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Helmholtz resonator? Intake manifold design

by Oppenheimer
Tags: design, helmholtz, intake, manifold, resonator
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Oppenheimer
#1
Nov3-11, 07:17 AM
P: 5
Hi guys,

I'm doing a bit of research into engine intake manifolds. The information I've found so far looks staright forward, but it's given out by turners and cottage industry designers, rather than engineers or physicists, so it's basic.

I understand the fact that air will stack at the valve when it closes and the reflected columns will ocsillate up and down the intake runners until the valve opens. The issue I have is that there are various runner length formulas that calculate the resultant length based on an assumption that there will be a clear reflection from inside the plenum.

Now I'm no PhD, but clearly the air leaving the runner trumpet will slow down by virtue of the venturi effect then carry on to the roof of the plenum at the lowwer speed. Then it will be reflected from the concave roof which will not have a clean reflection. Infact I would expect it to be rather turbulant, seeing as air will be flowing from a side direction as well from the intake. So after all this, what's left of the returning wave of air will then enter the runner venturi and accelerate, but now with a altered volume, velocity and wave characteristics etc.

None of these issues are taken into account by the basic principals of the intake design formulas I've seen.

Now, seeing as intake design seems of be more art than science, I'm not sure what I'm asking really. Basically, if anyone has any more advanced knowledge of intake design and could point me in the right direction for further research, then that would be most helpful.

Thanks for your help.
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AlephZero
#2
Nov3-11, 09:04 AM
Engineering
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P: 7,172
Quote Quote by Oppenheimer View Post
I'm not sure what I'm asking really.
Neither am I to be honest, but one thing you seem to have missed is that the speed of pressure waves travelling through the gas and reflecting from boundaries etc, and the speed of the gas itself, are two different things.

When you talk to somebody, the pressure waves (i.e. sound) travel through the air at about 1000 ft/sec, but that doesn't mean that some air leaves your mouth and travels all the way to the listener at 1000 ft/sec.
Oppenheimer
#3
Nov3-11, 09:27 AM
P: 5
You're right, I rather stupidly hadn't thought of it in that way. That makes sense in terms of fluid dynamics. The air will stack up and stay in the runner building pressure, but surely the pressure oscillating pressure wave will still have the same issue that I mentioned i.e. it will not refelect cleanly from the roof of the plenum?

The runner length formulas just assume the pressure wave will travel up and down the runners with no disruption i.e. ;

The length that a wave can travel at the speed of sound in the time that the valve is closed. Half that to account for it needing to get to the end and return. And then if that's too long (which it will usually be especially for low rev accoustic supercharging), then divide it down to an acceptable length.

I'm still not convinced, even with my rookie oversight that you pointed out , that the pressure wave wont be distorted or affected by the transition from the exit of the runner to the roof of the plenum and back.

Do you have any advice on where to find the most reliable information on the subject?

Is it just a case of 'finger in the air' and then tune the design after CFD simulation?

Thanks


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