def _calc_eddy_time(self):
        """ estimate the eddy turn-over time in days """
        ens = .5*self.H * self.spec_var(self.wv2*self.ph)
        return 2.*pi*np.sqrt( self.H / ens ) / year
def _calc_eddy_time ( self ) : ens = .5 * self . H * self . spec_var ( self . wv2 * self . ph ) return 2. * pi * np . sqrt ( self . H / ens ) / year
estimate the eddy turn-over time in days