Raymond Chow

Computational Investigations of Small Deploying Tabs and Flaps for Aerodynamic Load Control

The cost of wind-generated electricity can be reduced by mitigating fatigue loads acting on the blades of wind turbine rotors. One way to accomplish this is with active aerodynamic load control devices that supplement the load control obtainable with current full-span pitch control. Techniques to actively mitigate blade loads that are being considered include individual blade pitch control, trailing-edge flaps, and other much smaller trailing-edge devices such as microtabs and microflaps. The focus of this paper is on the latter aerodynamic devices, their time-dependent effect on sectional lift, drag, and pitching moment, and their effectiveness in mitigating high frequency loads on the wind turbine. Although these small devices show promise for this application, significant challenges must be overcome before they can be demonstrated to be a viable, cost-effective technology.

Unsteady computational investigations of deploying load control microtabs

Flow around an airfoil with a deploying microtab device has been numerically simulated by solving the unsteady turbulent compressible Navier-Stokes equations with the OVERFLOW-2 solver. Using a Chimera/overset grid topology, microtabs were placed at 95% ofchord of a symmetric NACA 0012 airfoil. Microtab heights on the order of 1% of chord, deployed on the order of one characteristic time unit were utilized. The unsteady effects of tab deployment time, deployment height, and freestream angle of attack on aerodynamic responses were also investigated. Validation studies with experimental results for static deployed microtabs and a dynamically deployed spoiler were also performed to ensure accurate temporal and spatial resolution of the numerical simulations.

Computational Investigations of Deploying Load Control Microtabs on a Wind Turbine Airfoil

Flow about a wind turbine airfoil with a deploying microtab device has been numerically simulated by solving unsteady turbulent compressible two-dimensional, Navier-Stokes equations with the OVERFLOW 2 solver. Using a Chimera/overset grid topology, microtabs were placed at 95% of chord of a S809 airfoil. Microtab heights on the order of 1% of chord, deployed on the order of one (1) characteristic time unit were utilized. The effects of freestream angle of attack on the aerodynamic response was also investigated. Validation studies with experimental results for static deployed microtabs and a dynamically deployed spoiler were also performed to ensure accurate temporal and spatial resolution of the numerical simulations.

ACTIVE AERODYNAMIC LOAD CONTROL OF WIND TURBINE BLADES

The cost of wind-generated electricity can be reduced by mitigating fatigue loads acting on the rotor blades of wind turbines. One way to accomplish this is with active aerodynamic load control devices that supplement the load control obtainable with current full-span pitch control. Thin airfoil theory suggests that such devices will be more effective if they are located near the blade trailing edge. While considerable effort in Europe is concentrating on the capability of conventional trailing edge flaps to control these loads, our effort is concentrating on very small devices, called microtabs, that produce similar effects. This paper discusses the work we have done on microtabs, including a recent simulation that illustrates the large impact these small devices can exert on a blade. Although microtabs show promise for this application, significant challenges must be overcome before they can be demonstrated to be a viable, cost-effective technology.© 2007 ASME

Active Load Control of a Wind Turbine Airfoil Using Microtabs

The dynamic response of an airfoil to microtab deployment has been studied in the University of California, Davis Aeronautical Wind Tunnel. Static tests were conducted at a Reynolds number of 1.0×106, whereas dynamic testing examined deploying microtabs under steady conditions and during simulated wind gusts. Experimental results are compared with two-dimensional Reynolds-averaged Navier–Stokes computational results generated using OVERFLOW2. Microtabs with a height equal to 1% of the airfoil chord are seen to produce a rapid change in the sectional lift coefficient sufficient to compensate for a 12% change in the freestream airspeed. The effect of tab deployment was modeled using a linear equation that related the lift force to the difference in surface pressure at 15% chord. The resulting function provided a usable approximation to the lift force during tab activation. Two modes of tab activation are examined: simultaneous deployment of all tabs along the airfoil span and sequential deployment of indivi...

Inboard Stall and Separation Mitigation Techniques on Wind Turbine Rotors

The aerodynamic characteristics of the NREL 5-MW rotor have been examined using a fully viscous, 3-D, Reynolds-averaged Navier-Stokes method, OVERFLOW2. The baseline NREL 5-MW rotor demonstrates significant radial flow and inboard separation up to 30% of span. Using overset grids, various passive geometric modifications have been made to the inboard blade region of the rotor. The approach attempts to limit the radial flow component and minimize flow separation in order to increase rotor power capture below rated speed. Compared to baseline NREL 5-MW rotor, a full airfoil section fenced configuration is shown to increase rotor power capture by 0.9% and 0.6% at U∞ = 8 and 11 m/s respectively. Suction side only fences perform similarly in terms of power capture but reduce the increase in rotor thrust by 0.2% at U∞ = 11 m/s. Fence heights from 0.5 to 17.5% of the maximum chord all demonstrate some level of effectiveness, with boundary layer scale fences (12.5%cmax) showing similar performance gains to taller fences with smaller penalties in thrust. Performance in terms of power capture is not very sensitive to spanwise location, with the fence located well within the region of separation. While increasing inboard blade twist does not increase power capture, it does appear to create a favorable benefit in terms of reducing thrust at a higher rate than power.

Realistic Leading-Edge Roughness Effects on Airfoil Performance.

Wind farm operators observe power production decay over time, with the exact cause unknown and difficult to quantify. A likely explanation is blade surface roughness, as wind turbines are continuously subjected to environmental hazards. Difficulty arises in understanding and quantifying performance degradation. Historically, wind turbine airfoil families were designed for the lift to be insensitive to roughness by simulating roughness with 2D trip strips. Despite this, roughness is still shown to negatively affect airfoil lift performance. Experiments have also illustrated that random-distributed roughness is not properly simulated by trip strips. Therefore, to better understand how real roughness effects performance, field measurements of turbine-blade roughness were made and simulated on an airfoil section in a wind tunnel. This data will serve to validate and calibrate a one-equation, computational roughness amplification model that interacts with the Langtry-Menter transition model. The observed roughness contains 2D steps, heavy 2D erosion, pitting, insects, and repairs. Of these observations, 2D steps from paint chips were characterized and recreated for this particular wind tunnel entry. The model was tested at chord Reynolds numbers up to 3.6 × 10. Measurements of lift, drag, and pitching moment were made with and without roughness contamination. Transition location was acquired with infrared thermography and a hotfilm array. The paint roughness yields a consistent increase in drag compared to the clean configuration. Numerical simulations are only compared to the clean configuration and match well to lift, drag, and transition for Rec = 1.6 × 10. However, drag is overpredicted at Rec = 3.2 × 10.

Experimental and Computational Analysis of a Wind Turbine Airfoil with Active Microtabs

This study aims to provide experimental and computation data to fully characterize the dynamic effects of deploying microtabs for active load control on wind turbine blades. A wind turbine airfoil model with linearly actuated microtabs has been tested in the UC Davis Aeronautical Wind Tunnel. Initial tests were conducted under static conditions at a Reynolds number of one million, followed by measurements of deploying microtabs under steady conditions and during simulated wind gusts. Wind tunnel results are compared with computational results generated using OVERFLOW2. Microtabs are deployed perpendicular to the surface of the airfoil, reaching a height 1% of the chord length. A microtab located at 95% chord on the pressure surface increases lift when deployed, while a tab at 90% chord on the suction surface decreases lift. Dynamic tests of microtabs on an airfoil subjected to rapid changes in airspeed demonstrated the ability of the tabs to control load excursions.

Aerodynamic Performance of Thick Blunt Trailing Edge Airfoils

An experimental and computational examination of the aerodynamic performance of a blunt trailing edge airfoil is presented. The UCD-38-095 airfoil is one of a family of thick airfoils designed using genetic and gradient-based optimization. Its maximum thickness is 38% of the airfoil chord, with a trailing edge thickness of 9.5%. In the present study, the UCD-38-095 was tested in the University of California, Davis aeronautical wind tunnel at Reynolds numbers of 333,000 and 666,000. Both free and fixed transition conditions were studied. The wind tunnel results are compared with computational predictions obtained in OVERFLOW, a Reynolds averaged Navier-Stokes solver using structured overset grids.

Thick Airfoils With Blunt Trailing Edge for Wind Turbine Blades

This paper addresses the primary concerns regarding the aerodynamic performance characteristics of thick airfoils with blunt trailing edges (or so-called flatback airfoils) and the utilization of these section shapes in the design of rotor blades for utility-scale wind turbines. Results from wind tunnel and computational fluid dynamic studies demonstrate the favorable impact of the blunt trailing edge on the aerodynamic performance characteristics including higher maximum lift coefficient and reduced sensitivity of lift to premature boundary layer transition. The negative effect of the blunt trailing edge on drag can be partially mitigated through simple trailing edge treatments such as splitter plates. Studies on the effect of these section shapes on wind turbine rotor performance show that at attached flow conditions this inboard blade modification does not adversely affect rotor torque output. Blade system design studies involving the collective optimization of aerodynamic performance, structural strength and weight, and manufacturing complexity demonstrate the overall favorable impact of the flatback concept.Copyright