The Role of the Laminar Separation Bubble in Wind Turbine Aeroacoustics and Dynamic Stall

Abstract

The implementation of airfoils in low Reynolds number (Re) conditions requires careful consideration of the natural boundary layer (BL) transition as it covers a significant portion of the airfoil surface and often includes the formation of a laminar separation bubble (LSB). This study focuses on the role of the LSB in two design challenges facing low Re applications including small wind turbines (SWT): the generation of tonal airfoil self-noise and the blade loading fluctuations that occur during dynamic stall. Computational fluid dynamic (CFD) and computational aeroacoustic (CAA) methods were applied to two low Re airfoils to first validate the ability of the chosen methods to predict the complex LSB behaviour and second investigate the role of the LSB in these processes. Multiple detailed experimental data sets were combined to form a comprehensive validation of the simulated aerodynamic and aeroacoustic behaviours.

The investigation of tonal airfoil self-noise was completed using the SD 7037 airfoil at a modest Re of 4.1x10⁴. The numerical methods of incompressible wall-resolved large eddy simulation (LES) and the Ffowcs-Williams and Hawkings (FW-H) acoustic analogy correctly predicted the tonal aeroacoustic noise and the Kelvin-Helmholtz (K-H) rolls that form in the LSB were identified as the acoustic source. Analysis of the transient BL development showed that the K-H rolls were amplified through a LSB pressure feedback process that altered the development of the laminar BL upstream of the LSB.

Later, the analysis of the deep dynamic stall process was completed using unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations of the SD 7037 airfoil at Re=4.1x10⁴ and a pitching reduced frequency of k=0.08, and the S833 airfoil at Re=1.7x10⁵ and k=0.06. The simulated timing of the dynamic stall agreed with experimental data for both airfoils which is an advancement from the early prediction of stall seen consistently in previous numerical studies. The accurate prediction of dynamic stall was found to be dependent on the correct simulation of the bursting of the LSB, which initiated the complete separation of the boundary layer and the formation of a leading edge vortex.

The airfoil self-noise and dynamic stall simulations both proved the important role the LSB plays in these behaviours for low Re airfoils. All simulation methods also required strict grid refinement at the leading edge of the airfoil to correctly develop the transient BL transition behaviours for the tonal noise prediction and to capture the bursting of the LSB in the dynamic stall simulations. Grid refinements offset from the airfoil surface were also required to capture the BL transition that occurs in the separated region of the LSB. The successful application of LES and FW-H for tonal noise prediction and URANS for dynamic stall prediction opens the possibility of incorporating these methods into the aerodynamic and aeroacoustic design of airfoils for low Re applications including SWTs.