Ventilation of an office room, Re 1078, isothermal, Confused by results with different dx dt

Palabos General
#1

Dear LBM colleague,

I have been working recently on a case about office ventilation, which is similar to the following one:
http://www.flowkit.com/showcases/example-applications/98-heatconvection
except that my case is isothermal - no coupling of temperature, and the air outlet is under the inlet on the same wall. In my case there is no table in the box either. Re = 1078, built by the height of the inlet.
I modified the channel3d example in the showCases. Used 3d poiseuille profile for inlet, and zero gradient (ConstantDensity) for outlet, D3Q19, BGK… It worked out first with the following config:
(I work with dimentionless problem setting here.)


Velocity in lattice units: u=0.05
Reynolds number:           Re=1078
Lattice resolution:        N=6  // N of the inlet height
Relaxation frequency:      omega=1.99667
Extent of the system:      lx=14.2857 // dimentionless length built by the inlet height
Extent of the system:      ly=11.9048
Extent of the system:      lz=20
Grid spacing deltaX:       dx=0.166667
Time step deltaT:          dt=0.00833333

The result looks like this, colored for U_mag
https://lh4.googleusercontent.com/-B2_K09CkC3k/VMEOdkDQY_I/AAAAAAAABew/rtojSNZ4Y3k/s600/u024000.gif
https://picasaweb.google.com/107020002235760504567/Plb#6107178470747431922

I was quite happy with that, until I increased the resolution to 8 and 10, tried different configs until it gets stable. However the result looks quite different now:


Velocity in lattice units: u=0.05
Reynolds number:           Re=1078
Lattice resolution:        N=10
Relaxation frequency:      omega=1.99445
Extent of the system:      lx=14.2857
Extent of the system:      ly=11.9048
Extent of the system:      lz=20
Grid spacing deltaX:       dx=0.1
Time step deltaT:          dt=0.005

https://lh3.googleusercontent.com/-fu6Er9xcaTE/VMEOdgz7h7I/AAAAAAAABe4/Cb7svaJVtKc/s600/u174000.gif
https://picasaweb.google.com/107020002235760504567/Plb#6107178469877843890

The experiment data shows something more like the first picture with N=6, it should be more turbulence than the second one.
My question is, WHY do the results look so different?? How can I get more turbulent results with the N=10 configuration? I tried different dt values and it does not work.

====================================================

I also need to do another calculation with Re = 4312. I tried N=12, 14, 16, 18 with dt=0.002 ~ 0.010
It has get stable yet. Should I increase the resoluation, or decrease dt more?
Is it possible that for every Re & N there is a best dt and when dt gets bigger or smaller than the optimum, the simulation will get unstable (t_NaN get smaller)? (For me it does not make sense. I thought too small dt values for a fixed N does not matter, but my lots of palabos experiments show me the principle above.)

Thanks a lot for any help.
Best regards, Peter

#2

Here is my code, you can try it out:

[code=“cpp”]
/* config maxT Re
*/

#include “palabos3D.h”
#include “palabos3D.hh”
#include
#include
#include
#include
#include
#include

…

#3

ok, I figured out the problem.
The poiseuille profile is quite stable and changing it to a block profile solved my problem.
Eventually I used BC type of Skordos and RegularizedBGKdynamics. The simulation gets quite stable for Re 1078, 4312. The results look good.

If some one is interested in the code of the simulation, I can send it per email.

#4

Hi guys.

I’m a beginner with Palabos and want to simulate a water jet in 2D. Now I have seen that you discuss here an air jet and hope you can help me with my questions.

I could not open your air jet code. Could you send it to me?

I used cavity2D as a template and tried to integrate the channel parameters from poisseuille showcase into it.

My dimensionless channel length lx is 6 and channel height ly is 1.
I used the resolution for channel height N=100,
but I saw that in your code you use N for inlet resolution. Could you explain why and how?

My relaxation frequency omega is 1.994, U = 0.02, Re = 100.
I don’t fully understand what means
const Plint nx = parameters.getNx ( )

Where are dx and dt entered in the code , is it in VTK file ? can someone explain it and help me?

Many thanks
tixiro