2.1.5 Numerical implementation
The average bulk flow velocity was ramped up at the beginning of the simulation (within 1 s) to reduce the computational difficulties. The mechanical parameters of the biofilm were obtained from rheometer tests (see Section 2.3), including the viscosity and shear modulus. All parameters are listed in Table 1. To accommodate the incompressibility, we used traditional Lagrange mixed finite elements (Hughes, 2012). In particular, we used the quadratic triangular elements for the velocity and extra stress components, and the linear elements for the pressure discretization. To alleviate the numerical difficulty caused by the Oldroyd-B equation, we implemented the Galerkin/Least squares stabilization (Craven, Rees, & Zimmerman, 2006; Rajagopal & Das, 2016). We also performed model verification and mesh convergence tests to ensure that the results were not affected by the mesh size (see Supplementary Information SI). Based on the results, a maximum mesh size of 50 µm was chosen for this study.