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| 2. |
- De Vincentiis, Luca, 1991-, et al.
(författare)
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Effects of upstream wakes on the boundary layer over a low-pressure turbine blade
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In the present work the evolution of the boundary layer over a low-pressureturbine blade is studied by means of direct numerical simulations. The set-upof the simulations follows the experiments by Lengani et al. (2017), aimingto investigate the unsteady flow field induced by the rotor-stator interaction.The free-stream flow is characterized by high level of free-stream turbulenceand periodically impinging wakes. As in the experiments, the wakes are shedby moving bars modeling the rotor blades and placed upstream of the turbineblades. To include the presence of the wake without employing an ad-hoc model,we simulate both the moving bars and the stationary blades in their respectiveframes of reference and the coupling of the two domains is done throughappropriate boundary conditions. The presence of the wake mainly affects thedevelopment of the boundary layer on the suction side of the blade. In particular,the flow separation in the rear part of the blade is suppressed. Moreover, thepresence of the wake introduces alternating regions in the streamwise direction ofhigh- and low-velocity fluctuations inside the boundary layer. These fluctuationsare responsible for significant variations of the shear stress. The analysis of thevelocity fields allows the characterization of the streaky structures forced inthe boundary layer by turbulence carried by upstream wakes. The breakdownevents are observed once positive streamwise velocity fluctuations reach theend of the blade. Both the fluctuations induced by the migration of the wakein the blade passage and the presence of the streaks contribute to high valuesof the disturbance velocity inside the boundary layer with respect to a steadyinflow case. The amplification of the boundary layer disturbances associatedwith different spanwise wavenumbers has been computed. It was found thatthe migration of the wake in the blade passage stands for the most part of theperturbations with zero spanwise wavenumber. The non-zero wavenumbers arefound to be amplified in the rear part of the blade at the boundary betweenthe low and high speed regions associated with the wakes.
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| 3. |
- De Vincentiis, Luca, 1991-, et al.
(författare)
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Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade
- 2023
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Ingår i: Journal of turbomachinery. - : ASME International. - 0889-504X .- 1528-8900. ; 145:5
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the low- and high-speed regions associated with the wakes.
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| 4. |
- Dellacasagrande, Matteo, et al.
(författare)
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A procedure for computing the spot production rate in transitional boundary layers
- 2022
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Ingår i: Experiments in Fluids. - : Springer Nature. - 0723-4864 .- 1432-1114. ; 63:8
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Tidskriftsartikel (refereegranskat)abstract
- The present work describes a method for the computation of the nucleation rate of turbulent spots in transitional boundary layers from particle image velocimetry (PIV) measurements. Different detection functions for turbulent events recognition were first tested and validated using data from direct numerical simulation, and this latter describes a flat-plate boundary layer under zero pressure gradient. The comparison with a previously defined function adopted in the literature, which is based on the local spanwise wall-shear stress, clearly highlights the possibility of accurately predicting the statistical evolution of transition even when the near-wall velocity field is not directly available from the measurements. The present procedure was systematically applied to PIV data collected in a wall-parallel measuring plane located inside a flat plate boundary layer evolving under variable Reynolds number, adverse pressure gradient (APG) and free-stream turbulence. The results presented in this work show that the present method allows capturing the statistical response of the transition process to the modification of the inlet flow conditions. The location of the maximum spot nucleation is shown to move upstream when increasing all the main flow parameters. Additionally, the transition region becomes shorter for higher Re and APG, whereas the turbulence level variation gives the opposite trend. The effects of the main flow parameters on the coefficients defining the analytic distribution of the nucleation rate and their link to the momentum thickness Reynolds number at the point of transition are discussed in the paper. [GRAPHICS] .
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| 5. |
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| 6. |
- Durovic, Kristina, et al.
(författare)
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Free-Stream Turbulence-Induced Boundary-Layer Transition in Low-Pressure Turbines
- 2021
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Ingår i: Journal of turbomachinery. - : ASME International. - 0889-504X .- 1528-8900. ; 143:8
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Tidskriftsartikel (refereegranskat)abstract
- The aerodynamic efficiency of turbomachinery blades is profoundly affected by the occurrence of laminar-turbulent transition in the boundary layer since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level, we can identify different paths toward a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behavior of the low-pressure turbine blade. In the simulations, the flow past only one blade is computed, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet. The free-stream turbulence is prescribed as a super-position of Fourier modes with a random phase shift. Two levels of the free-stream turbulence intensity were simulated (Tu = 0.19% and 5.2%), with the integral length scale being 0.167c, at the leading edge. We observed that in the case of low free-stream turbulence on the suction side, the Kelvin-Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high-turbulence case, where streaks broke down into turbulent spots and caused bypass transition. On the pressure side, we have identified the appearance of longitudinal vortical structures, where increasing the turbulence level gives rise to more longitudinal structures. We note that these vortical structures are not produced by Gortler instability.
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| 8. |
- Lengani, Davide, et al.
(författare)
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On the receptivity of low-pressure turbine blades
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In the present work, the laminar-turbulent transition of the flow evolving arounda low-pressure turbine blade has been investigated. Direct numerical simulationshave been carried out for two different free-stream turbulence intensity (FSTI)levels to investigate the role of free-stream oscillations on the evolution of theblade boundary layer. Emphasis is posed on identifying the mechanisms drivingthe formation and breakup of coherent structures in the high FSTI case andhow these processes are affected by the leading-edge receptivity and/or bythe continuous forcing in the blade passage. Proper orthogonal decomposition(POD) has been adopted to provide a clear statistical representation of theshape of the structures. Extended POD projections provided temporal andspanwise correlations that allowed us to identify dominant temporal structuresand spanwise wavelengths in the transition process.The extended POD analysis shows that the structures on the pressure sideare not related to what happens at the leading edge. The results on the suctionside show that the modes defining the leading edge and the passage basescorrelate with coherent structures responsible for the transition. The mostenergetic mode of the passage basis is strongly related to the most amplifiedwavelength in the boundary layer and breakup events leading to transition.Modes with a smaller spanwise wavelength belong to the band predicted byoptimal disturbance theory, they amplify with a smaller gain in the rear suctionside, and they show the highest degree of correlation between the passage regionand the rear suction side.
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