Investigation of piezoelectrically generated synthetic jet flow

The purpose of this paper is to investigate the compressible turbulent synthetic jet flow characteristics of a dual diaphragm piezoelectric actuator. Experimentally, a flow visualization system was established to obtain the particle streak images scattered from 10-μm red fluorescent spheres for observing the synthetic jet flowfield produced by a dual diaphragm piezo actuator. The centerline velocity of the synthetic jet was also measured by using a hot-wire anemometry system. In the analysis, the computational approach adopted the transient three-dimensional conservation equations of mass and momentum with the moving boundary specified to represent the piezo diaphragm motion. The standard k-∈ two-equation turbulent model was employed for turbulence closure. For the actuator operating at the frequency of 648 Hz, the particle streakline images in the cross-sectional plane visualized a turbulent jet flow pattern in the far-field area. The hot-wire anemometry data indicated that the measured centerline velocity of synthetic jets reached 3.8 m/s at y/d= 50. The predictions were compared with the visualized particle streak images and centerline velocity of the synthetic jet in order to validate the computer code. The numerical simulation revealed the time-periodic formation and advection of discrete vortex pairs. Caused by the outward movement of the piezo diaphragms, air near the orifice was entrained into the actuator cavity when the vortex pairs were sufficiently distant from the orifice.     

Ultrasonic particle size fractionation in a moving air stream

Identification of bio-aerosol particles may be enhanced by size sorting before applying analytical techniques. In this paper, the use of ultrasonic acoustic radiation pressure to continuously size fractionate particles in a moving air stream is described. Separate particle-laden and clean air streams are introduced into a channel and merged under laminar flow conditions. An ultrasonic transducer, mounted flush to one wall of the channel, excites a standing ultrasonic wave perpendicular to the flow of the combined air stream. Acoustic radiation forces on the particles cause them to move transverse to the flow direction. Since the radiation force is dependent upon the particle size, larger particles move a greater transverse distance as they pass through the standing wave. The outlet flow is then separated into streams, each containing a range of particle sizes. Experiments were performed with air streams containing glass microspheres with a size distribution from 2-22 μm, using a centerline air stream velocity of approximately 20 cm/s. An electrostatic transducer operating at a nominal frequency of 50 kHz was used to drive an ultrasonic standing wave of 150 dB in pressure amplitude. The microsphere size distributions measured at the outlet were compared with the predictions of a theoretical model. Experiments and theory show reasonable correspondence. The theoretical model also indicates an optimal partitioning of the particle-laden and clean air inlet streams.     

Hydrodynamic Behavior of Self-Propelled Particles in a Simple Shear Flow

The hydrodynamic properties of a squirmer type of self-propelled particle in a simple shear flow are investigated using the immersed boundary-lattice Boltzmann method in the range of swimming Reynolds number 0.05 ≤ Re_s ≤ 2.0, flow Reynolds number 40 ≤ Re_p ≤ 160, blocking rate 0.2 ≤ κ ≤ 0.5. Some results are validated by comparing with available other results. The effects of Re_s, Re_p and κ on the hydrodynamic properties of squirmer are discussed. The results show that there exist four distinct motion modes for the squirmer, i.e., horizontal mode, attractive oscillation mode, oscillation mode, and chaotic mode. Increasing Re_s causes the motion mode of the squirmer to change from a constant tumbling near the centerline to a stable horizontal mode, even an oscillatory or appealing oscillatory mode near the wall. Increasing the swimming intensity of squirmer under the definite Re_s will induce the squirmer to make periodic and stable motion at a specific distance from the wall. Increasing Re_p will cause the squirmer to change from a stable swimming state to a spiral motion or continuous rotation. Increasing κ will strengthen the wall’s attraction to the squirmer. Increasing swimming intensity of squirmer will modify the strength and direction of the wall’s attraction to the squirmer if κ remains constant.     

微通道内椭球颗粒惯性聚焦行为数值研究

基于浸没边界-格子Boltzmann方法,对方形截面微通道内椭球颗粒的惯性迁移与旋转动力学行为进行数值研究,发现微通道内椭球颗粒的惯性迁移存在两种主要的运动状态：①翻转状态,即椭球颗粒前进过程中长轴始终在中心对称平面内;②滚动状态,即椭球颗粒前进过程中长轴始终垂直于中心对称平面.研究表明：在低Re数（Re=10）下颗粒以两种状态随流体迁移至平衡位置;在较大Re数（50≤Re≤200）下最终均以翻转状态随流体迁移,随Re数增加,平衡位置先逼近壁面后远离壁面.通过对不同运动状态下椭球颗粒周围的微观流场进行分析,提示该微观流动在颗粒惯性聚焦行为特征中有重要影响,并从流体和颗粒的惯性角度对颗粒不同运动状态的转换机理给出解释.     

Lattice Boltzmann Simulations of Particle-Particle Interaction in Steady Poiseuille Flow

The moving behaviour of two- and three-particles in a pressure-driven flow is studied by the lattice Boltzmann simulation in two dimensions. The time-dependent values, including particles＇ radial positions, translational velocities, angular velocities, and the x-directional distance between the particles are analysed extensively. The effect of flow Reynolds number on particle motion is also investigated numerically. The simulation results show that the leading particle equilibrium position is closer to the channel centre while the trailing particle equilibrium position is closer to the channel wall. If Reynolds number Re is less than 85.30, the larger flow Reynolds number results in the smaller x-directional equilibrium distance, otherwise the x-directional distance increases almost linearly with the increase of time and the particles separate finally. The simulation results are helpful to understand the particle-particle interaction in suspensions with swarms of particles.     

Asymptotic analysis of wall modes in a flexible tube revisited

The stability of wall modes in fluid flow through a flexible tube of radius R surrounded by a viscoelastic material in the region R < r < HR is analysed using a combination of asymptotic and numerical methods. The fluid is Newtonian, while the flexible wall is modelled as an incompressible viscoelastic solid. In the limit of high Reynolds number (Re), the vorticity of the wall modes is confined to a region of thickness O(Re ^-1/3) in the fluid near the wall of the tube. Previous numerical studies on the stability of Hagen-Poiseuille flow in a flexible tube to axisymmetric disturbances have shown that the flow could be unstable in the limit of high Re, while previous high Reynolds number asymptotic analyses have revealed only stable modes. To resolve this discrepancy, the present work re-examines the asymptotic analysis of wall modes in a flexible tube using a new set of scaling assumptions. It is shown that wall modes in Hagen-Poiseuille flow in a flexible tube are indeed unstable in the limit of high Re in the scaling regime Re∼Σ^3/4. Here Σ is a nondimensional parameter characterising the elasticity of the wall, and Σ≡ρGR ²/η², where ρ and η are the density and viscosity of the fluid, and G is the shear modulus of the wall medium. The results from the present asymptotic analysis are in excellent agreement with the previous numerical results. Importantly, the present work shows that the different types of unstable modes at high Reynolds number reported in previous numerical studies are qualitatively the same: they all belong to the class of unstable wall modes predicted in this paper. Received 12 June 2000 and Received in final form 8 November 2000     

Transient electro-osmotic flow of generalized Maxwell fluids in a straight pipe of circular cross section

The transient electro-osmotic flow of a generalized Maxwell fluid with fractional derivative in a narrow capillary tube is examined. With the help of an integral transform method, analytical expressions are derived for the electric potential and transient velocity profile by solving the linearized Poisson-Boltzmann equation and the Navier-Stokes equation. It was shown that the distribution and establishment of the velocity consists of two parts, the steady part and the unsteady one. The effects of relaxation time, fractional derivative parameter, and the Debye-Hückel parameter on the generation of flow are shown graphically and analyzed numerically. The velocity overshoot and oscillation are observed and discussed.     

低速开式空腔自激反馈流场结构与流致噪声的风洞试验研究

本文主要针对低速开式空腔流动自激振荡产生噪声问题,在0.55 m×0.4 m航空声学风洞开展了不同低马赫数(0.1/0.15/0.2/0.25)条件下长深比为2的空腔腔内流场结构和噪声特性风洞试验研究。通过利用高频粒子图像测速技术捕捉腔内流场结构,分析了腔内声波传递路径;完成空腔远场噪声和壁面压力测试,分析了噪声自激振荡模态和简正波模态,并对空腔壁面脉动压力和远场噪声进行压/声相关性研究。结果表明:空腔内部除主涡外,在腔口前缘处剪切涡与腔口后缘处碰撞涡明显存在;在875 Hz,1288 Hz,1875 Hz,2050 Hz四个频率附近出现了由声腔共振所致的单频噪声;壁面压力与远场噪声密切相关,在壁面压力主频位置有明显单频噪声出现。     

Linear and nonlinear operation regimes of a laser with a transverse inhomogeneity of the cavity length and angular selection

Gaussian modes and pumping threshold values are found for a laser with spherical mirrors and angular selector. It is shown that, compared to the case of plane-parallel mirrors, the threshold overshoot is proportional to the square root of the mirror curvature. In the absence of the Kerr nonlinearity of the medium, exceeding the oscillation threshold of a stable cavity causes steady-state regimes to be established, whereas, for unstable cavities, pulsation regimes arise in a wide range of parameters. Calculations confirm the formation of moving localized structures in a laser with misaligned plane mirrors.     

Splitting dynamics of coherent structures in a transitional round-pipe flow

Direct numerical modeling of scenarios of the initiation, development, and mutual transitions of coherent turbulent structures in a round pipe at Reynolds numbers 1800 ≤ Re ≤ 4000 is performed. High-accuracy computations were performed for very prolonged time intervals, which made is possible to reveal the fundamental long-lived and statistically stationary flow modes in the transient region between the laminar and established turbulent modes. Reclassification of coherent structures describing the splitting dynamics of the subcritical laminar-turbulent transition is proposed.     

Viscous Interaction Between a Vortex Tube and a Rotating Spherical Particle

The transient advection of a cylindrical vortex tube in a viscous incompressible flow field and its interaction with a rotating/spinning spherical particle has been investigated numerically at Reynolds numbers in the range of 20≤ Re≤200 for angular velocities of 0≤Ω≤0.5. The effects of vortex parameters such as size, circulation strength and initial position relative to the particle, on the temporal behavior of the lift and drag forces are studied. Vortex‐sphere interactions bring about major changes in the flow field particularly when coupled with particle rotation. It is observed that the forces acting on the particle are significantly influenced during the time that the vortex core is in the vicinity of the particle. The extent of these local changes are about ±30% in the drag coefficient and about ±200% in the lift coefficient as compared to flow over a rotating solid sphere with no vortex interaction. It is also found that a vortex with core radius between one and two particle diameters creates the strongest temporal variations in the lift and drag coefficients. Furthermore, maximum lift variations occur for the vortex‐particle head on collision, while a vortex with an offset distance of about one diameter from the principal flow axis generates the maximum drag variations.     

Nonlinear oscillations of a charged jet of a conducting liquid under multimode initial deformation of its surface

The subject of consideration is a uniformly charged jet of an ideal incompressible conducting liquid moving with a constant velocity along the symmetry axis of an undisturbed cylindrical surface. An evolutionary expression for the jet shape is derived accurate to the second order of smallness in oscillation amplitude for the case when the initial deformation of the equilibrium surface is a superposition of a finite number of both axisymmetric and nonaxisymmetric modes. The flow velocity field in the jet and the electric field distribution near it are determined. The positions of internal nonlinear secondary combined three-mode resonances are found, which are typical of nonlinear corrections to the analytical expressions for the jet shape, flow velocity field potentials, and electrostatic field in the vicinity of the jet.     

微液滴振荡过程的光滑粒子动力学方法数值模拟

本文对传统的光滑粒子动力学方法进行了改进, 改进的光滑粒子动力学方法对传统粒子方法中的核梯度进行了修正, 采用了一种新型的核函数和离散格式, 应用改进的光滑粒子动力学方法对微液滴振荡过程进行了数值研究. 研究了不同纵横比和雷诺数(Re)下振荡阻尼与振荡的周期、振幅与Re数的关系. 研究表明: 对于纵横比λ≤ 4时的微液滴振荡过程, 其他参数恒定不变的前提下, Re数越大, 液滴形状变化越剧烈, 波的阻尼作用越弱, 液滴振荡周期变长; 在Re数一定的前提下, 随着液滴初始的纵横比的增大, 液滴振动的振幅增大, 液滴振荡的周期变长.     

Obstacle influence on the flow structure and mass transfer in a boundary layer with ethanol combustion on horizontal surface

Experiments with ethanol combustion on horizontal surfaces revealed the most general properties of a boundary layer with chemical and phase transformations. The list of flow features includes development of large-scale structures and manifestation of volumetric forces, which impact the flow stability and heat and mass transfer. It was demonstrated that the range of velocities ensuring flame existence is wider for flow past a rib than for flow past a backward-facing step. The nature of mass transfer in a reactive flow past an obstacle is transient and remains of that kind until the flame blow-off. For a flow above a horizontal wall at Reynolds numbers Re < 5·10⁴, the intensity of mass transfer is twice higher than for combustion below the wall. When the combustion occurs below the wall, the surface temperature gradients are higher.     

Drag and Lift Forces Acting on a Sphere in Shear Flow of Power-Law Fluid

Laminar flow of a power-law fluid over a sphere is considered for unbounded shear flow. The Navier–Stokes equations with power-law viscosity are solved numerically using an in-house developed CFD package. Vorticities structures downstream of particle are suppressed for powerlaw fluid. The shear rate influence on drag force is negligible for power index close to unit, and the drag force appreciably decreases with falling power index. For small Reynolds numbers, the lift force coefficient monotonically decreases against the power index and exhibits an opposite behavior for moderate values of Reynolds numbers. The results of the parametric studies are used to derive correlations for the drag force and to detect the hydrodynamic differences from uniform flow. The investigation parameters varied within the following ranges: power-law index 0.3 ≤ n ≤ 1, Reynolds number 0 < Re ≤ 150, and dimensionless shear rate 0.05 ≤ s ≤ 0.4.     

Spontaneous and induced Cherenkov radiation generated by electrons in cylindrical dielectrics

A theory of induced Cherenkov radiation in cylindrically symmetrical dielectrics in the case when an electron beam is moving close to the dielectric surface is presented. The spectrum of excited radiation modes has been investigated, and analytical expressions for the gain at the frequencies of various modes have been derived. Zh. éksp. Teor. Fiz. 111, 847–861 (March 1997)     

Submillimeter-wave large-orbit gyrotron

We develop a thermionic-emission electron-optical system forming a dense beam of electrons moving along helical trajectories round the axis of a gyrotron cavity. The maximum beam current is 4 A and the pitch-factor of electrons is 1.0 for a particle energy of 250 keV and a pulse duration of 10 µs. Using such a beam in a gyrotron operated at the third cyclotron harmonic, we obtain single-mode oscillation with a power of 10 µs in the TE_3,8 and TE_3,9 modes with frequencies 371 and 414 GHz, respectively.     

Dynamic analysis of bubble-driven liquid flows using time-resolved particle image velocimetry and proper orthogonal decomposition techniques

Abstract  

An experimental study to evaluate dynamic structures of flow motion and turbulence characteristics in bubble-driven water flow in a rectangular tank with a varying flow rate of compressed air is conducted. Liquid flow fields are measured by time-resolved particle image velocimetry (PIV) with fluorescent tracer particles to eliminate diffused reflections, and by an image intensifier to acquire enhanced clean particle images. By proper orthogonal decomposition (POD) analysis, the energy distributions of spatial and temporal modes are acquired. Time-averaged velocity and turbulent kinetic energy distributions are varied with the air flow rates. With increasing Reynolds number, bubble-induced turbulent motion becomes dominant rather than the recirculating flow near the side wall. Detailed spatial structures and the unsteady behavior of dominant dynamic modes associated with turbulent kinetic energy distributions are addressed.     

Inertial focusing and rotating characteristics of elliptical and rectangular particle pairs in channel flow

The lattice Boltzmann method is used to study the inertial focusing and rotating characteristics of two-dimensional elliptical particles and rectangular particles in channel flow. The results show that both elliptical particles and rectangular particles initially located on one side and two sides of channel centerline migrate first towards the equilibrium position. Then, the single-line particle train with an increasing spacing and the staggered particle train with stable spacing are formed. The axial spacing of the staggered particle pair increases with aspect ratio and Reynolds number increasing. The staggered elliptical or rectangular particle pairs form perpendicular orientation angles, which will be more obvious at larger aspect ratio and lower Reynolds number. The single-line particle trains with different shapes seldom form the perpendicular orientation angle.     

Applicability of quasi-static and Rayleigh approximations for spheroidal particles

Expressions are obtained for the efficiency of absorption and scattering of radiation by uniform spheroids in the quasi-static approximation and the region of their applicability is studied. The prolate and oblate nonabsorbing particles with refractive indices 1.1≤n≤10 and axial ratios 1.1≤a/b≤100 were studied. The approximation expressions are found for the particle size at which the efficiency factors are calculated in the quasi-static approximation with an accuracy of 1%.