Development of a Blade Element Method for CFD Simulations of Helicopter Rotors using the Actuator Disk Approach

Abstract

The flow field subjected to the influence of the helicopter rotor is characterized by its three-dimensional pattern and unsteadiness. The accurate modeling of the flow around the rotor blades addressed by Computational Fluid Dynamics (CFD) simulations is associated with high computational costs. The complexity of the analysis can be reduced by means of simplified methods such as the actuator disk approach, where the rotor is modeled as a zero-thickness porous surface.

The CFD solver TAU developed at the German Aerospace Center (DLR) includes among its features an actuator disk module which transmits to the flow field prescribed time-averaged load distributions both in axial and tangential directions by means of pressure and tangential velocity jumps. The computation of these loads can be achieved by using the Blade Element Analysis Tool (BEAT), a developed rotor code based on the blade element theory. In addition, since it is considered that changes in blade motion about the feathering bearing and flapping hinge induce variations in the aerodynamic loads acting on the blades and vice versa, the loads need to be calculated under trimmed or equilibrium conditions.

The coupling between BEAT and the TAU actuator disk is defined in a way that the velocity captured at the grid points of the actuator disk surface after each simulation performed in TAU is transferred to BEAT, which computes an updated aerodynamic load distribution. The performance of this approach is tested for an isolated rotor configuration in hovering and forward flight conditions. In hovering flight, the convergence of the CFD flow simulations towards the steady state solution is not satisfactory due to the stiffness of the compressible Navier-Stokes equations at low Mach numbers. Furthermore, reverse flow regions are determined by TAU at the inner and outer boundaries of the actuator disk. The recirculation flow entails high gradients in angle of attack between neighboring blade sectional elements, which yields to the unstable formation of new reverse flow regions. Nevertheless, in forward flight conditions the performance of the flow solver is robust and convergent solutions can be obtained. Moreover, as the flight speed is increased, the shed vorticity is displaced more quickly outside the rotor disk and, hence, its associated effects on the performance of the rotor are diminished.

The accuracy of the coupling approach is validated by comparing the computed results with those measured in a wind tunnel test campaign. The found differences in pitch control angles are assigned to the fact that the blade elastic deformations are neglected in the developed method. This statement constitutes the baseline to be developed for future work.

Finally, the reduction in computation time required by the coupling approach with respect to other more accurate methods enhances the idea of further developments. Therefore, the developed method can be regarded as a suitable strategy to tackle the problem in forward flight conditions in cases where high fidelity results are not needed, such as in the preliminary design stages.