[Geysers] Constrictions, bubbles, and pressure (RFK Part 2)

[Ron Keam]

Further to my earlier posting, I have now realised where and why the 
pressure drop occurs.  On side A of the tube, before the bubble 
reaches the restriction, the water of the upper part of the column 
fills the restriction.  This means that for the water above the 
bubble a given volume occupies a greater length of the column as 
compared with what would be the situation if there were everywhere a 
uniform cross section.  When the top of the bubble passes through the 
restriction, the length of the water column above it on side A 
suffers a shortening of length.  Thus the pressure exerted on the top 
of the bubble decreases significantly, this is transmitted by the air 
in the bubble to the bottom of the bubble, and hence to the rest of 
the water below the bubble, and the free surface in column B will 
therefore drop noticeably.  Thus we need consider only the 
quasi-static model I am describing, and dynamic effects which at 
first I thought might be crucial are in fact secondary and probably 
are unimportant in causing what is observed.

Ron Keam

>Here is an interesting experiment.  I believe it shows that when a bubble
>rises through a constriction in
>a geyser system, the pressure drops briefly.  If the pressure drops, the
>boiling rate should increase, and
>this might trigger an eruption.
>
>1) Obtain 4 feet of tygon tubing, surgical tubing, or similar material.
>2) Fill it with water.
>3) Get a bubble situated in the middle of the tubing section.
>4) Now hold both ends open, vertical, and next to each other.  The bubble will
>rise up one side of the U
>tube.
>5) The water level in the side containing the bubble will be highest.
>6) Pinch the tubing above the bubble so that the bubble must flow through the
>constriction on its way
>to the surface.
>7) Note that the water levels change while the bubble is flowing through the
>constriction.  The low side
>gets lower, but it rebounds once the bubble has passed through.
>
>I suspect this last observation means that the pressure dropped while the
>bubble moved through the
>constriction.  Why this happens isn't clear to me.
>
>I welcome explanations.
>
>I must get back to doing something productive.
>
>Have fun.
>
>Jeff Cross
>jacross at lamar.colostate.edu
>
>
>_______________________________________________
>Geysers mailing list
>Geysers at wwc.edu
>https://mailman.wwc.edu/mailman/listinfo/geysers

-- 


#####################
Ron Keam
The Physics Department
The University of Auckland
Private Bag 92-019
Auckland
New Zealand
Phone +64 9 373-7599 extension 87931
FAX +64 9 373-7445
EMail r.keam at auckland.ac.nz
#####################