Physics Asked by kevin Z on December 9, 2020
I am interesting to know whether there are analytical solutions for a piston gonging like a sine wave and generated shock wave and rarefaction. How the energy change during this process and how can we use mathematic equation to describe the process. More like the N-wave in the book “supersonic flow and shock waves”. Also is there any numerical approach?
Let us first start with some definitions of parameters, in no particular order.
We know for a regular, compressive shock wave that the following are true:
The change in entropy across a compressive, hydrodynamic shock is given by: $$ Delta S = C_{v} ln lvert frac{P_{dn} rho_{dn}^{gamma}}{P_{up} rho_{up}^{gamma}} rvert tag{1} $$
The relationship between the bulk flow speed and speed of sound is given by: $$ begin{align} left( frac{U_{up}}{C_{s,up}} right)^{2} & = frac{left( gamma - 1 right) + left( gamma + 1 right) tfrac{P_{dn}}{P_{up}}}{2 gamma} tag{2a} left( frac{U_{dn}}{C_{s,dn}} right)^{2} & = frac{left( gamma - 1 right) + left( gamma + 1 right) tfrac{P_{up}}{P_{dn}}}{2 gamma} tag{2b} end{align} $$
For a rarefaction wave, we know that:
Note that there is nothing in the Rankine-Hugoniot conservation relations that say anything about a rarefaction shock wave being disallowed. However, from Equations 1, 2a, and 2b we can see that:
The following is from pages 61-62 of Zel'dovich and Raizer, [2002]:
According to the second law of thermodynamics the entropy of a substance cannot be decreased by internal processes alone, without the transfer of heat to an external medium. This shows that it is impossible for a rarefaction wave to propagate in the form of a discontinuity...
The impossibility of the existence of a rarefactiion shock wave can be explained as follows. Such a wave would propagate through the undisturbed gas with the subsonic velocity $U_{up} < C_{s,up}$... any disturbances induced by the density and pressure jumps will begin to travel to the right with the speed of sound $C_{s,up}$, and will outrun the "shock wave." After a certain time the rarefaction region will include the gas in front of the "discontinuity", and the discontinuity will simply disappear. In other words, a rarefaction shock wave is mechanically unstable...
Mechanical stability can be present only when the wave is propagated through the undisturbed fluid with supersonic speed, otherwise disturbances induced by the shock wave would penetrate the initial gas at the speed of sound, overtake the shock wave, and thus "wash out" the sharp wave front.
I am interesting to know whether there are analytical solutions for a piston gonging like a sine wave and generated shock wave and rarefaction. How the energy change during this process and how can we use mathematic equation to describe the process. More like the N-wave in the book "supersonic flow and shock waves"
If each cycle of the piston causes a compressive shock wave, then the retreat of the piston will not create a rarefaction shock wave as I discussed above. The resultant wave profile is actually one like a sawtooth wave, which does have a well defined mathematical form given by: $$ xleft(tright) = frac{A}{2} - frac{A}{pi} sum_{n=1}^{infty} left( -1 right)^{n} frac{ sin{left( n omega t right)} }{n} $$ where $A$ is the amplitude and $omega$ is the angular frequency. There are more examples in the link above but the general point is that yes, it can be described mathematically.
Answered by honeste_vivere on December 9, 2020
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