bug: 1D flow perturbation applies random factor twice due to index aliasing

[sbryngelson]

Summary

In src/pre_process/m_perturbation.fpp, s_perturb_surrounding_flow applies a random perturbation to momentum:

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 (n == 0, p == 0), num_vels == 1, so eqn_idx%mom%end == eqn_idx%mom%beg. Line 1 writes rand * v0 into the only velocity slot, then line 2 reads that same (now-modified) slot and writes (1 + rand) * rand * v0. The perturbation is applied twice and compounded, giving a result of rand*(1+rand)*v0 instead of the intended (1+rand)*v0.

Fix

Swap the two lines so mom%beg is scaled first, then mom%end is set from the original value:

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

Discovered

Found during review of PR #1365. Pre-existing in master.