Order and realizability of impulse response filters for accurate identification of acoustic multi-ports from transient CFD

For analysis of (thermo-)acoustic systems and in particular combustion instabilities, so-called network models are popular. Such models comprise multi-ports, which are represented mathematically by transfer matrices. Transfer matrices may be reconstructed from transient CFD simulation data with system identification tools, i.e. unit impulse response filters are determined with correlation analysis. The present study is concerned with the optimal choice of parameters for accurate transfer matrix reconstruction. Recommendations for the optimal choice of sample increment and sample length as well as filter order are formulated. Remarkably, it is found that in many cases the use of non-causal filters is advantageous. It is argued that this is a consequence of the fact that causal interrelationships imposed by the underlying laws of fluid mechanics are not always represented properly with the standard acoustic variables.