[zwu249]

Recently I ran a CFD simulation to study the tumble flow motion in a SI engine using CONVERGE. I have two cases which are almost same, same geometry, same case setup, the only difference is the compression ratio(CR), one is 12 and 13. I moved the piston position at the BDC to get different CRs while keeping the engine stroke unchanged. The distance between the TDC positions of the two cases is around 0.6mm. However, I found the tumble ratio (especially during the compression stroke) has a significant difference between CR12 and CR13. Has anyone know the reason?

[zwu249]

I want to upload the tumble ratio result, anyone can tell me how to upload image?

[zwu249]

The tumble ratio comparison is shown below.IMG_8954.jpg

[SamWijey]

Hello,

Can you take one of your CR cases and run the exact same case on a different number of processors than you ran before? I want to see how much of a change this numerical perturbation have on your results before I try to answer your question.

Thanks,

[zwu249]

Hello Sameera,

Thank you for you reply. The numbers of processors for the two CR cases are same. Anyway, I will run the same case on a different number of processors to see the changes. The result will be post later on.

[zwu249]

Hello, Sameera,
The tumble ratio comparison between the same case on 16 cores and 32 cores is shown below.IMG_8961.jpg

[SamWijey]

Hi,

I invite you to read the below paper:

Richards, Keith, Probst, Daniel, Pomraning, Eric, Senecal, P. K., and Scarcelli, Riccardo. "The Observation of Cyclic Variation in Engine Simulations When Using RANS Turbulence Modeling." Proceedings of the ASME 2014 Internal Combustion Engine Division Fall Technical Conference. Volume 2: Instrumentation, Controls, and Hybrids; Numerical Simulation; Engine Design and Mechanical Development; Keynote Papers. Columbus, Indiana, USA. October 19–22, 2014. V002T06A010. ASME. https://doi.org/10.1115/ICEF2014-5605

Small perturbations are not smeared out when you use higher order numerics and finer mesh resolutions. This is what you are seeing and this paper is explaining this phenomena well.

Let me know if you have any additional questions after reading this paper,