tweaks CFL computation

TurTLE/DNS_statistics.py

@@ -460,8 +460,14 @@ def plot_basic_stats(
     with self.get_data_file() as data_file:
         if 'CFL_velocity' in data_file['statistics'].keys():
             self.statistics['CFL_velocity(t)'] = data_file['statistics/CFL_velocity'][ii0:ii1+1]
+            dx = ((2*np.pi/self.parameters['dkx'])/self.parameters['nx']) # Lx / nx
+            dy = ((2*np.pi/self.parameters['dky'])/self.parameters['ny'])
+            dz = ((2*np.pi/self.parameters['dkz'])/self.parameters['nz'])
+            if ((np.round(dx, 5) != np.round(dy, 5))
+             or (np.round(dx, 5) != np.round(dz, 5))):
+                print('WARNING: CFL computation does not make sense for non-isotropic grids')
             tresolution2 = (self.parameters['dt'] * self.statistics['CFL_velocity(t)']
-                            / (2*np.pi/self.parameters['dkx']/self.parameters['nx']))
+                            / dx)
             a.plot(self.statistics['t'] / self.statistics['Tint'],
                     tresolution2,
                     label = '$\\frac{\\Delta t}{\\Delta x} \\| u \\|_{CFL}$')