Ansys Employee

Does it mean that the whole tube is full with water at t= 0 ? If yes then you need to implement a momentum source term which would rely on the wall effects (expansion and contraction) ( I am explicitly not using the word " surface tension" as the latter is property of liquids in contact with other fluids and not alone). If the stress at the wall is caused by temperature gradients then you can go for the Marangoni Stress B.C. If you have your own theory then you can use UDF.