As a beginner for CFD modelling of plate subduction, I heard comsol to be a good software. Can anybody suggests alternatives and the demerits of using COMSOL Multiphysics? Thank you.
numerical modelling is a vast field and the tool to address your problem depends strongly on ... your problem and your approach!
Are you a master student? a PhD student with very short time to wrap up your thesis? A plumber with an intelectual interest in plate tectonics?
In short: Comsol may be an adequate tool. However, with large displacements (larger than the element volumes), you may want to avoid Finite Element method and probably better to have a look at other methods. You can find a nice description of alternative methods in the book "Introduction to Numerical Geodynamic Modelling" from Professor Taras Gerya. He sugests to use Matlab, but you can probably follow all the book simply with Python.