## 1 Introduction  
Designing hypersonic aircraft is inherently complex, owing to a multitude of factors, including mission diversity, power discrepancies, and variations in engine layout [1]. Consequently, designing the inlet layout presents a significant challenge. The design process necessitates a comprehensive consideration of various factors, such as external airflow, airframe structure, and propulsion system requirements [2], to ensure optimal and consistent inlet performance across diverse mission profiles. In recent years, numerous aerodynamic configurations have emerged for high-speed aircraft, some sharing similar characteristics. Notably, hypersonic aircraft like the Quarterhorse by Hermeus [3] and the SR-72 by Lockheed Martin [4] feature the ventral inlet mounted on the aft body of the hypersonic aircraft fuselage.  
The ventral inlet has garnered popularity in hypersonic vehicle design. This layout not only enhances the hypersonic vehicle's lift-to-drag ratio and pitch balance but also facilitates precompression of incoming airflow by utilizing the forebody to provide denser airflow for the engine [5]. Extensively studied by researchers globally, this layout has been investigated in various contexts, such as the X-43 [6] and X-51 [7] hypersonic verification vehicles in the United States. Furthermore, [PERSON] et al. [8] integrated the (REST) inlet design method into the integrated design of inward turning inlet and vehicle fuselage. You [9] employed streamlined tracing techniques to design multiple 3D integrated configurations of inward-turning inlet and fuselage, conducting comprehensive comparative analyses of single- and dual-module inlets. [PERSON] [10] also proposed a novel method for designing a hypersonic inlet, analyzing three inlet layouts for an axisymmetric vehicle and delvinginto the performance characteristics of each layout and its impact on the vehicle's overall aerodynamic properties.  
However, current research predominantly concentrates on the inlet mounted on the front and middle of the vehicle, with insufficient attention to aft-body inlet layouts. [PERSON] [11] highlighted the significant advantages of aft-body inlet layouts, including the substantial reduction in engine length and efficient utilization of fuselage internal space, for achieving compact and efficient designs. Nonetheless, this type of layout encounters challenges, particularly regarding the thick boundary layer and the non-uniformity airflow captured by the inlet. Wind tunnel tests conducted by [PERSON] [12] revealed that under certain flight conditions, the boundary layer thickness at the inlet entrance of the aft-body inlet may reach 30%-50% of the inlet height. [PERSON] [13] utilized mixed compressible flow theory to analyze the impact of non-uniform flow generated by the attached surface layer on inlet and combustion chamber performance, underscoring the significant effect even a thin attached surface layer can have on free-flow characteristics. [PERSON] [14] investigated the effect of non-uniform incoming airflow on side-pressure inlet. The result demonstrated that non-uniform incoming flow significantly reduces the starting performance and total pressure recovery of the side-compression inlet.  
However, the influence of compression direction on the performance of an inlet mounted on the aft body lacks extensive research. To address this gap, this study investigates the specific effects of compression direction on the performance of a 2D inlet. A simplified, integrated model of the body and 2D inlet, customized for this layout, will be developed. Numerical simulations will be employed to comprehensively analyze the significant impact of compression direction. Ultimately, this study aims to provide valuable insights into the design and optimization of inlet systems for hypersonic vehicles with aft-body inlet layouts.