What is sponge zone in Palabos?

Dear all
I’m learning Palabos recently, and I felt rather confused about “sponge zone” when reading code in rectangularChannelWithCylinder3d, what is it used for ?

Hi wxc,
The sponge zones are used to prevent numerical issues at the boundaries. For example, the viscosity sponge zones consist of an strong increase of the viscosity near the boundary, which erase the fluctuations and prevent from numerical instabilities. Of course, they should not be used if you are interested in the flow characteristics at the boundary.

Thanks for your help, Philippe.