Non-linear ring waves in a thin layer of magnetic fluid with variable thickness

The article presents the results of an experimental study of the heat transfer process in magnetic fluid under the influence of electric and magnetic fields of various configurations. It is shown that a constant electric field leads to an increase in the heat transfer coefficient when a certain threshold value of tension in a thin layer of magnetic fluid is reached. The possibilities of controlling the heat transfer process using the additional effects of homogeneous and inhomogeneous magnetic fields are found.

We consider the flow of two immiscible fluids lying between concentric cylinders when the outer cylinder is fixed and the inner one rotates. The interface is assumed to be concentric with the cylinders, and gravitational effects are neglected. We present a numerical study of the effect of different viscosities, different densities and surface tension on the linear stability of the Couette flow. Our results indicate that, with surface tension, a thin layer of the less-viscous fluid next to either cylinder is linearly stable and that it is possible to have stability with the less dense fluid lying outside. The stable configuration with the less-viscous fluid next to the inner cylinder is more stable than the one with the less-viscous fluid next to the outer cylinder. The onset of Taylor instability for one-fluid flow may be delayed by the addition of a thin layer of less-viscous fluid on the inner wall and promoted by a thin layer of more-viscous fluid on the inner wall.

Experimental investigation of surface instability of a thin layer of a magnetic fluid

Sonochemistry and sonoelectrochemistry are typically undertaken in a bulk fluid with sonication of sufficient intensity to generate visible cavitation. Sonoelectrochemistry was undertaken in a thin layer of fluid with a quartz crystal oscillator. In the thin layer configuration, no cavitation is visible and mass transport is not impacted, but impacts on electron transfer kinetics are observed. Because there is no visible cavitation, cyclic voltammetric responses typical of a quiescent electrolyte are observed. From the voltammograms for species that undergo rapid heterogeneous electron transfer such as outer sphere Ru(bpy)₃²⁺⇌Ru(bpy)₃³⁺+e, voltammetric morphology is unchanged on sonication of a thin layer of fluid. For Fe³+e⇌ Fe² that undergoes quasireversible (slower) electron transfer under cyclic voltammetric perturbation, sonication in a thin layer is found to increase the rate of heterogeneous electron transfer by roughly an order of magnitude. Sonoelectrochemical catalysis for inherently slow heterogeneous electron transfer processes is generated and observed in a thin layer of electrolyte. In the experiments reported here, the fluid layer was less than the wavelength of sonication. The aqueous electrolyte contains redox species and acid. Experiments were undertaken at a platinum wire electrode using a 41 kHz quartz crystal oscillator. For the same experimental conditions, no impact on the electron transfer reversible Ru(bpy)₃²⁺ was observed. All data were collected at a fixed frequency; impacts varied with oscillator intensity. As with many sonoelectrochemical experiments, subtle variation in experimental parameters complicate generation of reproducible results. Here, data generated by two different experimentalists yield similar outcomes. Best operating conditions differed slightly for the two experimentalists but the pattern of increased interfacial electron transfer rates for iron on sonication was found by both. Reference [1] Johna Leddy, Chester G. Duda, Jacob Lyon, and William J. Leddy III, "Thin Layer Sonoelectrochemistry and Sonoelectrochemistry Devices and Methods," published 28 May 2015 as US Patent Application 20150147594 A1.

Isolation cooling of slab geometry solid state lasers potentially solves a number of problems with direct cooling. We have tested a static gas cooled Nd:Glass slab laser to determine the advantages and limitations of this type of cooling system. This paper reports on these tests and discusses approaches to improve the average power of such a laser. The zig-zag slab geometry laser is an established method of reducing thermo-optical effects in solid state lasers. Eggleston et.al.,1 built and tested a small test-bed Nd:Glass slab laser that confirmed computer model performance predictions but demonstrated the need to carefully control the heat distribution within the slab. Eggleston et.al.'s work also indicated problems with direct fluid cooling the slab. In addition to coolant sealing difficulties a degradation of the total internal reflection faces due to contamination and etching was apparent. Since the zig-zag geometry involves many reflections off this surface any increase in loss is extremely undesirable. This under-standing led to the design and construction of a second generation of slab lasers that avoided the difficulties of direct fluid cooling by using cooling across a thin static aas gap.2 Joseph Chernoch was awarded the patent for conduction cooled slab lasers in July 19723 but to our knowledge a slab geometry laser using this cooling approach was never constructed. The laser built to examine the merits of static gas conduction cooling is shown in cross-section in Fig. 1. A (150 x 45 x 6.3) mm slab of Hoya LHG 5 phosphate glass with 8% Nd doping was used for these tests. Pumping was provided by two 15 cm long xenon flashlamps surrounded by second surface silver reflectors. The inner cooling pane was a 1 mm thick pyrex window supported and sealed by flexible elastomer such that it was free to be pressed against teflon spacers over the slab by the water flow. This allowed a variable gas gap thickness and easy mounting for the glass slab. This sealed static environment avoids slab surface contamination and provides a 'soft' thermal boundary against sudden changes in coolant temperature. One disadvantage of the gas conduction cooling design is the reflection loss of pump radiation at the solid to gas interfaces. We measured a slope efficiency increase of 33% when the gas gap was filled with water. Antireflection coatings on the window and the slab would improve flashlamp coupling but cannot prevent the rejection of radiation outside the critical angle for this solid to gas boundary. It can be noted that a permanent conduction layer of static fluid would be acceptable if a seal around the slab face could be made and convection of the thin static fluid layer in the flat plane of the slab could be effectively baffled.

Waves in thin fluid layers are important in various stellar and planetary problems. Due to rapid rotation such systems will become oblate, with a latitudinal variation in the gravitational acceleration across the surface of the object. In the case of accreting neutron stars, rapid rotation could lead to a polar radius smaller than the equatorial radius by a factor ∼0.8. We investigate how the oblateness and a changing gravitational acceleration affect different hydrodynamic modes that exist in such fluid layers through analytic approximations and numerical calculations. The wave vectors of g modes and Yanai modes increase for more oblate systems compared to spherical counterparts, although the impact of variations in the changing gravitational acceleration is effectively negligible. We find that for increased oblateness, Kelvin modes show less equatorial confinement and little change in their wave vector. For r modes, we find that for more oblate systems the wave vector decreases. The exact manner of these changes for the r modes depends on the model for the gravitational acceleration across the surface.

Upon the impact of a droplet onto a thin fluid layer, the fluid is pushed away around the impact point until it reaches a certain radius to go upward to form a thin liquid wall. At the tip of the liquid wall, the circumferential instability creates a several droplets, which is commonly known as The size and the height of the crown and the number of the tip droplets are affected by the conditions of a droplet and of a fluid layer. Presently, the fundamental characteristics of the milk crown have been extensively investigated based on the flow visualization by the high-speed camera. Moreover, the motions of the fluid layer and the liquid film have been measured with the aid of PIV. The result reveals several interesting features of the formation of a milk crown.

The transmission of explosive sound through a thin fluid layer above an infinitely deep elastic bottom is described by means of an asymptotic theory. Special attention is paid to early oscillatory arrivals at large horizontal distances for the case of a high-velocity bottom, and it is found that these arrivals do not decay exponentially with horizontal distance. When the shear wave velocity in the bottom is low, the over-all response is very similar to the case of a fluid bottom, except for the Rayleigh mode, which becomes an important feature of the late signal. The response at long times may involve standing waves of finite horizontal wavelengths.

Two-dimensional (2d) and quasi-2d flows occur at macro- and mesoscale in a variety of physical systems. Examples include stratified layers in Earth's atmosphere and the ocean, soap films and more recently also in dense bacterial suspensions, where the collective motion of microswimmers induces patterns of mesoscale vortices. A characteristic feature of turbulence in 2d and thin fluid layers is the occurrence of an inverse energy cascade. In case of weak Rayleigh damping the inverse energy cascade results in the formation of large-scale coherent structures, so-called condensates, which can take the form of jets or large-scale vortices. With a view towards atmospheric physics, we study the formation of the condensate in a rotating thin fluid layer with free-slip boundary conditions as function of the amplitude of the forcing, and we compare with results obtained in strictly 2d domains. Direct numerical simulations show that the condensate in the thin rotating layer appears in a first-order non-equilibrium phase transition, with rare transitions occurring towards and away from the condensate state. In contrast, condensate formation 2d can proceed by means of either a first or a second-order non-equilibrium phase transition, depending on the type of driving.

Understanding the properties of hydraulic fractures, for example their growth mechanism, is a main task in their investigation. We model a hydraulic fracture by a thin layer of an ideal fluid which is embedded between an elastic and isotropic solid with identical properties on each side of the crack. The main goal of this work is to find reflection coefficients for a wave reflected at this thin fluid layer. We present full analytical solutions for the reflections of an incident P-wave, the P-P and P-S reflection coefficients, as well as for an incident S-wave, the S-S and S-P reflection coefficients. We find that for parameters in the range of microseismic needs, which means a layer thickness of h=0.001-0.01m and frequencies of f=50-400Hz, the reflection coefficients change with Poisson's ratio. Furthermore the reflections of an incident S-wave are significantly high. This allows us to image hydraulic fractures using microseismic events as a source. The results presented in this paper will help to understand the properties of hydraulic fractures and can be used to characterize reservoirs or improve the imaging of hydraulic fractures.

We model a hydraulic fracture by a thin layer of an ideal fluid which is embedded between an elastic and isotropic solid with identical properties on each side of the crack. The main goal is to describe and analyze reflection coefficients for a wave reflected at this thin fluid layer. We present full analytical solutions for the reflections of an incident P-wave, the P-P and P-S reflection coefficients, as well as for an incident S-wave, the S-S and S-P reflection coefficients. We find that for parameters in the range of microseismic needs, which means a layer thickness of h = 0.001 − 0.01m and frequencies of f = 50 − 400Hz, reflection coefficients of an incident S-wave are remarkably high. This allows us to image hydraulic fractures using microseismic events as a source. It also suggests to solve the inverse problem, which means to interpret reflection coefficients extracted from microseismic data in terms of reservoir properties. We compare the analytical solutions for the P-P reflections with synthetic data that is derived using Finite Difference modeling and find that the modeling confirms our theoretical approach.

A theoretical description of the low-Reynolds-number collision and rebound of two rigid or elastic spheres separated by a thin layer of viscous fluid with pressure-dependent physical properties is presented. It has previously been shown by Davis et al. (1986) that the hydrodynamic pressure which builds up in the thin fluid layer must become large enough to elastically deform the spheres near the axis of symmetry, if they are to rebound subsequent to colliding. Under these extreme pressures, however, it is expected that the fluid may also compress and that its viscosity may increase by several orders of magnitude. It is shown that these pressure-dependent effects may significantly alter the minimum separation reached during approach of the spheres, as well as the maximum separation and relative velocity attained during rebound of the spheres. In particular, the pressure buildup during the collision process is predicted to become sufficiently large under some conditions so that the corresponding viscosity increase causes the fluid in the gap between the colliding spheres to behave nearly as a solid and to limit the close approach of the opposing surfaces. Also, the storage of energy via the compression of the fluid in the gap allows rigid spheres to bounce as this energy is released subsequent to their collision. However, it is found that this rebound is very weak relative to that which is predicted for elastic spheres.

An energy model for the transient flow boiling crisis under highly subcooled conditions at atmospheric pressure

We present a microscopic model of the interface between liquid water and a hydrophilic, solid surface, as obtained from ab initio molecular dynamics simulations. In particular, we focused on the (100) surface of cubic SiC, a leading semiconductor candidate for biocompatible devices. Our results show that in the liquid in contact with the clean substrate, molecular dissociation occurs in a manner unexpectedly similar to that observed in the gas phase. After full hydroxylation takes place, the formation of a thin (approximately 3 A) interfacial layer is observed, which has higher density than bulk water and forms stable hydrogen bonds with the substrate. The presence of this thin layer points at rather weak effects on the structural properties of water induced by a one-dimensional confinement between approximately 1.3 nm hydrophilic substrates. In addition, our results show that the liquid does not uniformly wet the surface, but molecules preferably bind along directions parallel to the Si dimer rows.

To identify the strength properties of a block structure including a bearing pair cage and the lubricated bearing itself, a model of a bearing with a defect is considered. The cage is a deformable base with a thin coating having defects. The coating is under the influence of a lubricant, which under operating conditions loses viscosity and can be modeled by a thin layer of an ideal incompressible fluid, on which the vertical external pressure from the bearing is applied. To study the local properties of the bearing in the vicinity of the defect, the base of the cage is considered as deformable layer with a defective coating containing on top a layer of ideal incompressible fluid. It is assumed that the coating has the most complex latent defect described by a crack, the plane of which is perpendicular to the boundary of the coating. The described block structure is investigated by the block element method, and the features of bearing behavior are revealed.