Visualization of impinging supersonic free jet on a tilt plate by LIF and PSP

The flow field structures of low density supersonic free jets impinging on a tilt plate are studied by hybrid use of LIF (Laser Induced Fluorescence) and PSP (Pressure Sensitive Paint). The jet through an orifice flows into a low pressure chamber and impinges on the tilt plate with angle from jet axis 45, 60 or 90 degrees. A plane including the jet axis and the normal of the plate is visualized by LIF of seeded iodine molecules, scanning a laser beam along the jet axis. On the other hand, the pressure distribution on the tilt plate is visualized by PSP. In comparing the results of the two methods, the complicated shock wave system is analyzed. Deformations of the Mach disk and the barrel shock are also confirmed.     

Investigation of the Flow Field in the Upper Part of a Cyclone with Laser and Phase Doppler Anemometry

The cyclone is a well known apparatus for separating particles out of a gas stream. With the modern laser diagnostic technologies of laser and phase Doppler anemometry (LDA and PDA), there is the potential to measure the flow and particle field inside the cyclone. The gas phase only measurements used micron‐sized oil seeding droplets, whereas the solid phase, chosen for the PDA particle size measurements, was limestone powder. To assess the possibility of measuring milled limestone particles with PDA, the measured size distribution was compared with those obtained by laser diffraction. The measurements inside the cyclone showed that the flow field in the upper part of the cyclone was different to that commonly thought. Therefore, the vertical height of the cyclone's vortex finder could be shortened without deterioration of the separation efficiency. The particles found in the hold‐up of the cyclone air flow were considerably larger than the average particle size in the feed pipe.     

Simultaneous Droplet Size and Gas-Phase Turbulence Measurements in a Spray Flow using Phase-Doppler Interferometry

A recently developed method to increase the sensitivity of a phase-Doppler interferometer (PDI) has been applied to an annular air flow interacting with a spray under isothermal conditions in order to demonstrate the applicability of the method to obtaining velocity and turbulence characteristics of the gas-phase in a two-phase flow. A conventional PDI system operated in the first order refraction mode cannot be configured to efficiently detect small seed particles in the presence of large droplets due to the limited dynamic range of the photodetectors. It is therefore difficult or impossible to obtain seed particle detection rates sufficient for turbulence measurements. Doping the spray with a very small quantity of dye preferentially attenuates the light intensity scattered by large droplets, thus allowing the PDI system to detect small seed particles without damaging the photodetectors when large particles are present in the flow. Application of this method to a two-phase flow produced by a pressure-swirl atomizer coaxially located within an annular air jet has resulted in profiles of axial mean and fluctuating velocities and shear stresses as well as integral time scales and turbulent energy spectra.     

实心与空心锥形喷雾与热多孔介质相互作用的数值研究

在KIVA-3V中增加了油滴碰撞热多孔介质壁面的碰撞模型、传热模型及空心喷雾的线性不稳定性液膜破碎模型(LISA).在多孔介质结构简化描述的基础上,详细模拟了实心喷雾与空心锥形油雾与热多孔介质之间的碰撞过程.针对Senda等人的实验进行了数值计算,油束碰壁后油滴和油蒸汽分布的数值计算结果与实验结果吻合得很好.计算结果表明油雾在碰撞到热多孔介质后,油束会发生分裂,为油滴的快速蒸发和油蒸汽与空气充分混合创造了前提.油滴初始动能相同的条件下,空心喷雾的油滴穿越多孔介质的可能性比实心喷雾要小.     

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.     

Dissipative Instability of Shock Waves

A new condition is obtained for the linear instability of a plane front of an intense shock wave in an arbitrary medium, which is determined by the finiteness of the viscosity. It is shown that the shock front instability occurs due to dissipative instability of the flow behind the front, which is analogous to the flow instability in the boundary layer. It is found that in the low-viscosity limit, one-dimensional longitudinal perturbations increase much faster than two-dimensional (corrugation) perturbations. The results are compared with the available data of experimental observation and numerical simulation of instability of shock waves. The comparison shows a better agreement between the new absolute shock instability as compared to the condition of such instability in the classical D’yakov theory disregarding viscosity.     

激波作用不同椭圆氦气柱过程中流动混合研究

在激波与气柱相互作用问题中,压力与密度间断不平行产生的斜压涡量会引起流动的不稳定性,从而促进物质间的混合.本文基于双通量模型,结合五阶加权基本无振荡(WENO)格式,求解多组分二维Navier-Stokes方程,分析激波作用面积相同结构不同的椭圆气柱所致的流动和混合.数值结果清晰地显示了激波诱导Richtmyer-Meshkov不稳定性引起的气柱界面变形和波系演化.同时定量地从界面运动、界面结构参数变化(长度和高度)、气柱体积压缩率、环量及混合率等角度分析激波诱导的流动混合机制,研究椭圆几何构型对氦气混合过程的影响.结果表明,界面及相关参数的演化与气柱初始形状密切相关.当激波沿椭圆长轴作用于气柱时,气柱前端出现空气射流结构,且射流不断增长并渗透到下游界面,致使气柱分离成两个独立涡团,离心率越大,射流发展越快;同时激波作用气柱后在界面处产生不规则反射现象.圆形气柱界面演化与这种作用情形类似.当激波沿椭圆短轴作用于气柱时,界面上游出现类平面结构,随后平面上下缘处产生涡旋,主导流动发展,激波在界面作用产生规则反射,离心率越大,这些现象越明显.界面高度、长度、体积压缩率也因此有所差异.对界面演化、环量和混合率的综合分析表明,激波沿长轴作用于气柱且离心率较大时,流动发展较快,不稳定性导致的流动越复杂,越有利于氦气与环境介质的混合.     

Models of anisotropically polarizable suspensions with irreversible processes: Anisotropy relaxation,dielectrophoresis, and electroviscous flows

We consider media consisting of an anisotropically polarizable particle of a disperse phase and a nonconducting liquid carrier phase. The particles exhibit easy axis or easy plane anisotropy. A complete electrohydrodynamic model describing the motion of the medium in an electric field is constructed with allowance for irreversible processes. The laws determining the polarization of the medium, dielectrophoresis of particles, and anisotropy relaxation are determined. Expressions are derived for the kinetic coefficients appearing in these laws and the expression for the free energy of the medium. The dependence of the effective viscosity on the electric field strength is determined for a flow in narrow channels.     

Interaction of Annular Air Jet with a Non-Evaporating Liquid Spray

The effects of a concentric annular air jet on the structure of a liquid spray have been examined under isothermal conditions using Phase Doppler interferometry, and flow visualization techniques. Local measurements of drop size, velocity, volume flux, and number density have been made simultaneously and non-intrusively. The results show that the initial spray cone widens substantially with increasing air flow while the overall spray envelope remains essentially the same. The air jet creates a narrow high mass flux region of small droplets in the core of the spray, surrounded by a nearly uniform flux region extending almost to the boundary of the spray. Line averaged mean diameters are found to be slightly smaller, indicating that the annular air jet plays a role improving the atomization process as well.     

Stereoscopic PIV measurements of a screeching supersonic jet

The effect of acoustic feed back on global flow response is illustrated through an example of a rectangular screeching jet operating at a nominal Mach number of 1.69. Using a stereoscopic Particle Image Velocimetry, the detailed flow characteristics within a screeching cycle are obtained with fidelity. To resolve the “bias” errors inherent with standard PIV image processing technique, a novel mesh-free and high spatial resolution scheme is implemented to yield accurate velocity measurements in a complex three-dimensional supersonic flow. The axis-switching phenomenon that arises due to unusual mixing enhancement in the minor axis plane of a rectangular jet is vividly displayed. Strong streamwise vortex structure in the jet shear layers, enhanced by the inherent instability of the shear layer, is reported.     

Numerical simulations of shock-wave interaction with a boundary layer in the plane supersonic flows with jet injection

A supersonic air flow in a plane channel with a transverse turbulent jet of hydrogen injected through a slot on the bottom wall is simulated. The algorithm for solving the Favre-averaged Navier-Stokes equations for the flow of a perfect multispecies gas on the basis of the WENO scheme is proposed. The main attention is paid to the interaction of the shock-wave structure with the boundary layers on the upper and lower duct walls under the conditions of an internal turbulent flow. Namely, a detailed study of the structure of the flow is done, and separation and mixing depending on the jet slot width are investigated. It is found that in addition to well-known shock-wave structures produced by the interaction of the free stream with the transverse jet and the bow shock interaction with the boundary layers near the walls, an additional system of shock waves and the flow separation arise on the bottom wall downstream at some distance from the jet. The comparison with the experimental data is performed.     

Instabilities and decay rates of charged viscous liquid jets

The eigenmodes and characteristic frequencies of surface deformations of real fluids in the geometry of a cylindrical jet, which is homogeneously charged, are calculated using the linearized Navier-Stokes equation with free surface boundary conditions. A stability phase diagram in the wavenumber-fissility plane is given. Two types of instabilities are described, surface tension and Coulomb force induced. The known modes for ideal potential flow are extended to arbitrary viscosity. In addition new types of modes are described (eigenfunctions and decay rates) which represent rotational flow and have no correspondence to Rayleigh's jet instability.     

Measuring the collective flow with jets

In nucleus-nucleus collisions, high-p(T) partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components can be disentangled by the measurement of particle production associated with high-p(T) trigger particles, jetlike correlations, and jets. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the eta x phi plane. We argue that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.     

Experimental evidence of a Rayleigh-plateau instability in free falling granular jets

A granular jet falling out of a funnel shaped container, subjected to small vertical vibrations, develops an instability farther downstream as may happen for ordinary liquid jets. Our results show that this instability is reminiscent of the Rayleigh-Plateau capillary instability leading to breakup of the jet at large scales. The first stages of this instability are captured in detail allowing a determination of the dispersion relation. Surface tensions measured in this unstable regime (of the order of mN/m) are in agreement with previously reported measurements carried out at much smaller scales. This instability and the breakup of the jet can be inhibited when the effect of the surrounding medium (air) is reduced by enclosing the jet in an evacuated chamber, showing that the effective surface tension measured is the result of a strong interaction with the surrounding air.     

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.     

Influence of air pressure on the performance of plasma synthetic jet actuator

Plasma synthetic jet actuator(PSJA) has a wide application prospect in the high-speed flow control field for its high jet velocity.In this paper,the influence of the air pressure on the performance of a two-electrode PSJA is investigated by the schlieren method in a large range from 7 k Pa to 100 k Pa.The energy consumed by the PSJA is roughly the same for all the pressure levels.Traces of the precursor shock wave velocity and the jet front velocity vary a lot for different pressures.The precursor shock wave velocity first decreases gradually and then remains at 345 m/s as the air pressure increases.The peak jet front velocity always appears at the first appearance of a jet,and it decreases gradually with the increase of the air pressure.A maximum precursor shock wave velocity of 520 m/s and a maximum jet front velocity of 440 m/s are observed at the pressure of 7 k Pa.The averaged jet velocity in one period ranges from 44 m/s to 54 m/s for all air pressures,and it drops with the rising of the air pressure.High velocities of the precursor shock wave and the jet front indicate that this type of PSJA can still be used to influence the high-speed flow field at 7 k Pa.     

均匀液滴尺寸及间距的实验研究

液滴发生器产生液滴的尺寸和间距影响液滴层的辐射和蒸发特性,液滴尺寸及间距的可控性值得重点关注。根据Weber的射流不稳定修正方程,确定了均匀液滴流产生的无量纲波数及扰动频率范围,结合射流质量守恒,分析了均匀液滴流中液滴的尺寸和间距与无量纲波数的关系。在不同喷孔直径和射流压力下,对理论和实验结果进行了对比,验证了液滴尺寸和液滴间距的理论计算结果,为液滴层辐射蒸发特性的研究提供了依据。     

Effects of combustible dust clouds on premixed flame extinction in normal- and micro-gravity

An experimental and numerical study was carried out on the effects of combustible solid particles on the extinction of atmospheric, strained, laminar premixed methane/air, and propane/air flames in normal- and micro-gravity. The study was conducted in the opposed-jet configuration in which single flames were stabilized either below or above the gas stagnation plane by counter-flowing a reacting mixture against ambient-temperature air. Spherical 50-μm glassy-carbon and 32-μm Lycopodium particles were injected from either the mixture or the air sides, and the flame extinction states were experimentally determined. The results provided insight into the effects of fuel type, gas-phase composition, strain rate, gravity, as well as particle type, number density, and injection orientation. The combustible particles could have a negative or positive effect on the gas-phase reactivity, depending on the prevailing strain rate and the orientation of injection. The effect of combustible particles on flame extinction was found to reverse when the orientation of the particle seeding is reversed. Experiments and simulations revealed that particle reactions that are not possible in upstream seeding become possible in downstream seeding due to differences in particle residence times and prevailing temperature fields. The effects of gravity on the particle–gas interactions were identified and explained. Gravity could notably modify the chemical response of reacting particles, which, in turn, affects the extinction behavior of the gas phase.     

Interaction of shock waves with a jet wake during gas injection into supersonic stream

In the present paper, we report the results of an experimental study of the interaction region of a planar compression shock produced by a wedge in stream with the wake formed behind a cocurrent gas jet (H₂, air, or Ar) injected into the flow. Depending on the gas jet parameters, three modes of interaction could be distinguished: a strong interaction, observed when the flow velocity in the wake was subsonic; a moderate interaction, observed when a subsonic flow region, bounded by a shock of almost conical shape, formed in the vicinity of the compression shock; and a neutral interaction. Three-dimensional non-stationary Euler equations were solved to numerically examine the interaction of an axisymmetric jet with an oblique shock wave. The obtained interaction regimes were found to be in a reasonable agreement with experimental data.     

Interaction of an Optical Discharge with a Shock Wave

The results of an experimental study of the impact of the focused pulsed-periodic radiation from a CO₂ laser on a gas-dynamic structure in a supersonic jet are presented. The radiation of the CO₂ laser is propagated across the stream and focused by a lens on the axis of the supersonic jet. To register the flow structure, a shadow scheme with a slit and a flat knife located along the flow is used. The image is fixed by a speed camera with an exposure time of 1.5 μs and a frame rate of 1000 1/s. In the flow, the plasma initiated by the pulsedperiodic laser is visualized in order to identify and determine the period of plasma development, as well as the motion of the initial front of the shock wave. It is shown that at the transverse input of laser radiation into the stream the periodic structure of the thermal trace is created with the formation of an unsteady shock wave from the energy release zone. At small repetition rates of laser radiation pulses, the thermal spot interacts with the flow in the pulsed mode. It is shown that elliptic nonstationary shock waves are formed only at low subsonic flow velocities and in a stationary atmosphere. The process of nonstationary ignition by an optical discharge of a methane–air mixture during a subsonic outflow into a motionless atmosphere is shown experimentally. The results of optical visualization indicate burning in the trace behind the optical discharge region.