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Hi, I'm trying to simulate the interraction between a box and waves in the sea. Thus I'm using the material LAW37 (BIPHAS) in order to model the interface between water and air which are modeled in ALE. The problem is that I don't know how to model the boundary conditions on the transverse faces in order to avoid wave reflexions on the wall (to simplify the computation, all DoFs are fixed on the longitudinal faces). I can't use MAT/BOUND (LAW11) because it doesn't work properly with the LAW37. So i'm trying to model these boundary conditions using a material LAW37 to create an "air outlet" and a "water outlet" with the same initial values as air except for : -the liquid bulk modulus C_Liquid = 0.0 - gamma = 0.0 The computation runs well at the beginning and i get a pressure gradient in the fluid volume, so it's okay. But then an error appears : " STOP : NEGATIVE RHO=-0.1322538772563E-09 FOR 3D-ELEMENT ID=3696 ERROR TERMINATION " In the results file I can see that there is a problem with a really big velocity vector on top of the volume in the air part. I think this is linked, but I don't know how to avoid this. Moreover, to model the interface between the box and the water, I use INTER/TYPE18 but I don't know how to set the right value for the StfVal. Using the formula written in the Hyperworks Help i.e. StfVal=(rho * v² * Sel)/GAP , the box is floating in the air, and not in the water. Do you have any suggestion? Thank you very much in advance, Hereby the .rad file ALE_05a_0000.rad