Problem to simulate a changing wind speed on 2D airfoil
 

Good evening to everyone,

I am trying to assess the Lift and Drag force for a 2D wind turbine airfoil in a changing wind speed regime with a transient analysis in Fluent. For example, during a certain period of time the wind speed at the inlet is fixed to 15 m/s and then suddenly it changes to 30 m/s for a shorter period of time (wind gust) and then again the inlet wind speed returns to the initial speed of 15 m/s.

If I simulate this two situations in two different cases (so initializing two cases with different inlet boundary conditions) , there's no problem and both simulations reach convergence (with higher Lift for the 30 m/s case as expected). If I try instead to simulate this changing wind speed in a single simulation, so using for the 30 m/s case the results after convergence of the 15 m/s case as initial condition, the solver seems very unstable and it doesn't reach convergence with the Lift starts oscillating, showing no significant pattern.

Does anyone know where the problem can be?

Thank you very much for your attention,

Lorenzo

PS: You can see attached the lift plot for the 30 m/s case, both for the case in which the simulation is starting from scratch and for the case in which the simulation is starting from the results of the 15 m/s case.