The Nucleation of Superfluid Turbulence at Very Low Temperatures by Flow Through a Grid

Recent experiments by Nichol et al. (cond-mat/0309245 v2) have been concerned with the dynamical behaviour of a grid oscillating in superfluid ⁴He at a very low temperature, where the normal fluid can be ignored. An interesting enhancement of the effective mass of the grid was observed above a first threshold velocity, without significant increase in damping. Only above a second larger threshold was there a large increase in damping, resulting, we presume, from the generation of turbulence. We show now how the increase in effective mass can be understood in terms of an adiabatic response of the remanent quantized vortices that are knoum to be present, usually, in superfluid helium. Only at the larger threshold is the adiabatic response replaced by a dissipative evolution into a turbulent tangle of vortex lines. We present a semi-quantitative analysis of the experimental results, which suggests strongly that the remanent vortices must take the form of a rather high density of vortex loops attached to the grid. But confirmation of our ideas must await the completion of further experiments and a programme of non-trivial computer simulations.     

Torsional oscillations of a rod in a layered medium of simple fluids

Free surface flow of a layered medium of simple fluids driven by a torsionally oscillating, cylindrical rod is investigated. The stress, expressed as a series of multiple integrals of polynomials in the strain histories, is expanded into a Fréchet series in terms of the amplitude of the oscillation of the rod. A domain perturbation method is used to perturb the rest state. The mean shapes and deviations from flatness at the rod of the interfaces between layers are determined together with the flow field up to and including second order in the amplitude of the oscillation.     

Two-layer flow in a corrugated channel

The flow of two superposed viscous fluid layers in a two-dimensional channel confined between a plane and a wavy or indented wall is studied by analytical and numerical methods at arbitrary Reynolds numbers. The interface between the two fluids may exhibit constant or variable surface tension due to an insoluble surfactant. The flow is computed from a specified initial condition using the immersed-interface method on a curvilinear grid constructed by conformal mapping. The numerical simulations illustrate the effect of geometrical nonlinearity and reveal that inertia may increase or decrease the amplitude of the interface profile at steady state depending on the flow parameters. Increasing either the Reynolds number or the wall amplitude above a certain threshold value provokes flow instability and overturning of the interface. In the Appendix, a linear perturbation analysis is performed for arbitrary Reynolds numbers on the assumption of small-amplitude sinusoidal undulations, and results for the amplitude and phase shift of the interfacial and surfactant concentration wave are documented for a broad range of flow conditions. It is found that inertia may have a mixed effect on the deformation and phase shift, while the surfactant promotes the deformation of the interface under most conditions.     

Acoustic Resonances and Non-linearity in Solid 4He

Direct measurements of the low frequency shear modulus of solid ⁴He show a remarkable increase below 150?mK, a regime where torsional oscillator (TO) experiments show evidence of mass decoupling. Acoustic resonance measurements at higher frequencies confirm the unusual elastic behavior. A striking feature of both TO and shear modulus measurements is their amplitude dependence—the low temperature anomalies are reduced as the drive amplitudes increase. We have studied the amplitude dependence of acoustic resonances in both standard and isotopically pure ⁴He crystals. The resonance peaks shift to lower frequencies as the drive amplitude increases. At high amplitudes the peaks are asymmetric and exhibit both bistability and hysteresis—classic features of nonlinear oscillators. At the lowest amplitudes the peak frequency and shape are independent of drive amplitude and are non-hysteretic. The threshold for nonlinear behavior is lower for the isotopically pure crystal. We also studied the effects of annealing on the acoustic resonances. The nonlinear behavior was qualitatively unchanged but annealing often affected the shape of the resonance peaks, which often had a complicated structure at low temperatures.     

科氏质量流量计模拟驱动方法研究

驱动系统是科里奥利质量流量计的重要组成部分之一。它为流量管提供驱动力,使流量管以其固有频率和稳定的振幅振动,并且能够跟随流量管固有频率的变化而变化。目前的驱动系统是基于模拟信号的工作方式。分析了这种驱动系统的工作原理,比较了各种波形的驱动信号,研究了几种驱动电路。     

Analysis of Film Condensation Along a Vertical Flat Plate Under Sinusoidal G-Jitter

Transient phenomenon of laminar film condensation along a vertical flat plate under sinusoidal g-jitter has been analyzed, based on the same assumptions of Nusselt’s analysis of film condensation where the heat transfer within the liquid film is by pure conduction. The momentum equation retains the transient term. The perturbed acceleration due to gravity has been assumed to be a sinusoidal function of time and frequency of oscillation for the simplicity. The resultant equation has been solved analytically and the velocity profiles and mass flow rate under such g-jitter has been simulated. It has been observed that at the different heights the velocity response with time is purely sinusoidal with different amplitude. Last but not the least; boundary layer thickness is also oscillating with time, hence heat transfer coefficient. However, the entire analysis is an extension of Nusselt’s analysis of film condensation which includes transient response.     

Integral methods of calculation of heat transfer and drag under conditions of turbulent pipe flow of liquid of variable properties: Pulsating high-frequency flow

An integral method is suggested for approximate calculation of oscillation-period average heat transfer and drag under conditions of pulsating high-frequency flow of gas in a pipe with constant density of heat flux to the wall. It is found that the flow rate oscillation superimposed on the flow has little effect on the period average Nusselt number and coefficient of friction drag; these quantities may be calculated by the method developed for a steady-state flow of liquid of variable properties. The oscillation affects significantly the period average coefficient of hydraulic drag whose values increase with the amplitude of superimposed oscillation.     

Magnetohydrodynamic mixed convection flow in vertical concentric annuli with time periodic boundary condition: Steady periodic regime

This work reports an analytical solution for fully developed mixed convection flow of viscous, incompressible, electrically conducting fluid in vertical concentric annuli under the influence of a transverse magnetic field, where the outer surface of inner cylinder is heated sinusoidally and the inner surface of outer cylinder is kept at a constant temperature. The analysis is carried out for fully developed parallel flow and steady-periodic regime. The governing dimensionless momentum and energy equations are separated into steady and periodic parts and solved analytically. Closed form solutions are expressed in terms of modified Bessel function of first and second kind. The influence of each governing parameters such as magnetic field parameter, Prandtl number and the dimensionless frequency of heating on flow formation and thermal behaviour are discussed with the aid of graphs. During the course of investigation, it is found that the oscillation amplitude of the friction factor is maximized at a resonance frequency near the surface of the concentric annuli where there is periodic heating. Furthermore, increasing transverse magnetic field decreases the oscillation amplitude of the friction factor.     

Acceleration waves in condensing gases

Summary The flow of a condensing gas is treated as a two-phase-flow, in which the size of the condensate-droplets may vary due to transfer of mass, momentum, and heat; the formation of new droplets is disregarded. An ordinary differential equation for the temporal variation of the amplitude of a one-dimensional acceleration wave is deduced, which holds along the path of the wave. Especially, if the wave propagates into a mixture at rest with spatial variation of the volume fraction of the droplets, the variation of the amplitude is given by the sum of three terms, one of which is quadratic in the amplitude and the others are linear. The quadratic term is solely determined by nonlinear effects in the pure gas and leads to a growth. The first linear term is given by the dissipative effect of the velocity relaxation; this term is the same as for the flow of a mixture of a gas and small solid particles. The second linear term is determined by the combined dissipative effects of the temperature relaxation and the mass transfer; both linear terms lead to a decay. Further, conditions are discussed, on which shock waves are formed.With 6 Figures     

热声斯特林发动机热动力学特性的CFD研究第二部分:热声转换特性及热声声流的研究

研究了热声斯特林发动机的声场特性和热力特性,计算了波动压力和体积流率振幅及两者的相位差、截面平均温度以及声功流在系统内的沿程分布,与实验及预期的趋势相符合.对热声斯特林发动机内部的Gedeon声直流进行了研究.通过监测系统内截面的质量流率周期分布特性观测到Gedeon直流的存在,计算了直流量的大小及在系统内的分布特性,并采用风扇模型抑制了直流,给出了直流抑制过程中的系统性能变化.进一步验证了CFD研究的有效性.     

The behaviour of excited plane jets

A plane subsonic jet can be excited to entrain more fluid from its surroundings by subjecting it to antisymmetric periodic disturbances. The essential feature in this phenomenon is the rolling-up motion of an initially flapping jet to form large vortices which are responsible for greater entrainment. Several methods developed to impart oscillations to the flow at the nozzle, such as the acoustic pressure oscillator, the vibration of a single vane in the potential core region, the reciprocating lip system and the twin vane exciter, are described in this article. A minimum threshold in amplitude is necessary for exciting the flow. However, the frequency of oscillation is much less than that predicted by stability considerations.     

回热器非线性热声动力学模型的研究

基于基本的守恒方程,研究了回热器在具有大振幅波动情况下的弱非线性热声动力学模型。其中,动量守恒方程采用理论性强的瞬态Darcy模型,能量守恒方程则包括了多孔介质的高阶耗散效应。采用摄动法,并在唯象的基础上获得了能描述包括声流（直流）在内的非线性动力学效应及由此而导致的非线性时均热声动力学效应的局部解析方程组,对加深回热器工作机理的理解具有一定的指导意义。     

Application of proper orthogonal decomposition to the discrete Euler equations

The response of a fluid moving above a panel to localized oscillation of the panel is predicted using reduced‐order modelling (ROM) with the proper orthogonal decomposition technique. The flow is assumed to be inviscid and is modelled with the Euler equations. These non‐linear equations are discretized with a total‐variation diminishing algorithm and are projected onto an energy‐optimal subspace defined by an energy‐threshold criterion applied to a modal representation of time series data. Results are obtained for a bump oscillating in a Mach 1.2 flow. ROM is found to reduce the degrees of freedom necessary to simulate the flowfield by three orders of magnitude while preserving solution accuracy. Other observed benefits of ROM include increased allowable time step and robustness to variation of oscillation amplitude. Published in 2002 by John Wiley & Sons, Ltd.     

一种考虑时延的双气泡耦合振荡模型

基于单气泡Keller-Miksis振荡方程,在考虑时延的情况下,建立了一种双气泡耦合振荡计算模型。该模型将气泡振荡的周期分成若干份,初始扰动引起第一个气泡的半径在极短时间内变化而产生振荡并辐射声压,声压在传播一定时间后作用到第二个气泡,第二个气泡同样在短时间内做耦合振荡并反馈到第一个气泡,然后重复此过程。利用数值仿真在此模型的基础上分别研究了气泡振幅、半径、间距等参数对耦合振荡的影响。结果表明:初始扰动越大、两个气泡半径越接近,气泡耦合效应越明显;初始半径和平衡半径较大的气泡对耦合振荡有显著影响,振荡的频率向低频移动;气泡间距越大,耦合效应越弱;在某个特定距离处,气泡耦合效应的阻尼会异常减小或者增大。     

Druckschwingungen in Seitenkanalpumpen

By the turbulent vortex forced by the impeller, strong high-frequency pressure oscillations occur in the side channel. These pressure oscillations result from the temporal modification of the vortex structure and the specific impulse flow transportation in the side channel. They are visible due to the high-frequency character neither at integral static pressures not in the characteristic lines of the pumps. However, they add essentially to the energy transfer to the pumped fluid and to the increase of the static pressure in the side channel. The dynamic pressure oscillations in the side channel with the characteristic exciter frequency by the blade revolution frequency are transmitted to the pump case and a part of it is emitted as effective acoustic pressure to the environment. The blade revolution frequency is by far recognizable as exciter frequency in the pressure oscillations, in the mechanical case oscillation and as effective acoustic pressure in the raised amplitude parts. The pressure oscillations refer to the unsteady flow in side channel pumps.     

A comparison of different methods to determine the threshold of fatigue crack propagation

Three different methods for determining the threshold value for fatigue crack growth — the load-shedding technique, the stepwise increase of load amplitude on specimens precracked in cyclic compression, and decrease of stress intensity range at a constant maximum stress intensity — were applied to a high-strength aluminium alloy. The load-shedding technique tended to lead to higher values of the threshold, especially at low R-ratios. The threshold determined with decreasing stress intensity range at a constant maximum stress intensity was larger than the effective threshold determined with stepwise increasing of load amplitude on specimens precracked in cyclic compression.     

Effective resistance of current-carrying superconducting wire in oscillating magnetic fields 2. Twisted multifilamentary composite conductor

The influence of oscillating magnetic fields of low frequency on the dc transport current in a twisted multifilamentary conductor has been studied by observing the terminal voltage of the conductor. An analysis of the voltage versus oscillating field characteristics, combined with theoretical considerations, shows that current distribution among the filaments becomes uniform due to the oscillation of the field, and an effective resistance appears in the transport current when the amplitude of the oscillating field exceeds a certain threshold value. The dependence of the effective resistance on the characteristic parameters of both the oscillating field and the filamentary conductor is clarified.     

一种锥形节流阀工作过程流-固耦合动力学响应的影响因素分析

基于三维流-固耦合有限元动力学仿真分析模型和直接耦合算法,分析了一种锥形节流阀在入口流速脉冲激励下由关闭状态开启而后重新关闭全过程的流量特性、压差特性及阀门开度的高频波动等非线性动力学响应特性,并采用小波分析方法等对阀门开度响应等进行了时-频域分析。选择不同的流体-结构模型的数值积分方法组合及时间步长对流-固耦合动力学求解算法进行了实际应用检验;然后对阀芯质量、弹簧参数与油液参数等系统参数以及激励速度幅值与脉宽等激励参数对其工作过程动力学响应的影响进行了细致的数值分析比较。结果表明：流体模型积分算法的选择对流-固耦合计算结果的影响较大;对该阀而言,阀芯质量与油液体积弹性模量的改变对阀芯振动频率的影响较为显著,油液粘度的改变对阀门开启的滞后量及振动相位的影响较大,而弹簧刚度及预紧力的改变对阀门的最大稳定开度的影响较大;阀芯与阀座间的碰撞使阀芯的振动频率提高。     

Numerical investigation of dilute aerosol particle transport and deposition in oscillating multi-cylinder obstructions

The transport and deposition of aerosol particles through a fibrous filter is encountered in many natural and industrial processes. As the filtration performance for a stationary filter has been extensively studied in the literature, the present work focuses on the effect of fiber oscillation in a filter where the fibers are allowed to vibrate periodically. The transport and deposition of dilute aerosol particles in such a system is simulated using an efficient numerical model, where an iterative immersed-boundary lattice Boltzmann method is applied to solve the background flow with finite-size moving fibers, and the motion of aerosol particles is then tracked by a one-way coupling Lagrangian approach. In the present scheme, the no-slip boundary condition at the fiber surface can be exactly enforced with an iterative approach and the numerical stability is improved by adopting the MRT collision model. After the model validation in the two special cases of flow over an oscillating fiber in a quiescent fluid and particle capture by a stationary fiber, the filtration performance of an oscillating multi-fiber filter is investigated to study the effects of fiber number, arrangement and vibration mode. It is found that the oscillating motion of fiber has significant influence on the filtration performance. For a single fiber, with larger oscillation amplitude, the distribution ranges of the release position and impact angle of captured particles both increase. On the other hand, a larger fiber oscillation frequency tends to reduce the width of release position but increase the width of impact angle of deposited particles. Furthermore, the collection efficiency is found to be linearly related to the oscillation amplitude or frequency. For multiple fibers, the collection efficiency always increases with larger fiber number, but it is a non-monotonic function of the arrangement parameters, i.e., the longitudinal and transverse spacings, and the vibration parameters such as the amplitude, frequency and vibration mode. It is interesting to find that the in-phase mode can usually lead to excellent collection efficiency.     

Phenomenology of the effective stress intensity related to fatigue crack growth thresholds

The fatigue crack growth threshold conditions for effective stress intensity amplitude are examined using simple phenomenological models for crack face interference and internal stresses. We show that behaviors correlating with all pure fatigue classifications can be generated from a single ‘ideal fatigue’ behavior by accounting for internal stress and crack face interference. The possible threshold or near-threshold manifestations of an intrinsic K_MAX threshold, independent of effective ΔK effects, are discussed.