Boundary Layer Separation in Laminar and Turbulent Flows

[Ali Baig]

When compared to laminar flows, the fluid "sticks" with the solid surface longer in case of turbulent flows. For example, the angle of separation for flow over a circular cylinder is 80 degrees for laminar flows, and 140 degrees for turbulent flows. What is the reason?

 

[aerohead_18]

The boundary layer in laminar flow is characterized by streamlines closer to the surface of the object moving more slowly than streamlines farther away from it. Laminar streamlines are moving more or less parallel to the surface. In turbulent flow, these streamlines are moving in many different directions as well as in the overall direction of the flow, so the net velocity of any given particle (or streamline) is higher than it would be in laminar flow. A turbulent streamline will still slow down as the local flow velocity decreases, but it will do so less quickly than laminar flow because the particles within it are moving faster. So turbulent flow will retain a higher velocity closer to the surface than laminar flow, and thus will separate later.

Attached is a pic of how the velocity profiles are different for laminar and turbulent flow. You can see how the turbulent flow closer to the surface is faster than the laminar flow.