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- Biasetti, Jacopo, et al.
(författare)
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Blood flow and coherent vortices in the normal and aneurysmatic aortas : a fluid dynamical approach to intraluminal thrombus formation
- 2011
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Ingår i: Journal of the Royal Society Interface. - : The Royal Society. - 1742-5689 .- 1742-5662. ; 8:63, s. 1449-1461
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Tidskriftsartikel (refereegranskat)abstract
- Abdominal aortic aneurysms (AAAs) are frequently characterized by the development of an intra-luminal thrombus (ILT), which is known to have multiple biochemical and biomechanical implications. Development of the ILT is not well understood, and shear-stress-triggered activation of platelets could be the first step in its evolution. Vortical structures (VSs) in the flow affect platelet dynamics, which motivated the present study of a possible correlation between VS and ILT formation in AAAs. VSs educed by the lambda(2)-method using computational fluid dynamics simulations of the backward-facing step problem, normal aorta, fusiform AAA and saccular AAA were investigated. Patient-specific luminal geometries were reconstructed from computed tomography scans, and Newtonian and Carreau-Yasuda models were used to capture salient rheological features of blood flow. Particularly in complex flow domains, results depended on the constitutive model. VSs developed all along the normal aorta, showing that a clear correlation between VSs and high wall shear stress (WSS) existed, and that VSs started to break up during late systole. In contrast, in the fusiform AAA, large VSs developed at sites of tortuous geometry and high WSS, occupying the entire lumen, and lasting over the entire cardiac cycle. Downward motion of VSs in the AAA was in the range of a few centimetres per cardiac cycle, and with a VS burst at that location, the release (from VSs) of shear-stress-activated platelets and their deposition to the wall was within the lower part of the diseased artery, i.e. where the thickest ILT layer is typically observed. In the saccular AAA, only one VS was found near the healthy portion of the aorta, while in the aneurysmatic bulge, no VSs occurred. We present a fluid-dynamics-motivated mechanism for platelet activation, convection and deposition in AAAs that has the potential of improving our current understanding of the pathophysiology of fluid-driven ILT growth.
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| 5. |
- Rezaeiravesh, Saleh, et al.
(författare)
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Estimating Uncertainty of Low- and High-Order Turbulence Statistics in Wall Turbulence
- 2022
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Ingår i: 12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP.
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Konferensbidrag (refereegranskat)abstract
- A framework is introduced for accurate estimation of time-average uncertainties in various types of turbulence statistics. A thorough set of guidelines is provided to adjust the different hyperparameters for estimating uncertainty in sample mean estimators (SMEs). For high-order turbulence statistics, a novel approach is proposed which avoids any linearization and preserves all relevant temporal and spatial correlations and cross-covariances between SMEs. This approach is able to accurately estimate uncertainties in any arbitrary statistical moment. The usability of the approach is demonstrated by applying it to data from direct numerical simulation (DNS) of the turbulent flow over a periodic hill and through a straight circular pipe.
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| 6. |
- Schlatter, Philipp, et al.
(författare)
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On the vortical structures of a turbulent boundary layer at high Reynolds number
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A recent data base from direct numerical simulation of a turbulent boundary layer up to Reθ = 4300 [Schlatter & Örlü, J. Fluid Mech. 659, 2010] has been analysed in an effort to educe the dominant flow structures populating the near-wall region. In particular, the question of whether hairpin vortices are indeed observable as a dominant building block of near-wall turbulence is addressed. It is shown that during the initial phase, dominanted by the specific laminar-turbulent transition induced via the tripping mechanism, hairpin vortices are very numerous, and can certainly be considered as the dominant structure. This is in agreement with previous experiments and low Reynolds number simulations such as Wu & Moin [J. Fluid Mech. 630, 2009]. At sufficient distance from transition, the flow is dominated by a staggered array of quasi-streamwise vortices which is the same situation as in previous channel flows. It turns out that even quantitatively, no major differences between boundary layers and channels can be detected; structures are about 200 viscous units in length, and inclined by about 9 degrees [Jeong et al., J. Fluid Mech. 332, 1997]. The present results clearly show that the regeneration process of turbulence does not involve the generation of (symmetric) hairpin vortices, and that their dominant appearance as instantaneous flow structures in the outer boundary-layer region is at least very unlikely.
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| 7. |
- Vincent, Jonathan, et al.
(författare)
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Strong Scaling of OpenACC enabled Nek5000 on several GPU based HPC systems
- 2022
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Ingår i: HPCAsia2022. - New York, NY, USA : Association for Computing Machinery (ACM). ; , s. 94-102
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Konferensbidrag (refereegranskat)abstract
- We present new results on the strong parallel scaling for the OpenACC-accelerated implementation of the high-order spectral element fluid dynamics solver Nek5000. The test case considered consists of a direct numerical simulation of fully-developed turbulent flow in a straight pipe, at two different Reynolds numbers Reτ = 360 and Reτ = 550, based on friction velocity and pipe radius. The strong scaling is tested on several GPU-enabled HPC systems, including the Swiss Piz Daint system, TACC's Longhorn, Jülich's JUWELS Booster, and Berzelius in Sweden. The performance results show that speed-up between 3-5 can be achieved using the GPU accelerated version compared with the CPU version on these different systems. The run-time for 20 timesteps reduces from 43.5 to 13.2 seconds with increasing the number of GPUs from 64 to 512 for Reτ = 550 case on JUWELS Booster system. This illustrates the GPU accelerated version the potential for high throughput. At the same time, the strong scaling limit is significantly larger for GPUs, at about 2000 - 5000 elements per rank; compared to about 50 - 100 for a CPU-rank.
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| 8. |
- Yao, Jie, et al.
(författare)
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Direct numerical simulations of turbulent pipe flow up to Re-tau approximate to 5200
- 2023
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 956
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Tidskriftsartikel (refereegranskat)abstract
- Well-resolved direct numerical simulations (DNS) have been performed of the flow in a smooth circular pipe of radius R and axial length 10 pi R at friction Reynolds numbers up to Re-tau = 5200 using the pseudo-spectral code OPENPIPEFLOW. Various turbulence statistics are documented and compared with other DNS and experimental data in pipes as well as channels. Small but distinct differences between various datasets are identified. The friction factor lambda overshoots by 2% and undershoots by 0.6% the Prandtl friction law at low and high Re ranges, respectively. In addition,. in our results is slightly higher than in Pirozzoli et al. (J. Fluid Mech., vol. 926, 2021, A28), but matches well the experiments in Furuichi et al. (Phys. Fluids, vol. 27, issue 9, 2015, 095108). The log-law indicator function, which is nearly indistinguishable between pipe and channel up to y(+) = 250, has not yet developed a plateau farther away from the wall in the pipes even for the Re-tau = 5200 cases. The wall shear stress fluctuations and the inner peak of the axial turbulence intensity - which grow monotonically with Re-tau - are lower in the pipe than in the channel, but the difference decreases with increasing Re-tau. While the wall value is slightly lower in the channel than in the pipe at the same Re-tau, the inner peak of the pressure fluctuation shows negligible differences between them. The Reynolds number scaling of all these quantities agrees with both the logarithmic and defect-power laws if the coefficients are properly chosen. The one-dimensional spectrum of the axial velocity fluctuation exhibits a k(-1) dependence at an intermediate distance from the wall - also seen in the channel. In summary, these high-fidelity data enable us to provide better insights into the flow physics in the pipes as well as the similarity/difference among different types of wall turbulence.
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