COOL AND QUIET USING SIMULATION TO OPTIMIZE VEHICLE HVAC SYSTEMS

Transcription

1 COOL AND QUIET USING SIMULATION TO OPTIMIZE VEHICLE HVAC SYSTEMS A whitepaper discussing the challenges acoustic engineers face and the use of new approaches in simulation technology to find noise sources and improve acoustic performance

2 COOL AND QUIET Using Simulation to Optimize Vehicle HVAC Systems EXECUTIVE SUMMARY Engineers have made huge strides over the past two decades in reducing powertrain and road noise which previously were the primary sources of vehicle noise. Their reward is an upsurge in consumer complaints over interior noises that were previously drowned out by engine and road noise. For example, owners are bringing their new vehicles in for expensive warranty repairs after hearing noise emanating from the HVAC system even though nothing is functionally wrong with the vehicle. In response, automotive Original Equipment Manufacturers (OEMs) are setting increasingly challenging HVAC noise targets. HVAC suppliers are struggling to meet these targets while at the same time wrestling with thermal management trade-offs. This task is made more difficult by increased crowding under the hood and around the dashboard which makes it necessary to reduce the size of HVAC units and run ducts around more obstacles. A further challenge is that even when engineers install a quiet HVAC module in the first vehicle prototype they may find that interactions between the module and vehicle cause noise targets to be exceeded. COVER IMAGE: Insight on automotive HVAC noise sources is now possible using SIMULIA aeroacoustic solutions. HVAC designers need the ability to simulate and diagnose their HVAC module designs from an acoustic standpoint early in the design process. But conventional simulation technology is not much help in predicting noise because a Computational Fluid Dynamics (CFD) code that simulates airflow must be linked to an aero-acoustic solver that simulates noise propagation, which takes too long and is not accurate enough to be much use in the design process. PowerFLOW addresses this challenge by accurately predicting the turbulent airflow within and around the HVAC system while simultaneously predicting noise propagation. The simulated noise signals are interactively played within PowerACOUSTICS in order to qualitatively assess the noise levels. The PowerACOUSTICS Flow Induced Noise Detection (FIND ) module processes transient information generated by PowerFLOW in order to pinpoint the location and magnitude of flowinduced noise sources providing engineers with a roadmap to reducing HVAC noise as perceived by the driver and passengers. These software solutions enable engineers to quickly iterate to an HVAC module design that will meet noise targets when integrated with the full vehicle in order to get the design right the first time and minimize expensive design and prototyping iterations. Flow perturbations and acoustics in truly rotating HVAC systems centrifugal fans are both captured with SIMULIA s PowerFLOW. 2

3 HVAC NOISE DESIGN CHALLENGES Recent Consumers Reports and JD Powers consumer surveys show that the HVAC system now ranks as the second greatest cause of noise complaints following closely behind wind noise. Frequent complaints include difficulties in hearing other passengers and the sound system as well as annoying sounds such as whistles and loud noises in the background. The complexity of HVAC modules mean there are many possible noise sources. For example, high-speed airflow around the front vents can interact with the HVAC doors and flaps to generate noise. The vents can generate whistles and high frequency noises. The cavity formed by the floor and knee bolsters can enhance low frequency sounds. The defroster can produce diffraction effects that amplify sound levels. Blowers needed to sustain airflow for rear passengers can also generate problems. These problems are of even bigger importance in Electric Vehicles and Hybrids, presenting special challenges because the absence of powertrain noise increases the prominence of the HVAC noise. A major complicating factor in designing HVAC systems is the fact that the ducts and components are distributed through a large part of the vehicle so they interact with many other systems. The proliferation of electronics gear within the engine compartment, for instance, often requires HVAC ducts to follow an increasingly serpentine path, which further increases noise generation. Another complicating factor is that OEMs have less and less space available for the HVAC system which forces units to be smaller. Shrinking HVAC module envelopes inherently increase flow velocity and turbulence, which in turn generates higher noise levels. An additional complicating factor in HVAC design is that the noise ultimately produced by the HVAC system is highly dependent upon its integration with the vehicle integration yet a vehicle prototype is usually not available when the HVAC system is being developed. There have been many occasions when HVAC engineers have met challenging acoustic targets during module-level testing only to find that when the module is installed in the first vehicle prototype, the noise has climbed back above the target values. At this stage, changing the vehicle design is usually not an option so the HVAC designers are forced to go back to the drawing board. It s also important to note that the HVAC supplier normally develops the mixing unit and the blower while the OEM is often responsible for the duct design. So, when the supplier designs the unit and the blower, it makes sure that the noise levels satisfy the requirements. But when the OEM plugs-in the ducts, the noise levels change and problems can occur. Engineers designing HVAC systems also have to carefully manage tradeoffs between thermal management and noise. For example, when the windshield is frozen the top priority is to clear it even at the expense of HVAC noise. Likewise, in the summer cooling a hot cabin takes priority over listening to the radio. HVAC flow structures and flow-generated noise radiating to the cabin are both accurately captured using SIMULIA PowerFLOW. 3

4 Using SIMULIA PowerACOUSTICS FIND module allows the quantification and ranking of the various noise-generating areas of the HVAC system. Image above shows the location of boxes delimiting areas of interest. Chart shows the acoustic power radiated within the system, each of these areas corresponds to a box color on left image. LIMITATIONS OF CURRENT DESIGN PRACTICES Current simulation solutions provide only limited assistance in addressing these issues. On one hand, acoustic solutions do not address the noise generated by flow inside the HVAC module and ductwork which are always a concern and sometimes the greatest contributor to noise as perceived by the vehicle passenger. On the other hand, traditional CFD software that takes the full geometry of the HVAC module into account is constrained by the difference in scale between the highly detailed geometry of HVAC modules, often on the order of a few millimeters, as contrasted to the much larger acoustic waves, for example 34 cm at 1 KHz. The CFD solver is similarly affected by differences in time scale between bulk mass flow that moves at tens of meters per second vs. noise that propagates at the speed of sound. CFD also needs to resolve a six to eight order of magnitude difference between the pressure field induced by the mass and acoustic wave amplitudes. These scaling differences mean that solution time when conventional CFD codes are used to predict bulk airflow and noise propagation in HVAC modules is typically measured in weeks if not months. For this reason, traditional CFD software is usually limited to steady state simulation of HVAC modules so it is not able to capture the turbulent mechanisms responsible for the noise production. The result is limited accuracy and the inability to identify noise problems occurring at specific frequencies. Engineers sometimes attempt to save time by using CFD to simulate the flow through individual components or assemblies of the HVAC module. The problem with this approach is that engineers have no way of knowing which flow structures generate significant noise so they often spend considerable time and effort in correcting flow structures only to discover later that the impact on overall noise levels was negligible. The best current approach is to work around the limitations of these two alternatives by combining them. The CFD code is used to predict bulk airflow through the HVAC system. The CFD results are then coupled to an acoustic solution that is used to quantify the noise. The weakness of this approach is that it typically requires licensing of two different codes, employing experienced users of each code, introducing modeling of the noise sources and creating two different simulation domains. The result is a long and tedious process that produces results of limited accuracy. As a result, HVAC module designers typically wait until the first module prototypes are built before they begin assessing the acoustic performance of a new design. They are hampered by the fact that physical tests are typically able to determine noise generated by an HVAC module prototype but usually provide only very limited information on the source of that noise. So engineers are often forced to rely upon a trial and error process, changing one design parameter at a time and hoping that the result will be a reduction in noise generation. They also typically have only very limited information at this point about how integration with the vehicle will affect acoustic performance and only obtain this information late in the design process when changes are very expensive. 4

5 NEW APPROACH PROVIDES DRAMATIC IMPROVEMENTS PowerFLOW provides dramatic improvements in simulation performance by using a proprietary Lattice-Boltzmann solver technology that efficiently bridges the scaling gap between the bulk flow and acoustic propagation through an HVAC module. This advanced solver efficiently simulates both the bulk flow and flow-generated acoustic field propagating within the fluid and inherently captures the interactions between the two, avoiding the need for a separate acoustic solver. The inherent low dissipation of the Lattice-Boltzmann method ensures accurate resolution of the acoustic field. The efficiency of this approach makes it practical to simulate not only the entire module but also relevant portions of the vehicle such as the cabin and engine compartment in hours or days, a timeframe that is relevant to the automotive design cycle. This new solution gives HVAC module designers the ability to evaluate the performance of as many proposed designs as they can develop in the early stages of the design process without building physical prototypes. The new FIND module makes it faster and easier to solve noise problems revealed either by simulation or physical testing. The FIND module pinpoints, quantifies and ranks flow-induced noise sources, first in terms of their generated acoustic power, and second in terms of their contribution to the noise at a frequency and point in space defined by the user. These points are typically the ears of the vehicle passengers at which the simulated noise signals can be interactively played in PowerACOUSTICS. A PowerVIZ module goes one step further by automatically creating clusters of noise sources, providing clear insight into the location and strength of the noise generation regions in the HVAC module. This information, which is virtually impossible to obtain by physical testing, provides engineers with detailed instructions on which part of the design needs to be changed in order to make noise reduction improvements within the system in the shortest possible timeframe. With the aid of SIMULIA PowerVIZ, the results can be quickly and clearly visualized in 3D and superimposed on the underlying design geometry. This makes it easy to convey the findings to engineers, managers and other stakeholders for the purpose of expediting decisions on how to improve the design. CUSTOMER CASE STUDIES In the past, BMW frequently performed flow simulation on standalone components such as ducts or blowers but found the results did not correlate well with the acoustic performance of the whole system. In many cases, engineers spent considerable time improving the flow in a particular component, only to discover that there was no significant overall noise reduction. On recent programs, engineers simulated a complete HVAC system with PowerFLOW. The results enabled them to visualize how the sound was generated and how it propagated through the system, from the initial source to the passenger s ear. The major acoustic sources were grouped into clusters and ranked by overall acoustic power radiation. The largest acoustic sources were in the blower region while significant sources were also found in the center ducts. Next the acoustic transfer functions between the respective acoustic sources and the target position the driver s left ear were computed. At low blower speed, blower noise was still the highest contributor to the sound at the target location. At high blower speed, on the other hand, the center ducts contributed almost as much noise at frequencies from 100 Hz to 2 khz, which dominated the overall sound pressure level. The results show that if engineers had targeted the blower alone for aeroacoustic optimization, the results would have been minimal. By instead targeting both the blower and center duct, BMW engineers achieved substantial noise reduction. BMW has now made the decision to use SIMULIA s product solution on all of their car programs. An HVAC engineer for another major automotive OEM, used POWERFLOW to simulate an upcoming HVAC module. The results showed that noise levels would be far above the targets. The HVAC engineer then used simulation to identify the sources of the noise and to evaluate a series of changes to the shape of the HVAC module and ducts. The simulation results were used to explain the problem and sell the proposed solution to the program team. These changes solved the problem and the vehicle was released with best in-segment noise levels. 5

6 CONCLUSION HVAC module designers have long been challenged by the inability of conventional fluid dynamics software to predict noise generation within the normal vehicle development cycle. By enabling engineers to accurately assess HVAC systems acoustic performance and efficiently optimize performance before a physical prototype is built, PowerFLOW enables engineers to achieve noise targets and noise quality with a higher level of certainty and with less time and effort. Engineers can evaluate a wide range of possible designs without having to build a prototype so they are able to meet more challenging noise targets and determine the ideal tradeoff between acoustic and thermal performance. The ability to accurately and quickly evaluate virtual prototypes in the early stages of the design process makes it possible to get the design right the first time, resulting in a decrease of the development costs and in getting the product to market faster. Our 3DEXPERIENCE platform powers our brand applications, serving 12 industries, and provides a rich portfolio of industry solution experiences. Dassault Systèmes, the 3DEXPERIENCE Company, provides business and people with virtual universes to imagine sustainable innovations. Its world-leading solutions transform the way products are designed, produced, and supported. Dassault Systèmes collaborative solutions foster social innovation, expanding possibilities for the virtual world to improve the real world. The group brings value to over 210,000 customers of all sizes in all industries in more than 140 countries. For more information, visit Dassault Systèmes. All rights reserved. 3DEXPERIENCE, the Compass icon, the 3DS logo, CATIA, SOLIDWORKS, ENOVIA, DELMIA, SIMULIA, GEOVIA, EXALEAD, 3D VIA, 3DSWYM, BIOVIA, NETVIBES, IFWE and 3DEXCITE are commercial trademarks or registered trademarks of Dassault Systèmes, a French société européenne (Versailles Commercial Register # B ), or its subsidiaries in the United States and/or other countries. All other trademarks are owned by their respective owners. Use of any Dassault Systèmes or its subsidiaries trademarks is subject to their express written approval. Americas Dassault Systèmes 175 Wyman Street Waltham, Massachusetts USA Europe/Middle East/Africa Dassault Systèmes 10, rue Marcel Dassault CS Vélizy-Villacoublay Cedex France Asia-Pacific Dassault Systèmes K.K. ThinkPark Tower Osaki, Shinagawa-ku, Tokyo Japan