Without cluttering these threads up with math - basically, Bernoulli’s equations are a fancy way of describing conservation of energy: Total Energy = Kinetic Energy + Potential Energy. In an “adiabatic” system - that is, when the total energy of the system is locked in, increasing the kinetic energy of a fluid (i.e. making it move) “sucks” energy out of the potential energy pool (i.e., its pressure).
But the shower problem isn’t an adiabatic system. The water not only speeds up the resting air in the shower, initially, but also heats it up.
<much math omitted here>
The amount of energy sucked out of the system to speed up air is negligible to the amount of energy deposited into it by warming. Therefore, the “Bernoulli vacuum” is overwhelmed by the increased pressure produced by heating the air. Bernoulli’s equations still work - and work fine here. But they don’t cause the airplane wing effect which caused our eyes to glaze over in high school.
BTW 
The term “suck” has a dignified & honored role in physics. Theorists in quantum gravitation coined the concept, “There is no gravity. The earth just sucks.” 
So how do you account for the same effect produced by a cold shower?
Oww! I’m not about to give it my ALL to science - I won’t stand in a cold shower.
I agree - I figure that the Bernoulli effect might be significant in a cold shower. You just don’t need to depend on it to explain airflow in a warm shower.
Haven’t got this one all figured out, yet.
Great news that intelligent people are working on this one.
When travelling I sometimes encounter the problem of a sticky, tacky, slimy, clammy, shower curtain - it really, well, sucks!
The big question for an ordinary guy like me is “How do I stop it sticking?”
Just to muddy the water a little bit (heh heh), There was an article in Discover magazine a few months ago detailing a study that shows that the Bernoulli effect is not actually at work when a plane is flying. The idea is, to get from the front of the wing to the back in the same amount of time, the air on top must move faster, which lowers the pressure, resulting in a net upward force. The question is, why would the air on top need to reach the back of the wing at the same time? Where does the energy to speed it up come from? Don’t ask me what the real mechanism at work is–I will work on finding a reference. I realize this is a tangent, but it is a pretty comical one.
Here is the link to the article:
http://www.discover.com/apr_01/featphysics.html
So I guess this is not new information. (what do you expect from a guy who reads Discover?) but it is still pretty funny to me that engineers have been using the wrong equations to design airplanes for ninety-some years.
Just to add my two cents, I think Professor Schmidt was overstating his case when he said it was solved “decisively.” This problem sounds deceptively simple, being about the common shower. But water flows and airflows in this system are really, really complex. I’d sure like to see the vortex confirmed by some empirical data.
Cecil’s column from Friday is interesting, but doesn’t address why even when someone (me) is actually in the shower, with the spray confined between myself and wall, the shower curtain still sucks in.
In my case, the total length of spray is only about 1 ft, and since the rest of the way down any vortices generated are confined between my body and the wall, how does that affect the shower curtain 3 ft away?
Moreover, the suction in my shower occurs only if I’ve got a good seal around the edges against the walls. If there is a gap between the curtain and the wall, no suction and a big draft comes in through the gap.
Maybe the answer is that there are two different things going on. Not knowing what the CFD simulation looks like, I wonder if the simulation simply showed a vortex and whether it actually showed that the vortex exerted sufficient force on the curtain to actually move it and hold it in place. Can you tell us what the velocity of the vortex is and why we don’t feel the draft from the vortex?
One further note is that in a good steamy shower, one does not see the steam swirling, implying no vortex effect, only slow motion upward, inplying chimney effect.
I’m still voting for the chimney effect.
To ProfFink:
It is about Energy. However, when humans observe systems we find it very difficult to directly measure Energy. We can easily measure pressure and velocity, so that is what we base our descriptions on. There is also the Property specifics of a system that need to be addressed as well when dealing with these systems. If we started out with just energy descriptions we would have little idea what was physically going on since energy has non-specific generic units that can be attributed to many physical properties, pressure, velocity, acceleration, heat, etc. So, a generic energy description, mathematically or otherwise, would give us little insight into what is going on in a system. But, you are right, it does have to do with Energy.
To the Coanda Turn Coats:
The Wing-Lift problem is far from being solved. This link provides a pro Coanda insight with a bit of a conservative cautionary note: http://www.amasci.com/wing/airfoil.html
Here is a small quote from the article:
Lets look at a couple of physical examples where the Bernoulli Effect is the only culprit and one where the “Attack Angle” theory is the only one to blame.
First we can observe the lifting power of the Bernoulli Effect by simply blowing on a piece of paper laying on a table. The breath of air increases the velocity of the air above the paper. This increase lowers the pressure and raises the paper off the table. No Coanda effect here, yet the paper if lifted off the table.
Now then, lets look at the Attack Angle theory - note: the Coanda Effect is negligible in this example. Water skiing is a great sport, but how is it that a ski can hold up a person. The answer is Attack Angle. The Ski deflects water downward causing a force that lifts the Skier up. Simple.
I agree with the author of the above mentioned article that it is both that cause a wing to lift and let us fly. However, it is important to note that the Bernoulli Equations are all that are needed to accurately describe the property values of the system. It is very interesting to me that though many people say that Bernoulli is wrong that they turn right around and use his incorrect mathematical description to correctly describe the end result, Lift.
I just performed an experiment in my bathroom that, while not resolving the problem, definitely casts doubt on Cecil’s theory.
First, the apparatus: I live in a somewhat old house, and thus my shower is an old freestanding bathtub with a curtain suspended from the ceiling, surrounding the whole tub (takes three standard-size curtains to go around). Each curtain overlaps the next quite a bit, so while there isn’t a seal as such, there isn’t very much air going between them. At the head of the tub is the faucet with the tube leading from the shower head rising from it.
Squatting outside the tub and curtains, I put the shower on full-blast, all hot water. The curtains, as expected, were sucked inwards, flapping six to eight inches from the side of the tub. Then I shut off the hot water, and ran an all-cold shower full-blast. There was a slight sucking effect, about an inch or two.
As has been noted here, though, the blast of water usually gets interrupted by a person, so I strung up a board to block the water flow, about a foot from the shower head, and repeated the experiment. There was less suckage for the hot water—about five or six inches—but about the same for cold water—a little over an inch.
In all cases (board or not, hot or cold), there was no curtain effect if I opened a substantial gap in the curtain.
Thus I conclude that the dominant effect is, in fact, the chimney effect due to the air heated by the hot shower rising, and being replaced from underneath the curtain. The reduction in the effect when the board was in place was (I speculate) due to the hot water spending less time and less surface area to the air, thus heating it less.
There clearly is some other effect at work, though, because the cold water also had slight effect. That effect was not decreased significantly by blocking the stream, which indicates to me that it isn’t a vortex effect; it may just be simple air currents stirred up by the shower. It certainly doesn’t have the power of the full, hot-water curtain effect.
This was exactly the thought that crossed my mind as I was fighting off the curtain this morning. I was picturing the posited vortex in my mind and it occurred to me that it should be located at right about the same place as my head. I have to say, it did not fill me with confidence.
I always love a scrap between the dedicated advocates of different perspectives, but, sad to say, the “Coanda Effect” and the “Bernoulli Principle” are one and the same. It’s like the difference between APPLES and apples.
The “Coanda Effect” can’t happen without air behaving Bernoulli’s laws :p. I won’t unload the explanation upon y’all unless you want it.
You can review the Coanda effect at http://www.spartechsoftware.com/reeko/Experiments/ExpCandleBlowAroundCan.htm
And thanks BIG to blahedo. How often elegant theories cannot overcome simple facts (to paraphrase an early 20th century French neurologist - I forgot his name).
Think of the “chimney effect” as a vortex, too - but one that RISES in the shower, spreads ACROSS the bathroom, SINKS outside the bathroom, and SCOOTS INTO the shower through that one-way valve, the shower curtain. This effect is greatly enhanced by warm showers, and diminished with cold showers (with the shivering, and the chilling, and the discomfort of personal body parts…
) Couple that in with the good UMass professor’s in-the-shower paddlewheel vortex, and you got yourself a yucky shower curtain stuck to your legs. :eek:
That oughta solve things.
So, two of the greatest questions of our time have to do with fluid flow. When, oh when will there be a great structural behavior mystery for me to solve for Uncle Cecil?
[sup]Or heat transfer, but I’m more of an expert in structural analysis.[/sup]
–Strainger, eagerly awaiting his moment in the spotlight.
Regarding Bernoulli and airplanes, the problem is not that Bernoulli is wrong, the problem is that the traditional Bernoulli explanation is wrong. It relies on an erroneous premise - that the air over the top of the wing must match up with the air stream under the wing. This is wrong. It does not.
What happens is the bottom of the wing deflects air downward, and the coanda effect also deflects air downward - the two combine to deflect air downward. So why does the Bernoulli equation work? Because the deflected air generates a vortex around the wing, with slow air in front/below and fast air behind/above - exactly what Bernoulli’s equation uses. Bernoulli’s equation is applicable, just not for the reason usually stated in these descriptions.
Sorry to bump this thread, but I came in with the intention of asking about the direction of the air, and saw that it had been asked already.
But without response. (excellent use of widdershins!!)
Would it flow in the same direction as water down a drain (clcocwise/anticlockwise depending on your hemispherical persuasion) Or would shower position come into it?
And as regards admitting to owning a shower curtain, I have a free standing folding shower door that also affected by this phenomenon.
TwistofFate said:
You are the victim of a myth. Water draining from tubs/sinks/showers is NOT controlled by Coriolis acceleration. Coriolis acceleration is the effect of the Earth rotating, but it is very tiny for objects smaller than hurricanes. It is inconsequetial as compared to other effects, such as the shape of the container and drain, the flow of water induced during filling, currents generated by standing in the tub, etc. And no, the air will not follow that effect, either.
A particular sham running in some places near the equator involves “demonstrating” coriolis. The person takes a small tub or bucket. He walks a few yards north of the “equator”, and water drains one direction. He then walks a few yards the other side of the “equator”, and water drains the reverse direction. This is a sham that is induced by the method by which the person handles the bucket. He rotates his body consistently to cause the proper fluid rotation. It can be demonstrated anywhere, and thus does not require the actual equator be anywhere nearby. Thus the demonstrations may be done anywhere in the country that is known to cross the equator, and you won’t know they aren’t actually at the equator or not.
Note that it may be possible to demonstrate Coriolis with a small bucket or tub, but it requires letting the water sit for some lengthy time, say a day or so, to allow all internal currents to damp out.
so the Simpsons (Bart vs. Australia) lied to me?
NOOOOOOOOOOO!!!
I would imagine there are several factors involved here. One that no one has brought up is air density. In spite of the high humidity introduced, the heated air is less dense than the external cold air. And although the vortex theory is plasible, it does not (forgive me) hold water.
My own empirical studies show that once the air inside the shower is heated, that the curtain will push in even after turning off the shower. The chimney effect would only account for the bottom of the curtain pushing in, not the middle.
Therefore I propose that:
-
The primary force acting on the curtain is the decreased air pressure inside the shower.
-
The chimney effect is a secondary cause limited to the bottom of the current.
-
The vortex effect is very small, and cannot be a factor at all when a body (i.e. mine, 6’1", 300lbs) interrupts the flow of water.
QED
David Schmidt has been awarded the 2001 Ig Nobel Prize for Physics for “his partial
solution to the question of why shower curtains billow inwards!”
Icefalcon wrote:
I thought I was going to be the first one to post the “humid air is lighter” theory to explain why a cold shower also pulls the curtain in, but you beat me to it. But I still have an issue with your description. #1 and #2 are the same thing. The chimney effect happens because of decreased pressure.
Warm, humid air is less dense, so it rises and pulls the curtain in. But cold, humid air is also less dense, so it pulls the curtain in too, although not as much.
The vortex theory can’t explain a very important fact. As the bathroom air warms up and becomes more humid, the curtain ceases to billow in. This should shoot it down as a viable explanation.
I propose that someone perform an experiment (my house, which has 3.5 “baths,” has only one bathtub, and it has sliding doors instead of a curtain). Using cigarette smoke, note the airflow going under the curtain, going out over the curtain, and going in/out if you leave a gap on the sides, observing that for both high and low. The chimney effect would predict inflow under the curtain, some low on the sides, and outflow above the curtain and high on the sides.
The lame vortex theory would predict very little flow, since the low-pressure center pulls the curtain in, but all around the edges, it would be near the high-pressure outer vortex. If you have a spare curtain, a hole cut in the center would have inflow, and that air would flow out around all four sides of the curtain.