Question

In flow over cylinders, why does the drag coefficient suddenly drop when the flow becomes turbulent?

Answer 1

For a body with an aerodynamic profile to reach a low resistance coefficient, the boundary layer around the body must remain attached to its surface for as long as possible. In this way, the wake produced becomes narrow. A high shape resistance results in a wide wake. In this type of bodies, if there is turbulence, the drag coefficient increases because the pressure drag appears.

However, in the case of cylinders, it happens that the separation point of the boundary layer will move towards to the rear of the body, which will reduce the size of the wake and there will reduce the magnitude of the pressure drag.