Mike Everman wrote:Oh, the plot I posted is not a snapshot of one point in time, nor is it a temporal plot. It indicates the points along the length where the richest harmonic content can be excited as a pressure pulse travels the length.
Maxima, for lack of a better description, are places where you'd want (excitation moving from left to right) pressure changes due to changes in cross-sectional area to occur, and pinches should occur at minima, where gas velocity is highest and therefore pressure is lowest.
Ah, I'm beginning to understand. That's a little bit like plotting "influence lines" for various points on a structure (like a bridge) under a moving load. (You could do influence lines for shear, tension, bending moment or whatever ... if I remember correctly.)
For instance, a small diameter would be positioned at a minima, and the area should increase, pressure increasing, until a maxima, where the area must stop rising, and either must move in to a cylindrical section or begin a new pinch. Crudely, the passing pulse has "sucked on the acoustic minimum", amplifying it, and "punched the maximum", amplifying IT.
Makes sense to me.
Sounds like for 1D analysis, the unfolding methodology has got to be dead on for any of this analysis to be meaningful.
Well, uh ... yes. We have to make "just right" assumptions about how to gyrate the intake around ;-) There are various reasons why it is impossible to make an "unfolded" version an exact model of the original "folded" one, but I think the important part is to get it close to spot on "acoustically" and neglect the effects caused by linearizing the mass flow. I mean, what else can we do? A "folded" engine is not a 1D flow system, after all, so reality can only be modeled approximately anyway.
The dimensional layout used for UFLOW modeling was as shown below. The velocity node location shown is just an "eyeball average" over time, chosen by observing the velocity curve as it changes over time in the cycle. How "good" this is can be roughly judged from the dark green line on the "Mach Number" graph in the UFLOW panel presented earlier, which was generated for that exact location, 185 mm in from the front edge.