The drag coefficient (with tripping) is significantly affected only at low aspect ratio. The boundary layer tripping is observed to decrease the lift curve slope and stalling angle of attack. The lift coefficient decreases with decreasing thickness ratio, while the drag coefficient increases. The lift and post-stall drag coefficients decrease with decreasing aspect ratio. The pitching moment is unstable beyond stall. The maximum drag coefficient is found to occur at an angle of attack of 90/sup 0/. Both initial and secondary stall are presented. Results of force and pitching moment measurements, over the angle of attack range, for all combinations of Reynolds numbers, thickness and aspect ratios, and the effects of boundary layer tripping, have been presented. The aspect ratios were 6, 9, 12 and infinity. The thickness ratios studied were 0.18, 0.15, 0.12 and 0.09. Tests were conducted at Reynolds number ranging from one-quarter million to one million. This report documents results of the wind tunnel investigation of constant chord blades more » having four aspect ratios, with NACA 44XX series airfoil sections, at angles of attack ranging from -10 to 110/sup 0/. The purpose of this wind tunnel study is to characterize the performance characteristics of a blade in stall as a function of its aspect ratio, airfoil thickness and Reynolds number. Peak power predictions upon which wind turbine components are sized depend on a good understanding of a blade's post stall characteristics. Unlike aircraft, a wind turbine's angle of attack range extends deep into stall where the three dimensional performance characteristics of airfoils are not generally known. Wind turbine blades operate over a wide angle of attack range. The combination of the two-equation transition model coupled with the Spalart–Allmaras (SA) RANS turbulence model is a good method for performance prediction of modern wind-turbine airfoils using CFD. However, a limitation of this model is observed at very high Reynolds numbers of around 12–15 million where the predictions are very sensitive to the inflow turbulent intensity. As a result, the two-equation model predictions are more comparable to the predictions from eN transition model. The two-equation transition model predicts the aerodynamic coefficients for airfoils of various thickness at higher Reynolds numbers up to 15 million more accurately compared to the one-equation model. However, at higher Reynolds numbers, the one-equation model fails to predict the natural transition behavior due to early transition onset. The two models exhibit similar behavior at Reynolds numbers around 3 million. Both transition models predict a larger L/D compared to fully turbulent results at all Reynolds numbers. We compare the predictions of the two transition models with available experimental and CFD data in the literature in the Reynolds number range of 3–15 million including the AVATAR project measurements of the DU00-W-212 airfoil. We present the performance of two existing local correlation-based transition models – one-equation model (γ− SA) and two-equation model (γ-Reθt‾- SA) coupled with the Spalart–Allmaras (SA) RANS turbulence model – for offshore wind-turbine airfoils operating at a high Reynolds number. more » The prediction of the drag bucket and the glide ratio is greatly affected by the choice of the transition model in RANS CFD of airfoils. While the lack of prediction of lift stall through Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) is well known, airfoil design using CFD requires the accurate prediction of the glide ratio (L/D) in the linear portion of the lift polar. Turbulence transition in the airfoil boundary layer is known to play an important role in the aerodynamics of these airfoils near the design operating point. The airfoils of current large offshore wind turbines operate with chord-based Reynolds numbers in the range of 3–15 million. Modern wind-turbine airfoil design requires robust performance predictions for varying thicknesses, shapes, and appropriate Reynolds numbers.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |