| 1. |
- De Vincentiis, Luca, 1991-, et al.
(författare)
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Free-Stream Turbulence Induced Boundary-Layer Transition In Low-Pressure Turbines
- 2022
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Ingår i: Proceedings of the ASME Turbo Expo. - : ASME International.
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Konferensbidrag (refereegranskat)abstract
- In the present work the evolution of the boundary layer over a low-pressure turbine blade is studied by means of direct numerical simulations. The set-up of the simulations follows the experiments by [1], aiming to investigate the unsteady flow field induced by the rotor-stator interaction. The free-stream flow is characterized by high level of free-stream 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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| 2. |
- Dellacasagrande, M., et al.
(författare)
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Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient
- 2021
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : AIP Publishing. - 1063-651X .- 1095-3787. ; 33:9, s. 094115-094115
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the free-stream turbulence (FST) induced transition of a flat plate boundary layer is studied using particle image velocimetry (PIV) under variable Reynolds number (Re), FST intensity, and adverse pressure gradient (APG). Overall, 10 different flow conditions were tested concerning the variation of these parameters. The streak spacing and the probability density function (PDF) of turbulent spot nucleation are computed for all cases. The streak spacing is shown to be constant in the transition region once scaled with the turbulent displacement and momentum thickness, with resulting values of around 3 and 5, respectively. Nucleation events are shown to occur near the position where the dimensionless streak spacing reaches such constant values. The streamwise position where most turbulent spots are formed is strongly influenced by the FST intensity level. Additionally, the PDF of spot nucleation becomes narrower with increase in the APG, while FST has the opposite effect. A common distribution of all the PDFs is provided as a function of a similarity variable accounting for the streak spacing, the shape factor of the boundary layer, and the FST intensity.
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| 3. |
- Durovic, Kristina, et al.
(författare)
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Free-stream turbulence induced boundary-layer transition in low-pressure turbines
- 2020
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Ingår i: Proceedings of the ASME Turbo Expo. - : American Society of Mechanical Engineers (ASME).
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Konferensbidrag (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 towards a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behaviour of the low-pressure turbine blade. The computational set-up was designed to follow the experiments by Lengani & Simoni [1]. 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 superposition 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 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 Görtler instability.
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| 4. |
- Lengani, D., et al.
(författare)
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Investigation on Strain and Stress Principal Axes in Unsteady DNS Turbine Data
- 2022
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Ingår i: Proceedings of the ASME Turbo Expo. - : ASME International.
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Konferensbidrag (refereegranskat)abstract
- In the present work, high-fidelity direct numerical sim- ulation (DNS) data has been adopted in conjunction with an extensive post-processing to provide a detailed descrip- tion of the turbulence characteristics and its production within a low pressure turbine (LPT) cascade blade pas- sage operating with unsteady inflow. Proper orthogonal decomposition is used at first to provide the statistical rep- resentation of the flow structures that occur in the blade passage. Different inlet turbulent scales are isolated and a representation of the turbulence produced in the passage is also provided. Principal axes of the Reynolds stress and the strain tensors have been analyzed to provide further insight on the turbulence production. Since each spatial POD mode captures a quota of the Reynolds stress ten- sor, the POD modes are well suited to provide reduced order models (ROMs) that represent the different scales of turbulence. Namely, four different scales are defined, and the eigenvectors of the stress tensor for each reduced model are discussed. The discussion includes the compar- ison with the principal axis of the strain rate tensor. It is shown that the spatial locations where the eigen- vectors of the strain and stress tensors are aligned lead to the largest production of turbulent kinetic energy. The de- terministic periodic perturbations induced at the inlet by the unsteady incoming wakes lead to the largest produc- tion of turbulence in the passage region where the highest strain is detected and where the eigenvectors of the two tensors are aligned. In the suction side boundary layers, the highest production is related to the local maximum of the Reynolds shear stress due to the stochastic perturba- tions. The deterministic perturbations do not contribute to the production of turbulence in the suction side bound- ary layer, even though their induced stress is not negli- gible, because the eigenvector directions have a maximum misalignment.
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| 5. |
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| 6. |
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| 7. |
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| 8. |
- Kern, Simon, et al.
(författare)
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Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels
- 2024
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 986
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 9. |
- Kleusberg, Elektra, et al.
(författare)
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Parametric dependencies of the yawed wind-turbine wake development
- 2020
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Ingår i: Wind Energy. - : John Wiley and Sons Ltd. - 1095-4244 .- 1099-1824. ; 23:6, s. 1367-1380
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Tidskriftsartikel (refereegranskat)abstract
- Yaw misalignment is currently being treated as one of the most promising methods for optimizing the power of wind farms. Therefore, detailed knowledge of the impact of yaw on the wake development is necessary for a range of operating conditions. This study numerically investigates the wake development behind a single yawed wind turbine operating at different tip-speed ratios and yaw angles using the actuator-line method in the spectral-element code Nek5000. It is shown that depending on the tip-speed ratio, the blade loading varies along the azimuth, resulting in a wake that is asymmetric in both the horizontal and vertical directions. Large tip-speed ratios as well as large yaw angles are shown to decrease the vertical asymmetry of the yaw-induced counter-rotating vortex pair. Both parameters have the effect that they increase the spanwise force induced by yaw relative to the wake rotation. However, while the strength of the counter-rotating vortex pair in the far wake increases with yaw angle, it is shown to decrease with the tip-speed ratio. The vertical shift in the wake center is found to be highly dependent on the yaw angle and the tip-speed ratio. These detailed insights into the yawed wake are important when optimizing potential downstream turbines.
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| 10. |
- Muld, Toma. W., et al.
(författare)
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Mode decomposition of flow structures in the wake of two high-speed trains
- 2012
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Ingår i: Civil-Comp Proceedings. - Stirlingshire, UK : Civil-Comp Press. - 1759-3433. ; 98
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Tidskriftsartikel (refereegranskat)abstract
- Two different train geometries, the Aerodynamic Train Model (ATM) and the CRH1, are studied in order to compare the flow structures in the wake. The flow is simulated with Detached Eddy-Simulation and then decomposed into modes with Proper Orthogonal Decomposition. This study has found that the flow structures are indeed different for the two train models although the tails are rather similar. For the CRH1 the dominant flow structures twist one of the counter-rotating vortices and leaves the other straight. The convergence of the modes are investigated and it is shown that approximately the same number of snapshots are needed for both trains.
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