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Ekman, P., Venning, J., Virdung, T. & Karlsson, M. (2021). Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations. Paper presented at 10/20/2020. Journal of Fluids Engineering, 143(1), Article ID 011501.
Open this publication in new window or tab >>Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations
2021 (English)In: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 143, no 1, article id 011501Article in journal (Refereed) Published
Abstract [en]

The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In this study, the flow around the 25 deg Ahmed body has been investigated using large eddy simulations and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used sub-grid scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Very good agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the wall-adopting local eddy-viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modeling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.

Place, publisher, year, edition, pages
ASME, 2021
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-170754 (URN)10.1115/1.4048351 (DOI)000595655600010 ()
Conference
10/20/2020
Available from: 2020-10-20 Created: 2020-10-20 Last updated: 2022-06-03Bibliographically approved
Fattahi, S., Ekman, P. & Karlsson, M. (2021). Influence of the Timber Shape on the Aerodynamics of a Timber Truck. SAE technical paper series, Article ID 2021-01-5045.
Open this publication in new window or tab >>Influence of the Timber Shape on the Aerodynamics of a Timber Truck
2021 (English)In: SAE technical paper series, ISSN 0148-7191, article id 2021-01-5045Article in journal (Refereed) Published
Abstract [en]

The aerodynamic improvement and efficiency of regular goods transportation trucks have been a topic of current interest; however, the timber transport industry has not been receiving as much attention. This is due to the small portion of timber transportation vehicles, compared to regular trucks, not justifying the cost of investigating these vehicles experimentally. Since these vehicles travel large parts of their journey at around 80 km/h, their fuel consumption is heavily affected by the aerodynamic resistance. In Sweden in 2015, there were around 2000 vehicles in operation transporting 6 billion ton-km with an average of 0.025 liter Diesel per ton-km. To understand these vehicles’ aerodynamics, and improve on these in the future, the modelling of the timber stacks is of utmost importance. Computational Fluid Dynamics (CFD) simulations have been utilized to conduct this investigation due to recent advancements and the relatively low cost of these simulations compared to an experimental approach. By investigating the influence of geometrical modifications of the stacks on the flow features and accumulated drag, a generic timber stack was created representative of a real stack for a loaded baseline vehicle. It was found that the shorter log length and a shuffling of the logs in the stack exhibit important flow features contributing to drag not present in the other cases. Based on this, a new baseline loaded truck configuration was created with all stacks being identical to each other. This generic stack was built with logs that were 4.25 m long and 0.35 m in diameter, had a smooth surface, and were stacked with a certain displacement in the lengthwise direction.

Keywords
Timber, Truck, Aerodynamics, CFD, Computational fluid dynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-183106 (URN)10.4271/2021-01-5045 (DOI)2-s2.0-85106040040 (Scopus ID)
Note

 Funding: The financial support by the Swedish Energy Agency is gratefully acknowledged.

Available from: 2022-02-21 Created: 2022-02-21 Last updated: 2022-06-03Bibliographically approved
Ekman, P., Wieser, D., Virdung, T. & Karlsson, M. (2020). Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations. Journal of Wind Engineering and Industrial Aerodynamics, 206, Article ID 104301.
Open this publication in new window or tab >>Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations
2020 (English)In: Journal of Wind Engineering and Industrial Aerodynamics, ISSN 0167-6105, E-ISSN 1872-8197, Vol. 206, article id 104301Article in journal (Refereed) Published
Abstract [en]

The introduction of the Harmonized Light Vehicles Test Procedure causes a significant challenge for the automotive industry, as it increases the importance of efficient aerodynamics and demands how variations of optional extras affect the car’s fuel consumption and emissions. This may lead to a huge number of combinations of optional extras that may need to be aerodynamically analyzed and possibly optimized, being to resource-consuming to be done with wind tunnel testing merely. Reynolds Average Navier-Stoles (RANS) coupled with Large Eddy Simulations (LES) have shown potential for accurate simulation for automotive applications for reasonable computational cost. In this paper, three hybrid RANS-LES models are investigated on the DrivAer notchback and fastback car bodies and compared to wind tunnel measurements. Several yaw angles are investigated to see the model’s ability to capture small and large changes of the flow field. It is seen that the models generally are in good agreement with the measurement, but only one model is able to capture the behavior seen in the measurements consistently. This is connected to the complex flow over the rear window, which is important to capture for accurate force predictions.

Keywords
CFD, DDES, IDDES, SBES, DrivAer, Notchback, Fastback, Yaw, Wind tunnel, Turbulence modelling
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:liu:diva-170753 (URN)10.1016/j.jweia.2020.104301 (DOI)000580638300013 ()
Available from: 2020-10-20 Created: 2020-10-20 Last updated: 2020-11-08Bibliographically approved
Casas Garcia, B., Lantz, J., Viola, F., Cedersund, G., Bolger, A. F., Carlhäll, C.-J., . . . Ebbers, T. (2020). Publisher Correction: Bridging the gap between measurements and modelling: a cardiovascular functional avatar. Scientific Reports, 10(1)
Open this publication in new window or tab >>Publisher Correction: Bridging the gap between measurements and modelling: a cardiovascular functional avatar
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2020 (English)In: Scientific Reports, E-ISSN 2045-2322, Vol. 10, no 1Article in journal (Other academic) Published
Place, publisher, year, edition, pages
Nature Publishing Group, 2020
National Category
Cardiac and Cardiovascular Systems
Identifiers
urn:nbn:se:liu:diva-174235 (URN)10.1038/s41598-020-58809-7 (DOI)000528916600001 ()31996774 (PubMedID)2-s2.0-85078668524 (Scopus ID)
Note

Correction to: Scientifc Reports https://doi.org/10.1038/s41598-017-06339-0

Available from: 2021-03-17 Created: 2021-03-17 Last updated: 2023-12-28Bibliographically approved
Ekman, P., Larsson, T., Virdung, T. & Karlsson, M. (2019). Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. In: WCX SAE World Congress Experience: . Paper presented at WCX SAE World Congress Experience. SAE International
Open this publication in new window or tab >>Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
2019 (English)In: WCX SAE World Congress Experience, SAE International , 2019Conference paper, Published paper (Refereed)
Abstract [en]

In aerodynamic development of ground vehicles, the use of Computational Fluid Dynamics (CFD) is crucial for improving the aerodynamic performance, stability and comfort of the vehicle. Simulation time and accuracy are two key factors of a well working CFD procedure. Using scale-resolving simulations, accurate predictions of the flow field and aerodynamic forces are possible, but often leads to long simulation time. For a given solver, one of the most significant aspects of the simulation time/cost is the temporal resolution. In this study, this aspect is investigated using the realistic vehicle model DrivAer with the notchback geometry as the test case. To ensure a direct and accurate comparison with wind tunnel measurements, performed at TU Berlin, a large section of the wind tunnel is included in the simulation domain. All simulations are performed at a Reynolds number of 3.12 million, based on the vehicle length. Three spatial resolutions were compared, where it could be seen that a hybrid element mesh consisting of 102 million cells only revealed small differences to the finest mesh investigated, well as showing excellent agreement with wind tunnel measurements. An investigation of the temporal resolution is performed, in order to see its effect on the simulation time/cost and accuracy of the results. The finest temporal resolution resulted in a Courant-Friedrichs-Lewy number less than unity, while the coarsest reached a CFL number of around 100. From these results, it is seen that it is possible to reduce the simulation time with more than 90 % (CFL 20) and still keep sufficient accuracy of the forces and important features of the flow field.

Place, publisher, year, edition, pages
SAE International, 2019
Series
SAE technical paper series, ISSN 0148-7191
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:liu:diva-164924 (URN)10.4271/2019-01-0639 (DOI)2-s2.0-85064594517 (Scopus ID)
Conference
WCX SAE World Congress Experience
Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2021-07-15Bibliographically approved
Lantz, J., Gupta, V., Henriksson, L., Karlsson, M., Persson, A., Carlhäll, C.-J. & Ebbers, T. (2019). Impact of Pulmonary Venous Inflow on Cardiac Flow Simulations: Comparison with In Vivo 4D Flow MRI. Annals of Biomedical Engineering, 47(2), 413-424
Open this publication in new window or tab >>Impact of Pulmonary Venous Inflow on Cardiac Flow Simulations: Comparison with In Vivo 4D Flow MRI
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2019 (English)In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 47, no 2, p. 413-424Article in journal (Refereed) Published
Abstract [en]

Blood flow simulations are making their way into the clinic, and much attention is given to estimation of fractional flow reserve in coronary arteries. Intracardiac blood flow simulations also show promising results, and here the flow field is expected to depend on the pulmonary venous (PV) flow rates. In the absence of in vivo measurements, the distribution of the flow from the individual PVs is often unknown and typically assumed. Here, we performed intracardiac blood flow simulations based on time-resolved computed tomography on three patients, and investigated the effect of the distribution of PV flow rate on the flow field in the left atrium and ventricle. A design-of-experiment approach was used, where PV flow rates were varied in a systematic manner. In total 20 different simulations were performed per patient, and compared to in vivo 4D flow MRI measurements. Results were quantified by kinetic energy, mitral valve velocity profiles and root-mean-square errors of velocity. While large differences in atrial flow were found for varying PV inflow distributions, the effect on ventricular flow was negligible, due to a regularizing effect by mitral valve. Equal flow rate through all PVs most closely resembled in vivo measurements and is recommended in the absence of a priori knowledge.

Place, publisher, year, edition, pages
Springer-Verlag New York, 2019
Keywords
Sensitivity analysis, Design-of-experiments, Computational fluid dynamics, In vivo measurements
National Category
Medical Image Processing Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-153252 (URN)10.1007/s10439-018-02153-5 (DOI)000456383600007 ()30362080 (PubMedID)2-s2.0-85055724908 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation
Available from: 2018-12-06 Created: 2018-12-06 Last updated: 2021-10-13Bibliographically approved
Berntsson, F., Karlsson, M., Kozlov, V. & Nazarov, S. A. (2018). A Modification to the Kirchhoff Conditions at a Bifurcation and Loss Coefficients.
Open this publication in new window or tab >>A Modification to the Kirchhoff Conditions at a Bifurcation and Loss Coefficients
2018 (English)Report (Other academic)
Abstract [en]

One dimensional models for fluid flow in tubes are frequently used tomodel complex systems, such as the arterial tree where a large numberof vessels are linked together at bifurcations. At the junctions transmission conditions are needed. One popular option is the classic Kirchhoffconditions which means conservation of mass at the bifurcation andprescribes a continuous pressure at the joint.

In reality the boundary layer phenomena predicts fast local changesto both velocity and pressure inside the bifurcation. Thus it is not appropriate for a one dimensional model to assume a continuous pressure. In this work we present a modification to the classic Kirchhoff condi-tions, with a symmetric pressure drop matrix, that is more suitable forone dimensional flow models. An asymptotic analysis, that has beencarried out previously shows that the new transmission conditions hasen exponentially small error.

The modified transmission conditions take the geometry of the bifurcation into account and can treat two outlets differently. The conditions can also be written in a form that is suitable for implementationin a finite difference solver. Also, by appropriate choice of the pressuredrop matrix we show that the new transmission conditions can producehead loss coefficients similar to experimentally obtained ones.

Publisher
p. 11
Series
LiTH-MAT-R, ISSN 0348-2960 ; 2018:5
National Category
Mathematics
Identifiers
urn:nbn:se:liu:diva-147718 (URN)LiTH-MAT-R--2018/05--SE (ISRN)
Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-05-07Bibliographically approved
Gupta, V., Lantz, J., Henriksson, L., Engvall, J., Karlsson, M., Persson, A. & Ebbers, T. (2018). Automated three-dimensional tracking of the left ventricular myocardium in time-resolved and dose-modulated cardiac CT images using deformable image registration. Journal of Cardiovascular Computed Tomography, 12(2), 139-148
Open this publication in new window or tab >>Automated three-dimensional tracking of the left ventricular myocardium in time-resolved and dose-modulated cardiac CT images using deformable image registration
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2018 (English)In: Journal of Cardiovascular Computed Tomography, ISSN 1934-5925, Vol. 12, no 2, p. 139-148Article in journal (Refereed) Published
Abstract [en]

Background Assessment of myocardial deformation from time-resolved cardiac computed tomography (4D CT) would augment the already available functional information from such an examination without incurring any additional costs. A deformable image registration (DIR) based approach is proposed to allow fast and automatic myocardial tracking in clinical 4D CT images.

Methods Left ventricular myocardial tissue displacement through a cardiac cycle was tracked using a B-spline transformation based DIR. Gradient of such displacements allowed Lagrangian strain estimation with respect to end-diastole in clinical 4D CT data from ten subjects with suspected coronary artery disease. Dice similarity coefficient (DSC), point-to-curve error (PTC), and tracking error were used to assess the tracking accuracy. Wilcoxon signed rank test provided significance of tracking errors. Topology preservation was verified using Jacobian of the deformation. Reliability of estimated strains and torsion (normalized twist angle) was tested in subjects with normal function by comparing them with normal strain in the literature.

Results Comparison with manual tracking showed high accuracy (DSC: 0.99± 0.05; PTC: 0.56mm± 0.47 mm) and resulted in determinant(Jacobian) > 0 for all subjects, indicating preservation of topology. Average radial (0.13 mm), angular (0.64) and longitudinal (0.10 mm) tracking errors for the entire cohort were not significant (p > 0.9). For patients with normal function, average strain [circumferential, radial, longitudinal] and peak torsion estimates were: [-23.5%, 31.1%, −17.2%] and 7.22°, respectively. These estimates were in conformity with the reported normal ranges in the existing literature.

Conclusions Accurate wall deformation tracking and subsequent strain estimation are feasible with the proposed method using only routine time-resolved 3D cardiac CT.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Cardiac computed tomography; 4D CT; Image registration; Strain analysis; Myocardial deformation; Torsion
National Category
Medical Image Processing
Identifiers
urn:nbn:se:liu:diva-147433 (URN)10.1016/j.jcct.2018.01.005 (DOI)000428247900008 ()29402736 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation, KAW 2013.0076
Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2021-12-28Bibliographically approved
Lantz, J., Gupta, V., Henriksson, L., Karlsson, M., Persson, A., Carlhäll, C. & Ebbers, T. (2018). Intracardiac Flow at 4D CT: Comparison with 4D Flow MRI. Radiology, 289(1), 51-58
Open this publication in new window or tab >>Intracardiac Flow at 4D CT: Comparison with 4D Flow MRI
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2018 (English)In: Radiology, ISSN 0033-8419, E-ISSN 1527-1315, Vol. 289, no 1, p. 51-58Article in journal (Refereed) Published
Abstract [en]

Purpose

To investigate four-dimensional (4D) flow CT for the assessment of intracardiac blood flow patterns as compared with 4D flow MRI.

Materials and Methods

This prospective study acquired coronary CT angiography and 4D flow MRI data between February and December 2016 in a cohort of 12 participants (age range, 36–74 years; mean age, 57 years; seven men [age range, 36–74 years; mean age, 57 years] and five women [age range, 52–73 years; mean age, 64 years]). Flow simulations based solely on CT-derived cardiac anatomy were assessed together with 4D flow MRI measurements. Flow patterns, flow rates, stroke volume, kinetic energy, and flow components were quantified for both techniques and were compared by using linear regression.

Results

Cardiac flow patterns obtained by using 4D flow CT were qualitatively similar to 4D flow MRI measurements, as graded by three independent observers. The Cohen κ score was used to assess intraobserver variability (0.83, 0.79, and 0.70) and a paired Wilcoxon rank-sum test showed no significant change (P > .05) between gradings. Peak flow rate and stroke volumes between 4D flow MRI measurements and 4D flow CT measurements had high correlation (r = 0.98 and r = 0.81, respectively; P < .05 for both). Integrated kinetic energy quantified at peak systole correlated well (r = 0.95, P < .05), while kinetic energy levels at early and late filling showed no correlation. Flow component analysis showed high correlation for the direct and residual components, respectively (r = 0.93, P < .05 and r = 0.87, P < .05), while the retained and delayed components showed no correlation.

Conclusion

Four-dimensional flow CT produced qualitatively and quantitatively similar intracardiac blood flow patterns compared with the current reference standard, four-dimensional flow MRI.

Place, publisher, year, edition, pages
Oak Brook, IL United States: Radiological Society of North America, Inc., 2018
National Category
Fluid Mechanics and Acoustics Cardiac and Cardiovascular Systems Medical Image Processing
Identifiers
urn:nbn:se:liu:diva-149320 (URN)10.1148/radiol.2018173017 (DOI)000444990900009 ()29944089 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation, Seeing Organ FunctionSwedish Heart Lung Foundation
Available from: 2018-06-28 Created: 2018-06-28 Last updated: 2021-10-13Bibliographically approved
Johannes, E., Ekman, P., Huge-Brodin, M. & Karlsson, M. (2018). Sustainable Timber Transport: Economic Aspects of Aerodynamic Reconfiguration. Sustainability, 10(6), 1-18
Open this publication in new window or tab >>Sustainable Timber Transport: Economic Aspects of Aerodynamic Reconfiguration
2018 (English)In: Sustainability, E-ISSN 2071-1050, Vol. 10, no 6, p. 1-18Article in journal (Refereed) Published
Abstract [en]

There is a need to reduce fuel consumption, and thereby reduce CO2-emissions in all parts of the transport sector. It is also well known that aerodynamic resistance affects the fuel consumption in a major way. By improving the aerodynamics of the vehicles, the fuel consumption will also decrease. A special type of transportation is that of timber, which is performed by specialized trucks with few alternative uses. This paper follows up on earlier papers concerning Swedish timber trucks where aerodynamic improvements for timber trucks were tested. By mapping the entire fleet of timber trucks in Sweden and investigating reduced fuel consumption of 2–10%, financial calculations were performed on how these improvements would affect the transport costs. Certain parameters are investigated, such as investment cost, extra changeover time and weight of installments. By combining these results with the mapping of the fleet, it can be seen under which circumstances these improvements would be sustainable. The results show that it is possible through aerodynamics to lower the transportation costs and make an investment plausible, with changeover time being the most important parameter. They also show that certain criteria for a reduced transportation cost already exist within the vehicle fleet today.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
timber trucks; fuel consumption; aerodynamic design; financial consequences
National Category
Business Administration Fluid Mechanics and Acoustics Transport Systems and Logistics
Identifiers
urn:nbn:se:liu:diva-150163 (URN)10.3390/su10061965 (DOI)000436570100277 ()2-s2.0-85048336587 (Scopus ID)
Note

Funding agencies: Energimyndigheten (the Swedish Energy Agency)

Available from: 2018-08-14 Created: 2018-08-14 Last updated: 2022-02-10Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-5526-2399

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