In response, the flow slows down. However, the fluid in the boundary layer has already given up some momentum because of viscous losses and viscous friction, and it does not have enough momentum to overcome the retarding force. Some fluid near the wall actually reverses direction, and the flow separates. A laminar boundary layer has less momentum near the wall than a turbulent boundary layer, as shown in figure 4, because turbulence is a very effective mixing process. More importantly, turbulent transport of momentum is very effective at replenishing the near-wall momentum.
Boundary layer profiles for laminar and turbulent flow.
The boundary layer over the front face of a sphere or cylinder is laminar at lower Reynolds numbers, and turbulent at higher Reynolds numbers. It follows that, if the boundary layer of a sphere can be made turbulent at a lower Reynolds number, then the drag should also go down at that Reynolds number. This is the case, as we can show by using a trip wire. A trip wire is simply a wire located on the front face of the sphere and it introduces a large disturbance into the boundary layer. This disturbance causes an early transition to turbulence, and it effect on the size of the wake, and the total drag is quite dramatic, as shown in figure 5 .