| 1. |
- Efraimsson, Gunilla, et al.
(författare)
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A note on the effect of artificial viscosity on solutions of conservation laws
- 1996
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Ingår i: Applied Numerical Mathematics. - : Elsevier. - 0168-9274 .- 1873-5460. ; 21, s. 155-173
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Tidskriftsartikel (refereegranskat)abstract
- We consider central difference schemes with artificial viscosity terms for nonlinear hyperbolic systems. We analyze the influence of artificial viscosity on solutions with shocks of nonlinear hyperbolic problems in one dimension. Both stationary and moving shocks are considered. The analysis shows that for the Euler equations one can obtain well behaved (sharp) shock layers when a scalar viscosity coefficient is used. This is not true for a general system. Numerical computations of Burgers' equation and the Euler equations are presented. They support the results from the linear theory.
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| 2. |
- Efraimsson, Gunilla, et al.
(författare)
-
Analysis of Stretched Grids as Buffer Zones in Aero-Acoustic Simulations
- 2009
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Ingår i: 15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference). - Reston, Virigina : American Institute of Aeronautics and Astronautics.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A zone of increasingly stretched grid is a robust and easy-to-use way to avoid unwanted reections at artificial boundaries in aero-acoustic simulations. There are two main damping mechanisms, dissipation and under-resolution a traveling wave, respectively. In this paper the behavior of acoustic waves and wave packages, respectively, are studied via analysis of solutions to a semi-discrete linear boundary value problem. The solutions of the linear problem is compared to solutions of the full non-linear Euler equations.
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| 3. |
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| 4. |
- Kreiss, Gunilla, et al.
(författare)
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Analysis of stretched grids as buffer zones in simulations of wave propagation
- 2016
-
Ingår i: Applied Numerical Mathematics. - : Elsevier BV. - 0168-9274 .- 1873-5460. ; 107, s. 1-17
-
Tidskriftsartikel (refereegranskat)abstract
- A zone of increasingly stretched grid is a robust and easy-to-use way to avoid unwanted reflections at artificial boundaries in wave propagating simulations. In such a buffer zone there are two main damping mechanisms, dissipation and under-resolution that turns a traveling wave into an evanescent wave. We present analysis in one and two space dimensions showing that evanescent decay through under-resolution is a very efficient way to damp waves. The analysis is supported by numerical computations.
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| 5. |
- Kreiss, Gunilla, et al.
(författare)
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Approximate solutions to slightly viscous conservation laws
- 2004
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Ingår i: Quarterly of Applied Mathematics. - : American Mathematical Society (AMS). - 0033-569X .- 1552-4485. ; 62:1, s. 117-133
-
Tidskriftsartikel (refereegranskat)abstract
- We study an approximate solution of a slightly viscous conservation law in one dimension, constructed by two asymptotic expansions that are cut off after the third order terms. In the shock layer, an inner solution is valid and an outer solution is valid elsewhere. Based on the stability results in [10], we show that for a given time interval the difference between the approximate solution and the true solution is not larger than o(epsilon), where epsilon is the viscosity coefficient. The result holds for shocks of any strength.
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| 6. |
- Kreiss, Gunilla, et al.
(författare)
-
Elimination of first order errors in shock calculations
- 2001
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Ingår i: SIAM Journal on Numerical Analysis. - : SIAM PUBLICATIONS. - 0036-1429 .- 1095-7170. ; 38:6, s. 1986-1998
-
Tidskriftsartikel (refereegranskat)abstract
- First order errors downstream of shocks have been detected in computations with higher order shock capturing schemes in one and two dimensions. Based on a matched asymptotic expansion analysis we show how to modify the artificial viscosity and raise the order of accuracy.
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| 7. |
- Bodén, Hans, et al.
(författare)
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Aeroacoustics research in Europe : The CEAS-ASC report on 2012 highlights
- 2013
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Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 332:25, s. 6617-6636
-
Forskningsöversikt (refereegranskat)abstract
- The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC) supports and promotes the interests of the scientific and industrial aeroacoustics community on an European scale and European aeronautics activities internationally. In this context, "aeroacoustics" encompasses all aerospace acoustics and related areas. Each year the committee highlights some of the research and development projects in Europe. This paper is a report on highlights of aeroacoustics research in Europe in 2012, compiled from information provided to the ASC of the CEAS. During 2012, a number of research programmes involving aeroacoustics were funded by the European Commission. Some of the highlights from these programmes are summarized in this paper, as well as highlights from other programmes funded by national programmes or by industry. Enquiries concerning all contributions should be addressed to the authors who are given at the end of each subsection.
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| 8. |
- Carbonne, Louis, et al.
(författare)
-
Use of Full Coupling of Aerodynamics and Vehicle Dynamics for Numerical Simulation of the Crosswind Stability of Ground Vehicles
- 2016
-
Ingår i: SAE International Journal of Commercial Vehicles. - : SAE International.. - 1946-391X .- 1946-3928. ; 9:2, s. 359-370
-
Tidskriftsartikel (refereegranskat)abstract
- The prediction in the design phase of the stability of ground vehicles subject to transient crosswinds become of increased concern with drag reduced shapes, lighter vehicles as well as platooning. The objective of this work is to assess the order of model complexity needed in numerical simulations to capture the behavior of a ground vehicle passing through a transient crosswind. The performance of a full-dynamic coupling between aerodynamic and vehicle dynamic simulations, including a driver model, is evaluated. In the simulations a feedback from the vehicle dynamics into the aerodynamic simulation is performed in every time step. In the work, both the vehicle dynamic response and the aerodynamic forces and moments are studied. The results are compared to a static coupling approach on a set of different vehicle geometries. Five car-type geometries and one simplified bus geometry are evaluated. The aerodynamic loads and moments are obtained using Detached Eddy Simulation (DES) where the motion of the vehicle is enabled using an overset mesh technique. This motion is calculated with a single-track model, including a driver model and handling two degrees of freedom, namely lateral translation and yaw motion.The results show that for vehicles undertaking large yaw moments and therefore large yaw motions, like the bus-type geometry, the full dynamic coupling is beneficial. In this case, a static coupling overestimates the aerodynamic loads and in turn the vehicle motion. On less crosswind sensitive vehicles, like the car-type geometries, the full-coupling approach does not modify the results in a significant way compared to a static coupling.
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| 9. |
- da Rocha-Schmidt, L., et al.
(författare)
-
Progress Towards Adaptive Aircraft Engine Nacelles
- 2014
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Ingår i: 29th Congress of the International Council of the Aeronautical Sciences, ICAS. - : International Council of the Aeronautical Sciences.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Emissions and noise of aircraft engineshave to be significantly further reduced andefficiency further increased in the future. Onemeans is the improvement of airflow though theengine and especially so in its inlet region byproper shapes. Due to changes in the flightconditions, the optimal nacelle shape varies. Itwould thus be beneficial to be able to change thenacelle shape. Evaluations on system and enginelevels including related flow simulations supportthe identification of proper shaping parameters.Initial concepts of possible morphingtechnologies are discussed as well.
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| 10. |
- Dahan, Jeremy A., et al.
(författare)
-
Numerical investigation of a realistic nose landing gear
- 2014
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Ingår i: 20th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A hybrid approach is used to study the noise generated by a realistic full-scale nose land- ing gear configuration. Compressible Detached-Eddy Simulations are performed to com- pute the flow field and the far-field noise is evaluated with the Ffowcs Williams-Hawkings acoustic analogy. Preliminary Reynolds-Averaged Navier-Stokes simulations are performed to evaluate the sensitivity of the steady solution to the computational grid. It is found that mesh independence is not obtained with the grids considered, although the agreement between the medium and fine grids is reasonable. The time-dependent solution obtained via DES is examined and the main noise sources on the gear surfaces are identified. The power spectral density of pressure uctuations on the tyre surface is found to be rather broadband on the noisier sides of the tyre, while distinct humps and a sharp peak near 1kHz are identified in the quieter regions of the tyre. This sharp peak is observed again in the far-field noise. Further work is needed to clarify the cause of this peak. Its frequency is too high to link it with shedding from a gear component, although it could be associated with unsteadiness near the tyre corner.
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| 11. |
- Dahan, Jeremy A., et al.
(författare)
-
Numerical investigation of the flow around a realistic nose landing gear
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Detached-eddy simulations of the flow around a realistic two- wheeled nose landing gear are conducted, at a Reynolds number corresponding to the final approach phase for a regional aircraft. The main focus is on the flow and associated sound fields in the inter-wheel region and in the wake of the landing gear. Three unstructured grids are designed to evaluate the sensitivity of the solution to grid resolution. The three flow solutions agree well, although the turbulent wake requires a fine mesh. In addition, comparisons with available experimental data on two other nose landing gear models, the PDCC and LAGOON gears, yield a good qualitative agreement. The wake of the landing gear assembly is strongly three-dimensional and exhibits a complex behaviour. The effect of the torque links and wheel axle on the flow dynamics and sound sources were examined via simulations on different configurations. The presence of the torque links and the wheel axle, often simplified or omitted in numerical landing gear studies, were found to strongly modify the flow in the inter-wheel region and in the wheel wakes. The noise was propagated to the far-field using the convective form of the FWH integral. The far- field noise at sideline receivers agrees well with wind-tunnel measurements and the overhead noise sources in the inter-wheel region are identified by considering the integrand of the FWH analogy.
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| 12. |
- Dahan, Jeremy, 1987-, et al.
(författare)
-
Aero-acoustic source analysis of landing gear noise via dynamic mode decomposition
- 2014
-
Ingår i: 21st International Congress on Sound and Vibration, ICSV21. - 9786165516822 ; , s. 1245-1252
-
Konferensbidrag (refereegranskat)abstract
- In this paper, we apply dynamic mode decomposition (DMD) on time accurate simulationsof the pressure distribution on a realistic full-scale noselanding gear configuration in order toidentify noise generating structures on landing gear surfaces. The simulated pressure data isobtained from DES simulations using the commercial software STAR-CCM+ by CD-adapco.The dynamics of the surface pressure on a tyre are discussed and the DMD modes are com-puted from instantaneous pressure snapshots. The far-fieldnoise is determined via the FfowcsWilliams-Hawkings analogy, where a given frequency band source term can be reconstructedby choosing an appropriate number of DMD modes.
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| 13. |
- Efraimsson, Gunilla
(författare)
-
A numerical method for the first-order wave equation with discontinuous initial data
- 1998
-
Ingår i: Numerical Methods for Partial Differential Equations. - : John Wiley & Sons. - 0749-159X .- 1098-2426. ; 138:3, s. 353-365
-
Tidskriftsartikel (refereegranskat)abstract
- We introduce a method, constructed such that numerical solutions of the wave equation are well behaved when the solutions also contain discontinuities. The wave equation serves as a model problem for the Euler equations when the solution contains a contact discontinuity. Numerical computations of linear equations and the Euler equations in one and two dimensions are presented
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| 14. |
- Efraimsson, Gunilla
(författare)
-
A Remark on Numerical Errors Downstream of Slightly Viscous Shocks
- 1999
-
Ingår i: SIAM Journal on Numerical Analysis. - : Society for Industrial and Applied Mathematics. - 0036-1429 .- 1095-7170. ; 36:3, s. 853-863
-
Tidskriftsartikel (refereegranskat)abstract
- Lower-order errors downstream of a shock layer have been detected in computations with nonconstant solutions when using higher-order shock capturing schemes in one and two dimensions [B. Engquist and B. Sjögreen, {SIAM J. Numer. Anal., 35 (1998), pp. 2464--2485].By analyzing the steady-state solution of slightly viscous hyperbolic systems of conservation laws we find that the solution can have an ${\cal O}(h)$-dependence downstream of a shock layer, although the viscous term in that region is of ${\cal O}(h^2)$. Numerical examples illustrate the analysis.
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| 15. |
- Efraimsson, Gunilla
(författare)
-
A 2D Analysis of the Influence of Artificial Viscosity Terms on Solutions of the Euler Equations
- 1997
-
Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 138:1, s. 103-120
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze the influence of artificial viscosity on solutions of the Euler equations in the neighborhood of oblique shocks in 2D by studying a discrete, linear model equation. Based on the linear analysis an artificial viscosity model is derived. It is tested on two different test cases with the Euler equations: flow over a wedge and Mach-3 flow in a wind tunnel with a step.
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| 16. |
|
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| 17. |
- Efraimsson, Gunilla, et al.
(författare)
-
Analysis of Aeroacoustic Wave Propagation Simulations Using a Higher-Order Accurate Method
- 2006
-
Ingår i: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 1563478099 ; , s. 922-931
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present analysis and numerical investigations of the influence of grid stretching together with artificial viscosity on solutions to higher order accurate centered finite difference schemes. Also, we numerically investigate the accuracy of a sixth order accurate scheme for varying Mach number. In the analysis the first-order wave equation together with artificial viscosity terms are considered. In the numerical calculations a simple model problem is solved for which detailed studies of the downstream and upstream acoustic waves is possible.
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| 18. |
- Efraimsson, Gunilla, et al.
(författare)
-
Analysis of stretched grids as buffer zones in aero-acoustic simulations in two space dimensions
- 2015
-
Ingår i: 22nd International Congress on Sound and Vibration, ICSV 2015. - : International Institute of Acoustics and Vibrations. - 9788888942483
-
Konferensbidrag (refereegranskat)abstract
- A zone of increasingly stretched grid is a robust and easy-to-use way to avoid unwanted reflections at artificial boundaries in aero-acoustic simulations. There are two main damping mechanisms, dissipation and under-resolution of a traveling wave, respectively. In this paper the behavior of acoustic waves in two space dimensions are studied via analysis of solutions to a semi-discrete linear boundary value problem. The solutions of the linear problem is compared to solutions of the linearized Euler equations.
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| 19. |
|
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| 20. |
- Efraimsson, Gunilla, et al.
(författare)
-
Simulations of Acoustic Waves in a Turbo-Fan Engine Air Intake
- 2010
-
Ingår i: 16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference).
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this work, propagation of acoustic waves in the air intake of an turbo-fan engine are simulated using a commercial Navier-Stokes solver. Three different acoustic modes are studied, the first and second radial mode with no azimuthal variation and the first radial mode with four lobes in the azimuthal direction. From the results it can be concluded that the propagation of sound waves in a curved intake can indeed be simulated using a commercial CFD solver. Also, the acoustic source, when given as a boundary condition, should be set at the fan plane. A strong influence of the flow or the curved geometry is identified, yielding a focusing of sound waves to the middle part of the duct. A transmission loss of the acoustic power from the fan plane to the inlet plane of around 5 dB is identified for the first radial modes for acoustic powers in the interval [128 db, 158 dB]. Non-linear effects are identified for powers of 148 dB and higher, which seems reasonable. Finally, a shielding effect of supersonic regions is identified.
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| 21. |
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| 22. |
- Favre, Tristan
(författare)
-
Aerodynamics simulations of ground vehicles in unsteady crosswind
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ground vehicles, both on roads or on rail, are sensitive to crosswinds and the handling, travelling speeds or in some cases, safety can be affected. Full modelling of the crosswind stability of a vehicle is a demanding task as the nature of the disturbance, the wind gust, is complex and the aerodynamics, vehicle dynamics and driver reactions interact with each other. One of the objectives of this thesis, is to assess the aerodynamic response of simplified ground vehicles under sudden strong crosswind disturbances by using an advanced turbulence model. In the aerodynamic simulations, time-dependant boundary data have been used to introduce a deterministic wind gust model into the computational domain. This thesis covers the implementation of such gust models into Detached-Eddy Simulations (DES) and assesses the overall accuracy. Different type of grids, numerical setups and refinements are considered. Although the overall use of DES is seen suitable, further investigations can be foreseen on more challenging geometries. Two families of vehicle models have been studied. The first one, a box-like geometry, has been used to characterize the influence of the radius of curvature and benefited from unsteady experimental data for comparison. The second one, the Windsor model, has been used to understand the impact of the different rear designs. Noticeably, the different geometries tested have exhibited strong transients in the loads that can not be represented in pure steady crosswind conditions.The static coupling between aerodynamics and vehicle dynamics simulations enhances the comparisons of the aerodynamic designs. Also, it shows that the motion of the centre of pressure with respect the locations of the centre of gravity and the neutral steer point, is of prime interest to design vehicles that are less crosswind sensitive. Recommendations on the future work on crosswind sensitivity for ground vehicles are proposed at the end of this thesis.
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| 23. |
- Favre, Tristan, et al.
(författare)
-
An Assessment of Detached-Eddy Simulations of Unsteady Crosswind Aerodynamics of Road Vehicle
- 2011
-
Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 87:1, s. 133-163
-
Tidskriftsartikel (refereegranskat)abstract
- An extensive study of the mesh requirements when simulating unsteady crosswind aerodynamics for industrial applications is conducted and reported in this article. Detached-Eddy Simulations (DES) of a simple car geometry under headwind, steady crosswind and time-dependent wind gust are analysed for different meshes and flow cases using a commercial software, STAR-CD. The typical Reynolds number of the cases studied is 2.0x106 based on the vehicle length. Mesh requirements for capturing the time development of the flow structures during a gust is provided. While respecting these requirements, the aerodynamic coefficients can be reliably calculated. Using turbulence methods like DES in order to resolve the flow scales provides a significant insight for designing a ground vehicle and, due to the reasonable computational times involved, can be incorporated in a design process in a near future.
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| 24. |
- Favre, Tristan, et al.
(författare)
-
Detached-Eddy Simulations Applied to Unsteady Crosswind Aerodynamics of Ground Vehicles
- 2010
-
Ingår i: Progress in hybrid rans-les modelling. - Berlin : SPRINGER-VERLAG. - 9783642141676 ; , s. 167-177
-
Konferensbidrag (refereegranskat)abstract
- Crosswind stability is an important safety issue for manufacturers of cars, buses and rail vehicles. Since side wind conditions are unsteady phenomena they require time-dependent techniques to simulate the flow. In this study, a hybrid RANS-LES methods, Detached-Eddy Simulation, is applied to evaluate headwind and unsteady crosswind situations for a simple model of car. A grid refinement study is carried out to evaluate the accuracy of the calculations. Convergence in the force coefficients while refining the grid suggests that a certain level of grid convergence is reached. A similar conclusion is drawn from the unsteady simulations.
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| 25. |
- Favre, Tristan, et al.
(författare)
-
Detached-eddy simulations for steady and unsteady crosswind aerodynamics of ground vehicles
- 2011
-
Ingår i: 20th AIAA Computational Fluid Dynamics Conference 2011. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 9781624101489
-
Konferensbidrag (refereegranskat)abstract
- This paper presents Detached-Eddy Simulations (DES) of two generic vehicle models with different design characteristics and different Re numbers under steady and unsteady crosswind conditions. The good agreement with the experimental data available on the first model in gusty conditions demonstrates the capabilities of the combination of transient boundary data together with DES to accurately simulate unsteady crosswind ows for ground vehicles. The second geometry serves to evaluate whether polyhedral meshes with a second order upwind scheme, that is a numerical framework usually employed for RANS in industry, can be used for DES on crosswind ows. The results with the polyhedrals show comparable results with the reference hexahedral mesh and can be effciently used to provide some insights on complex unsteady ows.
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| 26. |
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| 27. |
- Favre, Tristan, et al.
(författare)
-
Detached-Eddy Simulations of the Effects of Different Wind Gust Models on the Unsteady Aerodynamics of Road Vehicles
- 2010
-
Ingår i: ASME 2010 3rd Joint US-European Fluids Engineering Summer Meeting collocated with 8th International Conference on Nanochannels, Microchannels, and Minichannels (FEDSM2010). - 9780791849484 ; , s. 2605-2614
-
Konferensbidrag (refereegranskat)abstract
- Therelative flexibility of nowadays simulation techniques offers an alternative wayto experiments in order to investigate unsteady crosswind aerodynamics inan industrial framework. In this study, time-accurate simulations, Detached-Eddy Simulations(DES), are used to simulate the flow around a simplevehicle shape, the so-called Windsor model. The ReL of thecorresponding flow case is 2.0 · 106. The influence ofdifferent deterministic models of wind gusts on the aerodynamic loadsand moments are studied. The wind gusts are varied inthe stream-wise and the vertical direction. The magnitude of thegusts models corresponds to a yaw angle of 20°. Theaerodynamic loads calculated show a large excess of drag coupledwith a reduction of the pitch moment. In addition, althoughthe side force has a smooth variation in the gust,overshoots up to 18% higher than the steady value ofyaw moment are also observed.
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| 28. |
- Favre, Tristan, et al.
(författare)
-
Effects of Deterministic Wind Gusts on Unsteady Crosswind Aerodynamics of Road Vehicles
- 2009
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The relative flexibility of nowadays numerical methods offers an alternative way to experiments in order to investigate unsteady crosswind aerodynamics in an industrial framework. In this study, time accurate methods such as Detached-Eddy Simulations (DES) are used to simulate the flow. A simple vehicle shape, the so-called Windsor model, is chosen for the model. The ReL of the corresponding flow case is 2.0x106. Then, different deterministic models of wind gusts that are varied in the streamwise and the vertical direction are tested. The magnitude of the gusts models corresponds to a yaw angle of 20o. The aerodynamic loads calculated show a large excess of drag coupled with a reduction of the pitch moment. Although the side force has a smooth variation in the gust, overshoots up to 18% higher than the steady value of yaw moment are also observed.
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| 29. |
- Favre, Tristan, 1983-
(författare)
-
Numerical Investigation of Unsteady Crosswind Aerodynamics for Ground Vehicles
- 2009
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ground vehicles are subjected to crosswind from various origins such as weather, topography of the ambient environment (land, forest, tunnels, high bushes...) or surrounding traffic. The trend of lowering the weight of vehicles imposes a stronger need for understanding the coupling between crosswind stability, the vehicle external shape and the dynamic properties. Means for reducing fuel consumption of ground vehicles can also conflict with the handling and dynamic characteristics of the vehicle. Streamlined design of vehicle shapes to lower the drag can be a good example of this dilemma. If care is not taken, the streamlined shape can lead to an increase in yaw moment under crosswind conditions which results in a poor handling. The development of numerical methods provides efficient tools to investigate these complex phenomena that are difficult to reproduce experimentally. Time accurate and scale resolving methods, like Detached-Eddy Simulations (DES), are particularly of interest, since they allow a better description of unsteady flows than standard Reynolds Average Navier-Stokes (RANS) models. Moreover, due to the constant increase in computational resources, this type of simulations complies more and more with industrial interests and design cycles. In this thesis, the possibilities offered by DES to simulate unsteady crosswind aerodynamics of simple vehicle models in an industrial framework are explored. A large part of the work is devoted to the grid design, which is especially crucial for truthful results from DES. Additional concerns in simulations of unsteady crosswind aerodynamics are highlighted, especially for the resolution of the wind-gust boundary layer profiles. Finally, the transient behaviour of the aerodynamic loads and the flow structures are analyzed for several types of vehicles. The results simulated with DES are promising and the overall agreement with the experimental data available is good, which illustrates a certain reliability in the simulations. In addition, the simulations show that the force coefficients exhibit highly transient behaviour under gusty conditions.
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| 30. |
- Favre, Tristan, et al.
(författare)
-
Numerical Investigation of Unsteady Crosswind Vehicle Aerodynamics using Time-Dependent Inflow Conditions
- 2008
-
Ingår i: Seventh World MIRA International Vehicle Aerodynamics Conference. - 9781906400057
-
Konferensbidrag (refereegranskat)abstract
- Transient disturbances concerning ground vehicles are not only due to rail or road irregularities but are also caused by unsteady ambient wind conditions. This study presents a numerical investigation of unsteady crosswind aerodynamics by using the commercial software STAR-CD from CD-Adapco. The unsteadiness of the aerodynamics is introduced through time-dependent inflow boundary conditions that describe a jet flow according to Schlichting [1]. The purpose of this study is to explore the validity of the commonly used numerical methods (URANS and DES) for the current crosswind application. To this end, simplified vehicle geometries are utilized, for which the experimental results of Chadwick et al. [2] are used as reference data.
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| 31. |
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| 32. |
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| 33. |
- Favre, Tristan, et al.
(författare)
-
Static coupling between detached-eddy simulations and vehicle dynamic simulations of a generic road vehicle model with different rear configurations in unsteady crosswind
- 2016
-
Ingår i: International Journal of Vehicle Design. - : INDERSCIENCE ENTERPRISES LTD. - 0143-3369 .- 1741-5314. ; 72:4, s. 332-353
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, aerodynamic loads of a generic car model obtained from advanced computational fluid dynamics (CFD) simulations are coupled to a vehicle dynamics model to enable the assessment of the on-road response. The influence of four rear configurations is studied. The different configurations yield large differences in yaw moments and side forces, which in turn result in considerable discrepancies in lateral displacements as well as yaw rates. From the simulations, it is seen that through balancing the location of the centre of pressure, the stiffness of the suspension bushings and the cornering stiffness of the tyres, it is possible to obtain stable vehicles in strong crosswind conditions for all four rear designs. The results show that monitoring the location of the aerodynamic centre of pressure with respect to the centre of gravity and the neutral steer point is essential for the possibility of designing stable vehicles in transient crosswind.
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| 34. |
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| 35. |
- Futrzynski, Romain, 1988-, et al.
(författare)
-
Analysis of the wake of a half-cylinder by dynamic mode decomposition
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper analyzes the dynamic structures in the wake of a half-cylinder protruding from the ground. This relatively simple and smooth geometry allows to create a signicant wake, yet the the location of the detachment point is not predictable from the geometry. The flow over the half-cylinder has a Reynolds number of 32*10^3. It is considered to be incompressible and is simulated by Large Eddy Simulations (LES). The flow field is first described in terms of the time-averages of velocity, pressure, and turbulent kinetic energy. This is the most traditional way to study turbulent flows, and it enables to identify the recirculation regions upstream and downstream of the half-cylinder. The locations of separation and reattachment are also obtained. Then, dynamic structures are extracted by means of dynamic mode decomposition (DMD). The DMD modes have the particularity to oscillate in time at a single given frequency, which renders the dynamics of the flow field more intelligible. It is found that despite a broadband spectrum, all the DMD modes reveal the same type of phenomenon that varies only in scale. By observing the modes at different frequencies, vortices can be followed from their creation in the upstream recirculation region. As they are convected downstream, they merge with bigger and bigger vortices, until they are big enough to influence the whole wake.
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| 36. |
- Futrzynski, Romain, 1988-
(författare)
-
Drag reduction using plasma actuators
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is motivated by the application of active flow control on the cabin of trucks, thereby providing a new means of drag reduction. Particularly, the work presented strives to identify how plasma actuators can be used to reduce the drag caused by the detachment of the flow around the A-pillars. This is achieved by conducting numerical simulations, and is part of a larger project that also includes experimental.The effect of plasma actuators is modeled through a body force, which adds very little computational cost and is suitable for implementation in most CFD solvers. The spatial distribution of this force is described by coefficients which have been optimized against experimental data, and the model was shown to be able to accurately reproduce the wall jet created by a single plasma actuator in a no-flow condition.A half cylinder geometry - a simplified geometry for the A-pillar of a truck - was used in a preliminary Large Eddy Simulation (LES) study that showed that the actuator alone, operated continuously, was not sufficient to achieve a significant reduction of the drag. Nevertheless, a significant drag reduction was obtained by simply increasing the strength of the body force to a higher value, showing that this type of actuation remains relevant for the reduction of drag.In the course of finding ways to improve the efficiency of the actuator, dynamic mode decomposition was investigated as a post-processing tool to extract structures in the flow. Such structures are identified by their spatial location and frequency, and might help to understand how the actuator should be used to maximize drag reduction. Thus a parallel code for dynamic mode decomposition was developed in order to facilitate the treatment of the large amounts of data obtained by LES. This code and LES itself were thereafter evaluated in the case of a pulsating channel flow. By using the dynamic mode decomposition it was possible to accurately extract oscillating profiles at the forcing frequency, although harmonics with lower amplitude compared to the turbulence intensity could not be obtained.
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| 37. |
- Futrzynski, Romain, 1988-, et al.
(författare)
-
Dymode : A parallel dynamic mode decomposition software
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Dymode is a parallel program that computes dynamic mode decompositions. The code is written in C++ and relies on a number of libraries. Several parameters can be specified in order to control the computational aspects of the program as well as the input and output of the decomposition, particularly how the modes are sorted. Finally, dymode is almost entirely parallel and is therefore particularly suitable for computing the dynamic mode decomposition of large datasets.The dymode package also includes dymodem, a Matlab implementation of the code which accepts the same arguments as dymode, when they are relevant, and produces the same output. It can be useful to use dymodem when dealing with smaller datasets, or to validate the output from dymode.
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| 38. |
- Futrzynski, Romain, 1988-, et al.
(författare)
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Effect of a SDBD on the drag of a half-submerged cylinder in crossflow
- 2014
-
Ingår i: ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting, FEDSM 2014, Collocated with the ASME 2014 12th International Conference on Nanochannels, Microchannels, and Minichannels, 3 August 2014 through 7 August 2014. - : ASME Press. - 9780791846230
-
Konferensbidrag (refereegranskat)abstract
- In this paper the effect of a SDBD-type plasma actuator on the flow over a half-submerged cylinder is investigated numerically. The actuator is modeled via a body force, which is steady in time and where an exponential decay in space is assumed. First, the parameters in the numerical actuator model are determined for the case of no flow by optimization relative to experimental data. Thereafter, numerical solutions for the case with flow are studied numerically with and without actuation. A grid study is performed to check that the flow structures are resolved in both space and time. The effect of the actuator is examined. Although no significant change is observed when using the optimized parameters, using a stronger body force yields a reduction in drag of the order of 5%.
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| 39. |
- Futrzynski, Romain, 1988-
(författare)
-
Effect of drag reducing plasma actuators using LES
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work performed in this thesis explores new ways of reducing the drag of ground vehicles. Specifically, the effect of plasma actuators are investigated numerically with the intention to delay separation around a half-cylinder, a geometry chosen to represent a simplified A-pillar of a truck.The plasma actuators have to be included in turbulent flow simulations. Therefore, emphasis is first put on finding a numerical model that can reproduce the effect of the plasma without increasing the computational cost. This effect is modeled through a body force term added to the Navier-Stokes equations. To determine the strength and spatial extent of this body force, optimization was performed to minimize the difference between experimental and simulated profiles of plasma induced velocity. The plasma actuator model is thereafter used in Large Eddy Simulations (LES) of the flow around a half-cylinder at Reynolds number Re=65*10^3 and Re=32*10^3. Two types of actuation cases are performed. In the first case, a single actuator is used. In the second case, a pair of consecutive actuators are used, and their position on the half-cylinder is changed. It is found that a drag reduction of up to 10% is achievable. Moreover, the ideal location for actuation is determined to be near the separation point of the non-actuated flow. Finally, dynamic mode decomposition (DMD) is investigated as a tool to extract coherent dynamic structures from a turbulent flow field. The DMD is first used to analyze a channel flow where pulsations are imposed at a known frequency. It is found that DMD gives similar results to phase averaging done at the oscillation frequency. However, the presence of turbulence noise hinders the ability to identify modes at higher harmonics. The DMD is also used to post-process the half-cylinder flow case. There, it is found that the spectrum of the wake is broadband. Nevertheless, modes within distinct frequency ranges are found to be located in distinct spatial regions.
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| 40. |
- Futrzynski, Romain, 1988-, et al.
(författare)
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Numerical simulation of a plasma actuator on a half-submerged cylinder
- 2013
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper Large Eddy Simulations are used to study the reduction of drag that can be achieved on a half-submerged cylinder by using a type of plasma actuator: the single dielectric barrier discharge. Two body force models, one based on an exponential decrease of the force away from the plasma, the other based on a simplified electric field between the electrodes, are compared to experimental values when the actuator is positioned at the apex of the cylinder in an otherwise quiescent environment. The cylinder is then put in a crossflow, and the exponential-based model, which gives the velocity profiles the closest to the experimental data, is used to simulate the effect of the plasma actuator on such a flow. The reduction in drag is changed as the position of the actuator is varied.
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| 41. |
- Futrzynski, Romain, 1988-, et al.
(författare)
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Numerical study of the Stokes layer in oscillating channel flow
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Oscillating turbulent channel flows present particular physics that proves to be particularly difficult to understand. In this paper, a case where the amplitude of the oscillations at the center of the channel is approximately 15% of the mean velocity and the dimensionless angular forcing frequency is 0.01 was studied using several numerical methods. DNS was performed to serve as reference to which the results from an LES were compared. The LES data was post-processed using both phase averaging and the more recent dynamic mode decomposition (DMD), which extracts coherent structures based on their frequency. It was found that the DMD is not able to extract faint harmonic components of the oscillations, which have been observed with phase averaging and Fourier transforms. It is, however, able to extract accurate profiles of the mean and forcing frequency quantities. Compared to the DNS, the accuracy of the LES results was similar to analytical models, although no single model gives accurate result for every quantity investigated.
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| 42. |
- Futrzynski, Romain, 1988-, et al.
(författare)
-
Reduction of the wake of a half-cylinder using a pair of plasma actuators
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, the effect of plasma actuators on separated flows is studied via Large Eddy Simulations (LES) of the incompressible flow over a half-cylinder at a Reynolds number of 32*10^3. One plasma actuator is modeled by a steady body force distribution which is able to replicate the effect of the actuator in a quiescent environment without adding any significant complexity to the numerical simulations. This model is applied at two locations in order to simulate a pair of plasma actuators placed on the surface of the halfcylinder, separated by 20 degrees. Several simulations have been performed with the pair of actuators placed at different angles on the half-cylinder, and the drag reduction is reported for each configuration. It is determined that the actuation is able to achieve up to 10% of drag reduction when one actuator from the pair is placed a few degrees downstream of the separation point of the non-actuated flow. Mean flow quantities obtained in the wake and on the surface of the half-cylinder reveal that the reduction in drag is coupled to a reduction in the size of the recirculating zone as well as a delay of the separation point of up to 10 degrees.
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| 43. |
- Futrzynski, Romain, 1988-, et al.
(författare)
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Study of Plasma Actuator Efficiency by Simulation of the Detached Flow Over a Half-Cylinder
- 2016
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, the effect of a numerical model for plasma actuators, in the form of single dielectric barrier discharge, is evaluated. One such plasma actuator is modeled by a steady body force distribution able to replicate the effect of the actuator in a quiescent environment without adding any significant complexity to the numerical simulations. This model is used in Large Eddy Simulations (LES) of the flow over a half-cylinder at a Reynolds number of 32000 , where the actuation is expected to yield a measurable drag reduction. The flow without actuation is first analyzed by mesh refinement and by evaluation of different flow quantities in order the validate the simulation results. Thereafter, the model is used to simulate two actuators placed on the half-cylinder one after another and at four locations chosen so that the mean separation point of the non-actuated flow lies betweenthe two actuators. It is determined that the actuation is able to achieve up to 10% of drag reduction, although this value decreases to 6% when the actuation location is moved.
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| 44. |
- Gong, Jing, 1971-
(författare)
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Hybrid Methods for Unsteady Fluid Flow Problems in Complex Geometries
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, stable and efficient hybrid methods which combine high order finite difference methods and unstructured finite volume methods for time-dependent initial boundary value problems have been developed. The hybrid methods make it possible to combine the efficiency of the finite difference method and the flexibility of the finite volume method.We carry out a detailed analysis of the stability of the hybrid methods, and in particular the stability of interface treatments between structured and unstructured blocks. Both the methods employ so called summation-by-parts operators and impose boundary and interface conditions weakly, which lead to an energy estimate and stability.We have constructed and analyzed first-, second- and fourth-order Laplacian based artificial dissipation operators for finite volume methods on unstructured grids. The first-order artificial dissipation can handle shock waves, and the fourth-order artificial dissipation eliminates non-physical numerical oscillations efficiently.A stable hybrid method for hyperbolic problems has been developed. It is shown that the stability at the interface can be obtained by modifying the dual grid of the unstructured finite volume method close to the interface. The hybrid method is applied to the Euler equation by the coupling of two stand-alone CFD codes. Since the coupling is administered by a third separate coupling code, the hybrid method allows for individual development of the stand-alone codes. It is shown that the hybrid method is an accurate, efficient and practically useful computational tool that can handle complex geometries and wave propagation phenomena.Stable and accurate interface treatments for the linear advection–diffusion equation have been studied. Accurate high-order calculation are achieved in multiple blocks with interfaces. Three stable interface procedures — the Baumann–Oden method, the “borrowing” method and the local discontinuous Galerkin method, have been investigated. The analysis shows that only minor differences separate the different interface handling procedures.A conservative stable and efficient hybrid method for a parabolic model problem has been developed. The hybrid method has been applied to the full Navier–Stokes equations. The numerical experiments support the theoretical conclusions and show that the interface coupling is stable and converges at the correct order for the Navier–Stokes equations.
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| 45. |
- Hammar, Johan, 1979-
(författare)
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A Wave Expansion Method for Aeroacoustic Propagation
- 2016
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Although it is possible to directly solve an entire flow-acoustics problem in one computation, this approach remains prohibitively large in terms of the computational resource required for most practical applications. Aeroacoustic problems are therefore usually split into two parts; one consisting of the source computation and one of the source propagation. Although both these parts entail great challenges on the computational method, in terms of accuracy and efficiency, it is still better than the direct solution alternative. The source usually consists of highly turbulent flows, which for most cases will need to be, at least partly, resolved. Then, acoustic waves generated by these sources often have to be propagated for long distances compared to the wavelength and might be subjected to scattering by solid objects or convective effects by the flow. Numerical methods used solve these problems therefore have to possess low dispersion and dissipation error qualities for the solution to be accurate and resource efficient.The wave expansion method (WEM) is an efficient discretization technique, which is used for wave propagation problems. The method uses fundamental solutions to the wave operator in the discretization procedure and will thus produce accurate results at two to three points per wavelength. This thesis presents a method that uses the WEM in an aeroacoustic context. Addressing the propagation of acoustic waves and transfer of sources from flow to acoustic simulations. The proposed computational procedure is applied to a co-rotating vortex pair and a cylinder in cross-flow. Overall, the computed results agree well with analytical solutions.Although the WEM is efficient in terms of the spatial discretization, the procedure requires that a Moore-Penrose pseudo-inverse is evaluated at each unique node-neighbour stencil in the grid. This evaluation significantly slows the procedure. In this thesis, a method with a regular grid is explored to speed-up this process.
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| 46. |
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| 47. |
- Hammar, Johan, et al.
(författare)
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Simulation of aerodynamically generated noise propagation using the wave expansion method
- 2016
-
Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103865
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A numerical method to simulate aerodynamically generated sound and its propagation is presented in this paper. The flow is solved using both analytical expressions and numer- ical methods. The aeroacoustic source terms are then defined by using the aeroacoustic analogies of Lighthill, Ffowcs-Williams and Hawkings, Powell and Howe, and are evaluated from the flow solutions. The acoustic propagation with these sources is then performed using the wave expansion method. This is a discretization method suitable for solving wave propagation through inhomogeneous potential flows. Two different cases are considered in the study, a co-rotating vortex pair and the flow around a 2D cylinder at Re=150. The fo- cus of the work presented is to show a robust introduction of aeroacoustic sources in a wave expansion acoustic propagation solution procedure. The numerical results are compared to experimental and numerical results from other studies of the same configurations.
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| 48. |
- Hammar, Johan, et al.
(författare)
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Simulation of aerodynamically generated sound using hybrid aeroacoustic method
- 2015
-
Ingår i: Euronoise 2015. - : DC/ConfOrg. ; , s. 521-526, s. 521-526
-
Konferensbidrag (refereegranskat)abstract
- A numerical method to simulate aerodynamically generated sound and its propagation is presented in this paper. The transient flow field solution is established using a compressible 2D Navier Stokes solver. The source terms are then defined by using Howe's vortex sound aeroacoustic analogy and are evaluated from the flow solutions. The propagation of acoustic waves from these sources is then performed using the wave expansion method (WEM). This is a discretization method suitable for solving wave propagation through inhomogeneous potential flows. The method is tested on a flow of a rectangular open cavity. The flow conditions are a free stream Mach number of M=0.5 and Reynolds number of Re=1500.. The numerical results are compared to experimental and numerical results from other studies of the same configuration.
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| 49. |
- Herbst, A H, et al.
(författare)
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Aerodynamic prediction tools for high-speed trains
- 2014
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Ingår i: International Journal of Rail transportation. - : Taylor & Francis. - 2324-8378 .- 2324-8386. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- With high-speed trains, the need for efficient and accurate aerodynamic prediction tools increases, since the influence of the aerodynamics on the overall train performance raises. New requirements on slipstream velocities and head pressure pulse in the revised Technical Specification for Interoperability (TSI) for train speeds higher than 190 km/h are more challenging to fulfil for wide-body trains, like the Green train concept vehicle Regina 250, as well as higher trains, like double-deck trains. In this paper, we give an overview of the results from a project within the Green train programme, where the objective was to increase the knowledge on slipstream air flow of wide body trains at high speeds, to understand the implications of the new requirements on the front shape and to develop a prediction methodology in order to take this into account early in the design cycle. In addition, the front design was in parallel optimized with respect to head pressure pulse and drag.
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| 50. |
- Kierkegaard, Axel, et al.
(författare)
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A frequency domain linearized Navier-Stokes equations approach to acoustic propagation in flow ducts with sharp edges
- 2010
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Ingår i: Journal of the Acoustical Society of America. - : Acoustical Society of America (ASA). - 0001-4966 .- 1520-8524. ; 127:2, s. 710-719
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Tidskriftsartikel (refereegranskat)abstract
- Acoustic wave propagation in flow ducts is commonly modeled with time-domain non-linear Navier-Stokes equation methodologies. To reduce computational effort, investigations of a linearized approach in frequency domain are carried out. Calculations of sound wave propagation in a straight duct are presented with an orifice plate and a mean flow present. Results of transmission and reflections at the orifice are presented on a two-port scattering matrix form and are compared to measurements with good agreement. The wave propagation is modeled with a frequency domain linearized Navier-Stokes equation methodology. This methodology is found to be efficient for cases where the acoustic field does not alter the mean flow field, i.e., when whistling does not occur.
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