# How to create a moving 'fixed value' in FLUENT?

 A fixed value can be imposed via a source term and source term can depend on time. In this way it's possible to move a 'fixed value' around the domain during the simulation. The question becomes how to fix a variable value with a source term? Complete explanation can be found in Patankar's book page 145 (Numerical Heat Transfer and Fluid Flow, Suhas V. Patankar, McGraw Hill, ISBN: 0070487405). Since source term is: S = S_c + S_p * PHI_p if this source term is large enough to make other terms in the equation negligible, the discretization equation reduces to this : S_c + S_p * PHI_p ~= 0 (where ~= means approximatively) <=> PHI_p ~= -S_c / S_p Hence, if we use: S_c = 10^30 * PHI_p_desired and S_p = -10^30 source term is large enough to make other terms negligible and solver, by solving the reduced discterized equation will impose the desired value for PHI_p. This approach is illustrated here with a UDF example. The aim here is to impose a moving fixed temperature in a solid zone. This source of temperatrue is due to a moving laser. The easiest way to impose this moving fixed temperature in the solid slab is to use the following UDF: #include "udf.h" #define T 620.0 /*Desired Temperature (to be imposed in the slab)*/ #define CON 10.0e30 #define laser_radius 0.005 #define laser_init_pos 0.05 #define laser_vel 0.01 DEFINE_SOURCE(fix_temp, cell, thread, dS, eqn) { real source; real laser_position_x; real x[ND_ND]; laser_position_x = laser_init_pos + laser_vel * CURRENT_TIME; C_CENTROID(x,cell,thread); if((laser_position_x - laser_radius) <= x && (laser_position_x + laser_radius) >= x) { source = CON *T - CON * C_T(cell,thread); dS[eqn] = -CON; } else { source = 0.0; } return source; }