Viscosity
Added 2024-10-26 16:40:58 +0000 UTCThe upcoming version 0.7 adds a new simulation option - viscosity. It represents the internal friction caused by the movement of particles. Although this effect is not very important in planetary simulations, it can significantly affect the solution at smaller scales (i.e. in simulations of small asteroids) or when using materials with a much higher value of dynamic viscosity.
The SpaceSim solver implements the viscosity according to the paper
A Physically Consistent Implicit Viscosity Solver for SPH Fluids.
It's an implicit, iterative solver that uses a matrix-free version of the conjugate gradient to calculate the acceleration due to material viscosity. The implicit stepping allows a stable simulation even at higher time steps (similarly to the pressure solver of IISPH compared to the SPH solver). The downside is that the convergence can be quite slow and require a large number of iterations to converge to an acceptable error, especially when using a material with very high dynamic viscosity (>10^16 Pa·s).
Viscosity can be enabled in the simulation settings. Note that it is disabled by default to keep the same performance of the solver as in the previous versions.
