Introduce a ramp similar to `turek-hron-fsi3` in `perpendicular-flap`?

[BenjaminRodenberg]

I saw that the boundary condition in turek-hron-fsi3 is implemented as a cosine-shaped ramp. Why do we not use a similar kind of boundary condition for the perpendicular flap?

In https://github.com/precice/precice/issues/2172 I studied the case perpendicular-flap and made the observation that not all data equals zero at the beginning with the current boundary and initial conditions.

I implemented https://github.com/precice/precice/pull/2181 to plot the watchpoints at initial time. I get non-zero forces if I set <exchange data="Force" ... initialize="true" />:

  Time  Coordinate0  Coordinate1  Displacement0  Displacement1  Force0  Force1
 0.00000000e+00   0.00000000e+00   1.00000000e+00   0.00000000e+00   0.00000000e+00   2.29427968e+00  -3.02282515e-14

I assume that this is due to the initial condition (IC) where the fluid velocity in x direction is homogeneously set to 10 on the whole domain. However, the cross section of the channel is smaller at the x-coordinate of the flap. Here, one would expect a larger velocity due to mass conservation which makes the given IC unphysical. A ramp should lead to a more realistic scenario.

Additionally, I realized that using a ramp also leads to better results in a subcycling-based experiment I developed for my thesis. See here. I will probably also change this implementation to a cosine shaped ramp since this gives us C0 and C1 continuity in contrast to the piecewise linear ramp that is only C0.

Side note: Looking at the watchpoints I would claim that setting initialize="false" is actually wrong since it ignores the initial forces produced on the OpenFOAM side. Using a ramp "heals" this problem since here we really have zeros for all exchanged quantities. Related issues/PRs: https://github.com/precice/precice/issues/2033, https://github.com/precice/tutorials/pull/540