def archimedes(dp, rhog, rhos, mu):
    """
    Calculate the dimensionless Archimedes number.

    .. math:: Ar = \\frac{dp^3 \\rho_g (\\rho_s - \\rho_g) g}{\\mu^2}

    Parameters
    ----------
    dp : float
        Particle diameter [m]
    rhog : float
        Gas density [kg/m³]
    rhos : float
        Solid density [kg/m³]
    mu : float
        Dynamic viscosity [kg/(m⋅s)]

    Returns
    -------
    ar : float
        Archimedes number [-]

    Example
    -------
    >>> archimedes(0.001, 910, 2500, 0.001307)
    8309.1452

    References
    ----------
    Daizo Kunii and Octave Levenspiel. Fluidization Engineering.
    Butterworth-Heinemann, 2nd edition, 1991.
    """
    g = 9.81    # gravity acceleraton [m/s²]
    ar = (dp**3 * rhog * (rhos - rhog) * g) / (mu**2)
    return ar