the cs_user_head_losses function is used to compute the values of the head loss term, and is called at each time step for each previously defined head loss volume zone.
(cs_user_zones.c and cs_user_head_losses.cpp)
Volume zones may be defined using the GUI, or through the cs_user_zones (in cs_user_zones.cpp).
cku is the local head loss term.
It appears on the momentum as follows:
![\[ \rho \der{\vect{u}}{t} = - \grad p + \vect{headloss} \: (+\: \text{other terms})\]](form_654.png)
with
![\[ \vect{headloss} = - \rho \tens{cku}\cdot \vect{u} \,\: (\text{in } kg\cdot m^{-2} \cdot s^{-1})\]](form_655.png)
For a distributed head loss, let 

the source term tspdc is equal to 
we have 
For a singular head loss, let 

the source term tspdc is equal to
![\[\frac{\tens{dh_s}}{L} = - \frac{\tens{\xi_l}}{L} (0.5 \rho\vect{u}^2)\]](form_662.png)
. We have
![\[\tens{cku} = 0.5\frac{\tens{\xi_s}}{L}|\vect{u}|\]](form_663.png)
where 
Here is the list of examples: