Add solid-fake for perpendicular flap tutorial (#541)

perpendicular-flap/README.md

@@ -51,6 +51,8 @@ Solid participant:
 
 * OpenFOAM (solidDisplacementFoam). For more information, have a look at the [OpenFOAM plateHole tutorial](https://www.openfoam.com/documentation/tutorial-guide/5-stress-analysis/5.1-stress-analysis-of-a-plate-with-a-hole). The solidDisplacementFoam solver only supports linear geometry and this case is only provided for quick testing purposes, leading to outlier results. For general solid mechanics procedures in OpenFOAM, see solids4foam.
 
+* Fake. A simple Python script that acts as a fake solver and provides an arbitrary time-dependent flap displacement in the x-direction, i.e., it performs a shear mapping on the resting flap. This solver can be used for debugging of the fluid participant and its adapter. It also technically works with implicit coupling, thus no changes to the preCICE configuration are necessary. Note that [ASTE's replay mode](https://precice.org/tooling-aste.html#replay-mode) has a similar use case and could also feed artificial or previously recorded real data, replacing an actual solver.
+
 ## Running the Simulation
 
 All listed solvers can be used in order to run the simulation. OpenFOAM can be executed in parallel using `run.sh -parallel`. The default setting uses 4 MPI ranks. Open two separate terminals and start the desired fluid and solid participant by calling the respective run script `run.sh` located in the participant directory. For example:

perpendicular-flap/solid-fake/fake.py

@@ -0,0 +1,92 @@
+from __future__ import division
+
+import numpy as np
+import precice
+
+
+def displace_flap(x, y, t, flap_tip_y):
+    x_displ = np.zeros_like(x)
+    y_displ = np.zeros_like(y)
+    # get displacement independent of x, only dependent on y and t
+    max_x_displ = 0.5
+    period_fac = 3 * np.pi
+    damping_fac = 8  # damps the amplitude of the sine
+    # defines how much the sine is shifted in y-direction
+    shift = 0.95
+    # wiggles the flap periodically.
+    # the arcsin(-shift) in the sine evaluation is necessary to start at a flap displacement of 0 at t=0
+    # (i.e. sin(arcsin(-shift))+shift = 0)
+    x_displ = np.minimum(((np.sin(period_fac * t + np.arcsin(-shift)) + shift) /
+                         damping_fac), max_x_displ) * y / flap_tip_y
+
+    dimensions = 2
+    displ = np.zeros((len(x), dimensions))
+    displ[:, 0] = x_displ
+    displ[:, 1] = y_displ
+
+    return displ
+
+
+configuration_file_name = "../precice-config.xml"
+participant_name = "Solid"
+mesh_name = "Solid-Mesh"
+write_data_name = 'Displacement'
+
+solver_process_index = 0
+solver_process_size = 1
+
+# define mesh
+H = 1
+W = 0.1
+
+interface = precice.Participant(participant_name, configuration_file_name, solver_process_index, solver_process_size)
+dimensions = interface.get_mesh_dimensions(mesh_name)
+assert (dimensions == 2)
+
+x_left = 0.0 - 0.5 * W  # left boundary of the flap
+x_right = 0.5 * W  # right boundary of the flap
+y_bottom = 0.0  # bottom of the flap
+y_top = y_bottom + H  # top of the flap
+
+n = 24  # Number of vertices per side
+t = 0
+
+vertices = np.zeros((2 * n, dimensions))
+# define vertices of flap's left side
+vertices[:n, 1] = np.linspace(y_bottom, y_top, n)
+vertices[:n, 0] = x_left
+# define vertices of flap's right side
+vertices[n:, 1] = np.linspace(y_bottom, y_top, n)
+vertices[n:, 0] = x_right
+
+vertex_ids = interface.set_mesh_vertices(mesh_name, vertices)
+
+if interface.requires_initial_data():
+    # initially, there should be no displacement
+    interface.write_data(np.zeros_like(vertices))
+
+interface.initialize()
+# change if necessary
+solver_dt = np.inf
+# for checkpointing
+t_cp = 0
+
+while interface.is_coupling_ongoing():
+    if interface.requires_writing_checkpoint():
+        t_cp = t
+
+    precice_dt = interface.get_max_time_step_size()
+    dt = min([solver_dt, precice_dt])
+    # wiggle the flap
+    write_data = displace_flap(vertices[:, 0], vertices[:, 1], t, H)
+
+    interface.write_data(mesh_name, write_data_name, vertex_ids, write_data)
+    interface.advance(dt)
+
+    if interface.requires_reading_checkpoint():
+        t = t_cp
+    else:
+        # update t
+        t += dt
+
+interface.finalize()

perpendicular-flap/solid-fake/requirements.txt

@@ -0,0 +1,2 @@
+numpy >1, <2
+pyprecice==3

perpendicular-flap/solid-fake/run.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env sh
+set -e -u
+
+python3 -m venv .venv
+. .venv/bin/activate
+pip install -r requirements.txt
+python3 fake.py