Hi, I am trying to simulate two phase flow instability. The original CFD solver is tested okey for single phase flow, and I got a two layer couette flow simulated right, the upper layer is gas, the lower is water,they are immiscible. Now I try to give a initial disturbance to excite the instability, but the disturbance will disappear which is not right. the code solve two phase coupled. Do I need to put a bondary condition of Balance of normal stress across the interface? Can anybody here give me some suggestion? you help will be greatly appreciated Shaoping

In the absence of mass transfer, the gas and liquid phases should be explicitly coupled at the interface by continuity of velocities and shear stresses. For more info about interfacial jump conditions see:

Delhaye, J.M., Jump conditions and entropy sources in two-phase systems: Local instant formulation, Int. J. of Multiphase Flow, Vol. 1, p. 395, 1974.