Particle simulations model rarefied hypersonic flow by following millions of particles as they move, collide, and react in a simulated wind tunnel.
Unsteady problems require at least an order of magnitude increase in the number of particles.
The number of particles used in a particular simulation influences the need for computer resources in two ways. First, approximately 10 words of data are required for each particle being simulated, in addition to the data required for the simulation domain. For a simulation of 10 million particles, this amounts to 100 million words of memory just for particle data. The second effect is that the amount of computer time required by a particle simulation is proportional to the number of particles times the number of time steps through which the simulation is run. This means that doubling the number of particles in a simulation increases the amount of time required by a factor of two.