The distribution of forces on the surface of complex, deforming geometries isan invaluable output of flow simulations. One particular example of suchgeometries involves self-propelled swimmers. Surface forces can providesignificant information about the flow field sensed by the swimmers, and aredifficult to obtain experimentally. At the same time, simulations of flowaround complex, deforming shapes can be computationally prohibitive whenbody-fitted grids are used. Alternatively, such simulations may employpenalization techniques. Penalization methods rely on simple Cartesian grids todiscretize the governing equations, which are enhanced by a penalty term toaccount for the boundary conditions. They have been shown to provide a robustestimation of mean quantities, such as drag and propulsion velocity, but thecomputation of surface force distribution remains a challenge. We present amethod for determining flow- induced forces on the surface of both rigid anddeforming bodies, in simulations using re-meshed vortex methods and Brinkmanpenalization. The pressure field is recovered from the velocity by solving aPoisson's equation using the Green's function approach, augmented with a fastmultipole expansion and a tree- code algorithm. The viscous forces aredetermined by evaluating the strain-rate tensor on the surface of deformingbodies, and on a 'lifted' surface in simulations involving rigid objects. Wepresent results for benchmark flows demonstrating that we can obtain anaccurate distribution of flow-induced surface-forces. The capabilities of ourmethod are demonstrated using simulations of self-propelled swimmers, where weobtain the pressure and shear distribution on their deforming surfaces.
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