def helical_turbulent_fd_Mandal_Nigam(Re, Di, Dc, roughness=0):
    r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
    pipe such as a helical coil under turbulent conditions, using the method of 
    Mandal and Nigam [1]_, also shown in [2]_.
        
    .. math::
        f_{curv} = f_{\text{str,turb}} [1 + 0.03{De}^{0.27}]
        
    Parameters
    ----------
    Re : float
        Reynolds number with `D=Di`, [-]
    Di : float
        Inner diameter of the coil, [m]
    Dc : float
        Diameter of the helix/coil measured from the center of the tube on one
        side to the center of the tube on the other side, [m]
    roughness : float, optional
        Roughness of pipe wall [m]        

    Returns
    -------
    fd : float
        Darcy friction factor for a curved pipe [-]

    Notes
    -----
    Claimed to be valid for all turbulent conditions with 
    :math:`2500 < De < 15000`. At very low curvatures, converges on the 
    straight pipe result.

    Examples
    --------
    >>> helical_turbulent_fd_Mandal_Nigam(1E4, 0.01, .2)
    0.03831658117115902

    References
    ----------
    .. [1] Mandal, Monisha Mridha, and K. D. P. Nigam. "Experimental Study on 
       Pressure Drop and Heat Transfer of Turbulent Flow in Tube in Tube 
       Helical Heat Exchanger." Industrial & Engineering Chemistry Research 48,
       no. 20 (October 21, 2009): 9318-24. doi:10.1021/ie9002393. 
    .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
       Correlations for Convection Heat Transfer and Pressure Losses in 
       Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
       (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
    '''
    De = Dean(Re=Re, Di=Di, D=Dc)
    fd = friction_factor(Re=Re, eD=roughness/Di)
    return fd*(1. + 0.03*De**0.27)
r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
    pipe such as a helical coil under turbulent conditions, using the method of 
    Mandal and Nigam [1]_, also shown in [2]_.
        
    .. math::
        f_{curv} = f_{\text{str,turb}} [1 + 0.03{De}^{0.27}]
        
    Parameters
    ----------
    Re : float
        Reynolds number with `D=Di`, [-]
    Di : float
        Inner diameter of the coil, [m]
    Dc : float
        Diameter of the helix/coil measured from the center of the tube on one
        side to the center of the tube on the other side, [m]
    roughness : float, optional
        Roughness of pipe wall [m]        

    Returns
    -------
    fd : float
        Darcy friction factor for a curved pipe [-]

    Notes
    -----
    Claimed to be valid for all turbulent conditions with 
    :math:`2500 < De < 15000`. At very low curvatures, converges on the 
    straight pipe result.

    Examples
    --------
    >>> helical_turbulent_fd_Mandal_Nigam(1E4, 0.01, .2)
    0.03831658117115902

    References
    ----------
    .. [1] Mandal, Monisha Mridha, and K. D. P. Nigam. "Experimental Study on 
       Pressure Drop and Heat Transfer of Turbulent Flow in Tube in Tube 
       Helical Heat Exchanger." Industrial & Engineering Chemistry Research 48,
       no. 20 (October 21, 2009): 9318-24. doi:10.1021/ie9002393. 
    .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
       Correlations for Convection Heat Transfer and Pressure Losses in 
       Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
       (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.