Aeroacoustic simulations are divided into hybrid and direct simulations. In this chapter, the effects of freestream Mach number on flow and acoustic fields around a two-dimensional square cylinder in a uniform flow are focused on using direct and hybrid simulations of flow and acoustic fields are performed. These results indicate the effectiveness and limit of the hybrid simulations. The Mach number M is varied from 0.2 to 0.6. The propagation angle of the acoustic waves for a high Mach number such as M = 0.6 greatly differs from that predicted by modified Curle’s equation, which assumes the scattered sound to be dominant and takes the Doppler effects into consideration. This is because the acoustic field is affected by the direct sound, which is generated by quadrupoles in the original Curle’s equation. To clarify the effects of the direct sound on the acoustic field, the scattered and direct sounds are decomposed. The results show that the direct sound is too intense to neglect for M ≥ 0.4. Moreover, acoustic simulations are performed using the Lighthill’s acoustic sources.
Part of the book: Numerical Simulations in Engineering and Science