arb is described as 'software package designed to solve arbitrary partial differential equations on unstructured meshes using an implicit finite volume method'. There are three alternatives to arb for Windows, Linux, Mac, MATLAB and GNU Octave. The best alternative is COMSOL Multiphysics. Other great apps like arb are FEATool Multiphysics (Free Personal) and COSMOSWorks (Paid).
FEATool Multiphysics is a fully integrated Finite Element FEM CAE simulation toolbox for Matlab. With FEATool you can easily model and simulate heat transfer, fluid flow CFD, structural mechanics, chemical reactions, general PDEs, and more.