def Nu(Re, Pr):
    """
    Calculates the Nusselt criterion.
    Parameters
    ----------
    Re : float
        The Reynold criterion, [dimensionless]
    Pr : float
        The Prandtl criterion, [dimensionless]
    Returns
    -------
    Nu : float
        The Nusselt criterion, [dimensionless]
    References
    ----------
    Романков, формула 4.17, стр.152
    """      
    return 0.021 * (Re**0.8) * (Pr**0.4)