Sebastián Ubal

Numerical prediction of the film thickening due to surfactants in the Landau–Levich problem

In this work numerical solutions of the dip coating problem in the presence of a soluble surfactant are shown. Predictions of film thickening as well as thickening factors are in very good agreement with published experimental data, showing that pure hydrodynamic modeling suffices to mimic the process. Our numerical solutions provide a wealth of information on the functioning of the dip coating system; they show the appearance of a second stagnation point located in the bulk phase near the dynamic meniscus and they give clues about how the flow patterns might change as the surfactant becomes less soluble.

A numerical analysis of the influence of the liquid depth on two-dimensional Faraday waves

In this work a numerical analysis of two-dimensional Faraday waves is presented. This study is based on direct numerical simulation of Navier–Stokes and continuity equations with appropriate boundary conditions. Stability maps on the (F-α) plane for viscous liquid layers with equilibrium depths between 5×10−5 m and 10−5 m are presented; comparisons are made with the linear stability predictions obtained with Benjamin and Ursell’s model for an inviscid fluid and with Kumar and Tuckerman’s model for a viscous fluid. Regions in which time-periodic solutions are no longer obtained and nonlinear effects are relevant, and are also delimited and analyzed: in these zones the disintegration of the free surface into drops may take place.

A deeper insight into the dip coating process in the presence of insoluble surfactants: A numerical analysis

A numerical investigation is carried out to study the effects of an insoluble surfactant on the dip coating of a flat substrate. Predictions of both the film thickness and the concentration of surfactant in the film as a function of the capillary number compare well with the solutions of a simpler asymptotic model based on the lubrication approximation. Streamline patterns confirm the existence of a stagnation point located in the bulk phase in the region of the dynamic meniscus—a conjecture postulated forty years ago. The evolution of the flow patterns and the interfacial variables shows how the classical result of Landau and Levich is recovered as the coating speed is augmented. Finally, we show that the effect of inertia forces cannot be neglected when the viscosity of the coating liquid is low.

Elastic effects of an insoluble surfactant on the onset of two-dimensional Faraday waves: a numerical experiment

The elastic effects of an insoluble surfactant on the formation and evolution of two-dimensional Faraday waves is investigated numerically. We analyse the influence of the elasticity of the surface-active agent on the amplitude of the vertical vibration needed to excite two-dimensional standing waves on the free surface. The numerical solutions show that the interface is always subharmonically excited at the onset and that the presence of the surfactant requires a higher external force to induce standing waves. They also show that the magnitude of the external amplitude is related to the temporal phase shift that exists between the evolution of the surfactant concentration and the free-surface shape. A detailed description of the time-varying velocity fields and interfacial distribution of surfactants helps to provide insight into the mechanisms ruling the phenomenon.

The influence of inertia and contact angle on the instability of partially wetting liquid strips: A numerical analysis study

The stability of a thread of fluid deposited on a flat solid substrate is studied numerically by means of the Finite Element Method in combination with an Arbitrary Lagrangian-Eulerian technique. A good agreement is observed when our results are compared with predictions of linear stability analysis obtained by other authors. Moreover, we also analysed the influence of inertia for different contact angles and found that inertia strongly affects the growth rate of the instability when contact angles are large. By contrast, the wave number of the fastest growing mode does not show important variations with inertia. The numerical technique allows us to follow the evolution of the free surface instability until comparatively late stages, where the filament begins to break into droplets. The rupture pattern observed for several cases shows that the number of principal droplets agrees reasonably well with an estimation based on the fastest growing modes.

Streamline-averaged mass transfer in a circulating drop

Abstract Solute mass transfer is considered from the outside to the inside of a circulating drop in the context of liquid-liquid extraction. Specifically an internal problem is treated with resistance to mass transfer dominated by the liquid inside the drop. The Peclet number of the circulation is large, on the order of tens of thousands. A model is proposed by which the mass transfer into the drop begins in a boundary layer regime, but subsequently switches into a so called streamline-averaged regime. Solutions are developed for each regime, and also for the switch between them. These solutions are far easier to obtain than those of the full advection-diffusion equations governing this high Peclet number system, which are very stiff. During the boundary layer regime, the rate at which solute mass within the drop grows with time depends on Peclet number, with increases in Peclet number implying faster growth. However larger Peclet numbers also imply that the switch to the streamline-averaged regime happens sooner in time, and with less solute mass having been transferred to date. In the streamline-averaged regime, solute concentration varies across streamlines but not along them. In spite of the very large Peclet number, the rate of mass transfer is controlled diffusively, specifically by the rate of diffusion from streamline-to-streamline: sensitivity to the Peclet number is thereby lost. The model predictions capture, at least qualitatively, findings reported in literature for the evolution of the solute concentration in the drop obtained via full numerical simulation.

Numerical Solution of a 2D Lubrication Model with Sommerfeld Boundary Conditions for Hip Prostheses

Prosthetic in-contact surfaces wear is one of the main causes of hip replacements fallure. Lubrication is essen- tial to reduce the friction and consequently the wear. A two- dimensional model based on a modified Reynolds equation, was used to aoalyze the lubricated contact In a hypothetical hip prosthesis. It is assumed that prosthesis is made of a me- tallic component and a low rigidity elastic porous material that can absorb or exude Duid. The Duid pressure and channel Dow shape were simultaneously solved by a double precision computer code based on the rmite element method, Newton’s method and parametric continuation. The Information provided by this model is extremely difficult to obtain by experimental methods. Results show that a great- er deformation capacity of the elastic material promotes a thicker lubrication mm and induces a better load distribution reducing normal stresses; whlle exudation capacity reduces friction and wear. The sub-ambient pressures predicted and the oscillatory solution, show the need of more realistic boundary conditions and fmite element mesh refinement.

Computer Simulation of the Blood Flow in a Planar Configuration for a Pulsatile Ventricular Assist Device

Patients that suffer congestive cardiac insufficiency need to be assisted with assist devices (VAD) as a temporal solution when the cardiac transplantation is not possible. For that reason it is of great interest to know the performance of such devices, especially the potential blood damage that they can produce. In this work, a computational twodimensional simulation of a novel pulsatile VAD is performed. This new design of VAD has a double acting piston driven electromagnetically at 2.1 Hz. In addition, the VAD has four active valves. The flow velocities, the pressure and the shear stress developed in the blood are analyzed in the chambers and inflow and outflow conduits. The results suggest that the developed flow would not be dangerous for the blood.

Spacers in the treatment of hip joint infections: numerical analysis of their durability

Hip spacers are temporary implants having a geometry similar to the femoral component of a hip prosthesis, and they are manufactured with antibiotic-impregnated bone cement. The use of spacers in two stage revisions is the most effective treatment to eradicate infections and to avoid limb shortening. The most frequent complication associated with spacers is fatigue failure, for which doctors recommend patients to stay at rest. In this work, several spacer designs are analyzed in order to determine the feasibility of doing activities like walking, standing up or sitting down while performing the antibiotics treatment. Designs combine both different neck diameters and the presence/absence of an internal, stainless steel reinforcement. By means of computational simulations based on the finite element method, stress fields are calculated for various hip spacer designs under several load and fixing conditions. For this purpose, a 3D model of human femur is generated by processing tomographic images with segmentation techniques and inverse engineering. The results allow us to estimate the life expectancy of each design, by considering the fatigue behavior of the bone cement. Only the introduction of a reinforcement with a proper diameter into the bone cement matrix could assure the integrity of the spacer along the treatment period.

Effects of gravity on the stability of the steady propagation of a liquid plug in a small conduit

In this work we numerically study the stability of the steady state displacement of a liquid plug in a capillary tube when gravity, inertia and surface forces are important. The methodology employed is based on the analysis of steady state solutions and has been presented in previous publications. Gravity is assumed to act only along the axis of the tube.