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

Sunday, October 3, 2021

The many meanings of the word "shock." A series of posts will follow as time permits.

   This is the first of a series of articles on shock waves in physical systems and, over time, it will be updated to cross-reference to a parallel set of articles by my colleague who works with human systems. I will add more posts that discuss shock waves in various other physical systems. Please stay tuned to GeologyInMotion.com in the future! Here is my colleague, Steve Moddemeyer's corresponding post.

The word "shock" has developed many meanings and uses since it's origin in about the 1560's A.D. At that time, "shock" was a military term meaning a "violent encounter of armed forces or a pair of warriors." It derives from the French word "choc" (violent attack) and the Old French "choquer" (to strike against). Additional uses have arisen through history, such as: (1) a sudden or violent mental or emotional disturbance; (2) a medical state associated with a severe depression in vital processes such as blood pressure; (3) a convulsive contraction of muscles caused by discharge of electricity through an animal body; (4) a procedure in which a hot vegetable is plunged into ice water to stop its cooking.  Or, as shown to the left, a shock of corn!


In this discussion and related ones that will follow, I use the term to describe "a sudden change in the properties of a physical system."  The physical system might be a gas, such as air or the atmosphere of a star, in which case the thermodynamic variables such as pressure, temperature, density change suddenly over a narrow region, often only a few molecules in width. Or, the physical system might be water, in which case the depth of water changes suddenly. Or, in a commonly used analog, the system might be flowing traffic, or skiers on a slope (see image), where a sudden obstruction such as an accident (or a tree in the cartoon shown here) causes the velocity and density of vehicles to change suddenly.

Shock wave theory has developed in parallel with military needs. Quantitative analyses of shock waves are relatively young. Courant and Friedrichs wrote the classic book on supersonic flow in 1948--note the date relative to WWII and the use of bombs in that war.   A still-popular volume "Physics of Shock Waves and High-Temperature Hydrodynamic Phemomena" was published by two Russians, Zel'dovich and Raizer, in 1963, and translated into English in 1966. This English translation is "only" 915 pages long!!

An example of an
air shock occurs when an airplane reaches high velocity.*** An observer on the ground experiences this as a sonic boom that quickly comes and goes. The shock appears to be moving to an observer on the ground, but to the pilot in the aircraft, it is stationary, attached to the rear of his aircraft. In the image of a supersonic jet at the left, the white area contains