Flow of air through two disks

The situation is as shown above. Now, to explain why the bottom disk rises, I applied Bernoulli's Equation at a point just below and just above the disk.The pressure difference hence obtained should push the disk up. Even though this is definitely incorrect, as these two points are not on a streamline, I have no clue as to the correct explanation.

  • $\begingroup$ Apply Bernoulli equation to a radial streamline, with one point lying somewhere inside the gap between the disks, and another point just at the edge of the disk where the pressure is atmospheric. Using continuity you may calculate what radial speed must be at these two points. Pressure varies with radial distance so if you want to find total force you will have perform integration over the disk area. $\endgroup$ – Deep Sep 10 '16 at 4:45
  • $\begingroup$ So you're saying since it flows outwards, the pressure at the centre is higher than the periphery, which is atmospheric ? $\endgroup$ – Dhanvi Sreenivasan Sep 10 '16 at 4:47
  • $\begingroup$ Quite the opposite. Calculate and see for yourself. $\endgroup$ – Deep Sep 10 '16 at 4:50
  • $\begingroup$ Are you sure the diagram isn't upside down? $\endgroup$ – Chet Miller Sep 10 '16 at 12:09
  • $\begingroup$ This is the right side up $\endgroup$ – Dhanvi Sreenivasan Sep 10 '16 at 12:09

The air between the 2 disks is moving faster than the air below the lower disc. Moving air has a lower pressure than stationary air (Bernoulli Principle). So the pressure above the lower disk is lower than the pressure above it. Therefore there is a net upward force on this disk.


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