Star ccm

Numerical simulations of the blood flow within the HM3 with and without the AP were conducted in STAR-CCM+ (Siemens, Munich, Germany). The HM3 pump geometry and mesh were obtained from Boraschi et al. (17 (link)). The mesh contained 10 million polyhedral grid elements, including an 8-element boundary layer along the rotor, a 10-element boundary layer along the stator, and local mesh refinement where needed. Unsteady Reynolds-averaged Navier–Stokes simulations were conducted using implicit second-order temporal and spatial discretization, Menter's Shear Stress Transport (SST) k-ω turbulent modeling, dynamic time-stepping (programmed to correspond to 2° of rotation per time step), and a 10⁻⁵ convergence criterion for the residual error. Blood was considered as an incompressible Newtonian fluid (ρ = 1, 050 kg/m³ and μ = 3.5 mPa·s). For regional analysis, the pump fluid volume was divided into three parts: the inflow cannula, the pump core, and the outflow cannula (Figure 1). More detailed information on the HM3 geometry, grid independence tests, validation, and computational methods can be found in our previous studies (17 (link), 18 (link)).

The simulations were developed in the commercial CFD package STAR-CCM+ (V12, Siemens, Germany) and followed recommendations for development of cerebrovascular simulations (Berg et al. 2019 (link)). We used a combination of a polyhedral element mesh for the core of the fluid domain and 20 prism layer elements in the near wall boundary to sufficiently capture the velocity gradients and ensure accurate calculation of wall shear stress. In addition, we prescribed extrusions at the fluid boundaries equal to 11 times the boundary diameter to ensure adequate development of flow upstream and downstream of the fluid domain (Bluestein et al. 1997 (link)). Mesh core density was set proportional to local vessel diameter, and the mesh settings were prescribed as per our previously published work (Thomas et al. 2020 (link)). These settings were optimised to ensure mesh independence using the grid convergence index (Roache 1994 (link)) and used the subject with the greatest inlet velocity measurements for this optimisation. Optimal mesh size settings were deemed sufficient when the grid convergence index for wall shear stress within the CoW was found to fall below 3% (Thomas et al. 2020 (link)). Final mesh sizes ranged from 9.1 to 16.7 million cells per geometry. Further details regarding the computational mesh and extrusion specification are given in online resource 1.

The expected wall shear stress in the backward-facing step chamber was estimated by dynamical fluid simulations in a commercially available software environment (StarCCM+, Siemens). The geometrical negative of the internal channel volume was discretized into 9.63·10⁶ hexahedral cells, which were condensed around the step to ensure sufficient resolution in the step vicinity. The turbulence in the step wake was captured using a large eddy vortex model (LES) that resolves bigger structures in the interior of the flow, whereas smaller near-wall vortices were described with a simpler Reynolds-Averaged Navier-Stokes (RANS) approach. For further details of the discretization, the reader is referred to Supplementary Figures 1–3.