Even though the final goal is to run an unsteady simulation, the fluid-structure procedure has to run some preliminary steady couplings to provide a suitable initial condition. The steady run iterates until a converged sail shape and flow field are obtained, where converged means that there does exist a value of kc such that (11) is satisfied for every k > kc (within given tolerances on forces and/or displacements). When running steady simulations the velocity of the sails is required to be null at each coupling, thus somehow enforcing the convergence condition (which prescribes null
velocities at convergence). This explains why convergence is slightly faster when running steady simulations with respect to transient ones (clearly only when such a solution reflects a steady state physical solution).