Today’s vehicles no longer only fulfill the requirement to transport their passengers from A to B safely, they also have to offer a high level of comfort in line with low sound emissions. Additionally, they also need to use as little fuel - or power in the case of electric cars - as possible in order to protect the environment and keep running costs low. One of the main factors involved in achieving these goals is the way a car’s body is designed: even the slightest modification of the shape of the mudguards, exterior mirrors or rear can affect the airstream’s flow behavior. This means that car manufacturers have to achieve a fine balance between design and functionality with every new model they develop. Ascertaining the relevant variables involved with as much precision as possible requires extensive testing in a correspondingly designed test stand capable of achieving wind speeds of 250 km/h and above. This is where TLT-Turbo steps in: the globally operating industrial fan manufacturer from Zweibruecken in Germany builds high-quality aeroacoustic wind tunnel fans customized to the relevant industry and customer’s requirements in collaboration with specialized suppliers.
Energy efficiency, optimum road holding, and pleasant driving experience without any distracting internal or external sounds, even at high speeds – all of these criteria largely depend on a vehicle’s aerodynamics and associated airstream flow behavior. In order to measure the wind noises generated by cars with as much precision as possible, manufacturers consequently conduct extensive tests. “The accuracy of these test results depends extremely on the test environment," explains Thomas Dippold, M.Sc., project manager at TLT-Turbo. “It is only possible to accurately categorize and evaluate acoustic pressure and frequency measurements outside and inside a vehicle, if these measurements are affected as little as possible by other noises.” In concrete terms, that means using a wind tunnel fan with a diameter of several meters, which is consequently as high as an average two-story house, still has to work as silently as possible.
Well-coordinated cooperation of all constituent parts is essential
Dippold speaks from many years of experience: He has been planning and managing wind tunnel construction projects for the automotive industry at TLT-Turbo for 6 years, while the company itself has been successfully operating in this sector since the 1930s. Its product range includes climate wind tunnels as well as aerodynamic and aeroacoustic wind tunnels, every single one of which generally has to meet different requirements. TLT-Turbo provides its customers with help and support from planning right up to the commissioning stage. This includes developing the aerodynamic and acoustic as well as mechanical and electrical design and manufacturing the fan with its drive and control system. The mechanical components such as the rotor, housing, and rotor blades are produced in the company’s own production facilities distributed all over the world and then assembled with all the other components.
During a project, TLT-Turbo also collaborates with a number of highly specialized sub-suppliers. The experts at TLT-Turbo provide the specifications for the individual components and support the manufacturers with their services. These suppliers are responsible for assemblies such as the drive units, frequency converters, transformers, and switch cabinets. “The requirements for these wind tunnels can only be met if all constituent parts are perfectly coordinated," explains Dippold. During the planning and development phase, all components are extremely important and each one will later affect the fan’s performance. “This is why the only way to achieve our goals is to use high-end components throughout," says the project manager.
Automotive manufacturers demand wind speeds of up to 300 km/h
Customers tend to demand more and more very specific wind tunnel performance ranges with wind speeds of 250 to 300 km/h. To achieve these kinds of speeds, a fan has to process volume flows of up to 1,800 m3/s. At the same time, the associated sound pressure level generally has to be below 100 dB(A) in order to prevent it from affecting the test results – in particular, while performing aeroacoustic tests. On top of that, car manufacturers also expect a highly complex system to use as little energy as possible and to have an efficiency factor of over 89%. “In contrast to climate wind tunnels, in which vehicles are tested at different temperatures from -40 to +60°C, aerodynamic and acoustic wind tunnel have to keep the temperature as stable as possible – regardless of external conditions in the middle of the summer or winter," says Dippold. “For this reason, all of the energy that the fan converts into the airflow and hence into heat as a result of air friction is extracted from the tunnel using massive heat exchangers."
In addition to the above, TLT-Turbo is also constantly working to reduce the fans’ weight and sound emissions. The Zweibruecken-based company is increasingly relying on materials used in lightweight construction. Hence, rotor blades are now made of carbon-fiber materials and the nose cone of fiber-reinforced plastic. The blades and guide vanes also have special profiles, as well as the number and distances between them are decisive. “Another way of limiting the fan’s noise is to sound-insulate the entire tunnel and the area in front of and behind the fan," Dippold goes on to explain. “This prevents any noise from being created in the first place and creates perfect conditions on the measuring section.”
Global demand for the realization of major projects
What makes planning and actually building a wind tunnel and fan so challenging is their respective uniqueness. Every company sets different demands for the planners, which have to be taken into account with respect to the individual components’ performance. “Often, the most critical part of a project isn’t the feasibility of meeting a wind tunnel’s requirement specifications, it is increasingly the global procurement process and its installation on-site," says Dippold. “The individual fan components, which weigh several tons, often have to be fitted under extreme conditions and in very confined spaces, which requires working with a high level of precision to prevent the delicate components being damaged."
For example, TLT-Turbo recently installed a wind tunnel fan with a diameter of 8 m and a driving power of 3.4 MW for a well-known automotive manufacturer, of which the motor alone weighed 34 tons. The fan’s total weight was about 200 tons and the project involved 37 major segments weighing between 6 and 38 tons each. For assembly, they had to be lifted by a crane one after another through an extremely small 10 x 5-m opening, located at a height of 12 m. Working at a height of 10 m, the installers indeed often had no more than 10 cm of clearance from the concrete walls. “Our team constantly meets challenges of this kind – during the planning stage and installation," explains Dippold. At the moment, the company is increasingly realizing different types of wind tunnel projects for the automotive industry in China and the USA. “Thanks to our extensive experience and know-how gained over countless projects, we are always able to find a solution no matter what the challenge is," finished the project manager.