| 1. |
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| 2. |
- Demirkan, Ayse, et al.
(författare)
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Genome-Wide Association Study Identifies Novel Loci Associated with Circulating Phospho- and Sphingolipid Concentrations
- 2012
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Ingår i: PLoS Genetics. - : Public Library of Science (PLoS). - 1553-7390 .- 1553-7404. ; 8:2, s. e1002490-
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Tidskriftsartikel (refereegranskat)abstract
- Phospho- and sphingolipids are crucial cellular and intracellular compounds. These lipids are required for active transport, a number of enzymatic processes, membrane formation, and cell signalling. Disruption of their metabolism leads to several diseases, with diverse neurological, psychiatric, and metabolic consequences. A large number of phospholipid and sphingolipid species can be detected and measured in human plasma. We conducted a meta-analysis of five European family-based genome-wide association studies (N = 4034) on plasma levels of 24 sphingomyelins (SPM), 9 ceramides (CER), 57 phosphatidylcholines (PC), 20 lysophosphatidylcholines (LPC), 27 phosphatidylethanolamines (PE), and 16 PE-based plasmalogens (PLPE), as well as their proportions in each major class. This effort yielded 25 genome-wide significant loci for phospholipids (smallest P-value = 9.88 x 10(-204)) and 10 loci for sphingolipids (smallest P-value = 3.10 x 10(-57)). After a correction for multiple comparisons (P-value, 2.2 x 10(-9)), we observed four novel loci significantly associated with phospholipids (PAQR9, AGPAT1, PKD2L1, PDXDC1) and two with sphingolipids (PLD2 and APOE) explaining up to 3.1% of the variance. Further analysis of the top findings with respect to within class molar proportions uncovered three additional loci for phospholipids (PNLIPRP2, PCDH20, and ABDH3) suggesting their involvement in either fatty acid elongation/saturation processes or fatty acid specific turnover mechanisms. Among those, 14 loci (KCNH7, AGPAT1, PNLIPRP2, SYT9, FADS1-2-3, DLG2, APOA1, ELOVL2, CDK17, LIPC, PDXDC1, PLD2, LASS4, and APOE) mapped into the glycerophospholipid and 12 loci (ILKAP, ITGA9, AGPAT1, FADS1-2-3, APOA1, PCDH20, LIPC, PDXDC1, SGPP1, APOE, LASS4, and PLD2) to the sphingolipid pathways. In large meta-analyses, associations between FADS1-2-3 and carotid intima media thickness, AGPAT1 and type 2 diabetes, and APOA1 and coronary artery disease were observed. In conclusion, our study identified nine novel phospho- and sphingolipid loci, substantially increasing our knowledge of the genetic basis for these traits.
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| 3. |
- Ahlman, Daniel, 1978-
(författare)
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A study of turbulence and scalar mixing in a wall-jet using direct numerical simulation
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Direct numerical simulation is used to study the dynamics and mixing in a turbulent plane wall-jet. The investigation is undertaken in order to extend the knowledge base of the influence of the wall on turbulent dynamics and mixing. The mixing statistics produced can also be used to evaluate and develop models for mixing and combustion. In order to perform the simulations, a numerical code was developed. The code employs compact finite difference schemes, of high order, for spatial integration, and a low-storage Runge-Kutta method for the temporal integration. In the simulations performed the inlet based Reynolds and Mach numbers of the wall jet were Re = 2000 and M=0.5, respectively. Above the jet a constant coflow of 10% of the inlet jet velocity was applied. A passive scalar was added at the inlet of the jet, in a non-premixed manner, enabling an investigation of the wall-jet mixing as well as the dynamics. The mean development and the respective self-similarity of the inner and outer shear layers were studied. Comparisons of properties in the shear layers of different character were performed by applying inner and outer scaling. The characteristics of the wall-jet was compared to what has been observed in other canonical shear flows. In the inner part of the jet, 0 ≤ y+ ≤ 13, the wall-jet was found to closely resemble a zero pressure gradient boundary layer. The outer layer was found to resemble a free plane jet. The downstream growth rate of the scalar was approximately equal to that of the streamwise velocity, in terms of the growth rate of the half-width. The scalar fluxes in the streamwise and wall-normal direction were found to be of comparable magnitude.
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| 4. |
- Ahlman, Daniel, et al.
(författare)
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Direct numerical simulation of mixing in a plane compressible and turbulent wall jet
- 2005
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Ingår i: 4th International Symposium on Turbulence and Shear Flow Phenomena. ; , s. 1131-1136
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Konferensbidrag (refereegranskat)abstract
- Direct numerical simulation (DNS) is used to simulate the mixing of a passive scalar in a plane compressible and turbulent wall jet. The Mach number of the jet is M = 0.5 at the inlet. The downstream development of the jet is studied and compared to experimental data. Mixing in the inner and outer shear layers of the wall jet is investigated through scalar fluxes, the probability density function of the scalar concentration and the joint probability density function of the wall normal velocity fluctuation and the scalar concentration
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| 5. |
- Alvelius, Krister, et al.
(författare)
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LES computations and comparison with Kolmogorov theory for two-point pressure{velocity correlations and structure functions for globally anisotropic turbulence
- 2000
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 403, s. 23-36
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Tidskriftsartikel (refereegranskat)abstract
- A new extension of the Kolmogorov theory, for the two-point pressure–velocity correlation, is studied by LES of homogeneous turbulence with a large inertial subrange in order to capture the high Reynolds number nonlinear dynamics of the flow. Simulations of both decaying and forced anisotropic homogeneous turbulence were performed. The forcing allows the study of higher Reynolds numbers for the same number of modes compared with simulations of decaying turbulence. The forced simulations give statistically stationary turbulence, with a substantial inertial subrange, well suited to test the Kolmogorov theory for turbulence that is locally isotropic but has significant anisotropy of the total energy distribution. This has been investigated in the recent theoretical studies of Lindborg (1996) and Hill (1997) where the role of the pressure terms was given particular attention. On the surface the two somewhat different approaches taken in these two studies may seem to lead to contradictory conclusions, but are here reconciled and (numerically) shown to yield an interesting extension of the traditional Kolmogorov theory. The results from the simulations indeed show that the two-point pressure–velocity correlation closely adheres to the predicted linear relation in the inertial subrange where also the pressure-related term in the general Kolmogorov equation is shown to vanish. Also, second- and third-order structure functions are shown to exhibit the expected dependences on separation.
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| 6. |
- Aronson, Dag, 1959, et al.
(författare)
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A Shear-Free Turbulent Boundary Layer - Experiments and Modeling
- 1997
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Ingår i: Journal of Fluid Mechanics. ; 338, s. 363-385
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Tidskriftsartikel (refereegranskat)abstract
- The mean shear has a major influence on near-wall turbulence but there are also other important physical processes at work in the turbulence/wall interaction. In order to isolate these, a shear-free boundary layer was studied experimentally. The desired flow conditions were realized by generating decaying grid turbulence with a uniform mean velocity and passing it over a wall moving with the stream speed. It is shown that the initial response of the turbulence field can be well described by the theory of Hunt & Graham (1978). Later, where this theory ceases to give an accurate description, terms of the Reynolds stress transport (RST) equations were measured or estimated by balancing the equations. An important finding is that two different length scales are associated with the near-wall damping of the Reynolds stresses. The wall-normal velocity component is damped over a region extending roughly one macroscale out from the wall. The pressure-strain redistribution that normally would result from the Reynolds stress anisotropy in this region was found to be completely inhibited by the near-wall influence. In a thin region close to the wall the pressure-reflection effects were found to give a pressure-strain that has an effect opposite to the normally expected isotropization. This behaviour is not captured by current models.
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| 7. |
- Aronsson, D., et al.
(författare)
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Shear-free turbulence near a wall
- 1996
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 338, s. 363-385
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Tidskriftsartikel (refereegranskat)abstract
- The mean shear has a major influence on near-wall turbulence but there are also other important physical processes at work in the turbulence/wall interaction. In order to isolate these, a shear-free boundary layer was studied experimentally. The desired flow conditions were realized by generating decaying grid turbulence with a uniform mean velocity and passing it over a wall moving with the stream speed. It is shown that the initial response of the turbulence field can be well described by the theory of Hunt & Graham (1978). Later, where this theory ceases to give an accurate description, terms of the Reynolds stress transport (RST) equations were measured or estimated by balancing the equations. An important finding is that two different length scales are associated with the near-wall damping of the Reynolds stresses. The wall-normal velocity component is damped over a region extending roughly one macroscale out from the wall. The pressure–strain redistribution that normally would result from the Reynolds stress anisotropy in this region was found to be completely inhibited by the near-wall influence. In a thin region close to the wall the pressure–reflection effects were found to give a pressure–strain that has an effect opposite to the normally expected isotropization. This behaviour is not captured by current models.
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| 8. |
- Brethouwer, Geert, et al.
(författare)
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Recurrent Bursts via Linear Processes in Turbulent Environments
- 2014
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 112:14, s. 144502-
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Tidskriftsartikel (refereegranskat)abstract
- Large-scale instabilities occurring in the presence of small-scale turbulent fluctuations are frequently observed in geophysical or astrophysical contexts but are difficult to reproduce in the laboratory. Using extensive numerical simulations, we report here on intense recurrent bursts of turbulence in plane Poiseuille flow rotating about a spanwise axis. A simple model based on the linear instability of the mean flow can predict the structure and time scale of the nearly periodic and self-sustained burst cycles. Poiseuille flow is suggested as a prototype for future studies of low-dimensional dynamics embedded in strongly turbulent environments.
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| 9. |
- Brethouwer, Gert, et al.
(författare)
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Turbulence and cyclic bursts in rotating channel flow
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- DNS have been performed of turbulent channel flow with spanwise rotation and bulk Reynolds number Re up to 30000. At moderate rotation rates Ro the flow on one side of the channel is approximately laminar, or has turbulent patches or oblique turbulent-laminar patterns. Intense cyclic bursts of turbulence with long time intervals are in some cases observed at sufficiently high Re and Ro. A linear analysis indicates that the turbulence bursts are initiated by a linear instability of plane waves aligned with the rotation axis. This linear instability can develop even if parts of the flow are strongly turbulent.
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| 10. |
- Brethouwer, Gert, et al.
(författare)
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Turbulence, instabilities and heat transfer in rotating channel flow simulations
- 2012
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Ingår i: Proceedings of the International Symposium on Turbulence, Heat and Mass Transfer. - : Begell House Inc.. ; , s. 703-706
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Konferensbidrag (refereegranskat)abstract
- Direct numerical simulations of fully developed turbulent channel flow rotating about the spanwise direction are performed over a wide range of Reynolds numbers and rotation speeds. A scalar is also included to examine rotation effects on heat transfer. The Reynolds number based on the bulk mean velocity is up to 30 000, which is higher than in previous studies of rotating channel flow. The flow partly or completely relaminarizes on the stable channel side at sufficiently high rotation speeds, leading to distinct turbulent-laminar patterns in certain cases. Strong cyclic bursts of turbulence are observed at higher rotation speeds and sufficiently high Reynolds numbers. Further analysis showed that the burst are caused by plane spanwise waves with a rapidly growing amplitude, which become unstable after some time and break down into intense turbulence. Rotation strongly affects the direction of the turbulent scalar flux and significantly reduces the turbulent Prandtl number.
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