Convoluted Intake Distortion Measurements Using Stereo Particle Image Velocimetry

Abstract

The unsteady distorted flow fields generated within convoluted aero-engine intakes can compromise the engine performance and operability. Therefore there is a need for a better understanding of the complex characteristics of the distorted flow at the exit of S-shaped intakes. This work presents a detailed analysis of the unsteady swirl distortion based on synchronous, high spatial resolution measurements using stereoscopic particle image velocimetry. Two S-duct configurations with different centerline offsets are investigated. The high offset duct shows greater levels of dynamic and steady swirl distortion and a notably greater tendency towards bulk swirl patterns associated with high swirl distortion. More discrete distortion patterns with locally high swirl levels and the potential to impact the engine operability are identified. The most energetic coherent structures of the flow field are observed using proper orthogonal decomposition. A switching mode is identified which promotes the alternating swirl switching mechanism and is mostly associated with the occurrence of potent bulk swirl events. A vertical mode which characterizes a perturbation of the vertical velocity field promotes most of the twin swirl flow distortion topologies. It is postulated that it is associated with the unsteadiness of the centerline shear layer. Nomenclature a = POD temporal coefficient, m/s

Passive Flow Control Study in an S-Duct Using Stereo Particle Image Velocimetry

“…Zachos et al [15] identified that a biased flow toward one side were generally associated with large value of SI mean . These extreme events were also confirmed by Gil-Prieto et al [16] in which the study of the inner ring (r/R = 0.32) showed a tri-modal flow field: negative bulk swirl (SP=0.5, SD=-1), symmetric twin-swirl (SP=1, SD=0) and positive bulk swirl (SP=0.5, SD=+1). It was concluded from the SP-SD relationship that the flow experiences a bulk-to-twin switching mechanism in which the flow oscillates from one positive or negative bulk swirl to twin vortex pattern [16].…”

“…Therefore the inner section of the AIP is also subject to large swirl values due to high . Gil-Prieto et al [16] also reported the presence of peak values in absolute swirl angle higher than 25˚ along a circumferential locus of r/R = 0.32.…”

“…The assessment of the flow field based on each snapshot revealed the presence of bulk swirling structures rotating either clockwise or anti-clockwise. Gil-Prieto et al [16] identified the inner ring (r/R = 0.32) of the AIP section as the most critical in terms of unsteady swirl distortion for the same duct. The 5 analysis pointed out a tri-modal state of oscillation of the flow corresponding to a positive bulk swirl, twin swirl and negative bulk swirl.…”

“…2). The S-duct geometry used for this investigation was the high offset duct previously investigated by Zachos et al [15] and GilPrieto et al [16]. It is a circular section duct with a diffusion area of A out /A in =1.52, an offset to inlet diameter ratio of H⁄D in =2.44 and a length to inlet diameter ratio L/D in of 4.95 ( Table 1).…”

“…Peak values of swirl intensity in the inner rings were associated with the bulk swirl pattern. GilPrieto et al [16] also applied proper orthogonal decomposition (POD) on the velocity vector field at the AIP to identify the most energetic coherent structures of the flow field. This highlighted two main mechanisms present in the distorted flow field; a swirl switching mode and a vertical-perturbation mode [16].…”