No. I’m not talking about 6 year old boys sticking one end in their mouth and sucking.\u00a0 I’m talking about vacuuming-up old bits of uncooked spaghetti on the kitchen floor.<\/p>\n
So, on taking the vacuum cleaner over the kitchen floor at the weekend, I could see that there were a few broken strands of uncooked spaghetti to clean up, along with the usual dust and bits of cereal and apple pips and who-knows-what-else?<\/p>\n
But this is what was puzzling. While the vacuum cleaner had no problem with the dust and bits of cereal and apple pips and who-knows-what-else, it struggled with the spaghetti. It just wouldn’t suck them up, even if I carefully placed the nozzle over the spaghetti. I thought to start with that this was just because the pieces were too long and not getting through the opening of the flat floor-nozzle. So I broke them into smaller pieces. Still no good. Why is that – they aren’t that heavy – the vacuum cleaner tackles things such as\u00a0 buttons and shoelaces without issue. (The best I remember was a reel of cotton – left on the floor the vacuum cleaner did an impressive job of sucking up the thread and unwinding the reel in the process, causing a complete mess inside the hose.)<\/p>\n
So why not the spaghetti?<\/strong><\/p>\n What we have here is a case of fluid flow (the air) past a solid object (the spaghetti). The object is in the way of the fluid and so the fluid imparts a force onto the object.\u00a0 Fluid flow is characterized by the ‘Reynolds Number’, which is basically a measure of the relative importance of inertial and viscous effects. In this case the Reynolds number is moderately large. (Air moving at about 3 m\/s times spaghetti of width about 1 mm times air of density about 1 kg\/m^3 divided by viscosity of about 2 x 10^-5 kg\/(m s) gives us about 150). This means that inertial effects are most important – so we can consider the lifting of the objects from the floor to be due to the air hitting the objecting and giving it momentum, rather like the wind gives momentum to a yacht sailing directly downwind by hitting its sail.\u00a0 The size of the force depends on the ‘drag coefficient’. Drag coefficients can be different for different shaped bodies – put a flat object such as a button or a cornflake against the air stream and there’s a lot of drag – and up goes the button into the hose, but something streamlined like a sphere (or possibly a piece of spaghetti) would have less drag and it doesn’t lift so easily. At least, that’s my thinking – spaghetti is more ‘streamlined’ than a cornflake, so is harder to suck up.<\/p>\n Actually, it’s not that easy to get good numbers on this. The drag coefficient for a long cylinder (uncooked spaghetti) isn’t something that is readily quoted, possibly because of the difficulty in doing experiments with really long thin cylinders. (At low Reynolds number there also is a conceptual problem called ‘Stokes’ paradox<\/a>‘.) Also, if you do some web-searches on this, you’ll find a mess of information that may or may not be directly applicable to this problem. The Wikipedia page<\/a> leads the way in this regard. But streamlined bodies do certainly give lower drag than non-streamlined ones. So is that what is happening here?<\/p>\n I reckon the spaghetti problem is crying out to be analyzed as a science fair project.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" No. I’m not talking about 6 year old boys sticking one end in their mouth and sucking.\u00a0 I’m talking about vacuuming-up old bits of uncooked spaghetti on the kitchen floor. So, on taking the vacuum cleaner over the kitchen floor at the weekend, I could see that there were a few broken strands of uncooked […]<\/p>\n","protected":false},"author":20,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23],"tags":[36,35],"yoast_head":"\n