Modal Analysis of Serpentine Diffuser Distortion

Abstract

A double-bend, serpentine inlet diffuser is simulated to investigate the unsteady dynamics that contribute to reduced jet engine performance. The aggressive turning of the transonic flow, in conjunction with a varying non-axisymmetric cross-sectional area, induce separation along the upper and lower surfaces. The separated flow results in pressure distortion and total pressure losses at the Aerodynamic Interface Plane (AIP), i.e., the junction between the diffuser and the compressor. Results from the Large Eddy Simulation (LES) identify several related features influencing AIP distortion, including: shock-boundary-layer-interactions, counter-rotating vortices, and upstream-propagating acoustic waves. Proper Orthogonal Decomposition (POD) and Spectral POD (SPOD) identify the most energetic frequencies and pressure patterns at the AIP; these frequencies inform mode selection of a subsequent, three-dimensional SPOD analysis of the entire diffuser. Distortion modes are classified by low, middle, and high frequency bands, which are dominated by the upstream shock, lower-surface and upper-surface separated flow, respectively. These global SPOD modes reveal contributions to AIP pressure fluctuations through a variety of mechanisms, including symmetric and asymmetric coherent structures, wave-packets, oblique travelling waves, and standing waves. The upstream and downstream energy waves resonate to produce the largest tones in the high-frequency band at the AIP. Distortion patterns are also strongly correlated to the upstream shock motion that influences the initial flow separation on the bottom surface.