1,720,990 research outputs found
Large-eddy simulations of a mixed-flow pump at off-design conditions
No full textreduced flow-rates in turbopumps produce significant unsteady phenomena, characterized by separation and back-flow. In this study an LES approach coupled with an immersed-boundary methodology is utilized to investigate the changes in the flow physics, when compared to nominal flow-rates. The present methodology has been already validated for the design case through comparison with PIV experiments in the literature. It will be shown that for a reduced flow rate (40\% of the design one) separation phenomena are generated on the suction side of the rotor blades and on the pressure side of the stator ones. Significant spanwise non-uniformity is produced in the diffuser channels, with a displacement of the flow towards the hub side and back-flow on the shroud side. The values of turbulent kinetic energy are increased by an order of magnitude at off-design conditions and the main source of turbulence is not anymore the flow from the suction side and the trailing edge of the rotor blades: most turbulence is generated now at the leading edge of the diffuser blades. The increased interaction between rotating and stationary parts implies also a stronger dependence of the flow features on the relative position between impeller and diffuser blades
A hydrodynamic stress model for simulating turbulence/particle interactions with immersed boundary methods
No full textIn this work we propose a hydrodynamic stress model that accurately reconstructs the forces on an immersed body, with application to the simulation of turbulence interacting with finite-size particles of arbitrary shape. The proposed model is local and is based on the boundary layer approximation, where the effects of surface curvature on the pressure variation are accounted for. Numerical experiments show that the model requires about one or two grid points within the boundary layer to accurately reconstruct the hydrodynamic force distribution. This reduces significantly the cost of resolving the flow field around the particles. The accuracy of the proposed model is evaluated in a variety of flows with increasing complexity. In particular, results for the flows over stationary and oscillating circular cylinders, free falling cylinders and spheres, and the interaction of turbulence with spheres and ellipsoids will be presented. (C) 2019 Elsevier Inc. All rights reserved.</p
Direct numerical simulation of the pulsatile flow through an aortic bileaflet mechanical heart valve
No full textThis work focuses on the direct numerical simulation of the pulsatile flow through a bileaflet mechanical heart valve under physiological conditions and in a realistic aortic root geometry. The motion of the valve leaflets has been computed from the forces exerted by the fluid on the structure both being considered as a single dynamical system. To this purpose the immersed boundary method, combined with a fluid-structure interaction algorithm, has shown to be an inexpensive and accurate technique for such complex flows. Several complete flow cycles have been simulated in order to collect enough phase-averaged statistics, and the results are in good agreement with experimental data obtained for a similar configuration. The flow analysis, strongly relying on the data accessibility provided by the numerical simulation, shows how some features of the leaflets motion depend on the flow dynamics and that the criteria for the red cell damages caused by the valve need to be formulated using very detailed analysis. In particular, it is shown that the standard Eulerian computation of the Reynolds stresses, usually employed to assess the risk of haemolysis, might not be adequate on several counts: (i) Reynolds stresses are only one part of the solicitation, the other part being the viscous stresses, (ii) the characteristic scales of the two solicitations are very different and the Reynolds stresses act on lengths much larger than the red cells diameter and (iii) the Eulerian zonal assessment of the stresses completely misses the information of time exposure to the solicitation which is a fundamental ingredient for the phenomenon of haemolysis. Accordingly, the trajectories of several fluid particles have been tracked in a Lagrangian way and the pointwise instantaneous viscous stress tensor has been computed along the paths. The tensor has been then reduced to an equivalent scalar using the von Mises criterion, and the blood damage index has been evaluated following Grigioni et al. (Biomech. Model Mechanobiol., vol. 4, 2005, p. 249)
A strongly coupled, embedded-boundary method for fluid–structure interactions of elastically mounted rigid bodies
Full text linkIn the present paper, an embedded-boundary formulation that is applicable to fluid-structure interaction problems is presented. The Navier-Stokes equations for incompressible flow are solved on a Cartesian grid which is not aligned with the boundaries of a body that undergoes large-angle/large-displacement rigid body motions through the fixed grid. A strong-coupling scheme is adopted, where the fluid and the structure are treated as elements of a single dynamical system, and all of the governing equations are integrated simultaneously and interactively in the time domain. A demonstration of the accuracy and efficiency of the method is given for a variety of fluid-structure interaction problems.Fil: Yang, J.. University of Maryland; Estados UnidosFil: Preidikman, Sergio. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Estructuras; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Balaras, E.. University of Maryland; Estados Unido
Computational prediction of mechanical hemolysis in aortic valved prostheses
Full text linkThe paper reports the prediction of mechanical hemolysis by three different models for the case of blood flow through aortic valved prostheses. Two of the adopted models are based on the action of instantaneous shear stress on the blood cells (stress-based), while the third accounts for the finite response time of cell deformation and relaxation (strain-based). Two aortic Dacron grafts commonly adopted in clinical practice are considered, and both are equipped with a bileaflet mechanical valve. One of the grafts reproduces the three sinuses of Valsalva, while the other is a straight tube. A direct numerical simulation approach is utilized to solve the complex fluid-structure-interaction problem and obtain detailed information of the flow patterns. To evaluate hemolysis, a large number of Lagrangian tracer particles were released at the inlet of the computational domain (upstream of the valve), and blood damage was evaluated along each trajectory for each model. We found that stress-based models predict higher levels of blood damage than the strain-based one. The same level of blood damage is observed in the two geometric configurations we considered, indicating that the adopted mechanical valve is primary risk factor for hemolysis
Direct numerical simulation of the pulsatile flow through an aortic bileaflet mechanical heart valve
No full textThis work focuses on the direct numerical simulation of the pulsatile now through a bileaflet mechanical heart valve under physiological conditions and in a realistic aortic root geometry. The motion of the valve leaflets has been computed from the forces exerted by the fluid on the structure both being considered as a single dynamical system. To this purpose the immersed boundary method, combined with a fluid structure interaction algorithm, has shown to be an inexpensive and accurate technique for such complex flows. Several complete flow cycles have been simulated in order to collect enough phase-averaged statistics, and the results are in good agreement with experimental data obtained for a similar configuration. The flow analysis, strongly relying on the data accessibility provided by the numerical simulation, shows how some features of the leaflets motion depend on the flow dynamics and that the criteria for the red cell damages caused by the valve need to be formulated using very detailed analysis. In particular, it is shown that the standard Eulerian Computation of the Reynolds stresses, usually employed to assess the risk of haemolysis, might not be adequate oil several counts: (i) Reynolds stresses are only one part of the solicitation, the other part being the viscous stresses, (ii) the characteristic scales or the two solicitations are very different and the Reynolds stresses act on lengths much larger than the red cells diameter and (iii) the Eulerian zonal assessment of the stresses completely misses the information of time exposure to the solicitation which is a fundamental ingredient for the phenomenon of haemolysis. Accordingly, the trajectories of several fluid particles have been tracked in a Lagrangian way and the pointwise instantaneous Viscous stress tensor has been computed along the paths. The tensor has been then reduced to an equivalent scalar using the von Mises criterion, and the blood damage index has been evaluated following Grigioni et al. (Biomech. Model Mechanobiol., vol. 4, 2005, p. 249)
Large–eddy simulations in mixed–flow pumps using an immersed boundary method
No full textComputations of turbulent and transitional flows in rotating machinery applications are very challenging due to complexity of the geometry, which usually consists of multiple rotating and stationary parts. The application of well-established, body-fitted methods frequently utilizes overset grids and different reference frames, which have an adverse impact on the overall accuracy and cost-efficiency of the method. In the present work we explore the feasibility of performing computations of such flows using a single reference frame and an immersed-boundary approach. In particular, we report one of the first large-eddy simulation in this class of flows, where a structured cylindrical coordinate solver with optimal conservation properties is utilized in conjunction with an immersed-boundary method. To evaluate the accuracy of the computations the results are compared to the experimental measurements in [1]. Results using the standard Smagorinsky model and the Filtered Structured Function model are presented. We demonstrate that the overall approach is well suited for the flow under consideration and the results with the more advanced subgrid scale model are in good agreement with the experiment. We also briefly discuss some of the features of the instantaneous flow dynamics, to provide a glimpse of the wealth of information that can be extracted from such computations
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