# How to handle a moving porous medium?

 Moving porous media appears in many industrial applications. Since they are moving, they will induce the surrounding fluid to flow with them, at the same time, since they are porous, fluid can flow through them. In fluent, if you set a region as porous media, that region cannot move.To simulate a moving porous object, users can write a udf to modify the momentum source term used by FLuent in the porous medium model.For fixed porous media, the following source term will be added in the momentum equation:S_i=-(mu/alpha u_i + 0.5 C_2rho |u_i|u_i),If porous media are moving with a given velocity v_move, then we can modify the above equation:S_i=-(mu/ alpha (u_i-u_move)+0.5 C_2 rho | u_i-u_moe | (u_i -u_move) )In the following example, the porous region is moving in the Y-direction with a constant velocity.-------------------------------------------------------#include "udf.h"#define D 1.0e7#define Coef 3000.0#define V_MOVING 2.0DEFINE_SOURCE(xmom_source, c, t, dS, eqn){ real source; source = -1.0*(C_MU_L(c,t)*D*C_U(c, t)+ 0.5*Coef*C_R(c,t)*fabs(C_U(c, t))*C_U(c, t)); dS[eqn] = 0.0; return source;}DEFINE_SOURCE(ymom_source, c, t, dS, eqn){ real source; source = -1.0*(C_MU_L(c,t)*D*(C_V(c, t)-V_MOVING)+ 0.5*Coef*C_R(c,t)*fabs(C_V(c, t)-V_MOVING)*(C_V(c, t)-V_MOVING)); dS[eqn] = 0.0; return source;}DEFINE_SOURCE(zmom_source, c, t, dS, eqn){ real source; source = -1.0*(C_MU_L(c,t)*D*C_W(c, t)+ 0.5*Coef*C_R(c,t)*fabs(C_W(c, t))*C_W(c, t)); dS[eqn] = 0.0; return source;}