Gears might be one of the most important inventions ever made. But they have a problem. Without proper lubrication or with too much force you will turn that gear into a wheel. As a result, a lot of computational fluid dynamics (CFD) simulations end up testing gear boxes for how well they lubricate themselves in a bath of oil.
Unfortunately, the moving gears are hard to simulate in CFD. It traditionally needs the use of complex overlapping and rotating meshes that take a considerable amount of time to create. To simplify this method, engineers can use smoothed particle hydrodynamics (SPH) to eliminate the need of that pesky mesh.