docs: def _calc_eddy_time(self):
          """ estimate the eddy turn-over time in days """
  
          ens = .5*self.Hi[0] * self.spec_var(self.wv2*self.ph1) + \
              .5*self.Hi[1] * self.spec_var(self.wv2*self.ph2)
  
          return 2.*pi*np.sqrt( self.H / ens ) / 86400