Fix 1D flow perturbation applying random factor twice (index aliasing)

[sbryngelson]

Summary

Fixes #1369.

In s_perturb_surrounding_flow (src/pre_process/m_perturbation.fpp), the original code was:

q_prim_vf(eqn_idx%mom%end)%sf(i, j, k) = rand_real*q_prim_vf(eqn_idx%mom%beg)%sf(i, j, k)
q_prim_vf(eqn_idx%mom%beg)%sf(i, j, k) = (1._wp + rand_real)*q_prim_vf(eqn_idx%mom%beg)%sf(i, j, k)

In 1D, eqn_idx%mom%end == eqn_idx%mom%beg (same slot), so:

• Line 1 writes rand * v0 into the only velocity slot
• Line 2 reads the now-modified slot and writes (1 + rand) * rand * v0

The perturbation is applied twice, giving rand*(1+rand)*v0 instead of (1+rand)*v0.

Fix

Save the original mom%beg value before any writes, scale mom%beg first, then only assign mom%end when num_vels > 1:

v_beg = q_prim_vf(eqn_idx%mom%beg)%sf(i, j, k)
q_prim_vf(eqn_idx%mom%beg)%sf(i, j, k) = (1._wp + rand_real)*v_beg
if (num_vels > 1) q_prim_vf(eqn_idx%mom%end)%sf(i, j, k) = rand_real*v_beg

This also correctly preserves multi-D behaviour: mom%end gets rand * v_beg (the original x-velocity), not rand*(1+rand)*v_beg.

Verification

Ran a 1D case (m=9, v0=1.0, perturb_flow_mag=0.1) and read the post-pre_process velocity field. All 10 cell values fell in [1.0, 1.1] — consistent with (1 + rand) * v0 where rand ∈ [0, 0.1]. Under the old bug the values would have been near zero (rand*(1+rand)*v0 ≪ 1). Not in the test suite, so verified manually.

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📥 Commits

Reviewing files that changed from the base of the PR and between d513442 and 381ef1e.

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• src/pre_process/m_perturbation.fpp

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[sbryngelson]

Automated Code Review

Summary: Fix is correct. One suggestion.

Strengths

The fix is mechanically correct:

• num_vels > 1 is the right guard — it uses the same quantity that determines whether mom%beg and mom%end are aliased, and also correctly handles the MHD case where num_vels = 3 even in 1D
• v_beg correctly typed as real(wp) — promotes stp field data to wp for computation, consistent with project conventions
• 2D/3D behaviour exactly preserved: mom%beg gets (1+rand)*v_beg, mom%end gets rand*v_beg — identical to the original code's output in non-aliased cases
• num_vels is guaranteed initialized before s_perturb_surrounding_flow is ever called

Suggestion

No automated test covers perturb_flow. Since the bug went undetected, a programmatic test case in toolchain/mfc/test/cases.py would prevent regression — a simple 1D case with perturb_flow: T, perturb_flow_fluid: 1, perturb_flow_mag: 0.1 whose golden output should fall in [1.0, 1.1] for a unit-velocity IC.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.75%. Comparing base (d513442) to head (381ef1e).

Files with missing lines	Patch %	Lines
src/pre_process/m_perturbation.fpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1397      +/-   ##
==========================================
- Coverage   64.75%   64.75%   -0.01%     
==========================================
  Files          71       71              
  Lines       18721    18722       +1     
  Branches     1551     1552       +1     
==========================================
  Hits        12123    12123              
- Misses       5640     5641       +1     
  Partials      958      958              

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