
[Sponsors] 
August 30, 2005, 06:55 
You can do it that way or simp

#1 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
You can do it that way or simply rewrite the model in terms of btilde.


August 30, 2005, 12:16 
OK, good to know that.
I am

#2 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
OK, good to know that.
I am experiencing problems with bounding the progress variable though. I'm trying to compute a backwardfacing step; I've done a precomputation using turbFoam and have converted the data to run with my combustion code. My initial conditions for the ctilde variable are uniform 1.0 internal field, fixed value 0.0 at the inlet, zero gradient at the outlet and the walls (btilde is the inverse of this). However at the end of the first timestep the internal field ctilde has reached a value of 1.04 (pretty much throughout). This is clearly wrong, so... 1. Do my boundary conditions sound correct? If so, I assume that either my source term is incorrect in the ctilde equation, or I need to do something fancy in terms of bounding the ctilde equation. Are those the only options, or am I likely to be doing something else wrong that cound have this effect? (This is a rather difficult one to answer  I'm really just trying to pin down what might be going wrong with this solution). Gavin 

August 30, 2005, 12:24 
I would guess your problem is

#3 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
I would guess your problem is to do with omegaDot. Have you tried to run without it? Did it remain bounded then? Given that you have implemented omegaDot as an explicit source term how do you propose to ensure ctilda remains bounded?
I solved this problem of Sigmabased models by reformulating and remodelling in terms of the flame wrinking Xi which leads to a source term which can be implemnted as an implicit transport term and hence ensures boundedness by suitable choice of scheme. All this is described in detail in the technical reports and papers I have written on the subject. 

August 31, 2005, 12:04 
Just tried running without ome

#4 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
Just tried running without omegaDot  it does indeed remain bounded, although there do seem to be problems beyond that. I will investigate this in more detail.
Gavin 

September 1, 2005, 09:21 
Further (slow) progress...

#5 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
Further (slow) progress...
I've modified the omegaDot (source) term so that the first timestep remains bounded  ctilde now lies between 0 and 1 (chiefly around 1) and btilde between 1 and 0 (to within rounding error  see below). I've also included it as SuSp source rather than as purely explicit. However, now on the second timestep I get the following error : this is outside the PISO loop. Time = 0.50002 BICCG: Solving for Ux, Initial residual = 0.00778922, Final residual = 3.99268e07, No Iterations 1 BICCG: Solving for Uy, Initial residual = 0.0476625, Final residual = 1.67977e06, No Iterations 1 BICCG: Solving for ctilde, Initial residual = 0.215976, Final residual = 1.8554e09, No Iterations 2 Got this far! Got this far2! > FOAM FATAL ERROR : Maximum number of iterations exceeded Function: specieThermo<thermo>::T(scalar f, scalar T0, scalar (specieThermo<thermo>::*F)(const scalar) const, scalar (specieThermo<thermo>::*dFdT)(const scalar) const) const in file: /home/dm2/henry/OpenFOAM/OpenFOAM1.0/src/thermophysicalModels/specie/lnInclude/ specieThermoI.H at line: 83. FOAM aborting  AFAICT this traces through to a function in specieThermo which evaluates the temperature by inverting the equation of state using NewtonRaphson methods. Am I right? It crops up because I call thermo>correct() directly after solving for ctilde and btilde. I assume it has something to do with the state of the btilde field. btilde actually generates very small negative numbers (O(1e10)), presumably through rounding error. Is this likely to be what is causing the problems here? If so, what can I do about it? If not, what is the problem likely to be? Gavin 

September 1, 2005, 09:29 
It looks like you are violatin

#6 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
It looks like you are violating thermodynamic constraints. Are you saying ctilda goes from 0 to 1 in 1 timestep? With ctilde being chiefly around 1 after the first timestep you now have a fullyburnt system; that's an explosion not combustion and will blow the thermodynamics to pieces. Also I don't see the solution of the enthalpy equation.


September 1, 2005, 09:30 
Hi Gavin,
Yes, I would bet

#7 
Super Moderator
Niklas Nordin
Join Date: Mar 2009
Location: Stockholm, Sweden
Posts: 693
Rep Power: 22 
Hi Gavin,
Yes, I would bet on that. When we implemented the complex chemistry stuff I had similar problems when species fractions went negative ever so slightly. Did you try the sledgehammer approach btilde.max(0); after the transport eq. N 

September 1, 2005, 09:56 
Henry :
>It looks like you

#8 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
Henry :
>It looks like you are violating thermodynamic >constraints. Are you saying ctilda goes from 0 to 1 >in 1 timestep? With ctilde being chiefly around 1 >after the first timestep you now have a fullyburnt >system; that's an explosion not combustion and will >blow the thermodynamics to pieces. No, thats not what I mean. My initial condition for the ctilde field is uniform 1.0; i.e. the entire domain is filled with combusted gas. At t=0 my inlet starts introducing fresh gas (ie. ctilde=0.0). After one timestep this has not progressed very far, so the majority of the domain is still at ctilde=1.0, with just the region around the inlet dropping to ctilde=0.0. This seemed to be the most logical way of starting things up  please tell me if I am being stupid. (I'm doing combustion behind a backward facing step, not (hopefully) detonation or deflagration or anything really galloping). >Also I don't see >the solution of the enthalpy equation. I'm not solving it. My impresion from Poinsot and Veynante was that for relatively 'gentle' combustion it was not necessary  this being equivalent to an assumption of adiabatic conditions. The temperature can be derived by working back from the progress variable ctilde, which is in effect a reduced temperature. (I assume I do need to use the thermo database still anyway). Niklas : thanks, I'll try your suggestion in the first place. Thanks Gavin 

September 1, 2005, 10:00 
Niklas : just tried doing btil

#9 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
Niklas : just tried doing btilde.max(0). Now btilde is always 0 or greater  but I still get the same error on the second timestep.
Ho hum. Gavin 

September 1, 2005, 11:22 
I don't think your ignition ap

#10 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
I don't think your ignition approach will work because you are creating an infinite gradient of ctilde at the inlet which corresponds to an infinte reaction rate over an infinitesimal region. If you want to pursue this approach I recommend you ramp the inlet value of ctilte from 1 down to 0 over a period of time which introduces a flame of a sensible/physical/numerically resolved width.


September 1, 2005, 11:40 
That sounds feasible/sensible.

#11 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
That sounds feasible/sensible.
Your use of language suggests that there might be another way of doing this  is there? The only other option that I can think of would be to initialise the ctilde field with the value 0  but then how does the combustion get started? Gavin 

September 1, 2005, 11:51 
Yes the alternative is to init

#12 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
Yes the alternative is to initialise a flame within the domain, i.e. a profile going from 0 at the inlet to 1 somewhere downstream. Look at the ignition models proposed for the CFM model for SI engines, there are methods which involve the specification of Sigma and species distributions corresponding to reasonable (~5mm) sized kernel. However, these methods can also produce large initial perturbations due to the initial specification not being exactly consistent with the equations so I think the ramping method will be best/easiest if you choose an appropriate timescale.
You could improve on the simple ramping by choosing some kind of sigmoid shape for the ctilde distribution being transported into the domain at the inlet. My model has an analytical solution for the 1D flame, you could try using that profile, actually as I remember it was you who first solved my model analytically after I cast it into an appropriate form :) 

September 1, 2005, 12:11 
OK  a _really_ simple questio

#13 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
OK  a _really_ simple question this time.
I need to get the current time from the runTime database. How do I do that? I've found a function timeOutputValue() which claims to give the current time value, but it seems rather an odd name for what I want, and also it is returning a scalar rather than a dimensionedScalar (not really a problem, but I'm going to use startTime() which returns a dimensionedScalar, and I'd like to be consistent). Didn't there used to be a currTime() function which gave the current time? Gavin 

September 1, 2005, 12:16 
runTime is a Time which is der

#14 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
runTime is a Time which is derived from TimeState which is derived from a dimensionedScalar which is the current time. If you want the time value without dimensions simply use runTime.value() otherwise even more simply use runTime.


September 2, 2005, 02:20 
Just a thought...
What is y

#15 
Super Moderator
Niklas Nordin
Join Date: Mar 2009
Location: Stockholm, Sweden
Posts: 693
Rep Power: 22 
Just a thought...
What is your initial temperature? Seing how T0 and c=1 corresponds to a certain enthalpy H0, I just wonder what the temperature will be for H0 and c=0. N 

September 2, 2005, 09:32 
One further query.... how does

#16 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 10 
One further query.... how does one read a label (int) from a dictionary? I do seem to remember that there was a need for something like readLabel(), but I can't find the details.
Thanks Gavin 

September 2, 2005, 09:36 
refCelli = readLabel(dict.look

#17 
Senior Member
Hrvoje Jasak
Join Date: Mar 2009
Location: London, England
Posts: 1,806
Rep Power: 25 
refCelli = readLabel(dict.lookup(refCellName));
__________________
Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk 

September 2, 2005, 09:37 
Yes there is both readLabel an

#18 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
Yes there is both readLabel and readInt, for further details simply grep for them in say the OpenFOAM1.2/src/OpenFOAM/lnInclude directory.


October 14, 2005, 10:54 
Hi Henry
Repeating my messa

#19 
Member
Wladimyr Mattos da Costa Dourado
Join Date: Mar 2009
Location: Sao Jose dos Campos, SP, Brazil
Posts: 36
Rep Power: 10 
Hi Henry
Repeating my message for Tommaso, after long time working in many stupid matters I return to apply my attention and energy to use this wonderfull code: The FOAM! Please, I need your help. I would like in a first step to use the combustion solvers available in the OpenFOAM pack. But I need the literature which they are based. Could your help me to say which are the references which each solvers are based? I hope also to construct another solver for combustion flows. Many tanks in adavce for your help. 

October 14, 2005, 11:39 
I would very much like to have

#20 
New Member
Ramiro Brito Willmersdorf
Join Date: Mar 2009
Location: Recife, Pernambuco, Brasil
Posts: 16
Rep Power: 10 
I would very much like to have these references also.
Having a repository of these papers would be extremely useful, it could be hosted on the OpenFOAM wiki. I think, however, that many if not all of the papers cannot be posted on a web site for copyright reasons. At the very least, a simple list of the more important references would already be very useful. A lot of the work done on OpenFOAM seems to have been done during PhD thesis. Having all of them on the same spot would also be useful. I have Hrvoje's and Niklas' (both are stupendous works !) It would be nice if someone would collect the publicly available references and put them up on the wiki. It would be a simple and useful nonprogramming way of contributing to OpenFOAM. If no one else takes up the hint, I may volunteer myself in a couple of weeks time 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
combustion model in premixed combustion chamber  wuyu  FLUENT  7  October 21, 2015 13:18 
Hydrogen Air combustion in a combustion chamber  popi  CFX  7  July 11, 2007 18:40 
Sawdust CombustionNonpremixed Combustion Model  Jessy  FLUENT  1  June 19, 2007 10:59 
combustion in internal combustion engine  George  Main CFD Forum  0  September 7, 2006 14:41 
combustion  prasat  Main CFD Forum  1  June 16, 2003 13:17 