6.1. Differences between the experimental and numerical values
The agreement between the experimental and numerical values in the grazing flow validation case is poorest at 1000 Hz. This is visible in both the SPL and phase plots in Figs. 9 and 10. A relatively change in the acoustic field. Even with the improvement of the WEM educed impedance value, there remains a difference. This disagreement may be attributed to the presence of a hydrodynamic instability in the experimental test at this frequency. This instability has been reported previously in Refs. [26,14]. As it is not included in the present methodology, the disagreement with the data here is to be expected. Interestingly, it can be noted that the WEM educed impedance value is still comparable to a value which has been shown to give good agreement in propagation methods that include this instability – Burak et al. for example used an impedance value of Z = 0:48 þ 0:03i(compared to the WEM educed value of Z = 0:43 þ 0:07i) in a LES simulation and achieved excellent agreement with the same data. This strengthens the argument that the present methodology is robust, at least for impedance eduction.