.
Hello Graham --
Graham C. Williams wrote:
Thank you for all the time and effort you have put into this analysis. It's much appreciated.
No worries! It did delay the release of the sch-8.9e specification (Fredrik's entry in the 1.5 liter challenge).
Graham C. Williams wrote:
The sections of audio track I tested, near the end of the run (at full power) gave me peaks at the following...
354 Hz is not a long way from your results.
As I was very tired (and getting "foggy"
), when I checked the video I only scrutinised the
earliest tics.
This certainly could account for a 3 Hz delta. I will go back to check the
entire length, section by section.
The quality is good enough for basic analysis, but don't let that stop you from making a
better recording!
Graham C. Williams wrote:
To aid my understanding of your report would you please give me a little background in the interpretation
of the Sigma scores and how they relate to the real world? Same again for the Step ratios, Please.
Sure! Just not in this post; it's dinner time here.
I will leave you with the teaser that the sigma scores
mathematically
bind pulsejets to the other main jet engine types: (sc)ramjets, turbojets (and turbofans).
Graham C. Williams wrote:
Total length of Motor: 560mm.
After sleeping on it, I realised what is needed is for you to
audit this engine: measure each segment and
then report back any variances from the drawing you provided. The acoustic model is too precise to miss
by 6-7 Hz. Historically, these "misses" have always resulted in the audited engine not matching the plans!
I have worked with many builders, some of them are quite talented; but there are always build variances!
Edit: When I suggest that you audit this motor to determine its actual ("as built") dimensions, I mean all
dimensions: lengths and diameters, especially the lengths and diameters of the induction pipes, couplers
and (
most especially) the inlet flares. Despite Larry's
opinions to the contrary, the resonant frequency of
this duct is determined by the intake lock up ratio, which is a function of induction pipe acoustic lengths.
Edit: It does appear the subjective frequency is the synthetic value (ie., a weighted average of the open
and closed pipe modes). This is an interesting result, if true; it makes the whole process worth my while!
Graham C. Williams wrote:
Your interpretation of the CC length (100mm) does not surprise me.
My model shows late heat release in the motor suppressing the operating frequency.
I don’t know if this is an artefact of the system but how does this sit with the idea of limited combustion length?
Do you have any ideas?
Any statements containing the term
heat release will not
map onto my methodology. This concept does
not have any bearing on what I do. It is not needed to
determine or fix acoustic properties, thrust, fuel
consumption,
anything. I make many
thermokinetic calculations, but they do not (can not?) disrupt duct
acoustics. Much more the other way around: the combustion events are locked to the Helmholtz mode(s).
We can discuss this subject later, if you wish.
Graham C. Williams wrote:
A great read. Thanks.
You're welcome!
Graham C. Williams wrote:
Expect many more questions.
Did I mention my upcoming vacation?!
(joke!)
Cheers,
M.