def komogorov(r, r0):
    """Calculate the phase structure function D_phi in the komogorov approximation

    Parameters
    ----------
    r : `numpy.ndarray`
        r, radial frequency parameter (object space)
    r0 : `float`
        Fried parameter

    Returns
    -------
    `numpy.ndarray`

    """
    return 6.88 * (r / r0) ** (5 / 3)
