Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.

Comparative Study of Dynamic Stall under Pitch Oscillation and Oscillating Freestream on Wind Turbine Airfoil and Blade

Abstract: This study aims to assess the dynamic stall of the wind turbine blade undergoing pitch oscillation (PO) and oscillating freestream (OF), respectively. Firstly, a thin-airfoil theoretical analysis was performed to differentiate between these two dynamic effects. During upstroke, PO results in a positive effective airfoil camber, while OF has an additional negative effective airfoil camber, and yet in contrast during downstroke, PO decreases the effective camber, while OF increases the effective camber. Secondly, the equivalence relation between PO and OF is investigated by numerically solving the unsteady Reynolds-averaged Navier-Stokes equations. The difference between PO and OF mainly exists in the linear part of the aerodynamic loads. Because the difference is great at high reduced frequencies or angle of attack (AOA) amplitudes, PO and OF should be treated separately for dynamic stall from different aerodynamic sources. Thirdly, the Beddoes-Leishman dynamic model coupled with Bak’s rotational stall delay model was used to predict the yawed responses of the blade section. The obtained results show different aerodynamic responses between PO and OF, although consideration of rotational augmentation can greatly improve the accuracy of the lift and drag coefficients. To improve the understanding and coupling modeling of rotational augmentation and dynamic stall, an extended analysis of the coupled effect was performed as well.

Boundary-layer transition extent measurements on a cone and flat plate at Mach 3.5

Abstract: Wide excursions of the boundary-layer transition region are expected to occur on the X-30 National Aerospace Plane (NASP) due to the high Mach number, high temperature, and low density environment experienced during flight. Undesirable features of the transition region, such as the peak heat transfer rate, make it important to understand transition region physics. The current study investigates transition extent in 2D and axisymmetric bounday-layer flows. Surface-pitot and recovery temperature data obtained on a cone and flat plate at Mach 3.5 in the Supersonic Low-Disturbance Pilot Tunnel at NASA Langley are presented. Results show the effects of the unit Reynolds number, freestream disturbances, and nose/leading-edge bluntness on the extent of transition.

Navier-Stokes Computations of the NREL Airfoil Using a κ — ω Turbulent Model at High Angles of Attack

Abstract: This paper presents a two-dimensional numerical simulation of the turbulent flow fields for the NREL (National Renewable Energy Laboratory) S809 airfoil. The flow is modeled as steady, viscous, turbulent, and incompressible. The pseudo-compressible formulation is used for the time-averaged Navier-Stokes equations so that a time marching scheme developed for the compressible flow can be applied directly. The turbulent flow is simulated using Wilcox`s modified {kappa}-{omega} model to account for the low Reynolds number effects near a solid wall and the model`s sensitivity to the freestream conditions. The governing equations are solved by an implicit approximate-factorization scheme. To correctly model the convection terms in the mean-flow and turbulence model equations, the symmetric TVD (Total Variational Diminishing) scheme is incorporated. The methodology developed is then applied to analyze the NREL S809 airfoil at various angles of attack ({alpha}) from 1 to 45 degrees. The accuracy of the numerical results is compared with the available Delft wind tunnel test data. For comparison, two Eppler code results at low angles of attack are also included. Depending on the value of {alpha}, preliminary results show excellent to fairly good agreement with the experimental data. Directions for future work are also discussed.