I am working on annular flow modelling in horizontal pipe. How can I describe initial volume fraction with UDF in a horizonta pipe? is someone can help me? Thanks already

You may be able to get away without a UDF. A simple way to do this is to use the "Adapt Mesh - Mark" menu option to mark a part of the mesh, and then to patch the volume fraction in that part of the mesh.

Charles solution is possible. However, I guess you will have more difficulties. First, if the initial condition is an anluar pattern the 0 velocity will produce a liquid fall from the top part of the film at wall to the bottom part. It is the velocity which makes the filme to be in the anular pattern. May be you should start with stratified flow and start increasing the velocity and leave the anular flow be produced. Other possibility is to impose an initial condition with a high velocity which is enough to keep the annular pattern but I am not sure it is going to solve the problem.