UDF of natural convection in 3D cylinder
 

Hello all,

I'm completely new to CFD analysis, so I need some help with the following natural convection problem:
my geometry is 3D cylinder that water in it.
i want to model natural convection outside of cylinder in fluent, and i write this udf:
/************************************************** ****************
UDF that adds momentum source term and derivative to duct flow
************************************************** *****************/

#include "udf.h"


DEFINE_SOURCE(cell_Y_source, c, t, dS, eqn)
{
real source;

source = -9.8*C_T(c,t);
dS[eqn] = -9.8;
}

when i interpret it fluent i have no error and from material tab i add momentum as source term to air . but i don't see any natural convection around the cylinder.can any body helm me?
PD:I want to model with UDF without using gravity option in flunet,because when i use gravity option my solution is not converged.
PD2: TY CFD ONLINE froum.