[Paper Review] Numerical Investigation of the Effect of Airfoil Thickness on Onset of Dynamic Stall

Anupam Sharma, Miguel R. Visbal|arXiv (Cornell University)|Apr 29, 2017

Fluid Dynamics and Turbulent Flows4 citations

TL;DR

This study investigates how airfoil thickness affects dynamic stall onset using wall-resolved large eddy simulations (LES) at Re = 200,000. It finds that stall onset transitions from laminar separation bubble (LSB) bursting in thin airfoils to interaction between turbulent trailing-edge separation and the LSB in thick airfoils, blurring traditional classifications of leading-edge vs. trailing-edge stall.

ABSTRACT

Effect of airfoil thickness on onset of dynamic stall is investigated using large eddy simulations at chord-based Reynolds number of 200,000. Four symmetric NACA airfoils of thickness-to-chord ratios of 9%, 12%, 15%, and 18% are studied. The 3-D Navier Stokes solver, FDL3DI is used with a sixth-order compact finite difference scheme for spatial discretization, second-order implicit time integration, and discriminating filters to remove unresolved wavenumbers. A constant-rate pitch-up maneuver is studied with the pitching axis located at the airfoil quarter chord point. Simulations are performed in two steps. In the first step, the airfoil is kept static at a prescribed angle of attack ($=4^\\circ$). In the second step, a ramp function is used to smoothly increase the pitch rate from zero to the selected value and then the pitch rate is held constant until the angle of attack goes past the lift stall point. Comparisons against XFOIL for the static simulations show good agreement in predicting the transition location. FDL3DI predicts two-stage transition for thin airfoils (9% and 12%), which is not observed in the XFOIL results. The dynamic simulations show that the onset of dynamic stall is marked by the bursting of the laminar separation bubble (LSB) in all cases. However, for the thickest airfoil tested, the reverse flow region spreads over most of the airfoil and reaches the LSB location immediately before the LSB bursts and dynamic stall begins, suggesting that stall could be triggered by the separated turbulent boundary layer. The results suggest that the boundary between different classifications of dynamic stall, particularly leading edge stall versus trailing edge stall are blurred. The dynamic stall onset mechanism changes gradually from one to the other with a gradual change in some parameters, in this case, airfoil thickness.

Motivation & Objective

• To investigate the influence of airfoil thickness on the onset mechanism of dynamic stall at moderate Reynolds number.
• To determine whether dynamic stall is triggered primarily by laminar separation bubble (LSB) bursting or by interaction with turbulent boundary layer separation.
• To assess how the sequence of unsteady flow events—transition, LSB formation, DSV generation—varies with airfoil thickness.
• To evaluate the validity of traditional dynamic stall classifications (e.g., leading-edge vs. trailing-edge stall) across a thickness spectrum.

Proposed method

• Large eddy simulation (LES) using the FDL3DI solver with sixth-order compact finite difference spatial discretization and second-order time integration.
• Simulations performed on four symmetric NACA airfoils (9%, 12%, 15%, 18% thickness-to-chord) at Re_c = 200,000.
• A constant-rate pitch-up maneuver was simulated with pitching axis at 1/4 chord, using a ramp function to smoothly increase pitch rate.
• Spatial filtering via discriminating high-order low-pass filters removed unresolved wavenumbers to model sub-grid scale stresses implicitly.
• Static simulations at α = 4° were validated against XFOIL for transition and pressure coefficient (Cp) predictions.
• Spanwise coherence analysis confirmed that a 10% chord span length is sufficient to capture stall incipience without significant spanwise incoherence.

Experimental results

Research questions

• RQ1 How does airfoil thickness affect the onset mechanism of dynamic stall, particularly the role of laminar separation bubble (LSB) bursting versus turbulent boundary layer interaction?
• RQ2 To what extent does the transition process (especially two-stage transition) vary with airfoil thickness in wall-resolved LES?
• RQ3 Does the dynamic stall vortex (DSV) formation and convection speed depend on airfoil thickness?
• RQ4 How does the extent of reverse flow in the boundary layer before stall onset correlate with airfoil thickness?
• RQ5 Can the traditional classification of dynamic stall (e.g., leading-edge vs. trailing-edge stall) be maintained, or does it blur with increasing thickness?

Key findings

• For NACA-0009 and NACA-0012, FDL3DI predicts a two-stage transition not captured by XFOIL, indicating higher-resolution simulation sensitivity to thin airfoil transition physics.
• The laminar separation bubble (LSB) bursts immediately precedes dynamic stall onset in all airfoils, confirming its role as a universal precursor.
• For NACA-0018, the turbulent boundary layer separation region extends over 50% of the suction surface and reaches the LSB location just before stall, suggesting interaction-triggered stall onset.
• The peak suction pressure coefficient (Cp) increases with airfoil thickness, reaching its maximum for the thickest airfoil (NACA-0018).
• The dynamic stall vortex (DSV) convection speed is highest for the thickest airfoil (NACA-0018), indicating stronger unsteady vorticity dynamics.
• The skin friction coefficient (Cf) shows a sharp rise before stall, and its spatial correlation with Cp_rms confirms that Cp_rms can effectively detect transition and stall incipience.