Session F16: Aerodynamics: Unsteady Effects II

An investigation of the aerodynamics of an airfoil moving transversely through a uniform-shear approach flow

Aircraft commonly experience non-uniform approach flows, such as in wind shear or wake of other bodies. Yet, practically no studies exist on airfoils moving in the cross-stream direction of such flows, a situation that is encountered, for example, during landing through the air wake of an aircraft carrier. In this work, a NACA-0012 airfoil traverses steadily at fixed geometric angle of attack across a uniform-shear approach flow in a water tunnel. The lift force on the airfoil is measured with a load cell, and the streamwise flow velocity over the airfoil is measured with molecular tagging velocimetry. Results will be presented for the lift force on the moving airfoil as it traverses across the shear flow in comparison with the stationary airfoil in shear flow. Boundary-layer resolved streamwise velocity maps at various angles of attack will also be shown in order to illustrate the observed flow field differences.   

Aerodynamic Forces on a Steady Airfoil in a Two-Stream Shear Layer

Most problems in aerodynamics traditionally consider a uniform approach velocity, but in numerous circumstances of the complex flows found in nature the approach flow can be significantly non-uniform. As part of a larger investigation into airfoil aerodynamics in the presence of non-uniform approach flow, this study considers the aerodynamic loads on a steady NACA-0012 airfoil at chord Reynolds number of 1.2×10⁴, when placed inside a two-stream shear layer. Load measurements are carried out over a wide range of angles of attack with the airfoil placed at three downstream locations where the mean and fluctuating streamwise velocity profiles in the shear layer exhibit self-similar behavior typical of two-stream shear layers. The three measurement locations considered allow us to assess the influence of mean shear rate and shear zone thickness on the aerodynamic loads. The measured mean lift and drag forces are compared against our previous results in uniform freestream and a non-uniform approach flow with steady shear profile.   

Vortical Airfoil Wake Structure in Non-Uniform Approach Flow

Phase-resolved spatial map of the streamwise velocity from LDV measurements in the wake of an airfoil pitching at high reduced frequency placed inside a two-stream shear layer appears to show the signature of only a single sign vortex per oscillation cycle.  This is curious as it is counter to the alternating-sign vortex pair per oscillation cycle that is characteristic of the wake structure in uniform flow.  To elucidate the flow physics present in the experiment, computations are carried out in an attempt to mimic the conditions of the experiment.  The computed wake structure provides a clue to the actual vortical field and how this particular wake vortex arrangement can lead to incorrect inference based on the incomplete information from phase-resolved map of streamwise velocity.   

Numerical Investigation and Optimization of a Flexible Flapping Airfoil

Motivated by novel biomimetic technology inspired by the oscillating nature of fish locomotion, we present a computational framework for simulating the combined pitching and heaving of a two-dimensional airfoil with chordwise flexibility. A strongly coupled fluid-structure interaction (FSI) solver is developed using the existing FSI framework in the open-source toolbox OpenFOAM. With this model, we investigate the effects of varying a matrix of parameters on the thrust, lift and drag behavior of the flexible foil. The flexible material properties, as well as motion parameters, including heave/pitch amplitude, oscillating frequency, and degree of phase shift, have all been shown to significantly impact the lift and thrust characteristics of a chosen configuration. Finally, configurations are proposed which optimize thrust and efficiency within a range of realistic parameters.
  

Abstract Withdrawn

The pitching and heaving motions of multiple two-dimensional bodies are investigated, with a particular focus on a flat plate in ground effect. By establishing a conformal mapping from a canonical circular domain using the Schottky-Klein prime function, the complex potential for uniform flow past the bodies is calculated. The Schwarz problem for the velocity field is solved, which allows the no-flux boundary condition on the pitching and heaving surfaces to be included in the complex potential. The vortex shedding from the leading-edges is neglected, and the Kutta conditions at the trailing-edges are satisfied by the shedding of discrete Brown-Michael vortices of variable intensity, which become frozen when a subsequent vortex is shed. The results elucidate details of the underlying physics of multi-body interactions, including the enhanced lift-to-drag ratio enjoyed by foils operating close to the ground.   

Effect of stroke variations on the aerodynamic characteristics of a hovering wing

The wing kinematics of hovering insects are dominated by the stroke (back and forth motion of the wing) and  pitch (change in angle-of-attack). The manipulation of these motions is known to impart superior aerodynamic qualities to biological fliers. The wing kinematics are complicated and difficult to replicate in human-engineered aerial vehicles. Popular approximations that are used to mimic the flapping wing stroke kinematics are sinusoidal, trapezoidal, and linear profiles. The influence of different stroke kinematics on the aerodynamic characteristics of a hovering wing is experimentally investigated in this study. The time-varying aerodynamic forces acting on the hovering wing are measured using a dynamically scaled mechanical model in a quiescent flow in the insect flight regime. Phase-locked flow field measurements are carried out at the mid span of a wing with an aspect ratio of 3. The dynamically relevant flow features are identified and tracked. The results are correlated with the aerodynamic forces to explain the effect of stroke variations during hover.
  

Mechanism to Improve Propulsive Performance of an Oscillating Airfoil Employing Hybrid Heave Motions

A number of studies of unsteady airfoil dynamics have been inspired by animal locomotion. In the present research, we employ a sports-mimetic approach, where we study unsteady airfoil motions inspired by sail dynamics. Olympic sailors use various unsteady aerodynamic techniques to increase propulsion. One such technique is “sail flicking”, whereby sailors use their bodyweight to roll the boat about its longitudinal axis, flicking the sail periodically. This leads to a distinct high-lift mode and a low-lift mode, depending on the angle of the sail oscillations relative to the incoming apparent wind. By replacing the sail by a NACA 0012 airfoil, we show that this “hybrid-heave” motion, wherein the airfoil oscillates at non-normal angles to the incoming flow, exhibits strong lift amplification, up to 7-8 times the lift of a non-oscillating airfoil. Vortex dynamics and force measurements are made with reference to the airfoil trajectory through the fluid, indicating that the high-lift mode is associated with an in-phase variation of velocity and time-varying angle of attack of the airfoil.