[FLASH-USERS] Question about near-surface temperature dropping below initial value in LaserSlab run

[Loffelmann, Jiri]

Hello,

in FLASH, every material is treated as a liquid. So even though you setup the target with density of solid, in the simulation it behaves as dense liquid. That's why it expands into the gas above and mixes with it. Because you have different temperatures, the cell above the target will naturally have temperature in middle of those initial as it now contains mixture of the two materials.

You could try to implement some model to hold the solid phase from expanding, however, this expansion is actually needed for the energy deposition unit to work. Because the rays get reflected from over dense plasma and no energy is absorbed in the over dense cells. So you would be stuck in the initial state, only small amount of laser energy would get absorbed in the gas (depending on its density).

I believe you don't have to worry about this, because it just happenes in one cell layer above the target and should not have any impact on the result at later times..

Hope this helps,
Jiří Löffelmann

Sent from Outlook for Android<https://aka.ms/AAb9ysg>
________________________________
From: flash-users <flash-users-bounces at flash.rochester.edu> on behalf of yinru Wang <wangyinrujp at gmail.com>
Sent: Monday, September 22, 2025 4:28:38 AM
To: flash-users at flash.rochester.edu <flash-users at flash.rochester.edu>
Subject: [FLASH-USERS] Question about near-surface temperature dropping below initial value in LaserSlab run

E-maily z adresy wangyinrujp at gmail.com nedostáváte často. Přečtěte si, proč je to důležité<https://aka.ms/LearnAboutSenderIdentification>
Hi FLASH users，

I’m writing to ask for guidance on a behavior I’m seeing in a LaserSlab simulation.

Setup (summary):

  *   Code: FLASH (LaserSlab)

  *   Initial target surface temperature: 900 K

  *   Ambient gas temperature: room temperature

  *   Laser incidence: 45° from the lower-right side

  *   Laser fluence (energy density): 21.9 J/cm²

  *   I will attach screenshots (initial state and subsequent time steps) and my flash.par for full details of the run configuration.

Observed behavior:
Right after the simulation starts, a narrow row of cells immediately above the target surface shows a temperature drop. As the run proceeds, the near-surface temperature continues to decrease and stays around 400–700 K, i.e., below the initial 900 K, despite the ongoing laser irradiation.

Questions:

  1.  Is such an initial cooling band near the surface expected in LaserSlab under certain settings ?

  2.  Are there recommended parameters to avoid non-physical cooling below the initial surface temperature?

Attachments:

  *   initial_state.jpg – initial condition (surface set to 900 K).[electron temperature]

  *   temp_evolution_1.jpg, temp_evolution_2.jpg, temp_evolution_3.jpg – showing progressive decrease of near-surface temperature during the run.[electron temperature]

  *   flash.par – full configuration file.

Any pointers or examples would be greatly appreciated. Thank you for your time and support.

WANG YINRU
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://flash.rochester.edu/pipermail/flash-users/attachments/20250922/9f977112/attachment.htm>