| 44161. |
- Krajnovic, Sinisa, 1970
(author)
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Large Eddy Simulation Exploration of Passive Flow Control Around an Ahmed Body
- 2014
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In: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 136:12
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Journal article (peer-reviewed)abstract
- Large eddy simulations (LES) are used to study passive flow control for drag reduction in a simplified ground vehicle. Add-on devices in the form of short cylinders are used for the formation of streaks in the streamwise direction that lead to the separation delay. The results of the present numerical simulations are compared with the experimental data and show good agreement. The two-stage flow control mechanism is analyzed from the LES results. It was found to be in agreement with the previous experimental observations that the counter-rotating vortices behind the impinging devices influence the separation only indirectly through the longitudinal vortices further downstream.
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| 44162. |
- Krajnovic, Sinisa, 1970
(author)
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Large-Eddy Simulation for Computing the Flow Around Vehicles
- 2002
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Doctoral thesis (other academic/artistic)abstract
- The feasibility of the use of large-eddy simulation (LES) in external vehicle aerodynamics is investigated. The computational requirement for LES of the full size car at road conditions is beyond the capability of computers in the near future. Since LES cannot be used for quantitative prediction of this flow, i.e. obtaining the aerodynamic forces and moments, an alternative use of LES is suggested than can be useful in increasing our understanding of the flow around a car. It is found that making LES of the flow around simplified car-like shapes at lower Reynolds numbers can increase our knowledge of the flow around a car. Two simulations are made, one of the flow around a cube and the other of the flow around a simplified bus. The former simulation proved that LES with a relatively coarse resolution and simple inlet boundary condition can provide accurate results. The latter simulation resulted in a flow that agrees with experimental observations and displayed some flow features that were not observed in the experiments or steady simulations of such flows. This simulation afforded us the possibility to study the transient mechanisms that are responsible for the aerodynamic properties of a car. The knowledge gained from this simulation can be used by the stylist to tune the aerodynamics of the car's design as well as by CFD specialists to improve turbulence models. A mixed one-equation subgrid-scale model for LES is proposed. A new model is constructed following the observation that the chief transport of turbulent energy is a local process that occurs between the scales closest to the cut-off. This transport of energy is modeled with the scale-similarity part and the remaining non-local energy transport is represented with the eddy viscosity part.
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| 44163. |
- Krajnovic, Sinisa, 1970, et al.
(author)
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Large Eddy Simulation Investigation of the Hysteresis Effects in the Flow Around an Oscillating Ground Vehicle
- 2011
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In: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 133:12, s. 121103-
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Journal article (peer-reviewed)abstract
- This paper presents large eddy simulations (LES) of flow around a simplified vehicle model oscillating around its vertical axis. The frequency of the Strouhal number St = 0.068 and a relatively small amplitude of the oscillation are chosen to be representative for the crosswind conditions of vehicles on the road. The results were found to agree well with data from previous experimental investigations. Furthermore, the differences in LES flows between quasi-steady and dynamic flow conditions are presented and underlying flow mechanisms are explored. The cause of the phenomena of hysteresis and phase shift was found in the inertia of the flow to adjust to sudden changes in the direction of the oscillation of the body.
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| 44164. |
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| 44165. |
- Krajnovic, Sinisa, 1970, et al.
(author)
-
Large-Eddy Simulation of the Flow Around a Ground Vehicle Body
- 2001
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In: SAE Paper 2001-01-0702.
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Journal article (peer-reviewed)abstract
- Large Eddy Simulation of the the flow around bus-like ground vehicle body is presented. Both the time-averaged and instantaneous aspects of this flow are studied. Time-averaged velocity profiles are computed and compared with the experiments by Duell and George (SAE Paper 1999-01-0812) and show good agreement. The separation length and the base pressure coefficient are presented. The predicted pumping process in the near wake occurs with a Strouhal number St=0.073, compared with St=0.069 in the experiment. Unsteady results at two points are presented and compared with the experiments. The coherent structures are studied and show good agreement with the experiments.
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| 44166. |
- Krajnovic, Sinisa, 1970, et al.
(author)
-
Large ´-Eddy Simulation of the Flow Around A Simplified Bus
- 2001
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In: Proceedings of the 3rd AFOSR International Conference on DNS/LES Progress and Challenges. - 1570744882 ; , s. 775-782
-
Journal article (peer-reviewed)abstract
- Two large eddy simulations of the flow past a bus-like vehicle body were made and the results are compared with experimental data~\cite{duell:99}. The effect of the near wall resolution and the modeling of the unresolved coherent structures in the near wall flow were studied. It was determined that, although the wall functions are inadequate to represent the thin vortices close to the wall, their use leads to results in a near wake region that are similar to those in the simulation with a sufficient wall normal resolution. This study indicates that the wall normal resolution has little influence on the pressure coefficient at the rear face.
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| 44167. |
- Krajnovic, Sinisa, 1970, et al.
(author)
-
Large eddy simulation of the flow around a simplified train moving through a crosswind flow
- 2012
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In: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 110, s. 86-99
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Journal article (peer-reviewed)abstract
- Results of large eddy simulation of the flow around a simplified train model moving through a crosswind are presented. These results are compared with the experimental data and show good agreement. The Reynolds number based on the inlet velocity and the height of the model is 22 615. In addition to the simulation of the flow around the moving model, another simulation was made of the flow around a stationary model influenced by the maximum crosswind from the moving case and the results are compared with the moving case. Overshoots were observed in several aerodynamic coefficients of the moving case over the stationary case. The yawing moment coefficient displayed an overshoot of some 30% indicating the importance of performing the dynamic tests for the purpose of fulfilling safety standards.
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| 44168. |
- Krajnovic, Sinisa, 1970, et al.
(author)
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Large Eddy Simulation of the Flow around a Simplified Train Moving through a Crosswind Flow
- 2012
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In: Civil-Comp Proceedings. - Stirlingshire, UK : Civil-Comp Press. - 1759-3433.
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Journal article (peer-reviewed)abstract
- This paper presents the results from large eddy simulation (LES) of the flow around a simplified train model moving through a crosswind flow. The results are compared with the experimental data from previous experimental study by Suzuki et al. [1] who used a model train with 1/60 scale that was moved along a 4m linear guide rail placed in front of a wind tunnel nozzle. The train in the present numerical work is stationary and the influence of the train movement is simulated by changing the boundary conditions. In addition to the simulation of the flow around the moving model, another simulation of the flow around a steady model influenced by the maximum crosswind from the moving case was made and the results are compared with the moving case. Comparison of the LES results with the experimental data from [1] showed good agreement proving that LES is capable of accurately predicting the dynamic change of the aerodynamic coefficients caused by the motion of the model through the wind profile. The aerodynamic moments and forces are computed and the results from the dynamic and the steady case were compared. Overshoots in several aerodynamic coefficients of the moving case over the steady case were observed. The yawing moment coefficient displayed an overshoot of some 30% indicating the importance of performing the dynamic tests for the purpose of satisfying the safety conditions. The large difference between the front and the rear part of the body in the distribution of the flow structures attached to the lee side of the body were found to cause the overshoot in the yawing moment signal during the dynamic case simulations. The instantaneous trailing vortices responsible for much of the aerodynamic stability of the vehicle when it enters the crosswind region were analysed. Several differences between the trailing vortices resulting from the dynamic and the steady cases were observed. These flow structures were found have a shape that changed during the cross flow passage. During their evolution, some of them obtained, for some time instances, the shape of the vortices in the steady case while other (such as the trailing vortex around the lower front edge) never reached exactly the same shape of the structure as in the steady case.
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| 44169. |
- Krajnovic, Sinisa, 1970, et al.
(author)
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Large-Eddy Simulation of the Flow Around a Surface-Mounted Cube Using a Dynamic One-Equation Subgrid Mode
- 1999
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Conference paper (peer-reviewed)abstract
- Dynamic one-equation models enable us to predict the reverse transport of turbulent energy. The present paper provides an explanation of this ``backscatter'' phenomenon. LES was used to study the k equation with the aim of improving RANS models. This work is part of the project entitled ``Large Eddy Simulation for Computing the Flow Around Vehicles''(http://www.tfd.chalmers.se/~lada/projects/sinisa /proright.html) in collaboration between the Department of Thermo and Fluid Dynamics at Chalmers and the CFD group at the Volvo Car Corporation.
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| 44170. |
- Krajnovic, Sinisa, 1970
(author)
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Large eddy simulation of the flow around\\ a three-dimensional axisymmetric hill
- 2007
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In: Flow, Turbulence and Combustion. - 1573-1987 .- 1386-6184.
-
Journal article (peer-reviewed)abstract
- This paper investigates the use of LES for a flow around a three-dimensional axisymmetric hill. Two aspects of this simulation in particular are discussed here, the resolution and the inlet boundary conditions. In contrast to the LES of flows with sharp edge separations which do not require the near-wall dynamics to be fully resolved, the hill flow LES relies on the resolution of the upstream boundary layer in order to provoke the separation at a correct position. Although around 15 000 000 computational cells were used, the resolution of streaky structures in the near-wall region that are important for a LES is not achieved. Two different inlet boundary conditions were used: the steady experimental profile and the time-dependent boundary conditions produced from DNS results of low Reynolds number channel flow. No significant improvement in the results was obtained with the unsteady inlet condition. This indicates that, although the unsteady inlet boundary conditions may be necessary for a successful LES of this flow, they must be followed with the resolution of the boundary layer for a successful LES.
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