Original Free Surface Research Articles

Superficial Velocity in Heterogeneous Two-Phase Systems with Truncated Fractal Distribution of Particle Concentration

The interplay of particles in a heterogeneous multiparticle two-phase system and its effect on superficial velocity have not been well quantified. In this study, a new model is developed to examine the superficial velocity in a heterogeneous multiparticle two-phase system. To examine the heterogeneous effects to the potentially maximum extent, the particle concentration is assumed to follow a truncated fractal distribution, which is integrated into the free surface cell model. In a statistical sense, the multiparticle two-phase system is stationary, so the mean of spatial heterogeneity can be replaced by the ensemble mean. Since the underlying physical concept is rooted in the free surface cell model, the validity of the model should be, therefore, limited to the low-Reynolds number conditions. The developed model is compared to data from three representative experimental studies in the literature and it is found that the model can better capture the scatters in experimental data than the original free surface cell model. The model is also compared with three representative models and demonstrates reasonable results. While the deterministic free surface cell model underestimates the velocity, the cell model with truncated fractal distribution being incorporated can predict high velocity with a wide range of particle concentration heterogeneity.

Numerical Simulation of the Maneuvering Motion Wake of an Underwater Vehicle in Stratified Fluid

When a vehicle moves underwater, disturbance is generated and a wake remains that destroys the original free surface and produces a new wake. In order to study the mechanism and characteristics of the wave-making wake generated by the maneuvering motion of an underwater vehicle in density-stratified fluid, the k-ε model and the VOF method that is based on the RANS equation were used in this paper to analyze the SUBOFF model in stratified fluid at different drift angles. Numerical simulation of the maneuvering motion was carried out under these angles, and the corresponding changes in flow field caused by this motion were analyzed. The results from the comparison and analysis of the surface wave wakes under different drift angles in stratified fluid show that with the increasing drift angle, the motion wake of the vehicle still exhibits obvious Kelvin wave system characteristics. However, there are significant changes in hydrodynamic performance. The asymmetry of the surrounding flow field will increase with the increase in the drift angle. The pressure of the underwater vehicle is inversely proportional to the velocity of the surrounding flow field, and the amplitude of the peak and trough of the free surface wave is linearly related to the change in the drift angle. The numerical simulation can serve as a reference for the non-acoustic detection of the motion heading of an underwater vehicle and the motion trajectory of anti-reconnaissance underwater vehicles under actual sea conditions.

浮体を用いたLNG タンクのスロッシング防止に関する研究

Recent developments with offshore terminals and floating liquefied natural gas (FLNG) production, storage and offloading applications have included the new requirement that the storage tanks be able to handle unrestricted filling with LNG. If membrane-type tanks are used in such new systems, the occurrence of sloshing in the half-load condition becomes one of the most important design issues. In this paper, the authors propose a simple anti-sloshing floating device to be installed inside a liquid cargo tank. By setting the device on the free surface of the liquid cargo, the original free surface is divided into two or more sub-surfaces, and resonant sloshing of the liquid can be avoided by changing the natural frequency of its motion. This floating device can follow the liquid height change in the tank during loading and unloading operations of the liquid cargo, a feature that is especially preferable in FLNG applications. The authors examine the performance of the proposed anti-sloshing device by a series of model experiments and numerical simulations, which indicate that the device's performance is very promising.

Cast filling simulations of thin-walled cavities

Computer simulation models of casting processes which couple the velocity potential approach with a transient Bernoulli equation are developed. Instead of employing three primitive variables, displacement, velocity and pressure, only one variable-velocity potential is needed. When the thickness integrated equation is also employed, the method can provide practical results with very reasonable computing time and storage for the 3-D casting of an arbitrary shape thin-walled cavity. In this study, the flexibility of the finite element method in dealing with complex geometries and the efficient algorithm of volume of fluid (VOF) approach for tracking moving free surfaces are combined for solving casting filling problems. Multiple free surface contacts are developed when the stream finally turns back and impacts the original free surface. A double-node scheme is developed to treat these multiple free surface contacts. The movement of the interface is small and an inflexible wall across the separating surface is assumed. Because the casting process involves phase change, and the interface between the solid and liquid is generally an unknown curve, the enthalpy model with fixed mesh is used to determine the temperature distribution and the thickness of the filling. Numerical examples for prediction of filling patterns, effects of solidification on patterns and parametric studies are presented. Fairly good agreement between this method and experimental results and other numerical simulations have also been obtained. The computational techniques developed in this study can provide a powerful and flexible tool for analyzing the fluid flow and heat transfer in metal casting of thin-walled cavities and can help design engineers reduce the costly and time-consuming process of designing complex molds for the manufacture of casted parts.

Efficacy of dentin-bonding agents in relation to application technique

The gap-reducing efficacy of two dentin-bonding agents was examined in butt-joint dentin cavities prepared in extracted human teeth. The cavity wall and the surrounding dentin surface were treated with one of the dentin-bonding agents, and a light-activated microfilled restorative resin was either applied and polymerized or applied and then withdrawn from the cavity and once again applied before polymerization. Ten minutes after polymerization the width and the extent of the contraction gap were measured, using a light microscope, approximately 0.1 mm below the original free surface of the fillings. The marginal porosity was also registered and calculated in percentage of the total filling periphery. It was found that reapplication resulted in an increased marginal porosity and also that the width and extent of the contraction gap were increased when the restorative resin was reapplied.

Effect of postponed polishing on marginal adaptation of resin used with dentin-bonding agent.

Dentin cavities, prepared in extracted human teeth, were treated with two different dentin-bonding agents and filled with a light-activated microfilled resin. The maximum width of the contraction gap (MG) and the extent of the gap (GP) were then measured, using a light microscope, approximately 0.1 mm below the original free surface of the filling. The contraction gap was measured 30 s, 10 min or 60 min, and remeasured 65 min, after stop of irradiation. A positive correlation was found between the two variables, MG and GP. The product of MG and GP was chosen as basis for the statistical analyses. This "marginal index" was significantly reduced when polishing of the marginal area was postponed for 10 min with one of the dentin-bonding agents and for 60 min with the other. Even though the improvement in marginal adaptation was statistically significant, this improvement was considered clinically irrelevant. It is concluded that polishing of the marginal area should not be done before the hygroscopic expansion of the resin restoration has closed the contraction gap.

Influence of temporary filling materials on effect of dentin-bonding agents.

Dentin cavities, prepared in extracted human teeth, were filled with a dry mixture of zinc oxide-eugenol (ZOE) or a commercial temporary filling cement without eugenol (Cavit). The ZOE was removed 3 h later while the Cavit was removed after 7 days. The cavity wall and the surrounding dentin were then treated with a dentin-bonding agent before application of a restorative resin. The width and the extent of the marginal contraction gap were measured, using a light microscope, approximately 0.1 mm below the original free surface of the filling. It was found that the contraction gap was markedly increased in cavities previously filled with ZOE while the non-eugenol cement had no influence on the effect of the two dentin-bonding agents tested. The short-term test applied in the present study indicates that ZOE should not be used in cavities later to be restored with dentin-bonding agent and resin.

Intense Explosions at the Ocean Surface

An approximate solution to the problem of an intense point-source explosion at the interface separating two different fluids is described; in particular, it is applied to a nuclear explosion initiated on the surface of the ocean. Of special interest is the position of the deformed ocean surface as a function of time during the initial phase of the explosion. The equations of motion for an inviscid fluid with no heat conductivity are written in terms of the verticle angle and a similarity variable. The solution is valid for the early phase of the explosion, during which the counterpressure remains insignificant. It is found that the influence of the free surface on the vacuum interface propagated in the water is confined to a 45° sector adjacent to the original free surface. Below this, the vacuum surface retains very nearly its spherical form. This picture of the flow-field form is seen to be in good qualitative agreement with the type of flow fields calculated by Brode and Bjork for a megaton burst on the surface of the earth.

Smooth Spalls and the Polymorphism of Iron

When Armco iron is loaded with a transient wave in which the pressure exceeds 0.131 megabar, smooth spalls may be observed. The two distinct compression shocks which are transmitted by the iron are followed by a steep rarefaction which may be called a rarefaction shock. At a free surface, the first compression shock is reflected as a backward-facing rarefaction wave. This wave may also steepen into a rarefaction shock which eventually meets the forward-facing rarefaction shock. The interaction of these waves results in a localized tension of destructive magnitude, which produces a smooth fracture. Location of the plane of this interaction was calculated by applying the theory of hydrodynamics. These calculations are inexact because the speed of sound for relief waves was taken from Hugoniot data since other data are not available. Fair agreement was obtained between experimental and calculated results. The work indicates that, in the case of a double spall, the fracture most remote from the original free surface may sometimes be formed first.