Hanns Mueller-Vahl

Dynamic Stall Control on a Vertical Axis Wind Turbine Using Plasma Actuators

Dynamic stall was controlled on the blades of a small high-solidity vertical axis wind turbine, by means of dielectric barrier discharge plasma actuators installed on the blade leading-edges. A parametric study was conducted with the objective of increasing the net power output resulting from control in an open-loop manner. For the majority of experiments, the actuators were configured to control separation on the upwind half of the turbine azimuth while limited experiments were conducted on the downwind half. Turbine power was measured using a specially-designed dynamometer that allowed full characterization of its performance. Actuator duty cycle dependence observed previously on static airfoils was also observed on the turbine. However, optimum reduced frequencies showed substantially different dependence and this was traced to the importance of the plasma pulsation frequency relative to the turbine rotational frequency. Overall turbine power performance improvements of 38% and 20% were measured for upwind and downwind dynamic stall control, respectively. Based on the data acquired, up-scaling the turbine by a factor of 5 and 10, the percentage of plasma power required to produce comparable improvements was conservatively estimated at 3.3% and 1.7% respectively. A new switching-direction actuator was developed, together with high-speed plasma triggering, in order to produce combined upwind and downwind actuation that is phase-locked to turbine rotational frequencies. Flowfield measurements using particle image velocimetry are presently being performed.

Vortex Generators for Wind Turbine Blades: A Combined Wind Tunnel and Wind Turbine Parametric Study

Vortex generators (VGs) are passive flow control devices commonly employed to prevent flow separation on wind turbine blades. They mitigate the damaging fatigue loads resulting from stall while increasing lift and consequently lead to rotor torque increase. This work summarizes a research project aimed at optimizing the sectional as well as the full rotor-blade aerodynamics using VGs.The effects of chordwise position, spanwise spacing and VG size were studied with force balance measurements of a 2D wing section. Reducing the distance between adjacent VGs produced large increases in the static stall angle and maximum lift, but also resulted in a significant increase in drag as well as sharp lift excursions at angles exceeding the static stall angle. The optimal chordwise position of the vortex generators was found to be in the range of x/c = 15%–20%, where a comparatively low parasitic drag and a smooth post-stall lift curve were achieved. Particle Image Velocimetry measurements were conducted at various chordwise positions to provide insight into the interaction between adjacent streamwise vortices.The experimental aerodynamic performance curves of the optimal VG configuration were used to project their effect on wind turbine blade aerodynamics. Three different rotorblades were designed and several stall and pitch regulated wind turbine models were simulated by means of a Blade Element Momentum (BEM) code (QBlade) developed by Smart Blade GmbH. The performance of the rotorblades with and without VGs was simulated in order to assess their effect on the aerodynamic performance and loads. Finally, previously measured steady state performance curves under high-roughness conditions were used to simulate the detrimental effect of surface roughness on the performance of the aforementioned rotorblades. This allows for an estimate of the potential of the VGs to be employed as retrofit elements for performance recovery of blades with a contaminated surface.© 2012 ASME

Modeling Lift Hysteresis with a Modified Goman-Khrabrov Model on Pitching Airfoils

Wind tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman-Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasi-steady maneuvers, but static hysteresis was observed with the thick airfoil over a range of 16 < α < 22. Both airfoils exhibited dynamic hysteresis in the lift coefficient when the wing was pitching. The existence of static hysteresis had a strong effect on the lift response during periodic pitching, and must be accounted for in the model. A modified version of the Goman-Khrabrov model was introduced that captured the static hysteresis behavior and the large-scale features of the dynamic hysteresis for the thick airfoil. Large-scale variations in the lift trajectory at different pitching frequencies are reproduced by the modified model, even when the thick airfoil is in a deep stall condition. Some deviations between model and experiment are observed at the highest angles of attack, which are attributed to the dynamic stall vortex and trailing edge vortex shedding.

Development of a Low-Speed Oscillatory-Flow Wind Tunnel

A low-speed blow-down wind tunnel was developed with the capability of simultaneous unsteady changes in wind speed and angle-of-attack, while facilitating full optical access to the test section. Unsteady wind speed was achieved by means of a louver mechanism at the tunnel exit that was driven by a geared servo-motor. Unsteady angleof-attack variation was achieved by rotating two large rings, incorporating a NACA 0018 airfoil model, driven by a servo-motor via a series of two belt drives. The windows, floor and ceiling were constructed from Plexiglas, thereby rendering full optical access to the test section. The tunnel was characterized under quasi-steady conditions as well as unsteady conditions for a range of winds speeds by means of hot wire anemometers mounted at various locations in the test sections. Once the unsteady capability was quantified, experiments were performed on the pitching airfoil, with simultaneous unsteady free-stream. Unsteady surface pressure measurements and particle image velocimetry flow field measurements were made for different phases of the velocity and angle-of-attack.

Airfoil subjected to high-amplitude free-stream oscillations: theory and experiments

A combined theoretical and experimental investigation is carried out with the objective of validating the two-dimensional airfoil theories of Isaacs and Greenberg subjected to high-amplitude harmonically varying free-stream velocity. To date, these theories have eluded full validation due to the significant experimental challenges associated with reliably producing high-amplitude unsteady flow. The theoretical approaches assume a flat plate in potential flow that is subjected to an oscillating free-stream. Unsteady forces is calculated using the approach described by Isaacs and the generalisation of van der Wall. In order to further understand the unsteady lift mechanism, the theory is extended here for the purposes of calculating the unsteady bound vorticity sheet strength along the airfoil chord. A closed-form solution is obtained which is valid for arbitrarily reduced frequencies and amplitudes. To validate the theories, experiments are conducted on a NACA 0018 airfoil in a wind tunnel that is designed specifically for the purpose of generating high-amplitude velocity oscillations. Unsteady pressure transducers are located at nominally identical chordwise locations to facilitate an estimation of the bound vorticity strength and loads. All experimental data are phase-averaged over O(10) cycles. The phase variations of lift and moment coefficients are reasonably well predicted at a 2◦ angle-of-attack. At 8◦ however, the experiments under-predicted the theoretical load fluctuations. The theoretical and measured unsteady bound vorticity sheet show that the process of lift overshoot starts at the trailing edge and propagates upstream.

Mechanism of Dynamic Stall Control on a Vertical Axis Wind Turbine

Dynamic stall was controlled on a double-bladed H-Rotor vertical axis wind turbine (VAWT) using pulsed dielectric barrier discharge plasma actuators in a feed-forward control configuration. The azimuthal angles of plasma actuation initiation and termination, that produced the largest increases in power, were determined parametrically on the upstream half of the turbine azimuth in a low speed blow-down wind tunnel at wind speeds of 5m/s to 7m/s. Three different actuator setups were tested on the turbine. A model was used to estimate transient torque and power developed by the turbine under the influence of plasma actuation. Particle image velocimetry (PIV) data was acquired in order to further characterize and understand the effects of plasma actuation on dynamic stall effects. A remarkable result of this investigation was that a net turbine power increase of more than 10% was measured. This was achieved by systematically reducing plasma pulsation duty cycles as well as the plasma initiation and termination angles. Nevertheless PIV measurements of the flowfield showed that actuation was not fully successful in controlling the dynamic stall vortex. This indicates even greater potential for improvement with more powerful actuation.

Control of Unsteady Aerodynamic Loads Using Adaptive Blowing

Adaptive slot blowing was experimentally investigated on a NACA 0018 airfoil model as a method to minimize unsteady aerodynamic loads. Tests were conducted in a wind tunnel facility specifically designed to facilitate high amplitude wind speed fluctuations. Initially, the effect of steady blowing from a control slot located near the leading-edge was investigated under quasistatic conditions. At Reynolds numbers ranging from 1.5·10 to 5·10, a relative change in lift of ∆cl ≈ 0.5 was obtained over a wide range of angles of attack. Various dynamic test cases including a pitching motion, a sinusoidal variation of the wind tunnel speed and a combination of both were considered. The formation of the dynamic stall vortex was suppressed with blowing at moderate momentum coefficients. Dynamically adapting the momentum coefficient effectively counteracted the lift fluctuations resulting from the unsteady inflow conditions and virtually constant lift was obtained in all test cases. Furthermore, control significantly reduced the moment excursions in all dynamic pitching experiments.

Flow-Control-Induced Vibrations using Pulsed DBD Plasma Actuators

This paper describes flow-control-induced vibrations using pulsed dielectric barrier discharge (DBD) plasma actuators, in which boundary layer separation on a structure is actively controlled to produce periodic loads that lead to its vibration. The concept is intended for energy generation and is demonstrated experimentally using a one-degree-offreedom pivoted cylindrical body, mounted vertically within a blow-down wind tunnel. Subcritical Reynolds numbers, less than 10 5 , were considered where typical shedding frequencies were several times larger than the system natural frequency. Static deflection experiments were performed to determine the maximum imposed aerodynamic loads as a function of control parameters and these were complimented with flow-field measurements. Periodic loading of the cylinder was achieved by periodic modulation of the actuator. Large amplitude oscillations were observed when the modulation frequency was close to the system natural frequency. In contrast to natural vortex induced vibration, the large amplitude oscillations were achieved by alternating dynamic separation and attachment of the boundary layer. Estimation of the transient loads was performed using a system identification technique and the power generated by the system was estimated on the basis of a piecewise linear model. Peak estimated power coefficients were relatively small (0.042) but the system is amenable to up-scaling because the power coefficient increases with the squareroot of the system dimensions.

Tethered Cube Stabilization by Means of Active Flow Control

An experimental investigation was carried out to assess the effectiveness of active control as a means of suppressing oscillations of tethered cubes. To achieve an idealized experimental representation of the problem, several small 15cm square model cubes of different masses were constructed and two experimental configurations were considered: a static configuration involving surface pressure and flowfield measurements and a dynamic, tethered, configuration. Corner-mounted, pulsed dielectric barrier discharge (DBD) actuators were used at the leading-edges in an attempt to suppress the oscillations. The actuators were similar to the common asymmetric configuration where the upper (exposed) and lower (encapsulated) electrodes were separated by dielectric material. The actuators were driven at 10 kHz and 20 kHz and at 10 kV peak-to-peak. In addition, pulsation modulation frequencies were chosen to represent the reduced frequency range between O(0.1) and O(2), known to be effective for separation control. The duty cycle, namely the percentage (or fraction) of time that the actuator is operational, was varied in the range 0.1% to 50%. On the static configuration, actuation changed the direction of the side forces and virtually eliminated moment excursions. For the tethered cubes, the support wires also carried the high-voltages required to drive the DBD plasma actuators. In order to track the longitudinal, lateral and angular motions of the tethered cubes, their lower sides were painted matt black and colored markers were glued to the surfaces. These were filmed with a high resolution camera and image processing was used to extract the translation and angular motions. It was observed that active separation control at the leading-edges of a tethered cube can dramatically reduce the cube rotational motions. Reduced frequency and duty cycle had a marked effect on control effectiveness. Furthermore, transients following initiation of control acted over a longer period than transients following termination of control.

Active Control of an Incompressible Axisymmetric Jet using Flaps and Zero Mass-flux Excitation

An active flow control method of an axis-symmetric jet is being investigated which, when activated, generates streamwise vortices and thus enhances mixing of the jet flow with the ambient. The perimeter of the jet is equipped with six small flaps deflected away from the stream. Zero mass-flux perturbations are being used to excite the flow. These excitations are introduced in the flow through slots at the base of the flaps. Each of the flaps can be excited independently. In former studies, the shape and angle of a single flap were optimized and the local effect on the jet analyzed. It was shown that the local mixing can be increased. In the current investigations, the effect of an array of six individually controllable flaps on the global jet behavior is addressed. Each of the flaps can be excited in phase or with pre-fixed phase shift. Effects of frequency and amplitude on the flow momentum, streamwise vorticity, circulation and turbulence for a fixed flap deflection angle are part of the investigation. A stereo-PIV setup is being used to acquire complete flow field information. The emphasis is being placed on mapping the development of the trailing vortices in order to quantify the mixing achieved.