CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Nozzle rhoSimpleFoam transonic --> time step increase

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 3, 2013, 18:39
Question Nozzle rhoSimpleFoam transonic --> time step increase
  #1
New Member
 
Eric
Join Date: Mar 2013
Posts: 22
Rep Power: 4
erichu is on a distinguished road
Hello everyone,

I am trying to calculate the flow rate of air in a converging nozzle (10mm diameter --> 3 mm diameter, total length of nozzle/lance is 1000mm) using fixed pressures for inlet and outlet (0barg). There is a range with inlet pressures of interest as it can be compared with experimental data, 50mbarg -->2barg. 6 cases are therefore under construction.

I guess that it is not necessary to use a compressible solver for low pressure differences but for higher flows experiments indicated high velocities. However, I would like to stay with one solver for all cases. Currently, rhoSimpleFoam appears to be of interest (steady state/compressible/transonic).

The boundaries (for 1 specific case) are as following:

Pressure
Inlet: totalPressure 161325
Outlet: outletInlet 101325
Wall: zeroGradient

Velocity
Inlet: pressureInletOutletVelocity
Outlet: inletOulet (000)
Wall: fixedValue (000)

Temperature
Inlet: fixedValue 298
Outlet: zeroGradient
Wall: zeroGradient

For turbulence I have used compressible functions as used in tutorial cases but with modified initial values. Upwind schemes are also used.


Now to the problem;

The simulation converges (very slow) for low pressure differences ~max 400mbar but the time step continuity constantly increases for every iteration. Time step local sum error is 90 after reached convergence criteria.

Fo higher pressures I cannot reach convergence and the time step is steadily increasing. In general, I suspect that the pressure is problematic as its residuals decreases very slow, if at all. However, if I turn the transonic option off, the solution steadily but slowly approaches convergence.

I have tried coarse- and fine meshes (structured 4k to 200k cells), checkMesh OK, tried with/without turbulence, different initial conditions and BCs but I have hard time moving forward. The only partly working solution is transonic = no.

Does anyone have any suggestions of how to setup a case like this?
Why is not transonic = yes working?
How can I possibly speed up convergence?
Why is the time step increasing even though the residuals are decreasing?



Thanks!
erichu is offline   Reply With Quote

Old   April 4, 2013, 19:33
Default
  #2
New Member
 
Eric
Join Date: Mar 2013
Posts: 22
Rep Power: 4
erichu is on a distinguished road
Update:

Earlier today, I decided to run without the transonic option and used ParaView to look at the changes upon floating point exception as well as the convergence pattern. The slow convergence could be removed by using fewer cells upstreams. Calculations up to 1.5 barg is ok and correlates very good to experimental data. Outlet speed is close to sonic speed at 1.5barg.However, for 2 bar g there is almost no difference from 1.5 barg which I found strange.

If I run 2barg with transonic option activated, OpenFOAM returns floating point exception related to pressure I think. Relaxation factors, mesh are being changed but I see very little improvement.

I think the problem might be near the outlet as there is a sudden pressure drop, see attached image. Does anyone know how to come around this problem?
Attached Images
File Type: png Screenshot from 2013-04-05 01:22:37.png (68.2 KB, 47 views)
erichu is offline   Reply With Quote

Old   May 3, 2013, 12:54
Default Help
  #3
New Member
 
Fabian
Join Date: May 2013
Posts: 3
Rep Power: 4
fpierart is on a distinguished road
Quote:
Originally Posted by erichu View Post
Update:

Earlier today, I decided to run without the transonic option and used ParaView to look at the changes upon floating point exception as well as the convergence pattern. The slow convergence could be removed by using fewer cells upstreams. Calculations up to 1.5 barg is ok and correlates very good to experimental data. Outlet speed is close to sonic speed at 1.5barg.However, for 2 bar g there is almost no difference from 1.5 barg which I found strange.

If I run 2barg with transonic option activated, OpenFOAM returns floating point exception related to pressure I think. Relaxation factors, mesh are being changed but I see very little improvement.

I think the problem might be near the outlet as there is a sudden pressure drop, see attached image. Does anyone know how to come around this problem?
Hi,

How are you, I am trying to solve a similar case than you is a compressible flow with one inlet and two outlets, but I can do that rhosimpleFoam solve nothing, I try first with the tutorial of squareBend but didnt work, could you send me your case with the transient off just to believe that the rhosimpleFoam can be used, because I solve the same example for a incompressible flow and works perfect and when I made all the changes to run with the compressible solver dosent work.

Thank you in advance.
fpierart is offline   Reply With Quote

Reply

Tags
nozzle, rhosimplefoam, transonic

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
Time step size and max iterations per time step pUl| FLUENT 27 May 12, 2015 20:04
Multiple floating objects CKH OpenFOAM 10 September 21, 2011 23:13
Extrusion with OpenFoam problem No. Iterations 0 Lord Kelvin OpenFOAM 6 April 12, 2011 11:24
Full pipe 3D using icoFoam cyberbrain OpenFOAM 4 March 16, 2011 10:20
Problems with simulating TurbFOAM barath.ezhilan OpenFOAM 13 July 16, 2009 05:55


All times are GMT -4. The time now is 21:21.