User talk:Josephmpowers

Shock Wave
Hi Joseph. I noticed your edit to shock wave about detonation. Actually it's not totally right in that shock-induced combustion and detonation are the same phenomena. The chemical reaction only supports detonation in the case that the reaction length is short enough. This is the case when the initial gas temperature is high enough, and a strong enough shock is propogating into the mixture. In the case that these are not met, the mixture can combust behind the shock without actually driving it forward. There are rather a lot of pulse detonation engines, for example which have run into this problem. I think that it's worth mentioning the case to shock induced combustion separately, but I'll leave you to decide. If you answer here, I'll see it. Regards, AKAF 06:58, 11 May 2007 (UTC)

H AKAF. My main concern is this. It's difficult to argue that a detonation is NOT a shock-induced combustion process. It always is. And so when one tries to distinguish the two nomenclatures, it can falsely lead some to think that detonations are not shock-induced combustion. They are. In my opinion, it's the cleanest definition, when one adds the caveat that the combustion supports the shock (Detonation = combustion induced by shock in which the shock is supported by the combustion).

I am aware that the detonation community and literature often make the distinction you mention, but I have always seen it made in a way which requires an appeal to some device (e.g. a pulse detonation engine) with a geometric length scale which is extrinsic to an an actual detonation. I think a better definition should be device-independent. In forming the definition I advocate, reference to any geometry, other than the intrinsic length scales of chemical reaction zones, should be removed. That is to say, I think one ought to consider the phenomena in the context of a pure reactive material of infinite extent. Whether the detonation has a reaction zone thickness of a few millimeters in a high explosive, a few centimeters in a gas, or a few light years in a supernovae, there is always a subsonic zone following the shock in which acoustic signals from local exothermic reaction can propagate forward so as to support the shock. So when you say "short enough," it's unclear what that means. It can be QUITE long. Also, just as steady reaction zone thickness span a wide range, so do transition lengths to detonation. And these are highly problem-dependent.

I agree that some nomenclature is useful for processes you describe in which shocks are accompanied by some combustion process. These phenomena happen in devices in which the geometric length scales are not sufficiently long for a steady detonation to develop. Lastly, I might add that there was a lively discussion on this topic (with no consensus!) at the 1991 NASA Langley High Mach Number Combustion Workshop. The minutes of the workshop were published (J. Buckmaster, T. L. Jackson, and A. Kumar, eds., Combustion in High Speed Flows, Kluwer, 1994) and can be found in many technical libraries.

JMP