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FUNCTION FORWARD_CENTER_3D, DIFF=DIFF,del_x,del_y,del_z,del_t,times

ss=SIZE(DIFF.uu, /DIMENSIONS)
C_u = FLTARR(ss[0],ss[1],ss[2])
C_v = FLTARR(ss[0],ss[1],ss[2])
C_w = FLTARR(ss[0],ss[1],ss[2])



for i = 0, ss[0]-1 do begin
   for j = 0, ss[1]-1 do begin
          for k = 0, ss[2]-1 do begin
                 C_u[i,j,k] = DIFF.uu[i,j,k]*(del_t/del_x)
                 C_v[i,j,k] = DIFF.vv[i,j,k]*(del_t/del_y)
                 C_w[i,j,k] = DIFF.vv[i,j,k]*(del_t/del_z)
          endfor
   endfor
endfor

;A_FC=FLTARR(times, ss[0], ss[1],ss[2])
diff_ut = FLTARR(times,ss[0],ss[1], ss[2])
diff_vt = FLTARR(times,ss[0],ss[1], ss[2])
diff_wt = FLTARR(times,ss[0],ss[1], ss[2])
diff_tt = FLTARR(times,ss[0],ss[1], ss[2])

diff_ut(0,*,*,*)=REFORM(DIFF.uu(*,*,*))
diff_vt(0,*,*,*)=REFORM(DIFF.vv(*,*,*))
diff_wt(0,*,*,*)=REFORM(DIFF.ww(*,*,*))
diff_tt(0,*,*,*)=REFORM(DIFF.tt(*,*,*))

  FOR t=0, times-2 DO BEGIN
      FOR i=2, ss[0]-2 DO BEGIN
          FOR j=2, ss[1]-2 DO BEGIN
                FOR k=2, ss[2]-2 DO BEGIN
                      diff_ut(t+1,i,j,k)= diff_ut(t,i,j,k)+ ((0.5*(C_u[i,j,k])*(diff_ut(t,i+1,j,k)-diff_ut(t,i-1,j,k))) -(0.5*(C_v[i,j,k])*(diff_ut(t,i,j+1,k)-diff_ut(t,i,j-1,k))) -(0.5*(C_w[i,j,k])*(diff_ut(t,i,j,k+1)-diff_ut(t,i,j,k-1)))) 
                      diff_vt(t+1,i,j,k)= diff_vt(t,i,j,k)+ ((0.5*(C_u[i,j,k])*(diff_vt(t,i+1,j,k)-diff_vt(t,i-1,j,k))) -(0.5*(C_v[i,j,k])*(diff_vt(t,i,j+1,k)-diff_vt(t,i,j-1,k))) -(0.5*(C_w[i,j,k])*(diff_vt(t,i,j,k+1)-diff_vt(t,i,j,k-1)))) 
                      diff_wt(t+1,i,j,k)= diff_wt(t,i,j,k)+ ((0.5*(C_u[i,j,k])*(diff_wt(t,i+1,j,k)-diff_wt(t,i-1,j,k))) -(0.5*(C_v[i,j,k])*(diff_wt(t,i,j+1,k)-diff_wt(t,i,j-1,k))) -(0.5*(C_w[i,j,k])*(diff_wt(t,i,j,k+1)-diff_wt(t,i,j,k-1)))) 
                      diff_tt(t+1,i,j,k)= diff_tt(t,i,j,k)+ ((0.5*(C_u[i,j,k])*(diff_tt(t,i+1,j,k)-diff_tt(t,i-1,j,k))) -(0.5*(C_v[i,j,k])*(diff_tt(t,i,j+1,k)-diff_tt(t,i,j-1,k))) -(0.5*(C_w[i,j,k])*(diff_tt(t,i,j,k+1)-diff_tt(t,i,j,k-1))))                                               
                ENDFOR      
          ENDFOR
      ENDFOR
  ENDFOR
  
 difft={ut:diff_ut, vt:diff_vt, wt:diff_wt, tt:diff_tt}
 PRINT,'forward center done' 
 
RETURN, difft

END
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