Thank you very much for your answer! I hooked the mass source to the air-phase. So I think I online add it to phase level continuity. What do you mean by continuity? I only know the option to hook the udf to the phase level when using the vof model.nnThe first picture should visualize the fact that density for air ist not adapted in the air filled zones since it not drops to the reference value. Also there is a pressure bild up in the cells containing only air, where I thought the udfs will supress it. The VOF picture should show the air filled and partly air filled areas, where the water is rising.nRegards and thank you!n