Hi, i am mech undergrad student, currently i am doing my thesis for CFD simulation of wind flow in urban area by using FLUENT. I am trying to apply logarithmic velocity profiles for my inlet boundary condition. My question is that from the formula of logarithmic wind profile, the velocity will become negative when it less than certain height due to the ln function. Do i need to use power law profiles to describe the velocity profile under certain height? Thanks. Best regards.