Assessment of ForeFire/Meso-NH for wildland fire/atmosphere coupled simulation of the FireFlux experiment

Numerical simulations using a coupled approach between Meso-NH (Non-Hydrostatic) LES (Large Eddy Simulation) mesoscale atmospheric model and ForeFire wildland fire area simulator are compared to experimental data to assess the performance of the proposed coupled approach in predicting fine-scale properties of the dynamics of wildland fires. Meso-NH is a non-hydrostatic, large eddy simulation capable, atmospheric research model. ForeFire insures a front tracking of the fire front by means of Lagrangian markers evolving on the earth’s surface according to a physical rate-of-spread model. The atmospheric model forces the fire behavior through the surface wind field, whereas the fire forces the atmosphere simulation through surface boundary conditions of heat and vapor fluxes. The FireFlux experiment, an experimental 32 Ha burn of tall grass instrumented with wind profilers and thermocouples, was designed specifically to estimate the atmospheric perturbation introduced by wildland fire. Comparisons of the simulations at different resolutions with the large-scale experiment validate the chosen coupling methodology and the choice of a coupled approach with a meso-scale atmospheric model for the prediction of wildland fire propagation. Distinct fire propagation behavior is simulated between coupled and non-coupled simulation. While the simulations did not reproduce high frequency perturbations, it is shown that the atmospheric model captures well atmospheric perturbations induced by combustion at the ground level in terms of behavior and amplitude.     

Numerical investigation on radiation characteristics of discrete-frequency noise from scarf and scoop aero-intakes

Numerical studies have been carried out to investigate the detailed geometrical effects of full three-dimensional aero-intakes on the radiation of the discrete-frequency rotor-stator interaction noise. The near-field acoustic radiation characteristics and the far-field directivity patterns from the scarf and scoop aero-intakes with three different scarf/scoop angles are quantitatively analyzed and compared. The near-field predictions were obtained by solving the linearized Euler equations with computational aeroacoustic techniques consisting of high-order finite difference scheme, non-reflecting boundary conditions, overset grids and parallel computational methods. For the prediction of far-field directivity pattern, the Kirchhoff integral method was applied. By comparing the directivities of discrete-frequency noise radiating from the scarf and the scoop aero-intakes with that from an axisymmetric aero-intake, it is shown that the 7 dB noise reduction at downward peak radiation angle can be achieved by using the scoop aero-intake with scoop angle of 15°, and the 5 dB noise reduction by the scarf aero-intake with the scarf angle of 15°. The scattering of the radiating acoustic wave by the background mean flow around the aero-intakes shifts the peak lobe radiation angle toward ground and increases the amplitude of the acoustic pressure compared with the cases without mean flow effect. Overall, the scoop aero-intake was found to be more effective than the scarf and the axisymmetric aero-intakes in view of the lower noise radiation toward ground.     

1D Bessel-like beam generation by highly directive transmission through a sub-wavelength slit embedded in periodic metallic grooves

A new method to generate a quasi-one dimensional (1D) Bessel-like beam whose non-diffracting length extends to macroscopic scale is proposed by utilizing highly directive transmission through a sub-wavelength single slit embedded in periodic metallic grooves. We employed finite-difference time-domain methods for full-vectorial diffraction and spectral analysis. In optimal conditions, unique quasi-1D Bessel-like beams were generated in transmission for the incident p-polarized plane wave such that highly asymmetric far-field distribution was achieved; Bessel-like beam along the slit axis and a flat-top super-Gaussian beam along the other perpendicular axis. Detailed parametric studies for the proposed structure are reported in terms of the operating spectral range and a general window of optimal conditions.     

Near-Field Evaluation of a Quantum Size Effect in Self-Aligned GaN Whiskers Fabricated Using Photochemical Etching

We fabricated nanoscale GaN whiskers using photochemical etching. The fabricated GaN whiskers were conjugated and aligned perpendicular to the incident light polarization used for photochemical etching in a self-assembling manner. Their far-field photoluminescence spectra exhibited a blue-shifted photoluminescence peak at 3.60eV. Near-field photoluminescence spectra of individual GaN whiskers were obtained, for the first time. The evaluation of the near-field spectra identified several peaks from individual whiskers, corresponding to a diameter range of 5–10 nm, and revealed a stepwise change in the diameter along the axis of individual whisker.     

Method of resonance near-field optical microscopy

We have solved the problem in which a thin metal wafer (probe) with a nanohole interacts with the flat surface of a metastructured film consisting of metal nanoparticles in an external optical radiation field. Nanoparticles are considered as two-level atomic systems. This interaction of the wafer-probe and the flat surface in the external optical radiation field gives rise to optical near-field resonance, the frequency of which differs significantly from the natural frequencies of two-level atoms in the medium and the probe. The fields inside and outside the probe and metastructured film are calculated in the near-field and far-field zones. The maximum resolution, which is achievable in the suggested scheme of near-field optical microscopy, can reach about 10 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 499–506, July–August, 2007.     

Inclined fire whirls

Air entrainment, leading to strong fire whirls, is commonly thought to be caused by the buoyant rise of the hot combustion products under the influence of gravity. We have, however, created in the laboratory steady, axisymmetric strong fire whirls with axes inclined 30° from the vertical orientation, whirls which model an inclined fire whirl, about 30 m tall, observed in California wildland near the Cleveland National Forest. The results contradict the common notion of buoyancy being significant for the structure of the whirl, implying that strong fire whirls instead are dominated by rotation only, even if their axis is vertical. The new concept of rotation-controlled fire whirls is explained by a Rossby number displacing the Froude number (or the Richardson number) in describing the phenomenon.     

沟渠内杨木三合板架空燃烧的火灾行为的研究

建立了缩尺寸沟渠火灾试验模型,测量了沟渠以及平台入口附近的温度分布,研究了沟渠内杨木三合板架空燃烧的位置和沟渠的倾斜角度对火蔓延行为的影响,分析了火焰传播特征和火蔓延速率.试验结果表明:在沟渠效应和烟囱效应的综合作用下,火焰先从杨木三合板与沟渠底板之间的狭长空间传播到沟渠顶部,在入口处形成喷射状火焰;狭长空间内的温度先...     

合成近场校准基阵的方法研究

本文利用单根的束控线阵来构造一个虚拟近场校准基阵,使换能器能在它的近场直接测量辐射声场特性。通过利用改进的矩阵稳定技术,解决了求解束控线阵的束控系数时最小二乘解的稳定性和实用性问题。借助计算机仿真,研究了特定线阵条件下,合成近场测量基阵（Ｓ－ＮＦＣＡ）的近场空间的平面波均匀性与线阵间的间距、线阵的工作频率之间的关系。并以小球源为阵元合成一虚拟平面ＮＦＣＡ,用它对一平面喇叭换能器的近场测量得到远场指向性,并与远场实测结果比较,得到了相当吻合的结果。     

Flame sheet dynamics of bluff-body stabilized flames during longitudinal acoustic forcing

Bluff-body stabilized flames are susceptible to combustion instabilities due to interactions between acoustics, vortical disturbances, and the flame. In order to elucidate these flow-flame interactions during an instability, an experimental and computational investigation of the flame-sheet dynamics of a harmonically excited flame was performed. It is shown that the flame dynamics are controlled by three key processes: excitation of shear layer instabilities by the axially oscillating flow, anchoring of the flame at the bluff body, and the kinematic response of the flame to this forcing. The near-field flame features are controlled by flame anchoring and the far-field by kinematic restoration. In the near-field, the flame response grows with downstream distance due to flame anchoring, which prevents significant flame movement near the attachment point. Theory predicts that this results in linear flame response characteristics as a function of perturbation amplitude, and a monotonic growth in magnitude of the flame-sheet fluctuations near the stabilization point, consistent with the experimental data. Farther downstream, the flame response reaches a maximum and then decays due to the dissipation of the vortical disturbances and action of flame propagation normal to itself, which acts to smooth out the wrinkles generated by the harmonic flow forcing. This behavior is strongly non-linear, resulting in significant variation in far-field flame-sheet response with perturbation amplitude.     

Near-field scanning optical microscopy local luminescence studies of rhodamine dye

This article presents results of near-field scanning optical microscope measurement of local luminescence of rhodamine 3B intercalated in montmorillonite samples. We focus on how local topography affects both the excitation and luminescence signals and resulting optical artifacts. The Finite Difference in Time Domain method (FDTD) is used to model the electromagnetic field distribution of the full tip-sample geometry including far-field radiation. Even complex problems like localized luminescence can be simulated computationally using FDTD and these simulations can be used to separate the luminescence signal from topographic artifacts.     

Scale-model experiments of moving fire whirl over a line fire

A fire whirl, observed during a wildland fire in Brazil in 2010, occurred over a narrow but long line fire and moved along the line fire at nearly a constant speed. There appeared to be no mountains, tall buildings or trees near the scene, indicating that the fire whirl was generated merely by the interaction between the line fire and background wind. Scale-model experiments having different line fire configurations were designed and performed to reconstruct the above-mentioned Brazil fire whirl. Moving fire whirls were successfully reconstructed during the scale-model experiments, the mechanism and conditions of which are discussed herein.     

Surface plasmon mediated near-field imaging and optical addressing in nanoscience

We present an overview of recent progress in "plasmonics". We focus our study on the observation and excitation of surface plasmon polaritons (SPPs) with optical near-field microscopy. We discuss in particular recent applications of photon scanning tunnelling microscope (PSTM) for imaging of SPP propagating in metal and dielectric wave guides. We show how near-field scanning optical microscopy (NSOM) can be used to optically and actively address remote nano objects such as quantum dots. Additionally we compare results obtained with near-field microcopy to those obtained with other optical far-field methods of analysis such as leakage radiation microscopy (LRM).     

Near-Field Optical Transfer Function for Far-Field Super-Resolution Imaging

The far-field superlens based on surface plasmon polaritons （SPP） has shown great application potential, but it is difficult and time-consuming to reconstruct the far-field image. We derive a near-field optical transfer function （NOTF） of a silver slab and analyse its validity so that accurate information of nano-seale object in the near-field can be computed rapidly. The NOTF is helpful not only for analysing the super-resolution imaging process in far-field, but also for providing a track to describe the transmission of optical information from near-field to far-field by using the optical transfer functions theorv only.     

基于光导微探针的近场/远场可扫描太赫兹光谱技术

太赫兹技术已经成为涉及公共安全、军事国防和国民经济等国家核心利益的前沿研究领域.以往太赫兹测量技术中通常以远场测量为主,如常用的太赫兹时域光谱仪.近年来太赫兹近场技术得到了迅猛的发展,特别是基于光导天线的探针技术的发展,为可扫描的太赫兹近场测量提供了可能.本文详细报道了我们近期在可扫描太赫兹近场光谱仪研究中的进展.采用光纤耦合的光导微探针实现了方便灵活的太赫兹近场/远场三维扫描,并同时获得振幅和相位信息.该系统将有可能广泛应用于人工微结构、石墨烯、表面等离子激元、波导传输、近场成像、生物样品检测、芯片检测等研究领域.     

卫星激光通信终端光跟踪检测的数理基础

卫星激光通信光束的传播在空间属于远场衍射,而在地面光学跟踪检验中属于近场衍射。采用菲涅耳衍射理论研究了远场衍射和近场衍射对于产生光学跟踪位置误差信号的本质上的差别并且分析了等效条件,证明了近场检验中采用卫星相对角度运动轨迹的光束扫描可以准确模拟远场相对运动,同时也得到了光斑特性和卫星抖动模拟的分析。澄清了卫星激光通信终端光学跟踪性能检验中的基本概念,具有实际指导意义。     

Comparison investigation of near- and far-field properties for plasmon resonance of silver nanosphere dimers

Near- and far-field plasmon resonance properties of silver nanospheres dimers are calculated and compared based on the Generalized Multiparticle Mie-solution method. Greater differences between near- and far-field plasmon resonances are found than those of silver single nanoparticle, while the dimer of strongest near-field electric field amplitude and the dimer of highest far-field extinction coefficient almost have the same geometry parameter value under the illumination with fixed wavelength and polarization.     

Fourier treatment of near-field synchrotron radiation theory

In this paper, we couple synchrotron radiation (SR) theory with a branch of physical optics, namely laser beam optics. We show that the theory of laser beams is successful in characterizing radiation fields associated with any SR source. Both radiation beam generated by an ultra-relativistic electron in a magnetic device and laser beam are solutions of the wave equation based on paraxial approximation. It follows that they are similar in all aspects. In the space-frequency domain SR beams appear as laser beams whose transverse extents are large compared with the wavelength. In practical situations (e.g. undulator, bending magnet sources), radiation beams exhibit a virtual “waist” where the wavefront is often plane. Remarkably, the field distribution of a SR beam across the waist turns out to be strictly related with the inverse Fourier transform of the far-field angle distribution. Then, we take advantage of standard Fourier Optics techniques and apply the Fresnel propagation formula to characterize the SR beam. Altogether, we show that it is possible to reconstruct the near-field distribution of the SR beam outside the magnetic setup from the knowledge of the far-field pattern. The general theory of SR in the near-zone developed in this paper is illustrated for the special cases of undulator radiation. Using known analytical formulas for the far-field pattern and its inverse Fourier transform we find analytical expressions for near-field distributions in terms of far-field distributions.     

On the sound field of a resilient disk in free space

Radiation characteristics are calculated for a circular planar sound source in free space with a uniform surface pressure distribution, which can be regarded as a freely suspended membrane with zero mass and stiffness. This idealized dipole source is shown to have closed form solutions for its far-field pressure response and radiation admittance. The latter is found to have a simple mathematical relationship with the radiation impedance of a rigid piston in an infinite baffle. Also, a single expansion is derived for the near-field pressure field, which degenerates to a closed form solution on the axis of symmetry. From the normal gradient of the surface pressure, the surface velocity is calculated. The near-field expression is then generalized to an arbitrary surface pressure distribution. It is shown how this can be used as a simplified solution for a rigid disk in free space or a more realistic sound source such as pre-tensioned membrane in free space with non-zero mass and a clamped rim.     

Effect of flow circulation on combustion dynamics of fire whirl

In this paper, the effect of flow circulation on the combustion dynamics of fire whirl is systematically investigated by experiments. New correlations for the burning rate, flame height, radial temperature and mass flow rate are established for fire whirl. It is clarified that flow circulation helps increase both the fuel-flame contact area and the actual fuel surface area, which in turn increases both the heat feedback to the fuel surface and the radial velocity in the ground boundary layer, leading to increase of burning rate. A novel idea for correlation of fire whirl flame height is proposed by assuming that the ratio of the fire whirl flame height to the flame height without circulation solely characterizes the effect of circulation. This idea is fully verified, thereby a new formulation for flame height is established, which successfully decouples the burning rate and the circulation. It is indicated that the fuel-rich core in the flame body of fire whirl significantly affects the radial temperature distribution in the continuous flame region, and the flame body can be described by the combination of a cylinder and a cone. The flow circulation significantly suppresses fire plume radius and thus decreases its increasing rate with vertical distance. It is also demonstrated that the fire whirl flame involves laminarized regions in its lower section, coexisting with turbulent regions in the upper portion. The flow circulation enhances the air entrainment in the ground layer by altering the radial velocity profile and increasing the radial velocity. In the low section of flaming region, the significant decrease of mixture between the combustion products and surrounding air dominates the pure aerodynamic effect of flow circulation on the flame height. Finally, it is clarified that fire whirls maintain higher centerline excess temperature than general pool fires due to the effect of less air entrainment.