[FLASH-USERS] Simulating Low Density Gases in FLASH

[Hansen, Eddie]

Hi Conor,

The density that one uses for “vacuum” in a FLASH simulation is somewhat subjective and problem-specific. The code will always treat this region like a fluid, when in reality, it is empty or at the very least a region of low-density kinetic particles (ie, not a fluid). To approximate a vacuum, you just want the density to be low enough to not affect the dynamics or physics you are interested in.

To simulate a region that is purposely low-density but not approximating a vacuum, what you are doing sounds like a good approach. If the region is still collisional, then FLASH should be able to model it well, and the EOS is often the crucial piece. Even if the region is not quite collisional, FLASH might give decent results. Try calculating the mean free path, collision time, etc. to determine how fluid-like your low-density region is.

--
Eddie Hansen
Applications Group Leader
Flash Center for Computational Science


From: flash-users <flash-users-bounces at flash.rochester.edu> on behalf of Conor Fegan <cfegan23 at qub.ac.uk>
Date: Friday, November 10, 2023 at 5:16 AM
To: flash-users at flash.rochester.edu <flash-users at flash.rochester.edu>
Subject: [FLASH-USERS] Simulating Low Density Gases in FLASH
Dear FLASH users,

I am trying to improve my understanding of the capabilities of FLASH when working with low density background gases. I am working with a modified LaserSlab problem which includes a high energy nanosecond laser interacting a solid target surrounded by low density gas to model an experiment. The gas density required is 10^-9 g/cm^3, three orders of magnitude lower than what is typically considered vacuum in FLASH. To best model this, we have written a simple tabulated EOS for an ideal gas in an attempt to retain the ionisation physics. However, we are wondering if there are any other numerical artefacts or any nuances to consider when FLASH is dealing with low densities. I would be very grateful for any explanations to help improve my knowledge of FLASH.

Kind Regards,
Conor Fegan
PhD Student
Queen's University Belfast


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