STUDY OF AERODYNAMIC INTERACTION BETWEEN CARS AND ROAD

Summary

The project tackle an actual subject in the research fields of cars design, which also represents a constant preoccupation of the engineers in order to have an optimum airflow around cars, essentially in: - obtaining of a minimum drag for optimising of fuel consumption and minimising of the pollution; - minimising of the noise generated by the aerodynamic interaction between cars and air, for an adequate acoustic confront of the participants in traffic. - Increasing of the safety for passengers through increasing of cars stability, handling a. o. The approaching, specific for avehicles erodynamics in ground effect, is a modern one, being used in researches: - classical methods, namely the experimental and theoretical ones; the last mentioned are rather used in the aerodynamics of cars, which is mainly based on experiments; - modern methods, in virtual environments, using the facilities of the professional CFD codes. The main references for this study consist on the results of the researches presented in the doctoral thesis of the project manager, which give new opportunities for study in the field through: - establishing of an original theoretical method for aerodynamic interaction between cars and road; - improving of the CFD procedures for cars. In this context, the followings are the main goals of the project: - developing of the theoretical procedures for solving of the road vehicles aerodynamics in ground effect. - improving of the methods for cars analysis in virtual environments. - improving of the experimental procedures for cars evaluation - establishing of the noise level due to the aerodynamic interaction between cars and road in order to find solution for minimising of this.

Scientific content

Because the aerodynamic loads, which are acting on a car, play a significant part concerning the dynamic behavior of the latter as regards stability, handling, crosswind sensitivity, wind noise and not lastly upon the fuel consumption, today the aerodynamics becomes an important design consideration [1]. [1] Sumantran, V., Sovran, G., Vehicle Aerodynamics, PT-49, SAE International, 1996. The idea of applying aerodynamics to road vehicles cames up from the flight technology, when in this area were made considerable progress. For both aircrafts and cars, streamlined shapes were developed with lowered drag significantly, in order to permit higher cruising speed for a given engine power. The early attempts to streamline cars were made in Europe, 25 years ago from the first car building. Thus, in 1922, Klemperer was published the paper Investigations of the Aerodynamic Drag of Automobiles, and show a minimum drag coefficient, 15.0cx = for a half body of revolution, on wheels [2]. A cars having similarly shape was patented and build by the Romanian engineer Aurel Persu in 1922 [3] [2] Hucho, W,. H., Aerodynamik der Stumpfen Korper, ISBN 3-528-06870-1, 2002 [3] Cazacu, M., D., Arama, C., s.a., Aurel Persu Inventator al Automobilului Aerodinamic, Editura Tehnica,

Bucuresti, 1996, ISBN 973-31-664-X Chronologically, were made studies concerning the effect of the aerodynamic loads on cars stability, performed by a researchers team lead by Kamm [4], which founded The Research Institute of the Road Vehicles in Stuttgart (in present integrated in Technological University of Stuttgart. In USA, Chrysler and Ford Companies made the first studies of vehicles aerodynamics in the beginning of 1930s. These were systematized only in the 1950s, when the aerodynamics improves significantly the performances of the aircrafts. [4] Scibor-Rylski, Road Vehicle Aerodynamics, Pentech Press, London, ISBN 0-7273-18047, 1979 Till to the end of 1970s, the main preoccupation of the engineers was the improving of the cars mechanics, the

aerodynamics having a secondary role. The two oil crises of this decade generated great pressure for improving fuel economy drastically and provided a breakthrough for vehicle aerodynamics. Since then, drag coefficient has come down dramatically, this being one of the major problems in the design of cars. Till recently, the exterior shape of the vehicles was represented the main preoccupation of the engineers, the underbody geometry playing a secondary role, or being neglected for some type of vehicles, as for off-road ones. In the last decade, in order to reduce more and more the values of the drag coefficient of cars, the underhood flow management becomes a major problem of the designers, too. Recent studies show that for a modern car the basic shape stands for approximately 45%, wheels and wheel-housing for 30%, and the floor and detailing for 25% of the total drag [5, 6]. As there can be observed a significant potential for further improvements are to be found in the underbody flow. Today, this represent the main research issues of the designers [1], the team members of project having also contributions (see 7.2). [5] Emmelmann, H., J., Berneburg, H, Schuze, J., The Aerodynamic Development of the Opel Calibra, Vehicle

Aerodynamics PT-49, SAE Inc., ISBN 1-56091-594-3, 1996. [6] Perzon, S., Underbody Flow, VOLVO Report, 2002 Thus, there are performed complex studies to reveal the contribution on total drag for each component of the lower structure of car. These become possible due to the improving of the experimental techniques for underhood flow investigation [7], using moving belt devices. With this technique, the relative motion between cars and road can be simulated, also the properties of the boundary layer at the level of road [8, 9, 10]. [7] Graves, S., Investigation of a Technique for Measuring Dynamic Ground Effect in a Subsonic Wind Tunnel,

NASA Technical Report, NASA/CR-1999-209544, 1999. [8] Carr, G., Eckert, W., A Further Evaluation of the Groun-Plane Suction Method for Ground Simulation in

Automotive Wind Tunnels, Vehicle Aerodynamics PT-49, SAE Inc., ISBN 1-56091-594-3, 1996. [9] Cogotti Antonello, Ground Effect Simulation for Full-Scale Cars in the Pininfarina Wind Tunnel, Vehicle

Aerodynamics PT-49, SAE Inc., ISBN 1-56091-594-3, 1996. [10] Curry, R., Moulton, B., Kresse, J., An In-Flight Investigation on Ground effect on a Forward-Swept Wing

Airplane, NASA Technical Report, NASA TM-101708, 1989 Modern cars have very lowered drag coefficients, references being:

27.0cx = - BMW 318i, VW Passat, Mercedes C-class, Lexus LS400; 26.0cx = - Opel Calibra, Mercedes C180;

25.0cx = - Honda Insight, Lexus LS430, Audi A2. In the same time, doe to the developing of the computing machines and also of CFD (Computational Fluid Dynamics) codes, especially concerning their facilities for visualising of the results, the aerodynamics of cars is made successfully in the recent years in virtual environments, too [11, 12, 13]. [11] Akanni, S-D., Running RAMPANT. Computational Fluid Dynamics in Formula 1 Design, Journal Articles by

FLUENT Software Users, JA075, Fluent Inc. 1999. [12] Kleber, A., Simulation of Air Flow Around an Opel Astra Vehicle with FLUENT, Journal Articles by FLUENT

software users, JA132, 2001. [13] Huminic A., Chiru A., Comparative Study of Different Ground Simulation Techniques used in Numerical

Investigation of Vehicle Aerodynamics, Bucharest Politehnica Press, The 7th International Conference, FUEL ECONOMY, SAFETY and RELIABILITY of MOTOR VEHICLES, ESFA 2003, Volume 1, ISBN 973-8449-10-3

In today’s competitive environment, there is a need to shorten product development time and bring new designs to the marketplace more quickly. In consequence, the automotive designers need aerodynamic guidance on the shape of vehicle, very early in the design process. One proper answer to this need is represented by CFD analyses. The primary reason of using numerical methods in design of vehicles is that they can generate information before a testable model even exists. In addition, CFD analyses are not necessarily burdened with the limitation of size and geometry of the test section of the wind tunnels. In this sense, computational space can be made large enough to eliminate blockage effects, according with hard resources. Also, simulation of ground effect* and rotations of the wheels are comparatively easy to accommodate. On the other hand, once the equations of mathematical model have been solved, there is much more information (see Figure 2) available than from a routine experiment. But, because the accuracy of the CFD results is lower than experimental ones, using wind tunnels [14, 15], this method is used for additionally informations in the early design stage of cars. [14] Barlow, J., Rae, W., Pope, A., Low-speed Wind Tunnel Testing, Third Edition, USA, 1999. [15] ***, Aerodynamic Testing of Road Vehicle – Open throat Wind Tunnel Adjustement, SAE J2071 JUN94, SAE

Information Report. In this context, the followings are the main goals of the project, which is tackle an actual subject in the research fields of vehicles aerodynamics in ground effect: - developing of the theoretical procedures for solving of the road vehicles aerodynamics in ground effect. - improving of the methods for cars analysis in virtual environments. - improving of the experimental procedures for cars evaluation - establishing of the noise level due to the aerodynamic interaction between cars and road in order to find solution

for minimizing of this.

As aerodynamic phenomenon, the ground effect represents the effect of the air between a moving vehicle (air-vehicle and/or ground-vehicle) and a thick boundary. In many cases this cud be the ground, but also the surface of water, for example. It is results in a change of the aerodynamic behaviour and depending of vehicle has various ways of acting. Concerning the aviation’s point of view, two phenomena are involved when a wing approaches the ground. Ground effect is one name for both effects which is sometimes confusing. These two phenomena are referred to as span influence and chord influence ground effect. The former results in a reduction of induced drag and the latter in an increase of lift. When aeronautical engineers mention ground effect they usually mean span dominated ground effect. The two main sources of the global drag are friction drag and induced drag, which is sometimes also called lift induced drag because it is the drag due to the generation of lift. When a wing generates positive lift the static pressure on the lower side of the wing is higher than on the upper side. At the wingtip the high pressure area on the lower side meets the low pressure area on the upper side therefore the air will flow from the lower side to the upper side, around the wingtip. This is called the wingtip vortex. The energy that is stored in those vortices is lost and is experienced by the aircraft as drag. When the wing is approaching to the ground, there is not enough space for the vortices to fully develop and the induced drag is decreasing. The ground also pushes the vortices outward and apparently, the effective aspect ratio of the wing becomes higher than the geometric aspect ratio, having as results an increasing of the lift. This is a common way to account the influence of the span on ground effect. Concerning the influence of the cord, ground effect not always increases the lift. It is possible under certain conditions that lift is reduced when an airfoil approaches the ground. In the case when the bottom of the foil is convex and the angle of incidence is low, a Venturi tunnel is created between the wing and the ground and the high-speed low-pressure air sucks the airfoil down, revealed also by the study performed for Clark Y 11.7% airfoil [16].

[16] Huminic, A., Lutz, T., 2004, CFD Study Concerning the Behaviour of the Clark-Y Airfoil in Ground effect,

Research report, HPC-Europa, RII3-CT-2003-506079, Stuttgart University, Germany

This Venturi-type ground effect is used by the high-speed cars which have the under-body designed especially to make this effect. The phenomenon of the ground effect is very well emphases in the cases of the racing cars, as Formula 1. They combine the two ideas, mentioned in previously: to have a wing running near to ground level to exploit ground effect and to use the underside of the car to form a Venturi tunnel. The first ones who exploit this effect were the engineers of McLaren team in 1980s Concerning road-vehicle point of view, some researcher are using the term of ground effect when are mentioning the relative motion between the road and vehicles when these are experimental evaluated in wind tunnels. This is also called ground simulation.

Objectives and methodological approach

The approaching, specific for aerodynamics in ground effect, is a modern one, being used in researches: - classical methods, namely the experimental and theoretical ones; the last mentioned are rather used in the

aerodynamics of cars, which is mainly based on experiments; - modern methods, in virtual environments, using the facilities of a state of the art CFD code. The main references for this study consist on the results of the researches presented in the doctoral thesis of the project manager, Study of the Aerodynamic Interaction between Cars and Road, co-ordinated by Professor Anghel CHIRU, Transilvania University of Brasov. The thesis give new opportunities for study in the field through:

I. Establishing of an original theoretical method for aerodynamic interaction between cars and road throgh total drag D decomposition in two components, according with the Equation (1):

ubext DDD += (1)

The first term extD , is the drag due to the flow upon the external surface of the vehicle, having the rate flow extQ .

The second one ubD , represents the drag due to the flow under the body of vehicle, in the space determined by

the lower surface of the vehicle and the road, having the area hb× , treated as a convergent-divergent air nozzle with the flow rate ubQ ( infQ ), see figure 1.

Fig. 1 – Airflow aroun car

For the second component of the drag of vehicles was proposing the following Equation:

∞=

vv

2h bD

3n eub

ρζ (2)

where: enζ coefficient of the equivalent hydraulic rezistance of the nozzle;

v average velocity of the air through the section of the nozzle; ρ air mass density;

∞v reference air velocity.

Also, the following dimensionless indicators were defined to characterise the underhood airflow:

ubDK : coefficient what represent the ratio between underbody drag and global drag defined as

product of three dimensionless factors, Equation (3);

3

D

n eubD v

vAh b

CDD

Kub

==

∞

ζ (3)

where:

D

n e

Cζ

is the relative drag;

Ah b

is the relative area;

( )3v/v ∞ is the relative velocity.

ubQK is the coefficient what characterise the participation of the underbody flow rate on total flow

rate, Equation (4):

QQK ub

Qub= (4)

In this way, the underbody drag coefficient

ubDC can be expressed with the Equation (5):

3

n eDDD vv

Ah bCKC

ubub

=⋅=

∞

ζ (5)

II. Improving of the technique for CFD analyses of airflow around cars, taking into consideration:

The relative motion between cars and road; Wheels motion.

For these two additional movements**, there were studied the changing on aerodynamic characteristics of a car, ARO 26, experimental model or ARO SA, Romanian Automotive Company.

** Usually, the CFD engineers are representing the road as:

l b

h

8v

Qext

Qinfh avamh v

- solid, fixed surface; it is the simplest and most common way to represent the road; the relative motion

between car and road is not simulated, but this method is useful when the experiments for validation are performed in wind tunnel having the testing chamber with fixed walls.

- by means of mirror methods; the surface which represent the road is treated as symmetry plane (Euler wall);

this technique provides good result for bodies with no relative motion between their components, as aircrafts structures; for cars having the wheels in motion, this treatment of the ground affect in negative way the results; taking into consideration of the wheels’ motion is important for a better evaluation of the aerodynamic characteristics of the vehicle because this is the only way in which Magnus effect surround the wheels can be reproduced;.concerning these, the first reported CFD studies regarding the influence of ground effect, in order to simulate the relative motion between vehicle and ground combined with the motion of the wheels was made by the project team members [17, 18];

[17] Huminic, A., Huminic, G., Numerical Investigation of Vehicle Aerodynamics with Ground Effect Simulation -

Modelling and Optimization in the Machines Building Field MOCM – 8, ISSN 1224–7480, 2002. [18] Huminic A., Chiru A., Ground Effect in Design of Vehicle, FISITA 2004 World Automotive Congress,

F2004F130.

Theoretical and CFD studies were accompanied by experimental research on 1:6 scale model of car using the facilities of the subsonic wind tunnel, presented in Annex 7. In order to reproduce the relative motion between vehicle and ground, a moving belt device was design and build. The necessity for automation of this device was showed, as in similarly studies [19]

[19] Mercker, E., Wiedemann, J., Comparision of Different Ground Simulation Technique for Use in Automotive Wind Tunnels, PT-49, SAE Inc., ISBN 1-56091-594-3, 1996.

In this context, the followings are the main goals of the project: - Developing of the theoretical procedures for solving of the road vehicles aerodynamics in ground effect.

- Finding an analytical expression for the coefficient of the equivalent hydraulic rezistance of the nozzle enζ ;

- Establishing of the analytical dependences between the main parameters which are defining the interaction of cars with road from aerodynamic point of view:

- Geometry of the lower structure ob cars; - Type of the roadway; - Cruise speed; - Intensity of the road traffic.

- Finding an optimum configuration for lower geometry of cars from fuel consumption point of view.

- Improving of the methods for cars analysis in virtual environments, through:

- Carrying out of studies concerning the modern facilities offered by professional CFD codes; we mention as references: FLUENT, ANSYS CFX, CFDesign.

- Finding a procedure specific for CFD analyses of cars, concerning:

- Type and necessary dimension for computational domain and grid; - Boundary condition for surfaces which are defining the roadway; - Proper turbulence models; - Useful convergence criterias; - Post-processing of the results.

- Making of some CFD analyses with ground effect simulation for representative type of cars;

- Finding the influence on total drag for each component of car structure (wheels, chassis with reinforcing

frames and bracing rib, front and rear main suspensions, rear transmission, components of the exhaust of burnt).

- Developing of the experimental procedures for cars evaluation: in this sense the automation of the moving

belt device is an objective of this project; there will be study: - Influence of the used method in order to reproduce the ground, using a flat wall and the moving belt device. - Influence of the ground clearance on the main aerodynamic characteristics; - Influence of the wheels motion; - Validation of theoretical and CFD studies; - Practical solutions in order to improve the aerodynamic behaviour of cars in ground effect.

- Establishing of the noise level due to the aerodynamic interaction between cars and road in order to find

solution for minimising of the noise From managerial point of view, the objectives are the following:

- Increasing of the research team competences and national and international visibility;

- Developing of new relationship with automotive companies, for increasing of technological transfer from academic

institutes to industry.

- Developing of the research infrastructure for the Centre of Excellence CESAMET (Centre of Excellence in Automotive and Thermal Equipment), from Transilvania University of Brasov, coordinated by Professor Anghel CHIRU, PhD.

Original elements

As shown in previous sections, the originality of the project is given by the theoretical and CFD approaching. Also, from experimental point of view, tests of the level noise due to the interaction between lower structure of cars and ground represent newness. Although the experiments with moving belt devices were performed till now, these types of devices represent a necessity for validation of theoretical and CFD results, because this is the only technique, at least in principle, is capable of matching all the on-road properties, including the boundary layer [20].

[20] ***, Aerodynamic Testing of Road Vehicle – Testing Methods and Procedures, SAE J20784 JAN93, SAE Information Report.

In classical aerodynamics theory, the total drag D stands for the sum of two distinct components, as follows:

fp DDD +=

where

pD the drag due to the distribution of the pressure upon the surface of the vehicle, dependent on its

shape; fD the drag due to the friction between the air flow and the surface of the vehicle;

This decomposition of the drag force is purely formal as regards complex shapes as those of road vehicle, the separate determination of each component being practically impossible, requiring the precise knowledge of the distribution of pressures on the entire surface of the motor vehicle, of the tangential stress of friction from the boundary layer. Aerodynamic load can be computed analytically with some very complex procedures only if the geometry can be described exactly mathematically. This situation is specific especially for the aeronautical structures, as wings, empennages, propeller blade etc.

In this context the previously presented theory has the advantage of practical approaching with optimising of lower geometry in early design stages of cars [21], see Figure 3.

[21] HUMINIC, A., CHIRU, A., Aerodynamics of the Underhood Airflow for Road Vehicles, FISITA 2006 World Automotive Congress, Yokohama, Japonia (acceptata spre publicare: http://www.fisita2006.com).

Fig. 3 – Diagram of

infxc parametric variation as function of infQK for studied case,

ARO 26, for groung clearance in range h 100%)- % (65 , [21] Tte diagram reveal an ompinul value of

infxc versus infQK and h

Concerning the vehicles aerodynamics in virtual environments, with relative motion between cars and road considering also wheels rotation, we have published results [22] in the most prestigious scientific event of automotive industry, SAE Word Congress 2006, Detroit, USA.

[22] HUMINIC, A., CHIRU, A., On CFD Investigations of Vehicle Aerodynamics with Rotating Wheels' Simulation,

Vehicle Aerodynamics 2006, SAE Technical Papers SP1991, ISBN 0-7680-1726-2, Paper number 2006-01-0804 - SAE 2006 World Congress, April 2006, Detroit, USA

Obtained results till now and planed ones in this project, can led to optimising practical solution in design of vehicle, in order to minimising the fuel consumption and noise generated by the aerodynamic interaction between lower structure of cars and road.