The unsteady flowfields generated by convoluted aero-engine intakes are sources of instabilities that can compromise the performance of the downstream turbomachinery. Hence, there is a need for synchronous, high-spatial resolution measurements that will allow a greater understanding of the aerodynamics. In this work, Stereo Particle Image Velocimetry (S-PIV) is used in a new application to characterize the distorted flow at the outlet of complex intake. A suite of measurements and analyses for two S-duct configurations across a range of inlet Mach numbers are presented. The work demonstrates the feasibility of using SPIV techniques for determining the flow field at the exit of convoluted intakes with a higher spatial resolution than typical pressure measurements. Analysis of the distortion descriptors quantifies the dependency upon the S-duct configuration and highlights that the more high-offset duct generates greater levels of swirl distortion relative to the low-offset configuration. Distortion maps show that the flow is highly unsteady with a characteristic switching of the swirling flow. The results provide a new dataset which shows that the conventional assessments based on time-averaged data substantially underestimates the swirl distortion level and that the less frequent distortion events give rise to more potentiallysignificant threats to the compression system. Nomenclature A = area

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For many embedded engine systems, the intake duct geometry introduces flow distortion and unsteadiness which must be understood when designing the turbomachinery components. The aim of this work is to investigate the capabilities of modern computational methods for these types of complex flows, to study the unsteady characteristics of the flow field and to explore the use of proper orthogonal decomposition methods to understand the nature of the unsteady flow distortion. The unsteady flows for a range of S-duct configurations have been simulated using a delayed detached eddy simulation method. Analysis of the conventional distortion criteria highlights the main sensitivities to the S-duct configuration and quantifies the unsteady range of these parameters. The unsteady flow field shows signature regions of unsteadiness which are postulated to be related to the classical secondary flows as well as to the streamwise flow separation. A proper orthogonal decomposition of the total pressure field at the duct exit identifies the underpinning flow modes which are associated with the overall total pressure unsteadiness distributions. Overall, the unsteady distortion metrics are not found to be solely linked to a particular proper orthogonal decomposition mode, but are dependent on a wider range of modes. Nomenclature p = static pressure p o = total pressure r = cross section radius peak = maximum value of a temporal distribution max = maximum value of a spatial distribution 1