This paper aims to explore the feasibility of providing boundary layer propulsion and flow control by means of embedded aerofoils that are oscillating in the pure plunge mode. To this end, Navier-Stokes calculations of the low-speed flow over a flat plate with an oscillating small foil in close vicinity to the plate were performed to determine the influence of the wall distance, Reynolds number, and reduced frequency on the aerofoil thrust. The simulations were extensively validated against water tunnel experiments at Reynolds numbers between 440 to 5,940. Good agreement was obtained in terms of mean streamwise velocity profiles and the vortical wake patterns. Results indicate that the thrust increases from its value in unbounded flow with decreasing distance from the plate. The propulsive efficiency exhibits a consistent peak at a non-dimensional plunge velocity of about 0.55. For wall distances between one-half to one chord lengths, vortex pairs are shed in a slightly upward deflected direction independent of the starting motion of the aerofoil. As the wall distance increases further, these vortex pairs change into the well-known reverse Karman vortex street. Example calculations for a flat plate with two foils mounted close to the plate trailing edge and oscillating in counterphase confirm the device’s efficacy.

This work investigates the role of flexibility and resonant excitation on the deformation mechanism and aerodynamic performance of flapping wings. A hummingbird-inspired wing (HIW) is considered and designed to have a bone-like stiffener made of carbon fibre/epoxy (CF/E) composite attached to a membrane made of carbon nanotubes/polypropylene (CNTs/PP) nanocomposite representing the flexible part of the natural wing. The designed HIW model is analysed through fluid-structure interaction simulations performed at frequencies near and at resonant frequency. It is found that HIW generates desired bending and twisting deformations that are coupled. These deformation mechanisms are studied in detail with the help of time-varying deflections and bending-twisting angles. Further, the simultaneous effect of these parameters on the aerodynamic performance of the wing is also investigated. It is observed that the coupled nature of bending and twisting deformations is critical in enhancing the aerodynamic performance of flapping wings. In addition to that, the resonance generates higher amplitude of desired structural deformations that further enhances thrust as well as lift generation capability of the wing. The underlying mechanism for this is also explained by studying the flow around the deflected surface of the wing. Compared to off-resonant frequencies, vorticity and pressures are substantially higher for the wing at resonance. A physical model of HIW is realised using CNTs/PP and CF/E composites to perform experimental wing motion analysis and to validate the computational results. In conclusion, the present study provides a basis to design efficient biomimetic flapping wings for micro aerial vehicles (MAVs) by exploring flexibility and resonant excitation.