I am trying to use a adforce command to apply a stoke's drag force on a single particle using the following syntax
atom_style granular
atom_modify map array
communicate single vel yes
boundary f f f
newton off
units si
#region reg sphere 0.1 0.1 0.1 0.4 side out
#region reg block -0.05 0.05 -0.05 0.05 0. 0.15 units box
#create_box 1 reg
region reg block -0.005 0.005 -0.005 0.005 0.000 0.002 units box
read_data velo
#processors 2 2 1
neighbor 0.002 bin
neigh_modify delay 0
# Material properties required for new 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 0.95
fix m4 all property/global coefficientFriction peratomtypepair 1 0.05
fix m5 all property/global characteristicVelocity scalar 2
fix m6 all property/global cohesionEnergyDensity peratomtypepair 1 -3000000
# pair style
pair_style gran model hertz tangential history cohesion sjkr
pair_coeff * *
# timestep, gravity
timestep 0.0000001
#fix gravi all gravity 9.81 vector 0.0 0.0 -1.0
# CFD PART
variable tsteps equal 100000000
variable tf equal 360
variable tf1 equal 10
variable dt equal v_tf1/v_tsteps
variable t equal v_dt*step
variable tc equal (v_t)/v_tf
#defining parameters
compute kash all property/atom z
compute kashi all property/atom x
compute kish all property/atom y
variable b equal c_kash[1]
variable p equal sqrt((c_kish[1])^2+(c_kashi[1])^2)
variable uz equal 10^-7
variable uz equal 10^-7
variable uz equal 10^-7
variable fx atom (vx-v_ux)*0.0000000014
variable fy atom (vy-v_uy)*0.0000000014
variable fz atom (vz-v_uz)*0.0000000014
fix ali all addforce v_fx v_fy v_fz
here ux,uy and uz are the velocities of the fluid and vx,vy and vz are the velocities of the particles defined in the data file and 0.0000000014 is the stoke's drag coeffcient. The problem here is that after few iterations the velocity of the particle reaches to 6786m/s (for example). whereas the initial particle velocity is assigned 0 and initial fluid velocity is defined as 5*10^-7. the fluid velocity remains in ^10-7 but the particle velocity reaches so high. I am wondering how can particle move faster then fluid when I am applying stokes drag