What is an under resolved region?
 

Hello,
I'm trying to understand how a certain wave simulation program works (http://www.realflow.com/pdf/hybrido_whitepaper.pdf).

In this program splash particles are "generated automatically when an under-resolved region of the grid-based fluid is detected". In the case of the example shown in the paper it is a wave that breaks on a lighthouse.

I don't know too much about CFD. So could anyone here explain to me shortly, what is meant by "under-resolved region".

Thanks a lot!

CFD equations (such as Navier-Stokes) are solved numerically using finite volume, finite difference, or finite element methods. Each of these require that the domain in which the fluid moves is discretized into many many individual cells/elements. If the cells are too course to capture appropriate flow gradients (or waves in your case) the region is considered under-resolved. The documentation is indicating that in these regions the code switches from Eulerian (continuous) to Langrangian (particle-based) in order to adequately resolve the wave motion without blurring (clipping) the wave.