# FAQ

# Why do I get warning "CFL broken for W 1.208" ?

CFL stands for Courant–Friedrichs–Lewy condition. It is a necessary condition for stability while solving the equation of movement in Ichthyop.

We want at all time (U * dt) / dX stricly inferior to 1 with U the velocity (vertical in this specific case) and dX the move. **The warning informs you that the CFL condition has been broken for the vertical velocity** and that it might jeopardize the numerical stability of the model.

Try to decrease the time step (dt) in your configuration file (Time > Time step), let's say divide it by two. As explained in the configuration editor, an acceptable estimation for dt could be dt = 0.7 * dGrid / Umax with dGrid the average length of the grid cells and Umax the order of magnitude of the fastest current velocities in the hydrodynamic model (locally and punctually the vertical velocities can be intense). Nonetheless the smaller the better and as long as decreasing the time step does not slow down too much your simulations, you should always go for a smaller value that the previous estimation.