Thank you for your answer, I understand, so here is the code :
int ph_index_gas = 3; /* This is the phase ID that has my gas in Fluent (foud in "phase") */
Thread *mix_th, *subth; /* I believe mix_th is the mixture level thread pointer */
subth = THREAD_SUB_THREAD(mix_th, ph_index_gas);
/* Loop over all cells */
u = C_U(c,mix_th); /* get cell velocity */
uavg = uavg+u;
i = i+1;
uavg = uavg/i;
printf("n uavg = %g u = %g i = %gn",uavg,u,i);
The error is a "received signal SIGSEGV" type.
I am using a multiphase VOF model and the energy equation. The study is in 2D and the solver planar (so the bubble is more of an infinite cylinder).