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-   -   Difference between shock and expansion wave? (http://www.cfd-online.com/Forums/main/15142-difference-between-shock-expansion-wave.html)

jinwon park May 2, 2008 13:06

Difference between shock and expansion wave?
 
I hope someone can define it clearly. Help to distingish clearly between them physically, theromdynamically. Thanks in advance!

amine May 2, 2008 14:45

Re: Difference between shock and expansion wave?
 
hi,even if i'm not specialist,the shock wave is a compression wave through which the thermodynamical prms increase (statical ones but actually the stagnation pressure drops),in other hand the expansion wave drop the thermodynamical parms ,both cases are made by the need of the fluid to reach a themodynamical equilibrum..books of J.D Anderson ,shapiro ,liepmn & roshko are good references.i hope that it was usefull

sray May 6, 2008 05:06

Re: Difference between shock and expansion wave?
 
Physically, shock is a region in the flow field where the air undergoes rapid compression. Because the compression is too fast, it is not isoentropic thus making the flow process ir-reverisible. The compression increases the temperature, pressure and density of the fluid at the exit of the region compared to the inlet. The irreversibility of the flow process can be measured from the increase in the total pressure or increase in entropy. As the shock layer is too thin, the characteristic length in the flow region is very small, so it is safe to neglect any heat generation or viscosity inside that. These jumps in the flow condition can be mathematically expressed with the Rankine-Hugoniot condition. Shocks as well as the expnsion fans are created because of the non-linearity in the governing equation and the initial condition. Shock usually starts with a weak compression wave, when the speed of the characteristic variables at the upwind direction of the flow is higher than the speed at the downwind direction. Non-linearity rapidly makes them much closer thus creating shock.

On the other hand expansion fan is isoentropic and occurs when the speed of the characteristic variables at the upwind side is lower than that of the downwind side. Because the speed at the inlet is lower than that of the outlet, the region stretches(either linear or angular, as in case of Prandtl-Meyer centered expansion fan) in contrast to the shock region. Isoentropic relationship remains valid inside the expansion region.

Hope this helps. For more information on this perhaps you can look at the books suggested by Jinwon.


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