Study Of Aerodynamic Noise And Perforating Control Method Of Blade Vortex Interaction

Blade-vortex interaction(BVI)impulsive aerodynamic noise is one of the main sources of helicopter noise.Understanding the characteristics of BVI impulsive aerodynamic noise and further research on noise control methods can effectively reduce helicopter noise,which has important academic significance and engineering practical value.The direct method of aerodynamic noise direct calculation can clearly and intuitively reflect the process of noise generation and propagation.In this paper,the LBM-LES method combined with Lattice Boltzmann Method(LBM)and Large Eddy Simulation(LES)was used to directly calculate the parallel BVI impulsive aerodynamic noise at low Reynolds number.The generation mechanism,propagation and attenuation laws of noise were analyzed.The characteristics of BVI impulsive aerodynamic noise were studied.On this basis,the noise control was numerically analyzed and studied by means of leading edge perforating method.The applicability of the LBM-LES method for BVI flow field and sound field numerical calculation was analyzed.The parallel BVI flow field of NACA0012 airfoil was numerically simulated.The Lamb-Oseen vortex model was used.D2Q9 was used for the discrete velocities model of LBM.And the dynamic Smagorinsky sub-grid model was used for LES.The results show that the LBM-LES method has low numerical dissipation enough to preserve the characteristics of the vortex as it convects.The vortex has significant influence on larger scales of the airfoil flow field,whereas the influence on smaller scales is relatively small.The LES method is suitable for the simulation of the flow field of BVI.On this basis,the sound field is calculated numerically.The results show that the sound field of BVI aerodynamic noise can be obtained directly by LBM-LES method,which can be used to analyze and study the characteristics of noise.In order to study the impulsive aerodynamic noise characteristics of two-dimensional,parallel BVI,the generation mechanism,propagation and attenuation laws of BVI noise were analyzed in detail.When the vortex approaches and passes by the leading edge of the airfoil,two sudden changes of pressure near the leading edge of the airfoil occur with different intensity,which results in aerodynamic changes of the airfoil simultaneously giving rise to the dipole directional impulsive sound waves.The weaker sudden change of pressure can radiate sound waves more efficiently.By analyzing the sound fields of four different incoming velocities,it is found that the peak sound pressure of upper and lower far fields is inversely proportional to the propagationdistance,and proportional to the product of the third power of the incoming velocity and the fluctuation amplitude of lift coefficient.The formula for calculating the peak sound pressure in the far field is obtained,which provides another way for predicting the far-field sound pressure of BVI.Based on the study of the characteristics of BVI impulsive aerodynamic noise,a control method for reducing noise by perforating the leading edge of airfoil was designed.A NACA0012 airfoil has been used as the research object,four kinds of the NACA0012 airfoil model with the perforated leading edge have been designed.Numerical simulations of two-dimensional,parallel BVI have been performed for the perforated airfoils and the benchmark airfoil to examine the effects of the freestream velocity,vortex strength and the miss distance on the lift.The results show that the perforated leading edge airfoils can reduce BVI noise,but they also have an influence on the lift coefficient.The straight cavity with a width of 2.5% of the chord can effectively reduce BVI noise with low lift coefficient penalties under a wide application range.