Session J06: Aerodynamics: Active Flow Control

Transitory Aerodynamic Control of a Pitching Airfoil using Pulsed Bleed Actuation

The unsteady aerodynamic loads of a static and pitching 2-D airfoil (Clark-Y, Re_c = 6·10⁵) are regulated without moving control surfaces by impulsive actuation (~4 convective time scales) engendered by pulsed air bleed driven by pressure differences between the pressure and suction surfaces through the airfoil interior. The bleed air is driven through spanwise arrays of ports near midchord on both surfaces (2.3% of planform open area per side) and regulated using integrated louvers on the suction surface that are controlled at prescribed phases relative to the time-periodic pitch cycle. Despite its short duration, the bleed impulse leads to large-scale changes in the flow around the airfoil and consequently to significant temporal variations in the lift (C_L) and pitching moment (C_M). When the airfoil is static, a single bleed impulse leads to momentary changes in C_L (up to 27% at a=10°), and during time-periodic pitch motion yields programmable temporal variations in C_L and C_M that increase C_Lmax by up to 0.2 and reduce hysteresis (up to 30% in C_L and 20% in C_M).  Phase-locked measurements of the flow field in the near wake during dynamic pitch yield the cyclic variation in circulation that scales well with the corresponding changes in C_L.   

Machine Learning for Modeling and Control of an Unsteady Wing Motion using Active Aerodynamic Bleed

The complex temporal dynamics of the aerodynamic effects of distributed air bleed actuation on a 2-D airfoil that is undergoing unsteady pitching and plunging motions in wind tunnel experiments is modeled using an architecture based on machine learning. The model was developed using measurements of the aerodynamic loads and the angular motion characteristics for a dynamically pitching airfoil along with its corresponding static configurations. Two machine learning architectures were investigated a fully connected feedforward neural network and a long- and short-term time-series network (LSTNet). The fully connected network consisted of 4 hidden layers and just over 18,000 tunable parameters. The LSTNet is a deep learning framework for predicting time series with multiple layers including a convolution neural network, two recurrent neural networks, a fully connected dense layer, and a linear autoregressive component. It is shown that machine learning architectures are capable of modeling the complex wing unsteady aerodynamic characteristic including its lift during unsteady motion in the presence of temporal bleed and that the effects of the bleed are adequately modeled.   

Thrust recapture for morphing aerial vehicles with out of plane thrusters

Morphing aerial vehicles which rotate their thrusters out of plane during flight can suffer from reduced vertical thrust when the rotor axes are no longer pointed vertically upward. To circumvent this deficiency, a method based on flow redirection and conservation of linear momentum is presented which can significantly increase the amount of thrust that can be produced in challenging configurations. In particular, this study explores a novel aerodynamic configuration which recaptures a portion of the flow using aerodynamic surfaces. The resulting flow field and influence on the overall thrust force is studied.   

Control of Flexible 3-D Wing Bending Motion by Prescribed Spanwise Circulation

Controlled interactions between a 3-D flexible wing and the embedding cross flow are explored in wind tunnel investigations for effecting tunable structural and aeroelastic characteristics to control its bending motion by regulating the spanwise aerodynamic loads using distributed bleed actuation.  Air bleed driven by pressure differences between the airfoil’s pressure and suction surfaces through surface ports and its interior and is regulated by surface louvers on the pressure surface. The effects of time varying sectional bleed on spanwise and streamwise vorticity concentrations over a 3-D flexible wing, their advection into its near wake, and the mechanisms of spanwise spreading of the response to the actuation are explored using high speed 2-D and stereo PIV measurements. The temporal variations of the spanwise vorticity flux over each of the pressure and suction surfaces are used to assess the effects of unsteady actuation on the circulation of the spanwise vorticity. The balance between spanwise concentrations of streamwise and spanwise vorticity in the near wake is used to evaluate the unsteady circulation as a surrogate of temporal and spatial variations of the load along the wing and their dependence on the applied actuation.   

Temporal Control of the Aerodynamic Loads on a Cylinder at High Incidence using an Active Bleed Forebody

Dynamic variation of the aerodynamic loads on a slender cylinder at high angles of incidence (up to 60o) is investigated in wind tunnel experiments. The loads on the wire-mounted model are effected using azimuthally-segmented aerodynamic bleed actuation that is driven by the pressure distribution over the surface of the ogive forebody (20% porosity) and is regulated by a computer-controlled internal louver shell. Time-resolved measurements of the flow field about the cylinder and the aerodynamic loads show that the azimuthally segmented bleed can effectively alter the formation and evolution of the forebody vortices and their subsequent coupling and interactions with the cylinder’s near wake. Prescribed azimuthal actuation induces wake asymmetries along the cylinder that modify the aerodynamic loads and yields bi-directional control of the side force and yawing moment. This attribute enables direct control of the nominally-random formation of the forebody vortices and thereby override natural, unpredictable side forces and yawing moments in the absence of actuation. The temporally-varying bleed actuation enables attitude control on time scales that are commensurate with the characteristic convective time scale of the flow over the cylinder.   

Study of Serrated Backwards Facing Steps in an Adverse Pressure Gradient via Computational Methods

A thin layer of tape that acts as a backwards facing step (BFS) is applied to wind turbine blades for protection from wind, erosion, debris, etc. However, the addition of the thin layer is known to add drag, which reduces the electrical power output of a turbine. A novel tape design with serrated edges instead of traditional BFS, termed Serrated Backwards Facing Step (sBFS) can provides the protection while mitigating the negative impact of the BFS formed from traditional films. In a previous wind tunnel study, it was shown that the sBFS were able to produce strong coherent structures in the far wake region as well as produce less drag than a traditional BFS. Utilizing a commercial computational fluid dynamics (CFD) package (Star-CCM+), the current study looks to study the sBFS in adverse pressure gradients. Along with comparison to the experimental data, the pressure gradient will be varied and new sBFS configurations examined with the goal of finding an optimal configuration for drag.   

The self-stabilizing nature of a dart and its low Reynolds number aerodynamic characteristics

Throwing darts is a popular sport today and has also played a significant role as a hunting weapon, yet, the role of aerodynamic forces governing its motion remains unexplored. Our high-speed imaging of dart trajectories illustrates a self-stabilizing behavior in the longitudinal plane, enabling the dart to maintain its orientation mid-flight and pierce the dartboard. We also obtain the aerodynamic characteristics of a dart body estimated using numerical simulations and force measurement. We examined surface pressure and wall shear-stress distribution on flights obtained numerically. Our findings suggest that the aerodynamic performance of a dart is governed by a complex interaction between the multiple vortex pairs emerging over the dart: Primary vortex on horizontal flights, its interaction with Barrel vortex shed by cone-cylinder forebody, and the influence of vertical flights. Smoke flow visualization of the flowfield also substantiates the vortical structures and their mutual interaction in the presence of flights that act like walls, corroborating our findings from the simulations. The combination of a high pitching moment and a long moment arm enables quick pitch stabilization keeping the attitude small and preventing the body from undergoing high side forces, attributed to the axisymmetric bodies at large angles of attack, thus avoiding destabilization. Dart is, therefore, a refined design that evolved over centuries that can serve as an inspiration for self-stabilizing flying vehicles.