低速喷嘴动力学和声辐射研究
Experimental velocity measurements of a low-speed jet, performed using time-resolvedtomographic PIV, are used to study the dynamics of large-scale structures and their soundradiation. The experimental results show the roll-up of axisymmetric vortices that pairdownstream, and subsequently lose their azimuthal coherence. Models of linear instabilitywaves using both steady laminar and mean-field base flows flow are applied. While goodagreement can be obtained for the vortex roll-up frequency in the near-nozzle region usingthe laminar base flow, non-linear effects must be included, via the mean field, in orderto capture the downstream evolution of both the fundamental and subharmonic (vortexpairing). The velocity fluctuations for both frequencies have a wave-packet structure withsome jitter in the form of modulations of the spatial extent and amplitude of the envelope.The sound radiation is modelled using a jittering wave-packet model, an shows agreementwith the exponential directivity shape of Laufer and Yen (J. Fluid Mech. 134, 1983).