limit cycle oscillations (LCO). It is shown that linear control can be used to delay the flutter onset and reduce the LCO amplitude. Yet, its required gains remain a function of the speed. On the other hand, nonlinear control can be efficiently implemented to convert any subcritical Hopf bifurcation into a supercritical one and to significantly reduce the LCO amplitude.
- Deterministic Global Optimization of Flapping Wing Motion for Micro Air VehiclesGhommem, Mehdi; Hajj, Muhammad R.; Watson, Layne T.; Mook, Dean T.; Snyder, Richard D.; Beran, Philip S. (Department of Computer Science, Virginia Polytechnic Institute & State University, 2010-12-01)The kinematics of a flapping plate is optimized by combining the unsteady vortex lattice method with a deterministic global optimization algorithm. The design parameters are the amplitudes, the mean values, the frequencies, and the phase angles of the flapping motion. The results suggest that imposing a delay between the different oscillatory motions and controlling the way through which the wing rotates at the end of each half stroke would enhance the lift generation. The use of a general unsteady numerical aerodynamic model (UVLM) and the implementation of a deterministic global optimization algorithm provide guidance and a baseline for future efforts to identify optimal stroke trajectories for micro air vehicles with higher fidelity models.
- Modeling and Analysis for Optimization of Unsteady Aeroelastic SystemsGhommem, Mehdi (Virginia Tech, 2011-11-03)Simulating the complex physics and dynamics associated with unsteady aeroelastic systems is often attempted with high-fidelity numerical models. While these high-fidelity approaches are powerful in terms of capturing the main physical features, they may not discern the role of underlying phenomena that are interrelated in a complex manner. This often makes it difficult to characterize the relevant causal mechanisms of the observed features. Besides, the extensive computational resources and time associated with the use these tools could limit the capability of assessing different configurations for design purposes. These shortcomings present the need for the development of simplified and reduced-order models that embody relevant physical aspects and elucidate the underlying phenomena that help in characterizing these aspects. In this work, different fluid and aeroelastic systems are considered and reduced-order models governing their behavior are developed. In the first part of the dissertation, a methodology, based on the method of multiple scales, is implemented to show its usefulness and effectiveness in the characterization of the physics underlying the system, the implementation of control strategies, and the identification of high-impact system parameters. In the second part, the unsteady aerodynamic aspects of flapping micro air vehicles (MAVs) are modeled. This modeling is required for evaluation of performance requirements associated with flapping flight. The extensive computational resources and time associated with the implementation of high-fidelity simulations limit the ability to perform optimization and sensitivity analyses in the early stages of MAV design. To overcome this and enable rapid and reasonably accurate exploration of a large design space, a medium-fidelity aerodynamic tool (the unsteady vortex lattice method) is implemented to simulate flapping wing flight. This model is then combined with uncertainty quantification and optimization tools to test and analyze the performance of flapping wing MAVs under varying conditions. This analysis can be used to provide guidance and baseline for assessment of MAVs performance in the early stages of decision making on flapping kinematics, flight mechanics, and control strategies.