def Stokes_number(V, Dp, D, rhop, mu):
    """Calculates Stokes Number for a given characteristic velocity `V`, 
    particle diameter `Dp`, characteristic diameter `D`, particle density 
    `rhop`, and fluid viscosity `mu`.

    .. math::
        \\text{Stk} = \\frac{\\rho_p V D_p^2}{18\\mu_f D}

    Parameters
    ----------
    V : float
        Characteristic velocity (often superficial), [m/s]
    Dp : float
        Particle diameter, [m]
    D : float
        Characteristic diameter (ex demister wire diameter or cyclone 
        diameter), [m]
    rhop : float
        Particle density, [kg/m^3]
    mu : float
        Fluid viscosity, [Pa*s]
    
    Returns
    -------
    Stk : float
        Stokes numer, [-]

    Notes
    -----
    Used in droplet impaction or collection studies.

    Examples
    --------
    >>> Stokes_number(V=0.9, Dp=1E-5, D=1E-3, rhop=1000, mu=1E-5)
    0.5

    References
    ----------
    .. [1] Rhodes, Martin J. Introduction to Particle Technology. Wiley, 2013.
    .. [2] Al-Dughaither, Abdullah S., Ahmed A. Ibrahim, and Waheed A. 
       Al-Masry. "Investigating Droplet Separation Efficiency in Wire-Mesh Mist
       Eliminators in Bubble Column." Journal of Saudi Chemical Society 14, no.
       4 (October 1, 2010): 331-39. https://doi.org/10.1016/j.jscs.2010.04.001.
    """
    return rhop * V * (Dp * Dp) / (18.0 * mu * D)
