This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, January 6, 2011

China's stealth jet: How does it tie in to geology? !!

The purported Chinese stealth jet, J-20. Source of photo unknown.
Reported here.
News has been circulating on the WWW that China has tested a prototype stealth jet.  "Stealth" refers to a complex set of technologies used to make these jets relatively invisible to detection, not many of which are relevant to geologic processes! However, it is being said that the Chinese aircraft resembles the US F-22 supersonic jet, and supersonic flows are relevant to geology.

U.S. Navy photo of an Air Force F-22 Rapter,
June 22, 2009 in the Gulf of Alaska.
The F-22 is a supersonic aircraft (good Wiki summary here). Its roots date back to 1981 when the USAF developed a requirement for a new and superior fighter to replace the F-15 series. The production model was unveiled in April, 1997 and it first flew in September of that year.  There is a contract to Lockheed Martin for 183 jets by the end of 2011. Export of this plane is banned.Supersonic aircraft travel at Mach 1 or higher speeds.  The Mach number is the ratio of the speed of the aircraft to the speed of sound in the medium through which it is flying, in this case, air.  At room temperature, the speed of sound in air is about 343 m/s (about 760 miles per hour)--at high altitudes where it is colder, the sound speed would be slightly lower because it depends on the square root of the temperature.

From http://www.f-16.net/f-16_forum_viewtopic-t-8338.html
When an aircraft is flying at speeds much less than the speed of sound, it creates disturbances that spread out in all directions, like ripples on a pond.  The picture on the left of the graph on the left shows a limiting case when the jet is not moving at all. As the speed of the aircraft increases, the circles become distorted into ellipses, stretched away from the direction of motion, as shown in the middle picture.  When the jet reaches Mach 1, no disturbances radiate out in front of the aircraft--one major factor in making it stealthy.  No one on the ground hears it until it is "too late".

As the aircraft accelerates beyond Mach 1 (some aircraft can attain Mach 3-4) the circles of disturbance coalesce to form shock waves that stream off the nose and wings of the craft.  When you are on the ground and hear a "sonic boom", these shocks are sweeping across you.  The illustration on the right shows the nose and wing shocks from an F-22.  For stability, the craft cannot fly above speeds that would cause the shock from the nose to intersect the wings.  The shocks from the wing have a different shape than those from the nose because the angle of the wing is different than the angle of the nose, as illustrated in the graph on the right.

How does this tie into geology?  Some fluids, such as boiling water, bubbly magma, and dusty gases have very low sound speeds (here's a pdf of a paper that I published on this phenomenon).  The sound speed of boiling water can be as low as a few meters per second--this means that a fast track athlete could potentially run at speeds greater than Mach 1 if immersed in boiling water! Shock waves would be streaming off that runner like those in the photo above of the F-22!

Another way to envision the aircraft is from the pilot's view: he's not moving, but air is moving past him at Mach 1,2,3 or 4. The same applies to geologic situations.  Imagine that a fluid that has a low sound speed is flowing past an obstacle--a rock or a ridge, for example.  Shock waves will be generated around this obstacle that make the flow field very different from that created by low speed, subsonic fluids.  This fact has been under appreciated in general in the geologic community, and only over the past few decades have studies begun that include the possibility of supersonic flows.  I discussed one observation of shock waves earlier on this blog here.  Shock waves were observed by a number of people during the eruption at Eyjafjallajokul earlier this summer, for example, this You-Tube video.

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