By Erik Ferguson, Fluent News

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For those who might say “Ich bin ein gear-head”, it makes sense that the land of the Autobahn and other awesome automotive achievement would play host to a gathering of people who 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 their knowledge of how new fluid flow challenges in the realm of vehicle design and production are being met using computational fluid dynamics. Among the CFD-based designs highlighted at the conference were improvements in powertrain cooling, drag reduction, fuel-injection equipment, exhaust gas recirculation, and even how a car gets painted. These and many other contributions demonstrate a wide range of applied CFD work being carried out today in Europe and the increasing shift toward using flow simulations in automotive engineering.

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

First Gear

The Spray Painting Process

The “paintability” of a car body, that is, the compatibility of the painting process with the materials that are used in the body shop, has been investigated by Germany’s Fraunhofer Institute for Manufacturing Engineering and Automation (IPA). Within the painting process, spray painting plays an important role in creating the final technological and optical properties of the paint film. Specifically, the IPA has been interested in predicting the film thickness distribution and the efficiency of paint transfer to the work piece being painted. Using FLUENT, the coating processes of electrostatically- supported rotary bell atomizers and other paint-transfer processes can be simulated.

When modeling a rotary bell atomizer in particular, it is essential to define proper inlet conditions for both the flow of air and the flow of paint droplets. Operating on a hybrid unstructured mesh of 400,000 cells, FLUENT was used to calculate the full 3D multiphase, turbulent flow field and the electric field. In the practical application of spray painting, the atomizer moves across the target. This results in a dynamic film thickness distribution, which is the major atomizer characteristic used in the industry. Using the static spray pattern calculated by FLUENT, the dynamic film thickness was derived by artificially moving the spray pattern along a straight line and integrating the mass over the distance. Through this integration procedure, the calculated film thickness and transfer efficiency were found to be in excellent agreement with experimental results.

For now, the spray film patterns obtained from FLUENT are being used by the IPA to program the modules for dynamic painting on real car bodies. In the future though, it will be necessary to perform true unsteady calculations that will take the speed, acceleration, and pathlines of the painting robots into account as progress is made toward more advanced spray simulations and the development of new atomizer technology.

Cross-sectional view of the E1 injector and armature used in one of the CFD models
Courtesy of Delphi Diesel Systems

Absolute static pressure for an improved armature design that prevents the onset of cavitation
Courtesy of Delphi Diesel Systems

Second Gear

A More Durable Fuel Injection System

Moving inside the engine, Delphi Diesel Systems of the United Kingdom is using CFD modeling to assess damage to electronic unit injectors (EUIs) by cavitating fuel. Modern EUIs, which help increase fuel efficiency and decrease emission levels, employ ultra-fast solenoid-driven valves that enable injection pressures of 2000 bars while maintaining complete electronic 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 the pressure falls below the saturation point. The collapse of these bubbles in the liquid fuel surrounding the valves causes the solid surfaces of the valve 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 of low pressure on the valve actuator by placing holes in the interpole groove of the armature. After experiments with this approach did not achieve the desired result, Delphi decided to use FLUENT’s deforming mesh capability to provide a better understanding of the physics driving the observed cavitation damage. Simulation of designs both with and without vent holes predicted low-pressure areas that were in the same location as the cavitation damage observed in experiments.

From the results of the simulation, it was then hypothesized that oscillations in the pressure caused by the flow path of the diesel fuel were contributing to cavitation. Subsequently, it was proposed to remove both the vent holes and the interpole groove from the armature so that the pressure distribution in the actuator would be more even. These design changes were eventually accepted and proved successful in solving the cavitation problem while maintaining the performance of the injection system.

The EGR system (yellow, green, and red)

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

Third Gear

Analyzing Exhaust Gas Recirculation

Another engine-related challenge for reducing pollutant emissions is the cooling of recirculated exhaust gas. The Spanish firms CIDAUT and DAYCO ENSA, in collaboration with the University of Valladolid in Spain, have sought to optimize the geometry of an exhaust gas recirculation (EGR) cooling system to further improve the reduction of NOx and unburned hydrocarbons in diesel engines.

An EGR cooler is a multiple-tube heat exchanger that uses water to cool exhaust gas before it is recirculated to the engine. In a typical EGR cooler, the exhaust gas (hot fluid) flows inside the tubes, and the water (cold fluid) flows around the outside of the tubes. To produce more efficient heat exchangers, it is common to use corrugated tubes. The corrugations increase the surface area available for heat exchange. Additionally, they introduce a tangential velocity inside the tubes, which increases the level of turbulence in the gas flow and further improves the heat transfer. Along these lines, parametric studies of pressure loss and heat transfer were done using FLUENT to determine the influence of different corrugated tubes.

Experimental results confirmed the tendencies in heat transfer efficiency predicted by the FLUENT numerical model, with an overall difference of less than 5%. The findings demonstrated that as the degree of tube corrugation increased, so too did the efficiency of heat transfer and the pressure loss due to increased turbulence. Although pressure loss was an important parameter to consider, maximizing the heat transfer efficiency within the limits of the system was deemed the most important goal of the simulation. By corrugating the tubes, the size and cost of the heat exchanger equipment can also be reduced, thus making compliance with emissions regulations more cost-effective.

Contours of pressure coefficient on the Volvo XC90 and flow pathlines
Courtesy of Volvo Car Corporation

Fourth Gear

Making a “Cooler” New SUV

The class of automobiles known as sport utility vehicles (SUVs) has faced challenges in the area of emissions control and gas mileage, due mainly to the higher weight and increased drag of the four-wheel drive chassis that has been the standard base of construction. In 1999, Sweden’s Volvo Car Corporation instead decided to enter the SUV market using a saloon-car, or sedan platform as the base. The goal was to combine the level of ride comfort and crash safety of a sedan with the greater visibility and interior space of a standard SUV, while also offering predictable handling characteristics and competitive fuel consumption. A final challenge was to bring the new vehicle, known as the XC90, to market by the end of 2002, which led to the removal of early prototype testing and an increased reliance on CFD to aid in the final production design.

Given these challenges, it was identified that the flow rate of air required to cool the engine compartment would need to be increased by 50% compared to Volvo’s S80 sedan, upon which the XC90 design was based. CFD was chosen for use in the XC90 project, with a focus on the cooling air mass flow as the evaluation parameter.

Using FLUENT, an initial 2D study identified the parameters in the vehicle front-end design that played essential roles in affecting the flow rate of air entering the engine compartment. Subsequently, several full 3D simulations of the car addressed questions related to the fan position and performance, heat exchanger layout, and shape of the rear bumper beam. The target of 50% more cooling air flow for the Volvo XC90 compared to the Volvo S80 was successfully achieved for the final design. Another project was initiated to couple the cooling air flow to the hot side of the heat exchangers using user-defined functions (UDFs) in FLUENT. Favorable comparison with experiments has led Volvo to use CFD in all projects where cooling performance is addressed.

A major advantage of using CFD in the XC90 project was the ability to have questions about the effects of changing a parameter answered as quickly as the next day. Whether it was by fast 2D or highly-detailed 3D simulations, CFD played an important role in helping Volvo meet their cooling airflow targets by offering valuable background information that could be used to help balance increased airflow with the overall exterior design vision.

(Click on each thumbnail to view a larger image)
Aerodynamic development loops
Courtesy of Adam Opel AG

Fifth Gear

A New Diesel Concept

In the domain of high-performance sports cars, diesel engines have recently gotten a performance boost in the form of the ECOSpeedster. Developed by Adam Opel AG in Germany, and presented at the 2002 Paris Auto Show, the ECO-Speedster was a concept car intended to showcase the progress made in diesel technology, including a fuel consumption rate of 2.5 liters per 100 km (94 mpg) and a targeted top speed of 250 km per hour (155 mph). Achieving these goals required a low drag coefficient and a lightweight chassis in combination with an efficient engine. Having already developed a 112 horsepower 1.3 liter commonrail direct turbo-charged injection (CDTI) diesel engine and a carbon-fiber body, Opel’s remaining task was to optimize the external aerodynamics. Due to the short timeframe available for aerodynamic development, the decision was made to integrate CFD simulation into the design process to supplement the standard wind-tunnel testing.

In the beginning, early sketches of the car led to the creation of a one-fifth scale clay model, which was then digitized and converted to a CAD model. Using FLUENT and a hybrid volume mesh comprised of four million cells, an external airflow simulation was carried out. Here, regions were identified for improvement as a basis for the optimization that was then performed during subsequent tests in a model wind tunnel. As soon as the drag targets were met and the necessary body styling requirements implemented, construction of a full-scale prototype began. Five months later, on-road testing of the completed prototype showed that the engine and overall design concept produced a measured top speed of 264 km per hour (162 mph), which exceeded the target.

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

During its 24-hour record trial at Dudenhofen Proving Ground in June 2003 the ECO-Speedster finally managed to beat 17 world records for dieselpowered vehicles.

Pathlines colored by velocity around the 2003 SAUBER PETRONAS C22 CATIA CAD model
Courtesy of SAUBER PETRONAS Engineering AG

Overdrive

“The Pinnacle of Motorsports”

The Formula One (F1) World Championship is the premier global automotive racing series. Out of the early tubular-shaped designs of the 1950s, F1 race cars have evolved into multimillion dollar masterpieces of aerodynamic technology that are capable of reaching speeds greater than 360 km per hour (220 mph). Not surprisingly, speeds of this kind generate cornering and braking forces that range from 2.5g to 4g. Such forces might literally cause a car to fly off the track if not for the large amount of downforce generated by the car’s front and rear wings. However, because the sport’s governing body periodically changes the technical regulations, it is often necessary to modify the wings to optimize the overall aerodynamics within the boundaries of the rules.

At SAUBER PETRONAS Engineering AG in Switzerland, FLUENT is now playing a vital role in meeting this challenge. Leading up to full-car simulations, detailed CATIA CAD models were developed that resulted in a mesh with nearly 100 million cells. The results from these simulations helped provide initial conditions and boundary conditions for the smaller sub-models of the car. For the front wing, which produces downforce for the front of the car and also acts as an adjustable counterbalance to the rear wing, CFD offered the advantage of producing numerous flow-field and surface data that would normally be difficult to obtain from physical experiments. Such local analysis helped lead to further understanding of the interaction between the front wing and the overall aerodynamics, and allowed various front-wing configurations to be studied in a reasonable timeframe. With the rear wing, which generates the downforce that allows high cornering speeds, CFD simulations revealed aerodynamic shortcomings in the initial design of the lower wing element. Later simulations on the redesigned element led to successful testing of the car at the track.

Comparison with wind-tunnel measurements and track data have shown good agreement in several areas of aerodynamic importance, which has led to a high confidence in the adopted method. The successful integration of FLUENT into the design process at SAUBER PETRONAS has meant that more and more of its race car components have begun to be developed using CFD.

Downshifting

A Summary

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

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