Unsteady Characterization of Vortical Structures in Highly Distorted Intake Duct Flows

Unmanned aerial systems (UAS) are playing an increasingly crucial role in the advancement of aerospace technologies across multiple fields. Integrating complete engines within the aircraft fuselage, often using highly curved intake ducts, provides notable benefits, including a more compact design and reduced structural weight. However, such compact intake systems can cause significant flow distortions, compromising engine performance and stability. Researchers at the Institute of Jet Propulsion (IJP) at the University of the Bundeswehr Munich are addressing these challenges by studying a highly curved, compact intake, the so-called Military Engine Intake Research Duct (MEIRD), to investigate strong flow distortion and propulsion system integration effects. The aerospace industry typically assesses flow distortion using isolated pressure transducers at the aerodynamic interface plane (AIP) in combination with traditional metrics such as the DC60. However, this approach fails to capture the full spatial complexity of highly compact intake flows. It cannot represent the effects of secondary flow instabilities or more complex distortion patterns. Past studies have emphasized that the dynamic aspects of flow distortion play a crucial role in the stable operation of the intake and engine system. Specific frequency components can significantly influence how the engine’s fan responds to varying flow conditions. Furthermore, research has shown that methods based on steady-state RANS simulations fail to capture critical aerodynamic phenomena. A study also found that existing turbulence models may struggle to accurately represent unsteady flow characteristics when investigating the aerodynamic behavior within S-ducts. In this work, scale-resolved simulations were performed using the SAS-SST approach using ANSYS CFX to analyze the unsteady flow in the scaled-down MediMEIRD. The model combines the advantages of URANS and LES by dynamically adjusting according to local flow conditions. A high-resolution mesh with 189 million elements and a 8192 Hz sampling rate enabled accurate resolution of coherent structures and transient distortion mechanisms. Validation against previous experimental wall-pressure Kulite data confirmed that the SAS-SST approach is able to predict large- and small-scale unsteady flow features. The results reveal that the flow distortion is dominated by the interaction of lateral vortices and a central separation bubble, producing cyclic asymmetry and periodic regeneration of the vortex system. The spectral analyses identified dominant low-frequency separation-bubble breathing (St=0.06 - 0.07) and high-frequency shear-layer instabilities (St=0.65 - 1.2). These findings demonstrate that the scale-resolved SAS-SST simulation, combined with advanced distortion metrics, accurately captures both global and local unsteady flow dynamics. This approach provides a robust foundation for assessing intake and engine operability and developing future flow-control strategies for compact UAS propulsion systems.