One of the research areas at the Institute of Jet Propulsion focuses on the design and optimization of s-shaped engine inlet configurations. The distortion being evoked within such inlet ducts should be limited to ensure an optimal performance, stability, and durability of the engine's compression system. Computational fluid dynamics (CFD) play a major role in the design process of bent engine inlet ducts. The flow within such ducts can be computed, distortion patterns can be visualized, and related distortion coefficients are easily calculated. The impact of a distortion on flow phenomena within the compressor system can, however, only be computed with major computational efforts and thus the quality of an s-duct design in development is usually assessed by analyzing the evoked distortion with suitable distortion coefficients without a true knowledge of the duct's influence on the downstream propulsion system. The influence of inlet distortion on both the performance and stability of the Larzac 04 jet engine was parameterized during experimental investigations at the engine test bed of the Institute of Jet Propulsion. Both pressure and swirl distortion patterns as they typically occur in s-duct inlet configurations were reproduced with distortion generators. Pressure distortion patterns were generated using seven types of distortion screens. The intensity of the distortion varies with the mesh size of the screen whereas the extension of the distortion is defined by the dimensions of the screen in radial and circumferential direction. A typical counter rotating twin-swirl was generated with a delta-wing installed upstream of the compressor system. First, the development of flow distortion was analyzed for several engine operating points (EOPs). A linear relation between the total pressure loss in the engine inlet and the EOPs was found. Second, the flow within the compressor system with an inlet distortion was analyzed and unsteady flow phenomena were detected for severe inlet distortions. Finally, the effect of both pressure and swirl distortion on the performance and stability of the test vehicle was parameterized. A loss in engine performance with increasing inlet distortion is observable. The limiting inlet distortion with respect to engine stability was found; and moreover, it was shown that pressure distortion has a stronger influence on the stability of the compressor system compared to a counter rotating twin-swirl distortion. The presented parameterization was essential for the s-duct design, which was under development for an experimental setup with the Larzac 04 jet engine.

In many modern aircraft concepts, civil as well as military ones, the engine is fully integrated into the fuselage. This integration often requires a highly bent intake duct. Due to the high degree of curvature and also the diffusive character of the intake duct, the inflow at the engine’s fan is non-uniform and may feature severe flow distortions. The size, strength, and pattern of these flow distortions may affect the engine’s compressor system and its safety margins. In this paper the flow through a short highly bent intake duct geometry is analysed by means of CFD. The numerical simulations are validated against experimental data, which are obtained in extensive investigations at the institute’s engine test facility. The setup for the numerical investigations is based on previous studies of the aerodynamics of intake ducts at the Institute of Jet Propulsion, where it is shown that the shape of the entrance cross-section of the intake duct has a strong influence on the flow field throughout the entire intake duct. In this paper the flow throughout the duct is analysed in order to gain information on the flow features which cause the flow distortion at the aerodynamic interface plane (AIP) and how these flow features interact. Two main flow distortion patterns exist at the AIP, one of them is a system of two twin vortices, one on each side in the lower part of the AIP. These are caused by the particular shapes of the cross-sections in the front part of the duct. The dominating flow distortion in the AIP is caused by a large flow separation in the rear part of the duct, which resides in the upper half of the AIP and results in a large total pressure loss and axial velocity deficit, combined with a twin swirl. Although no direct interaction between these two flow patterns is present, it was found that the small vortices in the lower part are influenced by the flow separation at the upper wall in the rear section of the intake duct.

One of the research areas at the Institute of Jet Propulsion focuses on the design and optimization of s-shaped engine inlet configurations. The distortion being evoked within such inlet ducts should be limited to ensure an optimal performance, stability, and durability of the engine’s compression system. Computational Fluid Dynamics (CFD) play a major role in the design process of bent engine inlet ducts. The flow within such ducts can be computed, distortion patterns can be visualized, and related distortion coefficients are easily calculated. The impact of a distortion on flow phenomena within the compressor system can, however, only be computed with major computational efforts and thus the quality of an s-duct design in development is usually assessed by analyzing the evoked distortion with suitable distortion coefficients without a true knowledge of the duct’s influence on the downstream propulsion system. The influence of inlet distortion on both the performance and stability of the Larzac 04 jet engine was parameterized during experimental investigations at the engine test bed of the Institute of Jet Propulsion. Both pressure and swirl distortion patterns as they typically occur in s-duct inlet configurations were reproduced with distortion generators. Pressure distortion patterns were generated using seven types of distortion screens. The intensity of the distortion varies with the mesh size of the screen whereas the extension of the distortion is defined by the dimensions of the screen in radial and circumferential direction. A typical counter rotating twin-swirl was generated with a deltawing installed upstream of the compressor system. First, the development of flow distortion was analyzed for several engine operating points (EOPs). A linear relation between the total pressure loss in the engine inlet and the EOPs was found. Secondly, the flow within the compressor system with an inlet distortion was analyzed and unsteady flow phenomena were detected for severe inlet distortions. Finally, the effect of both pressure and swirl distortion on the performance and stability of the test vehicle was parameterized. A loss in engine performance with increasing inlet distortion is observable. The limiting inlet distortion with respect to engine stability was found and moreover it was shown that pressure distortion has a stronger influence on the stability of the compressor system compared to a counter rotating twin-swirl distortion. The presented parameterization is essential for the s-duct design, which is under development for an experimental set-up with the Larzac 04 jet engine.

Integrated propulsion plays a major role in future civil and military aircraft design. A key component of these systems are highly bent intake geometries. As the flow passes through such ducts, combined total pressure and swirl distortions are generated which have a negative impact on compressor performance, safety margin, and durability. Due to weight and space limitations, a close coupling of intake and compressor is necessary. An experimental test case including a highly bent intake geometry and a state of the art turbofan engine was established and extensive measurement data was acquired. This publication compares results of three different numerical approaches to this test data: Isolated intake simulations, isolated compressor simulations with distorted inflow conditions, and a coupled simulation of intake and three stage compressor. The isolated intake simulation is able to reproduce the static wall pressure field of the intake as well as the occurring flow separation. Towards the interface plane to the compressor however, significant deviations are observed. The upstream effect of the compressor working under the combined pressure swirl distortion is assessed via the second simulation approach. The influence of the swirl and total pressure distortion on the compressor is first simulated separately and then compared to the impact of the combined distortion. The coupled intake-compressor simulation reveals the manipulation of the intake flow field by an upstream static pressure field. In contrast to experiments a slightly unsteady operation point and an asymmetric intake flow field were observed.

In many modern aircraft concepts, civil as well as military ones, the engine is fully integrated into the fuselage. This integration often requires a highly bent intake duct. Due to the high degree of curvature and also the diffusive character of the intake duct, the inflow at the engine’s fan is non-uniform and may feature severe flow distortions. The size, strength, and pattern of these flow distortions may affect the engine’s compressor system and its safety margins. In this paper five highly bent intake duct geometries are analyzed by means of CFD. They evolve from the same baseline geometry but are defined by different crosssectional shapes. With this variation of the cross-sections, the influence of the cross-sectional shape on the aerodynamics of the intake duct is investigated qualitatively. Based on these analyses a sixth intake duct geometry was created as test vehicle for experimental investigation of intake-compressor interaction within the engine test facility. The defining cross-sectional shapes were selected in order to achieve a flow distortion at the duct outlet plane, that is small enough to ensure a safe engine operation, but is still strong enough to provoke interaction of the distorted flow and the compressor flow. The setup for these fully numerical investigations is based on previous studies of the aerodynamics of intake ducts at the Institute of Jet Propulsion. It is shown that the entrance cross-section has a strong influence on the flow throughout the whole intake duct. Additionally, it could be determined that the flow distortion caused by the strong curvature of the intake duct can be reduced in size and strength by a proper combination of cross-sectional shapes.