[FLASH-USERS] Growing error in magnetic field when updating fluid variables in wind tunnel simulation

[Rukmani Vijayaraghavan]

Hi everyone,

I've come across an error when updating fluid variables at the inflow 
edge of a wind tunnel simulation. I'm running a simulation of a galaxy 
(with active dark matter particles, gas, and passive particles) in a 
box, whose fluid is initialized to be identical to the incoming wind 
(with vx, vy, vz = 600 km/s, 0, 0). There is a small error (on the order 
of 1%) when updating grid cells near the inflow boundary (with both USM 
and PPM solvers), and this error is spatially correlated with  block 
boundaries. While this error itself is tolerable as far as the density 
and pressure go, this has bad consequences for the magnetic field which 
grows as the wind propagates through the box (see attached figure, 
xl_boundary). This figure shows slices of Bx at two timesteps (annotated 
with block boundaries and magnetic field vectors). The dynamic range of 
Bx in this image has been reduced to highlight these discontinuities. At 
the timesteps shown in the attached image, the fluctuations in Bx are 
~1%, but grow with time up to order unity. I've tried a variety of 
Riemann solvers (HLLC, HLLD, Roe, Hybrid), slope limiters (mc, minmod, 
etc.), interpolation orders, prolongation methods, turning on and off 
specific USM switches, but nothing seems to solve this issue so far. Has 
anybody else dealt with and/or successfully solved this issue?

Thanks,
Rukmani

-- 
Rukmani Vijayaraghavan
NSF Astronomy & Astrophysics Postdoctoral Fellow
University of Virginia
rukmani at virginia.edu

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