Abstract
The interaction of a helicopter rotor with the ground in hover flight is addressed numerically using a hybrid Eulerian–Lagrangian CFD model. When a helicopter takes off or lands, its wake interferes with the ground. This interaction, depending on the height-to-rotor diameter ratio, causes the altering of the rotor loading and performance as compared to the unconstrained case and gives rise to the development of a complex outwash flow field in the surrounding of the helicopter. The present study aims to characterize the interactional phenomena occurring in the early stages of the rotor wake development and in particular the interference of the starting vortex with the ground boundary layer and the effect of this interaction in the motion of the vortex in the rotor outwash flow. The hybrid CFD method employed combines a standard URANS compressible finite volume solver, the use of which is restricted to confined grids around solid bodies, and a Lagrangian approximation of the entire flow field in which conservation equations are solved in their material form, disctretized using particle representation of the flow quantities. The two methods are strongly coupled to each other through an appropriate iterative scheme. The main advantage of the proposed methodology is that it can conveniently handle complex configurations with several bodies that move independently from one another, with affordable computational cost. In this paper, thrust coefficient predictions of the hybrid model are compared to predictions of a free wake code and to experimental data indicating that consistent prediction of the rotor load requires the inclusion of the ground boundary layer in the analysis. Moreover, detailed comparisons of the rotor wake evolution predicted by the hybrid model are presented.
Highlights
When a helicopter executes hover flight during takeoff and landing, a complex flow field is developed in the wake of its main rotor which is affected by the blockage of the ground
The present study aims to characterize the interactional phenomena occurring in the early stages of the rotor wake development and in particular the interference of the starting vortex with the ground boundary layer and the effect of this interaction in the motion of the vortex in the rotor outwash flow
This paper addresses the near-ground hover flight and attempts to revise earlier potential flow predictions by introducing the effect of flow viscosity in the simulation, in particular the viscous boundary layer that develops on the ground as a result of the rotor down-wash
Summary
When a helicopter executes hover flight during takeoff and landing, a complex flow field is developed in the wake of its main rotor which is affected by the blockage of the ground. Where φ is the inflow angle; θt is the local twist angle; φp is the pitch angle of the blade; ae f f is the effective angle of attack; CL(ae f f ) and CD(ae f f ) are the lift and drag coefficient for the specific angle of attack, respectively; ρ is the density of the fluid; We f f is the local two dimensional inflow velocity; c is the blade element characteristic chord; ∆r is the width of the blade element; and eL, eD are the unit vectors in the direction of lift and drag, respectively The reaction of this body force and the corresponding power are imposed as source terms in the momentum and energy equations of the cells swept by the blades during their rotation. Coarser grid cab be considered when the priority of simulations focuses only on blade loads predictions
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