Textbook

Laminar, Turbulence, Transition and Flow Separation

Laminar Boundary Layer

For lower velocities, fluid flowing over a smooth surface that is relatively short and flat will only develop a very thin boundary layer. The flow inside the boundary layer will be smooth and orderly, meaning that the layers will basically stay in layers, without mixing. This condition is called laminar boundary layer.

Unfortunately, nature tends towards disorder, so it is rare to be able to maintain a laminar boundary layer for very long.

Turbulent Boundary Layer

As the air or liquid moves over a long, fairly flat surface, the boundary layer gets thicker, and the layers starts to mix and swirl around each other. This swirling, rolling layer is called a turbulent boundary layer. The mixing and swirling is called turbulence; if the swirling is regular and repeatable, it is called a vortex or an eddy.

Since most of the boundary layers over an airplane will tend to be turbulent, aerodynamicist try to design surfaces that minimize the amount of turbulence or disorder. They try to make the surfaces smooth and free from protrusions (like little bolts).

How does this apply to a tennis ball? The felt like covering is very rough and tends to create turbulent versus laminar (smooth) flow. Unlike a baseball's stiches that protrude on the surface of the ball, the tennis ball's seam is indented in the surface. The surface roughness of the ball will tend to mask the effects of the tennis ball seam.

Transition

The region in the boundary layer where the orderly laminar layers start to mix together, but before they really start swirling, is called the transition region. Most of the time it is a fairly small region.

Flow Separation

When a turbulent boundary really starts to swirl, the boundary layer thickness starts to grow even faster. Eventually, the flow is so mixed, it starts to flow back towards the front of the surface! When this happens, the outside, original fluid is moving over a large bubble created by the turbulence. Inside the bubble, the flow is moving back up the surface. This is called flow separation. The front of the bubble, where the outside fluid turns sharply away from the surface, is called the point of separation; the back of the bubble, where the outside fluid turns back to follow the surface again, is called the point of reattachment. If the region of flow separation extends past the surface, this region is called a wake.

You can see these different points in the pictures below.

Flow Separation Over an Airfoil

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