Radiation of cylindrical duct acoustic modes with flow mismatch

An analysis is presented for the radiation of spinning acoustic modes from a round duct. The model, based on a plug flow exhaust jet, allows for a flow temperature and velocity mismatch. Solutions to the convected wave equations within and outside the jet are devised, the continuity of acoustic pressure and particle displacement match at the jet interface being used. Formal solution is obtained by applying Fourier transforms, inversion being accomplished by using the Wiener-Hopf technique coupled with the Carrier-Koiter approximations. The analysis also is extended formally to include the inlet problem, but with a plug type inlet flow.The calculations suggest that the influence of the Mach number discontinuity is to offer a conflict between the effects of convection and refraction, the former becoming increasingly more significant at high subsonic Mach numbers by the presence of a strong lobe in the shadow zone. Effects of flight conditions are to weaken such conflicts so that in the limit of uniform Mach number, one recovers Carrier's well-known solution.The effect of a temperature mismatch is rather more dramatic, resulting in a sharp beaming of sound off to the side. This is consistent with the kinematic picture of a plane wave impinging on a temperature discontinuity.     

A study in the developing region of square jet

The developing region of a turbulent square jet is investigated using high-resolution particle image velocimetry (PIV). The mean velocity and turbulence stresses are presented in various horizontal planes, along the jet centerline covering the initial region of the jet as well as the transition to the self-similar region. To study the flow structure away from the central plane, velocity measurements in two additional horizontal planes, one located halfway from the jet central plane toward the edge and the other at the edge of the square jet, are also examined. Analysis of the instantaneous velocity fields reveal the presence of an arrow-like feature in the square jet due to the higher instability generated in the jet shear layer compared with a round jet. To elucidate the imprints of the vortex structures present in the jets, a swirling strength-based vortex identification methodology is applied on a large ensemble of instantaneous velocity fields. Statistical analysis of the number of vortex cores, and their size and rotational strength in the measurement plane is undertaken. Vortex population at the edge was found to be very different compared with that in the central plane.     

On the Mach number and temperature dependence of jet noise: Results from a simplified numerical model

Numerical simulations of sound radiation from perturbed round jets are used, firstly to explore the structure of the sound sources and then to carry out a parametric study of the effect of jet Mach number and jet temperature. The simplified model problem includes a steady base jet flow, maintained in the absence of disturbances, superimposed with instability waves that are free to interact nonlinearly. Simulations over a range of subsonic jet Mach numbers show that a nonlinear mechanism dominates over a linear mechanism for low-frequency sound radiation, while for supersonic Mach numbers the linear mechanism is dominant. Additional insight is gained from a frequency-wavenumber analysis, including a transformation in the radial direction. With this decomposition, the acoustic field is located by the arc of a circle in plots of radial against streamwise wavenumber for discrete frequencies. The transformation is applied to both the pressure field, showing the sound directivity, and to selected source terms, showing characteristic directivity patterns for the streamwise and radial quadrupole terms. Decreasing the Mach number leads to a reduction in amplitude of the sources and of the sound radiation. Simulations with broadband forcing show that the qualitative effects of Mach number and jet heating are captured by this approach, which requires less resolution than a direct numerical simulation. A significant increase in the strength of the acoustic radiation for cold jets is observed, which is worthy of further investigation.     

Sound propagation through a subsonic jet due to a source near the duct exit

Matched asymptotic solutions are constructed for the acoustic potentials of a periodic point source located in a two-dimensional subsonic jet near the exit of the duct with the ratio of the duct thickness to the acoustic wave length as the small parameter. The leading term of the far field solution has the same directionality effect as that for an infinite jet without the duct and that when the plane at the duct exit is considered to be a plane of symmetry. However, the intensity is different because of the wave propagation into the duct and is dependent on the location of the source.     

Caractérisation expérimentale et numérique des transferts de chaleur à travers un jet d'air plan horizontal

This study reports on heat transfer through a plan horizontal air jet. The jet maintained at a higher temperature than the ambience allows a thermal sealing at the upper side of the opening case.The thermal and hydrodynamic measurements taken on the experimental device and the numerical simulations carried out by using the Phoenics package distinguish two types of flow. For the small velocities of the jet or the high differences of temperature, the natural convection predominates: the hot jet diffuses upwards and can not seal the case. Inversely, for the high values of the jet velocities, the forced convection predominates: the case is correctly sealed but the power consumption increases like the flowrate. A dimensionless number called deflection modulus allows to characterize the transition from one flow pattern to another and it also allows to minimize the energy consumption.     

Confinement thermique d'une enceinte par jet d'air en convection forcée

A global model is proposed for the heat transfer through a plane air jet which is used as a thermal seal of a case at temperatures higher than the ambient one. The model considers that forced convection predominates over the natural convection and uses an analogy between heat and mass transfers. The heat balance calculation is based on an estimation of the velocity and temperature profiles at returning slot of the jet. Hydrodynamically, the jet is assimilated to a free jet. Thermally, a new formulation for the temperature profile in the jet is proposed for the most general case whatever the temperatures at the blowing slot and on each side of the jet — room and case —. The predictions by the model are in good agreement with experimental results from the point of view of:

• •- the changes in longitudinal and transversal temperature in the jet;
• •- the flux exchanged between all compartments;
• •- the energy consumption required to assure a given difference between the case temperature and the ambient one.

     

Use of a plane jet for flow-induced noise reduction of tandem rods

Unsteady wake from upstream components of landing gear impinging on downstream components could be a strong noise source.The use of a plane jet is proposed to reduce this flow-induced noise.Tandem rods with different gap widths were utilized as the test body.Both acoustic and aerodynamic tests were conducted in order to validate this technique.Acoustic test results proved that overall noise emission from tandem rods could be lowered and tonal noise could be removed with use of the plane jet.However,when the plane jet was turned on,in some frequency range it could be the subsequent main contributor instead of tandem rods to total noise emission whilst in some frequency range rods could still be the main contributor.Moreover,aerodynamic tests fundamentally studied explanations for the noise reduction.Specifically,not only impinging speed to rods but speed and turbulence level to the top edge of the rear rod could be diminished by the upstream plane jet.Consequently,the vortex shedding induced by the rear rod was reduced,which was confirmed by the speed,Reynolds stress as well as the velocity fluctuation spectral measured in its wake.This study confirmed the potential use of a plane jet towards landing gear noise reduction.     

PIV measurements of the flow and turbulent characteristics of a round jet in crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.     

超声速气流中液体横向射流的非定常特性与振荡边界模型

针对液体圆柱射流垂直喷入超声速横向气流中的非定常分布特性开展实验研究, 并建立穿透深度方向上的射流振荡分布模型. 利用脉冲激光背景成像方法“冻结”拍摄马赫2.1(Ma=2.1)气流中煤油射流/喷雾瞬态图像, 结合最大类间方差法(Otsu)和Canny算法提取瞬态图像特征, 基于统计方法并引入间歇因子(γ)定量描述射流振荡分布特性; 通过研究多参数协同作用下的射流振荡分布规律, 提出振荡分布数学模型, 研究的参数变量包括超声速来流总压(642-1010 kPa)、 液体喷注压降(0.36-4.61 MPa)、液体喷嘴流道直径 (0.48 mm/1.0 mm/1.25 mm/1.52 mm)、距离喷嘴的流向距离(10-125 mm)以及液气动量通量比(0.11-7.49). 研究中利用射流振荡分布模型成功预测出水射流在Ma=2.1气流中的的振荡分布, 预测分布与实验结果符合良好.     

Photonic Jet by a Near‐Unity‐Refractive‐Index Sphere on a Dielectric Substrate with High Index Contrast

Photonic jets are normally generated in transmission mode and are represented as a spatially localized high‐intensity region on the shadow side of a particle‐lens, with a background to medium refractive index contrast of 1.3–1.7 while illuminated by a plane wave. Here, a photonic jet is discovered in the opposing plane wave propagation direction and lies in the area in the upper boundary of a near‐unity refractive index sphere on a high refractive index dielectric substrate. The redistribution of the power flow is inhibited during the reflected wave passing the near‐unity refractive index sphere. This has led to a unique effect on the focus position and shape of the produced photonic jet in reflection mode which can be maximally maintained near the sphere regardless of the modulation of the refractive index for the dielectric substrate material.     

Slip boundary conditions and through flow effects on double-diffusive convection in internally heated heterogeneous Brinkman porous media

A model for double-diffusive convection in a heterogeneous porous layer with a constant throughflow is explored, with penetrative convection being simulated via an internal heat source using the Brinkman model. In particular, we analyse the effect of slip boundary conditions on the stability of the model. Because of the many applications in micro-electro-mechanical systems (MEMS) and other microfluidic devices, a study of this problem is necessary. Both linear instability analysis and nonlinear stability analysis are employed. We accurately analyse when stability and instability will commence and determine the critical Rayleigh number as a function of the slip coefficient.     

Influence of initial conditions at the nozzle exit on the structure of round jet

Experimental data concerning the influence of initial conditions at the nozzle exit on the structure and development characteristics of round jets are reported. Features in the development of laminar and turbulent round jets emanating from variously elongated nozzles at identical Reynolds numbers are revealed. Smoke visualization pictures obtained for jets formed under different initial conditions (with different distributions of mean and pulsating flow velocities at the nozzle exit) are discussed. It is shown possible to make the zone of laminar flow in the jet stream more extended, and to delay the jet turbulization process in space, by making the flow-velocity profile more parabolic at the exit of elongated nozzle. Features in the development of vortical structures in a jet under an acoustic action are identified. It is shown that, for a turbulent round jet to be produced right at the nozzle exit, the nozzle length must be increased in excess of a certain value so that to provide for spatial growth of turbulent boundary layer thickness, finally ending in the formation of a fully turbulent flow velocity profile across the channel. This work was supported by the Russian Foundation for Basic Research (Grant No. 08-01-00027), the Ministry of Education and Science of the Russian Federation (project RNP 2.1.2.3370), and by the grants of President of the Russian Federation (NSH-454.2008.1 and. MK-420.2008.1).     

Roles of initial condition and vortex pairing in jet noise

This is a study of the effect of initial condition on sound generated by vortex pairing in a low Mach number, cold air jet (0·15 ⩽ M ⩽ 0·35). Data has been taken, both flow velocity fields and sound pressure far fields, in a quality anechoic facility, with careful documentation of the effect of initial condition on the sound field of jets of two different geometries (i.e., circular and elliptic). Explanations are presented for most of the observed effects by applying Möhring's theory of vortex sound to vortex filament models of coherent structures in the jets. The explanations also draw upon experience with coherent structure dynamics. The sound source of interest here is that associated with the pairing of shear layer vortices. The evolution of these vortices is greatly affected by the initial condition as is their resultant sound field. The elliptic jets with laminar boundary layers show azimuthal directivity, namely, sound pressure levels in the minor axis plane were greater than in the major axis plane. This difference decreases as the nozzle boundary layer undergoes natural transition with increasing jet speed. When the nozzle boundary layer is tripped, making it fully turbulent and removing the shear layer mode of pairing, the elliptic jet sound fields become nearly axisymmetric. What appears to be the most acoustically active phase of vortex pairing has been modeled, and the resulting sound field calculated for the circular jet. Supporting evidence is found in the experimental data for the validity of this model. The model explains the connection between the initial condition and the far field sound of jets. Interestingly, a general result of Möhring's theory is that motions of vortex rings (of any arbitrary shape) can produce only axisymmetric sound fields if the rings remain in a plane. This implies that the observed asymmetric directivity of the laminar elliptic jet sound field must be due to non-planar ring motions of the vortical structures. The primary contribution of this paper is to examine quantitatively the role of vortex pairing in the production of jet noise; the results are used to reemphasize that “pairing noise” cannot be dominant in most practical jet sound fields, contrary to claims by other researchers.     

Experimental investigation of the power radiated by a monopole above a reflected plane

The presence of reflecting boundaries causes the acoustic power output of a point source to increase. As existing experimental data were found to be inadequate, experiments have been carried out in this study to determine the effect of a reflecting plane on the power radiated by a point source. A corona-type monopole was used in this study. Both source and receiver positions are varied; the statistical results of power radiated are compared with the theoretical predictions.     

Equilibrium configurations of a jet of an ideally conducting liquid in an external nonuniform magnetic field

Possible equilibrium configurations of the free surface of a jet of an ideally conducting liquid placed in a nonuniform magnetic field are considered. The magnetic field is generated by two thin wires that are parallel to the jet and bear oppositely directed currents. Equilibrium is due to a balance between capillary and magnetic forces. For the plane symmetric case, when the jet deforms only in the plane of its cross section, two one-parameter families of exact solutions to the problem are derived using the method of conformal mapping. According to these solutions, a jet with an initially circular cross section deforms up to splitting into two separate jets. A criterion for jet splitting is derived by analyzing approximate two-parameter solutions.     

侧面抽运板状激光介质的热汇冷却

高热负荷固体激光介质的热效应已经成为制约激光器功率进一步提高的严重障碍,只有对激光介质进行有效的冷却才能保证其安全运行。以不均匀换热系数模型为基础,研究了具有非均匀内热源的侧面双向抽运板状激光介质在狭窄通道强制对流冷却情况下的耦合换热问题,对热汇冷却方案下介质的温度分布和热应力分布进行了数值模拟和分析,并对复合介质、蓝宝石和金刚石三种热汇材料进行对比。结果表明,忽视换热系数的非均匀性将导致应力计算结果偏低。对于侧面抽运、侧面冷却的激光介质,金刚石热汇冷却方案最佳,蓝宝石热汇方案次之,而复合介质方案不宜采用。     

Laser velocimeter correlation measurements in subsonic and supersonic jets

This paper describes laser velocimeter cross-correlation measurements conducted in the flow field of a jet. The jet conditions were varied over a range of Mach numbers from 0·5 to 1·37, and in the case of the Mach 0·9 jet, tests were also run at an absolute exit temperature equal to 2·32 that of the ambient. From the families of cross-correlation curves, the integral length scales, the convection velocity and the integral time scale in the moving frame were determined. It was found that the axial and radial integral length scales changed almost linearly with the axial distance from the nozzle, at rates which were in agreement with Jones' [1]. The convection velocity and integral time scale also agreed with values previously obtained with hot wire anemometers. The effects of jet exit conditions on the variation of the length scales were not noticeable. However, in the case of the convection velocity and time scale, there was some dependence on the jet conditions.     

Non-modified propagation of optical mutual intensity in the Fresnel diffraction region

The propagation problem of mutual intensity from a source plane to an object plane is discussed. The propagated mutual intensity remains unmodified in any object plane in the paraxial approximation region of Fresnel diffraction if the mutual intensity in the source plane is space-invariant and propagated to a region of interest around the propagation axis in which the Fresnel diffraction effect by the edge of an aperture in the source plane is almost negligible.     

不相溶系统内酸碱中和反应驱动动力学的不稳定性

采用含Mach-Zehnder干涉光路和Hele-Shaw反应器的实验系统,研究了重力场作用下,在Hele-Shaw系统内沿水平界面发生的由酸碱中和反应驱动的动力学不稳定性.反应器内包含上下两层反应物,即下层密度较大的四甲基氢氧化铵水溶液和上层密度较小的溶解于有机相的丙酸溶液. 研究了在伴随有界面传质的中和反应过程中,化学组分对于动力学不稳定性的影响. 观察发现了由于反应物初始浓度不均引起的多种形式的Marangoni对流结构,包含有胞状结构和各种震动波形式的结构.测量了不稳定性发生过程中碱溶液的浓度. 结果表明不稳定性对流的产生可以显著提高系统内的传质效率,并造成传质结 构的剧烈变形.