Upstream velocity contour appears non-smooth regardless of simulation setup

[2000-lang]

Dear FDS Users,

I am working on an FDS model of wind flow through a street-canyon configuration with buildings and tree/fire regions. I am repeatedly observing that the upstream velocity contour is not smooth in the inflow region. Instead of a clean contour, the upstream field looks striped or patchy.

What is confusing is that this behavior seems to persist regardless of the simulation changes I make. Even after modifying the mesh arrangement and extending the upstream domain, the upstream contour still does not appear smooth.

My setup has:

• wind-driven inflow
• multiple nested mesh regions
• finer mesh near the fire/tree region
• coarser mesh in the outer domain
• velocity slices used to visualize the upstream field

The main issue is this:

Why does the upstream velocity contour remain non-smooth even before the flow reaches the main obstruction region? Is this expected because FDS is resolving instantaneous LES fluctuations, or could it be related to mesh transitions, inflow boundary treatment, or slice visualization/interpolation?

I would appreciate guidance on:

1. whether a non-smooth upstream contour is normal in FDS,
2. what mainly causes this behavior,
3. how to obtain a smoother upstream velocity contour for presentation or analysis.

Thank you.

[marcosvanella]

This is probably due to having a coarse mesh around the obstacle. Have you tried running the case with a finer mesh? Standing waves like these in coarse meshes are a result of the treatment of momentum advective fluxes; this behavior is not unexpected. You conserve momentum and kinetic energy but can get fluctuations at grid size wavenumbers with coarse resolution, even in LES. That said, is your inflow boundary at x=0?

[marcosvanella]

This is what a basic ChatGPT subscription will buy you on this topic :). In your case the large momentum fluxes happen around the front and corners of the object where the flow is being stopped. The AI took some artistic liberty.

[2000-lang]

Yes my inflow boundary is at x=0. I am running with a coarse mesh. Even for fine-coarse mesh fds inputs the upstream is not smooth.

&HEAD CHID='Tree_fire', TITLE='Street Canyon tree fire' /

! =====================================================
! 0.25 m FIRE REGION | Z = 0–20
! =====================================================
! ---------- FIRE1 (Z = 0–10, Y = 24–59) ----------
&MESH ID='FINE_MESH_1', IJK=36,70,40, XB=108,117,24,41.5, 0,10 /
&MESH ID='FINE_MESH_2', IJK=36,70,40, XB=117,126,24,41.5, 0,10 /
&MESH ID='FINE_MESH_3', IJK=36,70,40, XB=108,117,41.5,59, 0,10 /
&MESH ID='FINE_MESH_4', IJK=36,70,40, XB=117,126,41.5,59, 0,10 /

! ---------- FIRE2 (Z = 0–10, Y = 59–94) ----------
&MESH ID='FINE_MESH_5', IJK=36,70,40, XB=108,117,59,76.5, 0,10 /
&MESH ID='FINE_MESH_6', IJK=36,70,40, XB=117,126,59,76.5, 0,10 /
&MESH ID='FINE_MESH_7', IJK=36,70,40, XB=108,117,76.5,94, 0,10 /
&MESH ID='FINE_MESH_8', IJK=36,70,40, XB=117,126,76.5,94, 0,10 /

! ---------- FIRE3 (Z = 10–20, Y = 24–59) ----------
&MESH ID='FINE_MESH_9', IJK=36,70,40, XB=108,117,24,41.5, 10,20 /
&MESH ID='FINE_MESH_10', IJK=36,70,40, XB=117,126,24,41.5, 10,20 /
&MESH ID='FINE_MESH_11', IJK=36,70,40, XB=108,117,41.5,59, 10,20 /
&MESH ID='FINE_MESH_12', IJK=36,70,40, XB=117,126,41.5,59, 10,20 /

! ---------- FIRE4 (Z = 10–20, Y = 59–94) ----------
&MESH ID='FINE_MESH_13', IJK=36,70,40, XB=108,117,59,76.5, 10,20 /
&MESH ID='FINE_MESH_14', IJK=36,70,40, XB=117,126,59,76.5, 10,20 /
&MESH ID='FINE_MESH_15', IJK=36,70,40, XB=108,117,76.5,94, 10,20 /
&MESH ID='FINE_MESH_16', IJK=36,70,40, XB=117,126,76.5,94, 10,20 /

! =====================================================
! 0.5 m BUFFER RING | Z = 0–20
! =====================================================
&MESH ID='BUF_L', IJK=8,156,40, XB=104,108,20,98, 0,20 /
&MESH ID='BUF_R', IJK=8,156,40, XB=126,130,20,98, 0,20 /
&MESH ID='BUF_F', IJK=36,8,40, XB=108,126,20,24, 0,20 /
&MESH ID='BUF_B', IJK=36,8,40, XB=108,126,94,98, 0,20 /

! =====================================================
! 1 m TRANSITION RING | Z = 0–20
! =====================================================
&MESH ID='TR_L_Z0', IJK=8,94,20, XB= 96,104,12,106, 0,20 /
&MESH ID='TR_R_Z0', IJK=8,94,20, XB=130,138,12,106, 0,20 /
&MESH ID='TR_F_Z0', IJK=26,8,20, XB=104,130,12, 20, 0,20 /
&MESH ID='TR_B_Z0', IJK=26,8,20, XB=104,130,98,106, 0,20 /

! =====================================================
! 0.5 m LAYER | Z = 20–30
! =====================================================
&MESH ID='PLM05_1', IJK=52,78,20, XB=104,130,20,59,20,30 /
&MESH ID='PLM05_2', IJK=52,78,20, XB=104,130,59,98,20,30 /

! =====================================================
! 1 m TRANSITION RING UPPER | Z = 20–40
! =====================================================
&MESH ID='TR_L_Z20', IJK=8,94,20, XB= 96,104,12,106,20,40 /
&MESH ID='TR_R_Z20', IJK=8,94,20, XB=130,138,12,106,20,40 /
&MESH ID='TR_F_Z20', IJK=26,8,20, XB=104,130,12, 20,20,40 /
&MESH ID='TR_B_Z20', IJK=26,8,20, XB=104,130,98,106,20,40 /

! =====================================================
! 1 m LAYER | Z = 30–40
! =====================================================
&MESH ID='PLM10', IJK=26,78,10, XB=104,130,20,98,30,40 /

! =====================================================
! 2 m OUTER DOMAIN | Z = 0–90 / 40–90
! =====================================================
! =====================================================
! 2 m OUTER DOMAIN | Z = 0–90
! =====================================================

&MESH ID='OUT_NX1', IJK=25,32,45, XB=-200,-150, 0, 64, 0,90 /
&MESH ID='OUT_NX2', IJK=25,32,45, XB=-200,-150, 64,128, 0,90 /
&MESH ID='OUT_NX3', IJK=25,32,45, XB=-150,-100, 0, 64, 0,90 /
&MESH ID='OUT_NX4', IJK=25,32,45, XB=-150,-100, 64,128, 0,90 /
&MESH ID='OUT_NX5', IJK=25,32,45, XB=-100, -50, 0, 64, 0,90 /
&MESH ID='OUT_NX6', IJK=25,32,45, XB=-100, -50, 64,128, 0,90 /
&MESH ID='OUT_NX7', IJK=25,32,45, XB= -50, 0, 0, 64, 0,90 /
&MESH ID='OUT_NX8', IJK=25,32,45, XB= -50, 0, 64,128, 0,90 /

&MESH ID='OUT_L1', IJK=24,32,45, XB= 0, 48, 0, 64, 0,90 /
&MESH ID='OUT_L2', IJK=24,32,45, XB= 0, 48, 64,128, 0,90 /
&MESH ID='OUT_L3', IJK=24,32,45, XB= 48, 96, 0, 64, 0,90 /
&MESH ID='OUT_L4', IJK=24,32,45, XB= 48, 96, 64,128, 0,90 /

&MESH ID='OUT_F', IJK=21,6,45, XB= 96,138, 0, 12, 0,90 /
&MESH ID='OUT_B', IJK=21,11,45, XB= 96,138, 106,128, 0,90 /
&MESH ID='OUT_R1', IJK=57,32,45, XB= 138,252, 0, 64, 0,90 /
&MESH ID='OUT_R2', IJK=57,32,45, XB= 138,252, 64,128, 0,90 /
&MESH ID='OUT_TC', IJK=21,47,25, XB= 96,138, 12,106,40,90 /

! =====================================================
! STANDARD WOOD FUEL (pro duction-tested, proper soot)
! =====================================================

&SPEC ID='WOOD', FORMULA='CH1.7O0.74N0.002', MW=24.5 /
&SPEC ID='CARBON MONOXIDE', FORMULA='CO', MW=28.01 /
&SPEC ID='CARBON DIOXIDE', FORMULA='CO2', MW=44.01 /
&SPEC ID='SOOT', FORMULA='C', MW=12.0 /

&REAC ID='WOOD_FIRE', FUEL='WOOD', C=1.0, H=1.7, N=0.002, O=0.74, SOOT_YIELD=0.01, CO_YIELD=0.025 /

&SURF ID='INERT', RGB=128,128,128 /
&SURF ID='BUILDING_WALL', RGB=255,255,255 /

&SURF ID='TRUNK_WOOD',
MATL_ID(1,1)='DRY_PINE',
MATL_MASS_FRACTION(1,1)=1.0,
THICKNESS(1)=0.20,
HRRPUA=500.0,
SPEC_ID='WOOD',
COLOR='BROWN' /

&MATL ID='MOISTURE',
DENSITY=1000.,
CONDUCTIVITY=0.6,
SPECIFIC_HEAT=4.19 /

&MATL ID='DRY_PINE',
DENSITY=500.,
CONDUCTIVITY=0.1,
SPECIFIC_HEAT=1.0 /

! =====================================================
! WIND & VENTS
! =====================================================

&MISC TMPA=20.0 /

&TIME T_END=300.0 /

&WIND SPEED=6.0, Z_0=0.1, Z_REF=10.0, DIRECTION=270.0 /

&VENT PBX=-200.0, SURF_ID='OPEN' /
&VENT PBX=252.0, SURF_ID='OPEN' /
&VENT PBY=0.0, SURF_ID='OPEN' /
&VENT PBY=128.0, SURF_ID='OPEN' /
&VENT PBZ=90.0, SURF_ID='OPEN' /

! =====================================================
! BUILDINGS
! =====================================================

&OBST XB=90.0,108.0, 18.0,36.0, 0.0,18.0, SURF_ID='BUILDING_WALL' /
&OBST XB=126.0,144.0, 18.0,36.0, 0.0,18.0, SURF_ID='BUILDING_WALL' /
&OBST XB=90.0,108.0, 54.0,72.0, 0.0,18.0, SURF_ID='BUILDING_WALL' /
&OBST XB=126.0,144.0, 54.0,72.0, 0.0,18.0, SURF_ID='BUILDING_WALL' /
&OBST XB=90.0,108.0, 90.0,108.0, 0.0,18.0, SURF_ID='BUILDING_WALL' /
&OBST XB=126.0,144.0, 90.0,108.0, 0.0,18.0, SURF_ID='BUILDING_WALL' /

! =====================================================
! TREE TRUNKS & CROWNS (Proper wood fire)
! =====================================================

! Trunks
&OBST ID='TREE1_TRUNK', XB=116.9,117.9, 33.9,34.9, 0.0,4.0, SURF_ID='TRUNK_WOOD' /
&OBST ID='TREE2_TRUNK', XB=116.9,117.9, 62.9,63.9, 0.0,4.0, SURF_ID='TRUNK_WOOD' /
&OBST ID='TREE3_TRUNK', XB=116.9,117.9, 91.9,92.9, 0.0,4.0, SURF_ID='TRUNK_WOOD' /

&SURF ID='needle surface',
MATL_ID(1,1)='DRY_PINE',
MATL_MASS_FRACTION(1,1)=1.0,
MOISTURE_FRACTION=0.12,
SURFACE_VOLUME_RATIO=8000.,
LENGTH=0.10,
GEOMETRY='CYLINDRICAL',
HRRPUA=300.0,
SPEC_ID='WOOD',
BURN_AWAY=.TRUE.,
COLOR='FOREST GREEN' /

&PART ID='needles',
SURF_ID='needle surface',
DRAG_COEFFICIENT=2.8,
STATIC=.TRUE.,
COLOR='FOREST GREEN' /

&PROP ID='needle image',
SMOKEVIEW_ID='TUBE',
SMOKEVIEW_PARAMETERS='L=0.15,D=0.0015' /

! Three particle crowns

&INIT ID='TREE1_CROWN',
PART_ID='needles',
SHAPE='CONE',
XYZ=117.0,34.0,0.0,
CROWN_BASE_WIDTH=3.0,
CROWN_BASE_HEIGHT=4.0,
TREE_HEIGHT=8.0,
N_PARTICLES_PER_CELL=4,
MASS_PER_VOLUME=1.0,
DRY=.TRUE. /

&INIT ID='TREE2_CROWN',
PART_ID='needles',
SHAPE='CONE',
XYZ=117.0,63.0,0.0,
CROWN_BASE_WIDTH=3.0,
CROWN_BASE_HEIGHT=4.0,
TREE_HEIGHT=8.0,
N_PARTICLES_PER_CELL=4,
MASS_PER_VOLUME=1.0,
DRY=.TRUE. /

&INIT ID='TREE3_CROWN',
PART_ID='needles',
SHAPE='CONE',
XYZ=117.0,92.0,0.0,
CROWN_BASE_WIDTH=3.0,
CROWN_BASE_HEIGHT=4.0,
TREE_HEIGHT=8.0,
N_PARTICLES_PER_CELL=4,
MASS_PER_VOLUME=1.0,
DRY=.TRUE. /

!------------ OUTPUT ------------

&DUMP DT_SLCF=0.5, DT_HRR=5.0 /

! --------------------- DEVC (CRITICAL) ---------------------

&DEVC ID='U_REF_Z20', QUANTITY='U-VELOCITY', XYZ=110,63,20 /
&DEVC ID='U_REF_Z6', QUANTITY='U-VELOCITY', XYZ=110,63,6 /
&DEVC ID='TEMP_REF_Z20', QUANTITY='TEMPERATURE', XYZ=110,63,20 /

! --------------------- VELOCITY SLICES ---------------------

&SLCF QUANTITY='U-VELOCITY', PBY=63.0 /
&SLCF QUANTITY='U-VELOCITY', PBX=117.0 /
&SLCF QUANTITY='U-VELOCITY', PBZ=6.0 /
&SLCF QUANTITY='U-VELOCITY', PBZ=19.0 /

&SLCF QUANTITY='V-VELOCITY', PBY=63.0 /
&SLCF QUANTITY='V-VELOCITY', PBX=117.0 /
&SLCF QUANTITY='V-VELOCITY', PBZ=6.0 /
&SLCF QUANTITY='V-VELOCITY', PBZ=19.0 /

&SLCF QUANTITY='W-VELOCITY', PBY=63.0 /
&SLCF QUANTITY='W-VELOCITY', PBX=117.0 /
&SLCF QUANTITY='W-VELOCITY', PBZ=6.0 /
&SLCF QUANTITY='W-VELOCITY', PBZ=19.0 /

! ---------------------(FIRE INTENSITY) ---------------------

&SLCF QUANTITY='HRRPUV', PBZ=6.0 /

&TAIL /

[marcosvanella]

@2000-lang let's try a simple mesh without fine-coarse grid interfaces. For this case keep the mesh size at 1m throughout. That will give you a coarse ~8 cells in the obstacle you are showing. What do you see in terms of average velocities?

[2000-lang]

Subject: Non-smooth upstream velocity contour even in a simple rectangular FDS test

Dear FDS Users,

I have a question regarding velocity contours in FDS.

In my main case, I observed that the upstream velocity contour was not smooth and appeared patchy/irregular. At first I suspected that this was caused by my multi-mesh street-canyon setup, mesh transitions, or domain layout.

To check this, I created a much simpler test case:

• a small rectangular domain,
• uniform 0.25 m mesh,
• all outer boundaries open,
• one block placed at the center of the domain,
• wind-driven flow,
• velocity slices plotted in Smokeview.

However, even in this simplified case, the instantaneous velocity contour still appears non-smooth. The wake behind the block is visible, but there are also small irregular fluctuations in the surrounding field.

Because the same behavior appears in both the complex case and the simple rectangular test, I am now wondering whether this is simply expected behavior in FDS

&HEAD CHID='rect_test', TITLE='Simple open domain with center obstacle' /

! =====================================================
! SIMPLE RECTANGULAR DOMAIN
! Domain: 20 m x 10 m x 5 m
! Resolution: 0.25 m
! =====================================================

&MESH ID='M1', IJK=80,40,20, XB=0.0,20.0, 0.0,10.0, 0.0,5.0 /

! =====================================================
! TIME / AMBIENT / WIND
! =====================================================

&TIME T_END=300.0 /
&MISC TMPA=20.0 /

&WIND SPEED=6.0, Z_0=0.1, Z_REF=10.0, DIRECTION=270.0 /

! =====================================================
! OPEN BOUNDARIES
! =====================================================

&VENT PBX=0.0, SURF_ID='OPEN' /
&VENT PBX=20.0, SURF_ID='OPEN' /
&VENT PBY=0.0, SURF_ID='OPEN' /
&VENT PBY=10.0, SURF_ID='OPEN' /
&VENT PBZ=5.0, SURF_ID='OPEN' /

! =====================================================
! SURFACES
! =====================================================

&SURF ID='BLOCK_SURF', COLOR='GRAY' /

! =====================================================
! CENTER OBSTACLE
! Small block at domain center
! =====================================================

&OBST ID='CENTER_BLOCK', XB=9.0,11.0, 4.0,6.0, 0.0,2.0, SURF_ID='BLOCK_SURF' /

! =====================================================
! OUTPUT
! =====================================================

&DUMP DT_SLCF=0.5 /

&SLCF QUANTITY='U-VELOCITY', PBY=5.0 /
&SLCF QUANTITY='U-VELOCITY', PBZ=1.0 /

&SLCF QUANTITY='V-VELOCITY', PBY=5.0 /
&SLCF QUANTITY='V-VELOCITY', PBZ=1.0 /

&SLCF QUANTITY='W-VELOCITY', PBY=5.0 /
&SLCF QUANTITY='W-VELOCITY', PBZ=1.0 /

&TAIL /