Boundary layer separation

Imagine you are swimming in a pool and you have your arms on your sides while your feet are kicking in the water. As you move forward, water flows over your body and creates a thin layer of water around you. This layer is called the boundary layer.

Now, let's say you stop kicking your feet and start slowing down. As you slow down, the water around you starts to move slower too. But, the water that is closest to your body doesn't slow down as much as the water farther away from your body. This is because the water closest to your body experiences a lot of friction or drag, which makes it harder for it to slow down.

As you slow down even more, the water closest to your body slows down so much that it starts to pull away from your body. This is called boundary layer separation. When the boundary layer separates, a lot of turbulence is created and this can cause problems for airplanes or other objects moving through the air.

In order to prevent boundary layer separation and turbulence, engineers design the shapes of airplanes, cars and other objects to be smooth and streamlined. The smoother the shape, the less friction there is and the less likely the boundary layer is to separate.