Jonathon P. Baker

Two-dimensional wind tunnel and computational investigation of a microtab modified airfoil

*† ‡ A computational and wind tunnel investigation into the effectiveness of a microtab-based aerodynamic load control system is presented. The microtab-based load control concept consists of a small tab, with a deployment height on the order of 1% of chord, which emerges approximately perpendicular to a lifting surface in the vicinity of the trailing edge. Lift mitigation is achieved by deploying the tabs on the upper (suction) surface of a lifting surface. Similarly, lift enhancement can be attained by tab deployment on the lower (pressure) surface of a lifting surface. A sensitivity analysis using Reynolds-averaged NavierStokes methods was conducted to determine optimal sizing and positioning of the tabs for active load control at a chord Reynolds number of 1.0 million for the S809 baseline airfoil. These numerical simulations provide insight into the flow phenomena that govern this promising load control system and guided tab placement during the wind tunnel study of the S809 airfoil. The numerical and experimental results are largely in agreement and demonstrate that load control through microtabs is viable. Future efforts will include a study of the unsteady load variations that occur during tab deployment and retraction, and three-dimensional issues involving spanwise tab placement and tab gaps.

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

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

Effects of Splitter Plate Length on Aerodynamic Performance & Vortex Shedding on Flatback Airfoils

A two-dimensional experimental and computational examination of the effect of splitter plate length on a flatback airfoil on both aerodynamic performance and vortex shedding behavior is presented. The FB-3500-1750 airfoil is a thick airfoil with a blunt trailing edge designed for the inboard region of wind turbine blades. Its maximum thickness is 35% of the airfoil chord, with a trailing edge thickness of 17.5%. In the present study, the FB-3500-1750 was tested in the University of California, Davis aeronautical wind tunnel at Reynolds numbers of 666,000 with fixed transition. The wind tunnel results are compared with computational predictions obtained using OVERFLOW, a Reynolds-Averaged NavierStokes solver. Drag reductions of at least 27% from the baseline were observed both experimentally and computationally with the inclusion of a splitter plate with length 50% tTE. Additionally, increasing splitter plate length was shown to continue to decrease the base drag. Maximum lift reductions and earlier stall angles were also observed with increasing splitter plate length though in the wind tunnel tests with the 50% tTE case, the loss in was limited to 5%. Examining vortex shedding behavior, increases in the nondimensional shedding frequency, Strouhal number, were seen with increasing splitter plate length.