Wikipedia:Reference desk/Archives/Science/2023 July 23

= July 23 =

Draining water from a sink without swirling
Could we create a sink so solid and precisely crafted, up to fractions of mm, that water sinks straight down, without swirling? Or would that be like stacking 20 M&M's on top of each other? Bumptump (talk) 18:19, 23 July 2023 (UTC)


 * The construction of the sink is irrelevant. There will always be eddies that will produce a vortex. Shantavira|feed me 18:26, 23 July 2023 (UTC)


 * So, even on a non-rotating Earth, not to speak of on a flat Earth, we'll get a vortex? --Bumptump (talk) 22:01, 23 July 2023 (UTC)
 * See Coriolis force#Draining in bathtubs and toilets. {The poster formerly known as 87.81.230.195} 51.198.140.169 (talk) 00:23, 24 July 2023 (UTC)
 * Just to make things clear:  As it says in the article cited by poster 51.198.140.169, Coriolis force from the Earth's rotation is negligible with the area of a typical domestic sink.  Sinks draining in opposite directions in opposite hemispheres is an ancient urban myth that has even been taught in high school science classes - it was in my high school anyway.
 * The situation is this: It doesn't matter how smooth or precisely shaped the sink is, water going straight down without swirling is unstable if the flow rate and flow volume is sufficient.  If the flow rate down the pipe is sufficient, the Reynolds Number [Reynolds number - Wikipedia] will be high enough to create eddies/turbulence. Once an eddy has formed, the flow rate is locally increased - this will make the eddy quickly grow until it takes over the whole flow, as flow rate increases according to distance away from the drainpipe surface, in the same way as an ice skater increases spin speed by pulling her arms in.   The smaller the rotation radius, the higher the speed must be to have the same kinetic energy.
 * In symmetrical sinks, with a few cm of water you can usually change the swirl direction at will by swirling it vigorously in the direction you want with your hand. When you take your hand out, the new swirl direction will stay in place.  A fact I demonstrated to that fool of a science teacher we had at my school, thus proving Coriolis had nothing to do with it.
 * Dionne Court (talk) 02:08, 24 July 2023 (UTC)
 * Good answer, and the answer to the OPs question is in there. I was surprised to see that when filling my generator there was no vortex in the funnel. The end of the funnel rests on a filter gauze, so the flow speed is comparatively small. Greglocock (talk) 07:47, 24 July 2023 (UTC)
 * I once had the water just go straight down the drainhole in my bath without the swirling a number of years ago. I was amazed - it went down so fast. I hadn't done anything special, and despite trying I've never been able to get it to happen again. NadVolum (talk) 09:04, 24 July 2023 (UTC)


 * Another thing to consider here is to conceive of the problem as a function of the law of conservation of angular momentum. There will probably always be small eddies and vortices within the bulk of the fluid.  As the fluid drains down the smaller hole, these eddies and vortices will meet up; unless there is an exactly equal rotation among all of those little vortices to cancel out before they get to the drain, there will be a net rotation that gets intensified as the fluid gets constricted into the smaller drain (like the canonical example of the figure skater bringing their arms in during a slow turn and speeding up).  In the very very rare cases that all such tiny vortices cancel just right, you can get laminar flow down the drain (as noted above) but this is extremely rare and probably not reproducible.  In general what you get is a slight rotational imbalance, that gets intensified and intensified and intensified as the fluid becomes constricted on its way down the drain, and that's what leads to the classic drain swirl.  -- Jayron 32 13:11, 24 July 2023 (UTC)
 * The direction of the swirl is probably set by some small random initial thing, but I'm pretty definite the swirl has low angular momentum overall - and it probably is in the opposite direction to that assumed. The wate draining down the plughole will carry off the some of the fastest swirling water from the centre, so to conserve angular motion the swirl must be getting a bit of angular motion in the opposite direction. The energy for all this would come from the gravity acting on the water. The error I think that the angular motion argument has is thinking of the water as a solid bodym whereas if you put something small floating in the water you'll see it turn in the opposite direction to the swirl. Thus there is angular motion one way from the water swirling around - but each bit of water is turning around in the opposite direction.
 * What I think might be a nice simple little project for someone is an AI with a paddle to make the flow go straight down the plughole :-) It is hard to change the direction of the swirl when it is established - but it might be possible to make small movements that counteract one forming. NadVolum (talk) 16:30, 24 July 2023 (UTC)
 * I have this wonderful vision of a dozen wikipedians all sat in their baths tonight swirling water one way, then the other as the bath empties. :-) Martin of Sheffield (talk) 16:34, 24 July 2023 (UTC)
 * Didn't everybody do this at some point in their formative years? I certainly did.
 * On a related note, I imagine the tendency to vorticity could be countered by having a pattern of ripples or shallow flukes built in to the base of the basin, radiating from the drain hole. I wonder how shallow they could be while still doing so? This of course would be pointless in a domestic setting, but might be useful in some industrial context where rapid emptying might be advantageous. Is it actually employed, or am I fundamentally mistaken? {The poster formerly known as 87.81.230.195.} 51.198.140.169 (talk) 01:44, 25 July 2023 (UTC)
 * Jayron, you said "In the very very rare cases that all such tiny vortices cancel just right, you can get laminar flow down the drain (as noted above) but this is extremely rare and probably not reproducible"
 * You are completely wrong. It's not in the least bit rare. and it is completely reproducible.  You always get laminar flow when the Reynolds number is low enough.  For a given fluid of a certain density and viscosity (e.g., water) the Reynolds number rises with velocity and with pipe diameter. Therefore, non-swirl motion down a sink drain is reproducible by restricting the flow rate - either by turning down the tap or by putting in a restriction e.g., mesh in the pipe.
 * Generally, there are three ranges of how Reynolds number affects flow;- a) below a certain value turbulence cannot be sustained and the flow is always laminar;  b)  above a certain much higher number laminar flow cannot be sustained and the flow is always turbulent, which when a larger bore is terminated in a smaller bore, becomes a swirl flow; and c) a range in between where whatever flow is established remains in place.  in this range small influences (eg vibration0 can flip the flow from laminar to turbulent and vice versa.
 * I can see who does the dishes in your house, Jayron - it's not you. `Dionne Court (talk) 04:01, 25 July 2023 (UTC)
 * Thank you for the clarification. And no, that's why I produced children.  So I don't have to do dishes.  -- Jayron 32 13:16, 25 July 2023 (UTC)
 * Wouldn't it have been cheaper to buy a dish washer than this?  Bumptump (talk) 17:30, 25 July 2023 (UTC)
 * Yeah, but shopping for a dishwasher is not nearly as fun as making the children... -- Jayron 32 13:22, 26 July 2023 (UTC)
 * Not everyone has sludgy water after washing up dishes or blocks the drain! ;-) NadVolum (talk) 17:27, 25 July 2023 (UTC)