Is unloading shock possible?

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In summary, the speed of sound in low pressure gases is independent of pressure and depends only on temperature. Increases in pressure can propagate as a shockwave, while decreases in pressure cannot. However, in fluids, the speed of sound around critical point depends on pressure along adiabats and can enable a decrease in pressure to propagate as a shockwave. This is due to the isothermal compressibility diverging to infinity at and below the critical point, while the adiabatic compressibility stays finite everywhere. Though this phenomenon is more commonly seen in dense gas dynamics, it is possible for liquids as well since they can continuously transform passing around the critical point.
  • #1
snorkack
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In low pressure gases, speed of sound is independent on pressure and depends only on temperature.

Therefore increase of pressure will propagate as shockwave - front of the wave compresses and heats the gas, allowing the rear of the wave to travel faster and pile up into a shock.

By the same cause, a decrease of pressure CANNOT propagate as shockwave in ideal gas - initial expansion cools the gas and slows down the rear of the wave, spreading out the unloading.

How about fluids?

The isothermal compressibility diverges to infinity at and below critical point. It does not and cannot get negative.

Adiabatic compressibility therefore stays finite everywhere.

How does the speed of sound around critical poind depend on pressure along adiabats?

Is there any region where speed of sound increases on expansion, enabling decrease of pressure to propagate as shockwave?
 
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  • #3
cjl said:
It looks like there are circumstances under which it is possible to get a rarefaction shock, though the example I found involves dense gas dynamics rather than liquids (which is what I'm assuming you mean when you say "fluids").

Since liquid and gas can continuously transform passing around critical point, "fluid" is a term which covers both.
 
  • #4
snorkack said:
Since liquid and gas can continuously transform passing around critical point, "fluid" is a term which covers both.

I'm fully aware of that. The reason I made the distinction is because it sounded like you were using "fluid" as something which was distinct from "gas" in the OP, when in fact there is substantial overlap. Reading the OP again, it appears that I may have simply misunderstood what you were trying to say. Regardless, the paper linked in my previous post should give an example.
 
  • #5


I can confirm that the concept of unloading shock is possible in gases and fluids. In low pressure gases, the speed of sound is independent of pressure and depends only on temperature. This means that an increase in pressure will cause a shockwave to propagate, as the front of the wave compresses and heats the gas while the rear of the wave travels faster and piles up into a shock. However, in ideal gases, a decrease in pressure cannot propagate as a shockwave because the initial expansion cools the gas and slows down the rear of the wave, spreading out the unloading.

In fluids, the isothermal compressibility diverges to infinity at and below the critical point, meaning that it cannot become negative. Adiabatic compressibility, on the other hand, stays finite everywhere. This means that the speed of sound around the critical point will depend on pressure along adiabats, and there may be regions where the speed of sound increases on expansion, enabling a decrease in pressure to propagate as a shockwave.

In conclusion, the possibility of unloading shock depends on the properties of the medium, such as its pressure and temperature. In low pressure gases, it is possible for both increases and decreases in pressure to propagate as shockwaves. However, in ideal gases and fluids near the critical point, the conditions for unloading shock are more complex and may depend on the specific properties of the medium. Further research and experimentation may be needed to fully understand and predict the occurrence of unloading shock in these systems.
 

1. Is unloading shock a real phenomenon?

Yes, unloading shock is a well-documented phenomenon in the scientific community. It occurs when an object or material is suddenly unloaded or released from a high-pressure state, causing a sudden release of stored energy that can result in damage or deformation.

2. What causes unloading shock?

Unloading shock is caused by the rapid release of internal or external pressure on a material. This can happen due to a sudden change in temperature, sudden removal of a load or constraint, or a sudden change in the surrounding environment.

3. What are the effects of unloading shock?

The effects of unloading shock can vary depending on the material and the magnitude of the shock. In some cases, it can result in damage or deformation of the material. In other cases, it can cause changes in the material's microstructure or properties.

4. Can unloading shock be controlled or prevented?

Yes, unloading shock can be controlled or prevented through careful design and engineering. This can include using materials with higher strength and ductility, designing structures to reduce stress concentrations, and implementing proper loading and unloading procedures.

5. How is unloading shock studied in the scientific community?

Unloading shock is studied through experimental testing, numerical simulations, and theoretical models. Scientists and engineers use various techniques to measure and analyze the effects of unloading shock, such as strain gauges, high-speed cameras, and X-ray diffraction.

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