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

Incompressible to compressible flow simulation

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

Like Tree1Likes
  • 1 Post By zongtwi

Reply
 
LinkBack Thread Tools Display Modes
Old   March 3, 2009, 00:53
Default Incompressible to compressible flow simulation
  #1
Nishikant Tamrakar
Guest
 
Posts: n/a
hi, i have completed the incompressible flow simulation at 0.3 Mach. Now i want to do the compressible flow simulation with 0.7 mach.

What steps should i take?

i have done the following:

selected density based with node gradient implicit solver.

boundary conditions are: pressure inlet, pressure far field, pressure outlet, wall and interior.

k-epsilon model. It is not converging?

Thanks

  Reply With Quote

Old   March 3, 2009, 02:02
Default Re: Incompressible to compressible flow simulation
  #2
Yee-Hern.Tan
Guest
 
Posts: n/a
u need to apply ur fluid material as Ideal Gas.
  Reply With Quote

Old   March 3, 2009, 02:24
Default Re: Incompressible to compressible flow simulation
  #3
Nishikant Tamrakar
Guest
 
Posts: n/a
i am using Ideal Gas. please tell any other solution Thanks
  Reply With Quote

Old   March 3, 2009, 04:05
Default Re: Incompressible to compressible flow simulation
  #4
yufeng
Guest
 
Posts: n/a
i also have this question.if i want to do the compressible flow simulation with water ,i may select idea-gas?
  Reply With Quote

Old   March 3, 2009, 04:32
Default Re: Incompressible to compressible flow simulation
  #5
Nishikant Tamrakar
Guest
 
Posts: n/a
yes, you may have to select ideal gas. selecting ideal gas means varying density. but do you really want to do the compressible flow simulation for water, because water will be more or less incompressible over a large range of reynolds number.

  Reply With Quote

Old   March 3, 2009, 04:36
Default Re: Incompressible to compressible flow simulation
  #6
Yee-Hern.Tan
Guest
 
Posts: n/a
as i know the node gradient is harder to converge.

or maybe u can try to create a finer mesh for your model.

u using k-e standard model? maybe can try realizable.
  Reply With Quote

Old   March 3, 2009, 04:38
Default Re: Incompressible to compressible flow simulation
  #7
zongtwi
Guest
 
Posts: n/a
Compressible analysis requires a careful approach as it is very prone to divergence.

My recommendation would be:

1. Do the Mach 0.3 flow simulation with ideal gas flow and use the pressure based solver. (Don't use the density based solver...it will give a whole lot of problems) 2. Increase the speed to Mach 0.5, and use the mach 0.3 flow results as the initial condition. 3. Once converged, increase the speed to Mach0.6 using results from Mach0.5 as initial conditions. 4. Continue to Mach0.7, with mach0.6 as initial conditions.

Also try the following: 1. Use pressure far field instead of pressure inlet for the inlet. 2. Reduce under relaxation factors accordingly.

The trick is to be patient and increase your speed slowly for best convergence and results.

Hope that helps
Asura likes this.
  Reply With Quote

Old   March 3, 2009, 05:58
Default Re: Incompressible to compressible flow simulation
  #8
Nishikant Tamrakar
Guest
 
Posts: n/a
i hope this will work. i will try this. can you tell me what is difference in density based and pressure based because you are telling density based will give whole lot of problems. And can i use pressure based only for 0.7 mach. I dont have to go more than that, however i am expecting that in some places i may get mach number more than 1 due to the geometry.

Thanks in advance.
  Reply With Quote

Old   March 3, 2009, 10:56
Default Re: Incompressible to compressible flow simulation
  #9
zongtwi
Guest
 
Posts: n/a
It is very difficult to explain the difference between pressure based and density based solvers, I would suggest going through the users guide. But pressure based solvers can handle even supersonic flows, so you should be able to get decent results using them. You only need to use the density based solver should you want to do simulate complex highly coupled density and flow conditions, such supersonic simulation with multicomponent flow and combustion (like when you want to simulate the ramjet effect). Otherwise, pressure based should be OK.

If you are expecting local mach number to reach supersonic speeds, you will need to ensure that your mesh is fine enough to accurately predict any shocks that might appear. Your best bet would be to use mesh adaption, you can use the pressure gradient as your adaption characteristic.

Also, remember to include viscous heating with your turbulence model, as the high speeds will cause an increase in temperature, which will affect your local flow conditions.

Hope that helps.
  Reply With Quote

Old   March 3, 2009, 21:29
Default Re: Incompressible to compressible flow simulation
  #10
yufeng
Guest
 
Posts: n/a
yes!i want to make an simulation of piston-pump,so i must consider the compressible of water. if the media is water,we just select ideal gas or we will write a UDF for it?
  Reply With Quote

Old   June 16, 2009, 21:09
Default
  #11
New Member
 
TeddyNguyen
Join Date: Jun 2009
Posts: 1
Rep Power: 0
TeddyNguyen is on a distinguished road
simulation credit


Thanks for sharing this useful information. It's great.

TeddyNguyen is offline   Reply With Quote

Reply

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
Incompressible and compressible flow. Confused student. Main CFD Forum 22 January 30, 2014 07:32
compressible and incompressible flow chinababu FLUENT 4 November 13, 2008 06:56
NS-incompressible and compressible flow solvers ag Main CFD Forum 2 September 27, 2005 06:18
Incompressible and compressible flow. John. Main CFD Forum 1 December 15, 2004 12:29
Incompressible and compressible flow boris FLUENT 0 February 20, 2002 04:08


All times are GMT -4. The time now is 15:25.