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Abort on a fluid cell with no valid time step instead of dropping it from the dt reduction #1952

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Merged
sbryngelson merged 2 commits into from
Oct 9, 2026
Merged

Abort on a fluid cell with no valid time step instead of dropping it from the dt reduction #1952

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Abort on a fluid cell with no valid time step instead of dropping it …
sbryngelson Oct 7, 2026
9f83377
Merge branch 'master' into fix-dt-nan
sbryngelson Oct 8, 2026
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31 changes: 30 additions & 1 deletion src/simulation/m_time_steppers.fpp
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Original file line number Diff line number Diff line change
Expand Up @@ -684,6 +684,7 @@ contains
real(wp), dimension(5) :: dt_candidates_glb !< Global dt candidates (ICFL, VCFL, CCFL, TCFL, collision cap)
real(wp) :: dt_prev
logical :: is_fluid_cell !< Cell lies outside every immersed boundary
integer :: bad_cell !< 1 + linear index of a fluid cell with no valid dt, 0 if none
integer :: j, k, l !< Generic loop iterators
integer :: fl !< Fluid loop iterator

Expand All @@ -697,9 +698,10 @@ contains
ccfl_dt_local = huge(1.0_wp)
tcfl_dt_local = huge(1.0_wp)
coll_dt_local = huge(1.0_wp)
bad_cell = 0
$:GPU_PARALLEL_LOOP(collapse=3, private='[vel, alpha, alpha_rho, Re, rho, vel_sum, pres, gamma, pi_inf, c, qv, fl, &
& max_dt, is_fluid_cell]', reduction='[[icfl_dt_local, vcfl_dt_local, ccfl_dt_local, &
& tcfl_dt_local]]', reductionOp='[min]')
& tcfl_dt_local], [bad_cell]]', reductionOp='[min, max]')
do l = 0, p
do k = 0, n
do j = 0, m
Expand Down Expand Up @@ -733,6 +735,10 @@ contains

call s_compute_dt_from_cfl(vel, c, max_dt, rho, Re, alpha, alpha_rho, j, k, l)

! min() drops a NaN candidate, so a cell with a NaN or non-positive dt would otherwise vanish from
! the reduction and dt would come from the cells still valid.
if (.not. (max_dt(1) > 0._wp)) bad_cell = max(bad_cell, 1 + j + (m + 1)*(k + (n + 1)*l))

icfl_dt_local = min(icfl_dt_local, max_dt(1))
vcfl_dt_local = min(vcfl_dt_local, max_dt(2))
ccfl_dt_local = min(ccfl_dt_local, max_dt(3))
Expand All @@ -743,6 +749,8 @@ contains
end do
$:END_GPU_PARALLEL_LOOP()

if (bad_cell > 0) call s_abort_on_bad_dt_cell(bad_cell - 1)

! restrict the time step so an ongoing collision spans at least collision_temporal_resolution time steps; the collision
! flag is rank-local, so the cap enters as a candidate before the global elementwise min propagates it to all ranks
if (collision_model > 0 .and. collision_temporal_resolution > 0) then
Expand Down Expand Up @@ -775,6 +783,27 @@ contains

end subroutine s_compute_dt

!> Abort with the location of a fluid cell whose state admits no valid time step (NaN or non-positive inviscid dt).
impure subroutine s_abort_on_bad_dt_cell(idx)

integer, intent(in) :: idx !< Rank-local linear index j + (m+1)*(k + (n+1)*l)
integer :: j, k, l
real(wp) :: loc(3)
character(len=256) :: msg

j = mod(idx, m + 1)
k = mod(idx/(m + 1), n + 1)
l = idx/((m + 1)*(n + 1))
loc = 0._wp
loc(1) = x_cc(j)
if (n > 0) loc(2) = y_cc(k)
if (p > 0) loc(3) = z_cc(l)
write (msg, '(A,I0,A,3(I0,1X),A,3(ES12.4,1X),A)') 'No valid time step: rank ', proc_rank, ', local cell (j,k,l) = ', j, &
& k, l, ', x = ', loc, '. Its state is NaN or unphysical.'
call s_mpi_abort(trim(msg))

end subroutine s_abort_on_bad_dt_cell

!> Apply the body forces source term at each Runge-Kutta stage
subroutine s_apply_bodyforces(q_cons_vf, q_prim_vf_in, rhs_vf_in, ldt)

Expand Down
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