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The Role of Dynamic Stall in Aerofoil Shape Optimisation for Curvilinear Blade Kinematics

Benjamin Irwin, David Toal, Swathi Krishna

Abstract

This study investigates the influence of aerofoil shape optimisation on blade aerodynamic performance under curvilinear and unsteady kinematics characteristic of vertical-axis turbines and cycloidal propellers. Using a cyclorotor in hover as a representative configuration, aerofoil optimisation was performed using two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations coupled with Kriging. The optimised design was subsequently validated experimentally through force measurements and flow-field characterisation using particle image velocimetry. Performance was enhanced through the suppression of leading-edge vortex separation during the primary thrust peak. This finding also reveals a governing constraint: the effectiveness of aerofoil optimisation depends on dynamic stall severity. Under light dynamic stall, geometric modification promotes vortex attachment and improves aerodynamic loading. Under deep dynamic stall, flow separation dominates the blade aerodynamics, and aerofoil shape modification cannot suppress leading-edge vortex shedding. The stall severity is regulated by rotor solidity through its influence on the induced throughflow-to-blade-speed ratio and the resulting effective incidence. Aerofoil optimisation is therefore viable primarily in high-solidity configurations that operate within a moderated stall regime. These findings establish a physics-based condition for aerofoil optimisation in curvilinear dynamic stall environments.

The Role of Dynamic Stall in Aerofoil Shape Optimisation for Curvilinear Blade Kinematics

Abstract

This study investigates the influence of aerofoil shape optimisation on blade aerodynamic performance under curvilinear and unsteady kinematics characteristic of vertical-axis turbines and cycloidal propellers. Using a cyclorotor in hover as a representative configuration, aerofoil optimisation was performed using two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations coupled with Kriging. The optimised design was subsequently validated experimentally through force measurements and flow-field characterisation using particle image velocimetry. Performance was enhanced through the suppression of leading-edge vortex separation during the primary thrust peak. This finding also reveals a governing constraint: the effectiveness of aerofoil optimisation depends on dynamic stall severity. Under light dynamic stall, geometric modification promotes vortex attachment and improves aerodynamic loading. Under deep dynamic stall, flow separation dominates the blade aerodynamics, and aerofoil shape modification cannot suppress leading-edge vortex shedding. The stall severity is regulated by rotor solidity through its influence on the induced throughflow-to-blade-speed ratio and the resulting effective incidence. Aerofoil optimisation is therefore viable primarily in high-solidity configurations that operate within a moderated stall regime. These findings establish a physics-based condition for aerofoil optimisation in curvilinear dynamic stall environments.
Paper Structure (13 sections, 14 equations, 16 figures, 1 table)

This paper contains 13 sections, 14 equations, 16 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methodology
  3. Cyclorotor configuration and parametrization
  4. Optimisation via Kriging
  5. Computational methodology
  6. Blade mesh deformation
  7. Experiments
  8. Results
  9. Optimal aerofoil for the 4-bladed cyclorotor configuration
  10. LEV-dynamics of a single blade in curvilinear flow
  11. Impact of blade-count upon LEV characteristics
  12. Influence of LEV separation upon cyclorotor efficiency
  13. Conclusion

Figures (16)

  • Figure 1: Two rotors of fixed solidity ($\sigma=0.3395$) with (left) two blades with chord $c$ and (right) four blades with chord $c/2$
  • Figure 2: Blade-shape parametrization
  • Figure 3: Overview of the mesh used for 2D CFD. 800k cells after combination of outer, rotating and blade domains via overset meshing
  • Figure 4: Demonstration of mesh deformation for new aerofoil shapes
  • Figure 5: Setup of force and flowfield measurement of cyclorotor rig from (a) side view and (b) 3/4 view. (c) Close-up of 4-bar linkage system for blade pitch control
  • ...and 11 more figures