When pouring a really thick liquid–like molasses or honey–the stream changes depending on how far up the jar is held above the resulting puddle. When it’s down low close to the output puddle, the stream comes out thick and slow. If the jar is held up high, the stream comes out thin and quick. But is the actual flow volume the same in both cases? Or put another way, if I need a cup of molasses for a recipe, will the cup fill up at the same rate no matter how high the jar is above the cup?
For all most fluids yes. For very low Reynolds number fluids, maybe not.
For an ordinary fluid the jar has no knowledge of how high it is above the thing that catches the flow. But like any falling object, the longer the fluid has been falling the faster it is travelling. Hence the faster and thinner stream. But if you work out how much fluid passes a point on the stream in a given time, the value is the same no matter where you measure.
But for very viscous fluids the internal cohesion is a dominant characteristic. Honey, molasses and the like certainly count here. Now we have a situation where the mass of fluid below the jar is exerting a force on the fluid above it. So to some extent the additional height of the pour will result in more fluid leaving the jar. The more viscous the more coupling.
In the extreme you can pull something like taffy vertically upwards.
The cup doesn’t fill up any faster but the flow does some pretty amazing things, depending on the height.
Polyethylene glycol works well:
You can see that even starting as a thin stream, it gets faster as it pours, due to the stream getting fatter, which pulls harder, which makes the stream fatter yet, etc. He doesn’t vary the height in the demo, but it’s fairly clear that it wouldn’t pour nearly as well from a short distance.
I think Francis_Valghan has stated this in a technically correct way, but my anecdotal experience and minimal reading is somewhat more lay. A sufficiently dense column of fluid falling a short distance will tend to support itself and, in effect, slow down the rate of discharge.
My example is one I experience often. I have a 1200 gallon tank of water with a spigot about three feet off the ground. When I fill a bucket (of any size) I typically turn on a full flow. If my bucket is held close to the spigot the short, fatter, column of water hits the filling surface and “backs up” into the tank. If the distance is greater (say, >18 inches) the column of water has time to “stretch out” and does not back up into the tank. My bucket will fill faster if further away fom the spigot. I attribute this to molecular fluid density resisting separation and gravity not acting soon or strong enough to pull the fluid column apart.
If my tank were full of Mrs. Butterworth’s pancake syrup this effect would be more visually pronounced.
Not scientific, but I’m convinced this the explanation.
I’ve noticed this, too. If I’m pouring honey into a spoon with the jar really close, the honey in the spoon slows down the honey from the jar. I suppose the “spread rate” of the poured honey needs to be higher than the “pour rate” coming out of the jar.
Wanted to say that its the viscosity, and not the density, at play here.
For example, the density of honey is about 1.5 times that of water.
Viscosity of honey is about 2500 times that of water.