
214 Heat Transfer: Thermal Management of Electronics
action of viscous force reduces the uid velocity in layers near the wall and these
slower moving layers slow down uid layers above them. That is why, as shown
in Figure 9.5, the thickness of velocity boundary layer increases as ow moves
downstream the plate.
Equation 9.1 can be used to calculate viscous sheer stress if the variation of veloc-
ity U with distance from the surface, y, is known. A more practical approach that is
used in most engineering calculations is to dene a local friction coefcient, C
f
, that
relates viscous sheer stress to easily measurable quantities: