Climatic wind tunnels (CWTs) are nowadays a fundamental part of the development of new vehicles, since they enable all weather simulations at all times of the year, latitudes, altitudes, etc. Vehicle development research can be performed more targeted and development time can be reduced significantly, as modern climatic wind tunnels allow reproducible heat transfer analyses at all locations of a vehicle. In this work the influence of the ambient conditions on the heat transfer processes for passenger cars that can be simulated in CWTs is analyzed for the CWTs of the BMW Group. The influences of relative humidity, ambient temperature and altitude are considered with the aim to establish the required accuracy level for setting these parameters in CWTs such that road similar results can be obtained during CWT testing. Furthermore an investigation is performed on the flow quality simulated in the CWT and its influence on the heat transfer relevant to vehicle components when compared to the on-road conditions. Aspects which have been considered are the aerodynamic influences caused by the use of wheel fixation and the exhaust extraction system, as well as by the wind tunnel geometry and size. The aim was to establish the influence of these elements on the vehicle aerodynamics and consequently the different heat transfer rates on vehicle components induced when compared to the on-road conditions. The influences of both the ambient and aerodynamic parameters on the heat transfer have been studied by measuring the temperatures of vehicle components and cooling fluids throughout different vehicle zones, including components that act as heat sources, e.g. the engine and exhaust pipes, as well as those that are convectively heated. The analyses are performed at constant drive velocities and external temperatures on a standard, market-ready, BMW (F10 or F34) or Mini (R56). Depending on the investigation parameter, realistic and extreme conditions for relative humidity, altitude and temperature were set with high accuracy during CWT testing. The influence of the CWT hardware and geometry on the vehicle aerodynamics has been established by measuring the local surface pressures along the vehicle body in both the CWT as well as in a reference aerodynamic wind tunnel. As a comparison these effects are also studied trough CFD analyses. The analyses showed that for the tests performed within the conditions prescribed by the BMW operation envelope, changes in relative humidity have a negligible influence on the heat transfer process of the vehicle components and cooling fluids. On the contrary, small changes in ambient temperature were found to have a non-negligible effect on the component temperatures. Furthermore, no specific relations could be established between the changes in ambient and component temperatures, since actively and passively heated or cooled components showed different thermal behaviors. It was furthermore studied that altitude changes smaller than 500 m appear to have a negligibly small influence on the simulation quality of the tests. Instead, when the difference between local testing altitude and the desired ambient altitude is larger than 500 m then the altitude must be simulated at the correct altitude in order to have on-road realistic thermal behavior of the components and cooling fluids. The study of the pressure distributions along the vehicle body proved that both the vehicle fixation and the size and geometry of the wind tunnel have a strong influence on the vehicle aerodynamics and therefore the component cooling. These results have been confirmed by CFD analyses. As a consequence of the fixation or a smaller wind tunnel geometry the local pressure along the vehicle underbody is increased. The pressure increment strongly depends on the type of (front or rear wheel) vehicle fixation and the size of the wind tunnel. The results indicate that for CWT testing lower local flow velocities, i.e. less cooling, occur along the underbody than during on-road testing. The results correlate with the higher temperatures measured for several vehicle components during CWT testing when compared to on-road testing results. The exhaust extraction system was found to have a similar, but less strong influence. The results achieved under realistic ambient conditions can be used as an optimization parameter in planning and operating climatic wind tunnels for future use. Furthermore, a solution for the vehicle fixation and the modification of the vehicle aerodynamics in the CWT in order to obtain road similar aerodynamic conditions along the vehicle underbody is recommend.    «

Climatic wind tunnels (CWTs) are nowadays a fundamental part of the development of new vehicles, since they enable all weather simulations at all times of the year, latitudes, altitudes, etc. Vehicle development research can be performed more targeted and development time can be reduced significantly, as modern climatic wind tunnels allow reproducible heat transfer analyses at all locations of a vehicle. In this work the influence of the ambient conditions on the heat transfer processes for pass...    »