Infrared thermography is applied to measure the spiral vortices in the boundary layer over a rotating cone under axial inflow. The data sets are analysed using proper orthogonal decomposition (POD). A criterion based on the signal-to-noise ratio is defined for the selection of relevant POD modes, such that a low-order reconstruction with reduced measurement noise is obtained without affecting the thermal footprint of the spiral vortices. The resulting reconstruction still includes the largescale modulations in the local vortex strength, relating to low-frequency phenomena like amplification, changing vortex states, disturbances in outer flow, etc. The effect of coherent vortical structures is further separated from such phenomena by selective reconstruction of the POD modes based on the number of observed vortices (n) along the circumference. The counter-rotating nature of these vortices is confirmed by PIV measurements. The number of spiral vortices shows good agreement with previously reported methods in the literature. The spiral vortex angle is in good agreement with the previous methods at low rotation ratio (S) , but deviates towards the direction of the local wall shear for high values of S.

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Abstract

This work shows the behavior of an unstable boundary-layer on rotating cones in high-speed flow conditions: high Reynolds number [Formula: see text], low rotational speed ratio [Formula: see text], and inflow Mach number M =  0.5. These conditions are most-commonly encountered on rotating aeroengine nose cones of transonic cruise aircraft. Although it has been addressed in several past studies, the boundary-layer instability on rotating cones remains to be explored in high-speed inflow regimes. This work uses infrared-thermography with a proper orthogonal decomposition approach to detect instability-induced flow structures by measuring their thermal footprints on rotating cones in high-speed inflow. The observed surface temperature patterns show that the boundary-layer instability induces spiral modes on rotating cones, which closely resemble the thermal footprints of the spiral vortices observed in past studies at low-speed flow conditions: [Formula: see text], S >  1, and [Formula: see text]. Three cones with half-cone angles [Formula: see text], and [Formula: see text] are tested. For a given cone, the Reynolds number relating to the maximum amplification of the spiral vortices is found to follow an exponential relation with the rotational speed ratio S, extending from the low- to high-speed regime. At a given rotational speed ratio S, the spiral vortex angle appears to be as expected from the low-speed studies, irrespective of the half-cone angle.

In the pursuit of reducing the fuel burn, future aircraft configurations will feature several types of improved propulsion systems, e.g. embedded engines with boundary layer ingestion, high-bypass ratio engines with short intakes, etc. Depending on the design and phase of flight, the engine fan will encounter inflow distortion of varying strength, and fan performance will be adversely affected. Therefore, investigation of the flow phenomena causing performance losses in fan and distortion interaction is important. This experimental study shows the effect of varying distortion index on four aspects of fan performance: distortion topology, upstream redistribution, performance curve, and flow unsteadiness. A low speed fan is tested under 60° circumferential distortion of varying strength, generated using distortion screens. The flow field in the upstream redistribution region is measured using PIV (planar and stereo). The fan performance is obtained using total pressure measurements. The noise spectra measured by a microphone are used to quantify the unsteadiness in the flow field. The distortion index (DC60) varies linearly with the grid porosity at constant wall thickness and aspect ratio of the grid cells. However, the distortion topology is significantly different as a stream-wise vortex pair appears in distorted flow at higher DC60. The vortices are stronger at higher DC60, but their order of magnitude is much lower than the circulation corresponding to fan itself. The spinner, distortion index and topology significantly affect the upstream redistribution mechanism. The vortex pair redistributes the flow which results in lower asymmetry in the symmetry plane. With increasing distortion, the performance is reduced and the unsteadiness is increased.