A sharp immersed boundary method based on penalisation and its application to moving boundaries and turbulent rotating flows

This paper presents an Immersed Boundary Method (IBM) for handling flows in the presence of fixed and moving solids with complex geometries. The method is based on a penalisation approach and designed to preserve the sharpness of the immersed boundaries. Corrections of the boundary conditions are implemented at the interface to improve the accuracy of the solution in comparison to first-order methods and avoid the rasterisation issue on Cartesian grids. The current IBM is developed in the OpenFOAM library (-v 2.2) and its accuracy is first verified against the Wannier flow case. It is then applied to the flow in presence of fixed and moving circular obstacles. The computational results show good agreement with equivalent standard body-conforming simulations and other high order published IBM, and demonstrate as well that improvements can be achieved by correcting the boundary conditions for both velocity and pressure on the interface. Finally, the method is assessed by reference to a realistic engineering application involving rotating flow: a single-phase mixer. In this case, the method is coupled to a DES model for turbulence modelling, and results show again good comparison with experimental results

A naturally anti-diffusive compressible two phases Kapila model with boundedness preservation coupled to a high order finite volume solver

This paper presents a two phases flow model combined with a high order finite volume solver on unstructured mesh. The solver is highly conservative and preserves the sharpness of the interface without any reconstruction. Special care has been taken for boundedness preservation, as a high order scheme does not guaranty the boundedness of the volume fraction. The efficiency of the method is demonstrated with two numerical experiments: the simple advection test and the interaction between the shock and a bubble. Although experiments have been carried out with fine mesh, it is also demonstrated that the method allows satisfactory results to be obtained with coarse mesh

A Thin Film Fluid Structure Interaction Model for the Study of Flexible Structure Dynamics in Centrifugal Pumps

This paper describes a fluid-structure interaction (FSI) model for the study of flexible cloth-like structures or the so-called rags in flows through centrifugal pumps. The structural model and its coupling to the flow solver are based on a Lagrangian formulation combining structural deformation and motion modeling coupled to a sharp interface immersed boundary model (IBM). The solution has been implemented in the open-source library OpenFOAM relying in particular on its PIMPLE segregated Navier–Stokes pressure–velocity coupling and its detached eddy simulation (DES) turbulence model. The FSI solver is assessed in terms of its capability to generate consistent deformations and transport of the immersed flexible structures. Two benchmark cases are covered and both involve experimental validation with three-dimensional (3D) structural deformations of the rag captured using a digital image correlation (DIC) technique. Simulations of a rag transported in a centrifugal pump confirm the suitability of the model to inform on the dynamic behavior of immersed structures under practical engineering conditions

Evaluation of mixing and shear stresses in High Rate Algae Ponds for different paddlewheel designs

Achieving appropriate hydrodynamic mixing in the design and operation of large-scale High Rate Algae Pond (HRAP) ponds can be a delicate task which depends on a number of sometimes competing factors. Dealing with living cells require knowledge of their sensitivity to external conditions (shear stress, light, CO2, etc.). For example, vertical mixing is used to control the amount of light reaching the micro-organisms but must account for the organism sensitivity to excessive light exposure. The design and operation of mixing systems must also consider potential damage due to excessive strain induced by hydrodynamic shear stresses. Too much mixing can generate cell damage due to the sensitivity of certain strains to shear stress. From an operational efficiency point of view, it is also important to minimize the energy required by the mixing system. In standard HRAP, mixing is often provided by horizontal axis paddlewheels whose primary role is to drive the pond circulation. This project aims to study the full mixing process in a conventional HRAP design with a view to determining the flow conditions needed improve the productivity of the systems. A pilot experiment was built in Arava demo-site to study the fish wastewater treatment with spirulina. Numerical simulations of the fluid flow in a small (3.5m x 1.5m x 0.2 m) and a long (16.5 m x 1.5 m x 0.2 m) HRAP taking into account the paddle wheel using the immersed boundary method implemented by Specklin et al. [1]. A Large Eddy Simulation (LES) model have been used to model turbulence, a multiphase-particle-in-cell (MPPIC) method was included to capture the interaction of flow and moving boundaries with inert particles used to simulate the micro-algae. The results highlight that mixing occurs mostly in the bends and the neighbourhood of the paddlewheel. There is little mixing in the middle of the long pond. The quality of mixing with different paddlewheel geometries is also studied. The effect of the geometry (aligned and non-aligned blades) has previously been investigated by Hreiz et al. [2]. They show that by using the aligned-blades configuration the mixing is enhanced. In this study, we focussed on two different paddlewheel geometries with aligned-blades. A methodology to evaluate the vertical mixing has been developed by computing the average absolute velocity in several transversal sections all along the pond. Also, the particle light/dark cycles have been characterised by following the particle trajectories. Particles are injected in the fluid flow to determine the amount of particles that receive enough light and those that stay in the dark zones. The shear stress in all the pond has also been computed. The results show that maximum shear stress occurs in the bends, the walls and in the neighbourhood of the paddlewheel. The different configurations of paddlewheels are evaluated in terms of their ability to provide a suitable vertical mixing with a proper amount of energy and with minimum shear stress. This study was financed by the European project SaltGae [3] whose goal is to develop a viable solution to treat saline wastewater. [1] M. Specklin, R. Connolly, B. Breen and Y. Delauré, A versatile immersed boundary method for pump design, 3rd international Rotating Equipment Conference (Dusseldorf), Germany (sept. 2016). [2] R. Hreiz, B. Sialve, J. Morchain, R. Escudié, J.-P. Steyer and P. Guiraud, Experimental and numerical investigation of hydrodynamics in raceway reactors used for algaculture, Chemical Engineering Journal, 250 (2014) 230-239. [3] SaltGae, Algae to treat saline wastewater, saltage.e

Very high order finite volume solver for multi component two-phase flow with phase change using a posteriori Multi-dimensional Optimal Order Detection

In this work we propose a very high-order compressible finite volume scheme with a posteriori stabilization for the computation of multi-component two-phase flow with phase change. It is based on finite volume approach using moving least squares (MLS) reproducing kernels for high order reconstruction of the Riemann states. Increased robustness is achieved by using the multi-dimensional optimal order detection (MOOD) method to get a high-accurate and low-dissipation scheme while maintaining boundedness and preventing numerical oscillations at interfaces and strong gradient zones. The properties of the proposed framework are demonstrated on classical test problems starting with convergence order verification on simple scalar advection test cases. More complex shock and more stringent tube tests with various water, steam and air concentration are then simulated and compared with available references in the literature. Finally, the ability of the proposed approach to compute multi-component flows with phase change is illustrated with the simulation of a liquid oxygen jet in gaseous hydrogen

On the assessment of Immersed Boundary Methods for fluid-structure interaction modelling: application to waste water pumps design and the inherent clogging issues

The meshing stage of a Computation Fluid Dynamics (CFD) problem is of crucial importance. In realistic engineering applications, issues arise when dealing with complex, sharp and moving boundaries, removing the possibility of automatic creation of a high quality structured mesh for instance. For Fluid-Structure Interaction (FSI) problems, the standard body-fitted meshing approaches are also limited to low structure deformation and to simple geometries. In light of these limitations, Immersed Boundary Methods (IBMs) have shown to be good alternatives for a broad range of problems. The present work sets out to build a set of numerical methods based on IBMs to simulate both the motion of rigid bodies and the transport of thin flexible solids. This research focuses on the specific area of waste water pumps. Firstly, IBMs are used to provide estimates of the hydrodynamic performances of centrifugal pumps. Secondly, this type of method is used to give a first answer regarding the characterization of the physical mechanism that leads to clogging in such pumps. The numerical tool is coupled to two different solvers for the fluid equations: (i) Navier-Stokes (NS) and (ii) Lattice-Boltzmann (LB). In the NS context, a sharp IBM based on a penalization method is developed and implemented in the open source library OpenFOAM in order to model the flow around rigid bodies. The complete model includes correction of the boundary conditions at the fluid-solid interface to improve the accuracy of the solution and coupling with turbulence models. To model the transport and the deformation of flexible structures, a diffuse penalty based IBM coupled to a solid model based on the variational derivative of the deformation energy is considered. In the LB context, the diffuse IBM available in the open source library Palabos is assessed for one-way coupling problems with rigid bodies. The latter is extended and coupled to the same solid model as above in order to study flexible structures. The capabilities and the accuracy of the two IBMs are assessed and compared with several test cases dealing with rigid bodies. For the sharp NS-IBM, numerical results of academic cases highlight the benefits brought by the corrections of the interface boundary conditions. In engineering cases, the method leads to results in good agreement with experimental data and numerical data from standard body-fitted simulations. Finally, for the IBMs aimed at modelling the flexible structures, both physical approaches compare well with previous numerical models in literature, and are giving promising results regarding the clogging mechanism

On the assessment of Immersed Boundary Methods for fluid-structure interaction modelling: application to waste water pumps design and the inherent clogging issues

The meshing stage of a Computation Fluid Dynamics (CFD) problem is of crucial importance. In realistic engineering applications, issues arise when dealing with complex, sharp and moving boundaries, removing the possibility of automatic creation of a high quality structured mesh for instance. For Fluid-Structure Interaction (FSI) problems, the standard body-fitted meshing approaches are also limited to low structure deformation and to simple geometries. In light of these limitations, Immersed Boundary Methods (IBMs) have shown to be good alternatives for a broad range of problems. The present work sets out to build a set of numerical methods based on IBMs to simulate both the motion of rigid bodies and the transport of thin flexible solids. This research focuses on the specific area of waste water pumps. Firstly, IBMs are used to provide estimates of the hydrodynamic performances of centrifugal pumps. Secondly, this type of method is used to give a first answer regarding the characterization of the physical mechanism that leads to clogging in such pumps. The numerical tool is coupled to two different solvers for the fluid equations: (i) Navier-Stokes (NS) and (ii) Lattice-Boltzmann (LB). In the NS context, a sharp IBM based on a penalization method is developed and implemented in the open source library OpenFOAM in order to model the flow around rigid bodies. The complete model includes correction of the boundary conditions at the fluid-solid interface to improve the accuracy of the solution and coupling with turbulence models. To model the transport and the deformation of flexible structures, a diffuse penalty based IBM coupled to a solid model based on the variational derivative of the deformation energy is considered. In the LB context, the diffuse IBM available in the open source library Palabos is assessed for one-way coupling problems with rigid bodies. The latter is extended and coupled to the same solid model as above in order to study flexible structures. The capabilities and the accuracy of the two IBMs are assessed and compared with several test cases dealing with rigid bodies. For the sharp NS-IBM, numerical results of academic cases highlight the benefits brought by the corrections of the interface boundary conditions. In engineering cases, the method leads to results in good agreement with experimental data and numerical data from standard body-fitted simulations. Finally, for the IBMs aimed at modelling the flexible structures, both physical approaches compare well with previous numerical models in literature, and are giving promising results regarding the clogging mechanism

The evolution of long-term pediatric ventricular assistance devices: a critical review

International audienc

Numerical study of shear-based hemolysis in aorta with left ventricular assistance device

Ventricular assistance devices (VADs) for heart failure treatment have been paid high attention among researchers for decades. However, the follow-up complications such as hemolysis and thrombosis require further optimization for this technique. Shear stress has been demonstrated to be significantly related to the hemolysis because of the rupture of red blood cells membrane with a leaking of hemoglobin in the plasma. This issue has already been investigated inside the pump of VAD, but estimations are still lacking regarding hemolysis generation in the aorta itself after VAD implantaion. Thus, the present study aims to evaluate the hemolysis in aorta through establishing the 3D numerical model of aorta with left ventricular assistance device (LVAD). Non-Newtonian Carreau model has been adopted. Comparisons of hemolysis evaluation have been made with two different mathematical models existing in literature. Moreover, the flow topology and hemodynamic variations have been studied. Different working conditions of LVAD have been considered corresponding to different heart failure severities. The results reveal a relatively low level of hemolysis risks in aorta. The thrombosis is more prone to occur in the case of severe heart failure condition.</jats:p