1,720,991 research outputs found
Bioparticle separation in microfluidic devices: a numerical study on the role of viscoelastic properties and inertia
Full text linkRed blood cell transport in bounded shear flow: On the effects of cell viscoelastic properties
No full textRed 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
Simulation of hypersonic rarefied flows with the immersed‐boundary method
No full textThis paper provides a validation of an immersed boundary method for computing hypersonic rarefied gas flows. The method is based on the solution of the Navier‐Stokes equation and is validated versus numerical results obtained by the DSMC approach. The Navier‐Stokes solver employs a flexible local grid refinement technique and is implemented on parallel machines using a domain‐decomposition approach. Thanks to the efficient grid generation process, based on the ray‐tracing technique, and the use of the METIS software, it is possible to obtain the partitioned grids to be assigned to each processor with a minimal effort by the user. This allows one to by‐pass the expensive (in terms of time and human resources) classical generation process of a body fitted grid. First‐order slip‐velocity boundary conditions are employed and tested for taking into account rarefied gas effect
An immersed-boundary/isogeometric method for fluid–structure interaction involving thin shells
No full textA computational framework is designed to accurately predict the elastic response of thin shells undergoing large displacements induced by local hydrodynamic forces, as well as to resolve the complex fluid pattern arising from its interaction with an incompressible fluid. Within the context of partitioned algorithms, two different approaches are employed for the fluid and structural domain. The fluid motion is resolved with a pressure projection method on a Cartesian structured grid. The immersed shell is modeled by means of a NURBS surface, and the elastic response is obtained from a displacement-based isogeometric analysis relying on the Kirchhoff–Love theory. The two solvers exchange data through a direct-forcing immersed-boundary approach, where the interpolation/spreading of the variables between Lagrangian and Eulerian grids is implemented with a Moving Least Squares approximation, which has proven to be very effective for moving boundaries. In this scenario, the isoparametric paradigm is exploited to perform an adaptive collocation of the Lagrangian markers, decoupling the local grid density of fluid and shell domains and reducing the computational expense. The accuracy of the method is verified by refinement analyses, segregating the Eulerian/Lagrangian refinement, which confirm the expected scheme accuracy in space and time. The effectiveness of the method is then validated against different test-cases of engineering and biologic inspiration, involving fundamentally different physical and numerical conditions, namely: (i) a flapping flag, (ii) an inverted flag, (iii) a clamped plate, (iv) a buoyant seaweed in a free stream. Both strong and loose coupling approaches are implemented to handle different fluid-to-structure density ratios, providing accurate results
Self-excited flapping motion of wall-mounted valvular leaflets in a three-dimensional channel flow
No full textOn the unexplored relationship between kinetic energy and helicity in prosthetic heart valves hemodynamics
No full textSurgical 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
Isogeometric mixed collocation of nearly-incompressible electromechanics in finite deformations for cardiac muscle simulations
No full textWe 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
Development of accurate fluid-structure interaction models for aerospace problems
No full textThe research program deals with fluid-structure interaction (FSI), a challenging field of engineering, with a practical application on aerospace problems such as the flutter, an instability problem due to aeroelastic excitations. The research goal is to find a suitable way to deal with flutter in such a way the two macro fields, fluid and structure, are modelled with mid to high-level accuracy. Moreover, the creation of an interface could be useful to study other problems, such as the cabin comfort for an aircraft. To do that, the research activity is firstly split in two to study in depth the structural and the fluid problem, then they are merged together through the interface analysis: the main problem is that each field has an own scale and the union of both requests a suitable modelling and analysis. After a preliminary study on the state of the art of fluid-structure interaction in literature, which forms the pillar of the research project, the structural field is analysed first. The study of the structure system is carried out on the Carrera Unified Formulation (CUF), which allows a reduced degrees of freedom model with the same accuracy of the classical Finite Element Method (FEM). Analysis on possible adaptive mesh methods is needed in order to match the proper scale at interface with fluid dynamics system. This work could be a milestone for future investigation in problems that need a mesh refinement, beyond the aeroelastic field. Then, the fluid system is studied and analysed through the Navier-Stokes equations. In particular, a Dual Time Stepping model for non-stationary Favre Average Navier-Stokes is used. More in general, considering different order of magnitude for the Reynold’s number, three different analysis could be done: Reynolds Average Navier-Stokes (RANS) for only large-scale eddies resolved and other components modelled, Large Eddy Simulation (LES) which adds the resolution of the flux of energy with respect to the previous simulation and Direct Numerical Simulation (DNS) which resolves also the dissipating eddies, so representing the best performing simulation but with very high computational cost. Finally, a complete simulation of fluid-structure interaction is performed to find the flutter velocity and study the induced vibrations from turbulent boundary layer to the aircraft cabin and/or to the rocket nose
A fluid-structure interaction method for soft particle transport in curved microchannels
Full text linkA 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
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