Numeric Instability

[h7899]

The following code encountered a minimum density issue while running. How can I solve it? Thank you!
&HEAD CHID='1', TITLE='2'/

&MESH XB=47.4,56.6,-0.4,4.9,-0.4,2.7, IJK=92,54,62, MULT_ID='mesh1' /

&MULT ID='mesh1', DX=9.2, I_UPPER=22/1474560

&MISC TMPA=20 /
&OBST ID='1', XB=50.6 250.6 0 0.1 0 2.3 SURF_ID='CONCRETE'/
&OBST ID='2', XB=50.6 250.6 4.4 4.5 0 2.3 SURF_ID='CONCRETE'/
&OBST ID='3', XB=50.6 250.6 0 4.5 0 0.05 SURF_ID='CONCRETE'/
&OBST ID='4', XB=50.6 250.6 0 4.5 2.25 2.3 SURF_ID='CONCRETE'/

&TIME T_END=2000. /
&DUMP NFRAMES=2000, DT_HRR=1., DT_DEVC=1., /

&MATL ID ='CONCRETE',
SPECIFIC_HEAT=1.22,
CONDUCTIVITY_RAMP = 'wood_k'
DENSITY=360/
&RAMP ID = 'wood_k', T = 25, F = 0.06/
&RAMP ID = 'wood_k', T = 200, F = 0.06/
&RAMP ID = 'wood_k', T = 400, F = 0.09/
&RAMP ID = 'wood_k', T = 600, F = 0.12/
&RAMP ID = 'wood_k', T = 800, F = 0.15/

&SURF ID ='CONCRETE',
BACKING='INSULATED',
MATL_ID(1,1)='CONCRETE',
MATL_MASS_FRACTION(1,1)=1.0,
THICKNESS(1)=0.05/

&SPEC ID ='PVC', FORMULA = 'C2H3Cl' /

&SPEC ID ='AIR', BACKGROUND=.TRUE.,
SPEC_ID(1)='OXYGEN',
SPEC_ID(2)='NITROGEN',
VOLUME_FRACTION(1)=1.545,
VOLUME_FRACTION(2)=5.809/
##生成物
&SPEC ID = 'HYDROGEN CHLORIDE', LUMPED_COMPONENT_ONLY = T /
&SPEC ID = 'SOOT', FORMULA='C1', LUMPED_COMPONENT_ONLY = F /
&SPEC ID ='CARBON DIOXIDE', LUMPED_COMPONENT_ONLY=.TRUE./
&SPEC ID ='CARBON MONOXIDE', LUMPED_COMPONENT_ONLY=.TRUE./
&SPEC ID ='WATER VAPOR', LUMPED_COMPONENT_ONLY=.TRUE./
&SPEC ID ='NITROGEN', LUMPED_COMPONENT_ONLY=.TRUE./
&SPEC ID ='OXYGEN', LUMPED_COMPONENT_ONLY=.TRUE./

PVC

&SPEC ID ='PRODUCT_3',
SPEC_ID(1)='HYDROGEN CHLORIDE', VOLUME_FRACTION(1)=1.0,
SPEC_ID(2)='WATER VAPOR', VOLUME_FRACTION(2)=1.0,
SPEC_ID(3)='CARBON MONOXIDE', VOLUME_FRACTION(3)=0.11,
SPEC_ID(4)='CARBON DIOXIDE', VOLUME_FRACTION(4)=0.99,
SPEC_ID(5)='NITROGEN', VOLUME_FRACTION(5)=5.809
SPEC_ID(6)='SOOT', VOLUME_FRACTION(6)=0.9 /

&REAC FUEL='PVC',
HEAT_OF_COMBUSTION=17700,
SOOT_YIELD=0.172
CO_YIELD=0.05
RADIATIVE_FRACTION=0.456
SPEC_ID_NU='PVC','AIR','PRODUCT_3',
NU=-1,-1, 1 /

&MATL ID = 'pvc'
DENSITY = 1500
EMISSIVITY = 0.9
CONDUCTIVITY = 0.026
SPECIFIC_HEAT = 6.266
HEAT_OF_COMBUSTION = 2025/

&MATL ID = 'T'
DENSITY = 8940
CONDUCTIVITY = 387
SPECIFIC_HEAT = 0.38/

&SURF ID = 'DL'
COLOR = 'PURPLE'
BURN_AWAY = .TRUE.
MATL_ID(1:2,1) = 'pvc','T'
THICKNESS(1:2) = 0.006,0.004
HRRPUA = 270
RAMP_Q = 'ramp_q'
IGNITION_TEMPERATURE = 419/
&RAMP ID = 'ramp_q', T = 0, F = 0/
&RAMP ID = 'ramp_q', T =30, F = 1/
&RAMP ID = 'ramp_q', T =270, F = 1/
&RAMP ID = 'ramp_q', T =510, F = 0.5/
&RAMP ID = 'ramp_q', T =750, F = 0/
&SURF ID='fire',HRRPUA=9881,COLOR='RED',
BURN_DURATION=158/

&OBST ID = 'ra1 ' , XB = 55 245 0 1.5 0.6 0.7 SURF_ID = 'DL'
MULT_ID='1'/
&OBST ID = 'ra2 ' , XB = 55 245 3 4.5 0.6 0.7 SURF_ID = 'DL'
MULT_ID='2'/
&MULT ID='1'

DZ=0.3,K_UPPER=4/

&MULT ID='2'

DZ=0.3,K_UPPER=4/

&OBST ID='burn1', XB=149.8 150.2 0.7 1.1 0.3 0.4 SURF_ID6='INERT','INERT','INERT','INERT','INERT','fire'/
&SLCF PBY=2.3, QUANTITY='TEMPERATURE', CELL_CENTERED=.TRUE. /
&SLCF PBY=2.3, QUANTITY='VELOCITY', VECTOR=.TRUE. /
&SLCF PBY=2.3, QUANTITY='VOLUME FRACTION', SPEC_ID='OXYGEN', CELL_CENTERED=.TRUE. /
&SLCF PBY=2.3, QUANTITY='VOLUME FRACTION', SPEC_ID='CARBON MONOXIDE', CELL_CENTERED=.TRUE. /
&SLCF PBY=2.3, QUANTITY='PRESSURE', CELL_CENTERED=.TRUE. /
&VENT MB='XMIN', SURF_ID='OPEN' /
&VENT MB='XMAX', SURF_ID='OPEN' /
&VENT MB='YMIN', SURF_ID='OPEN' /
&VENT MB='YMAX', SURF_ID='OPEN' /
&VENT MB='ZMIN', SURF_ID='OPEN' /
&VENT MB='ZMAX', SURF_ID='OPEN' /

&TAIL /

[mcgratta]

I will run the case.

[mcgratta]

At what time during the simulation did the instability occur?

[h7899]

Hello, this case encountered a minimum density error at 241 seconds.

[mcgratta]

Then this case will run for several days.

[h7899]

Thank you very much. If you find a solution during operation, please let me know. Thank you again.

[mcgratta]

The case failed with a numerical instability at around 200 s with the latest source code. The cause was a locally high temperature that led to a low value of density that is often a sign a oncoming instability. FDS tries to adjust the time step to stop the instability, but it failed to do so. There is no obvious problem here, but several things to try

1. BURN_AWAY=T is often the cause of instabilities. If you do not absolutely need it, don't use it.
2. Custom or complex chemistry. Do you really need it? I suggest you use simple chemistry where you just specify a soot and CO yield. If you have other toxic gases, just assume them to be some fraction of the soot or CO. Plus, tracing all these gas species costs more CPU time.
3. I suggest you make the entire domain the interior of the tunnel and then use TUNNEL_PRECONDITONER=T on the PRES line. In any tunnel scenario, the pressures can oscillate significantly causing instabilities at mesh boundaries.
4. HRRPUA=9881? Are these cables made out of rocket fuel? This number seems way too high.