# Does a disk placed in a pipe perpendicular to flow direction of fluid feel dynamic pressure or total pressure?

There are two discs in the pipe but not at the same time. You can consider conditions with X but without Y, then with Y and without X.

I am pretty sure that disc X will feel less force from fluid than disc Y does. I am looking for its reason.

Total pressures (static + dynamic) are the same for both conditions, because it's the same flow. In R1 area, dynamic pressure is low, but static is high. In R2 area, dynamic is high, but static is low, but in conclusion, total pressure is the same.

If total pressure is the same but force on X and Y is not the same, then how to justify this observation?

Does it mean that discs feel only dynamic pressure, not total? When I go for momentum equations, I find that disc Y must feel more force than X does since flow has higher velocity in R2 than in R1, which means higher momentum.

So, in this case, discs feel only dynamic pressure, I think.

• My comment? Check out D'Alembert's Paradox. Mar 27, 2023 at 8:23
• "Force on a disc" implies that the flow is slowed down by pushing against the disc? In this case, the dynamic pressures will reduce, and the static pressures will remain, and the forces will be as you expect? Mar 27, 2023 at 8:26
• No, I mean total force trying to push discs to right. Which one faces more force from fluid? Which one starts to drag to right firs? Mar 27, 2023 at 8:49
• Ok, so if the fluid isn't being stopped by the disc, then the static pressure will be present on both sides of the disc - cancelling itself out! Mar 27, 2023 at 8:55
• But dynamic pressure will be only on left side pushing them to right? Mar 27, 2023 at 9:38