PIV and LDV evidence of hydrodynamic instability over a liner in a duct with flow

The acoustical behavior and the flow in a rectangular lined channel with grazing flow have been investigated. The liner consists of a ceramic structure of parallel square channels and is locally reacting. In the absence of flow, the liner has a classical behavior: the acoustic transmission coefficient has a minimum at the resonance frequency of the resonators. When the Mach number of the grazing flow increases, the material behavior becomes unclassical in the sense that its acoustic transmission increases strongly around the resonance frequency. To connect this behavior with flow features, the flow itself in the vicinity of a liner has been measured by means of laser velocimetry. Periodic structures have been observed along the liner that are phase-locked with the incident sound wave. The axial and transverse velocity of these structures bear the typical features of an instability. In particular, the wavelength, convection speed, and growth rate are given. This is the first time that an aeroacoustic instability resulting from the interaction of flow and sound over a liner is measured.     

A comparison of measured and computed sound pressure levels in a non-uniform acoustically lined duct

A comparison of measured and numerically calculated acoustical fields is presented for a non-uniform lined duct in the absence of appreciable mean flow. The frequency range investigated includes the “cut-on” frequencies of several transverse modes in certain portions of the duct. Measured pressure fields are compared to those predicted by one and two dimensional numerical models. The validity of the one dimensional model is confirmed for frequencies below cut-on of the first transverse mode. For higher frequencies the one dimensional model is clearly unsatisfactory, as might be expected. The two dimensional model gives reasonable results for frequencies below and above cut-on of the transverse modes, although it indicates a very strong sensitivity, of acoustical fields, to small variations in local wall impedance data.     

Failure of the Ingard-Myers boundary condition for a lined duct: an experimental investigation

This paper deals with experimental investigation of the lined wall boundary condition in flow duct applications such as aircraft engine systems or automobile mufflers. A first experiment, based on a microphone array located in the liner test section, is carried out in order to extract the axial wavenumbers with the help of an "high-accurate" singular value decomposition Prony-like algorithm. The experimental axial wavenumbers are then used to provide the lined wall impedance for both downstream and upstream acoustic propagation by means of a straightforward impedance education method involving the classical Ingard-Myers boundary condition. The results show that the Ingard-Myers boundary condition fails to predict with accuracy the acoustic behavior in a lined duct with flow. An effective lined wall impedance, valid whatever the direction of acoustic propagation, can be suitably found from experimental axial wavenumbers and a modified version of the Ingard-Myers condition with the form inspired from a previous theoretical study [Aure?gan et al., J. Acoust. Soc. Am. 109, 59-64 (2001)]. In a second experiment, the scattering matrix of the liner test section is measured and is then compared to the predicted scattering matrix using the multimodal approach and the lined wall impedances previously deduced. A large discrepancy is observed between the measured and the predicted scattering coefficients that confirms the poor accuracy provided from the Ingard-Myers boundary condition widely used in lined duct applications.     

Magnetostatic mode in a sheared magnetic field

The effect of magnetic shear on the magnetostatic mode is considered. It is found that the lifetime of the mode is shortened, leading to a reduction of the corresponding cross-field diffusion.     

Linear mechanism of surface gravity wave generation in horizontally sheared flow

An analysis is presented of a linear mechanism of surface gravity wave generation in a horizontally sheared flow in a fluid layer with free boundary. A free-surface flow of this type is found to be algebraically unstable. The development of instability leads to the formation of surface gravity waves whose amplitude grows with time according to a power law. Flow stability is analyzed by using a nonmodal approach in which the behavior of a spatial Fourier harmonic of a disturbance is considered in a semi-Lagrangian frame of reference moving with the flow. Shear-flow disturbances are divided into two classes (wave and vortex disturbances) depending on the value of potential vorticity. It is shown that vortex disturbances decay with time while the energy of wave disturbances increases indefinitely. Transformation of vortex disturbances into wave ones under strong shear is described.     

Dense granular flow around a penetrating object: experiment and hydrodynamic model

We present in this Letter experimental results on the bidimensional flow field around a cylinder penetrating into dense granular matter, together with drag force measurements. A hydrodynamic model based on extended kinetic theory for dense granular flow reproduces well the flow localization close to the cylinder and the corresponding scalings of the drag force, which is found to not depend on velocity, but linearly on the pressure and on the cylinder diameter and weakly on the grain size. Such a regime is found to be valid at a low enough "granular" Reynolds number.     

A mode matching method for modeling dissipative silencers lined with poroelastic materials and containing mean flow

A mode matching method for predicting the transmission loss of a cylindrical shaped dissipative silencer partially filled with a poroelastic foam is developed. The model takes into account the solid phase elasticity of the sound-absorbing material, the mounting conditions of the foam, and the presence of a uniform mean flow in the central airway. The novelty of the proposed approach lies in the fact that guided modes of the silencer have a composite nature containing both compressional and shear waves as opposed to classical mode matching methods in which only acoustic pressure waves are present. Results presented demonstrate good agreement with finite element calculations provided a sufficient number of modes are retained. In practice, it is found that the time for computing the transmission loss over a large frequency range takes a few minutes on a personal computer. This makes the present method a reliable tool for tackling dissipative silencers lined with poroelastic materials.     

Stability analysis of viscous Z-pinch plasma with a sheared axial flow

Within the magnetohydrodynamics （MHD） frame, we analyse the effect of viscosity on magneto-Rayleigh Taylor （MRT） instability in a Z-pinch configuration by using an exact method and an approximate method separately. It is demonstrated that the plasma viscosity indeed has a stabilization effect on the MRT mode in the whole wavenumber region, and its influence increases with the perturbation wavenumber increasing. After the characteristics and feasibility of the approximate method have been investigated, we apply it to the stability analysis of viscous plasma where a sheared axial flow （SAF） is involved, and we attain an analytical dispersion relation. It is suggested that the viscosity and the SAF are complemental with each other, and a wide wavenumber range of perturbation is possible to be restrained if the SAF and the viscosity are large enough. Finally, we calculate the possible value of viscosity parameter according to the current experimental conditions, and the results show that since the value of viscosity is much less than the threshold value, its mitigation effect is small enough to be neglected. The role of the viscosity in the stabilization becomes considerable only if special techniques are so developed that the Z-pinch plasma viscosity can be increased greatly.     

Experimental investigation of oscillatory air flow in a bronchial tube model with HFOV mode

Mechanical ventilations for artificial respiration have been developed to improve the medical treatment of the patients showing respiratory disorder. In various types of ventilation, High Frequency Oscillatory Ventilation (HFOV) is one of the most effective techniques of medical care for pulmonary disease patients, especially, infantsor premature infants. HFOV is a ventilation technique with high breathing rate in comparison with the normal breathing rate. Some successful studies have focused on the effect of treatment using HFOV However, the mechanism of gas exchange in bronchial tube under the medical treatment by HFOV has not been clarified. In this study, the oscillatory flow in a micro-channel model of bronchial with single bifurcation in HFOV mode has been investigated experimentally with micro Particle Image Velocimetry (micro PIV) technique. The phase averaged velocity profiles changing with the driving frequency of HFOV have been inves tigated.     

Numerical computation of an immiscible two-phase flow in comparison with experiment

The flow of fluids with interfaces or free surfaces is of great interest in both basic and applied research. Despite enormous efforts to develop numerical methods for solving such flow problems, no undisputed standards have emerged yet. We compare here three different approaches for the problem of a liquid film falling down along a vertical wall under the action of gravity. Due to an instability, waves form on the film surface and a nontrivial flow pattern emerges. This provides a test case for two-phase flow simulation methods under conditions of practical relevance. The methods compared are a moving boundary method and a volume of fluid method implemented in commercial software packages and a new phase field method. A set of experimental data is used as a reference for the comparison. All three methods correctly reproduce the main features of the experiment; however, none are free of quantitative deviations from the data. The text was submitted by the authors in English.     

光学微盘腔与三能级量子点系统中的模耦合研究

用一种全量子理论方法研究了波导、光学微盘腔与三能级量子点耦合系统的动力学过程,求出其耦合后的透射模和反射模的解析解. 由于微腔表面粗糙引起反向散射,在微腔内形成两简并回音壁耦合共振模,其耦合率为β;量子点的两激发态分别以耦合率g₁,g₂与回音壁耦合共振模产生耦合. 在实数空间里,得出透射光谱和反射光谱的数值解,这些三能级模型结果比二能级模型结果更接近真实光学微盘腔系统,能更好地显示耦合系统的动力学特性. 关键词： 模耦合 光学微盘腔 三能级量子点 全量子理论     

Theoretical comparison of magnetic and hydrodynamic interactions between magnetically tagged particles in microfluidic systems

Magnetic and hydrodynamic interactions between magnetic beads in microfluidic magnetic field gradient filters are compared theoretically and we find that the hydrodynamic interactions are of a longer range and dominate the magnetic ones. Hydrodynamic interactions aid the capturing of particles tagged with magnetic beads as the particles drag each other along, possibly easing requirements on magnetic parameters.     

Acoustics of a blocked duct with flow

An analysis is given for flow and acoustical motion of fluid in a duct, aimed at describing some features of the blowing mechanism of a woodwind instrument. The latter is simulated by a rigid rectangular duct, open at one end and nearly closed off at the other, where a local section of the side wall can undergo transverse displacement. It is supposed that such a piston-like oscillation superposes an acoustical perturbation on a steady flow through the duct. The principal results of the calculations, in which it is presupposed that the displacement amplitude is small compared to the piston width, which is in turn small compared with the duct width and wavelength, are contained in estimates for the natural frequencies and optimum flow speeds to excite these modes.