The best open source alternative to COMSOL Multiphysics is Elmer. If that doesn't suit you, our users have ranked more than 50 alternatives to COMSOL Multiphysics and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to COMSOL Multiphysics are CAD Sketcher, Xcos, OpenModelica and Calculix.
COMSOL Multiphysics alternatives are mainly 3D Modelers, but if you're looking for
Circuit Simulators or CAD Software you can filter on that. Other popular filters include
Open Source and
Free. These are just examples - use the filter bar below to find more specific alternatives to COMSOL Multiphysics.