Hydrodynamic instabilities and interface dynamics of two immiscible liquids driven by a rotating disk.

A volume of fluid (VOF) method is adopted to simulate the condensation of R134a in a horizontal single square minichannel with 1 mm side length. The effect of gravity, surface tension and gas-liquid interfacial shear stress are taken into account. The result denotes that condensation is first appeared at the corner of channel, and then the condensation is stretched at the effect of surface tension until the whole channel boundary covered. The effect of gravity on the distribution of the liquid film depends on the channel length. In short channel, the gravity shows no significant effect, the distribution shape of steam in the cross section of the channel is approximately circular. In long channel, due to the influence of gravity, the liquid converges at the bottom under the effect of gravity, and the thickness of the liquid film at the bottom is obviously higher than that of the upper part of the channel. The effect of surface tension on condensation is also analysed. The surface tension can enhance the condensation heat transfer significantly when the inlet mass flux is low. Whilst, at high mass flux, the enhancement of surface tension on heat transfer is unobvious and can be neglected.

Effects of surface tension on the dynamics of a single micro bubble near a rigid wall in an ultrasonic field

Reduction of drag torque in disengaged wet clutches is essential for transmission research because it is one of the potentials of efficiency improvement. Aeration of oil film between two closely rotating plates promotes the decrease of drag torque at high speed region. The effects of surface tension and static contact angles during aeration are non-negligible showed by test results. The traditional lubrication model does not adequately predict the experimental results with different surface tension and contact angles during aeration. Hence, in this present paper, contact angles between Aluminum and Teflon materials were firstly measured, and the drag torques under two different contact angles were examined experimentally. An improved lubrication model of drag torque based on Navier–Stokes equations at the gas-liquid interface was built. The lubrication boundary condition was modified to introduce the effects of surface tension and contact angle. The model shows that the effects at the beginning of aeration of oil film are significant. These effects almost occur at stationary plate due to low Reynolds number and Weber number. The model shows that an increase in the surface tension promotes aeration, but does not affect the peak drag torque. Increasing contact angle also promotes the aeration, and accelerates the decrease of drag torque. The larger contact angle is, the smaller the peak drag torque will be. A computational fluid dynamics (CFD) model based on volume of fluid (VOF) method was presented to validate the interface shape when aeration occurs. The model prediction has a good agreement with experimental observations for Aluminum plates and Teflon plates. The modified lubrication model of drag torque gives a convenient description of the effects of surface tension and contact angel, and lays down a frame to understand the beginning of aeration.

Dataset of flow experiment: Effects of density, viscosity and surface tension on flow regimes and pressure drop of two-phase flow in horizontal pipes

The discharge coefficient of small holes drilled through a rotating cylinder wall were studied experimentally, as a fundamental study of the centrifugal gas-liquid or liquid-liquid centactor.The results are as follows:(1) It is confirmed experimentally that the discharge pressure of liquid under the contrifugal force is represented by Eq.(1), and the discharge coefficient, by Eq. (2).(2) The discharge coefficient depends upon the diameter of hole, the thickness of the wall and Red, but as far as the discharge coefficient defined by Eq. (2) is concerned, no difference is recognized whether it is in the centrifugal field or in the gravitational field.(3) The effect of liquid viscosity on the discharge coefficient is covered by Red.(4) All the experimental results obtained of the effect of liquid surface tension agree with the results of the theoretical consideration introduced in the previous paper, as shown in Fig. 15. Taking into account the surface tension, the discharge coefficient is presented by Eq. (5): and the effect of surface tension is considered negligible when the Weber number is greater than 1, 000.

Two-dimensional numerical analyses were conducted and analyzed to simulate water splash produced by free falling object models starting from the resting position. The equilateral prism-shaped object models were allowed to fall onto the free surface of the water. The moving-particle semi-implicit (MPS) method was used to solve the unsteady Navier-Stokes equation for incompressible fluid flows with and without the surface tension effect. Froude numbers of 0.75, 1.0, and 1.25 were used with different model sizes for the entry velocity at the free surface. Splashes obtained by numerical computation closely agreed with the experimental results. The surface tension force, the Froude number, and the Weber number were found in these calculations to play major roles in determining various types of splash shapes. Model size was found to influence splash phenomena, even if the Froude number remained the same. The dependencies on these two nondimensional numbers and the fundamental law of similarity on water splash with and without the surface tension effect were thoroughly investigated in this research. Several two-dimensional numerical simulations are presented in this article to describe the hydrodynamic behaviors of water splash with and without the surface tension effect.

Effects of surface tension and inclined surface wettability on sliding bubble heat transfer

A boundary integral technique is developed for steady two-dimensional, irrotational, incompressible fluid flow with a free surface in a two-dimensional channel. The effects of surface tension and surface friction on the free surface profile are investigated. The numerical solutions indicate that the surface tension tends to increase the curvature of the free surface whilst the surface friction tends to smoothen out the free surface profile and it dampens the effect of the surface tension.

The first and clear indication that surface tension offers resistance to the initial lung aeration after birth came with von Neergaard’s study in 1929 [26]. He found that lungs that were collapsed would open up more readily if the effect of surface tension was completely nullified by using liquid rather than air as the expanding medium. Following this basic study, it took considerable time until Pattle in 1955 [19] could declare that he assumed there was an agent present in the lung that had the ability to depress surface tension to extremely low values. He extruded bubbles from the sectioned lung and found that when they were surrounded by saline, so that they could be studied under a microscope, they would quickly shrink 30% from their initial size, when their diameter was around 40 µ, but then the bubbles would persist for long periods of time (Fig. 1). Pattle reasoned that the high surface tension of water, 72 mN/m, ought to give an enormously high pressure in the tiny bubble, so that the gas inside the bubble should have been absorbed quickly by the surrounding saline solution. When this did not happen, but the bubble persisted, Pattle concluded that there must be a lining layer exerting surface pressure, almost completely counteracting the collapsing effect of water surface tension. The net surface tension according to Pattle must be very close to 0.

The effects of surface tension and initial input energy on cavitation properties based on a tunable-surface-tension large-density-ratio thermal lattice Boltzmann method pseudo-potential model are investigated. The validity and superiority of the proposed model in simulating the D2 law, Laplace law, and revised thermal two-dimensional Rayleigh–Plesset equation are demonstrated. Moreover, the lattice Boltzmann method was used to study the effects of varied surface tension on cavitation bubble properties for the first time, and the maximum surface tension-to-minimum surface tension ratio of 25 is utilized, which is highly improved compared with previous numerical simulations (<4) and makes our result more clear. The simulation results indicate that for an infinite liquid, the increase in the surface tension will improve the collapse intensity of cavitation bubbles, increasing the collapse pressure, velocity, and temperature and meanwhile reducing the bubble lifetime. For the cavitation bubbles collapsing near a neutral wall, with an increase in the surface tension, the collapse pressure, temperature, and cavitation bubble lifetime trends are the same as in the infinite liquid. However, the collapse velocity is affected by the neutral wall, and the micro-jet becomes wider and shorter. The maximum cavitation bubble radius in an infinite liquid is nearly linearly proportional to the input initial energy. An increase in the surface energy reduces the maximum radius of the cavitation bubbles, while increasing the pressure energy and thermal energy promotes the maximum radius of the cavitation bubbles. This series of simulations proves the feasibility of the proposed model to investigate the thermodynamic process of the cavitation bubbles with high density ratios, wide viscosity ratios, and various surface tensions.

Effects of dynamic surface tension on the droplet formation of surfactant solutions were studied. Test fluids used were aqueous solutions of CTAB at several surfactant mol concentrations (CD) and CTAB/NaSal aqueous solutions at CD=1.0 mM and at three mol concentrations of NaSal. A droplet formed when a surfactant solution was injected into air from a capillary tube was investigated and the relation between the droplet diameter and the injection velocity V was measured. The size of droplet was evaluated by an equivalent droplet diameter Dexp, which is the diameter of sphere whose volume is the same as that of a droplet injected. For the CTAB systems, Dexp increased with increasing V at relatively low velocities because the dynamic surface tension also increased. However, the diameter decreased with increasing the velocity at relatively high velocities. This phenomenon can be qualitatively predicted from an equation of the force balance at a capillary exit when the effect of surface tension is evaluated using the dynamic surface tension. For the CTAB/NaSal systems, Dexp increased with increasing V at relatively low velocities and reached a constant value. This phenomenon was also predicted qualitatively with the force balance equation.

We investigate the linear Kelvin-Helmholtz (K-H) instability of two streaming magnetized plasmas considering the effects of surface tension and suspended dust particles. The usual magnetohydrodynamic (MHD) equations are considered for the present configuration including the effects of suspended dust particles and surface tension. The general dispersion relation is obtained using the normal mode analysis by applying the appropriate boundary conditions. The dispersion relation is modified due to the presence of surface tension, magnetic field and suspended dust particles for three-dimensional perturbations. It is found that the effect of magnetic field appears in the dispersion relation due to consideration of three-dimensional perturbations of the present problem of K-H instability with suspended dust particles and surface tension. It is also found that the condition of K-H instability is affected by the presence of suspended dust particles, surface tension and magnetic field. Numerical calculations have been performed to show the effect of magnetic field, mass concentration and relaxation frequency of suspended dust particles on the growth rate of K-H instability. It is found that magnetic field, mass concentration and relaxation frequency of suspended dust particles have stabilizing influence on the growth rate of K-H instability.

Effect of surface tension and geometry on cavitation in soft solids

In a vertically oscillating circular cylindrical container, singular perturbation theory of two-time scale expansions was developed in inviscid fluids to investigate the motion of single free surface standing wave including the effect of surface tension. A nonlinear slowly varying amplitude equation, which incorporates cubic nonlinear term, external excitation and the influence of surface tension, was derived from potential flow equation. The results show that, when forced frequency is lower, the effect of surface tension on mode selection of surface wave is not important. However, when forced frequency is higher, the surface tension can not be neglected. This proved that the surface tension causes free surface returning to equilibrium location. In addition, due to considering the effect of surface tension, the theoretical result approaches to experimental results much more than that of no surface tension.

This brief report describes recent results obtained with the LDEC SPH module including the effects of surface tension. LDEC implements a quasi-incompressible approximation to the Navier-Stokes equations (Morris et al., 1997). The author previously developed an approach to surface tension with SPH that calculated the curvature directly by taking the divergence of surface normals obtained from the gradient of a color function (Morris, 2000). In contrast, the implementation demonstrated here is based upon that developed by Tartakovsky and Meakin (2005) who introduced an additional force between the particles which results in the effect of surface tension. A similar method was also employed by Becker and Teschner (2007) who replaced the cosine based functional form developed by Tartakovsky and Meakin (2005) with a form based upon the SPH kernel function itself. These formulations do not accommodate a specified surface tension value, rather the effect of surface tension is an emergent feature and thus the techniques must be calibrated. Figure 1 shows results of an initial simulation performed with the LDEC-SPH module in 2-D using the approach developed by Tartakovsky and Meakin (2005). A square volume of fluid is observed to transition to a circle due to the effects of surface tension.more » Due to being critically damped, this simulation proceeds directly to the circular stable state. Future work will include following the same validation steps that Tartakovsky and Meakin (2005) used. That is, performing numerical experiments to determine the relationship between the parameter of the surface tension formulation and an equivalent effective surface tension value. We will also evaluate the alternative functional form promoted by Becker and Teschner (2007) to see if it provides improved robustness as claimed by those authors.« less