Fortran Cantera: error while reading DME mechanism with floating-point exception flag

Solal Amouyal

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Oct 25, 2017, 10:23:31 AM10/25/17

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I am using cantera-2.4.0a1 so this might be a bug. I initially got this error from a very large code, it took me a while to figure out what was the issue.

I have written a very simple program in Fortran that reads a DME mechanism. If I add the floating-point exception flag -ffpe-trap=zero, the code throws an exception.

I have attached the program, mechanism and Makefile used to produce the error. Simply turn on or off the DEBUG variable in the Makefile in order to produce the error or not.

Note that I have added a test case with a Hydrogen-air mechanism which works perfectly with the flag in question.

Thanks,

DME_39species.xml

h2air.xml

main.f90

Makefile

Ray Speth

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Oct 25, 2017, 12:56:39 PM10/25/17

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Hi Solal,

This floating point exception is triggered by the use of 0 as the value for one of the falloff coefficients in one reaction (CH3 + CH3 (+ M) [=] C2H6 (+ M)). The value $T_1$ is used in a term computed as $\exp(-T/T1)$ . Since $T$ is always positive, $-T/T1$ will evaluate to $-\infty$ and the term will evaluate to 0. This is a case where the floating point rules for division by zero give exactly the desired limiting behavior, and trapping floating point exceptions leads to false identification of an error.

Regards,
Ray

Solal Amouyal

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Oct 26, 2017, 2:05:13 AM10/26/17

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Hi Ray,

Thanks for your rapid answer.

I located the reaction (#48) you mentioned in the xml file. Although not imperative, I do like to use floating-point exception when debugging my code. I changed to 0 falloff coefficients to a small value, 1e-10. Is that all right? Not being familiar with what happens under the hood, I want to make sure I do not get any unwanted effects down the line.