Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Monte McGlaun on 26/03/2011 16:30:03

Title: How can we better communicate fluid dynamics?
Post by: Monte McGlaun on 26/03/2011 16:30:03

Monte McGlaun asked the Naked Scientists:
   
Dave, I have a suggestion for you to illustrate with a Kitchen Science that may save lives by raising awareness of the hazards of the destructive forces of high speed winds and rapid flowing water.
 
Recent events in Japan with the tsunami highlight a need for awareness of the general population of a some basic physics of fluid motion.  I am a design engineer and use the equation for dynamic pressure to determine the loads applied to a structure by the relative motion of wind or water past an object.
 
I find that few outside engineering fields understand the nonlinear relationship of wind speed to wind pressure and, of course, water speed to pressure applied by the water.

½ Cd x rho x V^2  is the basic equation for dynamic pressure
 
Roughly speaking, a flat board 1 foot square has a drag coefficient of about 1 and will experience about 1 pound of force when faced into a 20 mph wind.  For those along our Texas coast willing to ride out a recent hurricane with 140 mph gusts, the same board would experience 7 times the wind speed but 7 squared the force or 49 pounds.  With wind gust approaching 200 mph in some conditions.  
 
But now with water.  The density of air is 1.204 kg/m^3 compared to water at the same 20C temperature, 998.2 kg/m^3.  Water is 829 times the density of air.  That would indicate that the same 1 foot square board placed in a 20 mph water stream will experience a whopping 829 pounds of force.
 
In Texas we see flash flooding of normally dry creek beds, and drivers will inevitably run their cars along roads that pass through moving waters and then be swept downstream, sometimes with tragic results.  I have long wished that weather reporters would give wind and water hazards in terms of dynamic pressure to properly warn the public.
 
Dave, would you do a Kitchen Science to demonstrate these physical realities?  Perhaps some lives will be saved by providing listeners with tools to make better judgments at time when wind and water are high.
 
Monte McGlaun
Mechanical Engineer
Abilene, Texas, USA
 
Long time listener (http://www.thenakedscientists.com/HTML/podcasts/) and one who greatly appreciates The Naked Scientists

What do you think?

Title: How can we better communicate fluid dynamics?
Post by: syhprum on 26/03/2011 17:18:01

The rapid rise in pressure is well illustrated  by the man falling sans parachute I would estimate the 85 kg man has an apparent area of about three square feet but the force of the air stream prevents him falling faster than 120 mph.
(apologies for the mixture of units)

Title: How can we better communicate fluid dynamics?
Post by: JMLCarter on 27/03/2011 22:37:32

I like the idea, pounds per square foot or kg/square meter would be a valuable complement to storm wind speeds in weather reports. I think the Beaufort scale sucks at raising public awareness because it is a log scale, rather like the richter scale. Good call.

How about starting by publishing some kind of conversion scale, gust speed to pressure etc on a web page somewhere?

Title: How can we better communicate fluid dynamics?
Post by: yor_on on 28/03/2011 06:33:20

Nice, make a interactive table where you can define your own density and wind/stream/motion. Would be cool to play with :)