Simulating Pitching Blade With Free Vortex Model Coupled With Dynamic Stall Model for Conditions of Straight Bladed Vertical Axis Turbines

Abstract

This study is on the straight bladed vertical axis turbines, which can be utilized for both wind and marine current energy. Vertical axis turbines have the potential of lower installation and maintenance cost. However, complex unsteady fluid mechanics of these turbines imposes significant challenges to the simulation tools. Dynamic stall is one of the phenomena associated with the unsteady conditions, and it is in the focus of the study. The dynamic stall effects are very important for vertical axis turbines, since they are usually passively controlled through the dynamic stall. A free vortex model is used to calculated unsteady attached flow, while the separated flow is handled by the dynamic stall model. This is compared to the model based solely on the Leishman-Beddoes algorithm. The results are assessed against the measured data on pitching airfoils. A comparison of force coefficients between the simulations and experiments is done at the conditions similar to the conditions of H-rotor type vertical axis turbines.

Numerical Validation of a Vortex Model against ExperimentalData on a Straight-Bladed Vertical Axis Wind Turbine

“…The procedure of calculating the angle of attack is described in greater detail in [25], and the method denoted "explicit method" is the one implemented here. This method adds the released vortex into the panel equations and solves for the Kutta condition together with the conservation of circulation, i.e., the total circulation of the released vortex, and the bound circulation should equal the bound circulation from the previous time step.…”

“…This approximation also increases the computational speed (compared to modeling the blades with panels) as no panel to vortex interactions have to be calculated. This approximation is evaluated in [25], and the results show that the difference between approximating the blade with a point vortex or with panels is very small; hence, the approximation is reasonable. All simulations were performed for 100 turbine revolutions to ensure convergence in the results.…”

“…The dynamic stall model uses experimental data of the lift and the drag coefficients for steady flow over airfoils [31]. This model was combined with the vortex model for a single pitching wing and showed reasonable agreement with experimental data on the pitching airfoils [25]. The inputs to the models are the angle of attack α and its rate of changeα, the velocity V rel , the chord length c, the Reynolds number and the blade airfoil.…”

“…The outputs of the dynamic stall model are the lift and the drag coefficients (C L and C D , respectively). The main principles of the model are described in this section, and the reader is referred to [25,29] for the details, including the empirical parameters.…”

“…Thus, the unsteady attached flow part of the Leishman-Beddoes model is not used when combined with the vortex model. This method is identical to the one described in [25].…”