Archive for the 'Physics' Category

Fixing Noise in Physics Engine

Recently I have fixed one long-standing issue in the physics used for Subtransit. Previously all of the physics variables (velocity, slip and other ones) were plagued by noise – the values which one would expect to be stable were actually jittery and noisy.

One of most notable results of this was the fact that acceleration experienced by the train body was so jittery that attempting to attach camera motion to it would result in a sickening constant camera movement.

Subtransit uses a custom physics engine for handling wheel versus rails physics response (the collision and forces between rails and the train wheels) and PhysX for handling the rigid body physics. In the current version of the game, PhysX is used to calculate the secondary suspension (between the train and the bogey) and coupling between the train cars.

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Train Aerodynamics (part 1)

The metro trains experience an interesting set of aerodynamic effects in the tunnels. The train itself is used to circulate air through the tunnels and ventilation systems (hence its shape is not very aerodynamic at all, not that it matters at these low speeds). Of course, the main aerodynamic effect on the train is the drag force. On long sloped down sections of track (where the train moves without any power input) the aerodynamic drag can provide about 5 km/h of speed difference as train accelerates from 30 km/h to 80 km/h.

For Subtransit, I’m working on an aerodynamic model that would account for both drag produced by the train as it moves through the tunnel as well as additional torque force produced when train passes by a ventilation shaft. Currently, the CFD results are only preliminary, although there are a few interesting results already.

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