At sufficiently high velocity, the flow of viscous fluid undergoes a transition from laminar to turbulent conditions. Under normal conditions, the drag force resisting motion is proportional to the velocity but under turbulent conditions, the drag force resisting motion is proportional to the square of the velocity.
The onset of turbulence is determined by the Reynolds’ number R e = vr /
For fluid flow in a tube or channel
For the motion of an object relative to a fluid.
if Re < 2000 , the fluid flowing in the tube or channel is steady. If Re > 2000 , the fluid flow in the tube or channel is turbulent