[HYT]

Hello, I am simulating the gust response of a 3-D wing with a flap.

The gust is given at the velocity-inlet and the elastic response of the wing is calculated by solving the equation in the user-defined function. A controller is used to obtain the motion of the flap. So, there are two motions given by UDF: 1. the elastic morphing of the wing and 2. the motion of the flap

At first 0.5 seconds, there is only elastic morphing. The controller is not open. Everything is normal.
From the point t=0.5, the controller is open and the spikes occur on the lift time series, as seen in the figure 1.

I have tried a lot of settings to observe the effect on the spikes:

1. reducing residual, it works when changing from 10-3 to 10-4, the spikes are smaller, but useless when changing from 10-4 to 10-5.
2. changing smoothing methods. I am using spring together and remeshing now. It shows a similar effect with only spring. Both of these two methods are better than the confusion.
3. refining the mesh. It does not work. By the way, the unstructured mesh is used in the case.

Testing:
I have made a testing case: given the flap deflection as shown in the figure 2 (y: deflection angle/deg, x: time step)
The lift turns to the blue line in the figure 3 (the y-axis means nothing, just noticing how big the spikes are)
It seems the calculating result is related to the sudden change in the mesh velocity. The Fluent manual said that the mesh velocity of the moving mesh should be given to solve the equations.

I have two guesses:
1. the sudden change in the mesh velocity leads to the spikes in lift. The equations may not converge. If so, is there anybody familiar with the theory, how can I solve the problem? Since the flap deflection is given by the controller at each step, I can not make it smooth.
2. the deflection of the flap is given too fast for Fluent to deal with it. Or there is still some problem in the mesh or motion definition.

Forgive me for the poor expression and language. I would be grateful if anyone could give me some suggestions or references.