Hello
I am learning the partitioned coupling process of the fluidstructure coupling process. By learning the FSI paper on the official website, I can roughly understand the coupling process of fluid and solid:
This picture is from this paper: Fluid Structure Interaction Modelling of Flapping Wings
I learned a few key parts:

The fluid solver transfers the force to the solid solver

Solid solver uses force vector to calculate displacement vector

The displacement vector is passed to the fluid solver. If it does not converge, the new displacement vector is used to resolve the fluid
Then I wanted to use a simple example to understand the fluidstructure coupling process. My idea was to increase the stiffness of the solidstate solver in the verticalFlap case (by a factor of 1000000, so that the structure is approximately rigid). The degree of freedom in the X direction is not fixed, other Settings are unchanged, so that under the action of the wind, the solid will move under the force of the fluid, by detecting the position of the object, and then calculating the acceleration of the object, it should be proportional.
The next two images are grid files in paraview, and you can see the motion of solids: the wind pushes things horizontally; Solid deformation is so small that it can be ignored
My solid solver is calculix, fluid uses openfoam, step size 0.01, and the observation point of my preciceconfig file is set to (0,0)
<watchpoint mesh="SolidMesh" name="FlapTip" coordinate="0;0" />
Down here is the preciceSolidwatchpointFlapTip file in the solid solver,
preciceSolidwatchpointFlapTip.log (27.7 KB)
I use displacement0 in the column, the difference between the next row and the previous row to △x, and take the second derivative with respect to △x to get the acceleration a.
here I find a problem: isn’t displacement in the x direction equal to mass times acceleration? Isn’t the acceleration proportional to the displacement in the x direction? Why is the acceleration in the figure below negative?
In other words, if the acceleration is negative, the force must be negative. Why is the force always positive
This diagram paraview draws the resultant force of the flap path