Experimental And Numerical Study On Blade Aerodynamic Noise Control By Bio-inspired Treatments

With the increasingly serious aeroacoustic problem of commercial aircrafts,wind turbines,and unmanned aerial vehicles,the use of innovative flow-control method to suppress noise at its source has aroused widespread concern.The bionic serrated trailing-edges and wavy leading-edges have been proven to be two passive flow-control devices that can effectively reduce aerodynamic noise.It is of great academic value and engineering application significance to intensively study the noise reduction law and mechanism.In this paper,the effects of bionic sawtooth structures on aerodynamic performance,trailing-edge self-noise,leading-edge interaction noise of an isolated airfoil are studied by means of experimental measurements and numerical simulations.The main research work and results are as follows:(1)Based on large eddy simulation and the acoustic analogy theory,a hybrid computational aeroacoustics method for the numerical prediction of blade turbulent broadband noise is developed successfully.Aiming at the problem that the spanwise length of numerical simulation is shorter than that of experiment,the acoustic spanwise correction equations are derived based on rectangular function,triangular function,Laplacian function,and Gaussian function,and the aeroacoustic spanwise correction theory is further improved.The differences between the four types of formulas are compared and the maximum difference between the four functions in SPL correction is 4dB.In this paper,the Gaussian function is chosen for acoustic spanwise correction,and the corrected numerical results are in good agreement with the present experiment.The reliability of the hybrid computational aeroacoustics method and the acoustic spanwise correction theory is confirmed.(2)The airfoil laminar boundary layer instability noise is studied in an anechoic wind tunnel,and the noise reduction law of serrated trailing-edge and wavy leading-edge is analyzed.The trailing edge bluntness-vortex shedding noise of the “cut-in” type serration is studied by using BPM trailing edge noise model.The generation mechanism of laminar boundary layer instability noise is attributed to an aeroacoustic feedback loop,which is sensitive to both Reynolds number and angle of attack.The peak frequency of the instability noise is proportional to the 1.5 power of the incoming velocity and inversely proportional to the 0.5 power of the chord length,but the variation of the peak frequency has no regularity with regard to the angle of attack.Serrated trailing-edge can effectively reduce instability noise,the larger the serration amplitude,the better the noise reduction effect.The maximum sound power reduction level is 42 dB,and the maximum OAPWL reduction level is 23 dB.However,the sawtooth root of the “cut-in” type serration is very thick,which may cause intensive bluntness-vortex shedding noise.The serration with larger amplitude has thicker root,leading to stronger bluntness-vortex shedding noise,and the maximum noise increase is 25 dB.Wavy leading-edge can also effectively mitigate instability noise,the larger the serration amplitude,and the smaller the wavelength,the better the noise reduction effect.The maximum sound power reduction level is 32 dB,and the maximum OAPWL reduction level is 18 dB.Although the effect of wavy leading-edge on suppressing instability noise is not as good as that of serrated trailing-edge,it does not cause bluntness-vortex shedding noise.(3)The noise reduction law of wavy leading-edge on rod–airfoil interaction noise is studied in an anechoic wind tunnel.The effects of serration amplitude,serration wavelength,inflow velocity,rod diameter,and rod–airfoil separating distance on noise reduction are analyzed.Wavy leading-edge can effectively reduce interaction noise,the larger the serration amplitude,and the smaller the wavelength,the better the noise reduction effect.The maximum sound power reduction level is 4dB,and the maximum OAPWL reduction level is 3dB.With the increase of the inflow velocity,the noise reduction effects shift towards high frequency range,but the velocity has little effect on the noise reduction amplitude.In general,the noise reduction decreases slightly with the increase of the velocity.For different separating rod–airfoil distance,the flow patterns can be divided into two different modes,and the critical spacing is observed to be L/d=3.5.For L d ?3.5,the flow undergoes a “shear mode”,the Karman vortex shedding is suppressed.For L d ?3.5,the flow goes through a “wake mode”,the Karman vortices shed from the upstream rod.For the two different modes,the acoustic radiation and noise reduction show quite distinct characteristics.A hybrid computational aeroacoustics method is applied to study the noise reduction mechanisms of wavy leading-edge on rod–airfoil interaction noise.In general,the main noise reduction mechanisms are that the wavy leading-edges reduce sound source level on the airfoil,reduce correlation and coherence along the span,and enhance the destructive phase interference effects.(4)The effects of wavy leading-edge on aerodynamic performance of 2D airfoil and 3D swept tapered wing are studied by using steady RANS and unsteady SAS methods.For 2D airfoil,the wavy leading-edge has negligible influence on the aerodynamic performance when the angle of attack is small.At moderate angles of attack,the wavy leading-edge reduces the lift coefficient and increases the drag coefficient,resulting in a decrease of 7.5%~34.7% of the maximum lift-to-drag ratio.When the attack angle is larger than the stall angle of the baseline airfoil,the wavy leading-edge decreases both the lift coefficient and drag coefficient,leading to an unchanged lift-to-drag ratio.Overall,the wavy leading-edge reduces the aerodynamic performance of the airfoil,and the airfoil with larger amplitude and smaller wavelength has worse performance.But the wavy leading-edge can cause a soft stall,especially when the amplitude is large enough,the lift coefficient of the wavy airfoil increases continuously with the increase of the angle of attack,and there is no stall occurrence.The effects of wavy leading-edge on swept tapered wing are similar to that of airfoil.At small angles of attack,the wavy leading-edge has little effect on lift and drag.With the increase of attack angle,the lift coefficient decreases,the drag coefficient increases,and the maximum lift-to-drag ratio decrases by 6%.The wavy leading-edge has great influence on the flow field around the airfoil and wing.The troughs between two adjacent tubercles are low-pressure regions and a pair of counter-rotating streamwise vortices arises from each trough.