\ \\ Seeing Sound: A New Way To Reduce Exhaust System Noise
Why Do You Need to See Sound? Vehicle comfort, safety, quality, and driver experience all rely on controlling the noise made by multiple systems. Engine noise and road noise are coming under control, making HVAC system noise the next target for noise reduction and improvement. Regulatory requirements limit the impact exhaust system and muffler noise can have on humans in the environment, like a law recently passed by the European Parliament requiring a 4 db reduction in passenger cars, buses and vans and a 3dB reduction in trucks. Improved fuel economy is a result of reduced exhaust system noise: for every inch of mercury of back pressure, approximately 1 or 2 horsepower is lost, resulting in an increase in fuel consumption and carbon monoxide and particulate matter emissions. Finding the source of noise and realizing its impact on the human ear are two essential components of correcting noise problems in vehicle design. Watch the video to see how Exa does it. 2
Controlling Exhaust System Noise: Challenges Two sources of unwanted noise from the exhaust system are: 1) acoustic pulses generated by periodic combustion pressures in the cylinders and 2) self-noise generated related to complex geometries and airflow paths. Controlling acoustic pulses. The periodic high amplitude pressure pulses originating in the combustion chambers are a source of noise contamination both inside and outside the vehicle. Muffler geometry creates acoustic resonance with interferences that reduce the intensity of the noise generated by the engine. Digital simulation makes it possible to compare the impact of two or more design alternatives on unwanted noise before a prototype is produced. Read the white paper to learn the challenges. Reducing system self-noise. The highly complex geometry required to accomplish this task generates airflow disturbances that can create new noise. The objectives of muffler design to cancel the pulsating components of the exhaust flow while generating as little noise as possible and keeping back pressure of the muffler as low as possible often compete, further challenging engineers. 3
Why Is It So Hard to See Sound? Sound = geometry + airflow. Muffler geometry is highly detailed like sheet metal with tiny (2-3 mm) perforations. But acoustic waves are much larger on the order of a 34cm at 1 khz. Conflicting time scales further complicate things, with bulk mass flow through the muffler at 10 20 meters/second and noise that propagates at the speed of sound, or 300-400 meters/second. Finding problems in test. By the time exhaust systems are typically tested, prototypes are already built and failures can be heard as unpleasant sounds. Physical tests can t see inside the muffler to tell what geometry is causing the unwelcome outcomes. Solving late-stage issues. The time-consuming process of re-engineering the exhaust system, then building and testing new prototypes, relies on costly trial-and-error, which suppliers and OEMs have reported can take as many as 6 to 12 months and some vehicles are even released with the issue when no solution could be found. Finding the source of noise and evaluating its impact on the ear are two essential components of correcting noise problems in vehicle design. Read: Exa customer successes aeroacoustics. 4
How does Exa Do It? PowerFLOW PowerFLOW by Exa accurately predicts the turbulent airflow driven by the most complex and intricate geometry details in the exhaust system while simultaneously capturing and propagating the noise inside and outside the exhaust system. Its proven and acknowledged accuracy for the prediction of aerodynamically-induced noise is enabled by key properties of the solver: transient, compressible, very low numerical dissipation, and ability to handle complex, detailed geometry. It can - Simulate transmission losses while accurately predicting the noise-generating turbulent flows driven by the smallest, most intricate geometric details of the exhaust system - Uses a proprietary and efficient Lattice-Boltzmann solver especially suitable for transient flow simulations - Employs proprietary turbulence and near wall models, extensively validated for hundreds of cases - Achieves very high accuracy of the flow and noise predictions - Performs transient and compressible simulations of the flow on fully realistic complex geometries - Computes the resulting noise sources accurately using an approach based on the dynamics of the flow structures. - Completes simulations in a time frame that satisfies the automotive development cycle, measured in hours or days. - Includes under and upper bodies in the simulation to evaluate the radiated noise levels and the insertion losses in order to assess the performance in installed conditions. - Measures the effect of absorbing materials in presence of the flow with Exa s Acoustic Porous Medium (APM) solution - Work as a pure acoustic solver in order to accurately estimate the transmission losses of the exhaust systems and quickly investigate the performances of various designs and architectures. 5
How does Exa Do It? PowerACOUSTICS FIND PowerACOUSTICS FIND (Flow Induced Noise Detection) provides diagnostic technology to pinpoint and quantify noise sources produced by the flow. Engineers can listen to sound generated by the simulated exhaust system before a prototype is built. They can quickly investigate the performance of different exhaust designs and architectures in order to achieve the right sound signature and meet noise and fuel economy standards with fewer design and prototyping iterations. It - Generates clear and meaningful 3-D maps of noise sources that can be visualized and intuitively analyzed - Pinpoints clearly the origin of noise and provides the ranking of the different sources in or around the system. - Simplifies the traditional analysis of the data using, for instance, unsteady flow and band-filtered animations or db maps. - Enables the automatic creation of clusters of noise sources - Offers the user clearer insight on the location and strength of the various noise-generating regions present in the system. EXA SOFTWARE USED FOR THIS APPLICATION: Simulation Preparation: Simulation: Results Analysis: PowerDELTA PowerFLOW PowerACOUSTICS PowerCASE PowerVIZ Read: Exa s FIND module in the news. PowerINSIGHT 6