LES Studies of Helicopter Blade-Vortex Mechanism of Interaction: The Icing Effect
48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition
The influence of icing effect on the helicopter blade-vortex mechanism of interaction is investigated using large eddy simulation (LES). The simulations were performed for a Reynolds number, Re = 1.3 x 10 6 , based on the chord, c, of the airfoil (NACA0012). Computations are carried out to investigate the influence of the icing effect on the aerodynamic coefficients and acoustic field. It was observed that the icing effect influences the aerodynamic coefficients in a specific manner. Larger
... manner. Larger amplitudes of aerodynamic coefficients were observed for the case of an iced leading edge when compared with the clean NACA0012 case. 2 conditions and the extreme flight conditions were not considered. However helicopters fly in different regimes and extreme flight conditions such as desert areas or Polar regions. One of the critical in flight conditions is the icing effect. In the present study, the influence of icing effect on the blade-vortex mechanism of interaction and implicitly on the aerodynamic coefficients and acoustic field is numerically investigated using a large-eddy simulation (LES) approach. Numerical simulation of BVI has been of interest to Computational Fluid Dynamics (CFD) for many years. 1-19 There are still difficulties concerning an accurate numerical prediction of BVI. One of the main issues is the inherent numerical dissipation of CFD turbulence models, which severely affects the preserving of the vortex characteristics. Accurate prediction of BVI aerodynamic loads and aeroacoustics using URANS is known to be very challenging due to the complex unsteady flow dynamics, involving boundary layer development on the suction side and flow separation. 7-10 The use of RANS methods, significantly rely on turbulence models to capture all the relevant turbulence scales. RANS methods predict the noise using the mean flow properties. Due to the fact that noise generation is a multi-scale problem, involving a wide range of length and time scales, the use of RANS-based prediction methods remains limited. Although RANS methods are useful for predicting the aerodynamic coefficients, holding accurately up to some extent, they are usually not suitable or reliable for an accurate noise prediction. The recent improvements in the processing speed of computers make the applicability of Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) to turbulent flows more feasible. However, due to a wide range of length and time scales present in turbulent flows, the use of DNS is still limited to low-Reynolds-number flows and relatively simple geometries. It is known that the number of grid points required for DNS is proportional to Re 9/4 . Direct Numerical Simulation of high-Reynolds number flows of practical interest would necessitate high resolution grid requirements that are far beyond the capability of the most powerful computers available now days. In order to overcome the grid requirements issues, turbulence has to be modelled in order to perform simulations for problems of practical interest. Large Eddy Simulation, with a lower computational cost, is a promising alternative method to DNS, for simulations of high Reynolds-number flows. LES methods are capable of simulating flows at high Reynolds number, LES method being independent of Reynolds number. In Large Eddy Simulation, the large scales are directly solved, while the small scales are modelled. Since noise generation is an unsteady process, LES is probably the most affordable computational tool to be used, since it is the only way, other than DNS, to obtain a time-accurate unsteady solution. The structure of the paper is as follows. In Section 2 the computational method and models are introduced with details regarding the numerical approach and computational domain. Section 3 presents the numerical results of the aerodynamic and aeroacoustic studies. The conclusions regarding the present study are summarized in Section 4.