CFD for restrictor or intake manifold.

[Jay Fleming]

Hey guys, we're having a little difficulty doing some CFD on a preliminary restrictor. Somehow, i was able to flow mach 3 at the throat. That's the first problem. Also, I hear that with, say, Fluent, you can simulate the pulsing that an engine produces. Anyone know how to do this? What program are y'all using? Fluent, FloWorks, Flow Wizard? Thanks in advance.

[Jay Fleming]

Hey guys, we're having a little difficulty doing some CFD on a preliminary restrictor. Somehow, i was able to flow mach 3 at the throat. That's the first problem. Also, I hear that with, say, Fluent, you can simulate the pulsing that an engine produces. Anyone know how to do this? What program are y'all using? Fluent, FloWorks, Flow Wizard? Thanks in advance.

[Denny Trimble]

I'm impressed, a question about CFD and ghetto-rigged lift kits by the same person on one night...

Sorry, I can't help in either area http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Jay Fleming]

The lift kit is due to a lack of funds (for that purpose) and time. Otherwise, I'd do it completely correct and buy a kit.

[BeaverGuy]

The first spot to look is at your boundary conditions. If you have an inlet boundary condition that indicates super sonic flow ahead of the restrictor then it might be possible that it isn't actually choked at the restrictor. Also you may wan't to look at the region really carefully to make sure that it is actually at the throat and not just past the throat. Besides that the only thing I could suggest would be to do the 1D steady state isentropic analysis of the restrictor and see what happens.

I have been using FloWorks for the CFD on our intake. The way I have gotten the pulsed flow is by doing a transient analysis with the pressure at the runners varying. It is a bit hokey because I haven't figured out a way to close off the runner ends in accordance with the cam timing but it seems to work.

[threehondas]

What are you using as you back pressure and at what area/length/angle of the throat?

A manifold absolute pressure reading at the runner or plenum can be signifacantly different then, say the pressure at the end of your diffuser section.

Bruce

[Jay Fleming]

Thanks for the info Beaver, I'll look into the 1D stuff.

threehondas, I have no idea what the back pressure is. We (the group I'm working with) haven't done anything like this before, so I don't know where to start for the boundary or initial conditions in order to do it correctly. The area is whatever 20mm diameter is, don't know off hand. Length is probably 30mm or so. I dont know what you mean by angle of the throat. I'm assuming we're using the same terminology as the throat being the smallest area in the restrictor. The angle after is probably 16* and before probably 30* or so.

[ben]

I know that some of the guys at Birmingham have cosimulated Ricardo Vectis CFD with a Ricardo Wave engine model. The animation they showed me had all the induction pulsing simulated.

Might be worth dropping them an email. (www.ubracing.co.uk (http://www.ubracing.co.uk))
Ben

[Marc Jaxa-Rozen]

Unless you have access to some serious parallel processing hardware or don't mind waiting a week to get results for the simulated powerband, I wouldn't recommend bothering with Vectis-Wave coupling at this stage. I don't think the difference between 1D and coupled results, which wasn't very significant for our geometries, warrants the processing time in most situations...might as well spend it on the dyno instead.

A 20 kPa pressure drop across the restrictor might be a good starting point for your analysis.

Marc Jaxa-Rozen
École Polytechnique de Montréal

[Jay Fleming]

Thanks Marc, I'll try that pressure drop next time. So far we've just been using a velocity inlet with a velocity of about 50 m/s. Well I say so far, we just used that value today. It finally converged to about 340 m/s, which made me very happy.

[ben]

I think you're probably right Marc. Once again though, go to a grad interview at a race engine manufacturer and say you spent a few more hours on your dyno or you did a co-simulation of intake CFD and engine combustion, which is better?

I guess it's a difference in philosophy, but a lot of these things are worth doing even if they don't make your FSAE car go quicker.

Ben

[Chris Allbee]

Jay, have you tried asking James? You know, the guy at your school who did the analysis for this year's intake system?

[Jay Fleming]

I know who you're talkin about, and no I haven't asked him. I don't ever think about it when I see him.

[Conall_Dodd]

Just to add a quick note to this discussion if it's of any use, I see BeaverGuy had issues with the modelling of the physical closing of the intake valves within CFD. I have previously modelled my team's intake system geometry as a 3D transient model in much the same way. I created a time varying pressure boundary condition through the use of Fast Fourier Transforms and applied this to the ends of the runners. Although the results were hopeful, there was still an element of reverse flow into the manifold between IVC and IVO which was to put it likely ˜unrealistic'. I have been supervising the engine manifold design team this year and we have made many approaches to simulate valve closure, the most promising of which was applying a variable loss coefficient to the runners as well as a pressure plot. Although we are still assessing the feasibility of this. At the moment we have spilt our simulation into multiple simulations, and modified the boundary conditions for each one. The results of one simulation become the starting input for the following and so on and so forth. In this way we can exactly control the point of IVC and IVO and ensure the boundary is ˜solid' between these points.
I'm not sure if I phrased this all correctly but sure if you want a more exact analysis follow it up.
Slán

[Ben Inkster]

If you are going to look into transient CFD then 1D/3D coupling has got to be the easiest (relatively) and most accurate, I used Wave and Vectis. The 1D model will compute all the boundary conditions from your valve coefficients and geometry and apply them to the 3D model.

Jay - I would continue to develope your steady state models, but once you find a practicle limit to their application I would definetly look into 1D transient and/or 1D/3D transient. I found it very useful for design work (and not just job applications) but make sure you can validate your results first.

Cheers

Ben Inkster
UWA Motorsport
University of Western Australia

[ahmadreza]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jay Fleming:
Thanks for the info Beaver, I'll look into the 1D stuff.

threehondas, I have no idea what the back pressure is. We (the group I'm working with) haven't done anything like this before, so I don't know where to start for the boundary or initial conditions in order to do it correctly. The area is whatever 20mm diameter is, don't know off hand. Length is probably 30mm or so. I dont know what you mean by angle of the throat. I'm assuming we're using the same terminology as the throat being the smallest area in the restrictor. The angle after is probably 16* and before probably 30* or so. </div></BLOCKQUOTE>
Hie jay.
What do you mean by lenght?
May be you've signaled to total lenght(from the inlet to outlet).
In some sites i saw that maximum degree was recommended as 12* for the outlet just after the throat.May you interprate why you selected 30 or from where.
as a newcomer I need some information about the manifold.

[kwancho]

Man... don't copy his numbers....

[Jersey Tom]

I question the worthwhile of fullscale CFD on the intake. The plenum and runners and all that jazz is a quickly-varying transient problem and is going to take an absolutely absurd amount of time to get any sort of meaningful data. Might be time better spent just building and testing assorted setups.

As for the restrictor venturi, there are design standards for critical-flow venturi meters. Bada bing.

[cmohn]

Did you say mach 3 at the restrictor throat? Thats impossible. =) In a converging-diverging nozzle the throat will reach a maximum of mach one, which is a choked flow.

CFD is a blessing and a curse at the same time. You can waste countless hours f ing around on it and get no where. However, once you get it figured out, its amazing.
chris

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by cmohn:
Did you say mach 3 at the restrictor throat? Thats impossible. =) In a converging-diverging nozzle the throat will reach a maximum of mach one, which is a choked flow. </div></BLOCKQUOTE>

No, it's possible, just need to have a smaller nozzle before it http://fsae.com/groupee_common/emoticons/icon_wink.gif

[ABelton]

We've used FloWorks to decent effect, imo, for modeling our restrictor. Numbers we got were reasonable, but there was some asymmetric flow seperation that I'm convinced is solely due to some rounding error; otherwise it was fine.

I'm pretty sure it was steady-state conditions without any pulsing.