We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

We study the effects of gravity modulation on the mixing characteristics of two interdiffusing miscible fluids initially in two vertical regions separated by a thin diffusion layer. We formulate the case of general gravity modulation of arbitrary orientation, amplitude $g$ and characteristic frequency $\omega$. For harmonic vertical modulation in two dimensions, the time-dependent Boussinesq equations are solved numerically and the evolution of the interface between the fluids is observed. The problem is governed by six parameters: the Grashof number, $\hbox{\it Gr}\,{=}\,({\Delta\rho}/{\bar{\rho}})g({l^{3}_{\nu}}/{\nu^{2}})$, based on the viscous length scale, $l_{\nu}\,{=}\,\sqrt{{\nu}/{\omega}}$; the Schmidt number, $\hbox{\it Sc}\,{=}\,{\nu}/{D}$; the aspect ratio, $A$; the non-dimensional length of the domain, $l$; the steepness of the initial concentration profile, $\delta$; and the phase angle of the harmonic modulation, $\phi$. When $\phi\,{=}\,0,\;\pi$, we observe four different flow regimes with increasing $\hbox{\it Gr}$: neutral oscillations at the forcing frequency; successive folds which propagate diffusively; localized shear instabilities; and both shear and convective instabilities leading to rapid mixing. In the last regime, the flow is disordered but not chaotic. By varying $\hbox{\it Sc}$, it was determined that the mechanism for the formation of these shear and convective instabilities is inertial. When $\phi \neq 0$ or $\pi$, the flow is similar to a modulated lock exchange flow.

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We study numerically the effects of zero-mean stochastic gravity modulation on the mixing characteristics of two interdiffusing miscible Boussinesq fluids initially separated by a thin diffusion layer. The gravity modulation has a Gaussian probability distribution and is characterized by an exponentially damped cosine autocorrelation function, i.e. $\langle g(t) g(t+\tau)\rangle/\langle g^{2}(t)\rangle\,{=}\,{\rm e}^{-\lambda \tau} \cos (\omega \tau)$. The associated power spectrum is a Lorentzian with peak at ω and width λ. The flow is found to depend on the following parameters: the Grashof number, Gr, based on the viscous length scale, $l_{\nu}\,{=}\,\sqrt{{\nu}/{\omega}}$; the Schmidt number, Sc; the correlation exponent, λ; and other geometric parameters. Even for extremely small Gr, we observe the propagation of gravity currents, Kelvin–Helmholtz (KH) and Rayleigh–Taylor (RT) instabilities. This is in contrast to the case of harmonic modulation considered in Part 1 (Siddavaram & Homsy J. Fluid Mech. vol. 562, 2006, p. 445) wherein these phenomena occur sequentially as Gr increases. The mixed volume is found to vary non-monotonically with the correlation exponent, λ, with narrow-band modulation having the largest mixed volume followed by harmonic modulation and then broadband modulation. This non-monotonicity of the mixed volume with λ is explained on the basis of the competition between the effects of excitation of lower frequencies, which lead to higher mixing, and the effects of the reduction in the energy content at the dominant frequency, which leads to reduced mixing. The value of the correlation coefficient, λ, at which the mixed volume is the largest is found to be independent of Gr. To understand the finer details of the mechanisms, we consider two- and three-frequency modulations. We find that increasing the amplitude of the secondary component when its frequency is smaller than that of the primary component leads to the occurrence of KH and RT instabilities at smaller Gr than that for the case of single-frequency modulation. We have understood the non-monotonic variation in the mixed volume by considering a three-frequency modulation, where one of the frequencies is smaller than the characteristic frequency and the other larger.