Multipoint Aerodynamic Design of Ultrashort Nacelles for Ultrahigh-Bypass-Ratio Engines

This paper presents a newly developed methodology for multipoint aerodynamic design of ultrashort nacelles for ultrahigh-bypass-ratio turbofan engines. An integrated aerodynamic framework, based on parametric geometry generation and flowfield solution via three-dimensional Reynolds-averaged Navier-Stokes equations, was built and used for designing several ultrashort nacelle shapes and to evaluate their aerodynamic performance. An approach for modeling the inlet-fan coupling is presented and validated. A design strategy is introduced, and various test cases are evaluated under the following critical operating conditions: midcruise, low speed/high angle of attack, and pure crosswind. The major design parameters are highlighted and their influence in the flowfield is discussed in detail for all the chosen flight conditions. Performance was evaluated by assessing inlet flow distortion and by bookkeeping of thrust and drag. The framework has proven to be suitable for designing high-performance nacelles capable of operating under critical flight conditions, without flow separation or high levels of distortion. Drooping the inlet by 4 deg is shown to reduce the drag at cruise by 1.9%, which also has a large beneficial impact on internal lip separation at high-incidence conditions. Furthermore, crosswind was identified as the most severe of the conditions, requiring a drastic reshaping of the nacelle to avoid internal lip separation. Two final nacelle designs were compared: the first allowed inlet separation under a 90 deg crosswind condition, whereas the second was reshaped to be separation-free under all operating conditions. Reshaping to avoid separation has increased drag by 5.1% at cruise