Hierarchy of structure functions for passive scalars advected by turbulent flows

A hierarchical model is proposed for the joint moments of the passive scalar dissipation and the velocity dissipation in fluid turbulence. This model predicts that the joint probability density function (PDF) of the dissipations is a bivariate log-Poisson. An analytical calculation of the scaling exponents of structure functions of the passive scalar is carried out for this hierarchical model, showing a good agreement with the results of direct numerical simulations and experiments.

Direct Numerical Simulation of Passive Scalar in Decaying Compressible Turbulence

The passive scalars in the decaying compressible turbulence with the initial Reynolds number (defined by Taylor scale and RMS velocity) Re=72, the initial turbulent Mach numbers (defined by RMS velocity and mean sound speed) Mt=0.2-0.9, and the Schmidt numbers of passive scalar Sc=2-10 are numerically simulated by using a 7th order upwind difference scheme and 8th order group velocity control scheme. The computed results are validated with different numerical methods and different mesh sizes. The Batchelor scaling with k(-1) range is found in scalar spectra. The passive scalar spectra decay faster with the increasing turbulent Mach number. The extended self-similarity (ESS) is found in the passive scalar of compressible turbulence.

Rapidly Infrared-Assisted Cooperatively Self-Assembled Highly Ordered Multiscale Porous Materials

In this paper, cooperative self-assembly (CSA) of colloidal spheres with different sizes was studied. It was found that a complicated jamming effect makes it difficult to achieve an optimal self-assembling condition for construction of a well-ordered stacking of colloidal spheres in a relatively short growth time by CSA. Through the use of a characteristic infrared (IR) technique to significantly accelerate local evaporation on the growing interface without changing the bulk growing environment, a concise three-parameter (temperature, pressure, and IR intensity) CSA method to effectively overcome the jamming effect has been developed. Mono- and multiscale inverse opals in a large range of lattice scales can be prepared within a growth time (15-30 min) that is remarkably shorter than the growth times of several hours for previous methods. Scanning electron microscopy images and transmittance spectra demonstrated the superior crystalline and optical qualities of the resulting materials. More importantly, the new method enables optimal conditions for CSA without limitations on sizes and materials of multiple colloids. This strategy not only makes a meaningful advance in the applicability and universality of colloidal crystals and ordered porous materials but also can be an inspiration to the self-assembly systems widely used in many other fields, such as nanotechnology and molecular bioengineering.

About generalized scaling for passive scalars in fully developed turbulence

In this paper we use a simple normal form approach of scale invariant fields to investigate scaling laws of passive scalars in turbulence. The coupling equations for velocity and passive scalar moments are scale covariant. Their solution shows that passive scalars in turbulence do not generically follow a general scaling observed for velocity field because of coupling effects.

Turbulent passive scalar field of a small prandtl number

The statistical-mechanics theory of the passive scalar field convected by turbulence, developed in an earlier paper [Phys. Fluids 28, 1299 (1985)], is extended to the case of a small molecular Prandtl number. The set of governing integral equations is solved by the equation-error method. The resultant scalar-variance spectrum for the inertial range is F(k)~x−5/3/[1+1.21x1.67(1+0.353x2.32)], where x is the wavenumber scaled by Corrsin's dissipation wavenumber. This result reduces to the − (5)/(3) law in the inertial-convective range. It also approximately reduces to the − (17)/(3) law in the inertial-diffusive range, but the proportionality constant differs from Batchelor's by a factor of 3.6.

A passive scalar field convected by turbulence

Classical statistical mechanics is applied to the study of a passive scalar field convected by isotropic turbulence. A complete set of independent real parameters and dynamic equations are worked out to describe the dynamic state of the passive scalar field. The corresponding Liouville equation is solved by a perturbation method based upon a Langevin–Fokker–Planck model. The closure problem is treated by a variational approach reported in earlier papers. Two integral equations are obtained for two unknown functions: the scalar variance spectrum F(k) and the effective damping coefficient (k). The appearance of the energy spectrum of the velocity field in the two integral equations represents the coupling of the scalar field with the velocity field. As an application of the theory, the two integral equations are solved to derive the inertial-convective-range spectrum, obtaining F(k)=0.61 −1/3 k−5/3. Here is the dissipation rate of the scalar variance and is the dissipation rate of the energy of the velocity field. This theoretical value of the scalar Kolmogorov constant, 0.61, is in good agreement with experiments.

Micromachining Of Passive Planar Micromixer On Poly (Methyl Methacrylate) Substrate With Excimer Laser Ablation

Excimer laser ablation technique was introduced into this work to fabricate a passive planar micromixer on the PMMA substrate. T-junction shaped and width-changed S-shaped microchannels were both designed in this micromixer to enhance mixing effect. The mixing experiment of distilled water and Rhodamine B with injection flow rate of 500 and 1,500 mu m/s validates the mixing effectivity of this micromixer, and indicates the feasibility of excimer laser ablation in the microfabrication of mu-TAS device.

Three-photon-excited upconversion luminescence of YVO4 single crystal by infrared femtosecond laser irradiation

We report on the upconversion luminescence of a pure YVO4 single crystal excited by an infrared femtosecond laser. The luminescent spectra show that the upconversion luminescence comes from the transitions from the lowest excited states T-3(1), T-3(2) to the ground state (1)A(1) of the VO43-. The dependence of the fluorescence intensity on the pump power density of laser indicates that the conversion of infrared irradiation to visible emission is dominated by three-photon excitation process. We suggest that the simultaneous absorption of three infrared photons promotes the VO43- to excited states, which quickly cascade down to lowest excited states, and radiatively relax to ground states, resulting in the broad characteristic fluorescence of VO43-. (c) 2005 Optical Society of America.

Fine interference fringes formed in high-order harmonic spectra generated by infrared driving laser pulses

We experimentally investigate the high-order harmonic generation in argon gas using a driving laser pulse at a center wavelength of 1240 nm. High-contrast fine interference fringes could be observed in the harmonic spectra near the propagation axis, which is attributed to the interference between long and short quantum paths. We also systematically examine the variation of the interference fringe pattern with increasing energy of the driving pulse and with different phase-matching conditions.

Formation of X-waves at fundamental and harmonics by infrared femtosecond pulse filamentation in air

We experimentally observe the formation of X-waves at fundamental, third harmonic, and fifth harmonic wavelengths by infrared (central wavelength at similar to 1500 nm) femtosecond laser pulse filamentation in air. By fitting the angularly resolved spectra of the fundamental and harmonic waves using X-wave relations, we confirm that all the X-waves have nearly the same group velocity, indicating that they are locked in space and time during their propagation in filament.

Accurate measurement of carrier-envelope phase drift for infrared femtosecond laser pulses

A novel scheme to eliminate the artificial background phase jitter is proposed for measuring the carrier-envelope phase drift of tunable infrared femtosecond pulses from an OPA laser. Different from previous methods, a reference spectral interference measurement is performed, which reveals the artificial phase jitter in the measurement process, in addition to the normal f-to-2f interference measurement between the incident laser pulses and it second harmonic. By analyzing the interference fringes, the accurate CEP fluctuation of the incident pulses is obtained. (c) 2008 Optical Society of America

Linear chirp control of infrared signal in a biased semiconductor thin film

Ultrashort light-matter interactions between a linear chirped pulse and a biased semiconductor thin film GaAs are investigated. Using different chirped pulses, the dependence of infrared spectra on chirp rate is demonstrated for a 5 fs pulse. It is found that the infrared spectra can be controlled by the linear chirp of the pulse. Furthermore, the infrared spectral intensity could be enhanced by two orders of magnitude via appropriately choosing values of the linear chirp rates. Our results suggest a possible scheme to control the infrared signal.

Spectral evolution of angularly resolved third-order harmonic generation by infrared femtosecond laser-pulse filamentation in air

We experimentally investigate the evolution of an angularly resolved spectrum of third harmonic generated by infrared femtosecond laser pulse filamentation in air. We show that at low pump intensity, phase matching between the fundamental and third-harmonic waves dominates the nonlinear optical effect and induces a ring structure of the third-harmonic beam, whereas at high pump intensity, the dispersion properties of air begin to affect the angular spectrum, leading to the formation of a nonlinear X wave at third harmonic.

Widely tunable quasi-phase-matched optical parametric amplification in infrared region using periodically poled lithium niobate

Widely tunable optical parametric amplification (OPA) in the IR region through quasi-phase-matching technology is demonstrated theoretically in periodically-poled lithium niobate (PPLN). For a 532nm pump wavelength and a broadband signal wavelength near 1300 nm, we can obtain the optimum grating period from phase-matching curves for different grating periods to achieve continuously tunable OPA by tuning the angle in a small range. Tunable OPA range of 200nm near 1300 mn can be obtained with a tuning incidence signal angle of 2.2 degrees.

X-shaped third harmonic generated by ultrashort infrared pulse filamentation in air

The authors report the measurement of the angularly resolved spectrum of the third harmonic generated in a femtosecond filament in air and its evolution with increasing pump power. Pumped by a focused infrared ultrashort pulse with a carrier wavelength of 1270 nm, a pulse duration of similar to 20 fs, and pulse energy up to 487 mu J, the generated third harmonic is composed of an on-axis emission and a conical ring emission. When the pump power is sufficiently high, angularly resolved spectra with significant X-like feature could be observed, indicating the formation of nonlinear X wave at third harmonic. (c) 2008 American Institute of Physics.