An Experimental Study on the Actuator Line Method with Anisotropic Regularization Kernel

Abstract

Nowadays, actuator line method (ALM) has become the most potential method in wind turbine simulations, especially in wind farm simulations and fluid-structure interaction simulations. The regularization kernel, which was originally introduced to ALM to avoid numerical singularity, has been found to have great influence on rotor torque predictions and wake simulations. This study focuses on the effect of each parameter used in the standard kernel and the anisotropic kernel. To validate the simulation, the torque and the wake characteristics of a model wind turbine were measured. The result shows that the Gaussian width ϵ (for standard kernel) and the parameter in chord length direction ϵc (for anisotropic kernel) mainly affect the normal velocity of each blade element when using ALM but have little effect on the tangential velocity calculation. Therefore, these parameters have great influence on the attack angle and rotor torque prediction. The thickness parameter ϵ t is the main difference between the standard kernel and the anisotropic kernel and it has a strong effect on the wind turbine wakes simulation. When using the anisotropic kernel, the wake structure is clearer and less likely to disperse, which is more consistent with the experimental results. Based on the studies above, a non-uniform mesh is recommended when using the anisotropic regularization kernel. Using a mesh refined in the main flow direction, ALM with anisotropic kernel can predict torque and wake characteristics better while maintaining low computational costs.

Evaluating the accuracy of the actuator line model against blade element momentum theory in uniform inflow

“…Typically, this force projection is performed using an isotropic Gaussian function, 4 although the use of nonisotropic Gaussian kernel function, in which the force projection width is different in each direction, has been proposed to represent the shape of the blades more accurately. 10,[24][25][26] Shives and Crawford 20 suggested that the spreading parameter ϵ should be related to the local chord length c, namely, as c/8≤ϵ≤c/4, for elliptical loaded wings. By minimizing the error between the potential flow around a two-dimensional airfoil and the flow generated by a Gaussian distributed body force Martínez-Tossas et al 25 concluded that ϵ ¼ 0:2c should be employed for a low angle of attack.…”

“…Subsequently, the forces are projected from the actuator points to the flow solver to ensure the effect of the rotating blades is evaluated by the flow solver. Typically, this force projection is performed using an isotropic Gaussian function, 4 although the use of nonisotropic Gaussian kernel function, in which the force projection width is different in each direction, has been proposed to represent the shape of the blades more accurately 10,24‐26 . Shives and Crawford 20 suggested that the spreading parameter ϵ should be related to the local chord length c , namely, as c /8≤ ϵ ≤ c /4, for elliptical loaded wings.…”

Special Issue “Numerical Simulation of Wind Turbines”

“…To achieve it, it is apparent that simulation will play a key role since it represents the common framework, within which the different disciplines and sciences can communicate. Developments are needed in each of them, from more accurate but computationally affordable simulation techniques [11,12], to innovative control logics [13] or effective methods for noise prediction [14]. Focusing more specifically on aerodynamics, it is apparent that the use of high-fidelity computational techniques is key to properly model the new generation of long and flexible blades, where aero-elastically-tailored designs are needed, and different flow control devices are often used to alleviate the loads [15,16].…”

Modelagem e simulação aerodinâmica de turbinas eólicas offshore usando modelos atuadores.

“…According to Ma et al (2020), there is still confusion about the optimal value of the Gaussian width, and the effect of each parameter used in an isotropic kernel on an ALM simulation result is still unclear. The smoothing parameter ϵ will strongly affect the torque result in ALM simulations and it may not converge for large values.…”