Study Of Aerodynamic Noise For The Vehicle With High Speed In The Exterior Flow

The aerodynamic noise around ground vehicles at high speeds hasbecome increasingly important and in more and more cases dominates thevehicle noise. It is attributable to the facts that the noises fromengine,tire and power train and other noise sources have beensignificantly reduced and the speeds of ground vehicles constantlyincrease in the past decades. It is therefore crucial to carry out thestudy of aerodynamic noise and explore proper countermeasures forsuppressing its contribution to the vehicle noise. The study of theaerodynamic noise is more and more favour in the world actually.The study actuality and conclusion in home and aboard is discussed.On the base of it ,the mostly content is put forward namely: recommendthe theories and study methods of the aerodynamic noise, make a try tohave a simulation of aerodynamic noise for the vehicle with high speedthe simultaneity.In this dissertation, the theory of classic aerodynamic acousticsis applied to study flow noise from a ground vehicle. First, by usinghalf-space Green's function, Lighthill's equation is solved to obtainthe integral formulation for the calculation of the flow noise outsidethe vehicle, with extensive consideration of the effects of roadconditions and the vehicle movement etc on it. Second, a systematictheoretical investigation is performed to understand the directivitypattern and velocity characteristic, as well as the dependence of theaerodynamic noise and fluctuating pressure on outside surface ofvehicle(FPOSV) upon the vehicle speed. Bernoulli's equation, Green'sfunction and correlation function theories are used for thistheoretical analysis purpose. The above-mentioned characteristics andthe flow pattern around the vehicle, especially in the region aroundthe A-pillar, are further investigated through in-situ noise level andflow field measurements on a scaled vehicle model and a side window model.Both wind tunnel and road testing which are carried out is recommendedAt the same time, the simulation of aerodynamic noise is the mostof the thesis, but it is only primary and tentative.Ahmed is the model which will be calculated. To catch the flow fieldcharacteristics, the computational domain must be big enough becausethe carbody is yawed. The dimensions is 6 times height of the model forthe height, 8 times the length of the model for the length and 5 timesthe width of the model for the width.The meshes combined of hexahedron meshes ,because the model issimple and the simulation resqust good meshes. The density of the meshesis designed according to the flow field, the meshes is compressded nearthe model and the block is pressed close to the surface of the model.The mesh examined is satisfying, now we can calculate.Star-cd is applied in the simulation by LES. The outer flow fieldcharacteristic of the car and the distribution of the fluctuatingpressure was researched. The numerical value of the fluctuatingpressure is exported and is calculated by FFT in matlab. Then theaerodynamic noise is analyzed by the spectral decomposition method suchas frequency spectrum analyse and the picture of one-third octave filterso that the connection between the fluctuating pressure and theaerodynamics noise and the distribution of the aerodynamics noise Wasdiscussed.It is known that when the vehicle run with high speed, there is aacute action between the vehicle and the outer flow field, and a boundarylayer comes into being. At the time, vortex, onflow and separate flowform. The vortex and onflow act on one another, then give birth tostrong fluctuating pressure. The fluctuating pressure out of thevehicle bring aerodynamic noise. The laminar flow from far away dividesinto two parts when meets the front of the car, and form vortex. As thecomplexity of the figure of the car, the flow occur a big separationat the big corner. The vortex flow exquisite at the A-pillar and theside window,then arose strong fluctuating pressure, it is the main partsof the aerodynamic noise, besides,in the accessories of a side window,many sub-distributary come into being ,and bring very complicateddistributary. Another flow through the clearance between the car andbackground forms complicated vortex among below the car and the cavumof the tyre In the back of the car ,two large tail vortex in theopposition diretion. All of these can bring aerodynamic noise. on theside,the accessories of surface can bring 5-20dB abourt noise.Major concluding remarks of the dissertation can be drawn as follows.The aerodynamically induced noise around a vehicle comprisesquardrupole, dipole and monopole sources. It is similar to noisecharacteristics generated by a fluid now in which there are immobilesolid boundaries in free acoustic field. However, the directivitypattern of aerodynamic noise around a vehicle is relevant to vehiclespeed,and the radiation and propagation of the aerodynamic noise ismore like that in a semi-free acoustic field. Under the same conditions,the aerodynamic noises of monopole, dipole and quardrupole tend toincrease with the fourth, sixth and eighth power of flow speedrespectively.Since most vehicles run at the speed of which the Mach number isless than 0.2, their aerodynamic noise is dominated by a dipole source.It tends to increase with the sixth power of the vehicle speed and hasa clear directivity. Because the dipole noise around vehicle isdetermined by the force with which a unit area of vehicle surface actsupon the fluid, the FPOSV is main aerodynamic noise source. Furthermore,Both correlative wind tunnel experiments and the numerical simulationsshow that The aerodynamic noise is bigger when FPOSV is stronger;Thereare separate areas and attachment areas,the separate areas is more 5-20dB than the attachment areas;FPOSV is also dominated by low frequencycomponents and decreases as frequency increases without evident mostlyfrequency, but frequency increases slowly from the 1000 Hz abourt.Because vehicle aerodynamic noise issue is very complicated, thereisn't much related research, the basic subjects involved in the problemsare not mature, present study work can't be put into practice, and thereare a lot of problems to be solved. This paper is just the introductionof aerodynamic noise appraisal.