Narrow Results

The two-dimensional laminar flow over a simultaneously pitching-heaving teardrop airfoil is studied numerically. The influence of frequency and amplitude on the wake structure and aerodynamic performance is investigated at a constant Reynolds number, $\mathrm{\text{Re}}=2640$. The computational modeling accurately depicts the changes in the wake structure that occur between the von Karman, reverse von Karman and deflected conditions. Investigations at low flapping frequencies and amplitudes identified the presence of additional wake structures that had not been reported previously. The interaction between the flapping frequency and vortex shedding frequency appears to govern the formation of such multiple vortex wakes. The wake structures identified are presented in the form of a wake map where the transitions between the wake structures are visible. This study correlates the wake arrangement with the variation in force coefficients and explains why the presence of a reverse von Karman street is necessary but not a sufficient condition for thrust production. The time history of the drag coefficient and the phase relation between lift and drag show the dynamics of the wake. The variation of force coefficients and efficiency at different flapping conditions is evaluated, and the influence of flapping parameters is assessed. The underlying vortex interactions that influence the aerodynamic performance are identified. The development and distribution of stress fields developed due to periodic fluctuations behind the flapping airfoil have not been discussed previously. The velocity fluctuations in the wake due to the periodic flapping are presented, and regions of maximum stress distribution are identified.

In this work, the performance of flapping airfoil propulsion at low Reynolds number of Re = 100–400 is studied numerically with the lattice Boltzmann method (LBM). Combined with immersed boundary method (IBM), the LBM has been widely used to simulate moving boundary problems. The influences of the reduced frequency on the plunging and pitching airfoil are explored. It is found that the leading-edge vertex separation and inverted wake structures are two main coherent structures, which dominate the flapping airfoil propulsion. However, the two structures play different roles in the flow and the combination effects on the propulsion need to be clarified. To do so, we adopt the dynamic mode decomposition (DMD) algorithm to reveal the underlying physics. The DMD has been proven to be very suitable for analyzing the complex transient systems like the vortex structure of flapping flight.