Study On Biomimetic Flow Control For Noise Reduction Of Thin Airfoil Blade And Its Application

Noise pollution not only affects people's interest,learning and work,but also seriously endanger human health.For instance,inducing heart disease,hypertension and other diseases.Modern transportation and industrial equipment are the main noise source of environmental noise.In many cases,the aerodynamic noise is more dominant in the total noise than the mechanical noise,due to the increase of the running speed of the working parts.As the basic elements of turbomachinery of the wing,fan and wind machine,and an important carrier of aeroacoustics research on the basic theory,the study on noise reduction technology is very significant for academic significance,and valuable for practical application.Flow control is one of the frontiers of fluid mechanics.The collision with bionics has given birth to an important research branch of flow control,called biomimetic flow control,namely,through imitating and learning the special function of creatures,furtherly enhance the flow control effect.Based on the different noise generation mechanism at the different Reynolds numbers and angles of attack,this paper proposes a concept of composite bionics with several biological characteristics,designs a biomimetic airfoil blade with low sensitivity for Reynolds numbers,performs the study on the aerodynamic noise reduction and its mechanisms,and carries out the experimental research on the application of airfoil axial fans.The noise components and spectral characteristics of NACA 0006 airfoil with different Reynolds number and different angles of attack were estimated by using NAFnoise softcode.The range of Reynolds number based on chord length of airofil is from 5×104-1.9×105,at the angle of attack(AOA)of 0°,laminar boundary layer vortex shedding(LBL-VS)noise with narrow bandwidth characteristics is dominant noise.At the AOA of 5°,the boundary layer transition and separation due to the increase of Reynolds number,which causes the increase of the frequency range with high sound pressure level(SPL),especially at the low and medium frequency range.The total sound pressure spectrum shows a broadband characteristic.At AOA of 10°,flow separation in large scale occurs,and the separation noise and turbulent boundary layer(TBL)noise caused by flow separation are dominant at low frequencies.Based on these conclusions,the LBL-VS noise could be reduced by changing the leading edge shape of airfoil blade.The TBL noise could be reduced by changing the surface and trailing edge shapes.Numerical simulation is an important research method in the field of acoustics.In order to accurately calculate the acoustic signal of the aerodynamic noise source and small scale vortexes,the numerical simulation method and the grid independent analysis were carried out.The aerodynamic sound source can be solved by LES method with the dynamic SmagorinskyLilly model,which has an ability to accurately calculate the small scale vortexes for the analysis of the noise reduction mechanism.The calculation domain is dispersed by C-type hexahedral mesh.The mesh density is determined through the study on the influence of drag coefficient and the surface pressure coefficient by the mesh cell number.Integrating the common bionic model such as owl wing,humpback whale flippers,and fish fins for the biomimetic flow control,a biomimetic airfoil blade with waved leading edge,serrated trailing edge and ridged surface is designed through changing chord length.Numerical simulations with LES/FW-H method indicate that the bionic structure can significantly reduce the aerodynamic noise of the NACA 0006 airfoil blade without significantly changing its aerodynamic performance.At the Reynolds number of 5×104 and the angle of attack of 0°,the OASPLs for the biomimetic airfoil blade at the observer points are reduced by 1.7-2.8d B.The airflow passes the trailing edge of the biomimetic blade,the two sides of the airflow with the same value flow and reverse direction in the spanwise direction in the valley between the two adjacent ridges.Under the restriction of the wall surface,the airflow after confluence has a certain normal velocity,which could prevent the formation of T-S waves.After the airflow shedding from the serrated edge of the biomimetic airfoil,the regular "blocky vortexes" are formed,which weakens the spanwise correlation of streamwise vortices.At the Reynolds number of 1.0×105 and the angle of attack of 5°,OASPLs are reduced by 1.7-3.2d B at all the valid observer points.In the suction side,the velocities of the two sides of airflow between the two adjacent ridges is much higher than those at low Reynolds number.With the reflection effect by the wall,normal velocity of the airflow in the valley is enough high to accelerate the laminar to turbulent transition of the boundary layer from laminar to turbulent,which reduces the pressure fluctuation caused by the instability of boundary layer transition.In the pressure side,the gathering effect by the ridged structures of wall can stabilize the boundary layer and eliminating the ? vortexes during the transition of boundary layer.The laminar boundary layer rapidly translates to turbulent boundary layer close to the trailing edge of the blade.At the Reynolds number of 1.9×105 and the angle of attack of 10°,large scale flow separation occurs after the airflow pass leading edge of the both blades due to the much higher Reynolds number and angle of attack.For the biomimetic airfoil blade,the flow is violently disturbed by the waved structures of the leading edge,which causes the increase of the noise at the observer points near the leading edge by 0.9d B.However,at the other observer points,especially near the trailing edge,the noise still is significantly reduced by 3.4d B.The noise reduction mechanism is very similar to flow condition of Reynolds number of 5×104 and the attack angle of 0°.The application research on the thin airfoil bionic flow control noise reduction technology was performed on airfoil axial fan.Biomimetic fan blade with the similar structure with the basic research model are designed by controlling the guide line of the leading edge.There are three kinds of biomimetic fan blades with the different guide lines of arc,straight line and broken line.Because the fan blade is quiet thin,to further improve the flow control effect,three derivative models are designed based on the understanding of the flow mechanism.The guide line of leading edge of these derivative models are spline curve,and the trailing edges are spline curve,straight line and broken line.Because the formation of bionic structure is generated by scaling the airfoil chord,in order to make the comparison more scientific,two fans with straight leading and trailing edges are designed as the prototype fans for comparison with the biomimetic models.The chords of the blades of the two prototype fans are the maximum and minimum chords for the design of biomimetic fan blade.The test results show that the performance of both the two prototype fans are good,and also,the performances of the two fans are not significant.Compared with the two prototype fans,the starting powers of all the biomimetic airfoil fans are significantly reduced,and the static efficiency are greatly improved at the four voltage modes of 13 V,16V,20 V,24V.The starting power could be reduced by 8%,the maximum difference of two static efficiency curves of efficient working range is 3.79%.According to the national standard of GB/T 2888-2008 "noise measurement method of fan and roots blower",the acoustic performance of prototype fan and bionic fans were tested by using the acoustic measurement system composed of MPA231 microphone and MC3242 data acquisition.The results showed excellent acoustic performance of all the biomimetic fans at the four voltage modes of 13 V,16V,20 V,24V.The biomimetic fans whose guide lines of leading edge and trailing edge are both spline curves are with significant low noise performance at the four voltage modes.Contrast with the prototype fan with the relatively large blade,whose aerodynamic noise are reduced by 3.39 d B,3.13 d B,4.14 d B,4.06 d B.During the experiments,the noise of the bionic fan can be obviously felt by the ears.