While ago I was studying drag coefficients (or pressure coefficients, as we call them in the context of wind loads in structural engineering) and I wondered how can they be more than 1, for example a small cylinder can have a drag coefficient of 1.15 according to Wikipedia. This seemed weird to me, since pressure upon a surface is caused by the fact that the presence of the surface brings moving fluid to rest, therefore applying force to the fluid, which in turn causes pressure to the surface according to Newton's third law. Wikipedia explains that for positive pressure, a drag coefficient of 1 would mean that all of the fluid coming into the front of the surface is brought to rest, and so all the momentum of the fluid is transferred to the object. In real surfaces, some fluid "escapes" over the sides of the object, continuing to move forward, keeping their momentum. Therefore not all of the momentum of the fluid is transferred to the object and the drag coefficient is less than 1. So this would mean that for drag coefficient over 1, we would need extra momentum from somewhere!
The explanation (from Wikipedia) lies in the fact that there is suction on the other side of the object, so the total pressure can be more than the momentum which the fluid transfers to the object. I'm curious, how does this mechanism work? How does the moving fluid cause suction on the other side of the object? One thing that comes to mind is that the fluid escaping over the sides of the object starts to flow to the back-side of the object instead of forward, therefore slowing the fluid and to conserve momentum, there has to be more force to the object, but this would only bring the coefficient closer to 1, not over it.