Published online by Cambridge University Press: 27 November 2018
Two temporally developing isothermal round jets at a Mach number of 0.9 and Reynolds numbers of 3125 and 12 500 are simulated in order to investigate noise generation in high-subsonic jet flows. Snapshots and statistical properties of the flow and sound fields, including mean, root-mean-square and skewness values, spectra and auto- and cross-correlations of velocity and pressure, are presented. The jet at a Reynolds number of 12 500 develops more rapidly, exhibits more fine turbulent scales and generates more high-frequency acoustic waves than the other. In both cases, however, when the jet potential core closes, mixing-layer turbulent structures intermittently intrude on the jet axis and strong low-frequency acoustic waves are emitted in the downstream direction. These waves are dominated by the axisymmetric mode and are significantly correlated with centreline flow fluctuations. These results are similar to those obtained at the end of the potential core of spatially developing jets. They suggest that the mechanism responsible for the downstream noise component of these jets also occurs in temporal jets, regardless of the Reynolds number. This mechanism is revealed by averaging the flow and pressure fields of the present jets using a sample synchronization with the minimum values of centreline velocity at potential-core closing. A spot characterized by a lower velocity and a higher level of vorticity relative to the background flow field is found to develop in the interfacial region between the mixing layer and the potential core, to strengthen rapidly and reach a peak intensity when arriving on the jet axis, and then to break down. This is accompanied by the growth and decay of a hydrodynamic pressure wave, propagating at a velocity which, initially, is close to 65 per cent of the jet velocity and slightly increases, but quickly decreases after the collapse of the high-vorticity spot in the flow. During that process, sound waves are radiated in the downstream direction.
Vorticity norm in the jet flow and of pressure fluctuations outside obtained for ReD=3,125.. The color scales range up to the level of 5uj/r0 for vorticity, and from -250 Pa to 250 Pa for pressure, from blue to red.
Vorticity norm in the jet flow and of pressure fluctuations outside obtained for ReD=12,500. The color scales range up to the level of 5uj/r0 for vorticity, and from -200 Pa to 200 Pa for pressure, from blue to red.
Space-time correlations of axial velocity fluctuations obtained at r=0 for ReD=3,125; solid line: dt=dz/(0.6uj); dashed line: dt=dz/uj. The color scale ranges from -1 to 1, from blue to red.
Space-time correlations of axial velocity fluctuations obtained at r=0 for ReD=12,500; solid line: dt=dz/(0.6uj); dashed line: dt=dz/uj. The color scale ranges from -1 to 1, from blue to red.
Space-time correlations obtained between centerline velocity fluctuations at t=t1 and pressure fluctuations at r=10r0 at t=t2 for ReD=3,125. The color scale ranges from -0.20 to 0.20, from blue to red; solid line: propagation at the ambient speed of sound; dashed line: t=tc.
Space-time correlations obtained between centerline velocity fluctuations at t=t1 and pressure fluctuations at r=10r0 at t=t2 for ReD=12,500. The color scale ranges from -0.20 to 0.20, from blue to red; solid line: propagation at the ambient speed of sound; dashed line: t=tc.
Velocity and pressure fluctuations obtained inside and outside the flow for ReD=3,125 using conditional averaging. The color scales range from -90 Pa up to 90 Pa for pressure and from -0.075uj up to 0.075uj for velocity, from blue to red.