Detonation waves in bubble-drop media

Wave processes in chemically active multicomponent media: liquid — gas bubbles — liquid drops have been studied experimentally. Existence of detonation waves in multicomponent (bubble-drop) media has been proved. Structure of detonation waves in bubble-drop and bubble media is qualitatively identical: detonation waves are solitary waves with pulsation profile the pressure behind which is close in value to the one in unperturbed medium. Propagation velocity of detonation waves in bubble and bubble-drop media drops with the increase in medium gas phase concentration and with the decrease in carrier liquid viscosity. Presence of liquid drops decreases detonation wave velocity compared with bubble medium that does not contain liquid drops. Detonation wave propagation in multicomponent media causes gas bubbles fragmentation as well as fragmentation of individual liquid drops. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 04-03-33106).     

Acoustic manifestation of bubbles frozen in an ice sheet

Hydrocarbon sources on the ocean floor produce buoyant bubble plumes, i.e., gas flares. In winter, bubbles reaching the surface freeze in an ice sheet. Such clouds of frozen bubbles are observable in Arctic seas and are usual elements of ice sheets of lakes, e.g., Lake Baikal. Based on the general solution of the problem of scattering by a sphere in an isotropic elastic medium, the frozen bubble scattering cross section is found. The theory of multiple scattering by frozen bubble plume is derived. The structure of low-frequency resonances corresponding to collective oscillations of a bubble cloud is described.     

Shock waves and formation of carbon dioxide hydrate at an increased pressure in the gas-liquid medium

The processes of breaking, solution, and formation of hydrates behind a shock wave of moderate amplitude were studied experimentally in water with carbon dioxide bubbles under different initial static pressures. It is shown that an increase in the static pressure in a gas-liquid medium leads to reduction of critical relative amplitude of the shock wave, corresponding to starting development of Kelvin — Helmholtz instability and bubble splitting into small gas inclusions behind the shock wave front. It is shown that the rates of carbon dioxide solution and hydrate formation behind the shock wave front are close by the value; their dependences on medium and wave parameters are determined. Calculations by the model of gas hydration behind the shock wave are presented. The work was financially supported by the Russian Foundation for Basic Research (grants Nos. 06-01-00142 and 06-08-00657).     

Kinetics of the formation of gas bubbles during polarization of copper and graphite electrodes in electrolytic aqueous solutions

The adsorption and kinetic processes of the formation of gas bubbles passivating the surface during polarization of copper and graphite electrodes in 1% aqueous solution of sulfuric acid have been investigated. Three stages of the process related to the recharging of the double electric layer—adsorption accumulation of the gas escaping from the surface, the critical nucleation of the gas bubbles, and their subsequent growth—have been revealed, distinguished, and quantitatively estimated. It has been shown that potential leveling at the steady-state value specified by the Tafel equation is unambiguously associated with achievement of the limiting surface area screened by the gas bubbles for each particular current density. The surface diffusion constants D _H = (1.5–4.4) × 10⁻⁴ and (0.1–3.8) × 10⁻⁵ cm²/s of hydrogen on copper and graphite, respectively, and D _O = (1.8–4.5) × 10⁻⁷ cm²/s of oxygen on graphite during the motion toward the drain (the gas bubbles) have been calculated.     

Dry microfoams: formation and flow in a confined channel

We present an experimental investigation of the agglomeration of microbubbles into a 2D microfoam and its flow in a rectangular microchannel. Using a flow-focusing method, we produce the foam in situ on a microfluidic chip for a large range of liquid fractions, down to a few percent in liquid. We can monitor the transition from separated bubbles to the desired microfoam, in which bubbles are closely packed and separated by thin films. We find that bubble formation frequency is limited by the liquid flow rate, whatever the gas pressure. The formation frequency creates a modulation of the foam flow, rapidly damped along the channel. The average foam flow rate depends non-linearly on the applied gas pressure, displaying a threshold pressure due to capillarity. Strong discontinuities in the flow rate appear when the number of bubbles in the channel width changes, reflecting the discrete nature of the foam topology. We also produce an ultra flat foam, reducing the channel height from 250 μm to 8 μm, resulting in a height to diameter ratio of 0.02; we notice a marked change in bubble shape during the flow.     

气泡在超声场中绕圈运动的高速摄影及其图像分析

借助高速摄影和图像分析技术对首次发现的附壁气泡的绕圈现象进行了实验研究,重点研究游移气泡的运动轨迹、附壁气泡的布阵过程、气泡的来源以及气泡的振动细节.研究发现游移绕圈气泡的运动轨迹呈现出不稳定、不规则、不光滑的特点.阵列气泡源于游移气泡,而游移气泡变成阵列气泡的方式主要是通过合并增大体积,从而减小所受的Bjerknes力,降低活性的方式实现的.游移气泡源于ALF(acoustic lichtenberg figure)空化云中大量空泡的合并,使以径向振动为主的空泡逐渐过渡到以表面波动为主的气泡.阵列气泡在Bjerknes力的作用下呈现出规则的表面波动,而体积更小受力更大的游移空泡的表面完全失稳,呈现极不规则的形貌,并对附近阵列气泡的表面波动产生影响.阵列气泡呈现出十分规则的排布,相邻阵列气泡之间的振动相位是相反的,表现为相互排斥.     

声场中空化气泡的耦合振动及形状不稳定性的研究

计算了两个具有非球形扰动的气泡所组成系统的能量,并基于Lagrange方程得到了有声相互作用的非球形气泡的动力学方程和形状稳定性方程,研究了声场中非球形气泡间相互作用力对非球形气泡的形状不稳定性和气泡形状模态振幅的影响.研究结果表明声场中具有非球形扰动的气泡之间的耦合方式有两种:形状耦合模式和径向耦合模式,气泡之间的耦合方式取决于气泡形状扰动模态.由形状耦合及径向耦合产生的气泡之间的相互作用力能够改变单个气泡的形状不稳定及形状模态振幅,具体影响因素取决于声场驱动条件、气泡形状模态、相邻气泡的初始半径.     

Garnet films for micron and submicron magnetic bubbles with low damping constants

Two types of films for small bubbles have been grown without making use of the rare earth ions which are the cause of increased damping of the Bloch wall motion in the usual small bubble film compositions. Firstly, Bi-containing films were grown on (111) orientated GGG for 1 μm bubbles and secondly, Mn³⁺-containing films were grown under compression on (100) orientated GGG substrates for 0.4 μm bubbles. The damping constants in these films have been obtained by measuring the FMR line width. The correlation between the damping of the FMR and of the Bloch wall motion is determined in films with bubble diameters of, 2 to 4 μm. The damping constants are a factor 10 to 100 smaller than in the existing device films for small bubbles.     

On the breakup of gas bubbles in a liquid under the action of a finite-amplitude pressure wave

A mechanism for breaking up gas bubbles in a liquid in a high-intensity pressure wave field is considered. Breakup criteria are obtained. An anomalous dependence of the breakup pressure on the initial bubble radius is found and explained. Zh. Tekh. Fiz. 69, 137–139 (January 1999)     

The wideband attenuation of short radio waves on mid-latitude paths during X-ray flares in January 2005

We present the results of the ionosphere oblique chirp sounding on the Cyprus—Nizhny Novgorod, Cyprus—Rostov-on-Don, and Moscow—Rostov-on-Don mid-latitude paths during X-ray flares in January 17, 19, and 20, 2005. It is found that during strong flares the blackout of short radio waves was observed over the entire frequency range of chirp sounding on the Cyprus—Nizhny Novgorod and Cyprus—Rostov-on-Don paths. Modeling of the electron-density profiles in the lower ionosphere based on absorption of short radio waves on the Moscow—Rostov-on-Don path at different stages of the decay of the X-ray radiation intensity is carried out. It is shown that at the instant corresponding to the maximum value of the flare radiation flux, the electron density in the lower ionosphere at altitudes 60–80 km increased by a factor of about 10 and 100 for flares with radiation flux densities 5·10⁻² and 3·10⁻¹ erg/(cm ²·s) in the wavelength range 0.5–4.0 Å which took place in January 19 and 20, respectively. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 1–8, January 2007.     

Computational study on cell structure evolution of random liquid and metal-melt foams

A method for geometrical and topological modeling the evolution of close-cell metallic foams based on the Voronoi tessellation in three-dimensional space is presented. Numerical computations were carried out to examine the evolution of the bubble size distribution and topological and geometric properties of aluminum foams in the liquid state, which were implemented by using McPherson’s new theory on coarsening of microstructures as well as the topological transition rules (T1 and T2 processes) in 3D foams, accounting for remarkable effects of both the gas diffusion and surface tension. Computational results show that the bubble size distributions of metallic foams are strongly coupled to the evolution of the cellular structure and dependent on the gas diffusivity and surface tension. The way of foam coarsening can be expressed as RR ₃₂=−mt ²+1 approximately, and gas diffusion between bubbles dominates the evolution of bubble sizes and foam structures. It is found that the average number of faces per bubble is 〈f〉=13.8, which is in good agreement with the values reported in the literature.     

Instability of a bubble in a liquid dielectric in an external electrostatic field

The sizes, charges, and number of daughter bubbles emitted during the development of instability with respect to the polarization charge in a uniform electrostatic field of a gas bubble in a liquid dielectric are found on the basis of the Onsager principle of minimum energy dissipation for nonequilibrium processes. Zh. Tekh. Fiz. 69, 10–13 (November 1999)     

导电流体中气泡径向振动行为的磁场控制

液态金属中气泡行为是磁流体力学的重要方面。为对磁场条件下导电流体中气泡动力学行为作全面理解,基于磁流体动力学方法建立了磁场条件下导电流体中气泡径向振动的无量纲化动力学方程,数值研究了磁场对导电流体中气泡径向非线性振动稳定性、泡内温度、泡内气压及液体空化阈值的影响。结果显示:磁场增强了气泡非线性振动的稳定性,随着磁场增强且当作用在泡上的电磁力与惯性力数量级可比时,气泡运动为稳定的周期性振动;同时,磁场引起泡内温度、泡内压力及液体空化阈值变化。研究表明,可用磁场调节和控制液态金属中气泡的运动使其满足工程应用需求。      

球状泡群内气泡的耦合振动

振动气泡形成辐射场影响其他气泡的运动, 故多气泡体系中气泡处于耦合振动状态. 本文在气泡群振动模型的基础上, 考虑气泡间耦合振动的影响, 得到了均匀球状泡群内振动气泡的动力学方程, 以此为基础分析了气泡的非线性声响应特征. 气泡间的耦合振动增加了系统对每个气泡的约束, 降低了气泡的自然共振频率, 增强了气泡的非线性声响应. 随着气泡数密度的增加, 振动气泡受到的抑制增强; 增加液体静压力同样可抑制泡群内气泡的振动, 且存在静压力敏感区(1–2 atm, 1 atm=1.01325×10⁵ Pa); 驱动声波对气泡振动影响很大, 随着声波频率的增加, 能够形成空化影响的气泡尺度范围变窄. 在同样的声条件、泡群尺寸以及气泡内外环境下, 初始半径小于5 μm 的气泡具有较强的声响应. 气泡耦合振动会削弱单个气泡的空化影响, 但可延长多气泡系统空化泡崩溃发生的时间间隔和增大作用范围, 整体空化效应增强.     

In-water gas combustion in linear and annular gas bubbles

A new pulsed-cyclic method of in-water gas combustion was developed with separate feed of fuel gas and oxygen with the focus on development of new technologies for heat generators and submerged propellers. The results of calorimetric and hydrodynamic measurements are presented. In-water combustion of acetylene, hydrogen, and propane was tested with the operation frequency of 2–2.5 Hz and with a linear injector. The combustion dynamics of combustion of stoichiometric mixture with propane (C₃H₈+5O₂) was studied for a bubble near a solid wall; the produced gas bubble continues expansion and oscillations (for the case of linear and annular bubbles). It was demonstrated that gas combustion in annular bubbles produces two same-magnitude pulses of force acting on the wall. The first pulse is produced due to expansion of combustion products, and the second pulse is produced due to axial cumulative processes after bubble collapse. This process shapes an annular vortex which facilitates high-speed convective processes between combustion products and liquid; and this convection produces small-size bubbles.     

II. Recirculation flow induced by a bubble stream rising in a viscous liquid

The recirculation flow induced by the rising motion of a bubble stream in a viscous fluid within an open-top rectangular enclosure is studied. The three-dimensional volume averaged conservation equations are solved by a control-volume method using a hybrid finite differencing scheme to describe the liquid phase hydrodynamics. The momentum exhange between the bubbles and the liquid phase is modeled with a source term equals to the volumetric buoyancy force acting on the gas in the bubble stream. The volumetric buoyancy force accounts for in line interactions between bubbles through the average gas volume fraction in the gas liquid column which depends on the size and the rising velocity of bubbles. The fluid flow within an open-top rectangular enclosure is further investigated by particle image velocimetry for a bubble stream rising in a water-glycerol solution. The measured fluid velocities in a vertical plane are compared with the predictions of the numerical model over a wide range of fluid viscosity (43 mPa s-800 mPa s) and gas flow rates. Finally, the recirculation flows resulting from the interaction of two neighbouring vertical bubble streams are studied. Received: 23 July 1997 / Revised: 19 December 1997 / Accepted: 11 May 1998     

气泡体积分数对沙质沉积物低频声学特性的影响

由于有机物质分解等原因,实际的海底沉积物中存在气泡,气泡的存在会显著影响沉积物低频段的声学特性,因此研究气泡对沉积物低频段声速的影响机理具有重要意义.考虑到外场环境的不可控性,在室内水池中搭建了大尺度含气非饱和沙质沉积物声学特性获取平台,在有界空间中应用多水听器反演方法首次获取了含气非饱和沙质沉积物300—3000 Hz频段内的声速数据(79—142 m/s),并同时利用双水听器法获取了同一频段的数据(112—121 m/s).在声波频率远低于沉积物中最大气泡的共振频率时,根据等效介质理论,将孔隙水和气泡等效为一种均匀流体,改进了水饱和等效密度流体近似模型.模型揭示了气泡对沉积物低频段声学特性的影响规律,理论上解释了沉积物中声速的降低.通过分析模型预报声速对模型参数的敏感性,根据测量得到的声速分布反演得到了沉积物不同区域的气泡体积分数,气泡体积分数从1.07%变化到2.81%.改进的模型为沉积物中气泡体积分数估计提供了一种新方法.     

无阀微泵腔内气泡对周期驱动压力的影响

气泡的存在使无阀微泵的工作性能和使用寿命大大降低, 甚至无法正常工作. 为了合理地预测无阀微泵腔内气泡对周期驱动压力的影响, 给出了用来描述收缩管/扩张管型无阀压电微泵的数学模型, 包括泵腔体积变化、连续性方程、流体有效体积弹性模量以及锥管阻力系数的计算. 同时, 分析了腔内不同气泡体积对无阀微泵周期驱动压力的影响, 并对两个气泡进入无阀微泵泵腔时压力脉动过程进行了仿真和试验研究. 通过仿真结果与试验数据的比较表明, 所提出的存在气泡时无阀微泵数学模型及仿真方法是合理的. 关键词： 无阀微泵 气泡 压力脉动     

次Bjerknes力作用下气泡的体积振动和散射声场

利用Lagrange方程得到了次Bjerknes力作用下气泡的体积振动方程,并探讨了次Bjerknes力作用下不同参数对气泡体积振动振幅和振动初相位的影响,研究了振动初相位差为π和0的气泡对在液体中形成的散射声场特征.结果表明:次Bjerknes作用力下,相邻气泡半径、气泡间距、多方指数均能影响气泡的体积振动振幅,气泡对的均衡半径、气泡间距和驱动频率则对气泡振动初相位产生明显影响;相距很近、相位相差为π的两个气泡的散射声压与气泡体积振动振幅、气泡间距、驱动频率和振动初相位有关,随声场距离成反比减小,与声场位置有关,其平均散射声功率是单个孤立气泡的1/6(kd_(12))~2半径相同、相距很近、相位相同的两个6气泡的散射声压与气泡振动初相位、体积振动振幅、气泡间距、驱动频率有关,随声场距离成反比减小,其平均散射声功率是单个孤立气泡的4倍.     

A new pressure formulation for gas-compressibility dampening in bubble dynamics models

We formulated a pressure equation for bubbles performing nonlinear radial oscillations under ultrasonic high pressure amplitudes. The proposed equation corrects the gas pressure at the gas–liquid interface on inertial bubbles. This pressure formulation, expressed in terms of gas-Mach number, accounts for dampening due to gas compressibility during the violent collapse of cavitation bubbles and during subsequent rebounds. We refer to this as inhomogeneous pressure, where the gas pressure at the gas–liquid interface can differ to the pressure at the centre of the bubble, in contrast to homogenous pressure formulations that consider that pressure inside the bubble is spatially uniform from the wall to the centre. The pressure correction was applied to two bubble dynamic models: the incompressible Rayleigh–Plesset equation and the compressible Keller and Miksis equation. This improved the predictions of the nonlinear radial motion of the bubble vs time obtained with both models. Those simulations were also compared with other bubble dynamics models that account for liquid and gas compressibility effects. It was found that our corrected models are in closer agreement with experimental data than alternative models. It was concluded that the Rayleigh–Plesset family of equations improve accuracy by using our proposed pressure correction.