For some embedded engine arrangements, the nature of the inlet distortion is influenced by the boundary layer characteristics at the inlet plane of the intake. This research presents the first quantitative assessment on the influence of inlet boundary layer thickness and asymmetry on the swirl distortion at the exit of an S-shaped intake. Measurements of high spatial and temporal resolution have been acquired at the outlet plane of the S-duct using time-resolved particle image velocimetry. When boundary layer profiles typical of embedded engines are introduced, the characteristic secondary flows at the outlet plane are intensified. Overall, the peak swirl intensity increases by 40% for a boundary layer which is 7 times thicker than the reference case. The unsteady modes of the S-duct remain, although the dominant fluctuations in the flow arise at a frequency 50% lower. When the inlet boundary layer profile becomes asymmetric about the intake centerline the peak swirl events at the hub are reduced by up to 40%. At the tip the peak swirl intensity increases by 29%. The results demonstrate that the effects of inlet boundary layer thickness and asymmetry must be carefully considered as part of engine compatibility tests for complex intakes. Nomenclature ai(t) = Temporal coefficient for i th mode in Proper Orthogonal Decomposition [ms -1 ] Ain = S-duct inlet plane area [m 2 ] Aout = S-duct outlet plane area [m 2 ] Din = S-duct inlet plane diameter [m]

Closer integration between the airframe and the propulsion system is expected for future aircraft to reduce fuel consumption, emissions, weight and drag. The use of embedded or partially embedded propulsion systems will require the use of complex intakes. However, this can result in unsteady flow distortion which can adversely affect the propulsion system efficiency and stability. Relative to conventional measurement systems, time-resolved Particle Image Velocimetry provides sufficient spatial and temporal resolution to enable the development of new methods to assess unsteady flow distortion. This work proposes a novel analysis approach to assess the unsteady flow distortion. For an S-duct configuration, the method was successfully used to evaluate the unsteady flow distortion in terms of idealized incidence angle perturbations. This example showed peaks up to ± 30° incidence and a duration equivalent to the passing time of 3 blades. The introduction of a non-uniform total pressure profile at the S-duct inlet increased the probability of peak distortion events with higher magnitude. The method provides an estimate of the likelihood, magnitude and duration of distortion events and is a new way to evaluate flow distortion that could induce instabilities for the propulsion system.

Convoluted aero-engine intakes are often required to enable closer integration between engine and airframe. Although the majority of previous research focused on the distortion of S-duct intakes with undistorted inlet conditions, there is a need to investigate the impact of more challenging inlet conditions at which the intake duct is expected to operate. The impact of inlet vortices and total pressure profiles on the inherent unsteady flow distortion of an S-duct intake was assessed with stereo particle image velocimetry. Inlet vortices disrupted the characteristic flow switching mode but had a modest impact on the peak levels and unsteady fluctuations. Non-uniform inlet total pressure profiles increased the peak swirl intensity and its unsteadiness. The frequency of swirl angle fluctuations was sensitive to the azimuthal orientation of the non-uniform total pressure distribution. The modelling of peak distortion with the extreme value theory revealed that although for some inlet configurations the measured peak swirl intensity was similar, the growth rate of the peak values beyond the experimental observations was substantially different and it was related with the measured flow unsteadiness. This highlights the need of unsteady swirl distortion measurements and the use of statistical models to assess the time-invariant peak distortion levels. Overall, the work shows it is vital to include the effect of the inlet flow conditions as it substantially alters the characteristics of the complex intake flow distortion.

No abstract

The need to reduce aero-engine emissions and direct operating costs is driving the civil aerospace sector towards considering more integrated propulsion systems. Many of the proposed novel aircraft architectures employ convoluted intakes for either the aero-engine or propulsion system. These intakes are characterized by unsteady distortion that can hinder the performance and operability of the propulsion system. This work assesses the impact of the inlet boundary layer on the unsteady aerodynamics of an S-duct intake using time-resolved particle image velocimetry at the aerodynamic interface plane. An increase in the boundary layer thickness at the intake inlet increases the flow unsteadiness on the swirl angle by up to 40% relative to the baseline case. The azimuthal orientation of the inlet boundary layer modifies the intensity and topology of the most frequent swirl distortion pattern. For a relatively thick inlet boundary layer, the reduction of the dominant frequencies associated with the unsteady swirl angle is postulated to be beneficial for the engine stability. Overall, this works gives guidelines for the integration between the intake and the engine across the range of potential inlet operating conditions.