Free-Stream Turbulence Induced Boundary-Layer Transition in Low-Pressure Turbines

Durovic, Kristina; Vincentiis, Luca De; Daniele, Simona; Lengani, Davide; Pralits, Jan O.; Henningson, Dan S.; Hanifi, Ardeshir

doi:10.1115/1.0002740v

Cited by 4 publications

(11 citation statements)

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“…The time mean results were presented and compared against experiments in Đurović et al. (2021) with satisfactory agreement. The main aim of the present paper is the analysis of the receptivity of the pressure and suction side boundary layers due to continuous forcing imposed by free stream turbulence.…”

Section: Introductionmentioning

confidence: 75%

“…Downstream of this position, the boundary layer faces an adverse pressure gradient (see Đurović et al. (2021) for more details), leading to a massive separation that fails to reattach in the low FSTI case. The separated flow region is identified by the dark blue area enclosed by the white line at zero velocity in the enlarged view on top of the pictures.…”

Section: Resultsmentioning

confidence: 99%

“…Inlet and outlet flow angles are approximately and , respectively, with a trailing edge thickness-to-pitch ratio of approximately 2 %, more details are available in Đurović et al. (2021).

Figure 1.Computational domain: spectral element mesh.…”

Section: Simulation Tools and Flow Geometrymentioning

confidence: 99%

“…A detailed description of the applied free stream turbulence and its behaviour can be found in Đurović et al. (2021).

Figure 2.Scaled kinetic energy of the free stream modes as a function of non-dimensional wavenumber, , given by von Kármán spectrum.…”

Section: Simulation Tools and Flow Geometrymentioning

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: Introductionmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 99%

“…Inlet and outlet flow angles are approximately and , respectively, with a trailing edge thickness-to-pitch ratio of approximately 2 %, more details are available in Đurović et al. (2021).

Figure 1.Computational domain: spectral element mesh.…”

Section: Simulation Tools and Flow Geometrymentioning

confidence: 99%

“…A detailed description of the applied free stream turbulence and its behaviour can be found in Đurović et al. (2021).

Figure 2.Scaled kinetic energy of the free stream modes as a function of non-dimensional wavenumber, , given by von Kármán spectrum.…”

Section: Simulation Tools and Flow Geometrymentioning

confidence: 99%

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

et al. 2022

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“…The FST is generated through a superposition of Fourier modes with random phase shift (Negi 2019; Durović et al. 2021). The spectrum is discretised using 40 concentric shells with their radius representing the magnitude of the wavenumber vector, and where the amplitude of the shells follows the von Kármán spectrum with the shell energy, the turbulent length scale and the total turbulent kinetic energy.…”

Section: Flow Configuration and Numerical Methodsmentioning

confidence: 99%

Disturbance growth on a NACA0008 wing subjected to free stream turbulence

et al. 2022

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The stability of an incompressible boundary layer flow over a wing in the presence of free stream turbulence (FST) has been investigated by means of direct numerical simulations and compared with the linearised boundary layer equations. Four different FST conditions have been considered, which are characterised by their turbulence intensity levels and length scales. In all cases the perturbed flow develops into elongated disturbances of high and low streamwise velocity inside the boundary layer, where their spacing has been found to be strongly dependent on the scales of the incoming free stream vorticity. The breakdown of these streaks into turbulent spots from local secondary instabilities is also observed, presenting the same development as the ones reported in flat plate experiments. The disturbance growth, characterised by its root mean squares value, is found to depend not only on the turbulence level, but also on the FST length scales. Particularly, higher disturbance growth is observed for our cases with larger length scales. This behaviour is attributed to the preferred wavenumbers that can exhibit maximum transient growth. We study this boundary layer preference by projection of the flow fields at the leading edge onto optimal disturbances. Our results demonstrate that optimal disturbance growth is the main cause of growth of disturbances on the wing boundary layer.

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Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels

Kern,

Blanco,

Cavalieri

et al. 2024

J. Fluid Mech.

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Thin airfoil dynamic stall at moderate Reynolds numbers is typically linked to the sudden bursting of a small laminar separation bubble close to the leading edge. Given the strong sensitivity of laminar separation bubbles to external disturbances, the onset of dynamic stall on a NACA0009 airfoil section subject to different levels of low-amplitude free stream disturbances is investigated using direct numerical simulations. The flow is practically indistinguishable from clean inflow simulations in the literature for turbulence intensities at the leading edge of ${Tu} = 0.02\,\%$ . At slightly higher turbulence intensities of ${Tu} = 0.05\,\%$ , the bursting process is found to be considerably less smooth and strong coherent vortex shedding from the laminar separation bubble is observed prior to the formation of the dynamic stall vortex (DSV). This phenomenon is considered in more detail by analysing its appearance in an ensemble of simulations comprising statistically independent realisations of the flow, thus proving its statistical relevance. In order to extract the transient dynamics of the vortex shedding, the classical proper orthogonal decomposition method is generalised to include time in the energy measure and applied to the time-resolved simulation data of incipient dynamic stall. Using this technique, the dominant transient spatiotemporally correlated features are distilled and the wave train of the vortex shedding prior to the emergence of the main DSV is reconstructed from the flow data exhibiting dynamics of large-scale coherent growth and decay within the turbulent boundary layer.

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Free-Stream Turbulence Induced Boundary-Layer Transition in Low-Pressure Turbines

Cited by 4 publications

References 0 publications

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

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

Disturbance growth on a NACA0008 wing subjected to free stream turbulence

Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels

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