Beam quality in a turbulent mixing medium

A criterion for evaluating beam quality is discussed.By dividing the mediumdisturbances into the ordered and random part, introducing the time-averaged variance ofphase distortion and fitting the wavefront with Zernike polynomials,a simplified formula forevaluating beam quality in a turbulent mixing medium is derived.     

Hyperdiffusion in nonlinear large- and small-scale turbulent dynamos

The generation of large-scale magnetic fields is generically accompanied by the more rapid growth of small-scale fields. The growing Lorentz force due to these fields backreacts on the turbulence to saturate the mean-field and small-scale dynamos. For the mean-field dynamo, in a quasilinear treatment of this saturation, it is generally thought that, while the alpha effect gets renormalized and suppressed by nonlinear effects, the turbulent diffusion is left unchanged. We show here that this is not true and the effect of the Lorentz forces is also to generate additional nonlinear hyperdiffusion of the mean field. A combination of such nonlinear hyperdiffusion with diffusion at small scales also arises in a similar treatment of small-scale dynamos, and is crucial to understand its saturation.     

Acoustic control of instability waves in a turbulent jet

The possibility of acoustic control of instability waves formed in the mixing layer of a jet is experimentally investigated. The feasibility of suppressing a hydrodynamic instability wave in a subsonic turbulent jet by an external acoustic action is demonstrated. This result can be used in designing active control systems for jet noise suppression.     

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.     

Small-scale anisotropy in turbulent shearless mixing

The generation of small-scale anisotropy in turbulent shearless mixing is numerically investigated. Data from direct numerical simulations at Taylor Reynolds' numbers between 45 and 150 show not only that there is a significant departure of the longitudinal velocity derivative moments from the values found in homogeneous and isotropic turbulence but that the variation of skewness has an opposite sign for the components across the mixing layer and parallel to it. The anisotropy induced by the presence of a kinetic energy gradient has a very different pattern from the one generated by an homogeneous shear. The transversal derivative moments in the mixing are in fact found to be very small, which highlights that smallness of the transversal moments is not a sufficient condition for isotropy.     

Hydrodynamic instability of multiple four-wave mixing

In the regime of normal dispersion and low-frequency detunings (or high powers), four-wave mixing is shown to undergo a hydrodynamic type of instability. Such instability involves the formation of shocks (steep fronts) from smooth initial data that are regularized through the appearance of trains of fast oscillations, which exhibit solitonlike behavior, colliding elastically.     

激波冲击V形界面重气体导致的壁面与旋涡作用及其对湍流混合的影响

基于多组分混合物质量分数模型,采用色散最小耗散可控的高分辨率有限体积方法,数值模拟了弱激波冲击V形空气/SF_6界面后,界面不稳定性生成的旋涡与固体壁面作用问题.激波冲击V形界面之后,因斜压效应诱导涡量沉积在界面附近,形成沿界面规则排列的多个涡对结构.旋涡的诱导作用使界面不断变形和卷起,同时旋涡之间不断发生相互并对,诱导更多更小尺度的旋涡产生.旋涡诱导作用的叠加效应,使界面尖端处的初始涡对向上下壁面发展.随后,涡结构开始与壁面发生复杂的相互作用.旋涡与壁面作用后沿壁面加速,使得物质界面沿壁面伸展,随后,旋涡从壁面回弹,并诱导二次旋涡产生.旋涡与壁面相互作用的过程,能够明显加剧物质混合.本文从物质混合的角度研究了该过程的机理,分析了旋涡与壁面作用对物质混合的影响.     

Topological evolution near the turbulent/non-turbulent interface in turbulent mixing layer

The topological evolution near the turbulent/non-turbulent interface (TNTI) in turbulent mixing layer is studied by means of statistical analysis of the invariants of velocity gradient tensor (VGT) based on direct numerical simulation data. The dynamics of topological evolution is investigated in terms of the source terms of the evolution equations for the invariants, including the pressure effect term, viscous effect term and interaction term among the invariants. It is found that the local topology of fluid particles at the TNTI evolves from non-focal region to focal region in the plane of the second (Q) and the third (R) invariants of the VGT. The topological evolution is mainly associated with the pressure effect term in the TNTI region. According to the analysis of the evolution of enstrophy and dissipation, the enstrophy increase and the dissipation decrease are revealed in the TNTI region, which are caused by viscous vorticity diffusion near the TNTI. A weak correlation between the strain rate and the rotation rate is found in the TNTI region which is related to the reduction of enstrophy production. The alignments between vorticity and strain near the TNTI are investigated and a strong alignment of the vorticity with the extensive eigenvector direction is identified in the TNTI region.     

Adjoint-based sensitivity and ignition threshold mapping in a turbulent mixing layer

Successful ignition in non-premixed turbulent flows remains a fundamental challenge in combustion systems. Current design strategies typically rely on iterative testing to map the spatial distribution of ignition probability. We propose to accelerate this by formulating the adjoint of the perturbed and linearised governing equations in such a way that sensitivity of an ignition indicator can be obtained with a cost comparable to the flow solution. A space–time discrete adjoint method for multi-component chemically reacting flows is developed, and the gradient formed via the corresponding adjoint solution is used to identify regions favourable to ignition in a direct numerical simulation of non-premixed turbulent free shear flow. This approach requires a specific definition of an ignition metric, although this can be problematic because ignition either succeeds or fails after some period and thus gradients for some metrics become ill-defined near the ignition threshold. To this end, a quantity of interest is designed to provide short-time sensitivity in conjunction with an indicator function over a long-time period that informs whether successful ignition occurred. The gradients are used in a line-search algorithm to map the ignition boundary under specific constraints. Finally, parametric sensitivity is evaluated at different flow realisations to analyse factors governing local sensitivity in unsteady chemically reacting flows.     

Small-scale-velocity-induced linear instability of large-scale turbulent flows

Analysis of a simplified equation derived previously for small-scale velocity components shows that any turbulent flow of an incompressible liquid becomes unstable against infinitesimal perturbations of small-scale velocity components if the strain rate tensor for the large-scale velocity is high. Such a statement comes into conflict with the classical stability theory, which specifically asserts that the Poiseuille flow in a circular tube is linearly stable against infinitesimal perturbations.     

Weakly turbulent instability of anti-de Sitter spacetime

We study the nonlinear evolution of a weakly perturbed anti-de?Sitter (AdS) space by solving numerically the four-dimensional spherically symmetric Einstein-massless-scalar field equations with negative cosmological constant. Our results suggest that AdS space is unstable under arbitrarily small generic perturbations. We conjecture that this instability is triggered by a resonant mode mixing which gives rise to diffusion of energy from low to high frequencies.     

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.     

Analysis of a turbulent jet mixing flow by using a PIV-PLIF combined system

In the present paper, the development of a high-resolution PIV-PLIF combined system for the simultaneous measurements of velocity and passive scalar concentration fields is described. The high-resolution PIV-PLIF combined system is used to perform the simultaneous whole-field measurements of velocity and concentration in the near field of a turbulent jet mixing flow. The characteristics of the mass transfer process and momentum transfer process in the near field of the jet mixing flows are discussed in the terms of ensemble-averaged velocity and concentration, turbulent intensity, concentration standard deviation and the correlation terms between the fluctuating velocities and concentration.     

Structure analysis of turbulent mixing patterns in inert and reactive turbulent liquid flows

The paper contains an extended summary of an invited plenary talk given at the Workshop on Active Chaos at the Los Alamos National Laboratory on 29-31 May 2001 by one of us (F.S.R.). (c) 2002 American Institute of Physics.     

Effects of turbulent mixing on the critical behavior

Effects of strongly anisotropic turbulent mixing on the critical behavior are studied by means of the renormalization group. Existence of new nonequilibrium types of critical regimes (universality classes) is established.     

In-flight simulation experiments on turbulent jet mixing noise

The effects of aircraft forward motion on pure turbulent mixing noise from unheated jets have been examined experimentally in the in-flight simulation mode. Both acoustic and flow characteristics were determined by testing model-scale nozzles in an anechoic free jet facility and a wind tunnel, respectively. Scaling laws were derived from each set of experiments and found to be complementary. The implications are discussed in detail. In particular, it is shown that the measured reduction in noise at 90° to the jet axis is a pure source alteration effect.