Unsteady aerodynamic response of pitching airfoils represented by Gaussian body forces

TL;DR

This paper develops a model to account for spanwise shed vorticity effects on unsteady aerodynamic forces of pitching airfoils represented by Gaussian body forces, improving accuracy in large-eddy simulations of wind turbines.

Contribution

The authors introduce a new model that explicitly incorporates spanwise shed vorticity effects into Gaussian body force representations, enhancing the accuracy of unsteady aerodynamic load predictions.

Findings

Unsteady lift amplitude can be up to 40% smaller than quasi-steady predictions.

The model's validity is confirmed through LES for pitch steps and periodic pitching.

Gaussian kernel width significantly influences unsteady lift amplitude.

Abstract

The actuator line method (ALM) is an approach commonly used to represent lifting and dragging devices like wings and blades in large-eddy simulations (LES). The crux of the ALM is the projection of the actuator point forces onto the LES grid by means of a Gaussian regularisation kernel. The minimum width of the kernel is constrained by the grid size; however, for most practical applications like LES of wind turbines, this value is an order of magnitude larger than the optimal value which maximises accuracy. This discrepancy motivated the development of corrections for the actuator line, which, however, neglect the effect of unsteady spanwise shed vorticity. In this work, we develop a model for the impact of spanwise shed vorticity on the unsteady loading of an airfoil modelled as a Gaussian body force. The model solution is derived both in the time and frequency domain and features an explicit dependence on the Gaussian kernel width. We validate the model with LES within the linear regime of the lift curve for both pitch steps and periodic pitching with reduced frequencies of k=0.1, 0.2 and 0.3. The Gaussian kernel width affects, in particular, the amplitude of the unsteady lift, which can be up to 40% smaller than the quasi-steady amplitude within the considered range of reduced frequencies and kernel widths.