# rhoSimpleFoam - Residuals do konverge, but oscillate all the way down

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

January 14, 2014, 05:30
rhoSimpleFoam - Residuals do konverge, but oscillate all the way down
#1
New Member

Join Date: Oct 2011
Posts: 19
Rep Power: 7
Hello guys,

I am calculating a simple laminar channel flow, with one inlet, one outlet, two walls, and periodic bc's in z-direction. Since i had stability issues, I'm starting with first order Schemes and a relaxation of 0.1 for p and 0.7 for U. Fortunetaly, the residuals do konverge (finally!). But they show oscillations as shown in the attached picture.

Could someone give me a hint about the way I could change my fvSchemes or fvSolution to dampen these oscillations. My fvSchemes and fvSolutions are attached in this Post.

okroud
Attached Images
 Residuals_0_2000.jpg (71.4 KB, 52 views)
Attached Files
 fvSolution.txt (1.8 KB, 22 views) fvSchemes.txt (1.7 KB, 10 views)

 January 14, 2014, 07:55 #2 Senior Member     Tobias Holzmann Join Date: Oct 2010 Location: Leoben (Austria) Posts: 1,548 Blog Entries: 6 Rep Power: 27 Hi, can you take a picture of your mesh and Initial conditions / velocity? Additionally a slice of U at 1000, 1010 and 1020 iterations?

January 14, 2014, 09:57
#3
New Member

Join Date: Oct 2011
Posts: 19
Rep Power: 7
Quote:
 Originally Posted by Tobi can you take a picture of your mesh and Initial conditions / velocity? Additionally a slice of U at 1000, 1010 and 1020 iterations?
Hi Tobi,
thank you for your interest. I will be able to post these things on Thursday.

Edit: Unfortunately i only have solutions of Timestep 1800,1900 an 2000. There is no visible difference between them. Indeed these timesteps could be interesting regarding the p-residual, I will reproduce them over the weekend. Pictures of the mesh are attached in this post.

Boundary conditions for U:

dimensions [0 1 -1 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{
in
{
type flowRateInletVelocity;
flowRate 0.17359;
value uniform (0 0 0); // placeholder
}
out
{
}

lid
{
type fixedValue;
value uniform (0 0 0);
}
bottom
{
type fixedValue;
value uniform (0 0 0);
}

PER_MERGED //periodic bc's
{
type cyclic;
}
}

Boundary conditions for p:

dimensions [1 -1 -2 0 0 0 0];

internalField uniform 101325;

boundaryField
{
in
{
}
out
{
type fixedValue;
value uniform 101325;
}

lid
{
}
bottom
{
}

PER_MERGED //periodic bc's
{
type cyclic;
}
}
Attached Images
 mesh_all.jpg (24.4 KB, 16 views) mesh_zoom1.jpg (60.5 KB, 14 views) mesh_zoom2.jpg (63.5 KB, 15 views) Velocity_Step2000.jpg (15.2 KB, 14 views) Velocity_Outlet_Step2000.jpg (15.0 KB, 13 views)

Last edited by okroud; January 16, 2014 at 05:37.

 January 28, 2014, 05:05 #4 New Member   Join Date: Oct 2011 Posts: 19 Rep Power: 7 I found the Solution to my Problem: The relaxation factor of the oscillating Solution for rho was 0.2. I deleted it completely (which i guess means that it is now 1). Now the residuals konverge without any oscillation. But this rises further questions: 1. Does anyone have a clue why this relaxation of rho is producing such great oscillations? My guess: Since i use the thermoPhysical Model: hPsiThermo>>>>; rho is just calculated with the equation of state for perfect gas. So after rho is underrelaxated, the solution of rho is not anymore in harmony with the equations of momentum and energy, which are solved in the following iteration. 2. But why is the underrelaxation of rho implemented in the solver? What is it's use? Would be great to hear some opinions

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Zigainer FLUENT 17 February 6, 2016 01:12 JR22 OpenFOAM Running, Solving & CFD 6 August 1, 2013 09:08 franzdrs FLUENT 5 March 21, 2013 04:59 MachZero Main CFD Forum 7 December 25, 2012 13:18 HS FLUENT 1 November 7, 2005 06:45

All times are GMT -4. The time now is 23:16.