Effect of Blade Cambering on Dynamic Stall in View of Designing Vertical Axis Turbines

Abstract

This paper presents large eddy simulations (LESs) of symmetric and asymmetric (cambered) airfoils forced to undergo deep dynamic stall due to a prescribed pitching motion. Experimental data in terms of lift, drag, and moment coefficients are available for the symmetric NACA 0012 airfoil and these are used to validate the LESs. Good agreement between computed and experimentally observed coefficients is found confirming the accuracy of the method. The influence of foil asymmetry on the aerodynamic coefficients is analyzed by subjecting a NACA 4412 airfoil to the same flow and pitching motion conditions. Flow visualizations and analysis of aerodynamic forces allow an understanding and quantification of dynamic stall on both straight and cambered foils. The results confirm that cambered airfoils provide an increased lift-to-drag ratio and a decreased force hysteresis cycle in comparison to their symmetric counterparts. This may translate into increased performance and lower fatigue loads when using cambered airfoils in the design of vertical axis turbines (VATs) operating at low tip-speed ratios.

Impact of Environmental Turbulence on the Performance and Loadings of a Tidal Stream Turbine

“…And whilst this method is unable to resolve explicitly the viscous part of the laminar boundary layer developing over the hydrofoil, and hence is unable to resolve the laminar-to-turbulent boundary layer transition, it nevertheless proved to be accurate enough when employing adequate grid spacings and time steps. This LES-IB method has been validated in complex fluid-structure interaction applications such as vortex-induced vibrations [50], dynamic stall in pitching airfoils [51], or flow around vertical axis tidal stream turbines [48]. Most relevant to this study is the validation of the current method for the horizontal axis tidal turbine operating over a flat bed by Ouro et al [11], which is exactly the same turbine as the one to be studied herein.…”

A simple model for deep dynamic stall conditions

“…Let us consider now the characteristic frequency 2 in Equation (13). Given the airfoil oscillation frequency ( in Equation 1), the time needed to reach point 5, and thus CL, min, from point 4 (see Figure 1) can be easily computed as follows: Note.…”

“…However, numerical simulations are difficult due to the intrinsic unsteadiness coupled with turbulence at high Reynolds numbers (approximately 10 6 ). A few large eddy simulations (LES) have been documented in the recent literature (see, eg, previous studies). On the other hand, simulations based on the discretization of the unsteady Reynolds average Navier‐Stokes equations (URANS) (see, eg, Martinat et al and Wang et al) imply significantly lower computational costs than LES, but they encounter accuracy problems for situations in which the boundary layer is massively separated with large recirculation areas, as typically occurring in deep stall conditions.…”

Low wind speed aerodynamics of asymmetric blade H-Darrieus wind turbine-its desired blade pitch for performance improvement in the built environment

“…Mohamed et al [14] investigated the performances of 25 different airfoils (symmetric and asymmetric) and concluded that asymmetric airfoils are better for turbine performance, and NACA 63-415 has higher power coefficient (C p ) and TSR range at low wind speed. Ouro et al [15] investigated the performance of both symmetric and asymmetric airfoils and recommended that asymmetric airfoil has higher lift-to-drag ratio compared to symmetric airfoil. Mazarbhuiya et al [16] also recommended asymmetric airfoil for achieving good performance at low wind speed condition.…”