Well, I just spent an interesting 2 hours looking at the links posted above and the sites and videos they then point to. The BBC video is extremely interesting and well done. The item I have the most trouble with is the FEA based heat transfer analysis posted by Dassault at The internal ramp: a question of temperature - Dassault Systèmes They show that the internal ramp case would have cool air circulation in the ramps that would cause a small but possibly detectable pattern on the exterior surface.
The questions this raises for me are:
1. Why do they think that there would be air circulation? This may be a lack of clarity of the definition, do they mean local convection cells, which I can accept, or a mass circulation as in air flowing through the ramp and generating a stack effect? (either up or down) If the mass stack flow, how is it getting in and out of the surface of the pyramid?
2. If the ramps are there, and they are using solar loading to detect them thermally, wouldn't a significant role be the lack of transfer through the air space into the pyramid under solar loading? This is the methodology used to detect delaminations in structures. It depends on the air space to not be as conductive as the surrounding matrix, causing a local hot spot over the the delamination under a heating transient. Now, delaminations are generally small and are certainly much smaller than the many feet of width that the ramps would have. This may be where the air circulation as a cell in the cross-section of the ramp comes into play. The question becomes whether the air circulation is sufficient to cause the heat transfer through the air space of the ramp to be greater than that through the surronding solid stone.
3. Dassault cites the "The air circulating there at a lower temperature than the exterior air should create "thermal bridges" detectable using infrared thermography". I am not sure whether I am willing to believe that the long term quasi-steady state air temperature is lower than the outside air, at least not under all conditions. The question of time lag also concerns me. With the sizes in question, did they model multiple 24 hour cycles and at what time of day or night does their image present the predicted thermal surface distribution?
4. As a pragmatic issue, would the rough exterior surface of the pyramid as it now exists, having been stripped of the limestone facing centuries ago, present a problem for the use of solar loading and IR for detection?
Overall, fascinating stuff! As some of you know, both IR and the kind of heat transfer analysis that Dassault shows are what I do, so this was particularly interesting to me. Thanks for posting it John.
Jack
Jack M. Kleinfeld, P.E.
Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
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Jack M. Kleinfeld, P.E.
Kleinfeld Technical Services, Inc.
Bronx, NY
718-884-6644
JKEngineer@KleinfeldTechnical.com
come see what we can do for you: http://www.KleinfeldTechnical.com