Typically, the evaluation of nacelle drag in preliminary design is required to find an overall optimum engine cycle and flight trajectory. This work focuses on the drag characteristics of aero-engine nacelles with separate jet exhausts. The main body of analysis comes from 3D numerical simulations. A new near-field method to compute the post-exit force of a nacelle is presented and evaluated. The effects of the engine size, Mach number, mass flow capture ratio and angle of attack are assessed. The results obtained from the numerical assessments were used to evaluate conventional reduced-order models for the estimation of nacelle drag. Within this context, the effect of the engine size is typically estimated by the scaling ratio between the maximum areas and Reynolds numbers. The effect of the angle of attack on nacelle drag is mostly a function of the nacelle geometry and angle of attack. In general, typical low-order models based on skin friction and form factor can underestimate the friction drag by up to 15% at cruise operating point. Similarly, reduced-order models based solely on Reynolds number, and Mach number can underestimate the overall nacelle drag by up to 74% for free stream Mach number larger than the drag rise Mach number.

中文翻译：

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独立喷气式航空发动机短舱阻力估计框架

通常，需要在初步设计中评估短舱阻力，以找到整体最佳发动机循环和飞行轨迹。这项工作的重点是具有独立喷气排气的航空发动机短舱的阻力特性。分析的主体来自3D数值模拟。提出并评估了一种计算机舱退出后力的新近场方法。评估了发动机尺寸、马赫数、质量流量捕获比和攻角的影响。从数值评估中获得的结果用于评估用于估计机舱阻力的常规降阶模型。在这种情况下，发动机尺寸的影响通常通过最大面积和雷诺数之间的比例来估计。攻角对机舱阻力的影响主要是机舱几何形状和攻角的函数。一般来说，基于皮肤摩擦和形状因子的典型低阶模型在巡航操作点会低估摩擦阻力高达 15%。类似地，仅基于雷诺数和马赫数的降阶模型对于自由流马赫数大于阻力上升马赫数的情况可能会低估整体机舱阻力高达 74%。