How to judge the velocity boundary layer thickness by numerical code? As well as we know,the definition of velocity boundary layer thickness is given as being at the point where the velocity reaches a certain percentage of the outer velocity , e.g.,99%. Well,in the numerical coding,how to realize that?How we know how many the outer velocity was? The following part ,on how we handle with the puzzle,is to be written, after your advice.

In my experience, the displacement boundary layer thickness is much easier to calculate. It's just a numerical integration.

You should find a definition in most modern fluids/heat transfer books.

A simple rule of thumb for laminar flows is delta ~ 1/sqrt(re). For most laminar flows this is a much more restrictive condition than the exact thickness of boundary layer. You can replace 1 with some constant less than 1 and also check.

Thank you for advice! Indeed the caculation of the displacement boundary layer thickness is much easier than that of the velocity boundary layer thickness.

Thank you for your expert advice! In our practice,we use the following rule of thumb during numerical caculation: (u(n+1)-u(n))/u(n+1)<0.02 (u(n+1)-u(n))/u(n+1)>0.001 u(n+1)-u(n)>1e-8

where u(n) means the velocity at the n-st point on the normal direction to the wall. That's all ,thank you!