'American Institute of Aeronautics and Astronautics (AIAA)'
Vortex formation during pure plunge was studied using constant plunge velocity and quarter-sine plunge velocity profiles and three fixed pitch angles. A plunging infinite carbon composite flat plate with square edges was used to simulate a simplified mode of flapping wing flight in quiescent water. No imposed freestream was used. The Reynolds numbers for all cases was 3000 based on an average plate velocity assumption for the freestream. Concepts developed in the study of vortex rings to characterize the vortex formation around flat plates in pure plunge were employed. In the study of vortex rings the formation parameter has been found to be a relevant scaling parameter for the growth and optimal circulation in vortices.
In the present study, measurements of the wake behind a flat plate were made using Digital Particle Image Velocimetry (DPIV) and a quantitative analysis was performed on unsteady vortex formation through the analysis of normalized circulation. The results showed that the formation parameter was unable to predict optimal circulation for all cases but the formation parameter was a relevant scaling parameter during the growth of vortices at low formation times (approximately T = 1-3). The plate plunge velocity profiles selected examined the dependence of normalized vortex circulation on plate velocity profile. The quarter-sine velocity profile produced larger maximum values for normalized circulation. The fixed pitch angle variation selected examined the sensitivity of normalized vortex circulation to deviations from symmetric vortex generation. When an angle of attack is used other than one that is normal to the induced freestream an upstream and downstream vortex were created where the upstream had preferential growth.
The two paramount observations of the DPIV results are twofold. First, the maximum circulation and separation of vortices was independent of formation time and dependent on parametric variations. Second, a region of linear normalized circulation exists for the upstream vortices from approximately T = 1-3 independent of parametric variation An ancillary observation of the DPIV was based on the approximate forces extracted from the velocity field for two of the constant velocity profile experiments. The forces compare well with the direct force measurement results from literature and the steady state drag coefficient for a normal flat plate