Session BM: Aerodynamics

Wake Vortex Alleviation Using Rapidly Actuated Segmented Gurney Flaps

A study to assess the potential for using rapidly actuated segmented Gurney flaps, also known as Miniature Trailing Edge Effectors (MiTEs), for active wake vortex alleviation is conducted using a half-span model wing with NACA 0012 shape and an aspect ratio of 4.1. All tests are performed with the wing at an 8.9 degree angle of attack and chord based Reynolds number around 350,000. The wing is equipped with an array of 13 MiTE pairs. Each MiTE has a flap that in the neutral position rests behind the blunt trailing edge of the wing, and in the down position extends 0.015 chord lengths perpendicular to the freestream on the pressure side of the wing. Dynamic PIV is used to measure the time dependent response of the vortex in the intermediate wake to various MiTE actuation schemes that deflect the vortex in both the spanwise and liftwise directions. A maximum spanwise deflection of 0.041 chord lengths is possible while nearly conserving lift. These intermediate wake results as well as pressure profile, five-hole probe, and static PIV measurements are used to form complete, experimentally-based initial conditions for vortex filament computations that are used to compute the far wake evolution. Results from these computations show that the perturbations created by MiTEs can be used to excite vortex instability.   

Active Control of a Flapping Wing in a Gust Setup

The aim of this experiment is to determine the response of a flapping Micro Air Vehicle wing to a wind gust while in forward and hovering flight and apply an active control to respond to the wind gust. The flapping wing is driven by a DC brushless motor which is geared to allow for flapping at frequencies up to 3 Hz. The wing is set up vertically in the wind tunnel, and can flap up to angles of 120 degrees. To simulate a wind gust perpendicular to the free stream flow a diffuser is set up on top of the wind tunnel. Strain gages are attached to the wing. It has been shown while simultaneously measuring the dynamical strain and the velocity field with a PIV system, that a realistic estimate of the wake flow field can obtained using low dimensional tools (POD, mLSE). The wing and the flapping mechanism are mounted directly on a force balance to calculate the lift being produced. In order to prevent flow separation on the wing during a sudden wind gust the wing is actively deformed by an attached piezoelectric actuator. The end result is to have closed loop control to produce stable hovering and forward flight.   

Evolution of Surface Topology and Flow Structure on a Delta Wing Due to Pitching Motion

Unmanned Combat Air Vehicles (UCAVs) and Micro Air Vehicles (MAVs) undergoing steady flight or unsteady maneuvers generate complex flow patterns, the physics of which must be clearly identified in order to optimize maneuverability and minimize unsteady loading that can lead to undesired vibrations and fatigue. Near-surface flow patterns of a basic delta wing having moderate sweep angle, representative of key features of the foregoing configurations, are visualized by a technique of high-image-density particle image velocimetry. Emphasis is on definition of the states of relaxation of the flow patterns following a pitch-up maneuver, in terms of patterns of velocity, streamline topology and vorticity, along the near-surface plane. The changes of the topological features observed during the early stages of the relaxation process are analogous to the alterations of the surface patterns obtained for the stationary wing at smaller angles-of-attack. These features include negative (separation) and positive (reattachment) bifurcation lines, saddle points, foci and nodes.   

The Drag Penalty of Lateral Asymmetries in Formation Flight

It has long been known that formation flight of birds and aircraft results in a significant energy saving due to reduction in induced drag. However measured gains have consistently been lower, usually explained by viscous effects neglected by the potential flow model for lift and induced drag. We show that the inherent asymmetry of the flow-field in the general case results in rolling and yawing moments, which need to be corrected by control surface reflection. This deflection results in an increase in drag, which partially cancels the gains mentioned above. Using classical lifting line theory and elliptical lift distributions on two or more wings flying in formation we show that the penalty incurred by these corrections can reduce the expected gains by up to 25{\%}. We also show that the gains for an individual in formation flight grow with the number of members of the formation, up to about 7 members, the added gains becoming negligible beyond that number. The present results are relevant for large aspect-ratio, fixed wing aircraft, and gliding bird flocks.   

Numerical Analysis of a Heaving flexible Airfoil in a Viscous Flow

A numerical model for two-dimensional unsteady viscous fluid flow around flexible bodies is used to analyze the effect of chordwise flexibility on heaving airfoils. Flexible airfoils proved to be more efficient than the rigid ones. Both the output power and the input power increase in the flexible cases. The gain in efficiency is realized as a result of the output power increasing more than the input power. The density of the airfoils was shown to be a key factor in determining efficiency and power. In the parameter range analyzed, heavier airfoils are shown to generate less output power and to require proportionately less input power. Thus, heavier airfoils are more efficient than lighter airfoils. In the numerical model the bodies are represented by a distributed body force in the Navier-Stokes equations. The body force density is found at every time-step so as to adjust the velocity within the computational cells occupied by the bodies to a prescribed value. The main advantage of this method is that the computations can be effected on a Cartesian grid, without having to fit the grid to the body surface. This approach is particularly useful when applied to the case of multiple bodies moving relatively to each other, as well as flexible bodies, in which case the surface of the object changes dynamically.   

Aeroelastic Deformation and Buckling of Inflatable Wings under Dynamic Loads

Inflatable wings have recently been used to control a vehicle in flight via wing warping. Internal pressure is required to maintain wing shape and externally mounted mechanical actuators are used to asynchronously deform the wing semi-spans for control. Since the rigidity of the inflatable wing varies as a function of inflation pressure, there is a need to relate the wing shape with aerodynamic loads. Via wind tunnel tests, span-wise deformations, twist and flutter have been observed under certain dynamic loading conditions. Photogrammetry techniques are used to measure the static aeroelastic deformation of the wings and videogrammetry is used to examine the dynamic shape changes (flutter). The resulting shapes can be used to determine corresponding aerodynamic characteristics. For particular inflation pressures, buckling can be induced at sufficiently high dynamic loads either through high dynamic pressure or large angle of attack. This results in a set of critical loading parameters. An inflatable winged vehicle would require operation within these limits. The focus of the presentation will be on defining and exploring the unsuitable operating conditions and the effects these conditions have on the operation of the wing.   

Complex flows over simple wings.

As the chord Reynolds number (\textit{Re}) of an airfoil section drops below 10$^{5}$, the global, averaged properties such as mean lift and drag, become strongly affected by the presence/absence of separation on portions of the upper surface. Such flows are difficult to measure and difficult to compute. As \textit{Re} decreases further, the lift:drag polars become increasingly odd in shape and difficult to replicate. At the same time, the amount of reliable literature data drops, so the aerodynamic performance becomes, in many ways, quite unpredictable. Since many practical small-scale flying machines, be they fixed or flapping wing designs, operate in this \textit{Re} regime, there is a clear need for an improved understanding of the basic performance based on the flow physics. An experimental program is described that characterizes the instantaneous flow fields and aerodynamic forces on two-dimensional and finite wings with various profile shapes. The objective is to provide a foundation for practical wing design at moderate \textit{Re}, and to provide a basis for rigorous comparisons with emerging computational capabilities.