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Assessment of a Nonlinear Unsteady Vortex Lattice-Vortex Particle Method for Predicting Helicopter Rotor/Multirotor Aerodynamics

Jinbin Fu, Eric Laurendeau

TL;DR

This work tackles the challenge of efficiently predicting rotorcraft aerodynamics by developing NL-UVLM-VPM, a mid-fidelity solver that couples a nonlinear unsteady vortex-lattice method with a mesh-free vortex-particle wake stabilized by a viscous-inviscid $\alpha$-coupling against a 2D RANS database and a Vreman SGS-based LES model. It introduces an adaptive wake-particle conversion and a multiple-reference-frame kinematic formulation to enable forward flight and multi-rotor interactions, and validates the approach against experiments and URANS across hover, forward flight, and multi-rotor configurations. Results show good agreement in aerodynamic loads and wake structures while achieving over two orders of magnitude reduction in computational cost, demonstrating strong potential for rapid rotorcraft design, parametric studies, and unsteady wake analysis. The framework lays the groundwork for future extensions to complex tip geometries, rotor-fuselage interactions, and multidisciplinary effects such as aeroelasticity and icing, enabled by the $\alpha$-coupled, stripwise approach.

Abstract

This study presents a nonlinear unsteady vortex lattice-vortex particle method (NL-UVLM-VPM) for the efficient prediction of helicopter rotor aerodynamics. The approach couples a nonlinear unsteady vortex lattice method for aerodynamic modeling with a mesh-free vortex particle method for wake evolution. Nonlinear viscous effects, derived from two-dimensional Reynolds-Averaged Navier-Stokes (RANS) sectional data, are incorporated through a low-cost viscous-inviscid alpha-coupling algorithm. To stabilize wake development, the Vreman eddy-viscosity subgrid-scale model is integrated with the particle strength exchange (PSE) method within a large eddy simulation (LES) framework, enabling robust and accurate resolution of the wake coherent, transient, and turbulent breakdown regions. The solver further includes a novel adaptive particle conversion technique, reducing computational wall-clock time by nearly 70%, and a multiple-reference-frame kinematics formulation, allowing simulations of complex flight conditions such as forward flight and multi-rotor interactions. Validation against experimental and unsteady RANS (URANS) data for three representative benchmark cases shows good agreement in aerodynamic loads and wake structures. Moreover, the NL-UVLM-VPM achieves comparable accuracy to URANS, with over two orders of magnitude reduction in computational cost, demonstrating its potential as a reliable and efficient tool for rotorcraft aerodynamic studies.

Assessment of a Nonlinear Unsteady Vortex Lattice-Vortex Particle Method for Predicting Helicopter Rotor/Multirotor Aerodynamics

TL;DR

This work tackles the challenge of efficiently predicting rotorcraft aerodynamics by developing NL-UVLM-VPM, a mid-fidelity solver that couples a nonlinear unsteady vortex-lattice method with a mesh-free vortex-particle wake stabilized by a viscous-inviscid -coupling against a 2D RANS database and a Vreman SGS-based LES model. It introduces an adaptive wake-particle conversion and a multiple-reference-frame kinematic formulation to enable forward flight and multi-rotor interactions, and validates the approach against experiments and URANS across hover, forward flight, and multi-rotor configurations. Results show good agreement in aerodynamic loads and wake structures while achieving over two orders of magnitude reduction in computational cost, demonstrating strong potential for rapid rotorcraft design, parametric studies, and unsteady wake analysis. The framework lays the groundwork for future extensions to complex tip geometries, rotor-fuselage interactions, and multidisciplinary effects such as aeroelasticity and icing, enabled by the -coupled, stripwise approach.

Abstract

This study presents a nonlinear unsteady vortex lattice-vortex particle method (NL-UVLM-VPM) for the efficient prediction of helicopter rotor aerodynamics. The approach couples a nonlinear unsteady vortex lattice method for aerodynamic modeling with a mesh-free vortex particle method for wake evolution. Nonlinear viscous effects, derived from two-dimensional Reynolds-Averaged Navier-Stokes (RANS) sectional data, are incorporated through a low-cost viscous-inviscid alpha-coupling algorithm. To stabilize wake development, the Vreman eddy-viscosity subgrid-scale model is integrated with the particle strength exchange (PSE) method within a large eddy simulation (LES) framework, enabling robust and accurate resolution of the wake coherent, transient, and turbulent breakdown regions. The solver further includes a novel adaptive particle conversion technique, reducing computational wall-clock time by nearly 70%, and a multiple-reference-frame kinematics formulation, allowing simulations of complex flight conditions such as forward flight and multi-rotor interactions. Validation against experimental and unsteady RANS (URANS) data for three representative benchmark cases shows good agreement in aerodynamic loads and wake structures. Moreover, the NL-UVLM-VPM achieves comparable accuracy to URANS, with over two orders of magnitude reduction in computational cost, demonstrating its potential as a reliable and efficient tool for rotorcraft aerodynamic studies.

Paper Structure

This paper contains 15 sections, 27 equations, 21 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Numerical Methods
  3. Nonlinear unsteady vortex-lattice method (NL-UVLM)
  4. Vortex particle method (VPM)
  5. Coupling strategy
  6. Multiple reference frames
  7. RANS solver -- CHAMPS (CHApel Multi-Physics Simulation)
  8. URANS solver -- ROSITA (ROtorcraft Software ITAly)
  9. Results and Discussion
  10. Adaptive panel-particle conversion assessment
  11. Hovering case: Caradonna-Tung rotor
  12. Forward flight case: AH-1G main rotor
  13. Multi-rotor interaction case: Large-scale side-by-side rotor
  14. Computational time
  15. Conclusions

Figures (21)

  • Figure 1: Descriptive diagram of the Unsteady Vortex-Lattice Method (UVLM).
  • Figure 2: The current VPM simulation capturing the wake evolution of a DJI Phantom II rotor at $J = 0.35$ from a coherent vortical structure to turbulent breakdown and mixing.
  • Figure 3: The conversion of wake panels to particles.
  • Figure 4: Schematic of multiple reference frames for helicopter.
  • Figure 5: Comparison of four rotor wake shedding cases after $\mathbf{10^{\circ}}$ azimuthal angle.
  • ...and 16 more figures