Parametric instability of dust lattice waves in a turbulent plasma sheath

It is shown that dust lattice waves (DLWs) become parametrically unstable in a turbulent dusty plasma sheath that contains random-phase ion plasma waves. Physically, the parametric instability arises because of the nonlinear coupling between ion plasma waves and DLWs. The parametric instability is identified as the possible reason for the observed "sublimation" transition of the plasma crystal from a solid to a gaslike state.     

Experimental study of propagation of instability waves in a submerged jet under transverse acoustic excitation

An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.     

Feedback control of instability waves in a transitional flat-plate boundary layer

A semi-automatic and active control of T-S waves and oblique waves in a transitional flat plate boundary layer is carried out in a wind-tunnel experiment and a numerical simulation. An array of piezo-ceramic actuators attached on a surface is used to generate counter waves that cancel the incoming instability waves. The actuator’s operating amplitudes and phases are successively updated using the velocity fluctuations monitored downstream by a rake of hotwires. Experimental results show that the system is effective in weakening these waves when their inclination angles are less than 15 degrees. However, the system encountered difficulty in controlling the waves of large inclination angles. In the numerical simulation, it is shown that the control can be accomplished much easier. The numerical results show that controllability of the large inclination angle waves can be improved by shortening the spanwise length each actuator piece. The danger of pursuing this kind of research solely by a numerical simulation is pointed out.     

Acoustic waves in a Cattaneo-Christov gas

A model is presented for an inviscid fluid using the recently proposed model for heat waves due to C. Christov. Christov's model generalizes the classical model of Cattaneo to the important case of a moving body. We here couple this to the equations of motion for a fluid. It is then shown that an acoustic wave will propagate together with a thermal wave. The theory allows for the possibility of a transverse wave in the heat flux. The wavespeeds are calculated exactly and one may fully determine the wave amplitudes.     

Effect of VGs on a turbulent hydrogen jet

The aim of this study is to investigate numerically the effects of four vortices on the dynamic, scalar, and turbulent fields of the hydrogen jet. These vortices, which appear in the vicinities of the nozzle, are created by the vortex generators (VGs), and they are assembled with periodicity or symmetry in order, respectively, to give four vortices of the same or opposite direction. A second-order Reynolds stress model is used to investigate asymmetric turbulent jet. The results indicate that the presence of the vortex near the emission jet section noticeably enhances mixing to ensure a good combustion.     

Free surface instability in a confined suspension jet

A suspension confined between two close parallel plates is studied in the Stokesian regime. The use of boundary integral equations and the lubrication approximation allows to compute the hydrodynamic forces acting on the particles. The forces are long ranged and depend on the orientation of the relative position and velocity of particles. This tensorial character predicts an “antidrag” that is observed in experiments. The effect of the computed hydrodynamic forces is studied in the dynamics of a jet of particles falling by a gravitational field, which shows a surface instability similar to the Kelvin–Helmholtz one. A theoretical model, based on hydrodynamic-like equations, is able to predict the instability that is produced by the interaction of the long-range forces and the free surface.     

Acoustic waves in a rotating ideal gas

The acoustic wave propagation in a rotating ideal gas with density stratification due to the rotation is considered. To describe the waves propagating normally to the rotation axis, an equation for a scalar quantity that is a function of density is proposed. The frequency dependences of the phase and group velocities of an acoustic-gyroscopic wave are presented. It is shown that the stratification of gas leads to an exponential amplitude decay for an acoustic wave generated near the rotation axis and has no effect on the inertial-gyroscopic wave.     

Intermittency in a turbulent low temperature gaseous helium jet

We analyse experimental velocity measurements on the axis of a low temperature gaseous helium jet. From independent increments arguments, we reproduce the behaviour of structure functions. We show where this approach fails and how the intermittency phenomenon is a small correction. The physical arguments under the multiplicative cascade models for this intermittency imply an acceleration of this cascade close to the dissipative range, which we are able to evidence. This acceleration could be responsible of the apparent Extended Self Similarity between structure functions of various orders. Received 13 October 1999 and Received in final form 19 May 2000     

Absolute instability of a liquid jet in a coflowing stream

Cylindrical liquid jets are inherently unstable and eventually break into drops due to the Rayleigh-Plateau instability, characterized by the growth of disturbances that are either convective or absolute in nature. Convective instabilities grow in amplitude as they are swept along by the flow, while absolute instabilities are disturbances that grow at a fixed spatial location. Liquid jets are nearly always convectively unstable. Here we show that two-phase jets can breakup due to an absolute instability that depends on the capillary number of the outer liquid, provided the Weber number of the inner liquid is >O(1). We verify our experimental observations with a linear stability analysis.     

Absolute parametric instability in a turbulent,inhomogeneous medium

It is shown analytically that the convective saturation of parametric instabilities in an infinite inhomogeneous medium can disappear in the presence of background turbulence. Absolute instability then occurs.     

Identifying turbulent jet sound sources

Theoretical models of the formation of the acoustic field of a nonisothermal turbulent flow are analyzed. The paper presents possible methods of constructing solutions to the convective wave equation for a shear parallel nonisothermal turbulent flow. A simplified mathematical model of noise sources in the field of a nonisothermal shear turbulent flow is considered, based on real physical notions on the processes occurring in the jet mixing range, making it possible to approximately estimate the spectral noise characteristics.     

Steady large-scale modulation of a moderately turbulent co-flow jet

The effects of a spatial modulation acting at the inflow of a moderately turbulent planar jet surrounded by a faster co-flow are investigated using direct numerical simulation of the Navier–Stokes equations. We adopt a superposition of spatially filtered small-scale random perturbations and a structured large-scale flow modulation. The large-scale modulation is characterised in terms of a Beltrami flow, specified by a wavenumber K. These large-scale modulations are steady and spatially periodic, while the random small-scale perturbations fluctuate in time and in space. The flow configuration studied in this paper is agitated by this combined large- and small-scale agitation at the inflow plane of a rectangular domain of size L × L × 2L in the x-, y- and streamwise z-directions. The inflow perturbation is focused on a strip of size L × D in the x- and y-directions. A parametric variation is carried out considering different choices for the wavenumber of the large-scale modulation. We focus on effects that the inflow modulation has on global characteristics of the flow, e.g. the width of the mixing region formed between the two streams and the dissipation rate, ?. Results show that the width of the mixing region increases faster compared to the case without the large-scale perturbation, when the flow is agitated by structures of size comparable to the integral scales of the flow. For the dissipation rate, results show the presence of a maximum response at a certain wavenumber K in case we apply a large-scale modulation. This maximum is attained at modulation scales that vary locally with respect to the distance from the inflow plane. Close to the inflow, the maximum response occurs at small modulation scales, while further into the domain a maximum response is present at comparably large modulation scales.     

Scattering of sound waves by a turbulent wake

In this investigation, the Doppler shifted power spectrum of the scattering cross-section is obtained for plane acoustic waves scattered by fluid flow fluctuations appropriate to a turbulent wake. The wake considered in this paper is assumed almost homogeneous and isotropic and of low Reynolds number.It is shown that the evaluation of the Doppler scattering cross-section essentially reduces to the calculation of the wave number converted and frequency shifted energy spectrum function of the turbulent flow fluctuations. In prescribing the low Reynolds number turbulence spectrum, inertial forces are assumed negligible. Convective effects of the macro-eddies, which cause a Doppler shift in the scattered waves, are considered using a Lagrangian-type of space-time velocity correlation.After finding the spectrum of turbulent fluctuations, the Doppler shifted power spectrum of the scattering cross-section, which characterizes the scattered waves, is obtained explicitly for the far field approximation.     

CMC simulations of lifted turbulent jet flame in a vitiated coflow

Lifted turbulent jet diffusion flame is simulated using Conditional Moment Closure (CMC). Specifically, the burner configuration of Cabra et al. [R. Cabra, T. Myhrvold, J.Y. Chen, R.W. Dibble, A.N. Karpetis, R.S. Barlow, Proc. Combust. Inst. 29 (2002) 1881–1887] is chosen to investigate H₂/N₂ jet flame supported by a vitiated coflow of products of lean H₂/air combustion. A 2D, axisymmetric flow-model fully coupled with the scalar fields, is employed. A detailed chemical kinetic scheme is included, and first order CMC is applied. Simulations are carried out for different jet velocities and coflow temperatures (T_c). The predicted liftoff generally agrees with experimental data, as well as joint-PDF results. Profiles of mean scalar fluxes in the mixture fraction space, for T_c=1025 and 1080 K reveal that (1) Inside the flame zone, the chemical term balances the molecular diffusion term, and hence the structure is of a diffusion flamelet for both cases. (2) In the pre-flame zone, the structure depends on the coflow temperature: for the 1025 K case, the chemical term being small, the advective term balances the axial turbulent diffusion term. However, for the 1080 K case, the chemical term is large and balances the advective term, the axial turbulent diffusion term being small. It is concluded that, lift-off is controlled (a) by turbulent premixed flame propagation for low coflow temperature while (b) by autoignition for high coflow temperature.     

Acoustic perturbations and turbulent pulsations

The space-time pulsation field of turbulent flow parameters is interpreted on the basis of acoustic perturbations. The process of perturbation formation is discussed. Two concurrent mechanisms of pulsation field formation are proposed. One of these mechanisms is traditionally related to vorticity; the other is caused by the pulsations of hydrodynamic parameters in the region of acoustic waves.     

Random vortex-street model for a self-similar plane turbulent jet

We ask what determines the (small) angle of turbulent jets. To answer this question we first construct a deterministic vortex-street model representing the large-scale structure in a self-similar plane turbulent jet. Without adjustable parameters the model reproduces the mean velocity profiles and the transverse positions of the large-scale structures, including their mean sweeping velocities, in a quantitative agreement with experiments. Nevertheless, the exact self-similar arrangement of the vortices (or any other deterministic model) necessarily leads to a collapse of the jet angle. The observed (small) angle results from a competition between vortex sweeping tending to strongly collapse the jet and randomness in the vortex structure, with the latter resulting in a weak spreading of the jet.