Effect of Eliminating Trailing Edge Vortices on Thrust Coefficient in a Plunging Flat Plate

Plunging motion, characterized by frequency and amplitude is a key component in the kinematics of many flying and swimming organisms. We studied plunging of a flat plate with a broad range of reduced frequencies 0.25$\le k\le $16 and plunge amplitudes 0.03125$\le h\le $8 giving plunge velocities of 0.25$\le $\textit{kh}$\le $4 at \textit{Re}$=$100. This study observed that, unlike previous investigations for small plunge amplitudes, thrust does not increase monotonically with \textit{kh} but reaches a maximum and then decreases. It is shown that Leading Edge Vortices (\textit{LEV}s) are responsible for thrust production whereas Trailing Edge Vortices (\textit{TEV}s) induce drag on the plate. At higher \textit{kh}, vortex induced velocities dominate the flow with strong nonlinear vortex-vortex interactions (\textit{VVI}). Three main \textit{VVI} mechanisms are identified; in two of them \textit{TEV}s adversely affect thrust production. It is shown that by introducing a splitter plate that eliminates the formation of \textit{TEV}s, the thrust coefficient ($C_{T})$ increases monotonically \quad with \textit{kh. }A parametrization of thrust coefficient is done with frequency ($k)$ and amplitude ($h)$ [$C_{T\thinspace }=$ $A$.$k^{\mathrm{1.4}}h -B$ where $A$ and $B$ are constants, with a $R^{2}=$0.96 for the proposed equation]. Additionally, a scaling analysis is done between $C_{T}$ and circulation to see the effect of eliminating \textit{TEV} on \textit{LEV} dynamics.