| 1. |
- Fu, Pan, et al.
(författare)
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Metasurface Enabled On-Chip Generation and Manipulation of Vector Beams from Vertical Cavity Surface-Emitting Lasers
- 2023
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- Metasurface polarization optics that consist of 2D array of birefringent nano-antennas have proven remarkable capabilities to generate and manipulate vectorial fields with subwavelength resolution and high efficiency. Integrating this new type of metasurface with the standard vertical cavity surface-emitting laser (VCSEL) platform enables an ultracompact and powerful solution to control both phase and polarization properties of the laser on a chip, which allows to structure a VCSEL into vector beams with on-demand wavefronts. Here, this concept is demonstrated by directly generating versatile vector beams from commercially available VCSELs through on-chip integration of high-index dielectric metasurfaces. Experimentally, the versatility of the approach for the development of vectorial VCSELs are validated by implementing a variety of functionalities, including directional emission of multibeam with specified polarizations, vectorial holographic display, and vector vortex beams generations. Notably, the proposed vectorial VCSELs integrated with a single layer of beam shaping metasurface bypass the requirements of multiple cascaded optical components, and thus have the potential to promote the advancements of ultracompact, lightweight, and scalable vector beams sources, enriching and expanding the applications of VCSELs in optical communications, laser manipulation and processing, information encryption, and quantum optics.
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| 2. |
- He, Kan, et al.
(författare)
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Assessment of LES, IDDES and RANS approaches for prediction of wakes behind notchback road vehicles
- 2021
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 217
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Tidskriftsartikel (refereegranskat)abstract
- The capability of Large Eddy Simulations (LES), Improved Delayed Detached Eddy Simulations (IDDES) and Reynolds-Averaged Navier–Stokes Equations (RANS) to predict the flow behind notchback Ahmed body is investigated in the present paper. Simulations consider two specific models, with effective backlight angles of β1=17.8° and β2=21.0°, respectively. The focus of the study is on the prediction of the expected lateral asymmetry or symmetry of the near-wake flows. Results show that IDDES using coarse computational grids predicts the flow in agreement with LES using finer computational grids. RANS results in inaccurate flow predictions, attributed to its steady formulation relying on turbulence modelling being incapable of dealing with the studied flow. Modal analysis applying Proper Orthogonal Decomposition (POD) suggests the consistency of the wake dynamics between IDDES and LES. The presence of the wake bi-stability is validated by the wind tunnel experiment.
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| 3. |
- He, Kan, 1991, et al.
(författare)
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Floor motion's influence on wake asymmetry of a notchback bluff body
- 2022
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:3
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Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulations are used to explore the influence of floor motions on asymmetric flows around a notchback bluff body. The focus of this study is on the aerodynamic forces and the extent of natural wake asymmetry presented under moving and stationary ground conditions. The different ground condition has a notable influence on the aerodynamic force and the surrounding pressure distribution of the body. On the other hand, the wake asymmetry, known to be a sensitive phenomenon, is not evidently affected by the floor motion. However, quantitative analysis of the averaged and the statistic flow still suggests slight differences in the degree of wake asymmetry between the two ground conditions. Modal analysis applying proper orthogonal decomposition confirms that the asymmetric wake dynamics and the wake shedding frequency are not sensitive to the floor motion. The accuracy of the numerical simulation is established by a grid-independence study.
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| 4. |
- He, Kan, et al.
(författare)
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Numerical investigation of the wake bi-stability behind a notchback Ahmed body
- 2021
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 926, s. A36-1-A36-29
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulations are used to investigate the origin of the wake asymmetry and symmetry behind notchback Ahmed bodies. Two different effective backlight angles, beta(1) = 17.8 degrees and beta(2) = 21.0 degrees, are simulated resulting in wake asymmetry and symmetry in flows without external perturbations, in agreement with previous experimental observations. In particular, the asymmetric case presents a bi-stable nature showing, in a random fashion, two stable mirrored states characterized by a left or right asymmetry for long periods. A random switch and several attempts to switch between the bi-stability are observed. The asymmetry of the flow is ascribed to the asymmetric separations and reattachments in the wake. The deflection of the near-wall flow structures behind the slant counteracting the asymmetry drives the wake to be temporarily symmetric, triggering the switching process of the bi-stable wake. The consequence of deflection that forces the flow structure to form on the opposite side of the slant is the decisive factor for a successful switch. Modal analysis applying proper orthogonal decomposition is used for the exploration of the wake dynamics of the bi-stable nature observed.
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| 5. |
- Kan, Siyi, et al.
(författare)
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Risk of intact forest landscape loss goes beyond global agricultural supply chains
- 2023
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Ingår i: One Earth. - : Elsevier BV. - 2590-3322 .- 2590-3330. ; 6:1, s. 55-65
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Tidskriftsartikel (refereegranskat)abstract
- The continued loss of unfragmented intact forest landscapes (IFLs) despite numerous global conservation initiatives indicates the need for improved knowledge of proximate and underlying drivers. Yet the role of non-agricultural activities in forest degradation and fragmentation has not received adequate attention. We focus on IFL loss caused by various economic activities and investigate the influence of global consumption and trade via the multi-regional input-output model. For IFL loss associated with the 2014 world economy, over 60% was related to final consumption of non-agricultural products. More than one-third of IFL loss was linked to export, primarily from Russia, Canada, and tropical regions to mainland China, the EU, and the United States. Of IFL loss associated with export, 51% and 26% was directly caused by logging and mining or energy extraction, respectively. The dispersed nature of IFL loss drivers and their indirect links to individual final consumers call for stronger government engagement and supply chain interventions.
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| 6. |
- Wang, Jiabin, et al.
(författare)
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An IDDES investigation of Jacobs bogie effects on the slipstream and wake flow of a high-speed train
- 2020
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 202
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Tidskriftsartikel (refereegranskat)abstract
- This study numerically investigates the effects of Jacobs bogies on the aerodynamic behaviors of a high-speed train using improved delayed detached eddy simulation (IDDES) at Re = 3.3 × 105. The results of the numerical simulations have been validated against the experimental data obtained from a previous reduced-scale moving model test and a wind tunnel test. The slipstream velocity, wake flow, underbody flow and aerodynamic drag of the HST are compared between the conventional bogie case and Jacobs bogie case. The results show that the use of Jacobs bogies can reduce the TSI values of the slipstream velocity at trackside and platform positions by 11.07% and 22.40%, respectively, which thereby shows a positive effect on improving the safety level of trackside workers and passengers standing on the platform. The Jacobs bogies are found to decrease the maximum values of the slipstream velocity and turbulence kinetic energy occurring at the intermediate bogie regions beneath the HST by 30.08% and 41.32%, respectively, which is beneficial for weakening the ballast flight phenomenon. The Jacobs bogies significantly narrow the scale of the longitudinal vortex structure in the wake propagation region. Additionally, the application of Jacobs bogies lowers the aerodynamic drag values of the vehicles and contributes to a 10% total drag reduction.
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| 7. |
- Wang, Jiabin, et al.
(författare)
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Impact of the bogies and cavities on the aerodynamic behaviour of a high-speed train. An IDDES study
- 2020
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 207
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the effects of bogie cavities and bogies on the aerodynamic behaviour of a high-speed train (HST) using improved delayed detached eddy simulation (IDDES) at Re = 3.3 × 105. The main aim of this work is to identify the individual influence of bogies and cavities on the surrounding flow, thereby revealing aspects to further improve the HST aerodynamic performance. The accuracy of the numerical method has been validated against the experimental data obtained from a previous reduced-scale moving-model test, a wind tunnel test and a full-scale field test. The underbody flow, wake flow, slipstream velocity, aerodynamic drag and the computational costs are compared for three cases. The results show that installing the bogies around the cavities in Case 1 and sealing the cavities in Case 3 can effectively reduce the turbulence kinetic energy (TKE) and slipstream velocity around the HST. The cavities in Case 2 produce the highest level of TKE and slipstream velocity distribution in both underbody flow region and the wake region, compared to other two cases. This presents the largest scales of the shear vortices in the cavities and longitudinal vortices in the wake. Compared to Case 2, the TSI values of the slipstream velocity at the trackside position decreases by about 18.9% in Case 1 and 56.9% in Case 3, respectively. The cavities account for approximately 65% of the aerodynamic drag of the HST. Installing bogies in Case 1 and sealing cavities in Case 3 gives a 24% and 56% drag reduction for the overall HST. It is recommended to invest 10% higher resources to achieve an accurate surrounding flow prediction of a HST in presence of bogies.
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| 8. |
- Wang, Jiabin, 1993, et al.
(författare)
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Validation of PANS and effects of ground and wheel motion on the aerodynamic behaviours of a square-back van
- 2023
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 958
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a numerical investigation of the effects of the moving ground and rotating wheels on the turbulent flow around a 1/10 scaled square-back van model. A comprehensive comparison among the partially averaged Navier-Stokes (PANS), large eddy simulation (LES) and particle image velocimetry (PIV) involving the aerodynamic drag, the wake topology, the velocity and the Reynolds stress profiles in the wake region is conducted. The proper orthogonal decomposition (POD) and fast Fourier transform (FFT) are applied to the shear layers shedding from the trailing edges to comment on the coherent structures and their frequency content. The Reynolds number for both simulations and experiments is set to Re = 2.5 × 105 based on the inlet velocity and the width of the model W = 0.17 m. The results show that PANS accurately predicts the flow field measured in experiments and predicted by a resolved LES, even with a low-resolution grid. The superiority of the PANS approach could provide good guidance for industrial research in predicting the turbulent flow around the square-back van model with affordable computational grids. The ground and wheel motion mechanism on the aerodynamic forces has been revealed by analysing the surface pressure distribution, the wheels' surrounding flow, the underbody flow characteristics and the turbulent wake structures. The effects of the ground and wheel motion on the frequency, evolution and development characteristics of the wake shear layers are analysed, thus providing relevant insights for future experimental investigations of square-back van models.
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