n I tried the below approach based on your suggestions to see if I could replicate the results of the solver's native VOF adaption, without any modification, using UDF to see if the approach is right. I tried out the simulation for just water and air.nI created property UDFs for both density and viscosity separately, as below, which I hooked onto both the fluids (air and water) and used the VOF of the primary phase.nrho = rho2 + ((rho1 - rho2)*C_VOF(c, pt);nmu = mu2 + ((mu1 - mu2)*C_VOF(c, pt);nMy residuals have a sawtooth profile to start with and as the flow time progresses the sawtooth profile is enlarging in magnitude (the oscillations are more) and after around 1e-4 seconds, I'm starting to get reversed flow on outlet and the number of faces with reversed flow seems to grow with time and the solution sharply diverges within a span of two time steps around 3e-4 seconds. I'm running with a time step of 1e-7 seconds and when I'm running a regular VOF case without the UDF there are no issues with the convergence.nCould you please suggest?n