Any time there is relative motion between a solid and a fluid, a small region near the surface will see a large change in velocity. This region, shown with smoke in the image above, is called the boundary layer. Here air flows from right to left over a spinning spheroid. At first, the boundary layer is laminar, its flow smooth and orderly. But tiny disturbances get into the boundary layer and one of them begins to grow. This disturbance ultimately causes the evenly spaced vortices we see wrapping around the mid-section of the model. These vortices themselves become unstable a short distance later, growing wavy before breaking down into complete turbulence. (Photo credit: Y. Kohama)
The Boundary Layer Visualized
