def getcf(lambda_val , Re_theta):
    """ Computes the skin friction coefficient, cf

    Assumptions:
    None

    Source:
    None

    Inputs: 
    lambda_val - thwaites separation criteria                        [unitless]
    Re_theta   - Reynolds Number as a function of momentum thickness [unitless]

    Outputs:  
    cf         - skin friction coefficient [unitless]

    Properties Used:
    N/A 
    """        
    l       = 0.22 + 1.402*lambda_val  + (0.018*lambda_val)/(0.107 + lambda_val ) 
    idx1    = (lambda_val>0.0)   
    l[idx1] = 0.22 + 1.57*lambda_val[idx1] - 1.8*lambda_val[idx1]**2 
    cf      = 2*l/Re_theta  
    return cf