A Hybrid Computational Fluid Dynamics Method for Unsteady Simulation of the Ship-Helicopter Dynamic Interface

Abstract

Helicopters operating from ships are exposed to turbulent airwakes which can determine ship-helicopter operating limits. During concurrent operations rotor-rotor interactions add to the complexity of the aerodynamics. Computational fluid dynamics solvers are able to predict these aerodynamics from first principles with the aid of turbulence-resolving approaches such as detached eddy simulation. Although it is possible to create body-fitted grids to resolve the rotor blades and move them, the fuselage, and the ship relative to one another, this is a computationally expensive and labour intensive method. To avoid this expense and while accurately predicting unsteady loading, a time accurate rotor model has been coupled to a Navier-Stokes solver by introducing momentum source terms to the governing equations. A novel coupling algorithm that accounts for the effects of unsteady aerodynamics as well as the induced velocity of the wake has been developed and validated. The coupled rotor model predicts performance, thrust and torque distributions, and unsteady aerodynamic loading of isolated and interacting rotors. A time accurate wake can also be generated by the model. The method requires far fewer grid points to resolve the rotor than a body-fitted grid and grids can be generated automatically. Navier-Stokes simulation of the ship airwake is a complex task and many of the parameters of importance for such simulations have been identified in the literature. A study of grid convergence of velocity spectra and analysis of finite sample error have been performed to add to this knowledge. A method for objectively assessing the finite sample error and determining the minimum sample time required to reach a certain error has been applied to ship airwake simulations for the first time and a minimum level of grid refinement for resolved velocity spectra suggested. The ship airwake and rotor model have been combined for ship-helicopter dynamic interface simulations of single helicopter operations and concurrent helicopter operations involving five rotors. These simulations demonstrate the ability of the method to predict the aerodynamic factors that influence ship-helicopter operating limits and, to the best of our knowledge, contain more vehicles than any previously published dynamic interface simulations.