[zahrae]

In my case study, I have to creat some udfs wich uses the radial velocity of the fluid and the radial gradient of velocity. I don’t know how to set these two parameters using fluent macros !
Please help !!

[AlexanderZ]

all depends on your case.
macros for velocities and gradients of velocities:

C_U(c,t)
C_V(c,t)
C_W(c,t)
G_C_U(c,t)
G_C_V(c,t)
G_C_W(c,t)

convert to cylindrical coordinates to get radial values

best regards

[zahrae]

Thanks you for your reply!

I am modeling deposition of particles by DPM in a surface of a tube and I have to code the forces responsible for this deposition, drag force depends on the radial velocity and the lift force depends on the radial gradient of velocity.
My geometry is a 3d tube inside of it the fluid flows in the Z direction.

I have created those macros for radial gradient and radial velocity:
du/dr=C_U_G(c,t)[0]

ur=C_U(c,t)[0]

0 means the x direction (I suppose the x direction is the radial direction) but I am not sure. should I use the cylindrical coordinates and how ?Can you please if possible give me some guidelines !

[AlexanderZ]

du/dr=C_U_G(c,t)[0]

ur=C_U(c,t)[0]

are wrong, read description of these macros,
cylindrical coordinates is your homework

best regards

[zahrae]

helloo!
Now I know how to express the velocities in cylindrical coordinates the difficulty lies in expressing their gradient.
The macro used for expressing the gradient in fluent is: C_U_G (c,t) [i] i=0, 1,2 for x, y, z positions respectively.
But for the cylindrical coordinates I don’t know how to express radial, tangential and axial positions.
please help I m stuck with this problem, I have to resolve this problem very queckly!!

[AlexanderZ]

google -> "cartesian coordinates to cylindrical coordinates"

first link: http://www.math.tamu.edu/~glahodny/M...ion%2013.9.pdf

best regards

[zahrae]

Thanks for your response!
But this isn't what I am searching for, By the program shown below I can found the radial,tangential and axial velocities and also the unit vectors in cylindrical coordiantes.

Now I should code the gradient of the radial velocity. the gradient of a function f in cylindrical coordiantes is writing as:

∆f=∂f/∂r e_r+1/r + ∂f/∂θ e_θ+∂f/∂z e_z

so the difficulty is how to write macros of these terms, specialy for the derivation in the radial and tangential directions (∂f/∂r e_r+1/r + ∂f/∂θ e_θ). I don't know how to deal with this. Any guidelines please!

real NV_VEC(origin), NV_VEC(axis);
real NV_VEC(V), NV_VEC(r), NV_VEC(R), NV_VEC(B);
real NV_VEC(er), NV_VEC(et);
real xc[ND_ND];
real Bmag, rmag, source;
real ua, ur, ut;

/* Get origin vector of fluid region */
NV_V (origin, =, THREAD_VAR(thread).cell.origin);
/* Get axis of fluid region */
NV_V (axis, =, THREAD_VAR(thread).cell.axis);
/* Store the 3 cartesian velocity components in vector V */
N3V_D(V, =, C_U(cell,thread),C_V(cell,thread),C_W(cell,thread) );

/* Get current cell coordinate */
C_CENTROID(xc,cell,thread);

/* Calculate (R) = (Xc)-(Origin) */
NV_VV(R, =, xc, -, origin);
/* Calculate |B| = (R) dot (axis)*/
Bmag = NV_DOT(R,axis);
/* Calculate (B) = |B|*axis */
NV_VS(B,=,axis,*,Bmag);
/* Calculate (r) = (R)-(B) This is the local radial vector*/
NV_VV(r, =, R, -, B);
/* Calculate |r|*/
rmag = NV_MAG(r);

if (rmag != 0.)
{
NV_VS (er,=,r,/,rmag);
NV_CROSS(et, axis, er);
ur = NV_DOT(V,er);
ut = NV_DOT(V,et);
ua = NV_DOT(V,axis);
}
else
{
ur = 0.0;
ut = 0.0;
ua = NV_DOT(V,axis);
}

[zahrae]

No one has the answer of my question!!
I really need help. I read all the manuel of the UDF's but I didn't find anything, any guidelines please !!

Thank you for your kind cooperation!