| 1. |
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| 2. |
- Basara, Branislav, 1964, et al.
(författare)
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Calculations of turbulent flow through a staggered tube bank
- 2017
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Ingår i: Proceedings of the Thermal and Fluids Engineering Summer Conference. - 2379-1748. ; 2017-April, s. 1563-1566
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Konferensbidrag (refereegranskat)abstract
- The primary aim of this paper is to numerically investigate the crossflow in a staggered tube bank by using a variable-resolution method. Experimental data of Simonin and Barcoude (1988) is available in the ERCOFTAC database. There are also few ERCOFTAC workshops, e.g. 1993, 19999, which were considering this test case primarily for checking the performance of the Reynolds-Averaged Navier-Stokes (RANS) models. Therefore, there are number of results with very different models which can be found in the literature. The work presented here aims to add one more set of results but this time with recently advanced variable resolution method, namely the Partially-Averaged Navier-Stokes (PANS). This method (Girimaji, 2006) belongs to so called bridging or seamless methods. The PANS approach adjusts seamlessly from the Reynolds-Averaged Navier-Stokes (RANS) to the Direct Numerical Solution (DNS) of the Navier-Stokes equation. The results are largely improved by using the PANS as for example shown in Basara (2015). This turbulence bridging method is derived from the RANS model equations. It inevitably improves results when compared with its corresponding RANS model if more scales of motions are resolved. This is done by varying the unresolved-to-total ratios of kinetic energy and dissipation. In the practice, the parameter which determines the unresolved-to-total kinetic energy ratio is defined by using the grid spacing and calculated integral length scale of turbulence. When the grid size is smaller, then more of the turbulent kinetic energy can be resolved. Usually, the integral scale of turbulence is obtained by summing up resolved turbulence, calculated as difference between instantaneous filtered velocity and the averaged velocity field, and unresolved turbulence obtained from its own equation. The turbulence model adopted in the present PANS variant is the four-equation ζ - f formulation (Hanjalic et al., 2004) which is the variant of more known v2-f model based on the elliptic relaxation concept.). As this model represents a practical and accurate RANS choice for a wide range of industrial applications, especially when used in conjunction with the universal wall approach (Popovac and Hanjalic, 2007, Basara, 2006), its PANS variant therefore guarantees that the proper near-wall model is used when fkis of a higher value. Therefore, the near-wall PANS variant of Basara et al. (2011) was used in the present study. The PANS model is implemented into the commercial CFD code AVL FIRE (AVL FIRE Manual, 2011).
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| 3. |
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| 4. |
- Basara, Branislav, 1964, et al.
(författare)
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Effects of convection schemes on hybrid RANS-LES calculations
- 2018
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Cham : Springer International Publishing. - 1612-2909 .- 1860-0824. ; 137, s. 145-155
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Tidskriftsartikel (refereegranskat)abstract
- Nowadays it is commonly accepted to report on convections schemes in the case of Large Eddy Simulations. However, this is still not mandatory for the hybrid Reynolds-Averaged Navier-Stokes (RANS)—LES calculations. Therefore, this paper intends to show that the effects of convection scheme is equally important for hybrid RANS-LES calculations as well as for LES runs. We choose here the Partially-Averaged Navier–Stokes (PANS) model as the representative hybrid RANS-LES model but the conclusions derived in this work are equally applicable to other methods. The aim of the paper is to provide a proper criterion from the employment of higher order differencing schemes. The paper will assess two approaches, namely step functions where central-differencing scheme is used in the regions with a lower unresolved turbulent kinetic energy and a continuous function which depends on the ratio between unresolved and total kinetic energy. The results will be presented for the flow around the square cylinder. Direct numerical simulation (DNS) data and measurements are available for comparisons.
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| 5. |
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| 6. |
- Basara, B., et al.
(författare)
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Near-Wall Formulation of the Partially Averaged Navier-Stokes Turbulence Model
- 2011
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Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics (AIAA). - 1533-385X .- 0001-1452. ; 49:12, s. 2627-2636
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Tidskriftsartikel (refereegranskat)abstract
- The variable-resolution partially averaged Navier-Stokes bridging strategy is applied to the four-equation k-epsilon-zeta-f turbulence model. In this approach, the popular two-equation model is enhanced with an additional transport equation for the velocity scale ratio zeta and an equation for the elliptic relaxation function f for the purpose of improved near-wall behavior. By using the elliptic relaxation technique to model the wall blocking effect, the new four-equation partially averaged Navier-Stokes model retains the simplicity of the previous two-equation partially averaged Navier-Stokes versions but significantly improves predictions in the near-wall region. The proposed partially averaged Navier-Stokes k-epsilon-zeta-f model is evaluated in a turbulent channel flow and flow around a three-dimensional circular cylinder mounted vertically on a flat plate. The results clearly show benefits of the improved near-wall modeling and extend partially averaged Navier-Stokes applicability to a broader range of smooth bluff-body separated flows.
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| 7. |
- Basara, Branislav, 1964, et al.
(författare)
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PANS methodology applied to elliptic-relaxation based eddy viscosity transport model
- 2010
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1612-2909 .- 1860-0824. - 9783642141386 ; 110, s. 63-69
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Konferensbidrag (refereegranskat)abstract
- The Partially-Averaged Navier-Stokes (PANS) approach is a recently proposed method which changes seamlessly from the Reynolds-Averaged Navier-Stokes (RANS) model equations to the direct numerical solution (DNS) of the Navier-Stokes equations as the unresolved-to-total ratios of kinetic energy and dissipation are varied. Two variants of the PANS model are derived up to now, one based on the k-epsilon formulation and the other based on the k-omega formulation. We introduce here another variant which is based on four equation eddy viscosity transport model, namely zeta-f turbulence model. Benefits of using such near wall model inside the PANS concept are clearly presented in this paper.
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| 8. |
- Basara, Branislav, 1964, et al.
(författare)
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PANS VS. LES FOR COMPUTATIONS OF THE FLOW AROUND A 3D BLUFF BODY
- 2008
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Ingår i: “7th International ERCOFTAC Symposium on “Engineering Turbulence Modelling and Measurements”, June 4-6, 2008, Limassol, Cyprus..
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Konferensbidrag (refereegranskat)abstract
- This paper presents comparisons between recentlyproposed Partially-Averaged Navier-Stokes(PANS) method with the Large Eddy Simulation(LES) for the flow around the Ahmed bluff body.PANS model changes seamlessly from Reynolds-Averaged Navier-Stokes (RANS) to the direct numericalsolution of the Navier-Stokes equations(DNS) as the unresolved-to-total ratios of kinetic energyand dissipation are varied. The parameter whichdetermines the unresolved-to-total kinetic energy ratiois defined based on the grid spacing and it is dynamicallyadjusted at each point at the end of everytime step. The main target of this paper is to show ifPANS obtains comparable results with LES calculationson finer meshes and improves results on coarsermeshes. In other words, the question to be answeredis whether PANS can provide the optimum solutionfor every computational mesh. This will be achievedby comparing PANS calculations on two differentcomputational meshes: 3.4 mil.(grid A), and 8.0 mil.(grid B), with LES calculations on grid A and additionaltwo grids consisting of 9.5 mil. cells (grid C)and 16.5 mil. cells (grid D).
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| 9. |
- Basara, B., et al.
(författare)
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Performance analysis of partially-averaged navier-stokes method for complex turbulent flows
- 2011
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Ingår i: 6th AIAA Theoretical Fluid Mechanics Conference, Honolulu, 27 - 30 June 2011. - Reston, Virigina : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- The paper evaluates the performance of the Partially-Averaged Navier-Stokes (PANS) method on complex industrial Computational Fluid Dynamics (CFD) applications. The PANS method changes seamlessly from Reynolds-Averaged Navier-Stokes (RANS) to the Direct Numerical Solution (DNS) of the Navier-Stokes equations and supports any filter width or scale resolution and therefore, it has potential to be efficient tool to solve large and complex applications on available computational resources. The PANS variant, which is used in the present study, is based on the RANS k-ε-ζ-f model (v2/k-f model). In the work presented here, the filter width is controlled by specifying one control parameter: unresolved-to-total ratios of kinetic energy fk. Furthermore, this parameter is defined based on the grid spacing. A dynamic update of fk at each computational point and at the end of every time step makes the computational procedure very simple and attractive for industrial CFD. The model's performance is shown for complex CFD cases: an external car aerodynamics, a train aerodynamics and engine intake ports.
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| 10. |
- Basara, Branislav, 1964, et al.
(författare)
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Scale-resolving simulations of the flow in internal combustion engines
- 2018
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Ingår i: Proceedings of the Thermal and Fluids Engineering Summer Conference. - 2379-1748. ; 2018-March, s. 245-248
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Konferensbidrag (refereegranskat)abstract
- The common practice in predicting engine flows is to use the Reynolds-averaged Navier-Stokes (RANS) models. However, the RANS models are single point closures relying on the assumption of self-similarity of the turbulence spectrum, the fact leading to only one characteristic turbulence length scale, defining the entire spectrum. Consequently, the complex physics of the flow in engines could not be captured well in such a way. In the case of running RANS model for multi-cycle engine calculations, calculation results will converge to one cycle results (the first 3-4 cycles are different due to different cycle starting fields, but then the results of the each next cycle will be the same) without predicting cycle-to cycle variations (CCV). Some of reported CCV results with RANS models, are probably due to numerical artefacts rather than physical background of the RANS models. In order to correctly capture all engine related flow phenomena, Large-Eddy Simulation (LES) has been recently more often used, but due to high computational costs, mainly as a research but not as a productive tool. An alternative approach could be found in the use of hybrid LES/RANS (HLR) models. Therefore, we use here a seamless HLR method denoted as Partial-Averaged Navier-Stokes (PANS) in conjunction with the universal wall treatment (Basara, 2006) in order to provide the optimum regarding the accuracy and computational costs. This turbulence bridging method, which supports any filter width or scale resolution, is derived from the Reynolds-Averaged Navier-Stokes (RANS) model equations. It inevitably improves results when compared with its corresponding RANS model if more scales of motions are resolved. The PANS (Girimaji, 2006) variant derived from the four equation near-wall eddy viscosity transport model (see Basara et al. 2011), namely k-ε-ζ-f turbulence model is used here (commercial software AVL FIRE®). This is done by varying the unresolved-to-total ratios of kinetic energy and dissipation given as (euqacation presented) This procedure is applied here on the full engine case. First, Figure 1 shows the resolution parameter fk which is based on RANS results and Eq. (2). This roughly shows that the mesh is in general coarse, for example for LES calculations, but fine enough to try PANS model (note that fk = 1 is in the 'RANS region' and fk = 0.4 is the minimum value). One could propose such mesh analysis prior to LES calculations in order to avoid long calculations, an assessment of the resolved energy and then again new meshing etc. Following the SSV approach applied on the same mesh, an instantaneous fk, which is different than one based on the RANS calculations, is shown in Figure 2, but overall, there are a lot of similarities. This is of course also due to different turbulence level predicted by two approaches. Calculations are performed here also by using spray and combustion modules. Predicted flame front propagation is shown in Figure 3, so the variations are visible for two neighbour cycles N and N+1. One can see that at the top most position of the piston towards head side of position (top dead center - TDC) and few degrees after TDC, the flame front is very different. This can be also seen at different cross-section as shown in Figure 4. This of course causes the pressure variations. It should be also reported that for this particular mesh, cycle to cycle variations obtained by LES are double larger than obtained with PANS calculations (20% vs. 10%) and measurements provided the value of 30%. However, having in mind Figures (1) and (2), and knowing that for proper LES calculations fk should be at least less than 0.2, those variations of 20% could be more addressed to the calculation error. The results presented here show the advantage of the variable resolution PANS method when compared to RANS models. With the help of the computational mesh and the resolution parameters incorporated used in PANS, cycle-to-cycle variations could be predicted. These results approve the PANS basic idea of providing the optimum fidelity on the given numerical mesh. It is also clear that in order to get the most accurate results, meshes should be further refine. However, many engine cases can be calculated with coarser meshes and the then last investigations could be made with refined meshes.
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| 11. |
- Bruzelius, Fredrik, 1974-, et al.
(författare)
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A simple real-time aerodynamic model for vehicles in overtaking situations
- 2013
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Ingår i: International Journal of Vehicle Systems Modelling and Testing. - 1745-6436 .- 1745-6444. ; 8:3, s. 241-259
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Tidskriftsartikel (refereegranskat)abstract
- Aerodynamic forces and moments play an important role inovertaking situations and affect the trajectory of the involved vehicles. One method of studying these effects and how the driver responds to these is via driving simulator studies. However, models that describe aerodynamic forces and moments are typically computationally complex and not suited for real time execution in driving simulators. The present work presents a simplified real time model of the moments and forces acting in an overtaking situation due to aerodynamic pressure imbalances. The model is compared with experimentally validated Navier-Stokes solutions. Despite the model’s simplicity, implemented as finite impulse response filters, it manages to capture the main features of the forces and moments. The approach used to derive the model is fairly general and might be used in similar applications where computational burden is a key issue.
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| 12. |
- Chen, Guang, et al.
(författare)
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Dynamic analysis of the effect of nose length on train aerodynamic performance
- 2019
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 184, s. 198-208
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Tidskriftsartikel (refereegranskat)abstract
- The improved delayed detached eddy simulation (IDDES) was used to study the influence of the train’s nose length on its aerodynamic performance. Both the time-averaged and instantaneous near-wake structures and the associated distribution of slipstream velocity are compared for three nose lengths. As the nose length increases, the mean and Std values of the drag and lift force are decreased. The shorter nose-length case results in a higher slipstream velocity. In particular, at the TSI track-side position, the TSI value U_2δ for the 5-m nose length case is 30% and 32% higher than the corresponding values for the 7.5-m and 10-m nose length cases, respectively. The dynamical flow topology in the wake reveals that the flow structures of the 5-m nose length are different from those of the other two cases in the tail streamline surface. As nose length increases, the longitudinal vortices are weaker, and the angle and distance between the longitudinal vortices are smaller. The shear production from the P_xy caused by the separation of the boundary layer at the lateral wall of the tail train is greater than that of the P_xz caused by the separation of the boundary layer at the top and bottom of the tail train.
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| 13. |
- Dai, Zhiyuan, et al.
(författare)
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Kriging-based multi-objective optimization on high-speed train aerodynamics using sequential infill criterion with gradient information
- 2024
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Ingår i: Physics of Fluids. - 1089-7666 .- 1070-6631. ; 36:3
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Tidskriftsartikel (refereegranskat)abstract
- For models with large numerical simulation costs, such as high-speed trains, using as few samples as possible to construct a high-precision surrogate model during aerodynamic multi-objective optimization is critical to improving optimization efficiency. This study proposes a sequential infill criterion (SIC) appropriate for the Kriging surrogate model to address this issue. Three multi-objective functions are employed to test the feasibility of constructing a surrogate model based on SIC, and the SIC surrogate model then performs multi-objective aerodynamic optimizations on the high-speed train. The findings indicate that the expected improvement infill criterion (EIC) in the first stage can enhance the global prediction accuracy of the SIC. An infill criterion based on EIC that fuses gradient information (PGEIC) in the second stage is proposed to seek samples in the Pareto front. The PGEIC surrogate model achieves the lowest generational distance and prediction error. The performance of EIC for global search, EIC for Pareto front search, and infill criterion for Pareto front search using only gradient information is poor. The final PGEIC-SIC surrogate model of train aerodynamics has less than 1% prediction error for the three optimization objectives. The optimal solution reduces the aerodynamic drag force of the head car and the aerodynamic drag and lift force of the tail car by 4.15%, 3.21%, and 3.56%, respectively, compared with the original model. Furthermore, sensitivity analysis of key parameters revealed that the nose height v1, cab window height v3, and lower contour line have a greater impact on aerodynamic forces. Moreover, the nose and cab window heights of the optimal model have been reduced, and the lower contour line is concave. Correspondingly, the streamlined shape appears more rounded and slender.
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| 14. |
- Diedrichs, Ben, et al.
(författare)
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Large eddy simulations of a typical European high-speed train inside tunnels
- 2004
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Ingår i: SAE Technical Paper Series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. - 0768013194
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Konferensbidrag (refereegranskat)abstract
- This article presents our results of external aerodynamics, obtained with Large Eddy Simulations(LES), about a typical European passenger-stock insidetunnels. The focal points are the aerodynamic forces andtheir typical frequencies applied to the tail. Two trainlengths and three tunnels are employed in the study tomodel the conditions of double and single-track bores.Owing to the relatively high numerical cost associated with LES for external train aerodynamics we could only afford sufficient spatial grid resolution on our shortest train. The flow simulations confirm the existence of coherent structures alongside the body that give rise to continuously propagating pressure disturbances. These disturbances with a relatively small amplitude and high spatial frequency cannot affect the ride comfort. Still,they are found to influence the flow separation about the tail, which is regarded as one of the candidate mechanisms to impair the ride comfort and running stability.
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| 15. |
- Diedrichs, Ben, et al.
(författare)
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Large eddy simulations of the flow around high-speed trains cruising inside tunnels
- 2004
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Ingår i: ECCOMAS 2004 - European Congress on Computational Methods in Applied Sciences and Engineering. ; , s. 1-21
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Konferensbidrag (refereegranskat)abstract
- Stability and ride comfort of high-speed rolling stock are, for reasons of external aerodynamics, dependent on the external design in conjunction with properties of the vehicle dynamics and the design of the infrastructure, herein referring to the confining tunnel walls. It is a fact that some Japanese high-speed trains are quite prone to tail vehicle vibrations only inside tunnels, while (to the authors' knowledge) other nation's comparable train systems are not. In this context the current work describes our results of external aerodynamics, calculated with large eddy simulations, about two simplified train models inside two double track tunnels with comparable blockage ratio. These models are based on the German Inter-City Express 2 and the Japanese Series 300 Shinkansen trains. The focal points of this study are the origin of unsteady aerodynamic tail forces, their spectral characteristics and the impact of spontaneously emerging coherent flow structures adjacent to the train's surfaces. Full scale tests of the above trains have shown that only the Shinkansen train is subjected to a reduced ride comfort inside tunnels, with a quite obvious lateral vibration at about 2 Hz at top speed (300 km/h). Our unsteady flow calculations have successfully predicted the more detrimental lateral aerodynamic forces. In addition, a spectral analysis of the unsteady side force resulted in a quite prominent frequency at the Strouhal number of 0.09 (based on the speed and height of the train), which correlates well with the Japanese full scale tests.
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| 16. |
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| 17. |
- Diedrichs, Ben, et al.
(författare)
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Vehicle dynamics of a high-speed passenger car due to aerodynamics inside tunnels
- 2007
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Ingår i: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit. - 0954-4097 .- 2041-3017. ; 221:4, s. 527-545
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Tidskriftsartikel (refereegranskat)abstract
- High train speeds inside narrow double-track tunnels using light car bodies can reduce the ride comfort of trains as a consequence of the unsteadiness of the aerodynamics. This fact was substantiated in Japan with the introduction of the series 300 Shinkansen trains more than a decade ago, where the train speed is very high also in relatively narrow tunnels on the Sanyo line.The current work considers the resulting effects of vehicle dynamics and ride comfort with multi-body dynamics using a model of the end car of the German high-speed train ICE 2. The present efforts are different from traditional vehicle dynamic studies, where disturbances are introduced through the track only. Here disturbances are also applied to the car body, which conventional suspension systems are not designed to cope with.Vehicle dynamic implications of unsteady aerodynamic loads from a previous study are examined. These loads were obtained with large eddy simulations based on the geometry of the ICE 2 and Shinkansen 300 trains.A sensitivity study of some relevant vehicle parameters is carried out with frequency response analysis (FRA) and time domain simulations. A comparison of these two approaches shows that results which are obtained with the much swifter FRA technique are accurate also for sizable unsteady aerodynamic loads. FRA is, therefore, shown to be a useful tool to predict ride comfort in the current context.The car body mass is found to be a key parameter for car body vibrations, where loads are applied directly to the car body. For the current vehicle model, a mass reduction of the car body is predicted to be most momentous in the vicinity of 2 Hz.
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| 18. |
- Dong, Tianyun, et al.
(författare)
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Effects of simplifying train bogies on surrounding flow and aerodynamic forces
- 2019
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 191, s. 170-182
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 Elsevier Ltd In this study, the numerical solution of a high-speed train with several simplified bogies is investigated. The time-averaged flow field around the train, the surface pressure, and the aerodynamic forces on the train are discussed. The results reveal that a simpler bogie structure can achieve a higher underbody flow velocity and change fluctuations beneath the train owing to the resulting turbulence level. The simplification of bogies has a smaller effect on the side slipstream velocity and pressure compare to which in underbody, and at 3 m away from the centre of the track, the simplified bogie with wheels and a simple side frame used in this study obtains similar results to cases wherein more complex bogies are used. The surface pressure under the train is affected by bogie simplification, especially in the bogie cabin end area, resulting in aerodynamic drag and lift variations. If underbody flow or aerodynamic drag and lift forces are the focus of study, then the geometry of the centre region of the bogie, i.e. its main structures features, should be maintained in simplified models.
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| 19. |
- Dong, Tianyun, 1990, et al.
(författare)
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Numerical investigation of a high-speed train underbody flows: Studying flow structures through large-eddy simulation and assessment of steady and unsteady Reynolds-averaged Navier-Stokes and improved delayed detached eddy simulation performance
- 2022
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:1
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Tidskriftsartikel (refereegranskat)abstract
- The underbody flow of a truncated, 1:10 scaled, CRH380A model is investigated at Re = 2.78 × 105 in this paper. The large-eddy simulation (LES) is used to study the main features of the development of the underbody flow under the snowplow, in the bogie/cavity region and after the cavity (equip-cabin region). A grid independence study and a validation against experimental data have been done prior to the investigation. The snowplow region is dominated by a pair of separated counter-rotating vortices, which further affects the downstream flow. A strong shear layer is observed in the cavity region, and the turbulent flow is intensively triggered by the shear instability and the complex bogie components within the cavity region. The equip-cabin region allows the turbulent flow to develop without any disturbance, decreasing the turbulence intensity. Moreover, the steady and unsteady Reynolds-averaged Navier-Stokes (RANS, URANS) model and the improved delayed detached eddy simulation (IDDES) are used to compute the same flow, and to compare the results to LES. The solution differences, in terms of aerodynamic forces and the underbody flow state, are analyzed. Specifically, the minimum velocity discrepancy, at line2, between RANS (URANS) and LES is 14.4%, while IDDES is 3.6%. The solution accuracy vs the computational cost is also reported.
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| 20. |
- Dong, Tianyun, et al.
(författare)
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The effect of ground clearance on the aerodynamics of a generic high-speed train
- 2020
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Ingår i: Journal of Fluids and Structures. - : Elsevier BV. - 1095-8622 .- 0889-9746. ; 95
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of ground clearance on the flow around a simplified high-speed train is investigated in this paper. Four clearance heights are studied using IDDES. After a grid independence study, the results of the simulations are validated against experimental data present in the literature. It is found that the drag decreases when reducing the clearance gap from the baseline height to a possibly critical height, while drag remains constant when the clearance is lower than this critical height. The negative lift (downforce) increases with the decreasing of the clearance gap. The flow is particularly influenced by the gap height at the underbody and wake regions, where a lower underbody velocity and a higher wake velocity are observed with lower clearance down to case h2. Therefore, the different topologies of the wake are presented and described. Particular attention is paid to the description of the wake flow and to the position and the formation of the flow mixing region. Specifically, with decreasing clearance, the mix of the tail downwash and underbody flows happen earlier, and the core of the counter-rotating vortices in the wake tends to develop with an increasing height trend. Overall, aerodynamic performance and flow structure descriptions show positive and negative effects when decreasing gap clearances, which should be taken into account for new design strategies.
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| 21. |
- Dong, Tianyun, et al.
(författare)
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The effect of reducing the underbody clearance on the aerodynamics of a high-speed train
- 2020
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 204
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Tidskriftsartikel (refereegranskat)abstract
- The effect of lowering the ground clearance, on train aerodynamics is investigated using IDDES. The high speed train geometry with its full underbody complexity is used in the investigation. The clearance is reduced by installing extra panels on the track. The numerical results are verified and validated by a grid independence study and experimental data. This work shows that, when lowered the clearance, the underbody velocity at the head car decreases, while the underbody velocity at the middle and tail car increases. The reduced clearance barely affects the time-averaged slipstream at 3 m away from the center of track. However, at a closer distance to the train body, the difference in velocity is observed to reach up to 50% between the two clearance configurations and the clearance has an opposite effect on the trackside and platform slipstream. Based on the analysis of ensemble-averaged slipstream, lowering the underbody clearance, the characteristic air speed at the trackside and platform height increases by 2.0% and 6.7%, respectively. The total drag is almost unaffected by the clearance, but the changed distribution of the drag indicates a larger drag depends on the bogie structure if longer grouped train is used. The total lift decreases 25.9% after the clearance is reduced. Specifically, 4.1% of the negative lift is increased at the head car, while the positive lift decreases 92.7% and 1.8% for the middle and tail car, respectively. Overall it is shown that reducing the underbody clearance barely affect the slipstream at standard positions, but affects more aerodynamic loads of the train.
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| 22. |
- Ferrari, S., et al.
(författare)
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Visualizing Vortex Clusters in the Wake of a High-Speed Train
- 2017
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Ingår i: 2017 IEEE INTERNATIONAL CONFERENCE ON SYSTEMS, MAN, AND CYBERNETICS (SMC). - 1062-922X. - 9781538616451 ; 2017-January, s. 683-688
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Konferensbidrag (refereegranskat)abstract
- Visualization of fluid flows at a high-Reynolds number (Re similar to 10(5)) presents difficulties for user comprehension due to density and ambiguous interactions between vortices. Prior work has used cluster-based reduced-order modelling (CROM) to analyze the wake of a High-Speed Train (HST) with Re = 86,000. In this paper, we present a novel surface visualization to convey the spatiotemporal changes undergone by clustered vortices in the HST wake. This visualization is accomplished through dimensional reduction of 3D volumetric vortices into 1D ridges, and physics-based feature tracking. The result is 3D surfaces visualizing the behavior of the vortices in the HST wake. Compared to conventional still-image representations, these surfaces allow the user to quickly compare and analyze the two shedding cycles identified via CROM. The spatiotemporal differences of the primary vortices in these shedding cycles provide analytic insight to influence the aerodynamics of the HST.
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| 23. |
- Han, Xingsi, 1981, et al.
(författare)
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A new very large eddy simulation model for simulation of turbulent flow
- 2012
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1612-2909 .- 1860-0824. ; 117
-
Konferensbidrag (refereegranskat)abstract
- Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one that was early proposed and is a unified simulation approach that can change seamlessly from RANS to DNS depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (1) RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation (DES) concept; (2) An LES subgrid scale model can be reached by the introduction of a third length scale, i.e. integral turbulence length scale. Thus the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard $k-\epsilon$ model and Wilcox's $k-\omega$ model. Applications are conducted for the turbulent channel flow at $Re_{\tau}=395$ and turbulent flow past a square cylinder at $Re=22000$. Results are compared with previous studies. It is demonstrated that the new method is quite effective in resolving the large flow structures, and can give satisfactory predictions on a very coarse mesh.
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| 24. |
- Han, Xingsi, 1981, et al.
(författare)
-
An efficient Very Large Eddy Simulation model for simulation of turbulent flow
- 2013
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Ingår i: International Journal for Numerical Methods in Fluids. - : Wiley. - 1097-0363 .- 0271-2091. ; 71:11, s. 1341-1360
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Tidskriftsartikel (refereegranskat)abstract
- Among the various hybrid methodologies, Speziale’s very large eddy simulation (VLES) is one that wasproposed very early. It is a unified simulation approach that can change seamlessly from Reynolds AveragedNavier–Stokes (RANS) to direct numerical simulation (DNS) depending on the numerical resolution. Thepresent study proposes a new improved variant of the original VLES model. The advantages are achieved intwo ways: (i) RANS simulation can be recovered near the wall which is similar to the detached eddy simula-tion concept; (ii) a LES subgrid scale model can be reached by the introduction of a third length scale, that is,the integral turbulence length scale. Thus, the new model can provide a proper LES mode between the RANSand DNS limits. This new methodology is implemented in the standard k " model. Applications are con-ducted for the turbulent channel flow at Reynolds number of Re D 395, periodic hill flow at Re D 10, 595,and turbulent flow past a square cylinder at Re D 22, 000. In comparison with the available experimen-tal data, DNS or LES, the new VLES model produces better predictions than the original VLES model.Furthermore, it is demonstrated that the new method is quite efficient in resolving the large flow structuresand can give satisfactory predictions on a coarse mesh.
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| 25. |
- Han, Xingsi, 1981, et al.
(författare)
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Comparison of URANS, PANS, LES and DNS of flows around simplified ground vehicles with passive flow manipulation
- 2015
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Ingår i: ERCOFTAC Series. - Cham : Springer International Publishing. - 1382-4309 .- 2215-1826. ; 20, s. 57-63
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Konferensbidrag (refereegranskat)abstract
- Flow control of ground vehicles has recently attracted large interest in both industryand academia. The potential in energy savings seems to be considerable and at themoment both passive and active control strategies are explored. Regardless of thechoice of control strategy, numerical methods are required for our understanding offlow control processes and improvement of its performance. The present work ex-plores applicability of whole spectra of numerical techniques from URANS, PANSto LES and DNS for prediction of two flows with an additional body.
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| 26. |
- Han, Xingsi, 1981, et al.
(författare)
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Large Eddy Simulation of Flow Control Around a Cube Subjected to Momentum Injection
- 2014
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Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 92:1-2, s. 527-542
-
Tidskriftsartikel (refereegranskat)abstract
- The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) applied to a cubic structure is numerically studied using Large Eddy Simulation at a Reynolds number of 6.7×104. Two small rotating cylinders are used to add the momentum at the front vertical edges of the cube. Two configurations are studied with the yaw angle of 0° and 30°, respectively, with ratio of the rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays the boundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2 % is observed in the 0° yaw angle case and about 44.1 % reduction in the 30° yaw angle case. In the case of 0° yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotating cylinders and the wake region is slightly changed due to MI. In the 30° yaw angle case, the flow field is changed a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced, which results in the large drag reduction. Meanwhile, the yaw moment is increased about 50.5 %.
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| 27. |
- Han, Xingsi, 1981, et al.
(författare)
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Large eddy simulation of flow control around a cube subjected to momentum injection
- 2012
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Ingår i: Proceedings of the International Symposium on Turbulence, Heat and Mass Transfer. - 2377-2816. ; 2012-September, s. 891-901
-
Konferensbidrag (refereegranskat)abstract
- The concept of Momentum Injection (MI) through Moving Surface Boundary-layer Control(MSBC) applied to a cubic structure is numerically studied using large eddy simulation at a Reynolds num-ber of 6.7 × 104 . Two small rotating cylinders are used to add the momentum at the front vertical edgesof the cube. Two configurations are studied with the yaw angle of 0◦ and 30◦ , respectively, with ratio ofthe rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays theboundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2% isobserved in the 0◦ yaw angle case and about 45.3% reduction in the 30◦ yaw angle case. In the case of 0◦yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotatingcylinders and the wake region is slightly changed due to MI. In the 30◦ yaw angle case, the flow field ischanged a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced,which results in the large drag reduction. Meanwhile, the yaw moment is increased about 44.7%.
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| 28. |
- Han, Xingsi, 1981, et al.
(författare)
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Numerical investigation of passive flow control around a D-shaped bluff body
- 2012
-
Ingår i: Proceedings of the International Symposium on Turbulence, Heat and Mass Transfer. - 2377-2816. ; 2012-September, s. 881-890
-
Konferensbidrag (refereegranskat)abstract
- A passive flow control around a two-dimensional D-shaped bluff body is studied at a Reynolds number of 13 000 using large eddy simulation with the Coherent Structure Model (CSM). A small circular control cylinder located in the near wake region behind the bluff body is employed as a local disturbance of the shear layer and the wake. Numerical results are compared with experimental data in detail, including global flow paramters of drag coefficient as well as mean and RMS velocities. The predictions suggest that the bubble recirculation length is significantly increased by the local disturbance of the small cylinder. A drag reduction of about 27.6% is observed in the numerical simulation. The small control cylinder was found to split the shear layer detached from the main bluff body and result in the flow structure like a jet which inflates the formation of the wake region behind the main bluff body.
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| 29. |
- Han, Xingsi, 1981, et al.
(författare)
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Study of active flow control for a simplified vehicle model using PANS turbulence model
- 2012
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1612-2909 .- 1860-0824. ; 117
-
Konferensbidrag (refereegranskat)abstract
- Flow control has shown a potential in reducing the drag in vehicle aerodynamics. Its effects are highly related to the flow structures, including small ones, and thus accurate predictions of the flow field are needed for numerical investigation. The numerical study reported in the present paper deals with active flow control for a quasi-2D simplified vehicle model by a synthetic jet. A newly developed PANS turbulence model is used, based on the $\zeta-f$ RANS turbulence model. The aim is to validate the performance of this model for the complex flow control problem. Results are compared with previous studies using LES and experimental data, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS model predicts a drag reduction of 9.8\%, which is close to previous LES results. The velocity profiles predicted by the PANS model agree well with experimental data for both natural and controlled cases. It is found that the PANS model is able to predict the flow control problem well, actually in a way not inferior to the LES method, and can thus be used for further flow control studies in vehicle aerodynamics.
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| 30. |
- Han, Xingsi, 1981, et al.
(författare)
-
Study of active flow control for a simplified vehicle model using the PANS method
- 2013
-
Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X. ; 42, s. 139-150
-
Tidskriftsartikel (refereegranskat)abstract
- Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present numerical study deals with active flow control for a quasi-2D simplified vehicle model using a synthetic jet (zero net mass flux jet). Recently developed near-wall Partially-Averaged Navier–Stokes (PANS) method, based on the ζ–f RANS turbulence model, is used. The aim is to validate the performance of this new method for the complex flow control problem. Results are compared with previous studies using LES and experiments, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS method predicts a drag reduction of approximately 15%, which is closer to the experimental data than the previous LES results. The velocity profiles predicted by the PANS method agree well with LES results and experimental data for both natural and controlled cases. The PANS prediction showed that the near-wake region is locked-on due to the synthetic jet, and the shear layer instabilities are thus depressed which resulted in an elongated wake region and reduced drag. It demonstrates that the PANS method is able to predict the flow control problem well and is thus appropriate for flow control studies.
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| 31. |
- Han, Xingsi, 1981, et al.
(författare)
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Validation of a novel very large eddy simulation method for simulation of turbulent separated flow
- 2013
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Ingår i: International Journal for Numerical Methods in Fluids. - : Wiley. - 1097-0363 .- 0271-2091. ; 73:5, s. 436-461
-
Tidskriftsartikel (refereegranskat)abstract
- The paper describes the validation of a newly developed very LES (VLES) method for the simulation of turbulent separated flow. The new VLES method is a unified simulation approach that can change seamlessly from Reynolds-averaged Navier–Stokes to DNS depending on the numerical resolution. Four complex test cases are selected to validate the performance of the new method, that is, the flow past a square cylinder at Re = 3000 confined in a channel (with a blockage ratio of 20%), the turbulent flow over a circular cylinder at Re = 3900 as well as Re = 140,000, and a turbulent backward-facing step flow with a thick incoming boundary layer at Re = 40,000. The simulation results are compared with available experimental, LES, and detached eddy simulation-type results. The new VLES model performs well overall, and the predictions are satisfactory compared with previous experimental and numerical results. It is observed that the new VLES method is quite efficient for the turbulent flow simulations; that is, good predictions can be obtained using a quite coarse mesh compared with the previous LES method. Discussions of the implementation of the present VLES modeling are also conducted on the basis of the simulations of turbulent channel flow up to high Reynolds number of Reτ = 4000. The efficiency of the present VLES modeling is also observed in the channel flow simulation. From a practical point of view, this new method has considerable potential for more complex turbulent flow simulations at relative high Reynolds numbers.
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| 32. |
- Han, Xingsi, 1981, et al.
(författare)
-
Very-Large-Eddy Simulation Based on k-omega Model
- 2015
-
Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics (AIAA). - 1533-385X .- 0001-1452. ; 53:4, s. 1103-U8
-
Tidskriftsartikel (refereegranskat)
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| 33. |
- Han, Xingsi, 1981, et al.
(författare)
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Very Large Eddy Simulation of Passive Drag Control for a D-Shaped Cylinder
- 2013
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 135:10
-
Tidskriftsartikel (refereegranskat)abstract
- The numerical study reported here deals with the passive flow control around a two-dimensional D-shaped bluff body at a Reynolds number of Re=3.6×104. A small circular control cylinder located in the near wake behind the main bluff body is employed as a local disturbance of the shear layer and the wake. 3D simulations are carried out using a newly developed very large eddy simulation (VLES) method, based on the standard k − ε turbulence model. The aim of this study is to validate the performance of this method for the complex flow control problem. Numerical results are compared with available experimental data, including global flow parameters and velocity profiles. Good agreements are observed. Numerical results suggest that the bubble recirculation length is increased by about 36% by the local disturbance of the small cylinder, which compares well to the experimental observations in which the length is increased by about 38%. A drag reduction of about 18% is observed in the VLES simulation, which is quite close to the experimental value of 17.5%. It is found that the VLES method is able to predict the flow control problem quite well.
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| 34. |
- He, Kan, et al.
(författare)
-
Assessment of LES, IDDES and RANS approaches for prediction of wakes behind notchback road vehicles
- 2021
-
Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 217
-
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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| 35. |
- He, Kan, 1991, et al.
(författare)
-
Blockage influence on bi-stable flows of a notchback bluff body
- 2021
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:12
-
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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| 36. |
- He, Kan, et al.
(författare)
-
Evaluation of LES, IDDES and URANS for prediction of flow around a streamlined high-speed train
- 2022
-
Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 223
-
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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| 37. |
- He, Kan, 1991, et al.
(författare)
-
Floor motion's influence on wake asymmetry of a notchback bluff body
- 2022
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:3
-
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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| 38. |
- He, Kan, et al.
(författare)
-
Influence of the rounded rear edge on wake bi-stability of a notchback bluff body
- 2021
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:11
-
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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| 39. |
- He, Kan, et al.
(författare)
-
Numerical investigation of the wake bi-stability behind a notchback Ahmed body
- 2021
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 926, s. A36-1-A36-29
-
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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| 40. |
- He, Kan, 1991, et al.
(författare)
-
On state instability of the bi-stable flow past a notchback bluff body
- 2022
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 931
-
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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| 41. |
- HELGASON, EYSTEINN, 1983, et al.
(författare)
-
A comparison of adjoint-based optimizations for industrial pipe flow
- 2014
-
Ingår i: American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM. - 0888-8116. - 9780791846230 ; 1C
-
Konferensbidrag (refereegranskat)abstract
- This paper presents results from adjoint-based optimization processes applied to an inlet pipe of an exhaust gas recirculation cooler in a diesel engine. The boundary conditions applied resemble those of a truck at cruising speed. Three implementations are considered for the gradient calculations with the objective of minimizing the total pressure drop through the pipe. In the first implementation the gradients are evaluated with respect to the motion of the center of the cell using a newly presented implementation based on the ALE formulation of the Navier-Stokes equations. The results are compared to the surface sensitivities, where the gradient of the cost function is evaluated with respect to the normal motion of the surface of the pipe. In the last approach a topological optimization is performed where the gradients are evaluated with respect to a momentum loss in each cell. This gives information that is used when blocking the cells.
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| 42. |
- HELGASON, EYSTEINN, 1983, et al.
(författare)
-
Adjoint method for transport of heat and species in internal automotive flows
- 2015
-
Ingår i: Proceedings of the Thermal and Fluids Engineering Summer Conference. - 2379-1748. ; 2015-August, s. 299-309
-
Konferensbidrag (refereegranskat)abstract
- The continuous adjoint formulation of the steady-state Navier-Stokes equations with scalar transport has been implemented in the open-source framework OpenFOAM™. The flow is solved using a steady-state compressible flow solver. A cost function based on uniform species distribution at a selected region on the surface of the geometry is presented. The continuous adjoint approach is applied to calculate the gradient of the cost function with respect to normal motion of the surface geometry. The main advantage of applying the adjoint method is the considerable decrease in simulation time compared to traditional methods, reducing the total simulation time for the whole sensitivity field to roughly two flow simulations. The current implementation of the adjoint equations with scalar transport is applied to a simple two dimensional channel. The sensitivity of a species distribution at a location at the surface of the channel with respect to the normal motion of the surface is evaluated. The results from the adjoint implementation are validated against gradients approximated using finite difference calculations. The comparison shows close agreement between the gradients obtained using the two methods, implying that the implementation of the adjoint equations can be used as a guideline when improving a design or in an optimization process for this type of flow.
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| 43. |
- HELGASON, EYSTEINN, 1983, et al.
(författare)
-
Aerodynamic Shape Optimization of a Pipe using the Adjoint Method
- 2012
-
Ingår i: Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition, IMECE 2012, November 9-15, 2012, Houston, Texas, USA. - 9780791845233 ; 7:PARTS A, B, C, D, s. 259-267
-
Konferensbidrag (refereegranskat)abstract
- Shape optimization of an inlet pipe to an engine re-circulator cooler using the adjoint method is presented. Themethod uses surface sensitivities calculated from an ad-joint flow field implemented in the finite volume CFD solverOpenFOAM R. This method allows for computation of thewhole sensitivity field with only two solver calls, a primal and an adjoint solver call. A RANS solver with the standard k-epsilon turbulence model applying standard wall functions was used for the primal flow solver. The adjoint surface sensitivities are calculated from the adjoint and the primal flow fields and give information about how the objective function is affected by normal motion of the surface. The surface sensitivities are coupled to a mesh morphing library in OpenFOAM diffusing the motion of the boundary nodes to the internal cells of the mesh. The resulting geometry gave a 6.5% decrease in the total pressure drop through the pipe.
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| 44. |
- HELGASON, EYSTEINN, 1983, et al.
(författare)
-
Implementation of an adjoint-based optimization with scalar transport
- 2014
-
Ingår i: ASME International Mechanical Engineering Congress and Exposition (IMECE2014), Montreal, Canada, Nov 14-20. - 9780791849545 ; 7
-
Konferensbidrag (refereegranskat)abstract
- A sensitivity analysis of a particle distribution at the outlet of a 2D-channel is presented. A convection-diffusion equation is used to represent the distribution of particles in a flow. The flow is governed by the Reynolds-Averaged Navier-Stokes equations. The particles follow the medium to the outlet, where the goal is to obtain uniform distribution of the particles. A goal function for the particle distribution at the outlet is presented, and the gradient of the goal function with respect to the normal motion of the surface is calculated. The calculation of the gradient is performed by applying the so-called adjoint method. For this, the adjoint scalar transport equation and the relevant adjoint RANS equations are implemented. The calculation cost of the entire sensitivity field is roughly twice the cost of a standard convection-diffusion particle simulation. This results in computationally cheap gradients compared to traditional methods. The results are validated by comparing the gradients calculated using the adjoint method to gradients obtained using numerical differentiation.
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| 45. |
- HELGASON, EYSTEINN, 1983, et al.
(författare)
-
Optimization using Arbitrary Lagrangian-Eulerian formulation of the Navier-Stokes equations
- 2015
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 137:6, s. Art. no. 061202-
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a new shape optimizationmethod by using sensitivities obtained from the ArbitraryLagrangian-Eulerian (ALE) form of the Navier-Stokesequations. In the ALE description the nodes of thecomputational domain may be moved with the fluid asin the Lagrangian description, held fixed in space as inthe Eulerian description or moved in some arbitrary wayin between. Applying the adjoint method with respectto mesh motion allows the whole sensitivity field for theshape changes to be calculated using only two solvercalls, a primal solver call and an adjoint solver call. Weshow that the sensitivities with respect to the meshmotion can be calculated in a post processing step to theprimal and adjoint flow simulations. The resulting ALEsensitivities are compared to sensitivities obtained usinga finite difference approach. Finally, the sensitivities arecoupled to a mesh motion smoothing algorithm, and aduct is optimized with respect to the total pressure dropusing the proposed method.
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| 46. |
- Hemida, Hassan, 1972, et al.
(författare)
-
Aerodynamic Shape Optimization of a Double-Deck Bus Subjected to a 30 Degree Side Wind Using CFD and Response Surface Models
- 2008
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Directional stability and passenger comfort are of great concern in the case of a double-deck bus subjected to a side wind. Since the shape of the bus to a large extend determines its aerodynamics, a proper design is needed to avoid the consequences of a large side force. In this paper, we proposed the use of CFD-based design with a response surface models to construct the optimum shape for a double-deck bus subjected to a side wind with a $30^o$ yaw angle. The primary objective is to minimize its aerodynamic coefficients by changing the radius and angle of its front face. The results show that increasing the curvature of the front face increases the side force and the yaw moment coefficients, while the frontangle has little influence on the bus aerodynamics. The Pareto-optimal front is constructed to relate two aerodynamic coefficients in the range of the design variables. In general, the CFD-based design with the response surface model shows a promising and cheap alternative to the wind tunnel experiments and the traditional trial and error method.
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| 47. |
- Hemida, Hassan, 1972, et al.
(författare)
-
DES of the Flow Around a Realistic Bus Model Subjected to a Side Wind with 30 Degree Yaw Angle
- 2007
-
Ingår i: The fifth IASME/WSEAS International Conference on Fluid Mechanics and Aerodynamics, Athens, Greece, 2007.
-
Konferensbidrag (refereegranskat)abstract
- Detached-Eddy Simulation (DES) was made for the flow around a 1/10 model of a double-deck bus of the type Scania K112 TL. The bus was subjected to a side wind with 30 degree yaw angle. The Reynolds number of the flow, based on the speed of the side wind and a reference length of sqrt(0.1) m, was 1300000. Detailed flow structures and global quantities such as lift, drag, side force, yaw moment and pressure coefficients were computed. Visualizations of time-averaged and instantaneous flow structures showed that the flow separates on the top and the lee-side faces of the bus. The flow separations on the lee-side face of the bus was found to be unsteady yielding large vortices that shed to the wake flow leaving disturbances on the surface pressure and hence the side force. The flow separation on the top-side face was found to be more stable in the first two third of the bus length. It became unsteady three-dimensional vortex shedding in the last one third of the length of the bus. The flow under the neath of the bus was found to be very complex and was dominated with small vortices shedding from the beams and the wheels of the bus. Good agreement was found between the DES results and the available experimental data.
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| 48. |
- Hemida, Hassan, 1972, et al.
(författare)
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Exploring Flow Structures Around a Simplified ICE2 Train Subjected to a 30 Degree Side Wind Using LES
- 2009
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Ingår i: Engineering Applications of Computational Fluid Mechanics. - : Informa UK Limited. - 1994-2060 .- 1997-003X. ; 3:1, s. 28-41
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulation (LES) is conducted to investigate the flow around a simplified ICE2 trainunder side wind conditions at 30° yaw angle. The Reynolds number based on the height of the train and the freestream velocity is 200000. Two computations on two different meshes with different numbers of nodes are made tocheck the effect of the mesh resolution on the results. The fine and the coarse meshes give similar results meaningthat the results are mesh independent. The results are also verified against available experimental data. Goodagreement is obtained between the LES results and the experimental data. The LES results show that two flowregimes exist in the wake. The first flow regime consists of steady vortex lines in the upper part of the wake flow. Itchanges into unsteady shedding after a distance of about five train heights from their onset on the surface of thetrain. The second flow regime is the unsteady movement of the lower part of the wake vortices. They attach anddetach from the surface of the train in a regular fashion. The time-averaged flow and the instantaneous flow aroundthe ICE2 train at 30° yaw angle are explored.
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| 49. |
- Hemida, Hassan, 1972, et al.
(författare)
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Exploring the Flow Around a Generic High-Speed Train Under the Influence of Side Winds Using LES
- 2006
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Ingår i: Fourth International Symposium on Computational Wind Engineering, July 16-19 Yokohama, Japan, 2006.. ; , s. 6-
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Konferensbidrag (refereegranskat)abstract
- Large-eddy simulation (LES) is made of the flow around a generic train model at two different yaw angles of 90 degree and 35 degree. The Reynolds number, based on the freestream velocity and the height of the train, is 300000 and 370000 for the yaw angles of 90 and 35 degrees, respectively. Both the time-averaged and instantaneous flows are explored. The LES results show that the influence of the three-dimensional flow from the nose of the train on the time-averaged wake flow at the 90 yaw angle is limited to a region of a length of 3.5 train heights from the tip of the nose in the direction of the length of the train. The instantaneous flow shows an unsteady vortex shedding due to the shear layer instabilities on the periphery of the recirculation region and the exterior flow. Weak vortex shedding is found in the case of the 35 yaw angle. Instead, unstable vortices are found in the wake flow. These vortices detach from andreattach to the train surface in a regular fashion leaving disturbances on the train surface and hence affecting the aerodynamic coefficients.
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| 50. |
- Hemida, Hassan, 1972, et al.
(författare)
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Large-Eddy Simulation of the Flow Around a Simplified High Speed Train Under the Influence of a Cross-Wind
- 2005
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Ingår i: 17th AIAA Computational Fluid Daynamic conference 6-9 June 2005, Toronto, Ontario Canada.. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 9781624100536
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Konferensbidrag (refereegranskat)abstract
- Large-eddy simulation (LES) was made to solve the flow around a simplified high speed train under the influence of a cross-wind. The Reynolds number of the flow was Re = 3 x 105 based the height of the train and the incoming air velocity. The results are obtained at a yaw angle (the angle between the relative side wind direction and the train cruising direction) of 90°. Both the instantaneous and the time-averaged flows are explored. Comparison of the LES flow patterns and aerodynamic forces with experimental data was reported. It was found that the LES predicted the flow in agreement with experimental observations. Aerodynamic forces are computed and their time histories are used to find the characteristic frequencies of the flow motion around the body. The results reveal that the dominated flow motion is very small and approaches the resonance frequency of trains.
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