arXiv subjects

Simon Mendez

Publications and source records attributed to Simon Mendez.

13 recordsLinked to original sources

arXiv2026

Enucleated incompressible red blood cells in shear flow: theoretical analysis of shape instabilities

Red blood cells (RBCs) are essential for oxygen transport, and their remarkable ability to undergo significant deformations during flow is a crucial feature for their physiological function. At intermediate shear rates typical of the microcirculation, RBCs can adopt complex, multi-lobed shapes, signifying a dynamic instability. Here we adopt a perturbative theoretical framework of a quasi-spherical RBC under external shear flow to study such shape instabilities. To better capture RBC maturation and enucleation, we first extend the framework to explicitly account for different excess areas between the stress-free and current membrane shapes. We revisit the reduced equations of motion obtained for an ellipsoidally-shaped RBC, and demonstrate the effect of different excess areas and initial orientation on the dynamical trajectories. Then, we introduce additional spatial modes and show that an emerging instability critically depends on the RBC's shear and bending moduli, the internal to external viscosity ratio, and the excess area, mainly through the RBC's membrane tension. We also study the instability-induced saturation of the membrane tension, and the resulting excess area redistribution at long times. The theoretical framework and the emerging picture of the different instabilities provide insights into the emergence of stomatocyte and trilobe shapes exhibited by RBCs under external flow.

cond-mat.soft↗

arXiv2024

A discrete de Rham discretization of interface diffusion problems with application to the Leaky Dielectric Model

Motivated by the study of the electrodynamics of particles, we propose in this work an arbitrary-order discrete de Rham scheme for the treatment of elliptic problems with potential and flux jumps across a fixed interface. The scheme seamlessly supports general elements resulting from the cutting of a background mesh along the interface. Interface conditions are enforced weakly à la Nitsche. We provide a rigorous convergence of analysis of the scheme for a steady model problem and showcase an application to a physical problem inspired by the Leaky Dielectric Model.

math.NA↗

arXiv2024

Physics-Guided Neural Networks for Intraventricular Vector Flow Mapping

Intraventricular vector flow mapping (iVFM) seeks to enhance and quantify color Doppler in cardiac imaging. In this study, we propose novel alternatives to the traditional iVFM optimization scheme by utilizing physics-informed neural networks (PINNs) and a physics-guided nnU-Net-based supervised approach. When evaluated on simulated color Doppler images derived from a patient-specific computational fluid dynamics model and in vivo Doppler acquisitions, both approaches demonstrate comparable reconstruction performance to the original iVFM algorithm. The efficiency of PINNs is boosted through dual-stage optimization and pre-optimized weights. On the other hand, the nnU-Net method excels in generalizability and real-time capabilities. Notably, nnU-Net shows superior robustness on sparse and truncated Doppler data while maintaining independence from explicit boundary conditions. Overall, our results highlight the effectiveness of these methods in reconstructing intraventricular vector blood flow. The study also suggests potential applications of PINNs in ultrafast color Doppler imaging and the incorporation of fluid dynamics equations to derive biomarkers for cardiovascular diseases based on blood flow.

eess.IV↗

arXiv2024

How a table modulates the risk of airborne transmission between facing individuals

Airborne transmission has been recognized as an important route of transmission for SARS-CoV-2, the virus responsible for the COVID-19 pandemic. While coughing and sneezing are spectacular sources of production of infected aerosols with far-reaching airflows, the prevalence of asymptomatic transmissions highlighted the importance of social activities. Gathering around a table, a common scenario of human interactions, not only fixes a typical distance between static interlocutors, but influences airborne transmission, by serving both as a flow diverter and a surface for droplet deposition. Here, we use high-fidelity large-eddy simulations to characterize short-range airborne transmission when two people face each other at a typical table. We show that compared to the natural distance travelled by free buoyant puffs and jets, the distance between the table and the emission constitutes a new length scale that modifies downward exhaled flows, common during nose breathing, speech, and laughter. When the table is close to the emitter, its main effect is to restrict the forward spread of emitted particles. However, if the distance between individuals is too short, particles reaching the recipient become more concentrated, rising transmission risks. Additionally, simulations of forceful exhalations, like laughter, demonstrate that the table acts as a filter that collects medium-sized particles that would have remained in the free jet otherwise, but can in that case be involved in the fomite transmission route. The table introduces a cut-off size for particles that depends on the inertia of the exhaled material, thereby modifying the size distribution of particles suspended in the air.

physics.flu-dyn↗

arXiv2023

Viral transmission in pedestrian crowds: Coupling an open-source code assessing the risks of airborne contagion with diverse pedestrian dynamics models

We study viral transmission in crowds via the short-ranged airborne pathway using a purely model-based approach. Our goal is two-pronged. Firstly, we illustrate with a concrete and pedagogical case study how to estimate the risks of new viral infections by coupling pedestrian simulations with the transmission algorithm that we recently released as open-source code. The algorithm hinges on pre-computed viral concentration maps derived from computational fluid dynamics (CFD) simulations. Secondly, we investigate to what extent the transmission risk predictions depend on the pedestrian dynamics model in use. For the simple bidirectional flow under consideration, the predictions are found to be surprisingly stable across initial conditions and models, despite the different microscopic arrangements of the simulated crowd, as long as the crowd evolves in a qualitatively similarly way. On the other hand, when major changes are observed in the crowd's behaviour, notably whenever a jam occurs at the centre of the channel, the estimated risks surge drastically.

cs.MA↗

arXiv2022

From microscopic droplets to macroscopic crowds: Crossing the scales in models of short-range respiratory disease transmission, with application to COVID-19

Short-range exposure to airborne virus-laden respiratory droplets is now acknowledged as an effective transmission route of respiratory diseases, as exemplified by COVID-19. In order to assess the risks associated with this pathway in daily-life settings involving tens to hundreds of individuals, the chasm needs to be bridged between fluid dynamical simulations of droplet propagation and population-scale epidemiological models. We achieve this by coarse-graining microscopic droplet trajectories (simulated in various ambient flows) into spatio-temporal maps of viral concentration around the emitter and coupling these maps to field-data about pedestrian crowds in different scenarios (streets, train stations, markets, queues, and street caf{é}s). At the scale of an individual pedestrian, our results highlight the paramount importance of the velocity of the ambient air flow relative to the emitter's motion. This aerodynamic effect, which disperses infectious aerosols and thus mitigates short-range transmission risks, prevails over all other environmental variables. At the crowd's scale, the method yields a ranking of the scenarios by the risks of new infections that they present, dominated by the street caf{é}s and then the outdoor market. While the effect of light winds on the qualitative ranking is fairly marginal, even the most modest ambient air flows dramatically lower the quantitative rates of new infections. The proposed framework was here applied with SARS-CoV-2 in mind, but its generalization to other airborne pathogens and to other (real or hypothetical) crowd arrangements is straightforward.

physics.bio-ph↗

arXiv2021

Full-volume three-component intraventricular vector flow mapping by triplane color Doppler

Intraventricular vector flow mapping (iVFM) is a technique for retrieving 2-D velocity vector fields of blood flow in the left ventricle. This method is based on conventional color Doppler imaging, which makes iVFM compatible with the clinical setting. We have generalized the iVFM for a three-dimensional reconstruction (3D-iVFM). 3D-iVFM is able to recover three-component velocity vector fields in a full intraventricular volume by using a clinical echocardiographic triplane mode. As with the 2-D version, the method is based on the mass conservation, and free-slip boundary conditions on the endocardial wall. These mechanical constraints were imposed in a least-squares minimization problem that was solved through the method of Lagrange multipliers. We validated 3D-iVFM in silico in a patient-specific CFD (computational fluid dynamics) model of cardiac flow, and tested its feasibility in vivo on volunteers. In both in silico and in vivo investigations, the dynamics of the intraventricular vortex that forms during diastole was deciphered by 3D-iVFM. Our results tend to indicate that 3D-iVFM could provide full-volume echocardiographic information on left intraventricular hemodynamics from the clinical modality of triplane color Doppler.

physics.med-ph↗

arXiv2021

Physics-constrained intraventricular vector flow mapping by color Doppler

Color Doppler by transthoracic echocardiography creates 2-D fan-shaped maps of blood velocities in the cardiac cavities. It is a one-component velocimetric technique since it only returns the velocity components parallel to the ultrasound beams. Intraventricular vector flow mapping (iVFM) is a method to recover the blood velocity vectors from the Doppler scalar fields in an echocardiographic three-chamber view. We improved our iVFM numerical scheme by imposing physical constraints. The iVFM consisted in minimizing regularized Doppler residuals subject to the condition that two fluid-dynamics constraints were satisfied, namely planar mass conservation, and free-slip boundary conditions. The optimization problem was solved by using the Lagrange multiplier method. A finite-difference discretization of the optimization problem, written in the polar coordinate system centered on the cardiac ultrasound probe, led to a sparse linear system. The single regularization parameter was determined automatically for non-supervision considerations. The physics-constrained method was validated using realistic intracardiac flow data from a patient-specific CFD model. The numerical evaluations showed that the iVFM-derived velocity vectors were in very good agreement with the CFD-based original velocities, with relative errors ranged between 0.3 and 12%. We calculated two macroscopic measures of flow in the cardiac region of interest, the mean vorticity and mean stream function, and observed an excellent concordance between physics-constrained iVFM and CFD. The capability of physics-constrained iVFM was finally tested with in vivo color Doppler data acquired in patients routinely examined in the echocardiographic laboratory. The vortex that forms during the rapid filling was deciphered. The physics-constrained iVFM algorithm is ready for pilot clinical studies.

physics.med-ph↗

arXiv2021

Model-based assessment of the risks of viral transmission in non-confined crowds

This work aims to assess the risks of Covid-19 disease spread in diverse daily-life situations (referred to as scenarios) involving crowds of maskless pedestrians, mostly outdoors. More concretely, we develop a method to infer the global number of new infections from patchyobservations of pedestrians. The method relies on ad hoc spatially resolved models for disease transmissionvia virus-laden respiratory droplets, which are fit to existing exposure studies about Covid-19. The approach is applied to the detailed field data about pedestrian trajectories and orientations that we acquired during the pandemic. This allows us to rank the investigated scenarios by the infection risks that they present; importantly, the obtained hierarchy of risks is conserved across all our transmission models (except the most pessimistic ones): Street caf{é}s present the largest average rate of new infections caused by an attendant, followed by busy outdoor markets, and then metro and train stations, whereas the risks incurred while walking on fairly busy streets (average density around 0.1 person/m${}^2$) are comparatively quite low. While none of our ad hoc models can claim accuracy, their converging predictions lend credence to these findings.} In scenarios with a moving crowd, we find that density is the main factor influencing the estimated infection rate. Finally, our study explores the efficiency of street and venue redesigns in mitigating the viral spread: While the benefits of enforcing one-way foot traffic in (wide) walkways are unclear, changing the geometry of queues substantially affects disease transmission risks.

physics.soc-ph↗

arXiv2020

Speech can produce jet-like transport relevant to asymptomatic spreading of virus

Many scientific reports document that asymptomatic and presymptomatic individuals contribute to the spread of COVID-19, probably during conversations in social interactions. Droplet emission occurs during speech, yet few studies document the flow to provide the transport mechanism. This lack of understanding prevents informed public health guidance for risk reduction and mitigation strategies, e.g. the "six-foot rule". Here we analyze flows during breathing and speaking, including phonetic features, using order-of-magnitudes estimates, numerical simulations, and laboratory experiments. We document the spatio-temporal structure of the expelled air flow. Phonetic characteristics of plosive sounds like 'P' lead to enhanced directed transport, including jet-like flows that entrain the surrounding air. We highlight three distinct temporal scaling laws for the transport distance of exhaled material including (i) transport over a short distance ($<$ 0.5 m) in a fraction of a second, with large angular variations due to the complexity of speech, (ii) a longer distance, approximately 1 m, where directed transport is driven by individual vortical puffs corresponding to plosive sounds, and (iii) a distance out to about 2 m, or even further, where sequential plosives in a sentence, corresponding effectively to a train of puffs, create conical, jet-like flows. The latter dictates the long-time transport in a conversation. We believe that this work will inform thinking about the role of ventilation, aerosol transport in disease transmission for humans and other animals, and yield a better understanding of linguistic aerodynamics, i.e., aerophonetics.

physics.flu-dyn↗

arXiv2016

A new look at blood shear-thinning

Blood viscosity decreases with shear stress, a property essential for an efficient perfusion of the vascular tree. Shear-thinning is intimately related to the dynamics and mutual interactions of red blood cells (RBCs), the major constituents of blood. Our work explores RBCs dynamics under physiologically relevant conditions of flow strength, outer fluid viscosity and volume fraction. Our results contradict the current paradigm stating that RBCs should align and elongate in the flow direction thanks to their membrane circulation around their center of mass, reducing flow-lines disturbances. On the contrary, we observe both experimentally and with simulations, rich morphological transitions that relate to global blood rheology. For increasing shear stresses, RBCs successively tumble, roll, deform into rolling stomatocytes and finally adopt highly deformed and polylobed shapes even for semi-dilute volume fractions analogous to microcirculatory values. Our study suggests that any pathological change in plasma composition, RBCs cytosol viscosity or membrane mechanical properties will impact the onset of shape transitions and should play a central role in pathological blood rheology and flow behavior.

physics.bio-ph↗

arXiv2014

On the damped oscillations of an elastic quasi-circular membrane in a two-dimensional incompressible fluid

We propose a procedure - partly analytical and partly numerical - to find the frequency and the damping rate of the small-amplitude oscillations of a massless elastic capsule immersed in a two-dimensional viscous incompressible fluid. The unsteady Stokes equations for the stream function are decomposed onto normal modes for the angular and temporal variables, leading to a fourth-order linear ordinary differential equation in the radial variable. The forcing terms are dictated by the properties of the membrane, and result into jump conditions at the interface between the internal and external media. The equation can be solved numerically, and an excellent agreement is found with a fully-computational approach we developed in parallel. Comparisons are also shown with the results available in the scientific literature for drops, and a model based on the concept of embarked fluid is presented, which allows for a good representation of the results and a consistent interpretation of the underlying physics.

physics.flu-dyn↗

arXiv2013

Elucidating the turbulence nature of the intracardiac flow: from medical images to multi-cycle Large Eddy Simulations

This brief article accompanies a fluid dynamics video presenting the results of a large-eddy simulation of the flow in a realistic left heart. The left heart geometry, from the pulmonary veins to the aortic root, is extracted from medical images and the endocardium movements are reconstructed through image registration. Large-eddy simulations are thus performed in a patient-specific heart model, where the patient-specific movements of the geometry are prescribed. The flow obtained is intermittent, showing both in the left atrium and in the left ventricle turbulent spots correlated to flow decelerations.

physics.flu-dyn↗