Noisy Inflow Advances Jet-Switching in the Wake of a Flapping Airfoil

The present work is focused on investigating the effect of noisy input fluctuations on a harmonically plunging elliptic foil in the low Reynolds number regime ($Re = 300$). Simulations are carried out using a discrete forcing Immersed Boundary Method (IBM) based in-house Navier-Stokes solver. The stochastic noisy inlet velocity is modeled as the Ornstein-Uhlenbeck process of chosen correlation length which governs the time-scales present in the velocity spectra. The unsteady flow-field is simulated at different dynamic plunge velocities ($kh$). A thrust producing reverse K\'arm\'an wake is observed at $kh = 1.0$ under steady uniform inflow. As $kh$ increases, at $kh = 1.5$, the reverse K\'arm\'an wake is seen to oscillate alternatively in the upward and downward directions with a small deflection angle, giving the notion of jet switching. However, in the presence of noisy inflow, the organized pattern of the reverse K\'arm\'an wake at $kh = 1.0$ gets distorted as the distances between the vortex cores get altered in a random fashion. At $kh = 1.5$, the onset of jet switching in the trailing wake gets advanced in the presence of incoming gust. Also, the deflection angle and the jet switching frequency are seen to increase compared to the case of the steady uniform inflow.