Simulate Rotor Hub Vortex Shedding In Helicopters

Although helicopter aerodynamics involves the same forces that arise in airplane aerodynamics, these forces occur differently due to fluid flow dynamics across the aircraft. Helicopters take advantage of the free stream flow along a rotor blade to produce lift and thrust. In rotating systems, the rotor hub clasps the blades and connects them to the main shaft of the system. The rotor in a helicopter allows it to fly, move laterally, make 360-degree turns, and change the flight altitude. The helicopter pilot communicates the necessary adjustments to the rotor blade angles through the swashplate assembly, which has two parts – the upper and lower swash plates. The upper swashplate is connected to the rotor shaft via linkages while the lower swash plates remain fixed with ball bearings separating the two plates and allowing the upper plate to rotate freely over the lower plate.

The flow around the rotor hub contributes a significant amount of total vehicle parasitic drag, i.e., about 20 – 30%, which needs to be analyzed for drag predictions during the design phase of a helicopter. The rotor hub wake affects the tail, or empennage, and can even hurt the structural stability of the helicopter.

Traditionally, the rotor hub wake analysis happens in a wind tunnel; however, with CFD solutions such as Fidelity CFD, the flow field computations can be done using overset meshes or sliding meshes that help capture the vortex shedding precisely.

Fig. 1: Rotor hub wind tunnel testing (to the left), rotor hub CAD model (to the right).

Challenges of rotor hub vortex shedding

Vortices form when a helicopter’s wake becomes unstable. The vortex shedding pattern in the wake of a rotor hub largely depends on the Reynolds number and the motion of the body.

Fig. 2: Isosurface of Q-criterion of the spinning rotor hub.

An important issue in helicopter aero-structural dynamics is the complex flow that is shed from the rotor hub. For a helicopter in forwarding flight, large flow features travel back toward the empennage and interact with it. These interactions are concerning because the structure often responds to the external forcing of the shed wake. These large flow features are highly vertical, and it is necessary to control and mitigate such fluid-structure interactions. Accurately predicting the behavior of vortex shedding using CFD solutions with overset structured meshes is of great benefit in helicopter design.

Overset mesh generation is conceptually split into off-body or background grids and near-body grids that resolve the geometry and the viscous effects. Often, structured hexahedral component grids are used for their efficiency and accuracy.

User benefits of overset meshing

Cadence Fidelity Pointwise Mesh Generation is a state-of-the-art meshing technology that generates structured, unstructured, overset, and hybrid meshes to suit the demands of the application. The Pointwise API provides the flexibility to integrate into external CFD workflows.

Fig. 3: Overset grids of rotor hub created in Fidelity Pointwise.

Using Overset grid assembly tools in Pointwise, high-quality structured meshes are created to accurately analyze the rotor hub turbulence in the form of vortex shedding. Although unstructured meshes are generally easier to generate than structured, this ease of creation comes at the expense of accuracy challenges for solvers in critical regions such as the boundary layer, and typically require significantly more cells than their structured counterparts.

Overset meshing and Overset Grid Assembly (OGA) tools work in tandem to achieve an optimal simulation model by selecting meshing tools that match the overset paradigm. The feedback from the grid assembly is used as a guide for mesh improvement. The near-body, off-body, and overset mesh remediation can be automated for maximum user efficiency. In figure 3, fine refinement of the rotor hub meshes at regions of fluid-structure interaction is critical for accurate drag predictions.

Summary

Simulation of rotor-hub vortex shedding for the accurate prediction of parasitic drag that affects the aero-structural dynamics of a helicopter is critical for its design. Fidelity Pointwise captures the regions of fluid-structure interactions and turbulence with the necessary refinement, using overset meshing and OGA tools. Automated meshing in Pointwise enables minimum user intervention and maximum meshing efficiency.