STAR-CD helps the Volkswagen Beetle keep its cool

August 7, 2001

magine a car moving at low speed under heavy load (for example when towing a trailer uphill) on a warm day. How does the car keep its cool? Is the airflow sufficient to keep all the components in the engine compartment in a thermally controlled environment: some of these can be operating at high temperatures (as in the case of a close-coupled catalytic converter), others will be sensitive to high temperatures (for example the engine control unit). How big should the grille be, in order to allow enough air to enter from outside? Is the ram effect sufficient, or does the air need the extra boost provided by a fan? If so, where is the best place to position the fan? What will happen if a fan fails?

It is the job of the vehicle design engineers to answer all these questions, and many other related issues. This branch of vehicle design, known as UTM (Underhood Thermal Management) is a crucial part of the development of any modern motor vehicle – and it is becoming ever more important as the number and complexity of components being packed into the limited space of the engine compartment increases.

The engineers at Volkswagen in Wolfsburg, Germany, use a variety of techniques to help them predict airflow rates and temperatures in the engine compartment of vehicles such as the new VW Beetle, shown here. One of the techniques employed, Computational Fluid Dynamics (CFD), allows engineers to simulate various scenarios on computer, and to address the "what-if?" questions. STAR-CD, widely used in the VW-Audi Group for applications ranging from in-cylinder analyses to passenger compartment climate control, is one of the tools applied in this part of the vehicle’s development.

In order to accurately simulate engine compartment flow, the analysis should include
:
* >A representation of the heat exchanger package(s);

* The effects of the fan(s);

* Reasonable boundary conditions (e.g. inlet turbulence, wall thermal boundary conditions, and moving wall boundary conditions);

* Thermal radiation effects must be included if they are deemed to be of importance. Note that if the goal is the prediction of the air temperature distribution, and reasonable wall thermal boundary conditions can be determined (i.e. boundary conditions which already include the effects of radiation), then it is not necessary to model the thermal radiation;

* Conjugate Heat Transfer may be required – that is, the calculation of the heat transfer both to and within the solid components;

* Practical modeling and solution issues, such as the turbulence model to be used, the treatment of the turbulent wall boundary conditions, and the differencing scheme used;

* The ablility to post-process the results to get the desired information, both visually, and as performance figures.