Thanks all, these are great suggestions! By the way, re- The Powder Toy - despite my comment in the question it actually seems like it's pretty realistic in ma

Thanks all, these are great suggestions! By the way, re- The Powder Toy - despite my comment in the question it actually seems like it's pretty realistic in many ways. The number of different things it models at once is impressive (interactions between substances, airflow and pressure, ambient heat and heat flow, etc) plus it has a ton of visualisation modes for seeing properties of the simulation (such as visualising temperature). Highly highly recommended - I've already spent a large amount of time in it! :)