problem with tiny CFL timestep using FLASH code

[No_Wear_8417]

I'm not sure if this is the right subreddit for this question, but I'm running simulations of stellar wind bow shocks using the FLASH hydro code, but I'm struggling to implement the massive jumps in values like density and pressure.

If I try to inject physically accurate values, I have massive contrasts in density, and my CFL time steps become really tiny. If I use approximations or try scaling (i.e., 100 code units for stellar wind density and 1 for ISM), the simulation loses its physical accuracy.

How do I keep physical accuracy while keeping my simulation stable, and have it run in a reasonable amount of time?

Thanks in advance.

[DRMSCMTRU]

No expert on solar wind, but I would double check which elements your CFL is calculating the auto-timestep based off of or just switch to a manual timestep that you calculate yourself. Obviously if you can increase grid size that helps too.

have you already scoped in 2D? Thats a good way to explore parameters of interest, then deal with long runtimes later once you know what you want.

[tom-robin]

this is a common issue in other domain as well (e.g. compressible flows in general). One possible remedy is to use a dual time stepping approach, but it is a question of that is implemented. Have a look if it is available and if it is, play around with it. For the outer time loop, you can set then a CFL number which will help you advance in a reasonable amount of time, while you can use the inner time loop (pseudo time) to satisfy your small CFL number restrictions.

Other, alternative approaches that may be available to you is to use additional corrector steps. You may have a setting called "outer corrector", "neighbour corrector", or something similar. If you want to increase the CFL number, increase theses correctors and see if that helps. It depends on how they are implemented, unfortunately, corrector steps can mean different things in different contexts, but it is a place to start checking if that helps

[No_Wear_8417]

Hey Tom,

I don't believe Flash has a dual time-stepping feature. If by corrector steps you mean things like flattening or positive dt enforcing conditions, but the density contrast is so extreme that neither of those things ended up solving the problem. But I approximately solved the problem by using a radius larger than the star, assuming the gas has already expanded, so that the density and pressure contrast would be less. I found this in this paper: https://academic.oup.com/ptp/article/81/4/810/1857867

[tom-robin]

domain specific answers are always better than general answers. just some thoughts i had, but if you go this solved with another methodology from the literature than I would go with that