mesh refinement in transient pb

student

Hi I have to study the form of a buble inside a tube where remains a poiseuille flow... So for this problem I use Free surface option on and I am in transient... The problem is that I don't need a lot of cells in my tube but a lot of cells around and in my buble... Also the buble is mooving in my tube... So I would like to know if is it possible to have a mesh that is changing the size of his cells at each time steps and put plenty of cells were for example the gradient of VOF (volume of fluid because I use free surfaces) is up and very few cells were the vof gradient is low? And how to do it please? thanks for your help it would make me save a lot of time in the time analysis