mod_dt.t

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module mod_dt
  implicit none
  private
  public :: setdt
 
contains
  !>setdt  - set dt for all levels between levmin and levmax. 
  !>         dtpar>0  --> use fixed dtpar for all level
  !>         dtpar<=0 --> determine CFL limited timestep 
  subroutine setdt()
    use mod_global_parameters
    use mod_physics
    use mod_usr_methods, only: usr_get_dt
    use mod_supertimestepping, only: set_dt_sts_ncycles, is_sts_initialized, sourcetype_sts,sourcetype_sts_split
    use mod_comm_lib, only: mpistop
  
    integer :: iigrid, igrid, ncycle, ncycle2, ifile, idim
    double precision   :: dtnew, qdtnew, dtmin_mype, factor, dx^d, dxmin^d
    double precision   :: dtmax, dxmin, cmax_mype
    double precision   :: tco_mype, tco_global, tmax_mype, t_peak
    double precision   :: trac_alfa, trac_dmax, trac_tau, t_bott
    integer, parameter :: niter_print = 2000
  
    if (dtpar<=zero) then
      dtmin_mype  = bigdouble
      cmax_mype   = zero
      tco_mype    = zero
      tmax_mype   = zero
      !$OMP PARALLEL DO PRIVATE(igrid,qdtnew,dtnew,dx^D) REDUCTION(min:dtmin_mype) REDUCTION(max:cmax_mype)
      do iigrid=1,igridstail_active; igrid=igrids_active(iigrid);
        dtnew=bigdouble
        dx^d=rnode(rpdx^d_,igrid);
        ^d&dxlevel(^d)=rnode(rpdx^d_,igrid);
        block=>ps(igrid)
        if(local_timestep) then
          ps(igrid)%dt(ixm^t)=bigdouble
        endif
        call getdt_courant(ps(igrid)%w,ixg^ll,ixm^ll,qdtnew,dx^d,ps(igrid)%x,&
             cmax_mype)
        dtnew=min(dtnew,qdtnew)
  
        call phys_get_dt(ps(igrid)%w,ixg^ll,ixm^ll,qdtnew,dx^d,ps(igrid)%x)
        dtnew=min(dtnew,qdtnew)
  
        if (associated(usr_get_dt)) then
           call usr_get_dt(ps(igrid)%w,ixg^ll,ixm^ll,qdtnew,dx^d,ps(igrid)%x)
           dtnew          = min(dtnew,qdtnew)
        end if
        dtmin_mype     = min(dtmin_mype,dtnew)
      end do
      !$OMP END PARALLEL DO
    else
       dtmin_mype=dtpar
    end if
  
    if (dtmin_mype<dtmin) then
       write(unitterm,*)"Error: Time step too small!", dtmin_mype
       write(unitterm,*)"on processor:", mype, "at time:", global_time," step:", it
       write(unitterm,*)"Lower limit of time step:", dtmin
       crash=.true.
    end if
  
    if (slowsteps>it-it_init+1) then
       factor=one-(one-dble(it-it_init+1)/dble(slowsteps))**2
       dtmin_mype=dtmin_mype*factor
    end if
  
    if(final_dt_reduction)then
       !if (dtmin_mype>time_max-global_time) then
       !   write(unitterm,*)"WARNING final timestep artificially reduced!"
       !   write(unitterm,*)"on processor:", mype, "at time:", global_time," step:", it
       !endif
       if(time_max-global_time<=dtmin) then
          !write(unitterm,*)'Forcing to leave timeloop as time is reached!'
          final_dt_exit=.true.
       endif
       dtmin_mype=min(dtmin_mype,time_max-global_time)
    end if
  
    if (dtpar<=zero) then
       call mpi_allreduce(dtmin_mype,dt,1,mpi_double_precision,mpi_min, &
                          icomm,ierrmpi)
    else
       dt=dtmin_mype
    end if
  
    if(any(dtsave(1:nfile)<bigdouble).or.any(tsave(isavet(1:nfile),1:nfile)<bigdouble))then
       dtmax = minval(ceiling(global_time/dtsave(1:nfile))*dtsave(1:nfile))-global_time
       do ifile=1,nfile
          dtmax = min(tsave(isavet(ifile),ifile)-global_time,dtmax)
       end do
       if(dtmax > smalldouble)then 
         dt=min(dt,dtmax)
       else
         ! dtmax=0 means dtsave is divisible by global_time
         dt=min(dt,minval(dtsave(1:nfile)))
       end if      
    end if
  
    if(mype==0) then
      if(any(dtsave(1:nfile)<dt)) then
        write(unitterm,*) 'Warning: timesteps: ',dt,' exceeding output intervals ', dtsave(1:nfile)
      endif
    endif   
    if(is_sts_initialized()) then
      if(sourcetype_sts .eq. sourcetype_sts_split) then
        qdtnew = 0.5d0 * dt 
        if (set_dt_sts_ncycles(qdtnew)) then
          dt = 2.d0*qdtnew
          !a quick way to print the reduction of time only every niter_print iterations
          !Note that niter_print is a parameter variable hardcoded to the value of 200
          if(mype==0 .and. mod(it-1, niter_print) .eq. 0) then
            write(*,*) 'Max number of STS cycles exceeded, reducing dt to',dt
          endif
        endif  
      else
        if(set_dt_sts_ncycles(dt))then 
         if(mype==0 .and. mod(it-1, niter_print) .eq. 0) then
           write(*,*) 'Max number of STS cycles exceeded, reducing dt to',dt
         endif
        endif
      endif
    endif
  
    ! global Lax-Friedrich finite difference flux splitting needs fastest wave-speed
    ! so does GLM: 
    if(need_global_cmax)   call mpi_allreduce(cmax_mype,  cmax_global,  1,&
         mpi_double_precision,mpi_max,icomm,ierrmpi)
  
    ! transition region adaptive thermal conduction (Johnston 2019 ApJL, 873, L22)
    ! transition region adaptive thermal conduction (Johnston 2020 A&A, 635, 168)
    if(phys_trac) then
      t_bott=2.d4/unit_temperature
      call mpi_allreduce(tmax_mype,t_peak,1,mpi_double_precision,&
           mpi_max,icomm,ierrmpi)
      if(phys_trac_type==1) then
        !> 1D TRAC method
        trac_dmax=0.1d0
        trac_tau=1.d0/unit_time
        trac_alfa=trac_dmax**(dt/trac_tau)
        tco_global=zero
        {^ifoned
        call mpi_allreduce(tco_mype,tco_global,1,mpi_double_precision,&
             mpi_max,icomm,ierrmpi)
        }
      endif
      if(.not. associated(phys_trac_after_setdt)) call mpistop("phys_trac_after_setdt not set")
      ! trac_alfa,tco_global are set only for phys_trac_type=1, should not be a problem when not initialized
      ! side effect of modifying T_bott from mod_trac -> T_bott sent as param
      call phys_trac_after_setdt(tco_global,trac_alfa,t_peak, t_bott)
    end if 
  
    contains
  
      !> compute CFL limited dt (for variable time stepping)
      subroutine getdt_courant(w,ixI^L,ixO^L,dtnew,dx^D,x,cmax_mype)
        use mod_global_parameters
        use mod_physics, only: phys_get_cmax, &
                               phys_get_tcutoff,phys_get_auxiliary, phys_to_primitive
  
        integer, intent(in) :: ixI^L, ixO^L
        double precision, intent(in) :: x(ixI^S,1:ndim)
        double precision, intent(in)    :: dx^D
        double precision, intent(inout) :: w(ixI^S,1:nw), dtnew, cmax_mype
  
        double precision :: courantmax, dxinv(1:ndim), courantmaxtot, courantmaxtots
        double precision :: cmax(ixI^S), cmaxtot(ixI^S), wprim(ixI^S,1:nw)
        double precision :: tco_local, Tmax_local
        integer :: idims
        integer :: hxO^L
  
        dtnew=bigdouble
  
        ! local timestep dt has to be calculated in the 
        ! extended region because of the calculation from the
        ! div fluxes in mod_finite_volume
        if(local_timestep) then
          hxomin^d=ixomin^d-1; 
          hxomax^d=ixomax^d; 
        else
          hxomin^d=ixomin^d; 
          hxomax^d=ixomax^d; 
        end if
  
        ! use primitive variables to get sound speed faster
        wprim=w
        call phys_to_primitive(ixi^l,ixi^l,wprim,x)
 
        if(phys_trac) then
          call phys_get_tcutoff(ixi^l,ixo^l,wprim,x,tco_local,tmax_local)
          {^ifoned tco_mype=max(tco_mype,tco_local) }
          tmax_mype=max(tmax_mype,tmax_local)
        end if
  
        ! these are also calculated in hxO because of local timestep
        if(nwaux>0) call phys_get_auxiliary(ixi^l,hxo^l,w,x)
  
        select case (type_courant)
        case (type_maxsum)
          if(slab_uniform) then
            ^d&dxinv(^d)=one/dx^d;
            do idims=1,ndim
              call phys_get_cmax(wprim,x,ixi^l,hxo^l,idims,cmax)
              if(need_global_cmax) cmax_mype = max(cmax_mype,maxval(cmax(ixo^s)))
              if(idims==1) then
                cmaxtot(hxo^s)=cmax(hxo^s)*dxinv(idims)
              else
                cmaxtot(hxo^s)=cmaxtot(hxo^s)+cmax(hxo^s)*dxinv(idims)
              end if
            end do
          else
            do idims=1,ndim
              call phys_get_cmax(wprim,x,ixi^l,hxo^l,idims,cmax)
              if(need_global_cmax) cmax_mype = max(cmax_mype,maxval(cmax(ixo^s)))
              if(idims==1) then
                cmaxtot(hxo^s)=cmax(hxo^s)/block%ds(hxo^s,idims)
              else
                cmaxtot(hxo^s)=cmaxtot(hxo^s)+cmax(hxo^s)/block%ds(hxo^s,idims)
              end if
            end do
          end if
          ! courantmaxtots='max(summed c/dx)'
          courantmaxtots=maxval(cmaxtot(ixo^s))
          if(courantmaxtots>smalldouble) dtnew=min(dtnew,courantpar/courantmaxtots)
          if(local_timestep) then
            block%dt(hxo^s) = courantpar/cmaxtot(hxo^s)
          end if
  
        case (type_summax)
          courantmax=zero
          courantmaxtot=zero
          if(slab_uniform) then
            ^d&dxinv(^d)=one/dx^d;
            do idims=1,ndim
              call phys_get_cmax(wprim,x,ixi^l,ixo^l,idims,cmax)
              if(need_global_cmax) cmax_mype = max(cmax_mype,maxval(cmax(ixo^s)))
              courantmax=max(courantmax,maxval(cmax(ixo^s)*dxinv(idims)))
              courantmaxtot=courantmaxtot+courantmax
            end do
          else
            do idims=1,ndim
              call phys_get_cmax(wprim,x,ixi^l,ixo^l,idims,cmax)
              if(need_global_cmax) cmax_mype = max(cmax_mype,maxval(cmax(ixo^s)))
              courantmax=max(courantmax,maxval(cmax(ixo^s)/block%ds(ixo^s,idims)))
              courantmaxtot=courantmaxtot+courantmax
            end do
          end if
          ! courantmaxtot='summed max(c/dx)'
          if (courantmaxtot>smalldouble)  dtnew=min(dtnew,courantpar/courantmaxtot)
        case (type_minimum)
          courantmax=zero
          if(slab_uniform) then
            ^d&dxinv(^d)=one/dx^d;
            do idims=1,ndim
              call phys_get_cmax(wprim,x,ixi^l,ixo^l,idims,cmax)
              if(need_global_cmax) cmax_mype = max(cmax_mype,maxval(cmax(ixo^s)))
              courantmax=max(courantmax,maxval(cmax(ixo^s)*dxinv(idims)))
            end do
          else
            do idims=1,ndim
              call phys_get_cmax(wprim,x,ixi^l,ixo^l,idims,cmax)
              if(need_global_cmax) cmax_mype = max(cmax_mype,maxval(cmax(ixo^s)))
              courantmax=max(courantmax,maxval(cmax(ixo^s)/block%ds(ixo^s,idims)))
            end do
          end if
          ! courantmax='max(c/dx)'
          if (courantmax>smalldouble) dtnew=min(dtnew,courantpar/courantmax)
        end select
      end subroutine getdt_courant
  end subroutine setdt
end module mod_dt