Linear Acoustic Exhaust System Simulation Using Source Data from Non Linear Simulation

Both linear (frequency domain) and non-linear (time domain) prediction codes are used for the simulation of duct acoustics in exhaust systems. Each approach has its own set of advantages and disadvantages. One disadvantage of the linear method is that information about the engine as an acoustic source is needed in order to calculate the insertion loss of mufflers or the level of radiated sound. The source model used in the low frequency plane wave range is the linear time invariant 1-port model. This source characterization data is usually obtained from experimental tests where multi-load methods and especially the two-load method are most commonly used. These measurements are time consuming and expensive. However, this data can also be extracted from an existing 1-D non-linear CFD code describing the engine gas exchange process. The pressure and velocity predictions from two acoustic load cases can be used to determine the source strength and impedance at a particular location in the exhaust line. This has been done at a location downstream of the turbocharger in the exhaust system of a heavy diesel truck over a number of speeds and engine loads. This source data is then used in a linear simulation of the exhaust line to predict sound pressure levels at a free field microphone position. The predicted source data and sound output at the microphone position is validated against measured data. The results show that you can obtain reasonably accurate source data and approximate free field sound pressure level predictions using non linear simulation in a linear acoustic model of the exhaust system. This technique can be used to extend the use of linear acoustic simulations to models of the complete exhaust line with the characterized engine as a source and exhaust sound output as a result.