Bruno Ogogrelec wrote:I wonder if it's all just different semantics. You say "an attempt at making the intake content so massive that choking even occurs at the fundamental frequency of the engine" and I say "drag so great that it equals peak internal pressure". It would be interesting to compare calculated mechanical drag predictions and acoustic predictions for total choking -- how much would they differ, I wonder. Foa's picture of the pulsejet cycle is rather interesting -- acoustic behavior superimposed over thermodynamic events. Very acute. (I hope I have not paraphrased him too badly.)
No, I don't think this is just a semantic difference. Drag / resistance would be a DC phenomenon, while reactance is ONLY an AC phenomenon or property. You can look at the drag on gas motion at a particular station in a pulsejet AT A PARTICULAR INSTANT IN TIME and it will be exactly the same as it would be in a ramjet station of the same diameter and same gas condition (temperature, density and relative velocity). Similarly, changing only the DC resistance has no effect on the frequency or basic operation of a simple coil & capacitor tuned circuit. On the other hand, a mass of gas in a pipe WILL act as a "delay line" for a pressure pulse introduced at one end, just like the delay line in a color TV that delays the monochrome signal so the more heavily processed chroma signal can stay sync'd up with it. This delay line is basically just a coil of carefully calculated reactance. It literally "slips" the phase of the signal back a few degrees at a certain frequency while leaving its shape unaltered.
Viv wrote:Hey guys!
Just before you get too excited if you look back in the old forum near the beginning you will see we went down the electrical theory acoustic theory analogue back then with an off forum email group to follow it up in detail.
It was fun with people like Don and Nicolas doing a lot of work but in the end it came down to the fact the maths could not describe a known engine with any real accuracy.
but we have moved on since then so maybe worth revisiting.
It's important to understand the detail differences in the two phenomena and the ways in which they make it hard to simulate the pipe with electronics. In the fine article Mike pointed out to us, the acoustic devices (e.g. organ pipes) are very simple to describe because there is the same air condition (temp & density) throughout the whole pipe. But it is not so with our favorite toys - there is very rapid and wild variation in temperature over very short periods of time at every engine station! That affects wave propagation and fluid drag, causes reflection and refraction effects, and probably a lot I've never thought of. There is practically no "density" or "temperature" equivalent in an electrical path, unless maybe they could be introduced by finely tuning the behavior of semiconductors or vacuum tubes or some such. Total mass in a particular section might not be so hard, as it can be represented by the charge in a capacitor.
It should be noted (as mentioned in the article) that a transmission line is represented by units of capacitance-inductance "tank circuits", and MAYBE a pipe with different temperatures (and possibly, cross section areas) along its length could be simulated by a series of little tank circuits of varying size and Q value (the Q of a tuned circuit is easily calculated). That might do it for us, but could get to be a pretty complex model if reasonable granularity is desired. It also has to be remembered that the chamber(s) of an engine is(are) also characterized by significant inductance, not just pure capacitance. And of course, in all of this there would be simple resistance, too.
This also says nothing about the non-linearity of drag, which probably has no electrical equivalent in pure resistance (though, again, some semiconductor might comply) nor about actual two- or three-dimensional effects like shear and turbulence (however, our one-dimensional solvers like UFLOW1D and NUDiS don't give us anything for these, either, and we still use them).