16 Fluent NEWS fall 2003

automotive

FLUENT “In Gear”By Erik Ferguson, Fluent News

The “paintability” of a car body, that is, thecompatibility of the painting process with the mate-rials that are used in the body shop, has been inves-tigated by Germany’s Fraunhofer Institute forManufacturing Engineering and Automation(IPA). Within the painting process, spray paintingplays an important role in creating the final tech-nological and optical properties of the paint film.Specifically, the IPA has been interested in predictingthe film thickness distribution and the efficiencyof paint transfer to the work piece being paint-ed. Using FLUENT, the coating processes of elec-trostatically-supported rotary bell atomizers andother paint-transfer processes can be simulated.

When modeling a rotary bell atomizer in par-ticular, it is essential to define proper inlet con-ditions for both the flow of air and the flow ofpaint droplets. Operating on a hybrid unstructuredmesh of 400,000 cells, FLUENT was used to cal-culate the full 3D multiphase, turbulent flow fieldand the electric field. In the practical applicationof spray painting, the atomizer moves across thetarget. This results in a dynamic film thickness dis-tribution, which is the major atomizer characteristicused in the industry. Using the static spray pat-tern calculated by FLUENT, the dynamic film thick-ness was derived by artificially moving the spraypattern along a straight line and integrating the

mass over the distance. Through this integrationprocedure, the calculated film thickness and trans-fer efficiency were found to be in excellent agree-ment with experimental results.

For now, the spray film patterns obtained fromFLUENT are being used by the IPA to programthe modules for dynamic painting on real car bod-ies. In the future though, it will be necessary toperform true unsteady calculations that will takethe speed, acceleration, and pathlines of the paint-ing robots into account as progress is made towardmore advanced spray simulations and the devel-opment of new atomizer technology.

For those who might say “Ich bin ein gear-head”, it makes sense that the land of the Autobahnand other awesome automotive achievement would play host to a gathering of peoplewho design the machines that we drive and those at which we marvel.

On June 25 – 26, 2003, engineers and researchers from many nations converged on Bingen,Germany for the inaugural European Automotive CFD Conference (EACC) to broaden theirknowledge of how new fluid flow challenges in the realm of vehicle design and productionare being met using computational fluid dynamics. Among the CFD-based designs highlight-ed at the conference were improvements in powertrain cooling, drag reduction, fuel-injectionequipment, exhaust gas recirculation, and even how a car gets painted. These and many othercontributions demonstrate a wide range of applied CFD work being carried out today in Europeand the increasing shift toward using flow simulations in automotive engineering.

Comparison between measured (left) and calculated (right) static film thickness distribution Courtesy of Fraunhofer Institute for Manufacturing Engineering and Automation (IPA)

First Gear – The Spray Painting Process

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automotive

with Car DesignersSecond Gear – A More Durable Fuel Injection System

Moving inside the engine, Delphi Diesel Systems of the United Kingdomis using CFD modeling to assess damage to electronic unit injectors (EUIs)by cavitating fuel. Modern EUIs, which help increase fuel efficiency anddecrease emission levels, employ ultra-fast solenoid-driven valves thatenable injection pressures of 2000 bars while maintaining complete elec-tronic control over the injection timing. The high speeds of the valves,however, produce pressure pulses in the fuel between the moving parts,which can lead to the creation of partial vacuums, or cavitation, if thepressure falls below the saturation point. The collapse of these bubblesin the liquid fuel surrounding the valves causes the solid surfaces of thevalve actuator, namely the stator and armature, to wear prematurely,making the injector less reliable.

A historical approach for avoiding cavitation is to vent the areas oflow pressure on the valve actuator by placing holes in the interpole grooveof the armature. After experiments with this approach did not achievethe desired result, Delphi decided to use FLUENT’s deforming mesh capa-bility to provide a better understanding of the physics driving the observedcavitation damage. Simulation of designs both with and without ventholes predicted low-pressure areas that were in the same location asthe cavitation damage observed in experiments.

From the results of the simulation, it was then hypothesized that oscil-lations in the pressure caused by the flow path of the diesel fuel werecontributing to cavitation. Subsequently, it was proposed to remove boththe vent holes and the interpole groove from the armature so that thepressure distribution in the actuator would be more even. These designchanges were eventually accepted and proved successful in solving thecavitation problem while maintaining the performance of the injectionsystem.

Another engine-related challenge for reduc-ing pollutant emissions is the cooling of recir-culated exhaust gas. The Spanish firms CIDAUTand DAYCO ENSA, in collaboration with the University of Valladolid in Spain, havesought to optimize the geometry of an exhaustgas recirculation (EGR) cooling system to fur-ther improve the reduction of NOx andunburned hydrocarbons in diesel engines.

An EGR cooler is a multiple-tube heatexchanger that uses water to cool exhaustgas before it is recirculated to the engine.In a typical EGR cooler, the exhaust gas (hotfluid) flows inside the tubes, and the water(cold fluid) flows around the outside of thetubes. To produce more efficient heat exchang-ers, it is common to use corrugated tubes.The corrugations increase the surface areaavailable for heat exchange. Additionally, theyintroduce a tangential velocity inside the tubes,which increases the level of turbulence in the

gas flow and further improves the heat trans-fer. Along these lines, parametric studies ofpressure loss and heat transfer were doneusing FLUENT to determine the influence ofdifferent corrugated tubes.

Experimental results confirmed the ten-dencies in heat transfer efficiency predictedby the FLUENT numerical model, with an over-all difference of less than 5%. The findingsdemonstrated that as the degree of tube cor-rugation increased, so too did the efficien-cy of heat transfer and the pressure loss dueto increased turbulence. Although pressureloss was an important parameter to consid-er, maximizing the heat transfer efficiency with-in the limits of the system was deemed themost important goal of the simulation. Bycorrugating the tubes, the size and cost ofthe heat exchanger equipment can also bereduced, thus making compliance with emis-sions regulations more cost-effective.

Cross-sectional view of the E1 injector andarmature used in one of the CFD modelsCourtesy of Delphi Diesel Systems

Absolute static pressure for animproved armature design thatprevents the onset of cavitationCourtesy of Delphi Diesel Systems

The EGR system (yellow, green, and red)Courtesy of CIDAUT, DAYCO ENSA, and University of Valladolid

EGR cooler

Contours of temperature for two types of corrugated tube Courtesy of CIDAUT, DAYCO ENSA, and University of Valladolid

Third Gear – Analyzing Exhaust Gas Recirculation

18 Fluent NEWS fall 2003

automotive

The class of automobiles known as sport util-ity vehicles (SUVs) has faced challenges in the areaof emissions control and gas mileage, due main-ly to the higher weight and increased drag of thefour-wheel drive chassis that has been the stan-dard base of construction. In 1999, Sweden’s VolvoCar Corporation instead decided to enter the SUVmarket using a saloon-car, or sedan platform asthe base. The goal was to combine the level ofride comfort and crash safety of a sedan with thegreater visibility and interior space of a standardSUV, while also offering predictable handling char-acteristics and competitive fuel consumption. Afinal challenge was to bring the new vehicle, knownas the XC90, to market by the end of 2002, whichled to the removal of early prototype testing andan increased reliance on CFD to aid in the finalproduction design.

Given these challenges, it was identified thatthe flow rate of air required to cool the enginecompartment would need to be increased by 50% compared to Volvo’s S80 sedan, upon whichthe XC90 design was based. CFD was chosen for

use in the XC90 project, with a focus on the cool-ing air mass flow as the evaluation parameter.

Using FLUENT, an initial 2D study identifiedthe parameters in the vehicle front-end design thatplayed essential roles in affecting the flow rate of air entering the engine compartment.Subsequently, several full 3D simulations of thecar addressed questions related to the fan posi-tion and performance, heat exchanger layout, andshape of the rear bumper beam. The target of 50%more cooling air flow for the Volvo XC90 com-pared to the Volvo S80 was successfully achievedfor the final design. Another project was initiat-ed to couple the cooling air flow to the hot side

of the heat exchangers using user-defined func-tions (UDFs) in FLUENT. Favorable comparison withexperiments has led Volvo to use CFD in all proj-ects where cooling performance is addressed.

A major advantage of using CFD in the XC90project was the ability to have questions aboutthe effects of changing a parameter answered asquickly as the next day. Whether it was by fast2D or highly-detailed 3D simulations, CFDplayed an important role in helping Volvo meettheir cooling airflow targets by offering valuablebackground information that could be used to helpbalance increased airflow with the overall exte-rior design vision.

In the domain of high-performance sportscars, diesel engines have recently gotten aperformance boost in the form of the ECO-Speedster. Developed by Adam Opel AG inGermany, and presented at the 2002 ParisAuto Show, the ECO-Speedster was a con-cept car intended to showcase the progressmade in diesel technology, including a fuelconsumption rate of 2.5 liters per 100 km(94 mpg) and a targeted top speed of 250km per hour (155 mph). Achieving thesegoals required a low drag coefficient anda lightweight chassis in combination withan efficient engine. Having already devel-oped a 112 horsepower 1.3 liter common-rail direct turbo-charged injection (CDTI) diesel engine and a carbon-fiber body, Opel’sremaining task was to optimize the exter-nal aerodynamics. Due to the short time-frame available for aerodynamic development,the decision was made to integrate CFD sim-ulation into the design process to supple-ment the standard wind-tunnel testing.

In the beginning, early sketches of thecar led to the creation of a one-fifth scaleclay model, which was then digitized andconverted to a CAD model. Using FLUENTand a hybrid volume mesh comprised of four

million cells, an external airflow simulationwas carried out. Here, regions were iden-tified for improvement as a basis for the opti-mization that was then performed duringsubsequent tests in a model wind tunnel.As soon as the drag targets were met andthe necessary body styling requirements imple-mented, construction of a full-scale proto-type began. Five months later, on-road testingof the completed prototype showed that theengine and overall design concept produceda measured top speed of 264 km per hour(162 mph), which exceeded the target.

After more than 200 shape variations inthe model wind tunnel and 25 CFD runs,the final test session in the full-scale windtunnel revealed that a drag level 5% belowthe targeted value was achieved. Additionally,the wind-tunnel tests showed that it was pos-sible to predict the drag coefficients of theECO-Speedster using FLUENT to within 0.5%, which was further validation for usingCFD as a part of the design process.

During its 24-hour record trial atDudenhofen Proving Ground in June2003 the ECO-Speedster finally managedto beat 17 world records for diesel-powered vehicles.

Contours of pressure coefficient onthe Volvo XC90 and flow pathlinesCourtesy of Volvo Car Corporation

Aerodynamic development loops Courtesy of Adam Opel AG

First clay model

Digitizing &surface modeling

3x

CFD Simulation

Windtunnel aero &styling optimizing

Fourth Gear – Making a “Cooler” New SUV

Fifth Gear – A New Diesel Concept

Fluent NEWS fall 2003 19

automotive

The Formula One (F1) World Championshipis the premier global automotive racing series.Out of the early tubular-shaped designs of the1950s, F1 race cars have evolved into multi-million dollar masterpieces of aerodynamic tech-nology that are capable of reaching speeds greaterthan 360 km per hour (220 mph). Not surpris-ingly, speeds of this kind generate cornering andbraking forces that range from 2.5g to 4g. Suchforces might literally cause a car to fly off the trackif not for the large amount of downforce gen-erated by the car’s front and rear wings. However,because the sport’s governing body periodical-ly changes the technical regulations, it is oftennecessary to modify the wings to optimize theoverall aerodynamics within the boundaries ofthe rules.

At SAUBER PETRONAS Engineering AG inSwitzerland, FLUENT is now playing a vital rolein meeting this challenge. Leading up to full-carsimulations, detailed CATIA CAD models weredeveloped that resulted in a mesh with nearly100 million cells. The results from these simu-lations helped provide initial conditions and bound-ary conditions for the smaller sub-models of thecar. For the front wing, which produces down-

force for the front of the car and also acts as anadjustable counterbalance to the rear wing, CFDoffered the advantage of producing numerousflow-field and surface data that would normal-ly be difficult to obtain from physical experiments.Such local analysis helped lead to furtherunderstanding of the interaction between thefront wing and the overall aerodynamics, andallowed various front-wing configurations to bestudied in a reasonable timeframe. With the rearwing, which generates the downforce that allowshigh cornering speeds, CFD simulations revealedaerodynamic shortcomings in the initial designof the lower wing element. Later simulations onthe redesigned element led to successful test-ing of the car at the track.

Comparison with wind-tunnel measure-ments and track data have shown good agree-ment in several areas of aerodynamic importance,which has led to a high confidence in the adopted method. The successful integration ofFLUENT into the design process at SAUBERPETRONAS has meant that more and more ofits race car components have begun to be devel-oped using CFD.

Downshifting – A Summary

All together, these examples from the conference give a good representation of how CFD isincreasingly being used within the automotive industry in Europe. In combination with state-of-the-art hardware and its software partners, FLUENT has been at the forefront of this increase, whichfuture generations of gear-heads may well come to rejoice. ■

Pathlines colored by velocityaround the 2003 SAUBER

PETRONAS C22 CATIA CAD model Courtesy of SAUBER PETRONAS Engineering AG

Overdrive – “The Pinnacle of Motorsports”