grid independence
 

I am checking grid independence in my case, which is a external air flow on the airplane. i am just wondering how much difference is acceptable among different mesh sizes. because no matter how fine the mesh is, the solution can't be exactly the same. there are always a tiny difference. it there any rule?

if any of your guys would like to have a look of my solution, i will send it to you.

many thanks

Re: grid independence
 

I believe there are a few ways to do grid checks. I did mine by comparing an average value of 'planes' that I had set up within my grid with different grids. For example, I would check the average temperature of the plane at different grid sizes. When I plotted the value vs number of cells I could see when the grid was too coarse. I would also check the value vs the changed grid count. For example, I would compare the value obtained at grid A with 1000 cells to that of grid b with 500 cells and have a criteria set that it would have to meet - ie..can only change by .001. Grid check used to be used to help with computational time as well as accuracy, but since computers have gotten so much more powerful it is really just an accuracy check now. that is my 2 cents worth anyway

Re: grid independence
 

thanks a lot, mettler,

your idea is really helpful for me. I am just wondering another thing, the criteria, is it a absolute one or a relative one. I guess it should be a relative one. what's your definition?

thanks again

Re: grid independence
 

my criteria depended on what I was looking at. If I were looking at temperature my results would be something like this.

Cell Count Temperature Plane ---------- ----------------- 5000 300.514 4000 300.515 3000 300.514 2000 300.485

Then I would look between 3000 and 2000 if I was trying to optimize it.

Again, this was just done to help with computing power. From the numbers above 4000 would be just fine, since it didn't really change above that or below it. In the case of temperature anyway.

Re: grid independence
 

ok, I see.

I also wonder if you have tried to compare the variables difference with your RMS residual. Do we have to keep the variables difference in the order of residual, say 1e-5 or 1e-6.

many thanks, really

anne

Re: grid independence
 

I think that will be up to you. I wouldn't waste too much time on this though. I would show that your answers do not change appreciably between different numbers of cells and then just go from there. If I remember correctly, I compared my variables to the order of my convergence criteria. You just need to find the cell count in which your results start to change and then increase that number of cells. The number of cells is going to be dependent on what you are looking at too. If it is a complicated geometry, then you are going to need a lot of cells to capture everything that is going on.

Re: grid independence
 

thanks a lot, mettler.

I will follow your guide and see what will happen.

all the best

anne

Re: grid independence
 

a plot is the best way to see it, and to show it (ie to a professor or something like that). pick a variable and record its value with different cell/grid counts and plot. You will get a horizontal line until the grid gets too coarse and then it will fall or go up - meaning the results are bad. You want your cell count to be on that straight line. My X-axis was my cell number and the Y-axis is the variable result. It gives you a nice graph to show how you arrived at the cell count that you did

Re: grid independence
 

mettler,

thanks again.

that's exactly what I am doing, like you said, the straight line will go up or go down, and it's a very nice show.

best regards,

anne

how to model a entrifugal pump using gambit
 

hello..

i am working on modelling a centrifugal pump (the impeller and its spiral casing).can anyone help me how to do this?i really need steps or procedures to draw the pump impeller in gambit.

thanks.

Re: how to model a entrifugal pump using gambit
 

don't draw it in Gambit..draw it in SolidWorks or ProE and import it.

Dear Sir ,

Can you send your example to me as I have no ideas how to do grid independency test ? Thanks you very much . Your cooperation is highly appreciate. :D

DeaR Anne ,

For your information , I would like to view , how you do your grid independency test. My e-mail is ser.tang@yahoo.com , hope to receive the feed back from you soon. Thanks you very much . :D

Hi mettler,

I would like to know how to do grid independency test? your ooperation is appreciated . Hope to hear from you soon. Thanks you very much.

Dear Sir,

can you guide me how to do independency test ? thanks alot :D :) ;)

reply:
 

hi !..to make a grid independancy test make the mesh finer ..increasing the no of meshes 2^n times the previous, and analyse everything..if u see the data does not change much,( a predefined value say 0.05) then u say the results have decayed to convergence in response to grid independancy test!..thank u :)

Dear Sir,

Can you guide me how to do the grid independency test as I don't know how to do it. Is it , I need to obtain the " grids " , meaning using the number of small grids devide with the bigger number of grids and the multiply by 100% , so I can obtain the results the independency test results . Is this consider as the independency test? Your cooperation is highly appreciate . :) :D ;) :* :P :( :O :/

Dear Sir,

I still not sure which icon i need to click ,so I can do the independency test.....meaning of this statement?? " a predefined value say 0.05) then u say the results have decayed to convergence in response to grid independancy test" .I hope you can guide me deeply as , my foundation is quite weak. Your cooperation is highly appreciate . Thanks you very much. :* :) :D :P :( :/

Dear Sir ,

Where can I see predefine value state in fluent software ?? :(

reply:
 

i just talked about the theory ..crudely speaking ,to make a grid indpendance test what u need to do is increase the mesh quantity manyfold, and see if u find any considerable change in the data..if not then ur solution has reached grid independance !..

i dont know if there is a switch to make the test..u can try it out by remeshing ur model every time in gambit !...:-):)

Dear Sir,

To increase the mesh quantity manifold in fluent & to remeshing my model every time in gambit is not the problem but I’m still not clear where to get data(s) to obtain/ do the independency test . Where is the reading should I refer , which reading should I take (name triangular cells??), where should I refer my reading ?? Hope you can guide me. Thanks you very much. :(

Dear Sir,

To increase the mesh quantity manifold in fluent & to remeshing my model every time in gambit is not the problem but I’m still not clear where to get data(s) to obtain/ do the independency test . Where is the reading should I refer , which reading should I take (name triangular cells??), where should I refer my reading ?? Hope you can guide me. Thanks you very much. :(

predefined value in fluent soflware??..ur question was a little confusing i guess?:(:confused:

ok !..after you complete the iterations in fluent, u plot the datas, and can use the Reports option( buttion) to calculate the fluxes or surface integrals u have predefined as a grid indepency measure..!..this is crudely the method..if u want just the grid independacy test u can rely on the Reports button in fluent !..u can try that i guess..this isthe simplest way !..:)

Dear Sir,

Which reading do I need to take , is it the ( eg. grids of 10,000 cells, 20,000cells , 50,000 , ...etc) and use the initial grids of 5000 cells ) to divide , then mutiply with 100% .

I had try to do it by selecting , surface icon , follow by report icon to determine the average of facet value by clicking compute. Before obtaining the result, I had rename it as point 1, continuous until point 20, Each point from point 1 till point 20 have the difference average of facet values. Then I selected adapt icon to determine the maximum & minimum values (all values for max & min had been obtain from point 1 to point 20). Everytime when I determine the max, min & average facet value, ,I will solve ,ilterate , & save in cas.gz file. BUT the problem , I'm NOT SURE WHICH READING SHOULD i TAKE IT 2 DO THE " INDEPENDENCY TEST" . please guide me , which value should I take it , to do the independency test. Thanks you.

Dear Sir,


I had try to do it by selecting , surface icon , follow by report icon to determine the average of facet value by clicking compute. Before obtaining the result, I had rename it as point 1, continuous until point 20, Each point from point 1 till point 20 have the difference average of facet values. Then I selected adapt icon to determine the maximum & minimum values (all values for max & min had been obtain from point 1 to point 20). Everytime when I determine the max, min & average facet value, ,I will solve ,ilterate , & save in cas.gz file. BUT the problem , I'm NOT SURE WHICH READING SHOULD i TAKE IT 2 DO THE " INDEPENDENCY TEST" . please guide me , which value should I take it , to do the independency test. Thanks you. :(

reply:
 

well i dont know which problem u are really upto, but i guess getting a grid independancy test is not that labourious??The purpose of grid independence test is to determine minimum grid resolution required to generate a solution that is independent of the grid used. Starting with a coarse grid the number of cells was increased in the region of interest until the solution from each grid was unchanged for successive grid refinements...so if urs is an uniform grid(simplest case!)..u just increase the number of grids..and for the crudest case, u can create appx say 5 planes in ur default-interior in the flow regime.and calculate the surface weighted velocity average or pressure averate,or anything in u wish , in the 5 planes..then u note that what is the minimum no. of meshi dimension after which these values dont change..u can also make a contour plot after iteration with a new finer mesh,and right click on the contour to find out if the values are changing with each neweer mesh or not!,,hope u can understand it now!..:)..:o..

another thing dont go for adaptivegrid generation first..it may confuse u..first u check for the inpependancy test seperately!..:)..and about the values.try to take almost all values..in the flow domain, if it is a laminar flow!..if it is a turbulent flow, use the values of turbulent intensity ,turbulent kinetic energy ,(sometimes omega, or intermittency)..u give the most importanca to them..well, if it is a reaction type flow in a combustorm then, it is really very difficult.. i hope u are not working on reaction type flows!

Dear Sir,

I'm doing simulation on the rectangular shape model using CFD . My mesh is unstructured. My model is 2D only . Inside the model, there are catalyst . This catalyst I model it as porous media. Is it difficult to do the independency test using 2D ,unstructure , model?

Dear Sir,

Thanks for your guidance, but now , I 'm stilltry to follow the steps you aregiving to me. I will try this method step by step . If I had any problems , I will willing to ask you . Thanks once again. :(

no it is not so difficult to do the independance test for the 2d unstructured model !..for unstructured grid u can also make it finer manyfold, by using GAMBIT!..and then export it to fluent!..but as u say,ur model is kind of reacting species!..so ur independance test ought to be a little cumbersome i guess!..u first identify which are the variables are of prime importance to you, and for the problem as well!..for example suppose,- average surface weighted x velocity , avg pressure pressure,and any reactive species concentration. .ok..now u can tentarively follow the steps i mentioned( making a plane..etc..) and find out that after making the mesh how much finer , u find that ur variables do not change significantly !..then u conclude that ur solution as grid independant !...u can do it , iguess !..
kind regards

Tanmoy
:)

Dear Sir,

I still can't follow the steps you had told me, can you tell me more detail : which icon do I need to click first.


My previous ways to get the results for independency test are:

Click Surface icon
click point tool
rename New surface Name, (e.g point-1)
click create
click close
Reason: create point 1

Click Report icon
select surface integrals
Select point-1
Select "Facet average" in report type
Select "Fluid" in surface types
Click compute
click close
Reading will appear on screen
Average of facet values : point 1
Pascal : -0.5492052

Click Adapt icon
select gradient icon
method : select gradient
Select "refine"
Key the value for refine threshold :(e.g 1864, 2200, ...etc)
Select "standard" in normalization
Click compute
Min & maximum reading obtain : (e.g Min:0.0004711999 , Max: 0.5091413)
Click adapt
"message appear in screen: Hanging node mode:Ok to adapt grid?
Click "yes"
click "close"
(once the adapt icon is click the previous value in minimum & maximum will be erase)


Click solve icon
Click icon iterate
type iteration values : 1000
Click "apply"
Click "iterate"


Click "file"
select "write--> case"
rename file: ( model1.cas.gz , model1.dat.gz )

Dear Sir,

My result are :

x(0)my(0)mIteration:20002nd upwind order0.005-0.005surface type: fluidrefine thresholdMinMaxStatic Pressure(pascal)18640.0002317110.04706485point-1 -0.02201333820000.0002306650.0470649point-2-0.02201886722000.0002302530.04706442point-3 -0.02204268224000.0002307690.04706373point-4 -0.02201150726000.0002323410.0470639point-5 -0.02197440328000.0002324830.04706398point-6 -0.02186676130000.0002307240.04706425point-7 -0.02179261132000.0002310990.04706431point-8 -0.02171541634000.0002309310.04706402point-9-0.02164241736000.0002287170.04706436point-10 -0.02156751638000.0002285910.0470643point-11-0.02149682140000.0002277820.04706394point-12-0.02143672942000.000228410.04706318point-13-0.02137597344000.0002280940.0470631point-14 -0.02133159746000.0002282830.0470633point-15-0.02128631448000.0002276090.04706281point-16-0.02127555850000.0002277050.04706288point-17-0.02124338452000.0002261490.04706301point-18 -0.02118280354000.0002266730.04706313point-19 -0.02111744556000.0002264430.04706273point-20 -0.0210474358000.0002265660.04706295point-21 -0.02098845160000.00022670.04706287point-22 -0.02094646562000.0002273980.04706257point-23 -0.02088112664000.0002263680.04706246point-24-0.02081430566000.0002259540.04706248point-25-0.02075119368000.0002268330.04706236point-26-0.02067450170000.0002264210.04706226point-27-0.02059461772000.0002265590.04706222point-28 -0.02052565974000.0002268760.04706224point-29-0.02046163676000.0002266690.04706162point-30 -0.02038286878000.0002263650.04706154point-31 -0.02029500980000.0002251710.04706158point-32-0.020237098


Hope you can help me :( :( :(

Dear Sir,


x(0)my(0)mIteration:20002nd upwind order0.005-0.005surface type: fluidrefine thresholdMinMaxStatic Pressure(pascal)18640.0002317110.04706485point-1 -0.02201333820000.0002306650.0470649point-2-0.02201886722000.0002302530.04706442point-3 -0.02204268224000.0002307690.04706373point-4 -0.02201150726000.0002323410.0470639point-5 -0.02197440328000.0002324830.04706398point-6 -0.02186676130000.0002307240.04706425point-7 -0.02179261132000.0002310990.04706431point-8 -0.02171541634000.0002309310.04706402point-9-0.02164241736000.0002287170.04706436point-10 -0.02156751638000.0002285910.0470643point-11-0.02149682140000.0002277820.04706394point-12-0.02143672942000.000228410.04706318point-13-0.02137597344000.0002280940.0470631point-14 -0.02133159746000.0002282830.0470633point-15-0.02128631448000.0002276090.04706281point-16-0.02127555850000.0002277050.04706288point-17-0.02124338452000.0002261490.04706301point-18 -0.02118280354000.0002266730.04706313point-19 -0.02111744556000.0002264430.04706273point-20 -0.0210474358000.0002265660.04706295point-21 -0.02098845160000.00022670.04706287point-22 -0.02094646562000.0002273980.04706257point-23 -0.02088112664000.0002263680.04706246point-24-0.02081430566000.0002259540.04706248point-25-0.02075119368000.0002268330.04706236point-26-0.02067450170000.0002264210.04706226point-27-0.02059461772000.0002265590.04706222point-28 -0.02052565974000.0002268760.04706224point-29-0.02046163676000.0002266690.04706162point-30 -0.02038286878000.0002263650.04706154point-31 -0.02029500980000.0002251710.04706158point-32-0.020237098

ok...i understand u are a bit confused!..i hope that u get a proper guide to explain u fluent in a bit more detailed fashion..and u r going for adaptation, this is not required here, in grid independancy test!..next u dont need to use the point tool,for surface average !..u can create a new 2d surface useing line/rake tool..your surface weighted average value can be gotten from report buttons!..:o

Hi Anne, I'm ser tang friends, robot . I have a big problem here. The problems is , I intend to use catalyts such as platinum-titanium oxide, platinum-aluminium oxide and platinum-carbon. But , there are no such catalysts in the FLUENT Database ( material type-solid). All that are listed are:

1.aluminium
2. ash
3. CaCO3
4. CaO
5. CaSO4
6. Cu
7. Dolomite( CaO_MgO_2002)
8. Gold
9. Gypsum
10. Nickel
11. Steel
12. Titanium
13. Wood

my model is using the catalysts for the chemical reaction . My catalyst is model as substrate.

The Zone Name: substrate
Material Name: ????
Porous zone (click)
Laminar Zone (click)

Hi Anne,

I share my e-mail with ser tang . The e-mail is ser.tang@yahoo.com . Thanks you very much.

Hi Anne,

How am I going to key in the catalysts such as platinum-titanium oxide, platinum-aluminium oxide and platinum-carbon as there is no SOLID catalyst in the FLUENT Database. Hope to hear from you. :(

Hi Tanmoy95,

I'm Ser Tang's friend,

I have a problem here, I'm using catalysts such as platinum-titanium oxide, platinum-carbon and platinum-aluminium oxide. When I click Fluent Database Materials in Material type: solid, there are no 3 catalysts listed in the fluent database materials. There are few catalysts in fluent database materials are:

1. Aluminium
2. ash
3. CaCO3
4. CaO
5. CaSO4
6. Cu
7. Dolomite [ CaO_MgO_2002]
8. Gold
9. Gypsum
10. Nickel
11. Steel
12. Titanium
13. Wood
The boundary condition I use is fluid , type:fluid ,zone:fluid, as my model catalyst are substrate. I also selected porous zone & laminar zone .

Dear Sir,

Dear Sir,

My model is square shape, 2D , simple model .The pink colour represent the reaction between reactant(e.g.glycerol with steam) , with the product (e.g. as hydrogen , carbon monoxide & carbon dioxide). I had key in the reactants & the products already by selecting:

->define
->materials
:name the each reactants & products
-Selecting material type : mixture
-Fluent mixture material : mixture-template
-key in all the reactants & products in properties (mixture species : name)
-key in number of mole for each reactant & product in properties(reaction:finite rate)

All the reactants & products can be obtain from the fluent database.

The red one represent the catalyst. I need to create the catalysts (e.g platinum-titanium oxide , platinum-silicon oxide and platinum-aluminium oxide ) . How should I do it?

CAn I directly change the mesh size in the gambit file ,than convert to mesh file ,to be open in fluent software??