def saturation_distance(mean_mu_squared, wavenumber, scale):
    """Saturation distance according to Wenzel.

    :param mean_mu_squared: Mean mu squared.
    :param wavenumber: Wavenumber.
    :param scale: Outer length scale.

    See Daigle, 1987: equation 5

    .. math:: r_s = \\frac{1}{2 \\langle \mu^2 \\rangle k^2 L}

    """
    return 1.0 / (2.0 * mean_mu_squared * wavenumber*wavenumber * scale)