Measurements of the dissipation coefficient at the wavemaker in the process of generation of the resonant standing waves in a tank

The complex dissipation at the wavemaker used for direct excitation of nonlinear standing waves in the vicinity of the cut-off frequency, is measured experimentally. The results indicate that the absolute value of this dissipation coefficient exceeds significantly the estimate based on purely viscous dissipation. This is attributed to the turbulent effects resulting from the vortex shedding at the discontinuities of the wavemaker surface. It is shown that incorporation of this dissipation in the boundary condition at the wavemaker as a nonlinear term in a form generally used to describe dissipation in a turbulent boundary layer, is more appropriate allowing to remove hysteresis in the value of the wavemaker dissipation coefficient. Such hysteresis is observed when a linear dissipation model is applied.     

Measurements of two- and three-dimensional waves in a channel,including the vicinity of cut-off frequencies

Measurements of water wave profiles were performed in a rectangular flume equipped with a modular wavemaker. This particular wavemaker could generate both two- and three-dimensional waves. A method is proposed to evaluate quantitatively the deviations of a spacial flow field from the two-dimensional one. Plane propagating waves, as well as pure sloshing waves with their crests parallel to the walls, were generated in the flume. In all cases the measured amplitudes were compared against linear theory predictions.     

Waves generated by an inclined-plate wave generator

This paper describes the characteristics of small-amplitude waves generated by a sinusoidally oscillating, inclined paddle-type wavemaker operating in a constant-depth channel. Two-dimensional, linearized potential flow is assumed. A semi-analytical method, the boundary collocation method, is used to establish the relationship between wave amplitude and paddle stroke. The numerical results are compared with the numerical results of the boundary integral equation method. It is found that the boundary collocation method is simpler and more flexible to implement and faster to compute. In addition, the numerical results are in reasonably good agreement with the laboratory experimental data. For the vertical wavemaker, which is a special case of the inclined wavemaker, an analytical series solution can be found. By using the boundary collocation method and the boundary integral equation method to solve the vertical wavemaker problem and comparing the results with the analytical series solution, it is found that the boundary collocation method yields a solution which is much more accurate than that from the boundary integral equation method. Finally, the relationships between wave amplitude and paddle stroke are established for different inclinations of the paddle-type wavemaker, based on the boundary collocation method.     

Wave motions near a translating plate in a stratified fluid with infinite depth

The dynamics of two horizontal inviscid liquid layers induced by a vertical wavemaker is studied analytically, followed by a numerical analysis. The focus is put on the time dependent motion of the two interfaces, formed between the two liquid layers and between the upper layer and the air above. The singularities that would appear in the two contact lines (the three-phase lines formed by the wavemaker, the liquids, and the air) in most variable-separating techniques are suppressed by a Fourier-integral method, which generates uniformly valid solutions; the surface elevations at the contact lines remain finite for all time. Various wavemaker velocities are considered for realistic applications of the results. The study is initially prompted by the oil-skimming problem, one of the main issues of which is to determine the optimum speed of the oil skirt, without leaving the spilt oil behind. By obtaining the locations of the two interfaces though this study, the motion of the oil layer (upper liquid) can be determined precisely, based on conservation laws.     

Direct simulation of multiphase flows with modeling of dynamic interface contact angle

We describe a modeling technique for dynamic contact angle between a phase interface and a solid wall using a generalized Navier boundary condition in the context of a front-tracking-based multiphase method. The contact line motion is determined by the generalized Navier slip boundary condition in order to eliminate the infinite shear stress at the contact line. Applying this slip boundary condition only to the interface movement with various slip ratios shows good agreement with experimental results compared to allowing full fluid slip along the solid surface. The interface slip model performs well on grid convergence tests using both the slip ratio and slip length models. A detailed energy analysis was performed to identify changes in kinetic, surface, and potential energies as well as viscous and contact line dissipation with time. A friction coefficient for contact line dissipation was obtained based on the other computed energy terms. Each energy term and the friction coefficient were compared for different grid resolutions. The effect of varying the slip ratio as well as the contact angle distribution versus contact line speed was analyzed. The behavior of drop impact on a solid wall with different advancing and receding angles was investigated. Finally, the proposed dynamic contact model was extended to three dimensions for large-scale parallel calculations. The impact of a droplet on a solid cylinder was simulated to demonstrate the capabilities of the proposing formulation on general solid structures. Widely different contact angles were tested and showed distinctive characteristic behavior clearly.     

Hydroelastic analysis of gravity wave interaction with submerged horizontal flexible porous plate

The present study deals with the surface gravity wave interaction with submerged horizontal flexible porous plate under the assumption of small amplitude water wave theory and structural response. The flexible porous plate is modeled using the thin plate theory and wave past porous structure is based on the generalized porous wavemaker theory. The wave characteristics due to the interaction of gravity waves with submerged flexible porous structure are studied by analyzing the complex dispersion relation using contour plots. Three different problems such as (i) wave scattering by a submerged flexible porous plate, (ii) wave trapping by submerged flexible porous plate placed at a finite distance from a rigid wall and (iii) wave reflection by a rigid wall in the presence of a submerged flexible porous plate are analyzed. The role of flexible porous plate in attenuating wave height and creating a tranquility zone is studied by analyzing the reflection, transmission and dissipation coefficients for various wave and structural parameters such as angle of incidence, depth of submergence, plate length, compression force and structural flexibility. In the case of wave trapping, the optimum distance between the porous plate and rigid wall for wave reflection is analyzed in different cases. In addition, effects of various physical parameters on free surface elevation, plate deflection, wave load on the plate and rigid wall are studied. The present approach can be extended to deal with acoustic wave interaction with flexible porous plates.     

Large eddy simulation in Code_Saturne of thermal mixing in a T junction with brass walls

Following on from Kuhn et al (2010) we study the capability of large eddy simulation with conjugate heat transfer to predict thermal fluctuations with thermal mixing. Wall functions are used to model the wall heat transfer. Comparison with experimental results show that the temperature variance on the outer skin of the solid is well predicted by the simulation. It is shown that the variance of thermal flux in the fluid closely maps the temperature variance at the outer boundary of the solid. Since the variance of thermal flux is closely related to the dissipation of temperature variance it can be concluded that the dissipation of temperature variance in the fluid is linked to temperature variance in the solid. Analysis of the equation of the temperature variance in the solid confirms this is indeed the case. It is the dissipation of temperature variance in the fluid that characterizes how the temperature variance penetrates the solid. Thus RANS modelling can be used to predict thermal variance in solids provided that there is an accurate model for the dissipation of temperature variance at the wall and an equation for the thermal variance in the solid is solved.     

Reflection and transmission of regular waves at a surface-pitching slotted barrier

The interactions between regular surface waves and a surface-pitching slotted barrier are investigated both analytically and experimentally.A quasi-linear theory is developed using the eigenfunction expansion method.The energy dissipation within the barriers is modeled by a quadratic friction factor,and an equivalent linear dissipation coefficient,which is depth-varying,wave-height dependent,is introduced to linearize the matching condition at the surface-pitching barrier.By comparing the theoretical results with laboratory experiments,it is shown that the present method can satisfactorily predict the variation of the reflection and transmission coefficients with wave height.     

Assessment of a three-component vorticity probe in decaying turbulence

Measurements in grid turbulence with a three-component hot-wire vorticity probe are compared with results obtained from a direct numerical simulation of decaying homogeneous turbulence at a similar Reynolds number. There is adequate agreement for both spectra and probability density functions of all three vorticity fluctuations. This supports the treatment of the measured data, in particular the corrections that need to be applied for the finite spatial resolution of the probe. The comparison does, however, highlight some inaccuracies in the way the instantaneous dissipation rate fluctuations were obtained experimentally.     

Generation of acoustic solitary waves in a lattice of Helmholtz resonators

This paper addresses the propagation of high amplitude acoustic pulses through a 1D lattice of Helmholtz resonators connected to a waveguide. Based on the model proposed by Sugimoto (1992), a new numerical method is developed to take into account both the nonlinear wave propagation and the different mechanisms of dissipation: the volume attenuation, the linear viscothermal losses at the walls, and the nonlinear absorption due to the acoustic jet formation in the resonator necks. Good agreement between numerical and experimental results is obtained, highlighting the crucial role of the nonlinear losses. Different kinds of solitary waves are observed experimentally with characteristics depending on the dispersion properties of the lattice.     

ON BREAKING WAVES AND WAVE-CURRENT INTERACTION IN SHALLOW WATER: A 2DH FINITE ELEMENT MODEL

A two-dimensional (horizontal plane) coastal and estuarine region model, capable of predicting the combined effects of gravity surface shallow- water waves (shoaling, refraction, diffraction, reflection and breaking), and steady currents, is described and numerical results are compared with those obtained experimentally. Two series of observations within a wave flume and a combined wave-current facility were developed. In the first case, the wave was generated via a hinged paddle located within a deepened section at one end of the channel, as, in the second case, the wave propagating with or against the current was generated by a plunger-type wavemaker; the re-circulating current was introduced via one passing tank connected to a centrifugal pump. Several comparisons for a number of 1D situations and one 2D horizontal plane case are presented.     

Effect of solid mass flux on anisotropic gas–solid flow in risers determined with an LES-SOM model

Within the framework of the two-fluid approach, gas was treated with a large-eddy simulation and a sub-grid-scale (SGS) turbulent kinetic energy model while particles were treated with a second-order-moment method to describe the anisotropy of the fluctuating velocity. A modified Simonin model was derived for the gas–solid interphase fluctuating energy transfer. The anisotropic gas–solid flow in a circulating fluidized bed was investigated. Predictions were in good agreement with experimental data. The distributions of the second-order moment of particles and SGS-turbulent kinetic energy of gas were simulated at different solid mass fluxes. The effects of the solid mass flux on the particle second-order moment, particle anisotropic behavior, gas SGS-turbulent kinetic energy and gas SGS energy dissipation were analyzed for the circulating fluidized bed.     

TiNi相变圆柱壳的径向压缩特性实验研究

对不同几何尺寸和边界约束条件的TiNi合金圆柱壳进行了准静态径向压缩实验研究,利用数字摄像和图像处理技术得到了不同位置柱壳的变形特征和应变分布,结果表明,在柱壳表面受压处先出现相变铰,随着名义应变的增大,相变铰将发展成相变铰区;在柱壳表面受拉处出现相变铰,但此处材料先进入马氏体相.当径厚比和边界约束不变时,随着名义应变的增大,柱壳的耗能率和比能不断增加.当名义应变不变时,柱壳的耗能率和比能随边界约束个数的增多而增加,吸能效果更好,随径厚比的增加而减小,吸能效率下降.     

Measurement of the acoustic velocity field of nonlinear standing waves using the synchronized PIV technique

The motion of gas within an air-filled rigid-walled square channel subjected to acoustic standing waves is experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the acoustic velocity fields at different phases over the excitation signal period. The acoustic velocity measurements have been conducted for two different acoustic intensities in the quasi-nonlinear range (in which the nonlinear effects can be neglected in comparison with the dissipation effects), and one acoustic intensity in the finite-amplitude nonlinear range (in which both the nonlinear term and the dissipative term play a role in the wave equation). The experimental velocity fields for the quasi-nonlinear cases are compared with the analytical results obtained from the time-harmonic solution of the wave equation. Good agreement between the experimental and analytical velocity fields proves the ability of the synchronized PIV technique to accurately measure both temporal and spatial variations of the acoustic velocity fields. The verified technique is then used to measure the acoustic velocity fields of the finite-amplitude nonlinear case at different phases.     

Solid damping in micro electro mechanical systems

This paper focuses on the problem of the numerical evaluation of dissipation induced by thermoelastic coupling in microelectromechanical systems. An ad hoc conceived, FE based, numerical procedure for the evaluation of the thermoelastic dissipation is proposed and the numerical results are compared with analytical solutions. In order to introduce in the numerical response a dependence on the size of the resonating devices, which is experimentally observed at very small dimensions, a new enhanced non-local coupled thermoelastic model is proposed and the first results are discussed. An erratum to this article can be found at     

Dissipation estimates in turbulent flows using the zero-wire-length technique

Azad and Kassab (1989) presented a new technique for estimating dissipation in turbulent flows and they referred to the method as the zero-wire-length technique. The validity of the approach has been here checked experimentally for the flow in the far wake of a circular cylinder for which Browne et al. (1987) had obtained reasonable estimates of the dissipation. It has been found that the zero-wire-length technique provides no more than an estimate of the isotropic dissipation: the actual dissipation values cannot be estimated by this technique.     

Linear and nonlinear gravity-capillary water waves with a soluble surfactant

Naturally occurring soluble-surfactant slicks influence the properties of water waves. This paper describes results from wave tank experiments involving a soluble surfactant, and linear and nonlinear gravity- capillary waves. Instantaneous surface deflections were measured using optical techniques to determine the damping, phase speed, and the frequency content of the waves for wavemaker frequencies from 4 to 22 Hz. Measured linear-wave phase speed and damping agree well with existing theory at surfactant concentrations away from that leading to maximum damping. Under conditions leading to nonlinear waves, an as-yet-unexplained subharmonic wave with one-sixth the wavemaker frequency was found only with soluble surfactant present. Received: 3 September 2000/ Accepted: 8 August 2000     

Effect of metachronal phasing on the pumping efficiency of oscillating plate arrays

An aqueous solution of sodium iodide and zinc iodide is proposed as a fluid that matches the refractive index of a solid manufactured by rapid prototyping. This enabled optical measurements in single-phase flow through porous structures. Experiments were also done with an organic index-matching fluid (anisole) in porous structures of different dimensions. To compare experiments with different viscosities and dimensions, we employed Reynolds similarity to deduce the scaling laws. One of the target quantities of our investigation was the dissipation rate of turbulent kinetic energy. Different models for the dissipation rate estimation were evaluated by comparing isotropy ratios. As in many other studies also, our experiments were not capable of resolving the velocity field down to the Kolmogorov length scale, and therefore, the dissipation rate has to be considered as underestimated. This is visible in experiments of different relative resolutions. However, being near the Kolmogorov scale allows estimating a reproducible, yet underestimated spatial distribution of dissipation rate inside the porous structure. Based on these results, the $k-\varepsilon$ model was used to estimate the turbulent diffusivity. Comparing it to the dispersion coefficient obtained in the same porous structure, we conclude that even at $Re_p=500$ the turbulent diffusivity makes up only a small part of mass transfer in axial direction. The main part is therefore attributed to Taylor dispersion.     

侧向爆炸冲击波加载作用下钢管吸能特性的实验研究

在爆炸冲击波作用下 ,对横卧单、双钢管吸能特性进行了实验研究。得出了影响钢管吸能效果的主要因素是几何尺寸、吸能元件配重的质量、结构形式等 ,且双钢管吸能元件的吸能效果好于单钢管的结论。     

TEMPERATURE PROFILES OF LOCAL THERMAL NONEQUILIBRIUM FOR THERMAL DEVELOPING FORCED CONVECTION IN POROUS MEDIUM PARALLEL PLATE CHANNEL

Based on the two-energy equation model, taking into account viscous dissipation due to the interaction between solid skeleton and pore fluid flow, temperature expressions of the solid skeleton and pore fluid flow are obtained analytically for the thermally developing forced convection in a saturated porous medium parallel plate channel, with walls being at constant temperature. It is proved that the temperatures of the two phases for the local thermal nonequilibrium approach to the temperature derived from the one-energy equation model for the local thermal equilibrium when the heat exchange coefficient goes to infinite. The temperature profiles are shown in figures for different dimensionless parameters and the effects of the parameters on the local thermal nonequilibrium are revealed by parameter study.