Boundary Layer Approximations velocity boundary layer ou av a u>>v:and Thermal boundary layer OT aT' Ox The magnitude of variables in the thermal boundary layer variables (main flow direction) magnitude 61 Heat Transfer Su Yongkang School of Mechanical EngineeringHeat Transfer Su Yongkang School of Mechanical Engineering # 21 The magnitude of variables in the thermal boundary layer variables x (main flow direction) y u v t magnitude 1 1 1 x T y T      Thermal boundary layer y v y u , x v y u , x u y u ; and                u  v Velocity boundary layer Boundary Layer Approximations  