Modeling Rotor Wakes with a Hybrid OVERFLOW-Vortex Method on a GPU Cluster

Abstract

The vortex core shed from rotorcraft blades maintains coherency—and thus dynamic relevance—many blade turns after its creation. This presents a challenge to traditional Eulerian computational methods, as fine grids are required to suppress numerical diffusion which would weaken the vortex cores after a small number of revolutions. Vortex methods have been used in the past to overcome these problems, as they require computational elements only in vorticity-containing regions, but suffer from greater computational cost per element. In the present work, we will solve these problems with a hybrid EulerianLagrangian method for modeling rotor wakes. An Eulerian OVERFLOW overset grid method computes the near-body flow, while a Lagrangian particle vortex method tracks the wake. The vortex method uses an anisotropic LES model to handle subgrid-scale dissipation explicitly. The computational cost of vortex methods is alleviated by using a parallel adaptive treecode on a cluster of machines each with multi-core CPUs and multiple costefficient graphics processing units (GPUs). Simulations of a low-Re sphere, finite wing, and 4-bladed rotor model are presented and are validated by comparisons with computational and experimental data. Rotorcraft operate in a highly complex vortex-dominated aerodynamic environment, characterized by unsteady non-homogenous turbulent flow interacting with the craft structure. The fuselage bluff body is often associated with unsteady separated flow. Further, the rotating blades generate highly energetic vortices, which invariably lead to the familiar phenomenon of blade-vortex interactions (BVI). BVI induces unsteady, non-periodic impulsive airloads along the length of the blades; thereby, increasing the vibration of the blades and the airframe. This has a strong impact on the stability of flight dynamics as well as the fatigue life of the vehicle. A comprehensive design and analysis tool that can predict the coupled fluid, structural, and vehicle dynamics of rotorcraft with high fidelity would greatly enhance the capability of the designer or analyst to understand the physics of the problem with better clarity, and it will ultimately lead to optimal aircraft designs. Eulerian computational fluid dynamics methods are very efficient in accurately resolvingthe flow in the immediate vicinity of the helicopter boundary, which is primarily anisotropic and essentially unidirectional in nature. Furthermore, mature technologies exist for Eulerian simulation of compressible flow, which, for rotor blades, is most significant within this same near body region. However, as is well known within the CFD community, the method is notoriously diffusive and tends to dampen high-intensity vortical structures within