Reattachment length in a backward facing step problem
 

I’m simulating a 2D backward facing step problem in CFX.

Could someone please suggest me a way to calculate the reattachment length for this problem in CFD Post.

There should be zillions of benchmark simulations and experiments on this flow. Go to your library and do a search for them - or these days you might be able to find it with google.

Finding the reattachment length
 

Find out the Wall shear stress at the plane where the flow is reattaching. The reattachment lenght is that location where the shear stress is changing its sign from -ve to +ve.