deleting atoms

Hello,

I'm creating a simulation with CFDEMcoupling, but I think this is more of a LIGGGHTS question. My goal is to have a tube (cross-section in XY, length in Z) with a vertical fluid flow in the Z direction. I'm inserting particles in the tube, which may drift to the top. I would like to delete the particles when they get "stuck" to the top wall. I've put in two delete_atom commands, but neither seems to be working the way I expect them to (or at all). Here's the code:

in.liggghts_init

# Pour granular particles into chute container, then induce flow
echo both
atom_style granular
atom_modify map array
communicate single vel yes
boundary m m m
newton off
units si

region reg block -0.015 0.015 -0.015 0.015 -0.001 0.151 units box
create_box 1 reg
neighbor 0.001 bin
neigh_modify delay 0

# Material properties required for granular pair styles

fix m1 all property/global youngsModulus peratomtype 5.e6
fix m2 all property/global poissonsRatio peratomtype 0.45
fix m3 all property/global coefficientRestitution peratomtypepair 1 0.3
fix m4 all property/global coefficientFriction peratomtypepair 1 0.5

# pair style
pair_style gran model hertz tangential history # Hertzian without cohesion
pair_coeff * *

# timestep, gravity
timestep 0.00001
fix gravi all gravity 9.81 vector 0.0 0.0 -1.0

# walls
fix zwalls1 all wall/gran model hertz tangential history primitive type 1 zplane 0.0
fix zwalls2 all wall/gran model hertz tangential history primitive type 1 zplane 0.0553 #0.1500
fix cylwalls all wall/gran model hertz tangential history primitive type 1 zcylinder 0.01385 0. 0.

# particle distributions and insertion
region bc cylinder z 0.0 0.0 0.012 0. 0.055 units box
fix pts1 all particletemplate/sphere 1 atom_type 1 density constant 2000 radius constant 0.0005
fix pdd1 all particledistribution/discrete 1 1 pts1 1.0

fix ins all insert/pack seed 100001 distributiontemplate pdd1 vel constant 0. 0. 0. insert_every once overlapcheck yes all_in yes particles_in_region 1000 region bc

# apply nve integration to all particles that are inserted as single particles
fix integr all nve/sphere

# screen output
compute rke all erotate/sphere
thermo_style custom step atoms ke c_rke vol
thermo 1000
thermo_modify lost ignore norm no
compute_modify thermo_temp dynamic yes

# insert the first particles so that dump is not empty
run 1
dump dmp all custom 5000 post/dump.liggghts_init id type x y z ix iy iz vx vy vz fx fy fz omegax omegay omegaz radius
#force : f_couple_cfd[0] f_couple_cfd[1] f_couple_cfd[2]
#node : f_couple_cfd[6]
#cell id : f_couple_cfd[7]

run 10000 upto
write_restart post/restart/liggghts.restart

_________________________________________________________________
in.liggghts_run:

# Pour granular particles into chute container, then induce flow
log ../DEM/log.liggghts
thermo_log ../DEM/post/thermo.txt

atom_style granular
atom_modify map array
communicate single vel yes

boundary m m m
newton off

units si
processors 2 2 1

# read the restart file
read_restart ../DEM/post/restart/liggghts.restart

neighbor 0.0005 bin
neigh_modify delay 0

# Material properties required for granular pair styles

fix m1 all property/global youngsModulus peratomtype 5.e6
fix m2 all property/global poissonsRatio peratomtype 0.45
fix m3 all property/global coefficientRestitution peratomtypepair 1 0.3
fix m4 all property/global coefficientFriction peratomtypepair 1 0.5

# pair style
pair_style gran model hertz tangential history # Hertzian without cohesion
pair_coeff * *

# timestep, gravity
timestep 0.00001
fix gravi all gravity 9.81 vector 0.0 0.0 -1.0

# walls
fix zwalls1 all wall/gran model hertz tangential history primitive type 1 zplane 0.0
#fix zwalls2 all wall/gran model hertz tangential history primitive type 1 zplane 0.1
fix zwalls2 all wall/gran model hertz tangential history primitive type 1 zplane 0.0553
fix cylwalls all wall/gran model hertz tangential history primitive type 1 zcylinder 0.01385 0. 0.

# change the particles density
set group all density 2000

#delete atoms that collect at top
region outlet cylinder z 0. 0. 0.01385 0.01 0.0553 units box
#region outlet cylinder z 0. 0. radius low high
delete_atoms region outlet compress yes

# cfd coupling
fix cfd all couple/cfd couple_every 100 mpi
fix cfd2 all couple/cfd/force/implicit
#fix cfd2 all couple/cfd/force/accumulator RongDrag 10 1.5e-3
#fix cfd2 all couple/cfd/force/implicit/accumulated #CrankNicolson 0.5

# apply nve integration to all particles that are inserted as single particles
fix integr all nve/sphere

# center of mass
compute centerOfMass all com

# compute total dragforce
compute dragtotal all reduce sum f_dragforce[1] f_dragforce[2] f_dragforce[3]

# screen output
compute rke all erotate/sphere
thermo_style custom step atoms ke c_rke vol c_centerOfMass[3] c_dragtotal[1] c_dragtotal[2] c_dragtotal[3]
thermo 10
thermo_modify lost ignore norm no
compute_modify thermo_temp dynamic yes

dump dmp all custom 5000 ../DEM/post/dump*.liggghts_run id type x y z vx vy vz fx fy fz f_dragforce[1] f_dragforce[2] f_dragforce[3] radius

run 1

region outlet2 cylinder z 0. 0. 0.01385 0.01 0.0553 units box
#region outlet cylinder z 0. 0. radius low high
delete_atoms region outlet2 compress yes

Thanks for any insight you can provide!