# G-force to pressure relationship inside a shock wave

• homedoc21k
Thread closed temporarily for Moderation...In summary, the pressure within a shock wave is quantified as a multiple of G, the gravitational constant. This one-to-one relationship is apparently simple to understand.f

#### homedoc21k

I need some help understanding shock waves, particularly the units of measure related to their pressure. Shock waves are frequently quantified as multiples of G, the gravitational constant. I need to understand how the G measurements related to the instantaneous pressure within the wave as expressed in atmospheres ("ATM"). Is it as simple as a one--to-one relationship? I.e. is the internal pressure in ATM simply equal to the G force? So, for example, if we have a 1,000G shock wave traveling in water at STP, is the instantaneous internal pressure simply 1,000 G = 1,000 atmospheres?

Never heard of a shock wave quantified as multiples of G. Can you show examples of sources using that?

berkeman
Suggest you google "[Search terms that lead to spammy websites redacted by the Mentors]" It's right there. I believe the question is valid and well-formed. Got an answer?

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Suggest you google "[Search terms that lead to spammy websites redacted by the Mentors]" It's right there. I believe the question is valid and well-formed. Got an answer?
I followed your suggestion and I got basically 3 hits on the first page of search results:
But with Google, results may vary from person to person.

So, no, I still don't have an answer because your question is most likely not as well-formed as you think it is. The fact that nobody else has answered it yet is also a hint that your question may not be valid as it is formulated right now.

Show us the link you are referring to; It will be a lot easier. Because converting G's to atm just sounds ridiculous.

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russ_watters

Suggest you google "[Search terms that lead to spammy websites redacted by the Mentors]" It's right there. I believe the question is valid and well-formed. Got an answer?

The OP may not have intended it, but the search terms he/she posted lead to lots of spammy product web pages, so that part has been redacted.

@homedoc21k -- Please post the links to the pages you want us to look at to try to understand how this is "standard" notation for Aerodynamic shock waves. Telling us to do a search with your terms makes more work for us, and can lead to the PF AI spam detector hits that we were alerted about. Thanks.

Hi, I'm an aerospace engineer who has a lot of supersonic wind tunnel experience. I've never in my career heard Gs used to characterize a shock wave. The most convenient ways to express shock strength are with a Mach number or a pressure ratio, both of which are dimensionless.

cjl, russ_watters and Delta2
OK, take a look at this
https://www.sciencedirect.com/topics/engineering/shock-measurement

Velocity Shocks
Velocity shock has two components: intensity, usually measured in g's (1 standard g = 9.80665 m/s2), and duration, measured in milliseconds. A drop from table-top height onto a hard floor can result in shock in excess of 1,000 g's over a period of about 3 milliseconds. A component in an artillery shell experiences about 16,000 g's for 12 milliseconds. Higher shock levels usually have shorter duration, perhaps fractions of milliseconds, and lower shock levels can have a duration as long as 20 milliseconds. Shocks resulting in crushing or bending of impact surfaces may last hundreds of milliseconds.

Well, what are you telling me? Is this source faulty? Or perhaps are there other measurement units used outside NASA and aerospace?

The shock waves under consideration here are fluid shock waves, so perhaps that is the source of the discrepancy. I provided only one source above, because of extant copyright restrictions imposed by the scientific journals , but I am deeply conversant with the full physics literature on the behaviour of nanobubbles related to shock waves in fluids, both with regard to their behaviour when hit by a shock waves and also with regard to the shock waves that nanobubbles generate after collapse (whether caused by an incident shock wave or by an oscillating acoustic fields above the necessary minimum pressure).

Further, there is a considerable body of knowledge related to the existence if ice phases (ice VI, ice VII etc.) which do not exist at STP but only at high pressures), and those reports related to the occurrences of these ice phases under the incidence of shock waves also universally quantify the shock waves in terms of g-forces. Reports related to the existence of these phases under static conditions refer to the pressure in their domain of existence in terms of Pascals.

So I can assure you that my question is not ill-founded and any thoughts you can provide on what I presumed would be a very simple question asked solely to verify my facts for an upcoming publication, would be greatly appreciated. I am starting to wonder whether I am overthinking this. After all, pressure is also measured in atmospheres. So, for example, the units expressed by a pressure of 10 ATM (approximately 1 MPa) is a force PER UNIT AREA, where the force is 10 g. So would it not be the case that the instantaneous pressure PER UNIT AREA inside a 10 g shock wave would be simply 10 ATM even though the 10 g force is being applied along the vector of the shock wave propagation rather than the earthward vector of gravity?

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The shock waves under consideration here are fluid shock waves, so perhaps that is the source of the discrepancy.
I'd originally moved your thread from the Thermodynamics forum to here in the Aerospace forum thinking you were asking about shock waves in gasses, but if you are asking about fluids, should I move your thread to the Mechanical Engineering forum? There are more discussions about fluids and fluid mechanics in the ME forum.

OK, take a look at this
https://www.sciencedirect.com/topics/engineering/shock-measurement

Velocity Shocks
Velocity shock has two components: intensity, usually measured in g's (1 standard g = 9.80665 m/s2), and duration, measured in milliseconds. A drop from table-top height onto a hard floor can result in shock in excess of 1,000 g's over a period of about 3 milliseconds. A component in an artillery shell experiences about 16,000 g's for 12 milliseconds. Higher shock levels usually have shorter duration, perhaps fractions of milliseconds, and lower shock levels can have a duration as long as 20 milliseconds. Shocks resulting in crushing or bending of impact surfaces may last hundreds of milliseconds.
I'd originally moved your thread from the Thermodynamics forum to here in the Aerospace forum thinking you were asking about shock waves in gasses, but if you are asking about fluids, should I move your thread to the Mechanical Engineering forum? There are more discussions about fluids and fluid mechanics in the ME forum.

berkeman
The link you provided is mechanical shock like when you drop a box on the floor and want to know if what is inside it broke. It has nothing to do with shock waves traveling through a fluid.

cjl, russ_watters, Delta2 and 1 other person
I'd originally moved your thread from the Thermodynamics forum to here in the Aerospace forum thinking you were asking about shock waves in gasses, but if you are asking about fluids, should I move your thread to the Mechanical Engineering forum? There are more discussions about fluids and fluid mechanics in the ME forum.
Gases are fluids

cjl, berkeman and Delta2
The link you provided is mechanical shock like when you drop a box on the floor and want to know if what is inside it broke. It has nothing to do with shock waves traveling through a fluid.
Can there be shock waves traveling through a solid?

G’s generally refer to acceleration, not pressure.

The second Hugoniot equation is Δpressure=ρ0CshockΔvelocity

One can approximate Δvelocity with G’s×duration.
Cshock (shock velocity, a material property determined from the EOS) is generally a function of the initial and final states (in this case Δvelocity).

So your question is nontrivial. If someone is using intensity (in G’s) and duration as a measure of pressure, it is a very specific subfield field with very specific conventions.

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Can there be shock waves traveling through a solid?
Given that solids are compressible, yes.

Delta2
Delta2
Further, there is a considerable body of knowledge related to the existence if ice phases (ice VI, ice VII etc.) which do not exist at STP but only at high pressures), and those reports related to the occurrences of these ice phases under the incidence of shock waves also universally quantify the shock waves in terms of g-forces. Reports related to the existence of these phases under static conditions refer to the pressure in their domain of existence in terms of Pascals.
Universally no.
For example: https://authors.library.caltech.edu/50677/1/jgre1889.pdf is about ice and uses the standard language of shock compression of condensed matter.

excellent and useful referene fr my work. Thank you!

Delta2