  We present an open-source adaptive front-tracking solver for biological
capsules in viscous flows. The membrane elastic and bending forces are solved
on a Lagrangian triangulation using a linear Finite Element Method and a
paraboloid fitting method. The fluid flow is solved on an octree adaptive grid
using the open-source platform Basilisk. The Lagrangian and Eulerian grids
communicate using an Immersed Boundary Method by means of Peskin-like
regularized Dirac delta functions. We demonstrate the accuracy of our solver
with extensive validations: in Stokes conditions against the Boundary Integral
Method, and in the presence of inertia against similar (but not adaptive)
front-tracking solvers. Excellent qualitative and quantitative agreements are
shown. We then demonstrate the robustness of the present solver in a
challenging case of extreme membrane deformation, and illustrate its capability
to simulate inertial capsule-laden flows in complex STL-defined geometries,
opening the door for bioengineering applications featuring large
three-dimensional channel structures. The source code and all the test cases
presented in this paper are freely available.
