We propose an isogeometric approach to model the deformation of active thin films using layered, nonlinear, Kirchhoff-Love shells. Isogeometric Collocation and Galerkin formulations are employed to discretize the electrophysiological and mechanical sub -problems, respectively, with the possibility to adopt different element and time -step sizes. Numerical tests illustrate the capabilities of the active -stress -based approach to effectively simulate the contraction of thin films in both quasi -static and dynamic conditions.

The onset of flow-induced oscillations in valve-like configurations remains not completely understood, despite the wide relevance in fluid transport across human physiology and various industrial applications. The present work explores the excitation mechanisms of self-sustained oscillation with key operating parameters in a general-purpose configuration by means of high-fidelity simulations. The investigation is carried out with a partitioned framework that resolves the fluid field by a finite-difference fractional step scheme, discretizes the structural domain via an isogeometric method, and considers an immersed boundary forcing through the interpolation/spreading kernel built by moving-least squares. Our findings confirm the onset of flapping motion in valvular shells, jointly influenced by geometric parameters, structural properties, and flow conditions. Specifically, at a Reynolds number (Re) of 800 and shell aspect ratio of 1.0, a critical reduced velocity exists at around 6, bifurcating static and periodic oscillation modes. After this criterion, flexible shells flutter in the third-plate-mode natural frequency, with oscillation amplitudes approaching an asymptotic value, coupled with intensified vortex shedding, as the reduced velocity increases. Re mainly imparts a destabilizing effect on the fluid-shell system; a lower Re suppresses flow-induced vibrations through viscous dissipation, while a higher Re introduces three-dimensional complexities, asymmetrical oscillations, and quasi-periodicity in the flapping dynamics, especially within the critical regime of reduced velocity. The impact of shell aspect ratio is intricate; in contrast, the case with an aspect ratio of 1.3 displays more intensive flapping motion compared to the reference case of 1.0, whereas further increasing to 1.6 mainly shows stabilizing effects in the shell dynamics.

A numerical framework is presented to predict the transport of soft elastic capsules immersed in an incompressible fluid with the aim of simulating inertial microfluidics applications. The flow evolution is modeled by a fully incompressible lattice Boltzmann method whereas a finite element model is considered for describing the dynamics of deformable structures. An immersed boundary technique is adopted to reconstruct the solution in the vicinity of the immersed surface and the time integration of the fluid-structure interaction problem is obtained by following an explicit procedure. The effectiveness of the framework is validated by means of several test cases involving: flows between two parallel plates for Reynolds numbers in the range 1÷100; capsules immersed in a fluid at Stokes regime exhibiting small and large deformations under shear; capsules migrating in a long straight microchannel with square cross-section at low-to-moderate Reynolds number. A very good agreement between the present results and literature data obtained using different numerical methods is found for all test cases. Finally, the method allowed to accurately simulate incompressible flows subjected to large pressure difference between the inlet and the outlet, characteristic of long curved microchannels, obtaining very smooth particle dynamics.

The TITAN project aims to improve immunotherapy, targeting the efficiency of methods to obtain genetically engineered T cells. Immunotherapy has achieved great success in clinical trials, but it is currently very expensive in terms of time required for analysis, reagents, and samples. TITAN aims to the continuous sampling of critical quality attributes, in order to quickly recognize deviations from the desired range and take appropriate corrective actions on process parameters for an optimal outcome. To achieve its aims, TITAN is currently developing microfluidic and sensing tools for the accurate and efficient real-time monitoring of the T cells amplification process.

Red blood cells (RBCs) are able to undergo significant shape changes when they flow in the microcirculation thanks to their ability to withstand large deformations. In this context, particular attention should be dedicated to the role of RBC deformability and membrane viscosity on the RBC fluid dynamics at the microscale. Experimentally investigating the impact of the RBC viscoelastic properties is challenging due to the overlapping effects of multiple viscous dissipation sources, making high-fidelity cell-resolved simulations crucial. This work focuses on (i) developing a fluid–structure interaction framework to predict the RBC dynamics at the microscale and (ii) evaluating the impact of the cell viscoelastic properties such as deformability, viscosity contrast and membrane viscosity on the RBC transport in bounded shear flow. An incompressible Lattice Boltzmann method (LBM) is adopted to resolve the fluid dynamics inside and outside the RBCs, alongside with a tagging procedure to assign a viscosity contrast between the cytoplasm and the plasma. A finite-element model coupled with the Standard-Linear-Solid model describes the viscoelastic behavior of the RBC membrane. The interaction between the fluid and the RBCs is enforced by means of an immersed-boundary (IB) technique. Benchmark tests are performed to simulate the deformation of a purely elastic capsule, a viscoelastic capsule, and a viscoelastic RBC subjected to a bounded shear flow. A good agreement is found between the present results and literature data obtained with similar IB-LBM methods. The analysis of the impact of the cell viscoelasticity on the RBC dynamics highlights the importance of including the membrane viscosity and a physiological viscosity contrast in the RBC model, especially when investigating the RBC time-dependent behavior. Overall, our findings revealed that adding a viscous term in the RBC model significantly impacts the migration timescale but has a minor effect on the RBC final equilibrium position.

In this work, we present a novel approach to perform the linear stability analysis of fluid-structure interaction problems. The underlying idea is the combination of a validated immersed boundary solver for the nonlinear coupled dynamics with Krylovbased techniques to obtain a robust and accurate global stability solver for elastic structures interacting with incompressible viscous flows. The computation of the leading eigenvalues of the linearized system is carried out in a matrix-free framework by adopting a classical Krylov subspace method. The proposed algorithm avoids the complex analytical linearization of the equations while retaining all the relevant aspects of the fully-coupled fluid-structure system. The methodology has been tested for several cases involving two-dimensional incompressible flows around elastically mounted circular cylinders. The obtained results show a good quantitative agreement with those available in the literature. Finally, the method was applied to investigate the linear stability of the laminar flow past two elastically mounted cylinders in tandem configuration at Re = 100, revealing the existence of two complex dominant modes. For low values of the reduced velocity U*, only one mode is found to be unstable and related to the stationary wake mode. The loss of stability of the second mode at U* = 4 marks the beginning of the lock-in region. We also show that for U* = 5 the modes interact, giving rise to the beating phenomenon observable in the nonlinear time evolution of the system. For larger values of the reduced velocity, the linear dynamics is governed by one dominant mode characterized by wider oscillations of the rear cylinder, matching the results of the nonlinear simulations.(c) 2022 Elsevier Ltd. All rights reserved.

Jellyfish are one of the earliest example of animal that actively regulate swimming, but the mechanisms governing the locomotion are still a matter of research. Jellyfish obtain locomotion by activating the subumbrellar muscle layer. Sensory inputs trigger the contraction of the bell and the fluid-structure interaction effects driving locomotion. These have been extensively studied, whereas a representation of the full neuro-mechanical locomotion chain, with focus on the actuation-locomotion dynamics, has not been proposed yet. A model of such a complex multi-physical phenomenon would be informative for several purposes, ranging from the comprehension of behavioral aspects to the design of soft actuators and bio-inspired devices. In this regard, we propose a computational framework to address the coupled electrophysiological, elastic, and fluid aspects of the locomotion of the Scyphozoan group. This relies on the sequential coupling of segregated solvers, such that each sub-problem can be addressed with the most computationally effective technique. The spatial discretization is addressed by isogeometric analysis for the electrophysiological and elastic sub-problems, and by finite differences for the fluid sub-problem. The active strain approach allows to distribute the active contraction of radial and coronal muscle fibers following the biological architecture. The inherent multi-scale nature of the model is addressed by means of a nested grid approach and multiple time-advancement lines. In view of a reasonable computational effort, we enforce the hypothesis of axial symmetry limiting the number of degrees of freedom used in the simulations. The effectiveness of the scheme employed for each sub-problem is verified against different test-cases of engineering and biologic inspiration. Finally, we carry out an extensive comparison between the simulation output and the in-vivo measurements on a 3-cm specimen of Aurelia Aurita.

Recent developments in additive manufacturing are moving toward a new trend in material extrusion (ISO/ASTM 52910:2018), namely, the possibility of printing thermoplastic strands directly from pellets. Pellet additive manufacturing (PAM) is a relatively new manufacturing process that realizes the aforementioned goal. Consequently, the development of models describing the behavior of the entire process remains a matter of research. The present study aims to propose a systematic and parametric analysis based on an analytical model that describes the entire process, from solid pellet conveying to the deposition of molten strands. First, a mathematical model that analytically describes the behavior of a system consisting of a single-screw extruder, a coupling element, and an extrusion nozzle is presented, without considering the effect of the printed strand. The proposed approach allows the calculation of important process variables, such as the mass flow rate, melting profile, and pressure profile for a given screw speed. An experimental setup aimed at predicting the mass flow rate of a real single-screw extruder and computational fluid dynamics simulations were used to validate the theory presented. The model was then extended to the PAM process, where an additional counterpressure exists because of the strand being deposited on the build plate. The goal is to predict the screw-speed which allows to extrude a prescribed mass flow rate. Subsequently, the effects of the printing nozzle speed and layer height on the process outcomes were investigated. A good agreement with similar trends already predicted in modeling the counterpressure in fused filament fabrication was found.

We present an extension of isogeometric collocation to coupled cardiac electromechanical problems. We develop a staggered solution scheme that takes advantage of isogeometric collocation to reduce the computational effort in the simulation of the mechanical step, guaranteeing high accuracy for all field variables. We mainly focus on (i) the strategy adopted to couple the electrical and mechanical sub-problems, (ii) the possibility of handling different meshes to better represent the spatial scales, (iii) and the mitigation of volumetric locking. To this end, we propose a suitable mixed formulation for finite elasticity. Several numerical tests demonstrate that the mixed formulation retrieves the expected convergence rates under h-refinement and the effectiveness of the proposed solution scheme for electromechanics. (c) 2023 Elsevier B.V. All rights reserved.

In this paper, mathematical models aiming at describing the fluid flow of non-Newtonian fluids inside ducts with both convergent and cylindrical sections have been formulated. The theory has been implemented for the description of industrially interesting fluids such as moisture-cured silicones and thermoplastics, which are well described by the Carreau and Cross-Williams-Landel-Ferry (Cross-WLF) rheological models, respectively. The viscous flow in a die has been considered, comparing the results with Computational Fluid Dynamics (CFD) solutions extracted through the Finite Element Method (FEM) software Comsol Multiphysics. The agreement with numerical data is very good, showing the potential for next experimental applications in predicting some very important and useful quantities, such as the extrusion force, which is critical for a wide range of 3D printing processes. The novelty relies in the general applicability of the theoretical models to viscous flows, no matter what its rheological behaviour is.

In this paper, the dynamic of inertial capsules into microfluidic bifurcations is studied. The fluid evolution is based on the solution of the BGK - lattice Boltzmann scheme including a forcing term accounting for immersed geometries. The dynamic-Immersed Boundary forc-ing strategy is adopted for imposing no-slip boundary conditions on moving deformable or rigid structures, while, on fixed immersed geometries the Bouzidi-Firdaouss-Lallemand second-order bounce back technique is implemented. The proposed computational frame -work is employed to detail dynamics and deformation of rigid and deformable capsules traveling into a branching duct. This journey is characterized in terms of i) the cap-sule/bifurcation interaction depending on the sharpness of the branching channels junc-tion; ii) daughter branches aperture angle; iii) occlusion ratio, the ratio between capsule size and main channel diameter; iv) flowing capsules stiffness; v) number of flowing par-ticles.(c) 2022 Elsevier Inc. All rights reserved.

We propose an isogeometric collocation approach to an efficient solution of the monodomain reaction–diffusion equation. The strong formulation, compared to standard Galerkin methods, easily models tissue composed of different cell types and enables an effective discretization of the reactive term reducing the computational effort due to the integration of the cellular model.We demonstrate the capabilities of the proposed approach with several numerical examples, ranging from the propagation of a planar wave front, to the simulation of complex tissue activation patterns and of the propagation of an action potential in a layered tissue.

Flow-induced vibrations (FIV) of an elastically mounted circular cylinder are investigated by means of two-dimensional simulations. A mechanical coupling between cross-flow translation and rotation provides a single degree-of-freedom system in which the coupled rotational oscillations affect the fluid–structure dynamics. The structural response of this system is investigated exploring the design space spanned by reduced velocity, coupling radius and phase density ratio. The kinematic coupling introduces the rotation-induced shear layer modifications, as well as an equivalent inertia effect connected to the coupling force. Such a computational campaign is carried out by means of direct numerical simulations with immersed boundary forcing at a Reynolds number equal to 100. The investigated system exhibits the wake-body synchronisation features typical of lock-in for non-rotating cylinders. However, the kinematic coupling provides a novel FIV scenario, in which the oscillation amplitude is magnified in the locked configurations with respect to the forced rotation case. Furthermore, it is found that there a significant widening of the reduced velocity domain where the lock-in condition takes place. In view of the proposed analyses, it is determined that the coupled rotation guarantees the phase alignment between lift and displacement necessary to sustain the lock-in condition, making the oscillation amplitude grow indefinitely with the reduced velocity. This is inherently achieved due to the rotational shear layer and the added mass contribution, which prevent the exact match between oscillation frequency and system natural frequency in vacuum. The outcomes of this study might potentially lead to an innovative water energy harvester offering larger power outputs and extended optimal operating regions.

Surgical replacement of the diseased aortic valve consists in the implantation of a prosthetic heart valve (PHV), either biological or mechanical (BHV and MHV, respectively). Risks of complication have been linked to high levels of turbulence and consequent energy dissipation induced by the PHV.As helicity is an emergent feature in cardiovascular flows, deemed to impact blood flow organization, stability and the turbulent energy cascade, in this study the interplay between the production/decay of phase-averaged and turbulent kinetic energy and helicity in the presence of a BHV or MHV was investigated. Technically, direct numerical simulations of the coupled fluid–structure interaction problem were conducted using the immersed boundary method. A quantitative description of phase-averaged and fluctuating helicity, mean and turbulent kinetic energy was adopted to explore the nature of the kinetic energy vs. helicity relationship.A clear PHV-type dependence of the helicity production/decay in the downstream hemodynamics emerged, with MHVs hemodynamics presenting larger phase-averaged and fluctuating helicity than BHVs. For both heart valve types strong linear correlations were found between volume-average kinetic energy and helicity when based on phase-averaged or fluctuating quantities (Pearson's correlation coefficient r up to 0.98, p<0.001). The generation of turbulent kinetic energy or fluctuating helicity for both heart valve types was delayed with respect to the inflow waveform or the generation of both mean kinetic energy and phase-averaged helicity (up to 5.4% of the cardiac cycle). The exploration of the link between helical and turbulent hemodynamic flow features expands the current understanding of the PHV hemodynamic features associated with clinical complications, potentially translating into improvements of the design of PHVs.

The reliability of cardiovascular computational models depends on the accurate solution of the hemodynamics, the realistic characterization of the hyperelastic and electric properties of the tissues along with the correct description of their interaction. The resulting fluid-structure-electrophysiology interaction (FSEI) thus requires an immense computational power, usually available in large supercomputing centers, and requires long time to obtain results even if multi-CPU processors are used (MPI acceleration). In recent years, graphics processing units (GPUs) have emerged as a convenient platform for high performance computing, as they allow for considerable reductions of the time-to-solution. This approach is particularly appealing if the tool has to support medical decisions that require solutions within reduced times and possibly obtained by local computational resources. Accordingly, our multi-physics solver has been ported to GPU architectures using CUDA Fortran to tackle fast and accurate hemodynamics simulations of the human heart without resorting to large-scale supercomputers. This work describes the use of CUDA to accelerate the FSEI on heterogeneous clusters, where both the CPUs and GPUs are used in synergistically with minor modifications of the original source code. The resulting GPU accelerated code solves a single heartbeat within a few hours (from three to ten depending on the grid resolution) running on premises computing facility made of few GPU cards, which can be easily installed in a medical laboratory or in a hospital, thus opening towards a systematic computational fluid dynamics (CFD) aided diagnostic.

Flow through mechanical aortic valves (MAVs) has been constantly associated to higher haemolysis and platelet activation levels with respect to native valves, due to non-physiologic haemodynamic features. Both computational and experimental investigations have correlated the blood damage to augmented levels of turbulent stress downstream of MAVs. This study provides a computational estimation, drawn from high-resolution direct numerical simulations, of turbulent and fluctuating viscous stresses in three different MAV configurations, at subsequent stages of the cardiac cycle. The configurations comprise a St. Judes Medical Regent valve (SJMV), a Lapeyre-Triflo FURTIVA valve (LTFV) with three leaflets, and a SJMV with vortex generators (VGs). Non-standard configurations are expected to mitigate the mean stress level on blood constituents reducing the turbulent production. Computations are carried out by means of a finite-difference flow solver with a direct-forcing immersed boundary technique to handle fixed and moving bodies. The VGs are found to provide instabilities which corrupt the Kármán-like vortex shedding downstream of the leaflets, reducing the intensity of turbulent kinetic energy at the peak flow rate, thus lowering the local Reynolds shear stress. Conversely, the LTFV configuration provides comparable haemodynamic performance at peak flow rate but further reduced stress level in the deceleration phase. These interpretations are supported by probability density distributions from three-dimensional fields, and further corroborated by a pointwise mapping of the Taylor length scale and local energy spectra. The outcomes of this study might potentially be exploited to improve the design of new-generation MAVs, with the aim of decreasing the risk of thromboembolic complications.

Purpose The purpose of this study is to introduce an alternative construction for microfluidic micromixers, where the effect of the extruded filaments in the fused deposition modeling (FDM) technique is used to enhance mixing performance identified as a challenge in microfluidic micromixers. Design/methodology/approach A simple Y-shaped micromixer was designed and printed using FDM technique. Experimental and numerical studies were conducted to investigate the effect of the extruded filaments on the flow behavior. The effects of the extruded width (LW), distance between adjacent filaments ( b ) and filament height ( h 1 ) are investigated on the mixing performance and enhancing mixing in the fabricated devices. The performance of fabricated devices in mixing two solutions was tested at flow rates of 5, 10, 20, 40, 80 and 150 µL/min. Findings The experimental results showed that the presence of geometrical features on microchannels, because of the nature of the FDM process, can act as ridges and generate a lateral transform through the transverse movement of fluids along the groove. The results showed the effect of increasing ridge height on the transverse movement of the fluids and, therefore, chaotic mixing over the ridges. In contrast, in the shallow ridge, diffusion is the only mechanism for mixing, which confirms the numerical results. Originality/value The study presents an exciting aspect of FDM for fabrication of micromixers and enhance mixing process. In comparison to other methods, no complexity was added in fabrication process and the ridges are an inherent property of the FDM process.

We propose an isogeometric approximation of the equations describing the propagation of an electrophysiologic stimulus over a thin cardiac tissue with the subsequent muscle contraction. The underlying method relies on the monodomain model for the electrophysiological sub-problem. This requires the solution of a reaction–diffusion equation over a surface in the three-dimensional space. Exploiting the benefits of the high-order NURBS basis functions within a curvilinear framework, the method is found to reproduce complex excitation patterns with a limited number of degrees of freedom. Furthermore, the curvilinear description of the diffusion term provides a flexible and easy-to-implement approach for general surfaces. At the discrete level, two different approaches for integrating the ionic current are investigated in the isogeometric analysis framework. The electrophysiological stimulus is converted into a mechanical load employing the well-established active strain approach. The multiplicative decomposition of the deformation gradient tensor is grafted into a classical finite elasticity weak formulation, providing the necessary tensor expressions in curvilinear coordinates. The derived expressions provide what is needed to implement the active strain approach in standard finite-element solvers without resorting to dedicated formulations. Such a formulation is valid for general three-dimensional geometries and isotropic hyperelastic materials. The formulation is then restricted to Kirchhoff–Love shells by means of the static condensation of the material tensor. The purely elastic response of the structure is investigated with simple static test-cases of thin shells undergoing different active strain patterns. Eventually, various numerical tests performed with a staggered scheme illustrate that the coupled electromechanical model can capture the excitation–contraction mechanism over thin tissues and reproduce complex curvature variations.

The need for accessible and inexpensive microfluidic devices requires new manufacturing methods and materials as a replacement for traditional soft lithography and polydimethylsiloxane (PDMS). Recently, with the advent of modern additive manufacturing (AM) techniques, 3D printing has attracted attention for its use in the fabrication of microfluidic devices and due to its automated, assembly-free 3D fabrication, rapidly decreasing cost, and fast-improving resolution and throughput. Here, fused filament fabrication (FFF) 3D printing was used to create microfluidic micromixers and enhance the mixing process, which has been identified as a challenge in microfluidic devices. A design of experiment (DoE) was performed on the effects of studied parameters in devices that were printed by FFF. The results of the colorimetric approach showed the effects of different parameters on the mixing process and on the enhancement of the mixing performance in printed devices. The presence of the geometrical features on the microchannels can act as ridges due to the nature of the FFF process. In comparison to passive and active methods, no complexity was added in the fabrication process, and the ridges are an inherent property of the FFF process.

Recent developments in additive manufacturing have moved towards a new trend in material extrusion processes (ISO/ASTM 52910:2018), dealing with the direct extrusion of thermoplastic and composite material from pellets. This growing interest is driven by the reduction of costs, environmental impact, energy consumption, and the possibility to increase the range of printable materials. Pellet additive manufacturing (PAM) can cover the same applications as fused filament fabrication (FFF), and in addition, can lead to scale towards larger workspaces that cannot be covered by FFF, due to the limited diameters of standard filaments. In the first case, the process is known as micro- or mini-extrusion (MiE) in the literature, in the second case the expression big area additive manufacturing (BAAM) is very common. Several models are available in literature regarding filament extrusion, while there is a lack of modeling of the extrusion dynamics in PAM. Physical and chemical phenomena involved in PAM have high overlap with those characterizing injection molding (IM). Therefore, a systematic study of IM literature can lead to a selection of the most promising models for PAM, both for lower (MiE) and larger (BAAM) extruder dimensions. The models concerning the IM process have been reviewed with this aim: the extraction of information useful for the development of codes able to predict thermo-fluid dynamics performances of PAM extruders.