Fluid Film Research Articles

Instability of Oldroyd-B Liquid Films with Odd Viscosity on Porous Inclined Substrates

In this paper, we investigate the effect of singular viscosity on the stability of a thin film of Oldroyd-B viscoelastic fluid flowing along a porous inclined surface under the influence of a normal electric field. First, we derive the governing equations and boundary conditions for the flow of the film and assume that the film satisfies the Beavers–Joseph sliding boundary condition when it flows on a porous inclined surface. Second, through the long-wave approximation, we derive the nonlinear interfacial evolution equation. Then, linear and nonlinear stability analyses are performed for the interfacial evolution equation. The stability analyses show that the singular viscosity has a stabilizing effect on the flow of the film, while the strain delay time of the Oldroyd-B fluid, the electric field, and the parameters of the porous medium all have an unsteady effect on the flow of the film. Interestingly, in the linear stability analysis, the parameters of the porous medium have an unsteady effect on the flow of the film after a certain value is reached and a stabilizing effect before that value is reached. In order to verify these results, we performed numerical simulations of the nonlinear evolution equations using the Fourier spectral method, and the conclusions obtained are in agreement with the results of the linear stability analysis, i.e., the amplitude of the free surface decreases progressively with time in the stable region, whereas it increases progressively with time in the unstable region

3D Jones-matrix thesiography of biological fluid facies

A new polarization–interference biomedical diagnostic three-dimensional (3D) Jones-matrix technology with digital Fourier reconstruction of layered maps of optical anisotropy (thesiograms) of dehydrated films (facies) of biological fluids of human organs is presented and experimentally tested. An original model of layered phase scanning of polycrystalline architectonics of supramolecular networks of biological fluid facies is proposed for the purpose of theoretical justification and prognostic use of the obtained results. On its basis, algorithms of Jones-matrix reconstruction of thesiograms of birefringence and dichroism of facies of synovial fluid, bile and blood are found. As a result, layered thesiograms of linear and circular birefringence and dichroism of facies with different spatial–angular architectonics of supramolecular networks are experimentally obtained for the first time. Within the framework of statistical analysis of experimental data, new objective markers (asymmetry and excess of optical anisotropy parameter distributions) for diagnostics of pathological changes in the optical anisotropy of biological fluid facies were defined and clinically tested. As a result, an excellent level of balanced accuracy of the developed polarization–interference Jones-matrix method of layer-by-layer reconstruction of thesiograms of polycrystalline supramolecular networks in differential diagnostics of bile facies (cholelithiasis), synovial fluid (reactive synovitis–septic arthritis) and whole blood (follicular adenoma–papillary thyroid cancer) was achieved.

Enhancing Lubrication Performance of Fluid Film Journal Bearings Using Combined Effects of Herringbone Groove and Porous Bush

Enhancing Lubrication Performance of Fluid Film Journal Bearings Using Combined Effects of Herringbone Groove and Porous Bush

Fluid-Structure Interaction Lubrication Analysis of Compliant Journal Bearing Lubricated with Non-Newtonian Plastic Fluid

Abstract High-viscosity plastic fluids and polymer liners are expected to improve the lubrication and load-carrying performance of low-speed and heavy-load journal bearings. The objective of this paper is to investigate the elastohydrodynamic lubrication performance of compliant journal bearings lubricated with non-Newtonian plastic fluids (following the Herschel-Bulkley model) using computational fluid dynamics (CFD) and fluid-structure interaction (FSI) methods. The following data are obtained: fluid film pressure, total bearing deformation, cavitation volume fraction, bearing capacity, and coefficient of friction. The simulation results are in close agreement with the data reported in the existing literature. A comparative analysis of the lubrication characteristics of journal bearings with Babbitt metal, PEEK, and PTFE liners at varying rotational speeds is presented. The effects of yield stress, power-law index, elastic modulus, and liner thickness on the lubrication characteristics are investigated. The results indicate that the power-law index of non-Newtonian plastic fluids has a greater influence on the bearing lubrication performance than the yield stress. Increasing the power-law index could improve the bearing capacity and reduce the coefficient of friction. The research provides a theoretical basis for the application of non-Newtonian plastic fluid-lubricated polymer journal bearings in low-speed and heavy-load equipment.

Stokes flow of an evolving fluid film with arbitrary shape and topology

The dynamics of evolving fluid films in the viscous Stokes limit is relevant to various applications, such as the modelling of lipid bilayers in cells. While the governing equations were formulated by Scriven (1960), solving for the flow of a deformable viscous surface with arbitrary shape and topology has remained a challenge. In this study, we present a straightforward discrete model based on variational principles to address this long-standing problem. We replace the classical equations, which are expressed with tensor calculus in local coordinates, with a simple coordinate-free, differential-geometric formulation. The formulation provides a fundamental understanding of the underlying mechanics and translates directly to discretization. We construct a discrete analogue of the system using Onsager's variational principle, which, in a smooth context, governs the flow of a viscous medium. In the discrete setting, instead of term-wise discretizing the coordinate-based Stokes equations, we construct a discrete Rayleighian for the system and derive the discrete Stokes equations via the variational principle. This approach results in a stable, structure-preserving variational integrator that solves the system on general manifolds.

Study on partial ER fluid hybrid two-lobe hole-entry journal bearing with genetic algorithm for bionic surface

This work focuses on ER (Electrorheological) fluid-based hybrid hole-entry journal bearings, which studies the effect of ER fluid with partial electric field on the tribological nature of two-lobe bearings with bionic textured surfaces. The design of bionic textures, by virtue of their capacity to influence fluid flow, load capacity, and friction reduction for the enhancement of bearing performance, has been analytically investigated. The integration of ER fluid subjected to partial electric field along with bionic textured surfaces displays an optimistic improvement in performance metrics of bearings, which clearly manifests through preliminary results. Notably, the optimization of bionic textures through genetic algorithms resulted in a 13.04% reduction in friction coefficient and a significant increase in load capacity. The combination of partial ER fluid lubrication and bionic textures also improved fluid film stiffness by up to 66.11%, further enhancing stability and overall bearing efficiency. The obtained results are expected to further enhance design and development activities of high-performance hybrid journal bearings for numerous industrial applications.

Neural network based prediction of lemon bore journal bearing performance with micropolar lubricants

The research paper aims to predict the static characteristics of lemon bore journal bearings lubricated with micropolar lubricant using an artificial neural network (ANN) and compare the results using traditional numerical methods. It commences by employing the Finite Difference Method (FDM) to solve the Reynolds equation for micropolar lubricants. Swift-Stieber boundary conditions were functional to evaluate pressure distribution, which enables the examination of the static characteristics by varying the eccentricity ratio (ε) = 0.25, 0.3, and 0.35, length of micropolar (lm) = 1 to 50, and coupling number (N2) = 0.1 to 0.9. Thereafter, the results from the FDM approach are used to train artificial neural network models. The study utilizes a dataset comprised of over 1560 data points with corresponding inputs and outputs. Three distinct feed-forward ANN architectures were considered: Levenberg-Marquardt optimization technique, Bayesian regularization technique, and Scaled Conjugate Gradient, and their performance was compared with results obtained through FDM. Results indicate a substantial reduction in computational effort and time when employing the trained ANN models compared to the FDM approach. Furthermore, it is found that the difference between the results given by the ANN models and the FDM results is not above 5%; the agreement between these two approaches thus appears good. This study reveals the effectiveness of ANNs in reducing computational time and effort while maintaining accuracy in evaluating the performance of lemon bore journal bearings lubricated with micropolar lubricant, and provides a new approach to predict the performance of fluid film bearings.

Instability of falling films of discontinuously shear-thickening fluid

Aqueous suspensions of cornstarch abruptly increase their viscosity on raising either shear rate or stress, and display the formation of large-amplitude waves when flowing down inclined channels. The two features have been recently connected using constitutive models designed to describe discontinuous shear thickening. By including time-dependent relaxation and spatial diffusion of the frictional contact density responsible for shear thickening, an analysis of steady sheet flow and its linear stability is presented. The inclusion of such effects is motivated by the need to avoid an ill-posed mathematical problem in thin-film theory and the resulting failure to select any preferred wavelength for unstable linear waves. Relaxation, in particular, eliminates an ultraviolet catastrophe in the spectrum of unstable waves and furnishes a preferred wavelength at which growth is maximized. The nonlinear dynamics of the unstable waves is briefly explored. It is found that the linear instability saturates once disturbances reach finite amplitude, creating steadily propagating nonlinear waves. These waves take the form of a series of steep, shear-thickened steps that translate relatively slowly in comparison with the mean flow.

Fluid-structure interaction analysis and density-based topology optimization of single pad externally adjustable fluid film bearing operating in high-speed application

Fluid-structure interaction analysis and density-based topology optimization of single pad externally adjustable fluid film bearing operating in high-speed application

Chiral π domain walls composed of twin half-integer surface disclinations in ferroelectric nematic liquid crystals

Ferroelectric nematic liquid crystals are polar fluids characterized by microscopic orientational ordering and macroscopic spontaneous polarizations. Within these fluids, domain walls that separate regions of different polarizations are ubiquitous. We demonstrate that the π walls in films of the polar fluids consist of twin half-integer surface disclinations spaced horizontally, enclosing a subdomain where the polarization exhibits left- or right-handed π twists across the film. The degenerate geometric arrangements of the twin disclinations generate kinks and antikinks, parting subdomains of opposite chirality, like the spin-up and spin-down in Ising chains. The hierarchical structures dictate that field-driven polar switching undergo two-step annihilations of the surface disclinations. These findings provide an insight for both comprehending other walls in the polar fluids and domain engineering crucial for advancing their nonlinear and optoelectronic applications.

Plateau–Rayleigh instability in a capillary: assessing the importance of inertia

Liquid plug formation in thin channels due to the Plateau–Rayleigh instability of a liquid film is observed in a variety of fields. In this paper, complementarity between theoretical solutions and direct numerical simulations (DNS) based on a front-tracking algorithm is explored to evaluate the importance of inertia for the case of a cylindrical capillary. A linear stability analysis is first performed and DNS results are then used to investigate the spatial distributions of inertial, convective and viscous terms of the Navier–Stokes equation. The existence of both viscous and inertial regimes is evidenced with a threshold given by the film thickness. The presence of the core fluid slows down the instability. In the viscous regime, predictions of the lubrication theory are verified. An example of liquid water as the outer fluid film and water vapour as the inner core fluid is simulated with application to the fuel cells.

On the Effect of Liquid Crystal Orientation in the Lipid Layer on Tear Film Thinning and Breakup

AbstractThe human tear film (TF) is thin multilayer fluid film that is critical for clear vision and ocular surface health. Its dynamics are strongly affected by a floating lipid layer and, in health, that layer slows evaporation and helps create a more uniform tear film over the ocular surface. The tear film lipid layer (LL) may have liquid crystalline characteristics and plays important roles in the health of the tear film. Previous models have treated the lipid layer as a Newtonian fluid in extensional flow. We extend previous models to include extensional flow of a thin nematic liquid crystal atop a Newtonian aqueous layer with insoluble surfactant between them. We derive the resulting system of nonlinear partial differential equations for thickness of the LL and aqueous layers, surfactant transport and velocity in the LL. We find that in the limit used here, the liquid crystal director field becomes orientated at a constant angle through the depth of LL. Evaporation is taken into account, and is affected by the LL thickness, internal arrangement of its rod-like molecules, and external conditions. Despite the complexity, this system still represents a significant reduction of the full system. We solve the system numerically via collocation with finite difference discretization in space together with implicit time stepping. We analyze solutions for different internal LL structures and show significant effect of the orientation. Orienting the molecules close to the normal direction to the TF surface results in slower evaporation, and other orientations have an effect on flow, showing that this type of model has promise for predicting TF dynamics.

Characteristics of rolling interface lubrication considering contact surface textures in mixed lubrication

This study comprehensively utilizes the mixed lubrication theory and rolling deformation theory to establish a finite element stiffness contact model that considers the surface texture characteristics of the rolling interface. The dynamic explicit finite element analysis is used to obtain the deformed solid, and the turbulent equation is used to control the fluid boundary conditions. The bidirectional Fluid-Structure Interacton (FSI) ansysis is used to study the influence of surface roughness, surface textures, and rolling parameters such as rolling speed on the distribution of oil film pressure and contact characteristics during the rolling process. The results confirm that transverse texture reduces the friction coefficient and dynamic pressure by enhancing the trapping effect of rough peaks and facilitating the entrainment of the fluid film. Under the same textural conditions, the contact area decreases with increasing roughness while the friction coefficient increases. As the rolling speed increases, the rolling pressure decreases to a certain extent, and the contact area ratio will decrease; Lubricants with higher viscosity can effectively reduce the friction and rolling force at the rolling interface. We believe that the insights obtained from the numerical simulations provided in this reaserch will lay a theoretical foundation for subsequent research on plate shape control.

Methodology for Optimal Design of Active Fluid Film Bearings Considering Their Power Losses, Stability and Controllability: Theory and Experiment

This study addresses the problem of the automated synthesis of active fluid film bearings optimized for their adjustable design for new generations of turbomachines. The developed methodology proposes a criterion describing the ability of a bearing’s mechanical design to effectively implement control actions along with its energy efficiency and stability properties considered in a solved multi-objective optimization problem. The design process of actively lubricated journal bearings was investigated in the context of the proposed approach. A multi-objective optimization problem was solved with heuristic algorithms. An analysis of the results obtained with the MOGA and MOPSO algorithm revealed their shortcomings emerging in such problems. The MOPSO algorithm was improved to expand the range and uniformity of the distribution of solutions in the resulting Pareto set and to speed up calculations. Four bearing configurations with significantly different properties were selected from the obtained set of solutions, manufactured and experimentally tested, showing the good agreement between the actual parameters and those set during the design procedure. The results substantiate the applicability of the proposed theoretical and computational tools for designing active fluid film bearings with pre-specified properties to meet the comprehensive requirements of the energy efficiency, reliability and service life of turbomachines.

On dewetting and concentration evolution of thin binary fluid films

On dewetting and concentration evolution of thin binary fluid films

Leakage Control Mechanisms and Strategies for Hydraulically Driven Controllable Rod Seals

Abstract The primary function of rod seals in hydraulic systems is to prevent the leakage of the hydraulic medium, which would otherwise result in environmental pollution and the loss of energy. However, the leakage control performance of conventional rod seals is limited by the level of product design. As a result, the performance of these seals will gradually degrade during the service process. Furthermore, it is not possible to adaptively adjust the performance of these seals in response to changes in operating conditions. Hydraulically controllable rod seals (HCRS) can be driven hydraulically to adjust the internal stress distribution of the seal to regulate the sealing performance. This may be an effective solution, although the leakage control mechanism and strategy remain unclear. This paper employs a thermoelastohydrodynamic mixed lubrication model to comparatively analyze the behaviors and performances of the HCRS with or without a cavity structure. The leakage control mechanism of the HCRS is revealed, namely that it is achieved by increasing the interface contact pressure and reducing the fluid film thickness through external pressurization. Furthermore, the functional relationship between the sealing performance and the regulating variables under different operating conditions is obtained through multivariate regression analysis, thereby forming the quantitative leakage control strategy of the HCRS.

Biomechanical drivers of the evolution of butterflies and moths with a coilable proboscis.

Current biomechanical models suggest that butterflies and moths use their proboscis as a drinking straw pulling nectar as a continuous liquid column. Our analyses revealed an alternative mode for fluid uptake: drinking bubble trains that help defeat drag. We combined X-ray phase-contrast imaging, optical video microscopy, micro-computed tomography, phylogenetic models of evolution and fluid mechanics models of bubble-train formation to understand the biomechanics of butterfly and moth feeding. Our models suggest that the bubble-train mechanism appeared in the early evolution of butterflies and moths with a proboscis long enough to coil. We propose that, in addition to the ability to drink a continuous column of fluid from pools, the ability to exploit fluid films by capitalizing on bubble trains would have expanded the range of available food sources, facilitating diversification of Lepidoptera.

CFD Modeling of Film Condensation from a Steam-Air Mixture in Vertical Channels

Steam condensation in the presence of a noncondensable gas is of vital importance for passive cooling containment systems. The noncondensable gas causes a significant reduction in the condensation rate and heat transfer across the containment, which is important for postulated loss-of-coolant accidents in a nuclear reactor. In this work, computational fluid dynamics models of condensation and the adjacent single-phase steam-air mixture flow are developed for laminar and turbulent flow in vertical channels by two distinct wall condensation modeling approaches using the commercial code STAR-CCM+. The first is the fluid film model available in STAR-CCM+, which solves liquid layer governing equations with connections to the adjacent gas mixture flow. The second is a user-defined wall condensation model that neglects the fluid film and instead accounts for mass, momentum, and heat transfer via user-defined volumetric sink terms adjacent to the cold wall. The condensation models are assessed by first comparing the calculated results with the numerical solution of laminar flow, solved using a complete two-phase model that solves parabolic equations based on conservation of mass, momentum, energy, and species for each phase. Next, the results of a two-dimensional analysis are compared with COPAIN experiments and existing numerical solutions from three-dimensional analyses. The comparisons include new, detailed results that have not been reported in previous analyses of a COPAIN case. These new results include local field profiles of velocity, temperature, and air mass fraction, and local mass flux.

A general model for spin coating on a non-axisymmetric curved substrate

We derive a generalised asymptotic model for the flow of a thin fluid film over an arbitrarily parameterised non-axisymmetric curved substrate surface based on the lubrication approximation. In addition to surface tension, gravity and centrifugal force, our model incorporates the effects of the Coriolis force and disjoining pressure, together with a non-uniform initial condition, which have not been widely considered in existing literature. We use this model to investigate the impact of the Coriolis force and fingering instability on the spreading of a non-axisymmetric spin-coated film at a range of substrate angular velocities, first on a flat substrate, and then on parabolic cylinder- and saddle-shaped curved substrates. We show that, on flat substrates, the Coriolis force has a negligible impact at low angular velocities, and at high angular velocities results in a small deflection of fingers formed at the contact line against the direction of substrate rotation. On curved substrates, we demonstrate that, as the angular velocity is increased, spin-coated films transition from being dominated by gravitational drainage with no fingering to spreading and fingering in the direction with the greatest component of centrifugal force tangent to the substrate surface. For both curved substrates and all angular velocities considered, we show that the film thickness and total wetted substrate area remain similar over time to those on a flat substrate, with the key difference being the shape of the spreading droplet.

Lubrication flow law and cooling mechanism considering cavitation in mechanical seals with Rayleigh step pattern

Lubrication flow law and cooling mechanism considering cavitation in mechanical seals with Rayleigh step pattern