Reconstructing the fluid flow by tracking of large particles

All the methods which estimate the unperturbed fluid flow velocity relying on particle suspensions address the same question: How can the fluid velocity be computed measuring the particles trajectory and/or their velocities? The tracking of a few large density-mismatched particles is here used to efficiently and accurately reconstruct the background fluid flow. Approximating the particulate phase space and taking the limit of vanishing Stokes number St -> 0, we retrieve the background flow for three test cases: a shear flow near a wall, a rigid-body vortex, and a strained vortex. The major advantages and the potentials of this approach are discussed in the end, highlighting how to overcome the classic shortcomings of experimental measurements faced for near-boundaries particle tracking

Particle accumulation in high‐Prandtl‐number liquid bridges

Particle accumulation in high‐Prandtl‐number (Pr = 68) thermocapillary liquid bridges is studied numerically. Randomly distributed small rigid non‐interacting spherical particles are found to cluster in particle accumulation structures. The accumulation is found to be caused by a finite‐particle‐size effect when the particles move close to the impermeable flow boundaries. The extra drag force experienced by a particle near the boundaries creates a dissipation in the dynamical system describing the particle motion. This causes particles to be attracted to regions in or near Kolmogorov‐Arnold‐Moser tori of the unperturbed flow field.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153263/1/pamm201900058.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153263/2/pamm201900058_am.pd

Peristaltic flow in the glymphatic system.

The flow inside the perivascular space (PVS) is modeled using a first-principles approach in order to investigate how the cerebrospinal fluid (CSF) enters the brain through a permeable layer of glial cells. Lubrication theory is employed to deal with the flow in the thin annular gap of the perivascular space between an impermeable artery and the brain tissue. The artery has an imposed peristaltic deformation and the deformable brain tissue is modeled by means of an elastic Hooke\u27s law. The perivascular flow model is solved numerically, discovering that the peristaltic wave induces a steady streaming to/from the brain which strongly depends on the rigidity and the permeability of the brain tissue. A detailed quantification of the through flow across the glial boundary is obtained for a large parameter space of physiologically relevant conditions. The parameters include the elasticity and permeability of the brain, the curvature of the artery, its length and the amplitude of the peristaltic wave. A steady streaming component of the through flow due to the peristaltic wave is characterized by an in-depth physical analysis and the velocity across the glial layer is found to flow from and to the PVS, depending on the elasticity and permeability of the brain. The through CSF flow velocity is quantified to be of the order of micrometers per seconds

Cuba: reformas y bienestar. Un análisis de los bienes no monetarios

Nuestro estudio es el primer análisis que muestra la tendencia de la distribución del bienestar en Cuba, en el periodo de las reformas económicas y sociales impulsadas por el presidente Raúl Castro. Los análisis revelan que el bienestar, premisa del desarrollo humano y medido como acceso a bienes y servicios fundamentales y no monetarios, ha aumentado, a pesar de la crisis económica; en tanto que las desigualdades en el acceso al bienestar han aumentado poco en el periodo 2006-2011 y han disminuido en el periodo siguiente, para llegar en 2014 a un nivel ligeramente inferior al de 2006

Liquid plug formation in an airway closure model

The closure of a human lung airway is modeled as an instability of a two-phase flow in a pipe coated internally with a Newtonian liquid. For a thick enough coating, the Plateau-Rayleigh instability creates a liquid plug which blocks the airway, halting distal gas exchange. Owing to a bifrontal plug growth, this airway closure flow induces high stress levels on the wall, which is the location of airway epithelial cells. A parametric numerical study is carried out simulating relevant conditions for human lungs, in either ordinary or pathological situations. Our simulations can represent the physical process from pre- to postcoalescence phases. Previous studies have been limited to precoalescence only. The topological change during coalescence induces a high level of stress and stress gradients on the epithelial cells, which are large enough to damage them, causing sublethal or lethal responses. We find that postcoalescence wall stresses can be in the range of 300% to 600% greater than precoalescence values and so introduce an important source of mechanical perturbation to the cells

Particle accumulation in incompressible laminar flows due to particle-boundary interaction

The accumulation of particles in laminar incompressible fluid flows is investigated. Boundary-driven closed systems are considered. We deal with a thermocapillary liquid bridge, a lid-driven cavity and a partially liquid-filled rotating drum. In all three the configurations we consider flows after the onset of a steady three-dimensional instability. The corresponding fluid dynamics systems are equivalent to a Hamiltonian system of 1.5 degrees of freedom and we consider flow parameters for which chaotic and regular regions coexist. Owing to the boundary-driving mechanism, the quasi-periodic streamlines (Kolmogorov-Arnold-Moser or KAM tori) are mainly located near the moving wall or the free-surface. Finite-size particles with small Stokes numbers are introduced in these fluid flow systems to study the so-called particle accumulation structures (PAS). In the following we extend the classical framework of investigation of PAS, passing from thermocapillary to boundary-driven flows. We further aim at clarifying the fundamental mechanism PAS is based on, regardless of the specific system in which it is considered. The main flow features basically required in these set-ups are KAM tori located near the boundaries and particles of finite-size. The particles which move close to a wall or a free-surface may be transferred from the chaotic to the regular regions of the flow because of the repulsion exerted by the boundaries. The particle--boundary interaction represents the main dissipative mechanism responsible for the formation of PAS. For simulating particles moving close to the driving boundaries we employ fully-resolved simulations produced via a discontinuous Galerkin finite element method (DG-FEM) in combination with the smoothed profile method (SPM). The simulations aim at clarifying the dependence of the lubrication gap width on particle size and density ratio after that a single particle is trapped (within a certain tolerance) in 2-D PAS. To this end, small particles in a shear--stress- and a lid-driven cavity are investigated. The fully-resolved simulation results are employed to improve an existing particle--boundary interaction (PSI) model. A one-way coupling approach which includes such an improved PSI model is used to simulate two- and three-dimensional particle-laden flows. A comparison of the numerically predicted PAS with experimental data is finally made to confirm the numerical results. All the main phenomenological explanations given for understanding PAS will be commented and discussed in details. Our principal aim is to show that the strong correlation between particle accumulation structures and flow topology, together with the particle--boundary interaction dissipative effect, provides a universal mechanism for explaining PAS for all set-ups investigated.19

Computational pulmonary edema: A microvascular model of alveolar capillary and interstitial flow

We present a microvascular model of fluid transport in the alveolar septa related to pulmonary edema. It consists of a two-dimensional capillary sheet coursing by several alveoli. The alveolar epithelial membrane runs parallel to the capillary endothelial membrane with an interstitial layer in between, making one long septal tract. A coupled system of equations uses lubrication theory for the capillary blood, Darcy flow for the porous media of the interstitium, a passive alveolus, and the Starling equation at both membranes. Case examples include normal physiology, cardiogenic pulmonary edema, acute respiratory distress syndrome (ARDS), hypoalbuminemia, and effects of PEEP. COVID-19 has dramatically increased ARDS in the world population, raising the urgency for such a model to create an analytical framework. Under normal conditions fluid exits the alveolus, crosses the interstitium, and enters the capillary. For edema, this crossflow is reversed with fluid leaving the capillary and entering the alveolus. Because both the interstitial and capillary pressures decrease downstream, the reversal can occur within a single septal tract, with edema upstream and clearance downstream. Clinically useful solution forms are provided allowing calculation of interstitial fluid pressure, crossflows, and critical capillary pressures. Overall, the interstitial pressures are found to be significantly more positive than values used in the traditional physiological literature. That creates steep gradients near the upstream and downstream end outlets, driving significant flows toward the distant lymphatics. This new physiological flow provides an explanation to the puzzle, noted since 1896, of how pulmonary lymphatics can function so far from the alveoli: the interstitium is self-clearing