# body force caused a unkown velocity

hi all
Recently,I used the multiphase program based on Xing’s single phase model to simulate the mix of gas and water. Under without external force and body force.it was a good results.

but now,when i tried to add the body force ,there was a problem .And i think the question is that the form of body force ‘rho*g(j)’ is not correct or something .

So i simulated the static water in the tanker without the body force and the result:no velocity is yielded in the tanker.
but when i simulated static water in the tanker with the body force and the result a unkown down velocity is yielded in the tanker.

I used the paper of “Discrete lattice effects on the forcing term in the lattice Boltzmann method” the form of the body force is:

F(i)=(1-1/(2*tau))w(i)[(e(i,j)-u(j))/cs^2+(e(i,k)*u(k))*e(i,j)/cs.^4]rhog(j)

f(i) = (1 -1/tau) * f(i) + 1/tau* feq(i) +F(i)

the quilibrium velocity uu and the fluid velocity u is given as follow
rhouu= rhou =sum_fe+rhog/2

where tau is the relaxation time, w(i) lattice weights, e(i,j) lattice speeds, u(k) fluid speed, cs sound speed, rho*g(j) physical force and F(i) lattice force.

If someone had used Guo’s body force model and please tell me if the form of body force is not right or something is wrong?

ray

Hello,

the expressions in the article are correct. There must be an error in your implementation. There is also a similar thread in the forum.

Timm

Dear Ray,

You wrote that your equilibrium velocity is shifted and called it uu, however in your equilibrium function you wrote just u (micro velocity). As I know it should be uu.

Hopefully, it will help,
Alex