Markus Holzer
--- a/lbmpy/boundaries/boundaryconditions.py
+++ b/lbmpy/boundaries/boundaryconditions.py
    def __call__(self, f_out, f_in, dir_symbol, inv_dir, lb_method, index_field):
        return Assignment(f_in(inv_dir[dir_symbol]), f_out(dir_symbol))

+
 # end class NoSlip

+class FreeSlip(LbBoundary):
+    """
+    Free-Slip boundary condition, which enforces a zero normal fluid velocity $u_n = 0$ but places no restrictions
+    on the tangential fluid velocity $u_t$.
+
+    Args:
+        stencil: LBM stencil which is used for the simulation
+        normal_direction: optional normal direction. If the Free slip boundary is applied to a certain side in the
+                          domain it is not necassary to calculate the normal direction since it can be stated for all
-                          domain it is not necassary to calculate the normal direction since it can be stated for all
+                          domain it is not necessary to calculate the normal direction since it can be stated for all
--- a/lbmpy/boundaries/boundaryconditions.py
+++ b/lbmpy/boundaries/boundaryconditions.py
+                          domain it is not necassary to calculate the normal direction since it can be stated for all
+                          boundary cells. This reduces the memory space for the index array significantly.
+        name: optional name of the boundary.
+    """
+
+    def __init__(self, stencil, normal_direction=None, name=None):
+        """Set an optional name here, to mark boundaries, for example for force evaluations"""
+        self.stencil = stencil
+
+        if normal_direction and len(normal_direction) - normal_direction.count(0) != 1:
+            raise ValueError("It is only possible to pre specify the normal direction for simple situations."
+                             "This means if the free slip boundary is applied to a straight wall or side in the "
+                             "simulation domain. A possible value for example would be (0, 1, 0) if the "
+                             "free slip boundary is applied to the northern wall. For more complex situations "
+                             "the normal direction has to be calculated for each cell. This is done when "
+                             "the normal direction is not defined for this class")
--- a/lbmpy_tests/test_boundary_handling.py
+++ b/lbmpy_tests/test_boundary_handling.py
+    normal_south = (0, 1)
+    normal_north = (0, -1)
+
+    normal_south_west = (1, 1)
+    normal_north_west = (1, -1)
+    normal_south_east = (-1, 1)
+    normal_north_east = (-1, -1)
+
+    for cell in index_array:
+        direction = stencil[cell[2]]
+        inv_dir = inverse_direction(direction)
+
+        boundary_cell = (cell[0] + direction[0], cell[1] + direction[1])
+        normal = (cell[3], cell[4])
+        # the data is written on the inverse direction of the fluid cell near the boundary
+        # the data is read from the mirrored direction of the inverser direction where the mirror axis is the normal
-        # the data is read from the mirrored direction of the inverser direction where the mirror axis is the normal
+        # the data is read from the mirrored direction of the inverse direction where the mirror axis is the normal
--- a/lbmpy_tests/test_boundary_handling.py
+++ b/lbmpy_tests/test_boundary_handling.py
+    dh = create_data_handling(domain_size=(4, 4), periodicity=(False, False))
+    src = dh.add_array('src', values_per_cell=len(stencil), alignment=True)
+    dh.fill('src', 0.0, ghost_layers=True)
+    method = create_lb_method(stencil='D2Q9', method='srt', relaxation_rate=1.8)
+
+    bh = LatticeBoltzmannBoundaryHandling(method, dh, 'src', name="bh")
+
+    free_slip = FreeSlip(stencil=stencil)
+    add_box_boundary(bh, free_slip)
+
+    bh.prepare()
+    for b in dh.iterate():
+        for b_obj, idx_arr in b[bh._index_array_name].boundary_object_to_index_list.items():
+            index_array = idx_arr
+
+    # normale directions
-    # normale directions
+    # normal directions