Viscous boundary layer
• An originally laminar flow is affected by
the presence of the walls.
• Flow over flat plate is visualized by introducing
bubbles that follow the local fluid velocity.
• Most of the flow is unaffected by the presence
of the plate.
• However, in the region closest to the wall, the
velocity decreases to zero.
• The flow away from the walls can be treated as inviscid, and can sometimes be
approximated as potential flow.
• The region near the wall where the viscous forces are of the same order as the
inertial forces is termed the boundary layer.
• The distance over which the viscous forces have an effect is termed the
boundary layer thickness.
• The thickness is a function of the ratio between the inertial forces and the
viscous forces, i.e. the Reynolds number. As Re increases, the thickness
decreases.