title: Mesh Independency Analyses and Grid Density Estimation for Ventricular Assist Devices in Multiple Reference Frames Simulations
Creators: name: Lopes Jr., G. B.
name: L.C. Gómez
name: E.G.P. Bock
Publisher: name: Otto von Guericke University Library, Magdeburg, Germany
Year: 2016

description: Technische Mechanik; 36; 3; 190-198; ISSN 2199-9244
description: The evolution of mechanical circulatory supports has been essential to improve life quality for heart disease  patients. The ventricular assist devices (VAD) developments, for example, incorporated computational  engineering in its process to reach more robust devices. A concern in VAD developments still being the  dynamics of the flow inside blood pumps, and so computational fluid dynamics (CFD) has being studied and  applied to these developments processes. In CFD, many aspects become relevant and essential, especially in  turbulent flows in which numerical and physical instabilities may cause divergence or false results. In order to  reduce the numerical error and guarantee an appropriate fluid domain representation, mesh independency is  essential to reach reasonable results. In this context this work is presented, as support for grid density  estimation from precision analysis, since accuracy to real data is not numerically relevant in mesh independency  determination. Ansys Fluent was used for performing 36 simulations with different grid densities for a VAD  prototype, applying multiple reference frames (MRF) methodology. Grid densities incorporate the number of  elements in a reference frame and the geometric scale of a VAD, what creates comparison parameters for use in  other fluid flow simulations using MRF applied to VADs. In each reference frame, although, this density might  be different for optimization of computational costs and to reach trustful results. Another concern felt in the  fluid-fluid interface between the two reference frames, keeping concordance and soft transitions between the  frames meshes, aiming to reduce locally numerical error propagations. The impeller Reynolds number is around  2800 and the inlet Reynolds number is 317. Despite the low Reynolds number, turbulence may occur and  standard k-e model is applied for turbulent flow analyses. The blood is considered as Newtonian and  incompressible fluid due to velocities and fluid deformations expected, thus mean values are applied for density  and effective viscosity: 1059kg/m³ and 0,004Pa.s. Simulations are compared in global analysis, verifying the  pressure difference between inlet and outlet, and in specific analysis, observing the fluid flow behavior, aiming  for changeless. Both analyses are based on numerical precision, what means the simulations are compared  mutually, changing the grid density scenarios. Results suggest independency of mesh size from 13 million of  elements, with variance coefficient around 0,7%, and grid density parameters are purposed as references for  mesh disposal in VAD simulations in same conditions. The use of 318,43 elements per mm³ and 210,14 elements  per mm³ are sufficiently to assure mesh size independency for non-inertial (near the rotor) and inertial frames,  respectively. The purpose of this work is to reach a grid density parameter for applying in future similar  simulations and analyze mesh influence on the specific results of the blood flow inside the prototype VAD. The  goals are to reach and as conclusion a minimum grid density is purposed for VAD simulations applying MRF in  the similar or lower impeller Reynolds numbers, and probably for higher impeller Reynolds numbers as well.