Linear and nonlinear mechanisms of sound generation in subsonic jets are investigated by numerical simulations of the compressible Navier–Stokes equations. The main goal is to demonstrate that low-frequency waves resulting from nonlinear interaction between primary, highly amplified, instability waves can be efficient sound radiators in subsonic jets. The current approach allows linear, weakly nonlinear and highly nonlinear mechanisms to be distinguished. It is demonstrated that low-frequency waves resulting from nonlinear interaction are more efficient in radiating sound when compared to linear instability waves radiating directly at the same frequencies. The results show that low-frequency sound radiated predominantly in the downstream direction and characterized by a broadband spectral peak near St = 0.2 can be observed in the simulations and described in terms of the nonlinear interaction model. It is also shown that coherent low-frequency sound radiated at higher angles to the jet axis (θ = 60°–707°) is likely to come from the interaction between two helical modes with azimuthal wavenumbers n = ±1. High-frequency noise in both downstream and side-line directions seems to originate from the breakdown of the jet into smaller structures.

The incompressible large eddy simulation technique, coupled with the Lighthill-Curle acoustic analogy, is used to investigate the oscillation mechanism and sound source of a two-dimensional cavity with a length-to-depth ratio of L / D = 4 and Reynolds number of Re D = 5000. It is demonstrated that the development of the three-dimensional flow field, initiated by the introduction of a random inflow disturbance, is eventually accompanied by transition from the wake to the shear layer oscillation mode, regardless of the amplitude and shape of the inflow disturbance. Once the transition to the shear layer mode is accomplished, the amplitude and frequency of oscillations are not very sensitive to the particular shape of the inflow disturbance. The effectiveness of controlling the flow oscillations by applying simultaneous steady injection and suction through the front and rear cavity walls, respectively, is demonstrated. The results show that, for injection levels exceeding a certain threshold value, the oscillations are quenched, and for levels below that value, the oscillation process is virtually unaffected. The major difference between the averaged uncontrolled and controlled velocity fields is the amount of reverse flow in the rear part of the cavity. With the aid of linear stability analysis, it is demonstrated that for injection levels leading to the quenching of the oscillations the mean velocity profiles in the cavity region are only convectively unstable, whereas for the uncontrolled case there is an absolutely unstable region. This suggests that, at least for incompressible flow, the reduction of the reverse flow inside the cavity can reduce or eliminate the oscillation process.

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The similarity of the coherent structures (streaks and hairpin vortices) naturally occurring in different fully developed bounded turbulent shear flows as well as in transitional flows suggests the existence of a basic mechanism responsible for the formation of these structures, under various base flow conditions. The common elements for all such flows are the shear of the base flow and the presence of a localized vortical disturbance. The objective of the present numerical study is to examine the capability of a simple model of interaction, between a localized vortical disturbance and laminar uniform unbounded shear flow, to reproduce the generation mechanism and characteristics of the coherent structures that naturally occur in turbulent bounded shear flows. The effects of the disturbance ‘localized character’ in the stream-wise and spanwise directions as well as its initial orientation relative to the base flow are investigated by using several geometries of the initial disturbance. The results demonstrate that a small-amplitude initial disturbance (linear case) eventually evolves into a streaky structure independent of its initial geometry and orientation, whereas, a large-amplitude disturbance (strongly nonlinear case) evolves into a hairpin vortex (or a packet of hairpin vortices) independent of its geometry over a wide range of the initial disturbance orientations. The main nonlinear effects are: (i) self-induced motion, which results in the movement of the vortical structure relative to the base flow and the destruction of its streamwise symmetry, and (ii) the alignment of the vortical structure with the vorticity lines. This is unlike the linear case, where there is a strong deviation of the vorticity vector from the direction of the vortical structure. Qualitatively, the disturbance evolution is sufficiently independent of its initial geometry, whereas the associated quantitative characteristics, i.e. inclination angle, centre and strength (which is governed by the transient growth mechanism), strongly depend on the disturbance geometry. The Reynolds number is found to have a negligible effect on the kinematics of the vortical structure, but does have a significant effect on its transient growth. Finally, the formation of the asymmetric hairpin vortex, due to minor spanwise asymmetries of the initial disturbance, is demonstrated.

The compression of a cylindrical gas bubble by an imploding molten lead (Pb) shell may be accompanied by the development of the RichtmyerMeshkov (RM) instability at the liquid-gas interface due to the initial imperfection of the interface. A converging pressure wave impinging upon the interface causes a shell of liquid to detach and continue to travel inwards, compressing the gas bubble. The efficiency of compression and collapse evo- existing theoretical models and good agreement has been found. While our main focus is on the effects of initial perturbation amplitude and azimuthal mode number, we also address differences between this problem and those usually considered, such as RM instability at an interface between two gases with a moderate density ratio. One important difference is the formation of narrow molten lead jets rapidly propagating inwards during the final stages of the collapse. Jet behaviour has been observed for a range of azimuthal mode numbers and perturbation amplitudes.