Fluid-Structure Interaction model for collapsing cavitation bubble near deformable solid boundaries

International audienceThe motivation behind this research lies in understanding the physical mechanism of cavitation erosion in compressible liquid flows, with applications in the field of aerospace, hydrodynamics, diesel injectors etc. As a consequence of collapsing vapor cavities in cavitating flow near solid boundaries, high pressure impact loads are generated. These pressure loads are believed to be responsible for the erosive damages on solid surface observed in most applications. For our investigation, the initial geometry is a single vapor bubble near a solid boundary collapsing due to the pressure difference between the bubble and surrounding liquid. The numerical approach employs a simplified homogenous mixture or 'single fluid' model with barotropic assumption in a fully compressible finite-volume fluid solver. The numerical method is validated against the well-known Rayleigh collapse of a pure 3D vapour bubble. It is then used for the simulation of a 2D vapour bubble collapsing in the proximity of a solid boundary placed at a specified distance from the centre of the bubble. The pressure loads are computed from the evolving dynamics of collapsing bubble near a solid boundary which can be used to determine the resulting surface deformation. The developed compressible cavitation solver in the CFD code 2 can efficiently model small and large scale cavitating structures in a fully resolved three dimensional flow

Modeling Fluid-Structure Interaction in Cavitation Erosion using Smoothed Particle Hydrodynamics

International audienceIn the present study a meshless Smoothed Particle Hydrodynamics cavitation solver is developed. The fluid bubble collapse solver is validated against analytical Rayleigh-Plesset equation and shows good agreement. The solid solver capable of solving elastic-plastic deformation and material damage is developed and is validated against FEM results. A fluid structure interaction solver capable of solving cavitation erosion is presented. A single bubble collapse is demonstrated in the paper

Rheology of non-Brownian suspensions of rough frictional particles under shear reversal: A numerical study

International audienceWe perform particle scale simulations of suspensions submitted to shear reversal. The simulations are based on the Force Coupling Method, adapted to account for short range lubrication interactions together with direct contact forces between particles, including surface roughness, contact elasticity and solid friction. After shear reversal, three consecutive steps are identified in the viscosity transient: an instantaneous variation, followed by a rapid contact force relaxation, and finally a long time evolution. The separated contributions of hydrodynamics and contact forces to the viscosity are investigated during the transient, allowing a qualitative understanding of each step. In addition, the influence of the contact law parameters (surface roughness height and friction coefficient) on the transient are evaluated. Concerning the long time transient, the difference between the steady viscosity and minimum viscosity is shown to be proportional to the contact contribution to the steady viscosity, allowing in principle easy determination of the latter in experiments. The short time evolution is studied as well. After the shear reversal, the contact forces vanish over a strain that is very short compared to the typical strain of the long time transient, allowing to define an apparent step between the viscosity before shear reversal and after contact force relaxation. This step is shown to be an increasing function of the friction coefficient between particles. Two regimes are identified as a function of the volume fraction. At low volume fraction, the step is small compared to the steady contact viscosity, in agreement with a particle pair model. As the volume fraction increases, the value of the viscosity step increases faster than the steady contact viscosity, and, depending on the friction coefficient, may approach it

Anomalous coalescence in sheared two-dimensional foam

We report an experimental study on shearing a monolayer of monodisperse bubbles floating on liquid in a narrow-gap Couette device. The bubbles in such a "bubble raft" coalesce only if the shear rate exceeds a threshold value. This is in contrast to the conventional wisdom that bubbles and drops coalesce for gentler collisions, at shear rates below a critical value. Furthermore, the threshold shear rate increases with the bubble size and the viscosity of the suspending liquid, contravening reasoning based on capillary number. Through visualization and scaling arguments, we investigate several plausible mechanisms for the anomalous coalescence. None explains all aspects of the observations. The most promising model is one based on inertial forces that compress the bubbles radially inward and accelerate film drainage

Clinicopathological predictors of recurrence in nodular and superficial spreading cutaneous melanoma: A multivariate analysis of 214 cases

Abstract Background Nodular melanoma (NM) accounts for most thick melanomas and because of their frequent association with ulceration, fast growth rate and high mitotic rate, contribute substantially to melanoma-related mortality. In a multicentric series of 214 primary melanomas including 96 NM and 118 superficial spreading melanoma (SSM), histopathological features were examined with the aim to identify clinicopathological predictors of recurrence. Methods All consecutive cases of histopathologically diagnosed primary invasive SSM and NM during the period 2005–2010, were retrieved from the 12 participating Italian Melanoma Intergroup (IMI) centers. Each center provided clinico-pathological data such as gender, age at diagnosis, anatomical site, histopathological conventional parameters, date of excision and first melanoma recurrence. Results Results showed that NM subtype was significantly associated with Breslow thickness (BT) at multivariate analysis: [BT 1.01–2 mm (OR 7.22; 95% CI 2.73–19.05), BT 2.01–4 mm (OR 7.04; 95% CI 2.54–19.56), and BT > 4 mm (OR 51.78; 95% CI 5.65–474.86) (p  5 mitoses/mm2 (OR 4.87; 95% CI 1.77–13.40) (p = 0.002)]. The risk of recurrence was not significantly associated with NM histotype while BT [BT 1.01–2.00 mm (HR 1.55; 95% CI 0.51–4.71), BT 2.01–4.00 mm (HR 2.42; 95% CI 0.89–6.54), BT > 4.00 mm. (HR 3.13; 95% CI 0.95–10.28) (p = 0.05)], mitotic rate [MR > 2 mitoses/mm2 (HR 2.34; 95% CI, 1.11–4.97) (p = 0.03)] and the positivity of lymph node sentinel biopsy (SNLB) (HR 2.60; 95% CI 1.19–5.68) (p = 0.007) were significantly associated with an increased risk of recurrence at multivariate analysis. Conclusions We found that NM subtype was significantly associated with higher BT and MR but it was not a prognostic factor since it did not significantly correlate with melanoma recurrence rate. Conversely, increased BT and MR as well as SNLB positivity were significantly associated with a higher risk of melanoma recurrence

Dermoscopic evaluation of nodular melanoma

Importance: Nodular melanoma (NM) is a rapidly progressing potentially lethal skin tumor for which early diagnosis is critical

Low in‑hospital mortality rate in patients with COVID‑19 receiving thromboprophylaxis: data from the multicentre observational START‑COVID Register

Abstract COVID-19 infection causes respiratory pathology with severe interstitial pneumonia and extra-pulmonary complications; in particular, it may predispose to thromboembolic disease. The current guidelines recommend the use of thromboprophylaxis in patients with COVID-19, however, the optimal heparin dosage treatment is not well-established. We conducted a multicentre, Italian, retrospective, observational study on COVID-19 patients admitted to ordinary wards, to describe clinical characteristic of patients at admission, bleeding and thrombotic events occurring during hospital stay. The strategies used for thromboprophylaxis and its role on patient outcome were, also, described. 1091 patients hospitalized were included in the START-COVID-19 Register. During hospital stay, 769 (70.7%) patients were treated with antithrombotic drugs: low molecular weight heparin (the great majority enoxaparin), fondaparinux, or unfractioned heparin. These patients were more frequently affected by comorbidities, such as hypertension, atrial fibrillation, previous thromboembolism, neurological disease,and cancer with respect to patients who did not receive thromboprophylaxis. During hospital stay, 1.2% patients had a major bleeding event. All patients were treated with antithrombotic drugs; 5.4%, had venous thromboembolism [30.5% deep vein thrombosis (DVT), 66.1% pulmonary embolism (PE), and 3.4% patients had DVT + PE]. In our cohort the mortality rate was 18.3%. Heparin use was independently associated with survival in patients aged ≥ 59 years at multivariable analysis. We confirmed the high mortality rate of COVID-19 in hospitalized patients in ordinary wards. Treatment with antithrombotic drugs is significantly associated with a reduction of mortality rates especially in patients older than 59 years

Dynamique et rhéologie d'une suspension de vésicules et globules rouges

The dynamics and the rheology of a suspension of vesicles (a model for red blood cells) in the limit of small Reynolds number are studied by means of two-dimensional numerical simulations, based on the boundary integral and phase field methods. The focus is on the link between the microscopic dynamics of the particles and the overall behavior of the suspension (i.e. rheology). A dilute suspension of vesicles in a linear shear flow is analyzed in detail and the influence of the three parameters governing the dynamics of a single vesicle (reduced volume; viscosity contrast; capillary number) is extensively described. The nontrivial behavior of the rheological quantities (effective viscosity and normal stress difference) is explained and the role of the membrane of the vesicle detailed. The influence of the curvature of the flow lines on the dynamics of the vesicles is investigated for the first time, and consistent inward migration is reported. The suggested interpretation remains valid for the flow of the majority of complex fluids, like emulsions and polymer suspensions, and is thus expected to have an impact in other fields. Moreover, the behavior of a suspension of vesicles in a microscopic Taylor-Couette cell is investigated, and a transition to ordered states is reported at very low volume fraction. The behavior of sets of vesicles in a parabolic flow, a setup that mimics red blood cells in the microvasculature, is presented. Vesicles submitted to sOn étudie la dynamique et la rhéologie d'une suspension de vésicules (un modèle pour les globules rouges) dans la limite de faibles nombres de Reynolds en utilisant des simulations numériques, basées sur les méthodes des intégrales au bord et du champ de phase. L'attention est mise sur le lien entre la dynamique microscopique des particules et le comportement d'ensemble de la suspension (c. à d. la rhéologie). On analyse une suspension diluée de vésicules dans un écoulement de cisaillement linéaire et on décrit en détail l'influence des paramètres qui en gouvernent la dynamique. On explique le comportement complexe des grandeurs rhéologiques (viscosité effective et différence des contraintes normales) et on détail le rôle de la membrane de la vésicule. On examine l'influence de la courbure des lignes d'écoulement sur la dynamique des vésicules, et on reporte une migration non-négligeable dans la direction de concavité. L'interprétation donnée reste valable pour la plupart des fluides complexes, comme les émulsions et les suspensions de polymères. De plus, on a investigué le comportement d'une suspension de vésicules dans un apparat de Taylor-Couette microscopique, et une transition vers des états ordonnés à été mise en évidence à des fractions volumiques très faibles. On étudie aussi le comportement d'ensembles de vésicules dans un écoulement parabolique, une situation qui imite les globules rouges dans les capillaires sanguins. Les vésicules, soumises au seules forces hydrodynamiques, forment des agrégats de taille finie, un fait qui pourrait être d'importance physiologique