def relative_roughness(D, roughness=1.52e-06):
    """Calculates relative roughness `eD` using a diameter and the roughness
    of the material of the wall. Default roughness is that of steel.

    .. math::
        eD=\\frac{\\epsilon}{D}

    Parameters
    ----------
    D : float
        Diameter of pipe, [m]
    roughness : float, optional
        Roughness of pipe wall [m]

    Returns
    -------
    eD : float
        Relative Roughness, [-]

    Examples
    --------
    >>> relative_roughness(0.5, 1E-4)
    0.0002

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
       Eighth Edition. McGraw-Hill Professional, 2007.
    .. [2] Cengel, Yunus, and John Cimbala. Fluid Mechanics: Fundamentals and
       Applications. Boston: McGraw Hill Higher Education, 2006.
    """
    return roughness / D
