Session L15: Flow Control: Aerodynamic Applications

Optimal cluster-based feedback control for separated flows

We propose a model-free self-learning cluster-based feedback control strategy from sensor measurements benchmarked for separation control simulations. Here, we leverage unsupervised clustering for in-situ learning  and optimization of coarse-grained control laws to manipulate turbulent post-stall flows over an airfoil in high-fidelity simulations. The approach partitions the baseline flow trajectories (force measurements) into clusters, which correspond to characteristic coarse-grained phases in a low-dimensional feature space.  A feedback control law is sought for each cluster state through iterative evaluation and downhill simplex search to minimize power consumption for aerodynamic flight. Re-routing the flow trajectories modifies the baseline Markov transition network to achieve aerodynamically favorable states with control. The approach is applied to two and three-dimensional separated flows over a NACA 0012 airfoil with large-eddy simulations at an angle of attack of $9^\circ$, Reynolds number $Re = 23,000$ and free-stream Mach number $M_\infty = 0.3$. The optimized control laws effectively minimize the power consumption for flight enabling the flows to reach a low-drag state.   

Optimal actuator selection for flow control based on empirical data

Actuator selection plays a central role in the efficacy and achievable performance of fluid flow control in practice. Here, we present a systems-theoretic approach for determining the optimal actuator location—among a set of candidate locations—for driving the flow to an arbitrary state with minimal input energy. The method only requires access to empirical data of the flow response to actuation—obtained either from numerical simulations or experiments—and is applicable in the context of stable and unstable flows alike. We use the approach to study optimal actuator selection for separation control using data gathered from high-fidelity numerical fluids simulations of a NACA 65(1)-412 airfoil. Specifically, data of lift and separation-angle responses to localized body-force actuation are used to determine the optimal actuator location on the upper-surface of the airfoil. We find that the optimal location for controlling lift differs from that for controlling separation angle. Further, when feedback separation control is being considered, our results suggest that performance can be improved by adopting separation-angle-tracking controllers over more conventional lift-tracking strategies.

  

A Numerical Study of Separation Suppression using Fluidic Oscillators

A numerical study of a fluidic oscillator was initiated to better understand its fluid dynamics, coupled with experimental validation. The study aims to increase the understanding of the flows within the fluidic oscillator using high-fidelity, unsteady computations and simultaneously evaluate the sensitivity of the flow to installation and geometric constraints. The characterization of the fluidic oscillator will permit accurate development of a boundary condition for use in larger scale computations. In addition, a curved surface test configuration was designed, defined by a streamline along the suction surface of a VR-12 airfoil at 13°. The design produces regions of mild and strong separation suitable for the evaluation of different fluidic active flow control devices. Computational simulations on this design will be completed, considering a spanwise array of fluidic oscillators to evaluate their ability to suppress separation and will be compared to experimental measurements.   

High-Frequency Fluidic Actuation of a Separation Bubble on a Curved Surface

The unsteady interactions between actuation jets and the vorticity concentration within a separation bubble formed by a subsonic cross flow over a curved surface is investigated experimentally. The nominally 2-D curved surface represents the suction side of a VR-12 airfoil at an angle of attack of 13^o such that its leading and trailing edge stagnation streamlines merge smoothly with the tunnel’s flat side wall. Dissipative (high-frequency) actuation is provided by a spanwise array of fluidic oscillating jets that is located upstream of the separation. The actuation controls the characteristic scale of the separated flow domain and the strength of the trapped spanwise vorticity concentration. The effect of the actuation on both the upstream and downstream edges of the separated bubble are assessed using high-speed particle image velocimetry, and analyzed using modal decomposition to reveal its underlying dynamical and structural characteristics. In particular, empirical mode decomposition (EMD) is used to determine the spectral content of the flow within distinct, characteristic frequency bands that separate between large- and small-scale motions of the separation bubble with specific emphasis on the evolution of the small scales as a result of the actuation.   

Steady blowing to control the lift and drag on a free shear layer airfoil

We report on the use of flow control to modulate the aerodynamic performance of a family of “free streamline” airfoils. The airfoil consists of two flat plates, the shorter plate (flap) is joined at an angle to the longer plate at the trailing edge. The flow separates at the leading edge, but the use of a steady wall jet, located on the flap close to the leading edge facilitates flow reattachment toward the back of the flap. The lift and drag generated by the free shear layer wing have been measured at low Reynolds numbers (~26,000) for different relative lengths and angles of the two plates. Without actuation, high lift coefficients (CL ~ 1.2-1.5) are obtained at high angles of attack but this comes with a high drag penalty (CD ~ 0.3-0.6).  Steady blowing can increase the maximum lift of the airfoil to values of CL = 2.8, while delaying stall. Blowing also reduces the drag coefficient (CD ~ 0.1-0.2) at small angles of attack (α up to 15°), although it is not effective at high angles of attack. The drag reduction at small angles of attack is primarily due to a pressure reduction in the cavity between the plates and also to the thrust generated by the jet.   

Bi-directional Control of the Aerodynamic Loads on an Airfoil at Low Angles of Attack using Fluidic Actuation

Controlled, bi-directional regulation of the aerodynamic loads on an airfoil at low angles of attack when the base flow is fully attached is investigated in wind tunnel experiments using fluidic actuation in the absence of moving control surfaces.  Control is effected by inducing transitory vorticity concentrations on the suction and pressure surfaces using pulsed actuation on time scales that are significantly shorter than the airfoil’s convective time scale.  Actuation is provided using individually-controlled spanwise arrays of miniature combustion-based actuators that are located near the leading edge and trailing edge of the suction and pressure surfaces.  The effects of actuation are assessed using phase-locked measurements of the time-dependent aerodynamic loads and particle image velocimetry.  Independent actuation on the suction or pressure surfaces near the airfoil’s trailing edge leads to momentary accumulation of vorticity and the concomitant, respective decrease or increase in lift is coupled with changes in the airfoil’s Kutta condition.  Therefore, bursts of successive actuation pulses enable bi-directional control authority without the need for mechanical control surfaces, and with lower drag penalty.    

Control of Flow Structure on Nonslender Delta Wing using Passive Bleeding: Effects of Orientation, Angle, and Solidity Ratio

Recently, we have demonstrated that the bleeding, which utilizes passages inside the wing to allow the fluid flowing from the pressure side to the suction side by using inherent pressure difference, could be used as an effective flow control method for nonslender delta wings. In the present study, this technique is investigated in detail for nonslender delta wings of 35 and 45-degree sweep angles in a low speed wind tunnel using smoke visualization, particle image velocimetry, and surface pressure measurements for broad ranges of angle of attack and Reynolds number. The effects of bleeding orientation, angle, and solidity ratio on flow structure are quantified in particular, where the solidity ratio signifies the level of bleed gap on the wing surface. The results indicate that the recovery of the leading-edge vortex with significant increases in the suction pressure coefficient, −C_p, along with the elimination of large-scale swirl pattern in near surface streamline topology are achieved with the proper bleeding configuration. Considering the effectiveness,the bleeding orientation and the solidity ratio are quite critical to achieve the successful flow control where the angles need to be adjusted according to the angle of attack of the wing to reach the utmost influence.   

Impulsive Jets for Dynamic Stall Suppression

Dynamic stall can occur on the retreating blade of a rotorcraft main rotor at high-speed forward flights with high disk loading. It limits the aerodynamic performance of the main rotor. Various flow-control techniques have been investigated for dynamic stall suppression. In the current study, impulsive jets are used for the stall control. Jets are generated from a combustion-powered actuator located on the suction side near the leading edge. The VR-12 airfoil is simulated with the compressible flow solver, SU2. A time-dependent boundary condition is implemented for impulsive jets. A pitching condition, relevant to the retreating-blade stall, is simulated in the current study. Unsteady RANS computations are conducted with the Spalart-Allmaras turbulence model. Current computational results show good agreement with the experimental data. Improved aerodynamic performances are observed in the downstroke of the pitching airfoil. Performance of the impulsive jet is compared to a steady-blowing jet with a similar time-averaged jet momentum. It is observed that the impulsive jet shows better performance than the steady-blowing jet does.