Active Flow Separation Control Research Articles

Generic Transport Aft-body Drag Reduction using Active Flow Control

Active flow-separation control is an effective and efficient mean for drag reduction and unsteady load alleviation resulting from locally or massively separated flow. Such a situation occurs in configurations where the aerodynamic performance is of secondary importance to functionality. The performance of heavy transport helicopters and aeroplanes, having a large, and almost flat, aft loading ramp suffer from the poor aerodynamics of the aft body. Hence, a combined experimental and numerical investigation was undertaken on a generic transport aeroplane/helicopter configuration. The experimental study provided surface pressures, direct drag measurements, surface and smoke flow visualization. The baseline flow was numerically analyzed, using finite volume solutions of the RANS equations. The baseline flow around the model was insensitive to the Reynolds number in the range it was tested. The flow separating from the aft body was characterized by two main sources of drag and unsteadiness. The first is a separation bubble residing at the lower ramp corner and the second is a pair of vortex systems developing and separating from the sides of the ramp. As the model incidence is decreased, the pair of vortex systems also penetrates deeper towards the centerline of the ramp, decreasing the pressure and increasing the drag. As expected, the ramp lower corner bubble was highly receptive to periodic excitation introduced from four addressable piezo-fluidic actuators situated at the ramp lower corner. Total drag was reduced by 3–11%, depending on the model incidence. There are indications that the flow in the wake of the model is also significantly steadier when the bubble at the lower ramp corner is eliminated. The vortex system is tighter and steadier when the ramp-corner bubble is eliminated.

Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat

DOI: 10.2514/1.24057 The concept of a self-governing smart plasma slat for active sense and control of flow separation and incipient wing stall is presented. The smart plasma slat design involves the use of an aerodynamic plasma actuator on the leading edge of a two-dimensional NACA 0015 airfoil in a manner that mimics the effect of a movable leading-edge slat of a conventional high-lift system. The self-governing system uses a single high-bandwidth pressure sensor and a feedback controller to operate the actuator in an autonomous mode with a primary function to sense and control incipient flow separation at the wing leading edge and to delay incipient stall. Two feedback control techniques are investigated. Wind tunnel experiments demonstrate that the aerodynamic effects of a smart actuator are consistent with the previously tested open-loop actuator, in that stall hysteresis is eliminated, stall angle is delayed by 7 deg, and a significant improvement in the lift-to-drag ratio is achieved over a wide range of angles of attack. These feedback control approaches provide a means to further reduce power requirements for an unsteady plasma actuator for practical air vehicle applications. The smart plasma slat concept is well suited for the design of low-drag, quiet, highlift systems for fixed-wing aircraft and rotorcraft.

Robust Multivariable Closed-Loop Control of a Turbulent Backward-Facing Step Flow

Active control of separated flows has become an attractive approach enabling the designer to meet increased performance demands for various systems. Whereas a significant part of the work published so far in the area of closed-loop flow control is based on simulation studies, this paper presents an example of a successful application of a multivariable controller in wind tunnel experiments. A robust H ∞ controller is used to control the spanwise variable reattachment length downstream of a benchmark problem, the backward-facing step. To reduce the conservatism of this approach, a nonlinear static precompensation is included, first. The synthesis of the controller is based on a family of identified linear black-box models, which describe the compensated input/output behavior of the plant. Tracking performance and disturbance rejection of the controller are tested in wind tunnel experiments and shown in this paper.

Étude du contrôle du décollement sur un profil d'aile par mesures PIV et analyse POD

The purpose of this study is the active control of flow separation on an ONERA D airfoil, using continuous blowing microjets. Particle Image Velocimetry (PIV) is used to measure velocity fields at midspan around the airfoil in both controlled and uncontrolled cases. Post-processing based on Proper Orthogonal Decomposition (POD) is developed to exhibit the main energetic features of the flow. Therefore, the detailed study of the POD modes for separated and stalled cases leads to a better understanding of the control mechanisms. Moreover, the POD basis computed for uncontrolled and controlled cases, opens the way to optimisation of control strategies. To cite this article: J. Favier, A. Kourta, C. R. Mecanique 334 (2006).

Dielectric barrier plasma dynamics for active control of separated flows

The dynamics of separation mitigation with asymmetric dielectric barrier discharges is explored by considering the gas flow past a flat plate at an angle of attack. A self-consistent model utilizing motion of electrons, ions, and neutrals is employed to couple the electric force field to the momentum of the fluid. The charge separation and concomitant electric field yield a time-averaged body force which is oriented predominantly downstream, with a smaller transverse component towards the wall. This induces a wall-jet-like feature that effectively eliminates the separation bubble. The impact of several geometric and electrical operating parameters is elucidated.

Active Control of Flow Separation from Supercritical Airfoil Leading-Edge Flap Shoulder

Zero-net mass-flux periodic excitation was applied at the leading-edge flap shoulder of a simplified high-lift airfoil to delay flow separation. The term simplified infers that no slat or Fowler flaps are used. The NASA energy efficient transport supercritical airfoil was fitted with a 15% chord simply hinged leading-edge flap and a 25% chord simply hinged trailing-edge flap. Initially, the cruise configuration data from previous experiments were reproduced. The effects of leading- and trailing-edge flap deflections on the airfoil integral parameters were quantified. Detailed flow features were measured to identify optimal actuator placement. The measurements included steady and unsteady model and tunnel wall pressures, wake surveys, arrays of surface hot films, flow visualization, and particle image velocimetry. Eventually, high-frequency periodic excitation was applied to delay the occurrence of leading-edge flap shoulder stall and increased the maximum lift by 10‐15%. Low-frequency amplitude modulation was used to reduce the oscillatory momentum coefficient by roughly 50% with similar aerodynamic performance gains. It is demonstrated that the efficacy of the amplitude-modulated excitation is due to the generation of low-frequency motion, which is amplified by the separating shear layer.

ON ACTIVE CONTROL OF SEPARATION FROM BLUFF BODIES(Keynote Lecture)

Active control of flow separation from a bluff object embodies the flow of curved jets and wakes both steady and unsteady and their effect on the forces experienced by the object. The efficacy of separation control by steady blowing, suction, or by periodic excitation is being extensively reinvestigated. The efficacy of steady blowing was measured by the increase in lift or the reduction in drag relative to the momentum added to the flow, while the efficacy of suction was assessed by the mass flow removed from the flow field. Recent experiments suggest that a combination of momentum and mass coefficients represents a better characterization of both methods of control. In the absence of mass flux, periodic excitation makes separation and circulation control much more effective than steady blowing provided the momentum input is small, but its advantage is smaller with respect to suction. When the momentum input is large blowing becomes advantageous because most of the input is recovered as thrust. Various experiments combining blowing suction and periodic excitation will be described.

MULTIVARIABLE CLOSED-LOOP CONTROL OF THE REATTACHMENT LENGTH DOWNSTREAM OF A BACKWARD-FACING STEP

Active control of separated flows has become a major challenge in recent years. Whereas most of the work published so far is based on simulation studies this paper gives an example of a successful application of a multivariable closed-loop flow control in wind tunnel experiments. A robust H∞-controller is used to control the spanwise reattachment length downstream of a benchmark problem, a so-called backward-facing step. The synthesis of the controller is based on a family of identified linear black-box models. To reduce the conservatism of this approach, a nonlinear static precompensation is included. Tracking performance and disturbance rejection of the controller are tested in wind tunnel experiments.

Active Flow Separation Control of a Stator Vane Using Embedded Injection in a Multistage Compressor Experiment

Active flow control has been applied to the suction surface of stator vanes in a low speed axial compressor. Injection from the suction surface has been shown to reduce separation on vanes that were induced to separate by increasing the vane stagger angle by 3° deg. Various configurations were investigated including injector geometry (slots versus holes) and type of injection (steady versus unsteady). Unsteady injection was realized using two different approaches; external actuation through a high frequency valve and embedded actuation using a fluidic device internal to the vane. Using total pressure loss through the vane passage as a metric, reductions in area-averaged loss of 25% were achieved using injected mass flow rates on the order of 1% of compressor throughflow. The development of a tracking control algorithm was also explored for the purpose of closed-loop control. A reliable method of detecting surface separation was implemented using unsteady pressure measurements on the compressor casing near the vane suction surface.

Active Flow Separation Control on Wall-Mounted Hump at High Reynolds Numbers

An active separation control experiment was conducted in a cryogenic pressurized wind tunnel on a wall-mounted bump at chord Reynolds numbers from 2.4 x 10 6 to 26 x 106 and a Mach number of 0.25. The model simulates the upper surface of a 20% thick Glauert-Goldschmied-type airfoil at zero incidence. The turbulent boundary layer of the tunnel sidewall flows over the model and eliminates laminar-turbulent transition from the problem. Indeed, the Reynolds number either based on the chord or boundary-layer thickness had a negligible effect on the flow and its control. Without control, a large turbulent separation bubble is formed at the lee side of the model. Periodic excitation and steady suction or blowing were applied to eliminate gradually the separation bubble. Detailed effects due to variations in the excitation frequency, amplitude, and the steady mass flux are described and compared to those of steady suction or blowing

Active flow separation control on wall-mounted hump at high Reynolds numbers

Active flow separation control on wall-mounted hump at high Reynolds numbers

The Mechanism of Active Boundary Layer Control Using Vortex Generator Jets.

The vortex generator jet method is active control of flow separation that has the ability to provide a time-varying control action. The mechanism for suppressing flow separation was studied experimentally by making a comparison between two effects of steady and pulsed jets on generating longitudinal vortices. The suppression of flow separation is accomplished by transporting high momentum fluid of the freestream to the lower wall by the secondary flow of longitudinal vortices. However, if the vortex detaches from the wall, a counter-rotating vortex of nearly equal strength is induced and, thus, an upwash region is produced by the effect of a vortex pair. The upwash makes ineffective the secondary flow toward the lower wall and as a result streamwise velocity decreases. Pulsed jets enhance the mixing process in comparison with steady jets and indicate effective separation control for the purpose of attaching the vortex to the lower wall.