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Anthony March 18, 2006 06:44

Newbie Unsteady Problem

I have what is probably a very silly question. Please bear in mind my fluent experience is limited entirely to steady state problems and is completely self taught as at my university we have no teaching support so I apologise if there are some basic things I overlook.

I am running an intake manifold for a 4 cylinder 600cc motorbike engine. The plenum has one inlet and four outlets with intake runners leading to the cylinders. My boundary conditions are set as one pressure inlet and four outlets leaving the flow to be driven by pressure difference. The problem has run ok as steady state with some outlets being changed to a wall representing the valve closed. However now I wish to run it as a unsteady time dependant problem to simulate the pistons firing order. I don't understand the manuals explanation of setting files for transient conditions and it looks like there is one file per boundary which doesn't make sense tome.

Please could someone post a brief example of how I can set the outflow gauge pressure and ideally change a given boundary zones type to a wall at a set time for multiple outflow boundaries. Also could you please indicate the basic steps of how to set up a really simple transient run.

Sorry if I have a crazy idea, please correct me if I need to construct the model differently or something else not covered.

Any help much appreciated

Thank you for your time


Ahmed March 18, 2006 09:52

Re: Newbie Unsteady Problem then review the intake tutorial

Anthony March 18, 2006 10:35

Re: Newbie Unsteady Problem
Thanks for your reply,

Found the manifold tutorial but it doesnt appear to deal with a pulsating flow from the outlets. It looks to run in steady state with constant outflow from each outlet. I have seen there is a 3D manifold tutorial on the main fluent website but at the moment I cant get to it (Need to register etc but not admin of FLUENT, we dont have one at uni cos they have left).


Ahmed March 18, 2006 11:48

Re: Newbie Unsteady Problem
1- If you wish to use a pulsating outlet pressure you have to write your UDF, Good (Read UDF Manual/Guide Book for examples, and do not get scared for writing a function using the C language, learning is beneficial doing things by yourself) 2- When You get that 3d manifold tutorial, drop me a copy, I do not have it on my system (Use the send email button) 3- Today is Saturday, what university does work on Saturdays?

Anthony March 18, 2006 12:08

Re: Newbie Unsteady Problem
Hi Ahmed,

Heh. Obviously the uni doesnt work on saturday but the students do I'll be working tmz too. :( Yep,i realise it has to be a UDF and I certainly dont have a problem learning the C language I need to do it but...I've been given this as a rush job cos the chap working on it for the last 3 months hasnt done anything and we need a final design by wed next week! Any chance of a basic code example to get me started? I've read both the User manual and UDF manual (Dont get much of the last one tho). Problem is im jumping in at the deep end with not much time but I dont have any other option at the moment other than just try it.

Yep, i'll certainly give you the tutorial if I can get hold of it.

Thanks for the replys


Ahmed March 18, 2006 12:34

Re: Newbie Unsteady Problem
You do not have to start a crash C language course now, just look the UDF guide book for a similar function Your pulsating pressure is a function of time, right, then the structure of your function will do two steps; 1- read flow time (See which macro does that) 2- assign the corresponding pressure to each outlet cell. This is a loop, but again you have to get the macro for that. But again, you need to have the UDF Guide book before your eyes( I always do that ), the C language is a pit picky, syntax errors etc. So get the guide book, read the index for these two macros and roll up your sleeves. Is there a MS C++ language compiler available on your system. If not do not worry, you can interpret the function. Define -> User-Defined -> Functions -> interpreted Good Luck

Anthony March 18, 2006 12:46

Re: Newbie Unsteady Problem
Many thanks for your help! Glad you explained its basic operation. I didnt realise it could act effectivly as a separate module (All my knowledge is based in VB). Right, off the library for me.

Thanks again


Ahmed March 18, 2006 13:39

Re: Newbie Unsteady Problem
Nice reply, Good luck and do not forget to calculate the pressure before assigning it (step 2) i.e within the loop a- calculate the pressure b- assign it Or you can calculate the pressure before starting the loop then do the loop. Chap is mainly used in UK, are you located near to UMIST

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