# Drag/lift force evaluation

I wonder if anyone could kindly give me some advices with Momentum Exchange method(MEM). I doubt maybe I didn’t understand well with the related paper. Renwei Mei’s paper–Force Evaluation in the Lattice Boltzmann Method Involving Curved Geometry

I made code to calculate Poiseuille flow, the velocity contour looks good. then I tried to calculate the Drag force and lift force on the top wall with MEM, as following fortran codes. But the result is totally wrong… Dragforce is around -1, and Liftforce is around -77., I think reasonally the drag force should be large value and liftforce should be very small…–!

Model: incompressible LBGK model.
Parameters: Re=20, Umax=0.05, nx=200,ny=40…
BCs: Top and bottom is simple bounce back; inlet: parabolic velocity profile; outlet: 0-gradient of velocity and fixed density to 1.
Calculation seququnce: Collision step–>>Boundary Condition treatment–>> Force evaluation–>>Stream step–>>Collision…
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subroutine evaluatingForce
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m(4)=0
n(4)=-1
m(7)=-1
n(7)=-1
m(8)=1
n(8)=-1
fdrag=0.d0
flift=0.d0

do i=1,nx
fdrag = fdrag + m(7)(f(7,i,ny+1)+f(5,i-1,ny)) + m(8)(f(8,i,ny+1)+f(6,i+1,ny))
flift = flift + n(7)(f(7,i,ny+1)+f(5,i-1,ny)) + n(8)(f(8,i,ny+1)+f(6,i+1,ny)) + n(4)*(f(4,i,ny+1)+f(2,i,ny))
enddo
return
end
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