Bionic Noise Reduction Of Small UAV Rotors

The four rotor unmanned aerial vehicle(UAV)is a kind of aircraft with the ability of vertical take-off and landing and air suspension.This kind of aircraft has attracted much attention because of its small size,low cost,convenient use and unique adaptability.With the increasing maturity of the four-rotor UAV control technology,the four-rotor UAV has begun to play an important role in our life and work.However,the noise problem of the four rotor unmanned aerial vehicle has not been solved well.The study of the low noise four rotor unmanned aerial vehicle will be of great significance to its further promotion in all walks of life.Owls are able to hunt silently,and the characteristics of silent flight bring many owl bionic engineering research.Based on the analysis of the noise composition of the small four-rotor UAV,the main noise source of the unmanned aerial vehicle is determined by the aerodynamic noise of the rotor.The noise level of the UAV is determined by the noise test of the four-rotor UAV under the low Reynolds number in the anechoic chamber and the open field.Based on the three unique bionic structures of the owl wing(tail feathers tip comb structure,front edge protruding structure and lower surface tubercle structure array),the three bionic structures are added to the UAV rotor by bionic method,and the effects on the aerodynamic and acoustic properties of the rotors are investigated.The design scheme of improving rotor noise is selected to reduce the noise of the four-rotor UAV.Based on the comb-like structure of the owl wing tail feather,this paper simulates the comb structure in two ways:one way is to use the brush fiber to fix it at the tail of the rotor at a certain gap,and the other is to paste the saw-tooth shaped PVC material at the tail of the rotor to simulate the comb structure.In each of these two ways,several groups of rotors experiments with different bionic structural parameters are set up.Through the experiments,it is found that the rotor noise with the brush comb structure is lower than that of the prototype rotor and the maximum can be reduced by 1.5dB.The bionic rotor with the saw-tooth structure can also reduce the noise.Compared with the prototype rotor,the rotor with the saw-tooth structure can reduce noise at the maximum of 2.5dB.The two ways of the bionic comb structure have a weak influence on the aerodynamic performance of the rotor,which could be neglected.At the same time,it is found that the bionic rotor with saw-tooth structure is superior to the bionic rotor with brush structure in reducing noise.The numerical simulation shows that the comb-like bionic structure can effectively improve the tail noise power of the airfoil,but it has no effect on the aerodynamic performance of the airfoil.The comb structure can comb the air flow in the tail,restrain the large eddy current,and reduce the noise of the tail of the rotor.The improvement effect of the saw-tooth structure is better than the brush structure.Based on the protruding structure of the owl wing front edge,this paper tries to use three-dimensional modeling to add the convex structure to the front edge of the prototype rotor,and then use the 3D printing technology,selecting the glass fiber material,to process the bionic rotor with the convex structure of the front edge.The front protruding bionic rotor experiments show that the lift force coefficient of the rotor with front protruding structure is improved to a certain extent.At the operating speed(3000r/min),the lift force of the prototype rotor is increased by 2.9%.However,at the same speed,the front flange structure does not reduce the aerodynamic noise of the rotor.On the contrary,it will slightly increase the noise of the rotor;according to the aerodynamic performance of the rotor,the aerodynamic performance of the rotor will be improved.A comprehensive comparison of dynamic and acoustic properties shows that the bionic rotors with protruding structure of the height of 8mm,the distance of 20mm and 40mm have the best improvement effect.With the same lift,the rotors produce the smallest noise.The numerical simulation of the front edge protruding bionic rotor is carried out through the dynamic grid method.The numerical simulation results show that the front protruding structure improves the aerodynamic performance of the rotor obviously,but the front protruding structure will increase the aerodynamic noise.It can be understood from two aspects that the protruding structure will increase thrust of rotors.One is that the added convex structure will increase the force area of the rotor and increase the lift force of the rotor;on the other hand,the front protruding structure is equivalent to the vortex generator,and a certain low pressure area is formed on the surface of the front edge of the rotor.This increases the pressure difference between the upper and lower surfaces of the rotor and enhances the lift force of the rotor.However,the vortex will increase the aerodynamic noise.For a convex structure array(or stripe structure)under the owl wing surface,the attempt is made to explore the performance improvement effect of this bionic structure on the rotor by adding a convex structure array in a certain region at the lower surface of the rotor.The experimental data show that the lower surface convex structure array can effectively improve the lift of the rotor.At the working speed,the lift force of the bionic rotor is increased by 12.9%,but at the same speed,the convex structure array of the lower surface cannot reduce the noise of the rotor.Comparing the diagram of the lift and the noise,we can find the second group of bionic rotors(the parameters of the convex structure of the lower surface are:the angle is 8 degrees,the space is 10mm,and the thickness is 1mm)has the best performance.At 3000r/min speed,the lift is increased by 8.2%compared to the prototype rotor of the 3D printing process,and raised by 6.2%compared to the prototype rotor of the carbon fiber material.The noise is reduced by 2dB than that of the carbon fiber prototype rotor.The simulation results of the lower surface convex structure array show that the lower surface convex structure array can effectively improve the lift of the rotor and have less influence on the acoustic power of the rotor surface.According to the contrast analysis of the pressure distribution on the surface of the rotor,it is found that the position of the protruding structure of the lower surface of the bionic rotor is larger than that of the prototype rotor,which indicates that the convex structure of the lower surface will have a certain adhesion to the air flow on the lower surface of the rotor,to postpone the separation of the air flow on the lower surface,the added the surface eddy current at the convex structure will increase the pressure.Considering the two aspects of acoustic and aerodynamic performances,the lower surface convex structure array can effectively improve the lift of the rotor.In the case of the same lift,the rotor speed will be reduced and the noise of the rotor can be reduced.