Exhaust Primary Length in WAVE

[Mikey Antonakakis]

So I'm getting the hang of WAVE, and got it to run experiments, which is a really cool feature. But changing the exhaust primary length in a 4-1 setup has hardly any impact on the system, and I'm changing them from say 8" to 20" and there is MAYBE a 1% difference. Right now I just have it set up with only a straight pipe after the 4-1 collector, because I don't have a muffler model, and that length has a much larger effect, about on the same level as intake runner length (I'm testing all three at the same time). Any ideas of what's going on, or is this supposed to happen? I'm going to check right now and make sure I don't have the exhaust primaries in mm instead of inches or something...

[Mikey Antonakakis]

So I'm getting the hang of WAVE, and got it to run experiments, which is a really cool feature. But changing the exhaust primary length in a 4-1 setup has hardly any impact on the system, and I'm changing them from say 8" to 20" and there is MAYBE a 1% difference. Right now I just have it set up with only a straight pipe after the 4-1 collector, because I don't have a muffler model, and that length has a much larger effect, about on the same level as intake runner length (I'm testing all three at the same time). Any ideas of what's going on, or is this supposed to happen? I'm going to check right now and make sure I don't have the exhaust primaries in mm instead of inches or something...

[Wesley]

The termination to ambient is a MUCH higher pressure drop than the transition from the header tube to the collector - because of that, you'll see a much larger pressure wave reflection from the open end than at the collector expansion. The intake is in a similar boat - larger expansion volume (and a larger pressure change,) stronger reflection.

Just for kicks, make the volume of your junction modelling the collector much bigger, and you'll see a higher impact on power as you adjust the lengths.

Get a muffler model, or make one if possible. If you read Blair's book, he talks about modelling one as a series of volumes connected by massless ducts, and that will at least get you closer to the actual pressure drop. WAVE will make a concentric silencer model for you, also. We've used both methods, the second works best, but both make a large impact, both on power and on calculation time. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Mikey Antonakakis]

Yes, this makes lots of sense http://fsae.com/groupee_common/emoticons/icon_smile.gif

As for calculation time, some of my experiments had 325 runs. Luckily I'm using a desktop now, and not just my five year old laptop lol.

Thanks for the help.

[Mikey Antonakakis]

So I slapped an exhaust on my model in WAVE, I just used the one from the tutorial. It made almost no difference whatsoever, except it took at little longer to run the simulation http://fsae.com/groupee_common/emoticons/icon_smile.gif it's either good news, or bad because I'm doing something completely wrong.

[Wesley]

If you're running a transient sim, you can run WNOISE to see if your muffler is actually doing anything.

If it's a straight-through, it won't affect power very much. But the characteristic length of it will change the reflection characteristics.

[samphlett]

My baby http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Charlie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mikey Antonakakis:
changing the exhaust primary length in a 4-1 setup has hardly any impact on the system, and I'm changing them from say 8" to 20" and there is MAYBE a 1% difference. </div></BLOCKQUOTE>

I don't understand what you mean by 1% difference? There should be a shift in peak that you see due to tuning; the overall peak might not change much.

That said there are setups that will prove insensitive to exhaust primaries. I would try simplifying the rest of your model, maybe trying the same experiments on one of the example models, to make sure your responses aren't totally out of the ballpark.

[Mikey Antonakakis]

It was a little hard to see a distinct change in peak, it seemed more like the entire graph was shifting up and down VERY slightly. It was too small of a change in any sense to be able to say something like "ah, the torque peak shifted right." When I tested a 4-2-1 configuration, primary length was pretty important, as were the other parameters. I tried several different exhaust configurations, on the same model, with basically the same intake each time. Wesley's answer seems to make sense for it. I'm most likely going to go back and run another experiment once I get a decent muffler model. I did check the units, they were correct.

[Mikey Antonakakis]

I'm a little confused by the graph it gave for the sound output. I'm using the intake and exhaust as sources, and I did a 5-second pull from 2000 to 12000rpm. I tried to adapt the tutorial to the model I had, but it was a little confusing.

http://i7.photobucket.com/albums/y294/hhspunter/noiseoutput1.png

[samphlett]

So I guess you have intake and exhaust orifices positioned appropriately and a single microphone receiving contributions from both.

The graph is the (unweighted) RMS sound pressure level expressed in dB, relative to 20uPa (RMS) as a function of engine speed.

What confuses you?

Why are you including the intake orifice if you are only considering exhaust noise?

[Mikey Antonakakis]

I was expecting the sound level to increase with RPM... maybe I'm missing something really obvious, but this is an area I know very little about.
I used the intake (positioned realistically) because, well, it gets picked up by the dB meter as well, right? I know the intake is hardly as loud as the exhaust can be, but I just want to get this right (or as close to it) the first time. Although you get an adrenaline rush from trying to get your exhaust to pass noise at the competition, it'd be more fun to be driving the car in the test area before the dynamic events start.

If I didn't make it clear, I'm confused because the sound pressure level decreases with RPM. Maybe I am confusing SPL with intensity? I know the amplitude of the pressure waves should decrease as VE decreases and whatnot, which is the case at higher RPM. If there is anything like a direct correlation, the chart I posted does NOT match our VE.

[Mikey Antonakakis]

Okay so I did some quick reading on wikipedia... I'm taking it that the chart I posted is unweighted, like you said. I THINK that means it is independent of frequency. If it were weighted, it would increase with frequency, like a human's perception of SPL, right? Which value does the meter used by the judges use?
Sorry if I'm being retarded. Here's a video of my friend to make up for it:
http://www.youtube.com/watch?v=-oBbOWPMvvE

[samphlett]

The sound pressure level normally goes up with speed, but not always. Simple systems will have resonances that get excited at the excitation frequency (e.g. the firing frequency) passes through them, giving clear humps and bumps in the SPL curve.

You are right in thinking that an A-weighting filter will skew the curve so that it tends to increase with speed. This is the filter that judges will almost certainly use. All the noise legs stipulate A-weighting. It comes some way close to imposing the response of a human ear.

You can get a much better idea of the sound "picture" by plotting a 3D SPL vs speed vs frequency "colormap". Confusing when you first see them, but they show where resonances are and describe the character of the sound.

Did you try dumping an audio file and listening to it? They can sound pretty good.

[Wesley]

Yeah, the human ear is a weird machine.

An increase in frequency creates an increase in loudness, so a sound of the same amplitude but at a higher frequency will sound louder, even though the sound pressure is the same.

Here's a colormap of our current system. The lines represent the nth orders of the primary engine frequency. It's pretty neat. I was playing with resonators, mufflers, and exhaust lengths to tweak the sound output levels for our car.
http://photos-b.ak.fbcdn.net/photos-ak-snc1/v1924/181/98/9619773/n9619773_38719161_728.jpg

[samphlett]

You need a higher sample rate pal, you have aliasing. Try simply increasing (doubling) the blocksize, keeping all other parameters constant. That will double the sample rate and should kill off those odd lines bouncing back off the right (the wierd criss-crossing at the top right).

[Wesley]

Yeah, that was my first go-round, and the only one I had an available screenshot of.

I used the tutorial settings for that one... based on a 6000RPM peak...

[Mikey Antonakakis]

Thanks for the help guys, I still have a lot to learn.

I did make a 3D colormap but the lines in mine were almost vertical, and I'm not entirely sure how to read it. I should have paid more attention in my physics class last semester...

As for the audio file, that's one of the first things I tried, but it gives me this error when I try to play it:
"The system DLL user32.dll was relocated in memory. The application will not run properly. The relocation occurred because the DLL C:\WINDOWS\system32\SHELL32.DLL occupied an address range reserved for Windows system DLLs. The vendor supplying the DLL should be contacted for a new DLL."
Sometimes I really don't like computers.

[Mikey Antonakakis]

ah, it's a problem with the player I'm using (VLC). I played it with WMP and it doesn't sound right, but I may be able to fix that... the file is less than two seconds long, and plays one frequency, then another higher one (an octave up I think), with no transition between.

[samphlett]

This is a transient run, right? With transient noise output activated. It sounds more like you've got two cases with steady-state noise activated in each.

[Mikey Antonakakis]

I don't think I changed anything, but opened it again in NOISE and now it works.... I've got diagonal lines in my colormap, the sound file was clearly an engine revving up, it worked! It didn't sound that great, compared to real life, but hey, I'm happy with it. Thanks for the help guys. OH one more quick question... when I am viewing a 3d colormap, when I right-click the SPL axis, which weighting should I use? LIN, A, B, or C?

[Wesley]

http://en.wikipedia.org/wiki/A-weighting

A-weighting is supposed to represent the human ear (though it does a poor job,) and is also what is used in tech.

[Mikey Antonakakis]

alright thanks

I was really liking my torque curve when I ran WAVE with 14 steady-state cases... but now that I did a transient it doesn't look so hot... several small local max and mins under 7000rpm. It might make it a little tricky to tune, but I can't imagine last year's intake and exhaust was much better...

[Mikey Antonakakis]

I've been doing some research, and I think I want to run a turbo, or two. Maybe the ones this guy has:
http://cache.gawker.com/assets/images/2008/12/custom_1229625816605_Spiro_Pappas_9_Camaro_Z28_RS_ Super_Street_071.jpg
I think the boost they'd create at the top end would offset the 40 year lag

[VFR750R]

I don't know...they might not spool up until after the last lap of endurance. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Wesley]

Maybe attach some foot pedals to help?