Also, I may have given the misimpression that the entire effect described in the OP was due to air in the drain above the trap. I only meant to say this would explain the increase in flow rate after some time. Maybe I edited it out up above but I was thinking it didn’t make sense that the sink would empty while the faucet was still on because the change in flow rate wouldn’t be that much unless some kind of physical clog passed through. The increasing momentum of the flowing water would explain it though.
Because Drano is NASTY stuff. If it splashes on you, it can be unpleasant.
Before I put Drano down anything, I’d try snaking the drain first. They even make skinny plastic strips with teeth on them, designed specifically to go down bathroom sink drains without removing the plug (which is often a major undertaking). For a larger drain or a toilet, you could use a traditional metal snake.
One example:
Life is full of nasty stuff, both literally and figuratively. Gasoline, paint, turpentine, bleach, and drain cleaners are all nasty. The trick is to use them appropriately and not splash them on your face!
Back to the subject of the OP, I tried testing the OP’s hypothesis and it doesn’t work on any bathroom in this house. Turn the tap on full, and the water runs down the drain. I have on occasion found that water doesn’t drain as fast as it comes in, but that’s always been a sign of a blockage that calls for drain cleaner, which clears it right up.
IANAP (not a plumber) and am not making recommendations, just saying what works for me. And I did note above that my kitchen-clog disaster did result in leaking drain pipes due to standing corrosive drain cleaner. The pipes were so clogged that many of the PVC joints had to be disassembled and the pipes mechanically cleaned out, so everything got reassembled and the joints re-cemented.
i quite often use a wooden BBQ-skewer (with whittled “feathers” (think: an arrow) by me) to do the same … the longish hair catch on to the feathers and twisting and poking around gets the worst of it out …
I have seen people using long zip-ties with “DIY-feathering” doing the same …
Water is not very viscous at all, so Reynolds number is high here. When you’re dealing with water, you need tiny length scales or creeping-low velocities to achieve laminar flow. For pipe flow, expect turbulence when Re > 2900.
Using this calculator:
For a pipe ID of 1.5 inches, you hit Re = 2900 at a flow velocity of about 4 inches per second. That assumes fully developed flow in a smooth-walled pipe, several diameters downstream of any flow obstructions, like bends in the pipe or drain plug components.
Here’s my theory about what’s going on when a sink is slow to drain for the first several seconds but then drains at a satisfactorily high rate:
I don’t think a large bolus of air is being trapped in the drain stem between the P-trap and the incoming water. The flow from the faucet is almost never perfectly centered on the drain stopper, and that asymmetry pretty much guarantees that air in the stem will rise up on one side while water flows down the other side.
Instead of trapped air, I think this is all caused by the aeration of the water as it’s coming out of the faucet. When you first open the faucet, you’re not pouring water down the drain, you’re pouring a foam composed of air dispersed in a water matrix. This is low-density stuff, and while it’s dense enough to displace the air that had heretofore been hogging all the space in the drain stem, it doesn’t have enough weight to shove the P-trap water downstream.
That is, until the water in the sink finally gets deep enough so that the foam falling out of the faucet can no longer drive through it all the way down into the sink’s drain stem. at that point, the foam in the stem has stagnated, and the bubbles can now rise up out of it, resulting in a sink stem full of non-foamy water, now heavy/dense enough to displace the water in the P-trap and start emptying the accumulation of water in the sink at the expected rate.
The edit clock on my previous post expired, so I’m adding this update here. To test my theory, you’d need to introduce non-aerated water to the bottom of the sink at roughly the same flow rate that the faucet does. Removing the aerator won’t do it, because the water roars out of the faucet at high flow rates (and generally not straight down). Instead, you’d need to pour water from a jug in a stream that’s nearly tangent to the porcelain, somewhere up toward the edge of the sink, at a rate comparable to what the faucet was doing. It’ll flow down the porcelain to the drain without getting substantially aerated along the way, allowing it to fill the drain stem with non-foamy water. My expectation is that this will develop the full steady-state drain flow rate more quickly than aerated faucet water does.
Yes, the water will have a lot of air in it and entrain more as it flows down the pipe. I’m not sure how this effects the flow. Entrained air makes the water less dense and the air wants to rise in the water and counters the effect of gravity on the water. I can’t begin to evaluate that effect in a downward flow.
As I keep considering this it doesn’t seem like the air space in the drain matter much in a clean drain and trap. I’ll note that some bathroom sinks have narrow drains, sometimes only 1-1/4" diameter. I would never expect this to happen in a kitchen sink drain. But then again I never expected this to happen at all.
I have been able to replicate this phenomenon.
I tried three sinks in our house without initial success. Then I tried partially closing the plug (the plug is a metal disc that can be levered up or down). With the drain flow slightly restricted in this way, I was able to observe the phenomenon described by the OP. At first there was an accumulation of water in the basin, as the flow from the tap exceeded the flow to drain. The water level rose to a depth about 0.5-1" above the top surface of the plug. Then after about 20-30 seconds the water level dropped. The surface of the plug was now completely exposed and all of the water flowing into the basin was flowing down the drain.
Trying again immediately afterwards, I found I couldn’t repeat it. Whatever conditions had allowed the sink to drain at the end of the first experiment appeared to be still in effect, and the water just flowed away without accumulating in the basin.
So it has to be something that changed between rapidly repeated trials.
The amount of aeration was a neat hypothesis but that is the same each time. Air bubble as well.
I don’t see it that way. My interpretation is that the condition persisted from the previous trial, such that at both the end of the previous trial and the start of the new trial, the water was able to drain more freely.
Two phenomena that may be relevant to this case:
- If you try to empty a bottle of water by turning it upside down, it empties slowly and unevenly as air bubbles up through the water inside the bottle. But if you impart some spin to the water inside the bottle, it flows out much faster. Possibly the water in the basin starts to swirl around the plug, allowing it to escape more rapidly
- Airlocks in a pipe. If there is air trapped at the high point of a water-filled pipe, water won’t flow over the high point. The airlock in this case could be the “bubble” of air above the water in the trap and below the water in the basin.
We also haven’t ruled out the possibility that, rather than the flow-rate to drain increasing, the flow-rate into the basin reduces for some reason.
I guess it depends on how we understand “immediately afterwards.”
If that means while the water was still draining then your interpretations may be valid. If afterwards means the water has all drained after the tap is off. Say 30 to 60 seconds later. Then swirling water or stack of air in the pipe are not reasonable explanations. The water is drained and back to standing water and air in the pipe above and below the trap.