Sound attenuation by multiple barriers

A method (tentatively called ‘the method of equivalent sound source’) of predicting the sound attenuation achieved by multiple barriers with knife edges and/or right-angled wedges is proposed. This paper shows that the sound pressure level in the shadow region behind the multiple barriers can be obtained by successively setting imaginary sound sources for respective edges and/or wedges. The locations of the imaginary sound sources are determined by means of the traditional solution for sound attenuation by a single edge. The calculated results for several kinds of small-scale model, obtained by ‘the method of equivalent sound source’, are compared with experimental results.     

随活性剂浓度变化的分离压对垂直液膜排液过程的影响

针对含不溶性活性剂的垂直液膜排液过程,基于文献实验结果进一步完善了受活性剂浓度影响的分离压(disjoining pressure)模型,应用润滑理论建立了液膜厚度、活性剂浓度和液膜表面速度的演化方程组,通过数值计算分析了在不同分离压作用下含不溶性活性剂液膜的演化特征.结果表明,垂直液膜的排液过程通常经历两个阶段:首先是厚膜阶段,此时重力对排液过程起主导作用.在随后的薄膜阶段,毛细作用和分离压作用影响逐渐增大,其中分离压将控制液膜的演化历程.分离压对垂直液膜排液过程的影响与活性剂类型及活性剂浓度与静电作用力的关联强度密切相关.当分离压与活性剂浓度正相关时,随斥力关联系数α增大,液膜的排液和变薄过程得以减缓,由此增强了液膜稳定性;当分离压与活性剂浓度负相关时,随斥力关联系数α绝对值增大,液膜排液过程加速,由此加大液膜失稳的风险.     

Fluids absorbed in structured pores

A model of fluids absorbed in a pore with walls patterned with parallel channels is used to demonstrate some of the unity that can be proved to hold between the statistical mechanics of fluids absorbed in structured pores and of fluids adsorbed at unstructured walls and at edges/wedges where walls meet. In particular, the work done to reversibly shear a corrugated pore immersed in liquid is related to the difference in the density profile structure of liquid adsorbed near the edges of the channels. When the channel dimensions are mesoscopic or macroscopic but the minimum pore width is microscopic, statistical mechanics generates remarkable links between the surface tension of planar wall-fluid interfaces or, more generally, the solvation free energy of a planar pore, and the density profile at the sides of a channel wall in the vicinity of edges and wedges.     

Numerical investigation of flow over two sweepback wedges at M = 4 and 6

Results of numerical simulation are discussed: simulation was carried out for a configuration of two wedges with sweepback leading edges placed on a pre-compression ramp in a way that skewed surfaces of the wedges deflect the compressed flows in the opposite directions. It was demonstrated that this configuration produces a flow with irregular interaction in the plane of symmetry for shock waves produced by sweepback wedges. The shock waves formed by the skew wedges induce 3D boundary layer separations along sweepback leading edges of the wedges. Flows in the separation zones are directed toward the plane of symmetry of this configuration; they interact and produce in the central part a “swollen” zone of separation flow with a typical S-shaped profile of velocity. Simulation data was obtained for the free stream flow with Mach number M = 4 and 6 and based upon Navier—Stokes equations and k-ωSST turbulence model using FLUENT computation code. Inviscid flow described by Euler equations was considered as well.     

On the influence of disjoining pressure on nonlinear oscillations of a water layer on the surface of a charged melting hailstone

The influence of a disjoining pressure on the nonlinear oscillations of a thin charged liquid layer on the surface of a spherical solid core is investigated by means of second-order asymptotic calculations. With the initial deformation governed by a kth mode in the spectrum of modes excited via nonlinear interaction, the disjoining pressure causes the frequencies of modes with numbers smaller than k to decrease and the frequencies of modes with numbers larger than k to increase. In the presence of the disjoining pressure, the amplitudes of all nonlinearly excited modes grow compared with the respective amplitudes without the pressure.     

Sound diffraction by multiple wedges and thin screens

A theoretical formula that is based on the geometrical theory of diffraction (GTD) is proposed for computing sound diffraction by multiple wedges, barriers, and polygonal-like shapes. The formula can treat both convex and concave edges, where edges may or may not be inter-connected. Comparisons of theoretical predictions with other results done by the BEM or experiments for scaled model confirm the accuracy of the present formula. Numerical examples such as double wedges and doubly inclined barrier show that when there exist several diffraction paths for given source and receiver positions, the insertion loss is dominated by the diffraction associated with the shortest propagation path. It is also found that although the partially inclined barrier increases the shadow zone as compared to the simple screen type of the same total height, it does not necessarily increase the insertion loss at all heights.     

The applicability of thermodynamic perturbation theory to the evaluation of the specific excess free energy and disjoining pressure in a wetting layer of nonpolar liquid with convex interface

The specific excess free energy and disjoining pressure in a wetting layer of nonpolar liquid on the surface of a solid spherical particle have been calculated on the basis of thermodynamic perturbation theory. The calculation results are compared with the data for a flat interface, obtained both experimentally and within the quasi-thermodynamic approach. The stability of the liquid layer on the convex surface is analyzed using the concept of reduced disjoining pressure.     

表面弹性和分离压耦合作用下的垂直液膜排液过程

针对含不溶性活性剂的垂直液膜排液过程,在考虑表面弹性和分离压耦合作用的基础上,采用润滑理论建立了液膜厚度、表面速度和活性剂浓度的演化方程组,通过数值计算分析了表面弹性和分离压单独作用和耦合作用下的液膜演化特征.结果表明:表面弹性与分离压均对垂直液膜排液过程有显著影响.表面弹性单独作用时,液膜初始厚度随弹性增大,黑膜仅在液膜顶部形成,长度较短且不能稳定存在;分离压单独作用时,活性剂随流体不断汇集在底端,液膜表面无法形成表面张力梯度,不发生逆流现象;当二者耦合作用时,可得到较稳定的液膜,排液前期增加表面弹性可提高液膜的厚度、降低表面速度和促使液体逆流,从而减缓排液过程;后期出现黑膜后,分离压中的静电斥力起主要作用,延缓液膜"老化".     

Effect of nanofluids on thin film evaporation in microchannels

A thin film evaporation model based on the augmented Young–Laplace equation and kinetic theories was developed to describe the nanofluid effects on the extended evaporating meniscus in a microchannel. The nanofluid effects include the structural disjoining pressure, a thin porous coating layer at the surface formed by the nanoparticle deposition and the thermophysical property variations compared with the base fluid. The results show that the nanofluid thermal conductivity enhancement mainly due to the Brownian motion tends to greatly increase the liquid film thickness and the thin film heat transfer. The structural disjoining pressure effect tends to enhance the nanofluid spreading capability and the thin film evaporation. The nanoparticle-deposited porous coating layer improves the surface wettability while significantly reducing the thin film evaporation with increasing layer thickness due to the thermal resistance across this layer. The nanofluid thermal conductivity enhancement together with the structural disjoining pressure effect can not counteract the thermal resistance effects of the porous coating layer when the coating layer thickness is sufficiently large.     

On the role of structural disjoining pressure to boiling heat transfer of thermal nanofluids

Research on nanofluids has progressed rapidly since their enhanced thermal conductivities were identified about a decade ago. For boiling heat transfer with nanofluids, however, many contradictory results have been reported, which cannot be explained by conventional theories developed for pure fluids. Recent progress in colloidal science shows that the presence of nanoparticles could enhance the spreading and wettability of base fluids through a long-range structural disjoining pressure. This article explores theoretically the influence of structural disjoining pressure to the nucleate boiling heat transfer through a four-zoned microlayer evaporation model. The influence of particle size, particle concentration, and heat flux on the structural disjoining pressure and the interfacial shape of the microlayer are investigated. The calculated equilibrium interfacial shape shows that the meniscus is displaced toward the vapor phase in the presence of nanoparticles, an implication of enhanced wettability. Such an improved wettability affects the number of active nucleate sites and bubble dynamics significantly, which could be one of the important parameters that is responsible for the controversy of boiling heat transfer with nanofluids reported in the literature.     

Localized vibration modes in free anisotropic wedges

Propagation of flexural localized vibration modes along edges of anisotropic wedges is considered in the framework of the geometrical-acoustics approach. Its application allows for straightforward evaluation of the wedge-mode velocities in the general case of arbitrary elastic anisotropy. The velocities depend on the wedge apex angle and on the mode number in the same way as in the isotropic case, but there appears to be additional dependence on elastic coefficients. The velocities in tetragonal wedges (with the midplane orthogonal to the four-fold axis) and in "weakly" monoclinic wedges are explicitly calculated and analyzed. Bounds of the wedge-wave velocity variation in tetragonal materials are established.     

Field-desorption microscopy study of the deformation of a tungsten tip subjected to thermal treatment in an electric field

Deformation of a tungsten tip 500 to 1000 nm in radius subjected to heating in an electric field is studied using field-emission microscopy and continuous-mode field-desorption microscopy. Measurements are performed immediately after the thermal field treatment without any smoothing of the tip by either heating or field evaporation. The edges of the tip (which is shaped into a polyhedron) are found to consist of monatomic steps about 1 nm wide and about 100 nm long. Microscopic protrusions about 10 nm in size are shaped like pyramids or wedges with single-atom apexes or monatomic edges and facets that are a continuation of the facets of the reconstructed tip or an outgrowth on which the protrusions are situated. The outgrowths are shaped like stepped truncated pyramids with monatomic edges. The observed phenomena are explained in terms of competing processes of surface diffusion, crystal growth in an electric field, and field evaporation.     

Influence of the ambient medium on the disjoining pressure of liquid metallic films

We have found the electron density of states in a thin liquid metallic film for an arbitrary form of the boundary conditions for the wavefunctions of the electrons on the film surface. Assuming a Fermi distribution of the electron energies, we have calculated the thermodynamic characteristics of the film and determined the electronic component of the disjoining pressure. It has been shown that the ambient medium and the state of the surface determine the value and the sign of the electronic component of the disjoining pressure. We have determined the conditions for which a stable state of liquid metallic films is ensured.     

Disjoining pressure in free-standing smectic-A films and its influence on their reflectivity

This paper reports the results of calculating the disjoining pressure and average thickness of smectic layers in free-standing liquid-crystal films heated above the temperature of breakdown of the smectic order in the bulk of the mesogens. The effect of the disjoining pressure on the reflectivity of free-standing smectic-A films with different numbers of smectic layers has been studied. The results of the calculations agree with the experimental study of the reflectivity of free-standing smectic-A films in the optical wavelength range.     

分离压作用下波纹状基底上含活性剂液滴铺展稳定性

针对波纹状基底上含不溶性活性剂液滴的铺展过程,引入受活性剂浓度影响的分离压模型,应用非模态稳定性理论分析液滴铺展稳定性.研究表明:与不计及分离压情形相比,分离压作用下的液滴铺展前沿高度明显下降,液滴铺展速率加快;长波扰动有利于增强液滴的演化稳定性,且随扰动波数增大稳定性增强;然而随短波波数增加,液滴演化稳定性逐渐减弱甚至转变为不稳定;较小波数下(k=3)减小引力强度系数α₁和提高斥力强度系数α₂液滴铺展稳定性增强;而较大波数下(k=30)增大α₁和减小α₂有利于液滴铺展稳定性.     

基于微肋管的微沟槽表面薄液膜沸腾理论模型

本文提出了基于微肋管的薄液膜蒸发沸腾的输运现象数学模型及其有限差分求解方法。数值模拟结果表明,在蒸发扩张半月形液膜中,非蒸发液膜区域液膜形状主要取决于分子膨胀压力;在薄液膜区域与非蒸发区域的连接处存在着一个强烈的蒸发点,这是膨胀压力和表面张力共同作用的结果;在本征半月形液膜区域压力梯度几乎完全取决于表面张力,因而在该区域内液膜形状可以假定为圆弧形状。     

The Young principle and integration-masked lumped sources of a diffracted field

A methodologically simple modification of the Young principle is proposed for describing diffracted field formation in problems of wave diffraction by the sharp edges of screens and wedges without using Sommerfeld’s two-sheeted space. The method of determining the diffracted field constructs the derivative of the field by introducing lumped sources positioned at given scattering edges with subsequent integration of the constructed field along the directions parallel to the wave fronts of the incident plane wave.     

分离压和表面黏度的协同作用对液膜排液过程的影响

针对含不溶性活性剂的垂直液膜排液过程,在考虑分离压作用的前提下,引入随活性剂浓度变化的表面黏度模型,应用润滑理论建立了液膜厚度、活性剂浓度和液膜表面速度的演化方程组,通过数值计算分析了常表面黏度和变表面黏度情形下的液膜演化特征.结果表明:表面黏度是影响液膜排液过程的重要因素,当不考虑表面黏度时,液膜表面呈"流动"模式,反之呈"刚性"模式,且随表面黏度增加,液膜排液速率明显减缓.分离压对"黑膜"的形成至关重要,分离压单独作用时,其形成的"黑膜"长度较短,而只考虑表面黏度时,则不能形成稳定的"黑膜".而在二者协同作用下,液膜中部形成了向下扩展、厚度很薄但非常稳定的"黑膜",且"黑膜"厚度、出现时间均随表面黏度的增大而增加.当考虑活性剂浓度对表面黏度的影响时,表面速度受此影响显著;在形成"黑膜"长度及出现时间方面与相应常表面黏度的情形基本类似,但其"黑膜"厚度小于相应常表面黏度,故在液膜排液过程中更容易发生失稳.     

Confined Coulomb Systems with Adsorbing Boundaries: The Two-Dimensional Two-Component Plasma

Using a solvable model, the two-dimensional two-component plasma, we study a Coulomb gas confined in a disk and in an annulus with boundaries that can adsorb some of the negative particles of the system. We obtain explicit analytic expressions for the grand potential, the pressure and the density profiles of the system. By studying the behavior of the disjoining pressure we find that without the adsorbing boundaries the system is naturally unstable, while with attractive boundaries the system is stable because of a positive contribution from the surface tension to the disjoining pressure. The results for the density profiles show the formation of a positive layer near the boundary that screens the adsorbed negative particles, a typical behavior in charged systems. We also compute the adsorbed charge on the boundary and show that it satisfies a certain number of relations, in particular an electro-neutrality sum rule.     

Capillary instabilities by fluctuation induced forces

The spontaneous break-up of thin films is commonly attributed to the destabilizing effect of van der Waals forces. Dispersion forces can be considered in terms of the confinement of the electromagnetic fluctuation spectrum. The principle of confinement is more general than the usual argument of interacting dipole fluctuations. It includes also disjoining pressures that are caused by thermal fluctuations. In this context, we review recent publications on the dewetting of thin polymer films, and argue that the presence of an acoustic disjoining pressure is necessary to adequately describe some of these experimental results.Received: 1 August 2003PACS: 47.20.Ma Interfacial instability - 68.15. + e Liquid thin films - 43.25.Qp Radiation pressure