Shanghai Automotive Wind Tunnel Center of Tongji University (Tongji-SAWTC) was the first aeroacoustic automotive wind tunnel in China. It is well-known in the field worldwide, and it has supported the ground vehicle industry for over 15 years since 2009.
In 2013, gfai tech provided Tongji-SAWTC with their first Acoustic Camera, a sophisticated and custom-built system with a single array of 120 microphones. We were honored by their trust, and are delighted to have been selected once again to deliver a state-of-the-art 3D wind tunnel system, further ensuring that their facilities remain at the cutting edge of technology. This article highlights some of the key aspects of this project.
As each wind tunnel presents unique characteristics, from varying sizes and wind speeds to specific shear layer dynamics, designing the 3D acoustic mapping system for Tongji-SAWTC was a highly collaborative effort.
At gfai tech, we recognize that a one-size-fits-all approach doesn't suffice. That’s why we customized several parts of the system, from hardware to software. We worked closely with the customer to ensure all project requirements and targets were met, and the system was delivered on time.
Three microphone arrays were specifically designed for this project. Each array consists of 168 optimally distributed microphones. Factors such as the size of the wind tunnel, the desired measurement distance, and the importance of different frequency ranges were all taken into account. In addition, our team used proprietary algorithms to determine the optimal distribution of the microphones.
The arrays were engineered for easy attachment and removal from the supporting traverse. They are also transportable through wheels and can be folded in order to facilitate transport and storage. Nonetheless, the traverse itself also includes a transporting mechanism, allowing to move it behind the exit of the tunnel´s nozzle, without removing the arrays, freeing the test section for other types of measurements.
The signals from the 504 microphones are captured in a synchronized manner at an exceptional sample rate of up to 192 kHz by gfai´s powerful data acquisition devices. The digitized audio data as well as the video camera signals are then reliably transmitted to the high-performance computer in the control room via a dedicated network.
The Acoustic Camera system enables the computation of acoustic maps over the external shell of a vehicle under test, as defined by its CAD model. For this, the first step is to accurately determine the position and orientation of the vehicle with respect to the microphone arrays. We ensured this occurs efficiently at Tongji-SAWTC by integrating a highly sophisticated and accurate 3D laser scanning and automatic model fitting system.
The process takes place after the vehicle enters the test section of the wind tunnel, and is locked securely in place over the large-scale turntable. With each change in yaw angle - the angle between the direction of the wind and the longitudinal axis of the vehicle - the system can scan the scene and automatically adjust the preloaded CAD model. This ensures the model precisely aligns with the vehicle's current position and orientation at the test stand.
The integrated system allows not only the automation of what would otherwise be a time-consuming task, which occurs with every rotation of the turntable and different vehicle, it also allows obtaining even more accurate acoustic mappings. That´s because by using CAD models the distances between the microphone arrays and each calculation data point over the vehicle´s surfaces are accurately determined and are taken into account by the 3D WindTunnel module of our NoiseImage software.
At the heart of the system lies a suite of advanced software, selected to meet the specific needs of Tongji-SAWTC.
Gfai's NoiseImage Controller empowers operators to oversee and manage the entire data acquisition process, including the smooth exchange of information with the model fitting system, while handling the storage of large volumes of data efficiently.
Once the data is acquired, which takes only a few seconds, it is processed and analyzed using our NoiseImage software and its specialized WindTunnel module. At this stage, utilizing gfai's advanced algorithms module for NoiseImage is essential for wind tunnel measurements, as it effectively addresses the challenge of localizing multiple noise sources, with distinct sound pressure levels, originating from various parts of the vehicle. This processing step can be performed outside the wind tunnel, allowing it to remain available for additional measurements. As the effective use of time in the wind tunnel is a critical aspect to be regarded, both the overall short-time measurement process and the possibility to post-process the data externally are indispensable.
After finalizing the design, our production team assembled the system, from microphone arrays to data acquisition devices, with meticulous attention to detail. Once assembly was complete, the entire system underwent a preliminary calibration and fully functional test at our Berlin facility, to ensure full functionality before shipment.
The completed Acoustic Camera System was securely packed in custom crates and transported to Shanghai, where our team oversaw its installation and calibration.
In under two weeks on-site, we meticulously assembled and installed the system, executing a series of processes with diligence, including:
With the installation and final adjustments complete, our team conducted a comprehensive in-house training on both hardware and software for Tongji-SAWTC’s personnel. Support by thorough documentation, the system was ultimately approved by Tongji-SAWTC in compliance with the established protocol.
The noise generated by aeroacoustic phenomena can be analyzed and evaluated from various perspectives using the tools integrated into gfai tech’s Acoustic Camera system.
Through acoustic maps - which take advantage of the high-resolution arrays, advanced signal processing algorithms, and comprehensive software tools - Tongji-SAWTC’s operators, engineers and researchers, can view and analyze aeroacoustic noise sources at various frequency bands in detail.
At higher speeds, the comfort of both drivers and passengers is predominantly affected by noise generated by the wind, making the identification and reduction of aeroacoustic noise sources a priority. Additionally, as typically the louder noise sources also indicate regions of considerable drag - the resistance force experienced by the vehicle moving through air – the acoustic maps can also be used to assist in aerodynamic analysis. Drag, for instance, plays a major role in the vehicle´s performance, particularly in its fuel efficiency.
It is also possible to investigate in detail the sound pressure level of a specific sound source, or sources within a region of the acoustic map, and the bandwidth of the noise it produces. This data allows for effective comparison and optimization of the shape and design of various vehicle components, such as its wing mirrors. Equipped with three high-resolution arrays, at Tongji-SAWTC vehicles can be evaluated from multiple angles. Whistle-like or tonal noises caused by wind turbulence, along with broadband noise from surface roughness, are quickly identified and their spectral characteristics evaluated.
For aeroacoustic engineers and specialists, hearing the sound coming from the vehicle with their own ears and taking advantage from their subjective impression can provide valuable insights. This can now be done at Tongji-SAWTC not only by entering the test section directly, which can be limiting, but also through the acoustic camera system. NoiseImage software and its powerful tools allows pinpointing a location in the acoustic map and hearing the sound coming only from that direction, as in this example below.
These are just a few examples of the measurements and results that the system is capable of delivering.
With the new system, Tongji-SAWTC achieved a major milestone, while retaining the flexibility to expand their hardware and software capabilities at any time.
With the addition of a Sphere Array such as the Sphere48 AC Pro, which can be placed inside of the vehicle, interior measurements can be performed simultaneously with exterior measurements. This capability allows for the correlation of internal noise sources with external ones, providing further insights into which sources most impact the comfort of passengers inside the cabin.
A/B tests can be easily performed using the NoiseImage Conclusions - a specialized tool to simplify the comparison of products designs.
Finally, the NoiseImage Controller software can be integrated with the wind tunnel operation system, enabling real-time communication and data transfer - such as wind speed and temperature - for a further streamlined workflow.
The combination of hardware and software is flexible depending on each customer´s needs, priorities and budget.
Working on custom projects with our clients is one of gfai tech's key strengths. Beyond providing ready-made products for acoustic measurements and structural dynamics, we specialize in creating tailored solutions.
As a leader in aeroacoustic measurements on wind tunnels, our acoustic camera has proven to be a valuable tool for R&D engineers, helping them understand and reduce noise. Its versatility extends beyond the automotive sector, making it a valuable asset across a wide range of industries.
We sincerely thank the Tongji-SAWTC team for their continued trust in our company and products.
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