AFRL-AFOSR-VA-TR-2016-0030 Cyber-physical systems to understand the dynamics of nonlinear aeroelastic systems for flexible MAVs and energy harvesting applications

The dynamics of an elastically mounted flat plate in a uniform stream undergoing flow-induced pitching oscillations is studied with the aid of a cyber-physical system which iscapable of simulating arbitrary structural dynamics. The system measures the angularposition and velocity, and combines them with appropriate gains to provide a torque to acomputer-controlled servomotor that emulates arbitrary torsional stiffness and damping.A series of experiments were carried out over a wide range of parameter space and theresults demonstrate that inertial scaling of the stiffness and damping effectively capturesthe system behavior. As the torsional stiffness decreases, the system exhibits severalbifurcations in its dynamical behavior. Firstly, a subcritical transition through a saddle-node bifurcation results in small-amplitude asymmetric limit-cycle oscillations and later,a subcritical transition through a Hopf bifurcation gives rise to large-amplitude symmetriclimit-cycle oscillations. At very low stiffness the plate does not oscillate. Energy harvestingfrom the flow is only 1% efficient, but is found to be optimized at a non-dimensionalstiffness close to unity while increasing the Reynolds number is found to extend the rangeof damping over which large scale oscillations and energy harvesting can be sustained.Although velocity measurements are not made, the details of torque-position phase planeare used to infer the details of the formation time and stability of the leading-edge vortexassociated with the rapidly pitching plate.& 2015 Elsevier Ltd. All rights reserved.