Altair Activate is described as 'software is an open & flexible tool for rapidly modeling and simulating products as multi-disciplinary systems in the form of 1D models (expressed as signal-based or physical block diagrams), optionally coupled to 3D models'. There are four alternatives to Altair Activate for Linux, Mac, Windows, Chrome OS and Online / Web-based. The best alternative is OpenModelica, which is both free and Open Source. Other great apps like Altair Activate are JModelica (Free, Open Source), Simulink (Paid) and StochSD (Free, Open Source).
StochSD (Stochastic System Dynamics) is a System Dynamics simulation software with focus on stochastics. It is a form of Continuous System Simulation (CSS) that in addition to model and simulate continuous flows between compartments, can handle transitions of discrete entities.