Abstract
To identify the spatiotemporal coherent structure of compressor tip leakage flow, spectral proper orthogonal decomposition (SPOD) is performed on the near-tip flow field and the blade surface pressure of a low-speed compressor rotor. The data used for the SPOD analysis are obtained by delayed-detached eddy simulation, which is validated against the experimental data. The investigated rotor near-tip flow field is governed by two tip leakage vortices (TLV), and the near-tip compressor passage can be divided into four zones: the formation of main TLV (Zone I), the main TLV breakdown (Zone II), the formation of tip blockage cell (Zone III), and the formation of secondary TLV (Zone IV). Modal analysis from SPOD shows that a major part of total disturbance energy comes from the main TLV oscillating mode in Zone I and the main TLV vortex shedding mode in Zone III, both of which are low-frequency and low-rank; on the contrary, modal components in Zones II and IV are broadband and non-low-rank. Unsteady blade forces are mainly generated by the impingement of the main TLV on the blade pressure surface in Zone III, rather than the detachment of the secondary TLV from the blade suction surface in Zone IV. These identified coherent structures provide valuable knowledge for the aerodynamic/aeroelastic effects, turbulence modeling, and reduced-order modeling of compressor tip leakage flow.
Highlights
Compressor tip leakage flow has long been known as the dominant mechanism in determining compressor work input, efficiency and stability.[1]
The investigated rotor near-tip flow field is governed by two tip leakage vortices (TLV), and the near-tip compressor passage can be divided into four zones: the formation of main TLV (Zone I), the main TLV breakdown (Zone II), the formation of tip blockage cell (Zone III), and the formation of secondary TLV (Zone IV)
Modal analysis from spectral proper orthogonal decomposition (SPOD) shows that a major part of total disturbance energy comes from the main TLV oscillating mode in Zone I and the main TLV vortex shedding mode in Zone III, both of which are low-frequency and low-rank; on the contrary, modal components in Zones II and IV are broadband and non-low-rank
Summary
Compressor tip leakage flow has long been known as the dominant mechanism in determining compressor work input, efficiency and stability.[1]. Modal analysis is a data mining tool that extracts the general spatial and temporal flow structures (i.e., modes) from the snapshots of unsteady flow field data. The most popular mode decomposition methods for compressor flows are proper orthogonal decomposition (POD) proposed by Lumley[11] and dynamic mode decomposition (DMD) proposed by Schmidt.[12]. POD extracts coherent structures in turbulent flows by identifying the optimal set of orthogonal modes ranked by their modal energy. From the perspective of a turbomachinery aerodynamicist, an ideal mode decomposition method should combine the advantages of both POD and DMD: the identified modes can be ranked according to their energy level with each mode having a characteristic frequency. Spectral proper orthogonal decomposition (SPOD) is an ideal mode decomposition method for this purpose, whose modes are frequency-resolved and energy-ranked. Scitation.org/journal/phf be valuable for understandings of the aerodynamic/aeroelastic effects, turbulence modeling, and reduced-order modeling of compressor tip leakage flow. The SPOD analysis of the near-tip flow field and the blade surface pressure will be presented and discussed
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