Hi! I'm learning fluid dynamics. I have a question!

OK, suppose that flow separates at the maximum thickness point of a cylinder, forming a large wake behind. Then, in the separated region, flow speed would be lower than the speed which would've been attained if the flow had been attached (right? Because flow is retarded and separates). Now, lower speed means higher pressure, and therefore it acts as a thrust instead of drag. ?????????? No way! I'm confused! What is wrong with my reasoning?

Your reasoning is only valid on a streamline - the pressure in the wake behind the cylinder will be approximately equal to the pressure where the streamlines separate from the wall - this is a low pressure/high velocity point thus the pressure in the wake will be lower than in the surrounding air.

The error in your reasoning is that you apply Bernoulli's equation between a point outside the wake, and another inside it, but here viscous effects cannot be neglected, thus making Bernoulli's theorem inapplicable. As a matter of fact , if viscous terms could be neglected thorought all the flow field, then separation would not occur, which is good, and no lift may be created when a 2D profile is set into motion from rest, which is generally not so good.

(1). The streamline before the separation point on the surface represent the surface. (2). The same streamline after the separation point will leave the surface and form an effective body shape, which is fatter than the original surface. (3). This effective body shape can be closed on the aft-portion of the surface and form a closed separation bubble. It can also form an open bubble with the separated streamline extended further downstream. (4). Since the flow inside the separation bubble or open bubble has relatively low speed and momentum, it contributes very little to the pressure change on the surface. (5). The flow outside the separated streamline will still have high speed and have strong effect on the pressure distribution. (6). It is easier to replace the region inside the separated streamline by solid body. In this way, the flow separation effectively changes the shape of the body , that is the flow is now flowing over a body with fat rear end. (7). After all, the streamline inside the bubble or the open bubble does not come from the free stream. So, the surface pressure comes from the separated streamline and transmitted through the nearly dead water region of separation bubble, with very little changes.

Also think about the well-known fact in the boundary layer theory that pressure is nearly constant across the boundary layer although the velocity changes very rapidly. Therefore, the simple reasoning based on Bernoulli's theorem, i.e. lower speed means higher pressure, does not work any more in viscous flows.