def Po_rect_duct_laminar_developed_flow(ac):
    """ Poiseuille number(Fanning friction factor times Re) for fully developed laminar flow rectangular ducts
        ac: aspect ratio
    """
    if ac < 1.:
        ac = 1./ac
    if ac < 1.5:
        return 14.23
    if ac < 2.5:
        return 15.55
    if ac < 3.5:
        return 17.09
    if ac < 5:
        return 18.23
    if ac < 7:
        return 19.7
    if ac < 10:
        return 20.58
    return 24.