Stepping on the Gas in the Wind Tunnel
Research in the new wind tunnel at Bosch is picking up speed: Airspeeds of up to 200 km/h (124 mph) can be selected. What’s unique is that this air jet is extremely quiet. And that’s a key requirement here, because these scientists want to study only those noises that are results of the interaction between the airflow and the test components.
Surprises are guaranteed: When Bosch scientists walk across the new aeroacoustics laboratory, they either have to duck or expose themselves to a deafening roar. That’s because an air jet traverses the room at shoulder height. And it’s an invisible and practically inaudible ‘whispering wind’ – exactly what the scientists need in these airflow experiments being conducted at Bosch. Researchers are using the new wind tunnel to study the noise that is caused by airflows, which is the reason that this wind itself has to be nearly silent. To achieve this, the planning and construction of the wind tunnel itself required a great deal of research. The airflow operates in a recirculating system. Air from the laboratory is sucked into a ceiling vent and then reinjected into the laboratory by an adjustable nozzle that controls the flow speed.
The most critical component is the fan that drives the circulation of the airflow. It has been flow-acoustically optimized but is clearly audible outside the laboratory. And it also generates heat, so air conditioning is required to maintain a comfortable temperature in the laboratory. Sound dampers were employed to optimally attenuate noise from the ventilation system on the basis of mathematical simulations.
Nonetheless, the first system start-up was suspenseful until there could be no doubt: No fan noise could be heard within the sound laboratory! Other features in the airstream further reduce noise and are used to control the well-defined air jet routed into the sound laboratory. The wind tunnel has become one of the most important experimental tools used in the advanced development of Bosch products that are exposed to a slipstream or where noise resulting from airflows is an important feature of the product’s quality. For example, researchers are investigating noise generation and sound propagation from windshield wipers that are exposed to the slipstream. In the models that they are using, the curved windshield is flattened and the wiper is turned into a straight bar.
There are two reasons for such simplifications: First, the aeroacoustics experts want to investigate the physical rules based on simplified physical models and also use analogous virtual models in computer simulations.
If the results of the computer modeling agree with the experimental findings, the scientists proceed to the next stage: “The physics looks OK. Now let’s check out some more complex geometries.” Like a realistic wiper shape, though there is never a guarantee that the physics “is OK.” While the time-dependent airflow and sound generation processes can be simulated in the computer, this approach involves an enormous amount of computing. More specifically: An ordinary PC would be crunching the numbers for months. So the scientists must simplify the algorithms to achieve reasonable computing times while also ensuring that the physics continue to check out and the results agree with experimental findings.
A complicating factor in this simulation is the interaction between the domains of airflow and sound. The first task is to use a fine-mesh mathematical grid containing millions of nodes to compute the courses of the airflows and the associated sound sources. And from these results, the scientists compute the sound waves measured over a period of time on a substantially coarser grid.
Initial results from the wind tunnel already demonstrate that the researchers’ computational approach is valid.
The most critical component is the fan that drives the circulation of the airflow. It has been flow-acoustically optimized but is clearly audible outside the laboratory. And it also generates heat, so air conditioning is required to maintain a comfortable temperature in the laboratory. Sound dampers were employed to optimally attenuate noise from the ventilation system on the basis of mathematical simulations.
Nonetheless, the first system start-up was suspenseful until there could be no doubt: No fan noise could be heard within the sound laboratory! Other features in the airstream further reduce noise and are used to control the well-defined air jet routed into the sound laboratory. The wind tunnel has become one of the most important experimental tools used in the advanced development of Bosch products that are exposed to a slipstream or where noise resulting from airflows is an important feature of the product’s quality. For example, researchers are investigating noise generation and sound propagation from windshield wipers that are exposed to the slipstream. In the models that they are using, the curved windshield is flattened and the wiper is turned into a straight bar.
There are two reasons for such simplifications: First, the aeroacoustics experts want to investigate the physical rules based on simplified physical models and also use analogous virtual models in computer simulations.
If the results of the computer modeling agree with the experimental findings, the scientists proceed to the next stage: “The physics looks OK. Now let’s check out some more complex geometries.” Like a realistic wiper shape, though there is never a guarantee that the physics “is OK.” While the time-dependent airflow and sound generation processes can be simulated in the computer, this approach involves an enormous amount of computing. More specifically: An ordinary PC would be crunching the numbers for months. So the scientists must simplify the algorithms to achieve reasonable computing times while also ensuring that the physics continue to check out and the results agree with experimental findings.
A complicating factor in this simulation is the interaction between the domains of airflow and sound. The first task is to use a fine-mesh mathematical grid containing millions of nodes to compute the courses of the airflows and the associated sound sources. And from these results, the scientists compute the sound waves measured over a period of time on a substantially coarser grid.
Initial results from the wind tunnel already demonstrate that the researchers’ computational approach is valid.
