Synchronization of multi-plane measurement data by means of POD: application to unsteady boundary layer transition

Lengani, Davide; Simoni, Daniele; Nilberto, Alessandro; Ubaldi, Marina; Zunino, Pietro; Bertini, Francesco

doi:10.1007/s00348-018-2642-9

Cited by 11 publications

(3 citation statements)

References 33 publications

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“…In real applications, like compressors or turbine blades, the flow subjected to the favourable and adverse pressure gradient conditions alter the dynamics leading to streak generation and propagation, thus nucleation of turbulent spots and transition. Recent experiments on LPT blades from Lengani et al (2018) show that streaky structures scale with the boundary layer integral parameters, also in the presence of a strong adverse pressure gradient and incoming wakes shed from upstream blade rows. Additionally, the higher the adverse pressure gradient, the higher the streak amplification rate in the pretransitional part of the boundary layer, as shown in Zaki & Durbin (2006).…”

Section: Structures In the Rear Part Of The Suction Sidementioning

confidence: 99%

“…Recent experiments on LPT blades from Lengani et al. (2018) show that streaky structures scale with the boundary layer integral parameters, also in the presence of a strong adverse pressure gradient and incoming wakes shed from upstream blade rows. Additionally, the higher the adverse pressure gradient, the higher the streak amplification rate in the pretransitional part of the boundary layer, as shown in Zaki & Durbin (2006).…”

Section: Introductionmentioning

confidence: 99%

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On the receptivity of low-pressure turbine blades to external disturbances

et al. 2022

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In the present work, the laminar–turbulent transition of the flow evolving around a low-pressure turbine blade has been investigated. Direct numerical simulations have been carried out for two different free stream turbulence intensity (FSTI) levels to investigate the role of free stream oscillations on the evolution of the blade boundary layer. Emphasis is placed on identifying the mechanisms driving the formation and breakup of coherent structures in the high FSTI case and how these processes are affected by the leading-edge receptivity and/or by the continuous forcing in the blade passage. Proper orthogonal decomposition (POD) has been adopted to provide a clear statistical representation of the shape of the structures. Extended POD projections provided temporal and spanwise correlations that allowed us to identify dominant temporal structures and spanwise wavelengths in the transition process. The extended POD analysis shows that the structures on the pressure side are not related to what happens at the leading edge. The results on the suction side show that the modes defining the leading edge and the passage bases correlate with coherent structures responsible for the transition. The most energetic mode of the passage basis is strongly related to the most amplified wavelength in the boundary layer and breakup events leading to transition. Modes with a smaller spanwise wavelength belong to the band predicted by optimal disturbance theory, they amplify with a smaller gain in the rear suction side, and they show the highest degree of correlation between the passage region and the rear suction side.

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Section: Structures In the Rear Part Of The Suction Sidementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On the receptivity of low-pressure turbine blades to external disturbances

et al. 2022

Self Cite

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“…The color levels provide the turbulent kinetic energy produced by the related POD mode, while the green lines superimposed to the plot indicate the boundary layer thickness. Previous applications have already shown the capability of the first POD mode to capture the deterministic part of the fluctuating field (see, for instance, [13][14][15]), identifying the wake as a negative jet and showing two large scale vortices, referred as Q1 and Q2 in the literature (see, for example, [4]), attached to the leading and trailing boundaries of the wake. Here, the application of POD at different phases of the wake passage period also provides the possibility to follow the temporal evolution of these deterministic structures, simply by looking at the first POD mode for different phases.…”

Section: Pod Analysismentioning

confidence: 99%

Flow Coefficient and Reduced Frequency Effects on Wake-Boundary Layer Interaction in Highly Accelerated LPT Cascade

Canepa

Lengani

Nilberto

et al. 2021

IJTPP

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The paper presents a detailed analysis of particle image velocimetry (PIV) measurements performed in a turbine cascade representative of highly accelerated low-pressure turbine (LPT) blades. Two cameras have been simultaneously used to observe a great portion of the suction side boundary layer with the highest possible spatial resolution, thus allowing us to solve the interaction process between impinging upstream wakes and the blade boundary layer. Four unsteady inflow conditions, characterized by different incoming wake reduced frequencies and flow coefficients, have been examined at fixed Reynolds number. The highly resolved flow fields have been processed to explore reduced frequency and flow coefficient effects on the boundary layer unsteady transition process and, consequently, on loss production. For a deep physical insight on the mechanisms responsible for loss generation, proper orthogonal decomposition (POD) has been applied at different phases of the wake passing period. This has provided the dominant structures affecting the cascade aerodynamics during the wake period. Moreover, the examination of POD modes has allowed us to show the effects induced by the parameter variation on the turbulent kinetic energy production and thus to the unsteady loss production mechanisms.

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Identification of coexisting dynamics in boundary layer flows through proper orthogonal decomposition with weighting matrices

et al. 2021

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A different version of the classic proper orthogonal decomposition (POD) procedure introducing spatial and temporal weighting matrices is proposed. Furthermore, a newly defined non-Euclidean (NE) inner product that retain similarities with the POD is introduced in the paper. The aim is to emphasize fluctuation events localized in spatio-temporal regions with low kinetic energy magnitude, which are not highlighted by the classic POD. The different variants proposed in this work are applied to numerical and experimental data, highlighting analogies and differences with respect to the classic and other normalized variants of POD available in the literature. The numerical test case provides a noise-free environment of the strongly organized vortex shedding behind a cylinder. Conversely, experimental data describing transitional boundary layers are used to test the capability of the procedures in strongly not uniform flows. By-pass and separated flow transition processes developing with high free-stream disturbances have been considered. In both cases streaky structures are expected to interact with other vortical structures (i.e. free-stream vortices in the by-pass case and Kelvin–Helmholtz rolls in the separated type) that carry a significant different amount of energy. Modes obtained by the non-Euclidean POD (NE-POD) procedure (where weighted projections are considered) are shown to better extract low energy events sparse in time and space with respect to modes extracted by other variants. Moreover, NE-POD modes are further decomposed as a combination of Fourier transforms of the related temporal coefficients and the normalized data ensemble to isolate the frequency content of each mode.

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Synchronization of multi-plane measurement data by means of POD: application to unsteady boundary layer transition

Cited by 11 publications

References 33 publications

On the receptivity of low-pressure turbine blades to external disturbances

On the receptivity of low-pressure turbine blades to external disturbances

Flow Coefficient and Reduced Frequency Effects on Wake-Boundary Layer Interaction in Highly Accelerated LPT Cascade

Identification of coexisting dynamics in boundary layer flows through proper orthogonal decomposition with weighting matrices

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