I'm trying to build a boundary ODE for my time dependent heat transfer problem. In my equation, there's a term ZL(t)-y, where y is the position on y direction. What I need here is to make ZL(t)-y=0 when y>ZL(t), in other words, in this equation, we only consider the area where y<=ZL. How to make this happen? I tried using Step Function but didn't find a way, because I have to define a time location for Step function.
Thanks a lot in advance.