CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

SIMPLE algorithm

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   February 7, 2010, 05:43
Default SIMPLE algorithm
  #1
New Member
 
Join Date: Feb 2010
Posts: 4
Rep Power: 7
SamR is on a distinguished road
Hi,

I have written the SIMPLE algorithm in Matlab (using an example 6.2 from Versteeg and Malalasekera: An Introduction to Computational Fluid Dynamics: The Finite Volume Method) for 1D, steady, incompressible, frictionless flow using a backward staggered grid and an upwind differencing scheme through a nozzle. The example uses 5 pressure nodes, and I get my program to converge to the Bernoulli solution (albeit quite inaccurately due to the coarse mesh).

When I changed the program to handle N nodes, I had problems with convergence for any more than 5 nodes. The example in Versteeg applies both velocity and pressure under-relaxation factors (urf) at the end of the algorithm before substituting the new u and p field back into the discretised momentum equations:

u = (1-urf).u_new + (urf).u_old
p = (1-urf).p_new + (urf).p_old

which for the 5 node case suggests using urf as 0.8. I later found that for 5 nodes I can use urf=1 and the solution still converges.

Then, having looked at Patankar (Numerical Heat Transfer and Fluid Flow), I scrapped the pressure urf from this bit of the algorithm and moved it to:
p=p*+p' =>to become=> p = p* + urf.p'

However by doing this I found that urf is very sensitive to the number of nodes that I wish to use. I built in a loop to find the optimal urf by reducing the urf (from 1) by 0.01 each time the program started to diverge, and I did this for up to 100 nodes. This told me that I do not need to apply an under-relaxation to velocity (so I kept it at 1), only to pressure (urf_p). To give an example of a few...
5 nodes: urf_p = 1
10 nodes: urf_p = 0.49
20 nodes: urf_p = 0.20
25 nodes: urf_p = 0.15
40 nodes: urf_p = 0.08
50 nodes: urf_p = 0.06
75 nodes: urf_p = 0.04
80 nodes: urf_p = 0.04
100 nodes: urf_p = 0.03

Applying this and I can get convergence for up to 100 nodes, with the velocity and pressure fields becoming ever more accurate (with Bernoulli). However to increase the number of nodes further, I have to drop the urf_p down even further. The optimal urf_p seems to be proportional to a 1/N relationship (N=number of nodes). So if I want an even more accurate solution, say for 10,000 nodes, I have to use urf_p=0.0002 in order to get convergence, where I get an error in the mass flow rate on the order of 0.01%. Speed it not really a problem at the moment, even for 10,000 nodes I can get a converged solution in around 10 minutes.

Basically the question is to whether this sort of behaviour is normal?
SamR is offline   Reply With Quote

Old   February 19, 2010, 16:39
Default
  #2
New Member
 
Scott
Join Date: Feb 2010
Posts: 1
Rep Power: 0
Scott_M is on a distinguished road
This behavior IS to be expected from the SIMPLE algorithm. I'm working on my masters in thermo/fluids and am also writing code for control volume methods. The slow rate of convergence and poor prediction of pressure fields are due to dropping the velocity contributions from neighboring nodes and using only the pressure field for corrections.

Patankar addressed this issue through the SIMPLER alg. (SIMPLE Revised). SIMPLER requires additional computation steps, but achieves the correct velocity and pressure fields in the same time as SIMPLE.

I got this information from a book written by Majumdar called Computational Methods for Heat and Mass Transfer. It describes several of these algorithms in detail including an example. I hope this has been helpful.

-Best Regards
Scott_M is offline   Reply With Quote

Old   February 21, 2010, 11:20
Default
  #3
New Member
 
Join Date: Feb 2010
Posts: 4
Rep Power: 7
SamR is on a distinguished road
Thanks for getting back to me, Scott.

Unfortunately my college library does not have the book you mentioned, so I've been looking at SIMPLER in both Versteeg and Patankar.

For the SIMPLER algorithm, in the calculation of the pseudo-velocities, am I right to assume that momentum source term 'b' is zero in case of 2D convection-diffusion?? In that case I would be left with:

uPSEUDO = (aW.uW* + aE.uE* + aN.uN* + aS.uS*)/aP

where aP = aW + aE + aS + aN + (Fe-Fw) + (Fn-Fs)

Therefore in the calculation of the pseudo velocity, you do not need to a pressure field. Again, am I right in saying this?

Thanks
SamR is offline   Reply With Quote

Old   February 23, 2010, 02:06
Default please help me
  #4
New Member
 
prashant kadam
Join Date: Dec 2009
Location: Pune
Posts: 7
Rep Power: 7
prashant810 is on a distinguished road
hi i am trying to write the code SIMPLE in the c language by using the upwind scheme but i am not getting how to start the code
so please send me theSIMPLE code






Quote:
Originally Posted by SamR View Post
Hi,

I have written the SIMPLE algorithm in Matlab (using an example 6.2 from Versteeg and Malalasekera: An Introduction to Computational Fluid Dynamics: The Finite Volume Method) for 1D, steady, incompressible, frictionless flow using a backward staggered grid and an upwind differencing scheme through a nozzle. The example uses 5 pressure nodes, and I get my program to converge to the Bernoulli solution (albeit quite inaccurately due to the coarse mesh).

When I changed the program to handle N nodes, I had problems with convergence for any more than 5 nodes. The example in Versteeg applies both velocity and pressure under-relaxation factors (urf) at the end of the algorithm before substituting the new u and p field back into the discretised momentum equations:

u = (1-urf).u_new + (urf).u_old
p = (1-urf).p_new + (urf).p_old

which for the 5 node case suggests using urf as 0.8. I later found that for 5 nodes I can use urf=1 and the solution still converges.

Then, having looked at Patankar (Numerical Heat Transfer and Fluid Flow), I scrapped the pressure urf from this bit of the algorithm and moved it to:
p=p*+p' =>to become=> p = p* + urf.p'

However by doing this I found that urf is very sensitive to the number of nodes that I wish to use. I built in a loop to find the optimal urf by reducing the urf (from 1) by 0.01 each time the program started to diverge, and I did this for up to 100 nodes. This told me that I do not need to apply an under-relaxation to velocity (so I kept it at 1), only to pressure (urf_p). To give an example of a few...
5 nodes: urf_p = 1
10 nodes: urf_p = 0.49
20 nodes: urf_p = 0.20
25 nodes: urf_p = 0.15
40 nodes: urf_p = 0.08
50 nodes: urf_p = 0.06
75 nodes: urf_p = 0.04
80 nodes: urf_p = 0.04
100 nodes: urf_p = 0.03

Applying this and I can get convergence for up to 100 nodes, with the velocity and pressure fields becoming ever more accurate (with Bernoulli). However to increase the number of nodes further, I have to drop the urf_p down even further. The optimal urf_p seems to be proportional to a 1/N relationship (N=number of nodes). So if I want an even more accurate solution, say for 10,000 nodes, I have to use urf_p=0.0002 in order to get convergence, where I get an error in the mass flow rate on the order of 0.01%. Speed it not really a problem at the moment, even for 10,000 nodes I can get a converged solution in around 10 minutes.

Basically the question is to whether this sort of behaviour is normal?
prashant810 is offline   Reply With Quote

Old   March 3, 2010, 10:38
Default Colocated grid - boundary condition
  #5
Member
 
jk
Join Date: Jun 2009
Posts: 64
Rep Power: 8
jyothishkumar is on a distinguished road
Hi Scott,

I have written a solver (FVM) with staggered grid arrangement and it is working well (compared with fluent solver also). But it is not robust for different geometries. So i started working on the colocated grid. I have taken a simple 1d grid (assuming a diffuser or a nozzle) with varying area at different nodes. I am writing a cell centered FVM. I have tried with different conditions (bc's). Solution is diverging for all the condition. I think i have problem in my exit bc. It is as follows:

Assuming I have 8 cell centered nodes (or 8 control volumes).

1. Node no.1 is the ghost node. similarly node no.8 is the ghost node.

2. U-Momentum equation is solved for the nodes 2 to 7.

3. At the inlet (CVno.2, basically west face velocity of control volume 2) i am giving the velocity inlet bc.

4. Pressure at the ghost node 1 is taken from node 2 (i.e p(1)=p(2))

5. Similarly for velocity u(1)=u(2) (these velocities are nodal and not face)

6. At the exit i am extrapolating nodal velocity like u(8)=u(7) and the face velocity equal to u(7) (i.e ue(7)=u(7) where ue(7) is the east face velocity of node 7)

7. My pressure at the exit is p(8)=0

Please tell me Whatever i said above is right or do i need any in my boundary condition. Please help me in this regard

thanks

Jyothish
jyothishkumar is offline   Reply With Quote

Old   October 26, 2010, 03:10
Default
  #6
New Member
 
alex
Join Date: Oct 2010
Posts: 4
Rep Power: 6
ahazbavi is on a distinguished road
Hello,
please send your source code SIMPLE / SIMPLER scheme through my email, ahazbavi@ymail.com.
ahazbavi is offline   Reply With Quote

Old   October 26, 2010, 16:07
Default
  #7
Senior Member
 
morteza08's Avatar
 
Morteza
Join Date: May 2010
Location: Iran,Islamic Republic of
Posts: 159
Rep Power: 7
morteza08 is on a distinguished road
hi i have written a code that used simple code in fortran.
it is ac ode for flow in a tube that there are two rectangular obstacle inside.
i can debug your code if i see it....send it to me if you like.
morteza08@gmail.com
morteza08 is offline   Reply With Quote

Old   September 27, 2011, 10:17
Default SIMPLE algorithm code
  #8
New Member
 
Azeldin El-sawi
Join Date: Sep 2011
Posts: 2
Rep Power: 0
Azeldin is on a distinguished road
Hi everyone;

I have worked on phase change materials and I have done some simulations on Fluent. At this time I need to use SIMPLE algorithm code in order to get velocity- pressure coupling. However, I have difficulty in coding SIMPLE algorithm as tool. So, if anyone has worked on SIMPLE algorithm, I really appreciated send it to me on my e-mail,
azeldin.elsawi@gmail.com
Azeldin is offline   Reply With Quote

Old   September 29, 2011, 07:52
Default Fluent for Phase change materials
  #9
New Member
 
Azeldin El-sawi
Join Date: Sep 2011
Posts: 2
Rep Power: 0
Azeldin is on a distinguished road
Hi everybody;

I would like to know the viscosity could be changed in Fluent as case of phase change materials.
Azeldin is offline   Reply With Quote

Old   October 7, 2011, 23:14
Default simple
  #10
Member
 
kabilan.B
Join Date: Jul 2010
Location: chennai
Posts: 77
Rep Power: 6
kabilan is on a distinguished road
hi
i need that coding, could u please send me,
it should be very useful for me. i just try in the fortran.
thanks........................
kabilan is offline   Reply With Quote

Old   March 28, 2014, 17:16
Default Example 6.2 matlab file
  #11
New Member
 
Banma Kadachha
Join Date: Mar 2014
Posts: 3
Rep Power: 3
banmakadachha is on a distinguished road
Can somebody send me the matlab file for this please. I really need it. I will really appreciate it.
banmakadachha is offline   Reply With Quote

Old   March 31, 2014, 15:15
Default please send me this code
  #12
New Member
 
Banma Kadachha
Join Date: Mar 2014
Posts: 3
Rep Power: 3
banmakadachha is on a distinguished road
Could you please send me this code at banmakadachha@gmail.com
banmakadachha is offline   Reply With Quote

Old   January 5, 2015, 05:30
Default
  #13
New Member
 
Join Date: Jan 2015
Posts: 1
Rep Power: 0
hoangbh86 is on a distinguished road
Could you please send me this code
thanks alot
my email: hoangbh86@gmail.com
hoangbh86 is offline   Reply With Quote

Reply

Tags
matlab, simple algorithm, under-relaxation factor

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
SIMPLE algorithm in 3D cylindrical coordinates zouchu Main CFD Forum 1 January 20, 2014 18:02
SIMPLE algorithm jika Main CFD Forum 2 October 22, 2009 06:46
About Phase Coupled SIMPLE (PC-SIMPLE) algorithm Yan Kai Main CFD Forum 0 April 18, 2007 03:48
About Phase Coupled SIMPLE (PC-SIMPLE) algorithm Yan Kai FLUENT 0 April 13, 2007 23:17
SIMPLE algorithm Jonathan Castro Main CFD Forum 3 December 10, 1999 05:59


All times are GMT -4. The time now is 06:42.