Testsuite: Backport faster lb stokes sphere test from #2392

testsuite/python/lb_stokes_sphere.py

@@ -44,9 +44,9 @@
              'tau': TIME_STEP}
 # System setup
 radius = 5.4
-box_width = 44
+box_width = 36 
 real_width = box_width + 2 * AGRID
-box_length = 50
+box_length = 36
 v = [0, 0, 0.01]  # The boundary slip
 
 
@@ -61,8 +61,6 @@ def test_stokes(self):
         self.system.actors.clear()
         self.system.lbboundaries.clear()
         self.system.actors.add(self.lbf)
-        # The temperature is zero.
-        self.system.thermostat.set_lb(kT=0)
 
         # Setup walls
         walls = [None] * 4
@@ -102,19 +100,21 @@ def size(vector):
                 tmp += k * k
             return np.sqrt(tmp)
 
-        self.system.integrator.run(800)
+        self.system.integrator.run(200)
 
         stokes_force = 6 * np.pi * KVISC * radius * size(v)
-        print("Stokes' Law says: f=%f" % stokes_force)
 
         # get force that is exerted on the sphere
-        for i in range(4):
-            self.system.integrator.run(200)
-            force = sphere.get_force()
-            print("Measured force: f=%f" % size(force))
-            self.assertLess(abs(1.0 - size(force) / stokes_force), 0.06)
-
-##Invoke the GPU LB
+        force = sphere.get_force()
+        np.testing.assert_allclose(
+            force,
+            [0,
+             0,
+             stokes_force],
+            rtol=0.06,
+            atol=stokes_force * 0.06)
+        self.system.integrator.run(300)
+        np.testing.assert_allclose(sphere.get_force(), force, atol=0.02)
 
 
 @ut.skipIf(not espressomd.has_features(
@@ -124,8 +124,6 @@ class LBGPUStokes(ut.TestCase, Stokes):
     def setUp(self):
         self.lbf = espressomd.lb.LBFluidGPU(**LB_PARAMS)
 
-#Invoke the CPU LB
-
 
 @ut.skipIf(not espressomd.has_features(
     ['LB', 'LB_BOUNDARIES', 'EXTERNAL_FORCES']), "Skipping test due to missing features.")