The influences of spatial and temporal discretization in flow simulation on Lighthill’s aeroacoustic source terms applied to a turbocharger

For investigating source term mechanisms and the location of source terms in a turbocharger compressor, Lighthill’s analogy is applied. The influence of different time steps, as well as the influence of different element sizes onto results of the compressible and transient Computational Fluid Dynamic (CFD), and of Lighthill’s stress tensor are investigated. Therefore, all three parts of Lighthill’s source term, (1) the Reynolds stress term, (2) the excess term, and the (3) viscous term were analyzed in time and frequency domain. It could be shown that for an operating point close to the highest efficiency, the most dominant source term is the excess term. The Reynolds stress term is about one order of magnitude smaller, and the viscous source term is about two orders of magnitude smaller than the excess term. Furthermore, it could be shown that a time step corresponding 360 steps per rotation is sufficient for resolving frequencies in die audible range. Regarding the mesh, 3 · 10 ⁶, and 7 · 10 ⁶ cells could not sufficiently resolve relevant scales of turbulence. Consequently, the mesh with 11 · 10 ⁶ cells and 360 time steps per revolution should be used for a turbocharger of similar size.