# Computational fluid dynamics, nah

Today I was working on physics type stuff again. Actually I had two issues, the first was how to combine inertia’s, that is, if you have a rotational velocity that’s then geared to another rotational velocity, what’s the total inertia only taking the first velocity into account? I’ll leave that as an exercise to the reader. The second was modelling a turbo charger. Now I have a large sample that takes into account the air pressure, temperature, chamber size, density of air and turbine dynamics. Basically it uses full computational fluid dynamics (CFD) to model a turbo charger. This is rather complicated. So what’s a turbo charger after all? Well its a turbine powered by an exhaust which drives a compressor, the output of this forces air back into the engine. So a couple of things to take into account, a turbo is basically a big wheel, this has mass and a radius so has an inertia. Inertia is rather useful as it gives a ‘willingness’ or rather lack of it, to change direction. This means it takes a while for it to spin up and spin down again. So if you drive the wheel from the potential energy of the engine (which is quite handy as when it’s not under load the potential energy is quite low, when under load it’s greater, thanks to inertia again). So if you tap off this energy then make a deduction from it based on the square of the wheel velocity (most things that offer resistance are squared, air resistance for example), use that to drive the wheel. Then use the wheel velocity to add to the torque of the engine. You then have a fairly good turbo model which has lag and boost and acts pretty much how a turbo should. And no fluids are involved.

Apart from that I walked the dog and went to Body Combat. That was fun. My paint stripper appears to have done its job. Unfortunately I still have half the room to do. Did some more Japanese. Watched ‘Celebrity Wedding Planner’ with Harry from TOWIE. He’s cute but I think I’d have to gag him.