Expert: Mark Janus Date: 7/11/2008 Subject: Spool and card demonstration
Question Hi, and thanks for any help you can offer. I'm taking a fluid mechanics class, where the book mentions the demonstration where you blow air through a sewing thread spool, you put a playing card on the other side, and the card sticks to the spool. The explanation they give is that the fast air that flows between the spool and the card has lower pressure than the ambient air due to Bernoulli's equation, therefore the card sticks to the spool, however this makes no sense to me. If the air between the spool and the card has lower pressure than ambient pressure, why doesn’t the air blow in, rather than out? Also I don’t think it’s correct to say that the pressure is lower due to Bernoulli's equation, because in saying that the faster air's pressure is lower than the ambient air’s pressure through Bernoulli, we would be making the unfounded and wrong assumption that the total pressure between the streamlines of the fast air and the stagnant air is equal. It’s wrong to apply Bernoulli’s equation between the two streamlines because they have different total pressures. It makes much more sense to me that the air inside the spool has a higher total pressure and also a higher static pressure; since its static pressure is higher than outside, the air will accelerate until it exits into the ambient, at which point its static pressure reaches exactly ambient pressure but however at that point it has a higher velocity than the surrounding air due to the fact that this flow has a higher total pressure than the surrounding air. But if that is true, then why would the card stick to the spool? Is it due to another effect? Can you help me clear up my confusion? Thank you.
Answer If I understand the demonstration project, the spool is held with its axis (hole) vertical and the card is flat covering the lower hole.... blow across (not down the hole) the top of the spool and let go of the card.... the card stays because the static pressure inside the spool (hole) is lower than the ambient pressure. The pressure on the "other side" of the card is ambient pressure, the pressure inside the hole is lowered similar to the Venturi effect. The flow outside moving over the hole in effect pulls some of the air out of the hole and hence the pressure is lower inside. If you blow to much at an angle (rather than across the hole) this will not be the case, you will actually be blowing air into the hole (and increasing the pressure)...
