ABSTRACT
Nowadays, outer surface design of pasnger cars is not just a matter of styling and safety but air flow around car body and exterior accessories has significant effect on fuel consumption, performance and dominantly on the wind noi.In recent years, pasnger comfort is one of the most challenging and important automotive attributes for car makers. Controlling the turbulence eddies that caus aerodynamic noi can remarkably affect pasnger's comfort quality. Identification of aerodynamic sources is considered as the first step in order to control the wind noi.
In this rearch computational fluid dynamics method is applied to simulate the wind flow around the car and the investigation of aerodynamic noi pattern is performed by numerical method which is the most prevalent way that is ud by auto industries. By the advent of virtual simulations and by implementing the methods for the purpo of predicting and modifying in the whole car design pha, a considerable reduction in the automotive design process time and cost has been achieved. This study includes two main ctions: Firstly, identification of aerodynamic noi source around a coupe pasnger car is investigated. For this purpo after CAD modeling, preparing model for simulation is performed in preprocessing CAE software and numerical calculations are done by using finite volume method. In fact,fluctuations of pressure on external surfaces of body are considered as th
e main cau for aerodynamic noi and in practice this phenomena is detected for the identification purpo. Hence, acoustic power level is the reference parameter for studying the wind acoustic quality. In order to investigate acoustic power, broad band noi model is applied for acoustics and realizable k-ε model is ud for solving turbulence fluid. In the cond ction, rear spoiler is added
to the vehicle and acoustic effects are studied. Results are compared with each other and the acoustic effects of the rear spoiler on the rear ction of the car surface including the windshield, trunk lid and rear end parts are summarized using CAE tools.
INTRODUCTION
辉的词语Pasngers comfort is one of the most important aspects in design process for auto-industries and car makers. Noi reduction in vehicle cabin is one of the dominant attributes for pasnger comfort. Accordingly, the first step in order to achieve this purpo is Noi Source Identification (NSI).There are different main noi sources in a vehicle which cau occupant annoyance. Aerodynamic noi is one of dominant noi sources in high speed where air flow with high velocity strikes the exterior surface of the car. In fact,fluctuations of air pressure on the surfaces of car cau noi.There are many literatures in which aerodynamic noi around the car outer surface in near-field or far-field ha
ve been studied. Chien-Hsiung Tsai et al. [1] have investigated the effect of rear spoiler on aerodynamic noi in the rear side of dan car. They conclude that adding spoiler in a suitable angle has positive effect in the rear side (far-field) of the car.Yiping Wang et. al. [2] investigated aerodynamic noi that is caud by vehicle side mirror. They implemented numerical method in order to study air flow noi around side mirror.They have compared different turbulence models in order to find the location of noi sources for side mirror. They also predicted noi propagation generated by side mirror in far-field.
Zhenxu Sun et. al. [3] studied aerodynamic noi around a high speed train in near- and far-field. They have
identified
Aerodynamic Noi Source Identification for a Coupe Pasnger Car by Numerical Method Focusing on the Effect of the Rear Spoiler
2013-01-1013
Published 04/08/2013
Sajjad Beigmoradi, Kambiz Jahani, Arash Keshavarz and Mohn Bayani Khaknejad
CAE Engineer, R&D Center of SAIPA
Copyright © 2013 SAE International
doi:10.4271/2013-01-1013
locations of aerodynamic noi by using numerical method.The results indicated that the flow in the inter-coach space has greater noi level existing in the trailing surface, due to the interaction between the strong shear layer of the leading edge and boundary layer on the trailing wall. In addition, they found that the overall noi level of rear power car is very high due to the unsteady flow structures in the rear flow.Lu Dun-min et. al. [4] studied aerodynamic noi sources around a dan car by using CFD method. They have shown that fluctuating pressure has a clo relationship with flow velocity and static pressure, and flow paration is the direct cau of the vehicle external aerodynamic noi.
Yu Zheng et. al. [5] applied BEM with CFD in order to predict aerodynamic noi source around a car. They have achieved that head and tail of the car are the main aerodynamic noi radiation areas, and most of the dipole sources' SPL value is more than 70dB; the variation in car speed greatly impacts on the directivity of aerodynamic noi field near the car's tail surface.克服困难的作文
The best way in order to reduce the noi in car cabin is to reduce the power of noi sources. Hence, prior to controlling the noi sources, identification of the candidates is of high importance. In this study aerodynamic noi sources of a coupe car body are investigated by CFD method. Afterwards,variations of acoustic power level for spoiler source in different car speed and rear spoiler angles is studied. In this paper noi source identification is investigated for near-field noi and for a coupe vehicle that normally experiences high speed, which in turn could be considered as potential aerodynamic noi source. Moreover, effects of different spoiler angles are investigated in near-field noi that can be ud in controlling interior noi of car cabin.
BACKGROUND THEORY
Proudman [6] ud Lighthill's acoustic analogy to derive a formula for acoustic power generated by isotropic turbulence without mean flow. More recently, Lilley [7] re-derived the formula by accounting
for the retarded time difference, which was neglected in Proudman's original derivation. Both derivations yield acoustic power due to the unit volume of
isotropic turbulence (in W/m) as:
(1)
Where u and l are the turbulence velocity and length scales,respectively, and a 0 is the speed of sound. In Equation (1), αis a model constant. In terms of turbulence kinetic energy, k,and turbulence dissipation rate, ε, Equation (1) can be
rewritten as:
(2)
Where
(3)
The rescaled constant, αε, is t to 0.1 bad on the calibration of Sarkar and Hussaini [8] in using direct numerical simulation of isotropic turbulence.
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Acoustic power can also be reported in dB, which is
computed from Equation (4):
(4)
Where P ref is the reference acoustic power (P ref = 10−12 W/m 3).
The Proudman's formula gives an approximate measurement of the local contribution to total acoustic power per unit volume in a given turbulence field. Special consideration,however, should be taken into account when interpreting the results in view of the assumptions made in the derivation,such as high Reynolds number, small Mach number, isotropy of turbulence, and zero mean motion.
In fact, Proudman's formula is a steady state model of broad band noi source among other broad band models that can predict acoustic power level by appropriate estimation, which is the most common approach in industrial applications due to time and cost saving in vehicle design process.
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Power level is lected in this paper as a reprentative of generated noi since this parameter is the most common parameter in order to treat noi sources in near-field nois'studies. Reducing power of sources yields in overall inner and outer noi reduction, as the best and most effective solution in this regard.
SIMULATION PROCEDURE Mesh Generation
In this rearch a coupe vehicle is considered for the ca study. In order to avoid cumbersome calculations and reducing the complexity of model, the model is simplified while avoiding large deviat老鹰的英语
ions between the simulation and real ca, e.g. the effect of tires is neglected since it is intentionally disregarded to purely study the effect of the rear spoiler. Simulation of aerodynamic noi is done in virtual wind tunnel that is 17.25m×2.3m×2.3m in which the car model is located in 4312 (mm) distance from the tunnel entrance. Virtual simulation is performed via ANSYS FLUENT 14 software.
Triangle mesh is applied for surface meshing of the car model and wind tunnel. Five layers are considered to reprent the boundary layer region in order to detect the flow near the walls of body accurately and hence tracking the air flow paration pattern on the car body. In order to mesh the wind tunnel space, HEXA element type is applied for volumetric elements. Becau of the need for accuracy in the solution of air flow in the vicinity of the vehicle, two size-boxes are considered in the wind tunnel. Fine elements are generated in the box clor to the car surface and for the farther space coar meshes are generated. Mesh independency factor is checked for ba and spoiler models. In order to develop different models with different spoiler angles, morph technique is applied that facilitates changing the geometry of model without re-meshing.
Mesh independency is checked for model and final element size for car surface mesh is about 1.5cm. After solving air flow around the car and computing turbulence kinetic energy,k, and turbulence dissipation rate, ε, using equation (2),acoustic power of vehicle model can be computed.
Number of elements for ba ca is about 2.7 millions and for car model with installed spoiler is about 2.9 millions.
Figure.1 illustrates the vehicle model in wind tunnel.
Figure.1. Mesh model of the reference coupe vehicle in
wind tunnel.
Boundary Condition
In this rearch, in order to reduce the time of calculations, a symmetry plane is ud as the boundary condition.Accordingly, shear stress in tunnel side walls is assumed to be zero. Velocity for the inlet flow is considered in three different values: 90, 120 and 140 km/h. Pressure value in the outlet flow is t to 0 Pa. Model's faces are suppod as stationary walls without slip and the moving non-slipping boundary is defined for the floor surface since in practice it has a significant effect on the aerodynamic respon of the system.
Far-field density and speed of air (as wind flow properties)are considered 1.225 kg/m 3 and 340 m/s, respectively. In this investigation, realizable k-ε turbulence model with standard wall function is applied for all of the simulations. The reason behind this decision is that this model has good ability in prediction of paration flows as well as form of the eddies.Pressure-velocity coupling is defined as the simple type and standard pressure type, the first order upwind, is ud for momentum, turbulent kinetic energy & turbulent dissipation rate as spatial discretization parameters. Broad band noi source using Proudman's model is applied in order to simulate acoustic levels on the vehicle model.
Turbulence realizable k - ε model is validated for AHMED bluff body model with experimental results that is prented in literatures [9]. It is indicated that numerical model with turbulence realizable k - ε model has good agreement with experimental results.
RESULTS AND DISCUSSION
As stated before, this study includes two ctions. At first, the aerodynamic noi identification is done around car by considering rear spoiler. The rear spoiler has significant positive effect on vehicle stability especially for a high speed car such as the one in this rearch. In this investigation studies are performed in three different speeds for all cas:90, 120 and 140 (km/h).
Aerodynamic Noi Source Identification
As illustrated in figure.2, for the coupe car without rear spoiler, front bumper region can be introduced as the first rank among other aerodynamic noi sources. Considering vehicle dynamics and lateral stability concerns, adding rear spoiler is inevitable in sport vehicles. Therefore studying this ca by considering rear spoiler is more applicable.
Figure.2. Contour of Acoustic power level around car
without spoiler As shown in figure.3, the regions around car body in which speed of air flow increa
s is likely to generate aerodynamic noi. In the regions paration occurs and accordingly powerful turbulent eddies are generated. In practice, rear region of the car, where air slides between trunk and rear spoiler, velocity of flow increas.
Generally, rear spoiler and front bumper of vehicle can be identified as first and cond powerful sources of noi,respectively. Conquently, edges of wheel hou and A-pillar can be placed in the third and forth ranks of
aerodynamic noi sources.
Figure.3. Contour of Acoustic power level around car
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with spoiler
Aerodynamic Noi Source Power of Spoiler in Different Angles
Secondly, in order to investigate the aerodynamic noi sources that can cau interior and exterior noi, the acoustic power level is computed in different cas (illustrated in Table. 1).
The effect of spoiler is investigated in ven different angles and three different speeds as well. Each state of rear spoiler (with its endplate) is shown in Figure.4.
The main purpo of introducing the spoiler to the rear end of a car is to increa its lateral dynamic stability by controlling the down force in high-speed maneuvers. On the other hand,installing rear spoiler will result in a new dominant aerodynamic noi source in car. As illustrated in Figure.5, in each angle noi acoustic power ris by increasing the vehicle crui.
Table.1. Different Cas
Figure.4. Configurations of rear spoiler at different农民英语怎么写
angles
Figure.5. Maximum acoustic power level in different
rear spoiler angle As it is depicted in Figure.5, the ca with +5-degree spoiler angle (Ca 3) has the minimum value of acoustic power level in all speeds; therefore this ca can be considered as the favorable ca from interior and exterior noi perspective. Acoustic power level distribution for +
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5-degree rear spoiler angle at 90km/h is depicted in Figure 6 as the optimum ca for acoustic source among all different cas of rear spoiler. Moreover, by increasing the spoiler angle from 5up to 17 degrees, acoustic power level corresponding to the
rear spoiler increas.
Figure.6. Acoustic power level around car model with 5-degree Spoiler [90 km/h]As shown in Figure.5, it is conceived that the angle within the range of ±5 degrees (in accordance with chord line) has the
optimum acoustic power level according to the results.
Figure.7. Pressure coefficient around car model with +5-degree Spoiler (Ca 3) [90 km/h]Variations of static pressure on body surfaces create aerodynamic noi. The pressure fluctuations are generated by changes in surface topology, e.g. height, sharp edges and corners. As depicted in Figure.7, the locations where the pressure coefficient has negative value can be considered as corresponding aerodynamic noi sources. This means that by optimizing the shape of body where the negative pressure coefficient has its maximum value, the appropriate shape can be achieved from the air-born noi point of view. The path-
lines over the model are depicted in Figure.8.
Figure.8. Stream lines of air around model with +5degree (Ca 3) angel. (Trailing Vortex) [90 km/h]
