[FLASH-USERS] boundary condition for symmetrical ablation

[Dean Townsley]

Hi,

It is known that at least some hydro schemes used in Flash can produce 
anomalies near the symmetry axis for specific flows: Blondin & Lufkin 
https://ui.adsabs.harvard.edu/abs/1993ApJS...88..589B/abstract

Basically it turns out that treating the symmetry axis as a reflecting 
boundary is not quite good enough.

I'm pretty sure that the "fix" discussed in that paper is not 
implemented for the split solver in flash but I'm less sure of the 
unsplit solver.

Or what you are seeing could be something else, but I figured it might 
be useful to mention this known issue.

Dean


On 2/22/21 3:53 AM, Thibault Goudal wrote:
> Hi everyone,
>
> I've been runing 2D cylindrical FLASH simulations based on the 
> LaserSlab setup.
> I would like to study the evolution of hydrodynamic instabilities at 
> the ablation front.
>
> I attached bellow an illustration of the density map results. I only 
> run the right side. The left part is built by the symmetry with the 
> white axis at x=0.
> In white color, I circled what is my concern. This bubbly structure 
> seems to show that the boundary symmetry at the left edge of the 
> numerical box isn't what I expected to be a flat laser irradiation.
> It seems that at x=0, the laser deposition is lower. At late times, 
> this impacts the instability growth on the axis.
>
> I used the "reflect" or "axisymmetric" hydro boundary on the axis.
> I also attached some parameters from the flash.par which could be 
> relevant to fix my issue.
>
> I guess this is kind of a typical problem when you deal with such 
> geometry.
> If you have any idea, I'll be glad to discuss about all of this.
>
> Thank you!
>
>
>
> # RADIATION/OPACITY BOUNDARIES
>
> rt_mgdXlBoundaryType = "reflecting"
> rt_mgdXrBoundaryType = "vacuum"
> rt_mgdYlBoundaryType = "vacuum"
> rt_mgdYrBoundaryType = "vacuum"
> rt_mgdZlBoundaryType = "reflecting"
> rt_mgdZrBoundaryType = "reflecting"
>
> #CONDUCTION BOUNDARIES
>
> diff_eleXlBoundaryType = "neumann"
> diff_eleXrBoundaryType = "neumann"
> diff_eleYlBoundaryType = "neumann"
> diff_eleYrBoundaryType = "neumann"
> diff_eleZlBoundaryType = "neumann"
> diff_eleZrBoundaryType = "neumann"
>
> #HYDRO BOUNDARIES
>
> # Hydro boundary conditions:
> xl_boundary_type = "reflect"
> xr_boundary_type = "outflow"
> yl_boundary_type = "outflow"
> yr_boundary_type = "outflow"
> zl_boundary_type = "reflect"
> zr_boundary_type = "reflect"
>
> #    LASER PARAMETERS
>
> ed_laser3Din2D                     = .true.
> ed_laser3Din2DwedgeAngle = 0.1
>
> ed_lensX_1                                   =  1000.0e-04
> ed_lensY_1                                   =  0.0e-04
> ed_lensZ_1                                   = -1000.0e-04
> ed_lensSemiAxisMajor_1             =  1000.0e-04
> ed_lensSemiAxisMinor_1             =  1000.0e-04
> ed_targetX_1                                =  0.0e-04
> ed_targetY_1                                =  0.0e-04
> ed_targetZ_1                                =  1400.0e-04
> ed_targetSemiAxisMajor_1          =  1000.0e-04
> ed_targetSemiAxisMinor_1          =  1000.0e-04
> ed_pulseNumber_1                      =  1
> ed_wavelength_1                         =  0.351
> ed_crossSectionFunctionType_1 = "gaussian2D"
> ed_gaussianExponent_1              =  4.0
> ed_gaussianRadiusMajor_1         =  900e-04
> ed_gaussianRadiusMinor_1         =  900e-04
> ed_numberOfRays_1                    =  16384
> ed_gridType_1                              = "radial2D"
> ed_gridnRadialTics_1                    =  512
> ed_semiAxisMajorTorsionAngle_1 =  0.0
> ed_semiAxisMajorTorsionAxis_1   = "x"
>
>
>
>
>
> --
> Thibault Goudal
> PhD Student CEA
> CELIA CNRS UMR5107 laboratory
> +33664160842
> thibault.goudal at u-bordeaux.fr
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