VENT not attached to solid construction and OPEN VENT must be exterior boundary condition

[leenkne...@gmail.com]

Dear all,

I am new to FDS and am trying to set up a simple case:
- rectangular geometry with thin obstacles defining floor, ceiling and walls
- mesh dimensions similar to geometry, except that the domain extends 2 m above the room, as well as in both x and y directions, OPEN surface id applied to all mesh boundaries except the bottom side
- fire source in the center of the room
- 4 inlets with OPEN surfaces, one in each wall
- 2 outlets with defined volume flow rate, in the ceiling

I am having problems with:
- defining the OPEN surface to the mesh boundaries: FDS complains that the OPEN VENT must be an exterior boundary condition, although I have assigned it through MB='XMIN' etc.
- defining the outlet patches: FDS complains that they are not attached to a solid construction, although I have defined the ceiling on the same z-level with OBST

Specifically, following elements are resulting in errors:
- VENT_MESHBOUNDARIES_xxx
- VENT_OUTLET_x

Feedback is greatly appreciated, I am probably missing something basic..

Here is my code:

&HEAD CHID='Testmodell_SLE'
      /
     
&TIME T_END=2.
      /

!!! --- Geometric entities (from Blender Objects)

&MESH ID='MESH_Meshblock'
      IJK=162,87,44
      XB=0.000,28.400,0.000,13.400,0.000,6.800

!!! --- Fire Reaction

&REAC FUEL='PROPANE'
    SOOT_YIELD=0.01
    /

!!! --- Surfaces with special boundary conditions (not inert)
     
&SURF ID='SURF_FIRE'
      RGB=204,29,1
      HRRPUA=500.
      /     

&SURF ID='SURF_OUTLET'
      RGB=204,204,204
      VOLUME_FLOW=2
      / 
     
!!! --- Surfaces with inert boundary conditions but with assigned color

&SURF ID='SURF_BODEN'
      RGB=204,204,204
      /
&SURF ID='SURF_DECKE'
      RGB=204,204,204
      /

&SURF ID='SURF_IW'
      RGB=204,204,204
      /     

!!! --- Obstructions: define geometry of SOLID walls inside mesh domain and refer to surf_id to assign BDC and/or color

&OBST ID='OBST_BODEN'
      SURF_ID='SURF_BODEN'
      XB=0.000,28.4,0.000,13.400,0.000,0.000
      /
&OBST ID='OBST_DECKE'
      SURF_ID='SURF_DECKE'
      XB=0.000,28.4,0.000,13.400,4.800,4.800
      /
&OBST ID='OBST_IW_LEFT'
      FYI='internal walls'
      SURF_ID='SURF_IW'
      XB=0.000,28.400,13.400,13.400,0.000,4.800
      /     
&OBST ID='OBST_IW_RIGHT'
      FYI='internal walls'
      SURF_ID='SURF_IW'
      XB=0.000,28.400,0.000,0.000,0.000,4.800
      /
&OBST ID='OBST_IW_FRONT'
      FYI='internal walls'
      SURF_ID='SURF_IW'
      XB=0.000,0.000,0.000,13.400,0.000,4.800
      /
&OBST ID='OBST_IW_BACK'
      FYI='internal walls'
      SURF_ID='SURF_IW'
      XB=28.400,28.400,0.000,13.400,0.000,4.800
      /         

!!! --- Vents: define geometry of special surfaces inside mesh domain and refer to surf_id to assign BDC and/or color     

! fire vent     
&VENT ID='VENT_FIRE'
      XB=14.0,15.0,5.000,6.000,0.000,0.000
      SURF_ID='SURF_FIRE'
      /

&VENT ID='VENT_MESHBOUNDARIES_TOP'
      MB='ZMAX'
      !XB=-2.000,30.400,-2.000,15.400,6.800,6.800
      SURF_ID='OPEN'
      MESH_ID='MESH_Meshblock'
      /

&VENT ID='VENT_MESHBOUNDARIES_LEFT'
      MB='YMAX'
      !XB=-2.000,30.400,15.400,15.400,-2.000,6.800
      SURF_ID='OPEN'
      MESH_ID='MESH_Meshblock'
      /

&VENT ID='VENT_MESHBOUNDARIES_RIGHT'
      MB='YMIN'
      !XB=-2.000,30.400,-2.000,-2.000,-2.000,6.800
      SURF_ID='OPEN'
      MESH_ID='MESH_Meshblock'
      /

&VENT ID='VENT_MESHBOUNDARIES_FRONT'
      MB='XMIN'
      !XB=-2.000,-2.000,-2.000,15.400,-2.000,6.800
      SURF_ID='OPEN'
      MESH_ID='MESH_Meshblock'
      /

&VENT ID='VENT_MESHBOUNDARIES_BACK'
      MB='XMAX'
      !XB=30.400,30.400,-2.000,15.400,-2.000,6.800
      SURF_ID='OPEN'
      MESH_ID='MESH_Meshblock'
      /      
     
! inlet vents
&VENT ID='Inlet_0'
      XB=0.000,0.000,11.400,13.400,0.000,3.200
      SURF_ID='OPEN'
      /

&VENT ID='Inlet_1'
      XB=0.000,2.000,0.000,0.000,0.000,3.200
      SURF_ID='OPEN'
      /

&VENT ID='Inlet_2'
      XB=26.400,28.400,13.400,13.400,0.000,3.200
      SURF_ID='OPEN'
      /

&VENT ID='Inlet_3'
      XB=28.400,28.400,0.000,2.000,0.000,3.200
      SURF_ID='OPEN'
      /

! outlet vents

&VENT ID='VENT_OUTLET_1'
     XB=3.400,3.900,3.200,10.200,4.800,4.800
     SURF_ID='SURF_OUTLET'
      OBST_ID='OBST_DECKE'
      /
&VENT ID='VENT_OUTLET_2'
      XB=24.400,24.900,3.200,10.200,4.800,4.800
      SURF_ID='SURF_OUTLET'
      /

&TAIL /

[leenkne...@gmail.com]

Dear all,

FYI, I have fixed the problem with OPEN mesh boundaries. In fact, they were correctly defined, but the error referred to the VENT Inlet_x, which were indeed trying to put an OPEN surface on a surface inside the domain. This was solved by replacing the VENTS for the inlets by punching HOLES in the walls:
&HOLE XB=-0.200,0.200,11.400,13.400,0.000,3.200 /
&HOLE XB=0.000,2.000,-0.200,0.200,0.000,3.200 /
&HOLE XB=26.400,28.400,13.200,13.600,0.000,3.200 /
&HOLE XB=28.200,28.600,0.000,2.000,0.000,3.200 /

My other problem remains: VENT_OUTLET_x produces an error without further specification.

&SURF ID='SURF_OUTLET'
      RGB=204,204,204
      VOLUME_FLOW=2
      / 

&SURF ID='SURF_DECKE'
      RGB=204,204,204
      /

&OBST ID='OBST_DECKE'
      SURF_ID='SURF_DECKE'
      XB=0.000,28.4,0.000,13.400,4.800,4.800      /

&VENT ID='VENT_OUTLET_1'
     XB=3.400,3.900,3.200,10.200,4.800,4.800
     SURF_ID='SURF_OUTLET'
      OBST_ID='OBST_DECKE'      /

Thanks!

Sarah

[Dave McGill]

Hi Sarah,

A vent cannot be backed by a zero-thickness obstruction. If you change,

&OBST ID='OBST_DECKE'
      SURF_ID='SURF_DECKE'
      XB=0.000,28.4,0.000,13.400,4.800,4.800      /

to 

&OBST ID='OBST_DECKE'
      SURF_ID='SURF_DECKE'

      XB=0.000,28.4,0.000,13.400,4.600,4.800      /

Then it should work.

Dave

[leenkne...@gmail.com]

Hi Dave

Thanks for the feedback!
I changed it as you said, and the case runs without errors, but my outlet is now only assigned to the top surface of the ceiling slab (z 4.8) and no air is extracted from the room.
I tried making a HOLE through the ceiling, but then again, the VENT is no longer assigned to a solid OBST.

I guess the easy solution is to move the VENT to the bottom side of the ceiling: z 4.6? That seems to work. :-)

Thanks again,

Sarah

[Dave McGill]

Sarah,

That sounds right. I made the (incorrect) assumption that the vent was on the upper side of the obstruction. It's good practice to put a velocity vector slice file through all vents to confirm that the air is moving in the desired direction.

Dave