def compute_form_no(stroke_length, orifice_d=25.6, piston_d=160., num_orifices=1):
    """
    Compute formation number (L/D) of a vortex ring
    Parameters
    ----------
    stroke_length: float
        stroke length of piston in mm
    orifice_d: float
        diameter of orfice in mm
    piston_d: float
        diameter of piston in mm
    num_orifices: int or float
        Number of orfices in a box

    Returns
    -------
    ld: float
        formation number of vortex ring
    """
    ld = (piston_d / orifice_d)**2 * stroke_length / orifice_d / float(num_orifices)
    return ld