Numerical Study Of Laminar Airfoil Tonal Aerodynamic Noise At Moderate And Low Reynolds Numbers

The industrial equipment using laminar airfoils,such as micro or small unmanned vehicles and wind turbines,will produce strong tonal noise at moderate and low Reynolds numbers,which will lead to serious noise pollution.Therefore,further understanding of tonal noise of laminar airfoils at moderate and low Reynolds numbers is of great significance for low-noise design of related machinery and equipment.Based on the LBM-LES method combined with Lattice Boltzmann Method(LBM)and Large Eddy Simulation(LES),the laminar airfoil flow field and sound field were directly calculated,and the reliability of the numerical method was verified.The standard Smagorinsky,dynamic Smagorinsky and WALE subgrid models were used to conduct numerical simulation on the complex multi-scale flow field of NACA0012 airfoil,a subgrid model which suitable for reflecting the complex multi-scale flow field characteristics of airfoil was summarized by analyzing relevant flow field parameters and pressure spectrum characteristics.In order to further explore the characteristics of tonal noise of laminar airfoil at moderate and low Reynolds numbers,the flow field of NACA0012 airfoil was simulated numerically and the aerodynamic noise of airfoil was calculated directly.The effects of inflow angle of attack and Reynolds number on unsteady flow characteristics and radiated noise of airfoil flow field are studied.For unsteady flow of airfoil,dynamic Smagorinsky model shows good symmetry in calculating lift coefficient in time domain.The analysis of flow field structure shows that the Dynamic Smagorinsky model has lower dissipation than the other two subgrid models in numerical simulation of airfoil flow,which is more in line with the real flow around airfoil at moderate and low Reynolds numbers,and can well reflect the influence of boundary layer disturbance on airfoil.In addition,the dynamic Smagorinsky model can capture the spectral characteristics of the tonal noise caused by the trailing edge vortex shedding of the airfoil.Based on the dynamic Smagorinsky model,the flow field and tonal noise of airfoil at different inflow angles of attack and free inflow Reynolds number are studied.It is found that acoustic source of the airfoil is mainly located at the separation zone and the trailing edge of the airfoil.The acoustic radiation characteristics at different angles of attack and Reynolds number have the characteristics of the dipole acoustic field.The increase of the angle of attack will cause a large vortex scale and boundary layerdisturbance,and the acoustic source area of the suction surface will move forward.The sound pressure level spectrum analysis shows that the tonal noise gradually disappears and the noise spectrum finally presents broadband characteristics with the angle of attack increases.With the Reynolds number increases,the trailing edge pressure fluctuation increases.The primary frequency in the sound pressure level spectrum increases with the increase of the Reynolds number,and conforms to the power law relationship of the Paterson formula.In addition,the spectral characteristics of the sound pressure level show a tendency to change from discrete characteristics to broadband characteristics with the Reynolds number increases.