Velocity Boundary Layer or Velocity Boundary Layer Thickness:
The boundary layer is formed when the fluid is viscous, viscous fluid means it has the resistance to flow i.e viscosity. When the viscous fluid flows over the surface, the layer of fluid closer to the surface sticks to the surface, sticks means it sticks partially to the solid surface and flows, hence the layer of fluid sticks to the surface experiences the inability to flow because of that sticking [ viscous] property, it tends to reduction in velocity of that particular layer not to the all layers of fluid flow. At this point velocity is about zero.
Now first layer of fluid flow is stopped to flow partially and the second layer( the flow layer above the layer which sticks to the surface i.e 1st layer) tries to flow over the first layer. Since both layer of fluid is viscous now the second layer sticks to the first layer partially, so 2nd layer experiences the restriction to flow from the first layer.But this restriction is some what lesser than the restriction to flow between 1st layer and surface, hence the second layer has velocity greater than first layer. The phenomenon that happened between 1st and 2nd layer will occur between the layers 2and3, 3and4……..n-1 and n layers. At the end the nth layer will have greater velocity than n-1 layer. After the nth layer there would not be any restriction to flow i.e the restriction is zero, at that point the velocity will be maximum than all the layers beneath the level of nth layer.
After the nth layer, since there is no restriction to flow, the fluid layers will flow freely, hence they have maximum velocity.Now from the point at zero velocity layer to the point at 99% of free stream velocity layer there is a region, that region is called as velocity boundary layer or velocity boundary thickness.