| 1. |
- He, Kan, 1991, et al.
(författare)
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Blockage influence on bi-stable flows of a notchback bluff body
- 2021
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:12
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Tidskriftsartikel (refereegranskat)abstract
- The expected flow asymmetry behind a three-dimensional notchback Ahmed body is numerically investigated using large-eddy simulations with different blockage ratios of 0%, 5 %, 10 %, and 20 %. The focus of the study is on the natural bi-stable flow influenced by the blockage ratio. Although the wake asymmetry can be observed under the blockage ratio within 0 % - 10 %, the significant blockage influence on the sensitive bi-stability is indicated by the wake structures, pressure gradients, and wake dynamics achieved with the proper orthogonal decomposition. The higher blockage ratio increases the turbulence kinetic energy, velocity, and negative pressure in the near-wake region, resulting in the decrease in the asymmetry degree. The consistency of the asymmetric wake is found with blockage ratios between 0% and 5 %. However, a 20 % blockage ratio symmetrizes the bi-stable wake. Several existing drag corrections for the blockage influence are discussed. The wind tunnel experiment verifies the bi-stable flow with low blockage ratios and the wake symmetrization with a 20 % blockage ratio.
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| 2. |
- He, Kan, et al.
(författare)
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Evaluation of LES, IDDES and URANS for prediction of flow around a streamlined high-speed train
- 2022
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 223
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Tidskriftsartikel (refereegranskat)abstract
- The turbulent flow past a simplified Intercity-Express 3 high-speed train at ReH=6×104 is investigated by a combination of wind tunnel experiments and numerical simulations using the large-eddy simulation (LES), the improved delayed detached eddy simulation (IDDES) and the unsteady Reynolds-averaged Navier-Stokes (URANS) simulation. This work aims to compare the predictive capabilities of LES, IDDES and URANS for the flow over a streamlined high-speed train. Numerical simulations are compared to experimental data for validation. Results show that the well-resolved LES is more accurate among the numerical methods used. Compared to the well-resolved LES, IDDES and URANS using the coarser mesh can produce similar mean flow, although IDDES and URANS are found to be slightly inaccurate for the coherent wake structures near the wall. However, for the near-wall flow instability concerning wake dynamics, Reynolds stresses, turbulence kinetic energy and the fluctuation of pressure, IDDES is found to be inapplicable. Overall, this study suggests that the well-resolved LES is appropriate to the flow of a streamlined high-speed train. Moreover, IDDES and URANS are proved to apply to the mean field of the studied flow.
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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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Influence of the rounded rear edge on wake bi-stability of a notchback bluff body
- 2021
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:11
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Tidskriftsartikel (refereegranskat)abstract
- The wake bi-stability behind notchback Ahmed bodies is investigated by performing wind tunnel experiments and large eddy simulations (LESs). The focus of this study is on the suppression of bi-stable wakes achieved by rounding the roof's trailing edge of the body. The suppression effect is found to depend on the Reynolds number (R e). The wake behind a sharp edge remains bi-stable for all tested R e. However, for a rounded edge with small radius, wake bi-stability at R e = 0.5 × 10 5 and wake symmetrization with 0.75 × 10 5 ≤ R e ≤ 1.5 × 10 5 are observed. Increasing R e with R e ≥ 1.75 × 10 5, the wake returns to the bi-stable state. Particularly, with R e ≥ 2 × 10 5, a stable asymmetric wake state with no switches is observed for long periods. Performing LES confirms the expected wake asymmetry at R e = 0.5 × 10 5 and symmetry at R e = 1 × 10 5 for the case of rounded edge with a small radius. Besides, another wake symmetry is observed for the rounded edge with a large radius at R e = 0.5 × 10 5. For the two wake symmetries shown in the LES results, the symmetrization is attributed to wake suppression in the notchback region, forcing the flow separation from the rear roof to attach to the slant on both sides of the body.
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| 5. |
- He, Kan, 1991, et al.
(författare)
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On state instability of the bi-stable flow past a notchback bluff body
- 2022
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 931
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Tidskriftsartikel (refereegranskat)abstract
- The wake of a notchback Ahmed body presenting a bi-stable nature is investigated by performing wind tunnel experiments and large-eddy simulations. Attention is confined to the Reynolds number (Re) influence on the wake state instability within 5 x 10(4) <= Re <= 25 x 10(4). Experimental observations suggest a wake bi-stability with low-frequency switches under low Re. The wake becomes 'tri-stable' with the increase of Re with the introduction of a new symmetric state. The higher presence of the symmetric state can be considered as a symmetrization of the wake bi-stability with an increasing Re. The wake symmetry under high Re attributed to the highly frequent switches of the wake is extremely sensitive to small yaw angles, showing the feature of bi-stable flows. The wake asymmetry is confirmed in numerical simulations with both low and high Re. The wake asymmetries are indicated by the wake separation, the reattachment and the wake dynamics identified by the proper orthogonal decomposition. However, the turbulence level is found to be significantly higher with a higher Re. This leads to a higher possibility to break the asymmetric state, resulting in highly frequent switches showing symmetry.
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| 6. |
- Su, Xinchao, 1991, et al.
(författare)
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Comparison of flow characteristics behind squareback bluff-bodies with and without wheels
- 2023
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:3
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Tidskriftsartikel (refereegranskat)abstract
- The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels is numerically studied by performing improved delayed detached eddy simulation. Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bi-stability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bi-stable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of low-order flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of this present work suggests that the wake bi-stability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in real-world upstream conditions.
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| 7. |
- Wu, Yitong, et al.
(författare)
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Numerical and experiment study on ventilation performance of the equipment compartment of Alpine high-speed train
- 2023
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Ingår i: Engineering Applications of Computational Fluid Mechanics. - 1994-2060 .- 1997-003X. ; 17:1
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Tidskriftsartikel (refereegranskat)abstract
- Alpine High-Speed Train serves on the Lanzhou-Urumqi Line in northwest China, where the terrain is mainly the Gobi Desert. To adapt to this complex environment, the independent-air duct is mainly used for the electrical facilities inside the equipment compartment to prevent the spread of sand particles. This isolated air duct makes ventilation characteristics of the equipment susceptible to the external environment. For this reason, this work aims to clarify and investigate the ventilation characteristic of electrical facilities. A two-step simulation method using IDDES (Improved Delayed Detached Eddy Simulation) and a real-vehicle tracking test using the T-typed pitot tubes were conducted. In the simulation, it is found that the ventilation performance can be influenced by the location of the equipment compartment, facilities and the fan mounted inside. By comparing the results of the test and simulation, they share the same characteristic that the air outlet volume of the converter near the head car is being promoted while it near the tail car is being inhibited. The maximum deviation ratio between the test and simulation is 9%. Therefore, the measurement method in this study is relatively reliable.
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| 8. |
- Xia, Yutao, 1995, et al.
(författare)
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Aerodynamic influences of typical windbreak wall types on a high-speed train under crosswinds
- 2022
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 231
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Tidskriftsartikel (refereegranskat)abstract
- The effect of windbreak wall types on their windproof effects on trains has been systematically investigated by comparing the flow field around a high-speed train behind three typical windbreak walls. These three typical windbreak walls are widely used along the windy railway lines in China, which include the earth embankment–type windbreak wall (EW), the road cutting–type windbreak wall (RW), and the straight reinforced concrete–type windbreak wall (SW). We compared the time-averaged crosswind-induced flows and aerodynamic performances of high-speed trains behind these three windbreak walls with the same height using numerical simulations. The results revealed that the windbreak wall geometric shape effects on the aerodynamic load coefficients varied according to the type of aerodynamic load and carriage marshalling positions but barely varied with yaw angles. The total drag coefficients of the train in the EW and RW were approximately 50–60% of that in the SW under the two smaller yaw angles and were only 30–40% for the largest yaw angle. For the absolute value of the side force and rolling moments coefficients, the maximum values for the head and middle cars both appeared in the RW, and the corresponding minimum values were obtained in the SW. The maximum and minimum values for the tail car were obtained at the SW and EW, respectively. The maximum of the rolling moment coefficient among three carriages in the SW was approximately 45–60% of that in the EW and only 30–40% of that in the RW. The time-averaged train surface pressure coefficient and flow patterns were similar between the EW and RW, which showed apparent differences from those in the SW. The SW provided a strong blocking effect on the incoming windward airflow and avoided the direct impact on the train. Compared with the RW, the uplifting effect of the EW's windward slope on the incoming flow further reduced the crosswind effect on trains. In addition, the dominating frequency characteristics of the aerodynamic loads were significantly affected by windbreak wall types. These findings provide a systematic understanding of the time-averaged aerodynamics of trains behind these three typical windbreak walls.
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| 9. |
- Xia, Yutao, 1995, et al.
(författare)
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Investigation of the dynamic airtightness coefficient of high-speed trains traveling through a tunnel: A field study
- 2023
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 236
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Tidskriftsartikel (refereegranskat)abstract
- Determining the dynamic airtightness coefficient (τdyn) of train carriages using a high-accuracy and high-efficiency method is essential regarding the study of the dynamic airtightness performance of high-speed trains (HSTs) traveling through tunnels. This study set out to provide a systematic elaboration and investigation of the whole process of establishing the calculation methodology of τdyn of HST carriages. Moreover, the applicability of three different typical optimization algorithms in the calculation of τdyn was explored and compared. These were the traversal algorithm (TRAA), the trichotomy algorithm (TRIA), and the genetic algorithm (GA). Using the established calculation method, the values of τdyn of different HST carriages for tunnels with different lengths were calculated based on field test data of pressure outside and inside HST carriages. Results revealed that the selection of optimization algorithms had significant effects on the calculation process of τdyn because of their different optimization mechanisms. These three optimization algorithms were proven to be feasible for establishing the calculation method of τdyn and gave the same calculation result of τdyn for the calculation example within the expected accuracy. The elapsed time of the TRAA and GA were approximately 100 and 80 times that of the TRIA regarding the calculation example. The TRIA was the preferable choice for the calculation of τdyn among the three algorithms because of its uncomplicated implementation logic and high efficiency. Within the tunnel-length range involved, the tunnel length itself had no significant effect on τdyn of HST carriages. A significant increasing trend in τdyn of HST carriages was found along the train length. Compared to the head car, the increasing ratio of τdyn of the second car was varying from 0.2 to 0.5, while the increasing ratio of the seventh car was ranging from 0.8 to 1.7 considering different tunnel lengths. These findings provide a reference basis for the design of the calculation method of τdyn of HST carriages and also enhance understanding of τdyn of HST carriages under different operating circumstances.
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| 10. |
- Xu, Kewei, 1992, et al.
(författare)
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Active flow control of the airflow of a ship at yaw
- 2023
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Ingår i: Ocean Engineering. - : Elsevier BV. - 0029-8018. ; 273
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Tidskriftsartikel (refereegranskat)abstract
- This paper implements the steady Coanda effect active flow control (AFC) on the Chalmers ship model (CSM) to study its influence on the ship's side force and airwake under the yaw effect. The study is conducted numerically using Large Eddy Simulation (LES) with Wall-Adapting Local-Eddy Viscosity (WALE) model. Numerical methods are validated by the experimental data acquired from the baseline CSM under 10∘ port-side wind. The model with AFC is created by modifying the square-shaped hanger base to the Coanda surface and added with injection slots along the base's roof edge and two side edges. The results show that the base-shape modification significantly alters the vortex structure on deck from z-direction vortex (ZV) to streamwise vortex (SV), and the steady Coanda effect with a momentum coefficient (Cμ) of 0.02 further enhances the SV with the removal of port-side vortex (PV). The side force and yaw moment are reduced by 5.27% and 7.97%, respectively in the AFC case due to the reduction of port-side (windward) ship-surface pressure. Furthermore, the current AFC can suppress the low-speed region and alleviate the velocity gradient in the lateral direction, which mitigates the regions of high TKE (turbulent kinetic energy) and high shear stress along the port-side deck.
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