Natural flyers are capable of complex aerial maneuvers that is one of the critical factors for miniature surveillance equipment such as Micro Aerial Vehicles (MAVs). Scientists and engineers are interested to understand the physics associated with such natural flight to apply them in the design of MAVs. Researchers have demonstrated that flexible wings can generate more lift and thrust than the rigid ones. Recently it is reported in the literature that the flow-field of a harmonically oscillating rigid wing undergoes aperiodic/chaotic transitions as the stroke amplitude is increased. However, the effect of wing flexibility on the dynamic transitions of the flow-field is largely unknown. A rigid elliptic foil (with major axis length as c) with a flexible tail of length 0.3c has been considered to model a flexible wing configuration in the present study. This canonical design is referred to as a teardrop model of a wing. A sinusoidal plunge motion is provided at the center of the elliptic foil. The trailing part of the teardrop is modeled as an inextensible flexible filament and is allowed to oscillate passively. The present study is focused on examining the dynamics of the flow-field over the undulatory flexible tail as compared to the rigid configuration where tail flexibility is infinite. The flow around the oscillating body is governed by the incompressible Navier-Stokes equations. An in-house Fluid-Structure Interaction (FSI) solver has been developed following a discrete forcing type Immersed Boundary Method (IBM). In the present FSI framework, the structural solver and the IBM solver are coupled through a weak coupling approach where both the solvers' exchange information explicitly at every time-step. The Reynolds number is kept constant at 300 throughout the study. The effects of plunge velocity (kh) and flexibility of the tail on the qualitative and quantitative dynamics of the flow-field are systematically studied in terms of flow separation at the foil surface, vortex shedding at the trailing edge and their interactions. Here, k (= 2πfc/U∞) is the reduced frequency and h (=h0/c) is the nondimensional stroke amplitude. The simulations are performed for two different bending rigidity (γ) of the tail with γ=∞ (rigid case) and 0.01 (flexible case). For an elliptic foil with-tail of γ=∞, the aerodynamic performance is enhanced but it triggers the system to enter into aperiodicity at much lower stroke amplitude (at kh= 1.2) compared to an equivalent without-tail scenario. However, at γ= 0.01, the aperiodic onset gets delayed and the flow-field remains periodic till a higher stroke amplitude (till kh= 1.65) with a decrement in the performance compared to the perfectly rigid case. In the presence of a rigid tail strong leading-edge vortices (LEVs) are formed and after separating they interact with the trailing edge vortices (TEVs). A phase delay in this chronology of events triggers aperiodicity in the near-field. The flexible tail (γ= 0.01) alters the near-field in such a way that no LEV separation takes place. The LEVs always remain attached to the body and vortices shed from the trailing edge alone. As a result, there is no LEV/TEV interactions and the flow field remains periodic. The characteristics of different dynamical states at different parametric values will be reported by using various tools from dynamical time series analysis, such as the recurrence plot, Poincare sections, phase-space reconstructions, and Lyapunov exponents in the full-length paper.


Delaying the Chaotic Onset in the Flow-Field of Flapping Foil With Flexible Aft Tail

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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