Two-dimensional flow analysis over a flat plate serves well to illustrate several key concepts in forced convection heat transfer.
The viscosity of the fluid requires that the fluid have zero velocity at the plate's surface. As a result a boundary layer exists where the fluid velocity changes from in the free stream (far from the plate) to zero at the plate. Within this boundary layer, the flow is initially laminar but can proceed to turbulence once the Reynolds Number Re of the flow is sufficiently high. The transition from laminar to turbulent for flow over a flat plate occurs in the range,