I am simulating forced vortex induced vibration by Nektar++ (latest version) and found jumps in the lift force trace as shown below. The jumps occur every 5 seconds, which is half of the period of the forced sinusoidal vibration (when displacement of the body is zero). The XML session file is also attached. I tried to find the reason from the formulation of additional body force due to the usage of inertial coordinate system fixed on the vibrating body, but can't explain these jumps. Do you have some ideas about this issue? Thanks a lot.