Thank you so much, KarthiknHere is my dimension and how I set it up. Water initially fill up the upper part and air is in the groove. Without the CSS or CSF, it converges (1e-3) very well. After a few time step, It takes just around 5 trails then converges. The Courant number is around 0.9 and seems decreases slowly.nnWith CSF, the Courant number is like around 90. It takes much longer to converge.nnI just tried to set the surface tension coefficient to be none. And I run it for the same time, then I got a similar result (not sure if the same). nI am confused how the interface sustains if no surface tension is modeled.nBestnJianhuan