Abstract
The unsteady flow characteristics and responses of an NACA 0012 airfoil fitted with a bio-inspired morphing trailing edge flap (TEF) at near-stall angles of attack (AoA) undergoing downward deflections are investigated at a Reynolds number of 0.62 × 106 near stall. An unsteady geometric parametrization and a dynamic meshing scheme are used to drive the morphing motion. The objective is to determine the susceptibility of near-stall flow to a morphing actuation and the viability of rapid downward flap deflection as a control mechanism, including its effect on transient forces and flow field unsteadiness. The dynamic flow responses to downward deflections are studied for a range of morphing frequencies (at a fixed large amplitude), using a high-fidelity, hybrid RANS-LES model. The time histories of the lift and drag coefficient responses exhibit a proportional relationship between the morphing frequency and the slope of response at which these quantities evolve. Interestingly, an overshoot in the drag coefficient is captured, even in quasi-static conditions, however this is not seen in the lift coefficient. Qualitative analysis confirms that an airfoil in near stall conditions is receptive to morphing TEF deflections, and that some similarities triggering the stall exist between downward morphing TEFs and rapid ramp-up type pitching motions.
Highlights
Most natural flyers used shape changing, or morphing, mechanisms in order to harness the full capabilities of their wings [1,2] (Figure 1)
The Fluent predictions are in better agreement with the test data in the linear region for both lift and drag predictions, the experimental data of Sheldahl et al [36] seem to show an earlier stall for the airfoil at about angles of attack (AoA) = 11◦, while the Fluent predictions show a higher stall angle at AoA = 13◦, resulting Fluids 2021, 6, x FOR PEER REVIEWin an 8.6% higher CL,max than the measurement
For all AoA, both the lift and the drag coefficient responses showed a proportional relationship between the morphing frequency and the slope of response at which these quantities evolve
Summary
Most natural flyers (insects, birds, bats, or even extinct Pterosaurs) used shape changing, or morphing, mechanisms in order to harness the full capabilities of their wings [1,2] (Figure 1). To the authors’ best knowledge, the study of near-stall dynamic flow response to a rapid morphing flap deflection rather than a hinged flap deflection has not yet been addressed in open literature by means of either experimental or numerical work at high or low Reynolds numbers. The paper presents an exploratory CFD study, using a hybrid RANSLES model, investigating the responses of both lift and drag forces to rapid downward morphing TEF deflection at various frequencies. To the authors’ best knowledge, this is the first time SBES has been used to investigate the near-stall aerodynamic performance of an airfoil in response to rapid morphing deflections. For a fixed large morphing amplitude, flow responses at three angles of attack near stall (8◦, 10◦, and 12◦) and at a range of morphing frequencies are presented and discussed
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