Hacker News Reader

Comment by lossolo | Hacker News Reader

If you like things like fluid simulation and you would like to create real simulation then I would recommend RealFlow. It's so easy to model real life looking huge fluid simulations with physical objects in this software. You can choose to create things like foam automatically. You don't need any knowledge about programming or physics, just set the source, set your scene, click simulate and watch the magic happens. It's only hobby for me but it's great fun.

Some simple examples:

Seaplane Water Simulation: https://www.youtube.com/watch?v=R9TZNiZYhq0

Some nice features: https://www.youtube.com/watch?v=PuS1zD6kCyc

Made with realflow: https://www.youtube.com/watch?v=nnv-95w1d5A

[1] http://www.nextlimit.com/realflow/

View on HN

It's worth pointing out that particle-based fluid simulators are not physically accurate. They compute the positions of zero-volume particles in space at every frame (fluid has volume). This is convenient for performance purposes since you can evaluate each particle position mostly in parallel and the results look convincing enough for a movie or a game.

In contrast, mesh-based (and grid-based like in the article) simulators approximate the Navier-Stokes equations and converge to the exact solution as the mesh element sizes approach zero.

Mesh-based simulators solve a global system of equations in every frame. This system has variables equal in number to total mesh elements over the entire domain. Solving such a large system (especially in 3D) is very expensive. Furthermore, the size and shape of the mesh elements drastically affect the numerical stability of the solution, and generating good meshes is not a solved problem in all cases. For these reasons, we really only use mesh-based simulators when we need a high-degree of scientific accuracy.

> It's worth pointing out that particle-based fluid simulators are not physically accurate.

Do you have some references for this? I was under the impression that convergence results have been shown for purely Lagrangian approaches like SPH.

> It's worth pointing out that particle-based fluid simulators are not physically accurate.

Depends on what you mean by "particle-based". Lattice Boltzmann models, given specific constraints, can be easily shown to converge to the Navier-Stokes (and, if desired, even higher order fluid dynamics models) equations via the Chapman-Enskog expansion.

How can you show convergence when it is not shown that there is a unique solution to Navier-Stokes?

There's no contradiction. You show that what it converges to satisfies Navier-Stokes.

(Maybe there's another Navier-Stokes solution with the same initial conditions, maybe it's unique, doesn't matter!)

Reply on news.ycombinator.com