[medaouarwalid]

hello every one, the residuals after 1144 iterations still like this , without any message of "the solution is converged" , what should i do ?

[BlnPhoenix]

Residuals do look OK. So whats the problem exactly?

[medaouarwalid]

Originally Posted by BlnPhoenix

Residuals do look OK. So whats the problem exactly?

There is no message , " solution converged " . I am stil waiting

[BlnPhoenix]

Are you doing steady stat or transient simulation here?

[medaouarwalid]

Originally Posted by BlnPhoenix

Are you doing steady stat or transient simulation here?

Here was steady .

[BlnPhoenix]

Then it's converged. You see it from the residuals not going down anymore. It's not saying "Convergence reached" because you may have not defined Automatic Convergence Detection or the values that are defined are to low to reach with your current setup. You need to check also other important parameters in your simulation if they're converged or not. If they have, that's the result of your simulation. You can now compare with experiment, find agreement or redo simulation with better mesh, better BC, better Solution Shemes etc. It's an iterative process also, you see..

[Светлана]

Just like BlnPhoenix said, there are different things to try (and ways to check them)

- improve your mesh, then the flow would be resolved more accurately (do mesh convergence plot called 'H-convergence plot' or grid convergence index to quantify this effect),
- try a different solver in case it suits your problem better (compare results from different models and solvers of different order),
- try a different boundary condition or change the computational domain size in case (compare results from different boundary conditions),
- compare your results with experiments, so that you know that you're solving the same problem on the computer and in the experimental data and the problem set on the computer is not irrelevant.

These processes are explained at, for example,

https://www.grc.nasa.gov/WWW/wind/va.../overview.html validation and verification
https://www.grc.nasa.gov/WWW/wind/va.../tutorial.html the full tutorial

[medaouarwalid]

What parameter to improve in the mesh, i have 1milion tetrahedral cells, skewnes is 0.1 . Also , if i want to use the LES model the problem will be the time step, i read that the best time step should be less than "the minimal size of mesh/velocity" so in my case the time step should be less than 0.0001 , so if visualising a result of 10 minutes flow i need 6 milion time step :O !!!!

[BlnPhoenix]

LES is very demanding! I would not go down that road if your limited with your resources (CPU speed, HDD space etc.). Instead use RANS it works with fewer cells and is accurate enough for engineers

A tip to reduce cell count: Try to use a hexaeder mesh type. This saves a lot of cells and is also more accurate than tet mesh at the same time. Ansys Workbench has the "Cutcell" Mesh method which generates hexader-dominant mesh for your geometry.

[medaouarwalid]

Originally Posted by BlnPhoenix

LES is very demanding! I would not go down that road if your limited with your resources (CPU speed, HDD space etc.). Instead use RANS it works with fewer cells and is accurate enough for engineers

A tip to reduce cell count: Try to use a hexaeder mesh type. This saves a lot of cells and is also more accurate than tet mesh at the same time. Ansys Workbench has the "Cutcell" Mesh method which generates hexader-dominant mesh for your geometry.

Ok i will try it, meanwhile i am doing a simulation with the standard k-epsilone and the results seems to be acceptable, just i want you to confirm me one thing, if i use in LES the time step of 10^-5 so i need 6 milions incrementation of the time step to see the jet properties after 1 minute of flow, is this true or i am not understanding the time step very well ?

[BlnPhoenix]

1 min total simulation time with a time step of 1e-5 secs means:

60s / 1e-5 secs = 6e6 iterations. Yes.

[medaouarwalid]

Originally Posted by BlnPhoenix

1 min total simulation time with a time step of 1e-5 secs means:

60s / 1e-5 secs = 6e6 iterations. Yes.

Hahaha , 6 milion iteration if the solution converges after 1 iterations, but if for example it converges after 9 iterations , the times step will increments every 9 iterations so i need 9*6 = 54 milions iterations to see just 1 minute of the simulation !!!!!! I need a nasa stations to do that . if i want to see just 1 seconde, it will not be sufficient ?