Probably the old "draw tube" trick. The flow is slow at first until the
downcomer pipe below fills up. Then the "head" or weight of water in the
"draw tube" exerts a negative pressure on the top of the drain. To help
explain this, the pressure at the bottom of the downcomer (waste pipe) is
atmospheric pressure, so the pressure at the top must be less by however
many feet of pipe are full below it, about 0.5 psi per foot.

Rod
"The Visitor" wrote in message
...
Plugged (partially)roof vent/stack.

John

AES wrote:

This is totally OT for this group -- except the group contains a lot of
obviously clever people who are professionally interested in aerodynamic
and fluid flows and pressures, pipes, valves, and the like, and maybe
someone will be entertained by the following odd bathroom sink behavior.

Our newly remodeled bathroom has a circular glass above-counter sink like
a hemispherical glass salad bowl, about 18" in diameter at the top and 7"
deep, with a push down-pop up drain plug in the bottom center, and a
pretty high capacity faucet above it. Standard elbow fitting and drain
pipe going into the wall underneath the sink and counter top.

With the drain open, turn the faucet on full force: water gets dumped
into the sink considerable faster than it can drain out and the water
level in the sink rises rapidly up to the rim, on the verge of
overflowing, in 20 or 30 seconds.

At the last second turn the faucet part way off -- down to roughly 50% of
full flow, more or less -- then trim the flow until inflow rate just
equals outflow, so the water level stays just 1/4 inch or so below the
rim. Then leave it in this steady-state condition, and wait.

For approximately *eight minutes* (by the watch) the resulting situation
remains perfectly stable, with water level hovering just below the
overflow point. Then, all of sudden, water level starts dropping. Turn
faucet back up to full flow. Water level continues dropping, keeps
dropping faster in fact, until sink is essentially empty, and the full
force input that initially caused the sink to fill now roars down the
drain with only 1/2' or so of water swirling around the drain in the
bottom. This continues as long as I want to watch.

This doesn't seem to result from just blowing some temporary clog out of
the drain: I've repeated it three times, several hours apart, with
essentially identical behavior. I'm at a loss to explain how it happens,
except to hypothesize that maybe there's some point underground and quite
a ways further down the drain where the drain pipe has a long slow rise,
then a drop, and in some way the initial slower flow has to fill the
rising section until a siphon action gets going over the top? That
doesn't really sound persuasive, however. Anyone have any other ideas?