By Adrien Mann
If you’ve been in a new car lately, you know that automotive OEMs are focused on reducing noise from sources like the engine, wind, and rolling. This means that drivers and passengers now notice noise from other systems more, presenting OEMs with a new acoustic challenge: the HVAC system.
What makes addressing noise in the HVAC system such a challenge? From a technical point of view, the HVAC system is distributed throughout a huge part of the vehicle, and its packaging interacts with many other systems. From an operational point of view, drivers and passengers rely on it the most under extreme environmental conditions, and today’s technology offers different temperatures and fan speeds for driver and passengers – all of which requires the HVAC system to be both complex and extremely durable.
Because it’s so connected throughout the vehicle, changes to an HVAC system once a hardware prototype is completed are difficult, costly, and usually insufficient. They can delay time to market, setting back launch deadlines, and add parts’ cost and other unanticipated outcomes like weight. Finally, late-stage changes can’t account for potential impacts on related systems because it’s too late in the product development lifecycle to do so.
It is therefore critical to achieve a robust design for the HVAC system early in the development stage in order to ensure a successful prototype test and avoid late-stage failures. Only digital simulation can meet these needs, since it digitally re-creates both functional and acoustical properties of 3D geometry, and replicates real-world conditions so engineers can understand and improve vehicle systems’ performance. Design engineers from BMW and Exa reported on how they addressed this challenge in a 2015 paper at the 3rd International ATZ Conference in Zurich, titled “Acoustic Source Detection for Climate Systems via Computational Fluid Dynamics for Improved Cabin Comfort,” . (See Images 1 and 2, below: Validation of PowerFLOW flow-induced noise predictions for a BMW HVAC system sub-unit.)
The necessary prediction capabilities for such an aeroacoustics approach have been validated both against subsystems of HVAC units on testing rigs and full systems of to-be-built vehicles. In close collaboration between BMW AG and EXA, acoustic comparisons for characteristic sub-units of an HVAC system were completed under operating conditions such as defrost, fresh air ventilation, and air recirculation mode, obtained by altering the flap positions. The variation of the blower rotation speed also played an important role in the resulting operating conditions in order to better understand the system’s behavior.
Finally, this paper presents an example of analyzing the flow-induced noise sources present in the HVAC system using the patented Flow-Induced Noise Detection (FIND) module in Exa’s PowerACOUSTICS software. The noise sources are detailed as contributions per component, corrected by the acoustic transfer function between the source and a defined target point or target space, such as the driver’s ear and the passenger’s ear. (See images 3 and 4 below: Exa Standard Vehicle HVAC System and Isosurface of Acoustic Power Volume Density at 400Hz 1/3rd octave frequency band for the smoothed ducts geometry; (Left) Baseline; (right) Improved Design.)
The FIND analysis module, which is unique to Exa, provides both an assessment of the performance of a given HVAC system and a ranking of the noise sources according to the perceived noise at the passenger’s ears, providing guidance toward an efficient design and optimization process. Only digital simulation in Exa can uniquely combine flow properties and acoustic impact under realistic operating conditions, help engineers identify sources of unacceptable noise levels, and improve their design with critical changes – all before a physical prototype is produced – helping to reduce costly late-stage failures during prototype testing.
All that should be music to the ears of automotive OEMs – and their customers.
 J. Biermann, B. Neuhierl, A. Mann, M. S. Kim, “Acoustic Source Detection for Climate Systems via Computational Fluid Dynamics for Improved Cabin Comfort”, 3rd International ATZ Conference, Zurich, Switzerland, June 23-24, 2015
 A. Mann, M.S. Kim, F. Pérot, M. Meskine, “Designing quieter HVAC systems coupling LBM and flow-induced noise source identification methods”, 10th FKFS-Conference, Progress in Vehicle Aerodynamics and Thermal Management, Stuttgart, Germany, September 29-30, 2015