Ultra-high bypass nacelle geometry design space exploration

The design of a civil aeroengine nacelle is a complex multi-objective constrained problem, where several variables interact to determine the final aerodynamic performance during cruise and off-design operation. This paper presents a design space exploration of a ultra-high bypass ratio nacelle to study the relations between key geometric factors that affect the flow field past the external cowl. An in-house geometry parameterisation tool allowing a full control on the set of curves is employed in a series of sub-tasks. The investigations of axisymmetric isolated nacelles are conducted using a computational fluid dynamics tool, first studying the influence of global dimensions on the performance metrics, to restrict the feasible design space. A first optimisation with a lean numerical model is then performed to rapidly assess the effect of design variables locally changing the aerolines shapes and thus more finely controlling the pressure and isentropic Mach number distributions. Finally, a second optimisation-driven exploration employing a more advanced genetic algorithm is run, refining the feasible design space using a higher-fidelity computational model. The resulting features of the Pareto optimal solutions and the relationships between the nacelle drag coefficients with the geometric ratios can provide useful guidelines for the design of future compact nacelles for ultra-high bypass ratio turbofans.