Learning Forum

[faroos]

Hi, 

I am trying to understand what the difference between the Basic and Initial Grid under grid properties mean, and under intital grid there are three options. 

• Based on Temperature Profile Estimate 
• Number of Uniform Grid Points 
• User-Specified Grid Profile

Could you please what is the difference between the three approaches and how do we even determine which of the three to use? The default number of uniform grids in the second approach is 6. How do we determine which is the best number to use and what does that mean?

[Rob]

Please can you elaborate on what you're looking at? Which solver? 

[faroos]

I am running a premixed flat flame simulation, and you can specify the grid properties such as gradient, curvature and maximum number of grid points under the basic tab, and then there is a specific Initial Grid, under which these three calculation options are available. I am wondering what each of these mean, and the difference in each method?

[Ren]

Chemkin employs an adaptive grid to solve the equations. The number of grid points depends on the solution and the controls in the "Basic" tab. These control parameters affects the accuracy and the number of grid points employed and the calculation time (more grid points, more calculation time). The parameters in "Initial Grid" specifies the initial number of grid points and their spatial distribution. As the parameter names imply:

• Based on Temperature Profile Estimate: will use the grid points in the initial temperature profile estimated/specified
• Number of Uniform Grid Points: user specified number of uniform grid points 
• User-Specified Grid Profile: A user specified non-uniform grid

As a general guideline, the user should first try the default settings in Chemkin. You need to tweak the settings only when experiencing convergence issues. Please always follow a tutorial that is similar to what you plan to do and read the comments/tips in the tutorial.

Chemkin has a context-sensitive tips tool. Just hover the mouse pointer over the items on the panels and a tips pop-up dialog box will provide the information.

 

[faroos]

Hi, 

Thanks for that! Just to clarify, does that mean that number of grids in the initial tab does not affect the accuracy of the final result and should only be tweaked if I run into convergence errors?

Also, what exactly does intitial grid determine? Does it determine the number of intial grids utilised at the beginning of the calculation and how/does it affect the final result? 

I had a Chemkin file that utilised 500 initial grid and the file converged up until Oct 2025, but when I re-ran the same file (with no changes to the mechanism/thermo/transport files), it no longer converges. However, when I reduce the intial number of grids to 20, it converges. Why is this? What has changed? Is this a software update problem?