How should I determine dt in natural convection problem?
 

I have prepared a code for solving free convection cavity. My code has written based on non-uniform mesh. My question is about determining dt for solve the problem. Because of using non-uniform mesh sometimes the minimum size of mesh reach to 1e-15.
What should I do in these cases?

I defined dt = 1e-15, it worked but I had a very thin boundary layer (at the second cell) after 600000 iteration !!

What is the method for dealing with this situation? :confused:

Sorry , I'm newbie and just started to learn CFD.