Region 4 Discussion
We can see that in region 4, the drag coefficient suddenly decreases to 0.10. What happened? How can the effects from region 3 be different to region 4? Logically, these two operations should look similar in effect, so why the sudden decrease?
The decrease arises because of one reason: once a critical Reynold’s number is reached, the boundary layer surrounding the object goes from laminar to turbulent. Since the boundary flows turbulently in this region, the fluid particles now have more velocity and momentum, which better equips these fluid particles to flow against a pressure gradient.
What we would see then that there is a narrower wake region solely because the particles are able to stay attached to the surface of the sphere longer. Furthermore, the turbulent boundary layer causes for the pressure behind the object to be higher. Combining the higher pressure behind the object, and a narrower wake region, the amount of drag the object experiences is reduced, which is why we see this sudden decrease in the coefficient of drag.