drag and flow separation explained
A flat board flying at the same speed pushes the air hard, while a drop shape slices through smoothly. The difference is the wake the flow leaves behind as it separates, and drag the speed to see how the flow splits behind the body. When the back is empty, the front pushes.
The flow wraps the front of a round body smoothly and then, at some point on the back, cannot follow the surface and separates, and behind it a slow swirling wake appears. Inside this wake the pressure is low, so a high pressure pushes from the front and a low pressure supports less from the back, and the body is dragged backward. That is pressure drag. Drag the speed and the wake grows fiercer. An empty back pulls the body.
Even with the same frontal area, the shape sets the size of the wake. Switch with a button and a bluff body separates early and makes a wide wake and a large drag, while a streamlined shape has a gentle tail, the flow stays attached to the end, the wake is almost gone, and the drag is small. That is why airplanes, fish, and cars all stretch a long tail. A long tail keeps the flow attached.
Why does the flow separate? On the back of a body the flow widens and slows and the pressure rises instead, an adverse pressure gradient. The slow layer near the wall cannot beat this uphill pressure. It stops, flows backward, and leaves the surface. Drag how steep the adverse gradient is and the separation point is pulled forward and the reversed region grows. The layer that cannot beat the uphill turns around.
The size of the drag gathers in one formula. FD = ½ ρ v² CD A, the product of the fluid density, v², the drag coefficient CD, and the frontal area A. Drag the speed v and the drag shoots up as v², and double the speed and the drag is four times. So at high speed air resistance dominates.
Why does a golf ball have dimples? A smooth ball has a laminar boundary layer that separates early and makes a wide wake and a large drag, while the dimples deliberately make the boundary layer turbulent, so the turbulent layer bears the adverse gradient better and separates later. Then the wake shrinks and the pressure drag falls a lot. Skin friction grows a little, but the wake reduction is much larger, so a dimpled ball flies farther, and when you compare wake and drag with a button the difference shows. Dimensional analysis, which ties all these flow facts into dimensionless numbers, follows.