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Description
You can use DEFINE_DPM_BC to specify your own boundary conditions for particles. The function is executed every time a particle touches a boundary of the domain, except for symmetric or periodic boundaries. You can define a separate UDF (using DEFINE_DPM_BC) for each boundary.
Usage
| DEFINE_DPM_BC( name, p, t, f, f_normal, dim) |
| Argument Type | Description |
| symbol name | UDF name. |
| Tracked_Particle *p | Pointer to the Tracked_Particle data structure which |
| contains data related to the particle being tracked. | |
| Thread *t | Pointer to the face thread the particle is currently hitting. |
| face_t f | Index of the face that the particle is hitting. |
| real f_normal[] | Array that contains the unit vector which is normal to the |
| face. | |
| int dim | Dimension of the flow problem. The value is 2 in 2d, for |
| 2d-axisymmetric and 2d-axisymmetric-swirling flow, | |
| while it is 3 in 3d flows. | |
| Function returns | |
| int | |
There are six arguments to DEFINE_DPM_BC: name, p, t, f, f_normal, and dim. You supply name, the name of the UDF. p, t, f, f_normal, and dim are variables that are passed by the ANSYS FLUENT solver to your UDF. Your UDF will need to compute the new velocity of a particle after hitting the wall, and then return the status of the particle track (as an int), after it has hit the wall.
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Pointer
p can be used as an argument to the particle-specific macros (defined in Section
3.2.7) to obtain information about particle properties.
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Example 1
This example shows the usage of DEFINE_DPM_BC for a simple reflection at walls. It is similar to the reflection method executed by ANSYS FLUENT except that ANSYS FLUENT accommodates moving walls. The function must be executed as a compiled UDF.
The function assumes an ideal reflection for the normal velocity component (
nor_coeff
1) while the tangential component is damped (
tan_coeff
0.3). First, the angle of incidence is computed. Next, the normal particle velocity, with respect to the wall, is computed and subtracted from the particles velocity. The reflection is complete after the reflected normal velocity is added. The new particle velocity has to be stored in
P_VEL0 to account for the change of particle velocity in the momentum balance for coupled flows. The function returns
PATH_ACTIVE for inert particles while it stops particles of all other types.
/* reflect boundary condition for inert particles */
#include "udf.h"
DEFINE_DPM_BC(bc_reflect,p,t,f,f_normal,dim)
{
real alpha; /* angle of particle path with face normal */
real vn=0.;
real nor_coeff = 1.;
real tan_coeff = 0.3;
real normal[3];
int i, idim = dim;
real NV_VEC(x);
#if RP_2D
/* dim is always 2 in 2D compilation. Need special treatment for 2d
axisymmetric and swirl flows */
if (rp_axi_swirl)
{
real R = sqrt(P_POS(p)[1]*P_POS(p)[1] +
P_POS(p)[2]*P_POS(p)[2]);
if (R > 1.e-20)
{
idim = 3;
normal[0] = f_normal[0];
normal[1] = (f_normal[1]*P_POS(p)[1])/R;
normal[2] = (f_normal[1]*P_POS(p)[2])/R;
}
else
{
for (i=0; i<idim; i++)
normal[i] = f_normal[i];
}
}
else
#endif
for (i=0; i<idim; i++)
normal[i] = f_normal[i];
if(p->type==DPM_TYPE_INERT)
{
alpha = M_PI/2. - acos(MAX(-1.,MIN(1.,NV_DOT(normal,P_VEL(p))/
MAX(NV_MAG(P_VEL(p)),DPM_SMALL))));
if ((NNULLP(t)) && (THREAD_TYPE(t) == THREAD_F_WALL))
F_CENTROID(x,f,t);
/* calculate the normal component, rescale its magnitude by
the coefficient of restitution and subtract the change */
/* Compute normal velocity. */
for(i=0; i<idim; i++)
vn += P_VEL(p)[i]*normal[i];
/* Subtract off normal velocity. */
for(i=0; i<idim; i++)
P_VEL(p)[i] -= vn*normal[i];
/* Apply tangential coefficient of restitution. */
for(i=0; i<idim; i++)
P_VEL(p)[i] *= tan_coeff;
/* Add reflected normal velocity. */
for(i=0; i<idim; i++)
P_VEL(p)[i] -= nor_coeff*vn*normal[i];
/* Store new velocity in P_VEL0 of particle */
for(i=0; i<idim; i++)
P_VEL0(p)[i] = P_VEL(p)[i];
return PATH_ACTIVE;
}
return PATH_ABORT;
}
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Example 2
This example shows how to use DEFINE_DPM_BC for a wall impingement model. The function must be executed as a compiled UDF.
#include "udf.h"
#include "dpm.h"
#include "surf.h"
#include "random.h"
/* define a user-defined dpm boundary condition routine
* bc_reflect: name
* p: the tracked particle
* t: the touched face thread
* f: the touched face
* f_normal: normal vector of touched face
* dim: dimension of the problem (2 in 2d and 2d-axi-swirl, 3 in 3d)
*
* return is the status of the particle, see enumeration of Path_Status
* in dpm.h
*/
#define V_CROSS(a,b,r)\
((r)[0] = (a)[1]*(b)[2] - (b)[1]*(a)[2],\
(r)[1] = (a)[2]*(b)[0] - (b)[2]*(a)[0],\
(r)[2] = (a)[0]*(b)[1] - (b)[0]*(a)[1])
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DEFINE_DPM_BC(bc_wall_jet, p, thread, f, f_normal, dim)
{
/*
Routine implementing the Naber and Reitz Wall
impingement model (SAE 880107)
*/
real normal[3];
real tan_1[3];
real tan_2[3];
real rel_vel[3];
real face_vel[3];
real alpha, beta, phi, cp, sp;
real rel_dot_n, vmag, vnew, dum;
real weber_in, weber_out;
int i, idim = dim;
cxboolean moving = (SV_ALLOCATED_P (thread,SV_WALL_GRID_V) &&
SV_ALLOCATED_P (thread,SV_WALL_V ) );
#if RP_2D
if (rp_axi_swirl)
{
real R = sqrt(P_POS(p)[1]*P_POS(p)[1] +
P_POS(p)[2]*P_POS(p)[2]);
if (R > 1.e-20)
{
idim = 3;
normal[0] = f_normal[0];
normal[1] = (f_normal[1]*P_POS(p)[1])/R;
normal[2] = (f_normal[1]*P_POS(p)[2])/R;
}
else
{
for (i=0; i<idim; i++)
normal[i] = f_normal[i];
}
}
else
#endif
for (i=0; i<idim; i++)
normal[i] = f_normal[i];
/*
Set up velocity vectors and calculate the Weber number
to determine the regime.
*/
for (i=0; i < idim; i++)
{
if (moving)
face_vel[i] = WALL_F_VV(f,thread)[i] + WALL_F_GRID_VV(f,thread)[i];
else
face_vel[i] = 0.0;
rel_vel[i] = P_VEL(p)[i] - face_vel[i];
}
vmag = MAX(NV_MAG(rel_vel),DPM_SMALL);
rel_dot_n = MAX(NV_DOT(rel_vel,normal),DPM_SMALL);
weber_in = P_RHO(p) * SQR(rel_dot_n) * P_DIAM(p) /
MAX(DPM_SURFTEN(p), DPM_SMALL);
/*
Regime where bouncing occurs (We_in < 80).
(Data from Mundo, Sommerfeld and Tropea
Int. J. of Multiphase Flow, v21, #2, pp151-173, 1995)
*/
if (weber_in <= 80.)
{
weber_out = 0.6785*weber_in*exp(-0.04415*weber_in);
vnew = rel_dot_n * (1.0 + sqrt( weber_out /
MAX( weber_in, DPM_SMALL )));
/*
The normal component of the velocity is changed based
on the experimental paper above (i.e. the Weber number
is based on the relative velocity).
*/
for (i=0; i < idim; i++)
P_VEL(p)[i] = rel_vel[i] - vnew*normal[i] + face_vel[i];
}
if (weber_in > 80.)
{
alpha = acos(-rel_dot_n/vmag);
/*
Get one tangent vector by subtracting off the normal
component from the impingement vector, then cross the
normal with the tangent to get an out of plane vector.
*/
for (i=0; i < idim; i++)
tan_1[i] = rel_vel[i] - rel_dot_n*normal[i];
UNIT_VECT(tan_1,tan_1);
V_CROSS(tan_1,normal,tan_2);
/*
beta is calculated by neglecting the coth(alpha)
term in the paper (it is approximately right).
*/
beta = MAX(M_PI*sqrt(sin(alpha)/(1.0-sin(alpha))),DPM_SMALL);
phi= -M_PI/beta*log(1.0-cheap_uniform_random()*(1.0-exp(-beta)));
if (cheap_uniform_random() > 0.5)
phi = -phi;
vnew = vmag;
cp = cos(phi);
sp = sin(phi);
for (i=0; i < idim; i++)
P_VEL(p)[i] = vnew*(tan_1[i]*cp + tan_2[i]*sp) + face_vel[i];
}
/*
Subtract off from the original state.
*/
for (i=0; i < idim; i++)
P_VEL0(p)[i] = P_VEL(p)[i];
if ( DPM_STOCHASTIC_P(P_INJECTION(p)) )
{
/* Reflect turbulent fluctuations also */
/* Compute normal velocity. */
dum = 0;
for(i=0; i<idim; i++)
dum += p->V_prime[i]*normal[i];
/* Subtract off normal velocity. */
for(i=0; i<idim; i++)
p->V_prime[i] -= 2.*dum*normal[i];
}
return PATH_ACTIVE;
}
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Hooking a DPM Boundary Condition UDF to
ANSYS FLUENT
After the UDF that you have defined using DEFINE_DPM_BC is interpreted (Chapter 4) or compiled (Chapter 5), the name of the argument that you supplied as the first DEFINE macro argument will become visible in the appropriate boundary condition dialog box (e.g., the Velocity Inlet dialog box) in ANSYS FLUENT. See Section 6.4.1 for details on how to hook your DEFINE_DPM_BC UDF to ANSYS FLUENT.
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