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Publications (9 of 9) Show all publications
Stylianou, F., Pečnik, R. & Kassinos, S. C. (2016). Analyzing a turbulent pipe flow via the one-point structure tensors: Vorticity crawlers and streak shadows. Computers & Fluids, 140, 450-477
Open this publication in new window or tab >>Analyzing a turbulent pipe flow via the one-point structure tensors: Vorticity crawlers and streak shadows
2016 (English)In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 140, p. 450-477Article in journal (Refereed) Published
Abstract [en]

Efforts to identify and visualize near-wall structures typically focus on the region y+≳5, where large-scale structures with significant turbulent kinetic energy content reside, such as the high-speed and low-speed streaks associated with sweep and ejection events. While it is true that the level of the turbulent kinetic energy drops to zero as one approaches the wall, the organization of near-wall turbulence does not end at y+≈5. Large-scale structures with significant streamwise extent and spatial organization exist even in the immediate proximity of the wall y+<5. These coherent structures have received less attention so far, but it would be both useful and enlightening to bring them to focus in order, on one hand, to understand them, but also to analyze their interaction with the energetic structures that reside at somewhat higher distances from the wall. We have recently developed a rigorous mathematical and computational framework that can be used for the calculation of the turbulence structure tensors in arbitrary flow configurations. In this work, we use this new framework to compute, for the first time, the structure tensors in a fully-developed turbulent pipe flow. We perform Direct Numerical Simulation (DNS) at Reynolds number Reb=5300, based on the bulk velocity and the pipe diameter. We demonstrate the diagnostic properties of the structure tensors, by analyzing the DNS results with a focus on the near-wall structure of the turbulence. We develop a new eduction technique, based on the instantaneous values of the structure tensors, for the identification of inactive structures (i.e. large-scale structures without significant turbulent kinetic energy). This leads to the visualization of “vorticity crawlers” and “streak shadows”, large-scale structures with low energy content in the extreme vicinity of the wall. Furthermore, comparison with traditional eduction techniques (such as instantaneous iso-surfaces of turbulent kinetic energy) shows that the structure-based eduction method seamlessly captures the large-scale energetic structures further away from the wall. We then show that the one-point structure tensors reflect the morphology of the inactive structures in the extreme vicinity of the wall and that of the energy-containing large-scale structures further away from the wall. The emerging complete picture of large-scale structures helps explain the near-wall profiles of all the one-point structure tensors and is likely to have an impact in the further development of Structure-Based Models (SBMs) of turbulence

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-63740 (URN)10.1016/j.compfluid.2016.10.010 (DOI)2-s2.0-84994056892 (Scopus ID)
Available from: 2017-06-05 Created: 2017-06-05 Last updated: 2017-11-24Bibliographically approved
Stylianou, F., Sznitman, J. & Kassinos, S. C. (2016). Direct numerical simulation of particle laden flow in a human airway bifurcation model. International Journal of Heat and Fluid Flow, 61, 677-710
Open this publication in new window or tab >>Direct numerical simulation of particle laden flow in a human airway bifurcation model
2016 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 61, p. 677-710Article in journal (Refereed) Published
Abstract [en]

During the delivery of inhaled medicines, and depending on the size distribution of the particles in the formulation, airway bifurcations are areas of preferential deposition. Previous studies of laminar flow through airway bifurcations point to an interplay of inertial and centrifugal forces that leads to rich flow phenomena and controls particle deposition patterns. However, recent computational studies have shown that the airflow in the upper human airways is turbulent during much of the respiratory cycle. The question of how the presence of turbulence modifies these effects remains open. In this study, we perform for the first time Direct Numerical Simulations (DNS) of fully developed turbulent flow through a single human airway bifurcation model, emulating steady prolonged inspiration and expiration. We use the rich information obtained from the DNS in order to identify key structures in the flow field and scrutinize their role in determining deposition patterns in the bifurcation. We find that the vortical structures present in the bifurcation during expiration differ from those identified during inspiration. While Dean vortices are present in both cases, a set of three dimensional “carinal vortices” are identified only during expiration. A set of laminar simulations in the same geometries, but at lower Reynolds numbers, allow us to identify key differences in aerosol deposition patterns between laminar and turbulent respiration. We also report deposition fractions for representative Stokes numbers for both laminar and turbulent conditions. Given the suspected role of external mechanical stress on the airway epithelium in determining mucus clearance and chronic disease development, here we report wall shear stress distributions for both the turbulent and laminar cases. Finally, we also perform Large Eddy Simulations (LES) and Reynolds-Averaged Navier-Stokes (RANS) simulations for the same configuration in order to asses their performance as compared to DNS. We find that LES and RANS perform well and that they are able to capture the key characteristics of the flow field. The agreement between DNS and RANS holds true only for the mean flow field, which is primarily influenced by curvature effects

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-64101 (URN)10.1016/j.ijheatfluidflow.2016.07.013 (DOI)2-s2.0-84995486684 (Scopus ID)
Available from: 2017-06-16 Created: 2017-06-16 Last updated: 2018-06-14Bibliographically approved
Stylianou, F., Pečnik, R. & Kassinos, S. C. (2015). A general framework for computing the turbulence structure tensors. Computers & Fluids, 106, 54-66
Open this publication in new window or tab >>A general framework for computing the turbulence structure tensors
2015 (English)In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 106, p. 54-66Article in journal (Refereed) Published
Abstract [en]

Good measures of the turbulence structure are important for turbulence modeling, flow diagnostics and analysis. Structure information is complementary to the componentality anisotropy that the Reynolds stress tensor carries, and because structures extend in space, structure information is inherently non-local. Given access to instantaneous snapshots of a turbulence field or two-point statistical correlations, one can extract the structural features of the turbulence. However, this process tends to be computationally expensive and cumbersome. Therefore, one-point statistical measures of the structural characteristics of turbulence are desirable. The turbulence structure tensors are one-point statistical descriptors of the non-local characteristics of the turbulence structure and form the mathematical framework for constructing Structure-Based Models (SBM) of turbulence. Despite the promise held by SBM, the tensors have so far been available only in a small number of DNS databases of rather simple canonical flows. This inhibits further SBM development and discourages the use of the tensors for flow analysis and diagnostics. The lack of a clear numerical recipe for computing the tensors in complex domains is one the reasons for the scarce reporting of the structure tensors in DNS databases. In particular, the imposition of proper boundary conditions in complex geometries is non-trivial. In this work, we provide for the first a time a rigorous and well-documented description of a mathematical and computational framework that can be used for the calculation of the structure tensors in arbitrary turbulent flow configurations

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-64105 (URN)10.1016/j.compfluid.2014.09.042 (DOI)2-s2.0-84908587177 (Scopus ID)
Available from: 2017-06-16 Created: 2017-06-16 Last updated: 2017-11-24Bibliographically approved
Alexandrou, C., Constantinou, M., Korzec, T., Panagopoulos, H. G. & Stylianou, F. (2012). Renormalization constants of local operators for Wilson type improved fermions. Physical Review D. Particles and fields, 86(1), Article ID 014505.
Open this publication in new window or tab >>Renormalization constants of local operators for Wilson type improved fermions
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2012 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 86, no 1, article id 014505Article in journal (Refereed) Published
Abstract [en]

Perturbative and nonperturbative results are presented on the renormalization constants of the quark field and the vector, axial-vector, pseudoscalar, scalar, and tensor currents. The perturbative computation, carried out at one-loop level and up to second order in the lattice spacing, is performed for a fermion action, which includes the clover term and the twisted mass parameter yielding results that are applicable for unimproved Wilson fermions, as well as for improved clover and twisted mass fermions. We consider ten variants of the Symanzik improved gauge action corresponding to ten different values of the plaquette coefficients. Nonperturbative results are obtained using the twisted mass Wilson fermion formulation employing two degenerate dynamical quarks and the tree-level Symanzik improved gluon action. The simulations are performed for pion masses in the range of 480-260 MeV and at three values of the lattice spacing, a, corresponding to β=3.9, 4.05, 4.20. For each renormalization factor computed nonperturbatively we subtract its perturbative O(a2) terms so that we eliminate part of the cutoff artifacts. The renormalization constants are converted to MS̄ at a scale of μ=2GeV. The perturbative results depend on a large number of parameters and are made easily accessible to the reader by including them in the distribution package of this paper, as a Mathematica input file.

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-64573 (URN)10.1103/PhysRevD.86.014505 (DOI)
Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2017-11-29Bibliographically approved
Constantinou, M., Dimopoulos, P., Frezzotti, R., Jensen, K., Gimenez, V. C., Lubicz, V., . . . Vladikas, A. (2011). BK-parameter from Nf=2 twisted mass lattice QCD. Physical Review D. Particles and fields, 83(1), Article ID 014505.
Open this publication in new window or tab >>BK-parameter from Nf=2 twisted mass lattice QCD
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2011 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 83, no 1, article id 014505Article in journal (Refereed) Published
Abstract [en]

We present an unquenched Nf=2 lattice computation of the B K parameter which controls K0-K̄0 oscillations. A partially quenched setup is employed with two maximally twisted dynamical (sea) light Wilson quarks, and valence quarks of both the maximally twisted and the Osterwalder-Seiler variety. Suitable combinations of these two kinds of valence quarks lead to a lattice definition of the BK parameter which is both multiplicatively renormalizable and O(a) improved. Employing the nonperturbative RI-MOM scheme, in the continuum limit and at the physical value of the pion mass we get BKRGI=0.729±0.030, a number well in line with the existing quenched and unquenched determinations.

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-65219 (URN)10.1103/PhysRevD.83.014505 (DOI)2-s2.0-79551511472 (Scopus ID)
Available from: 2017-08-21 Created: 2017-08-21 Last updated: 2017-11-29Bibliographically approved
Constantinou, M., Dimopoulos, P., Frezzotti, R., Lubicz, V., Panagopoulos, H. G., Skouroupathis, A. & Stylianou, F. (2011). Perturbative renormalization factors and Oða2Þ corrections for latticefour-fermion operators with improved fermion/gluon actions. Physical Review D. Particles and fields, 83(7), Article ID 074503.
Open this publication in new window or tab >>Perturbative renormalization factors and Oða2Þ corrections for latticefour-fermion operators with improved fermion/gluon actions
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2011 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 83, no 7, article id 074503Article in journal (Refereed) Published
Abstract [en]

In this work we calculate the corrections to the amputated Green's functions of four-fermion operators, in 1-loop lattice perturbation theory. One of the novel aspects of our calculations is that they are carried out to second order in the lattice spacing, O(a2). We employ the Wilson/clover action for massless fermions (also applicable for the twisted mass action in the chiral limit) and a family of Symanzik improved actions for gluons. Our calculations have been carried out in a general covariant gauge. Results have been obtained for several popular choices of values for the Symanzik coefficients (Plaquette, Tree-level Symanzik, Iwasaki, TILW and DBW2 action). While our Green's function calculations regard any pointlike four-fermion operators which do not mix with lower dimension ones, we pay particular attention to ΔF=2 operators, both parity conserving and parity violating (F stands for flavor: S, C, B). By appropriately projecting those bare Green's functions we compute the perturbative renormalization constants for a complete basis of four-fermion operators and we study their mixing pattern. For some of the actions considered here, even O(a0) results did not exist in the literature to date. The correction terms which we calculate (along with our previous O(a2) calculation of Z Ψ) are essential ingredients for minimizing the lattice artifacts which are present in nonperturbative evaluations of renormalization constants with the RI-MOM method. Our perturbative results, for the matrix elements of ΔF=2 operators and for the corresponding renormalization matrices, depend on a large number of parameters: coupling constant, number of colors, lattice spacing, external momentum, clover parameter, Symanzik coefficients, gauge parameter. To make these results most easily accessible to the reader, we have included them in the distribution package of this paper, as an ASCII file named: 4-fermi.m; the file is best perused as Mathematica input. The main results of this work have been applied to improve nonperturbative estimates of the BK-parameter in N F=2 twisted mass lattice QCD

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-64575 (URN)10.1103/PhysRevD.83.074503 (DOI)2-s2.0-79960770124 (Scopus ID)
Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2017-11-29Bibliographically approved
Alexandrou, C., Constantinou, M., Korzec, T., Panagopoulos, H. G. & Stylianou, F. (2011). Renormalization constants for 2-twist operators in twisted mass QCD. Physical Review D. Particles and fields, 83(1), Article ID 014503.
Open this publication in new window or tab >>Renormalization constants for 2-twist operators in twisted mass QCD
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2011 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 83, no 1, article id 014503Article in journal (Refereed) Published
Abstract [en]

Perturbative and nonperturbative results on the renormalization constants of the fermion field and the twist-2 fermion bilinears are presented with emphasis on the nonperturbative evaluation of the one-derivative twist-2 vector and axial-vector operators. Nonperturbative results are obtained using the twisted mass Wilson fermion formulation employing two degenerate dynamical quarks and the tree-level Symanzik improved gluon action. The simulations have been performed for pion masses in the range of about 450-260 MeV and at three values of the lattice spacing a corresponding to β=3.9, 4.05, 4.20. Subtraction of O(a2) terms is carried out by performing the perturbative evaluation of these operators at 1-loop and up to O(a2). The renormalization conditions are defined in the RI⊃′-MOM scheme, for both perturbative and nonperturbative results. The renormalization factors, obtained for different values of the renormalization scale, are evolved perturbatively to a reference scale set by the inverse of the lattice spacing. In addition, they are translated to MS̄ at 2 GeV using 3-loop perturbative results for the conversion factors

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-65223 (URN)10.1103/PhysRevD.83.014503 (DOI)2-s2.0-79551522973 (Scopus ID)
Available from: 2017-08-21 Created: 2017-08-21 Last updated: 2017-11-29Bibliographically approved
Constantinou, M., Dimopoulos, P., Frezzotti, R., Herdoisa, G., Jensen, K., Lubicz, V., . . . Vladikas, A. (2010). Non-perturbative renormalization of quark bilinear operators with N f = 2 (tmQCD) Wilson fermions and the tree-level improved gauge action. Journal of High Energy Physics (JHEP), 8, Article ID 068.
Open this publication in new window or tab >>Non-perturbative renormalization of quark bilinear operators with N f = 2 (tmQCD) Wilson fermions and the tree-level improved gauge action
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2010 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, Vol. 8, article id 068Article in journal (Refereed) Published
Abstract [en]

We present results for the renormalization constants of bilinear quark operators obtained by using the tree-level Symanzik improved gauge action and the N f = 2 twisted mass fermion action at maximal twist, which guarantees automatic O(a)-improvement. Our results are also relevant for the corresponding standard (un-twisted) Wilson fermionic action since the two actions only differ, in the massless limit, by a chiral rotation of the quark fields. The scale-independent renormalization constants Z V, Z A and the ratio Z P /Z S have been computed using the RI-MOMapproach, as well as other alternative methods. For Z A and Z P /Z S, the latter are based on both standard twisted mass and Osterwalder-Seiler fermions, while for Z V a Ward Identity has been used. The quark field renormalization constant Z q and the scale dependent renormalization constants Z S, Z P and Z T are determined in the RI-MOM scheme. Leading discretization effects of O(g 2a 2), evaluated in one-loop perturbation theory, are explicitly subtracted from the RI-MOM estimates

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-65426 (URN)10.1007/JHEP08(2010)068 (DOI)2-s2.0-84856381757 (Scopus ID)
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2017-11-24Bibliographically approved
Constantinou, M., Lubicz, V., Panagopoulos, H. G. & Stylianou, F. (2009). (a 2) corrections to the one-loop propagator and bilinears of clover fermions with Symanzik improved gluons. Journal of High Energy Physics (JHEP), 10, Article ID 064.
Open this publication in new window or tab >>(a 2) corrections to the one-loop propagator and bilinears of clover fermions with Symanzik improved gluons
2009 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, Vol. 10, article id 064Article in journal (Refereed) Published
Abstract [en]

We calculate corrections to the fermion propagator and to the Green's functions of all fermion bilinear operators of the form , to one-loop in perturbation theory. We employ the Wilson/clover action for fermions and the Symanzik improved action for gluons. The novel aspect of our calculations is that they are carried out to second order in the lattice spacing, (a 2). Consequently, they have addressed a number of new issues, most notably the appearance of loop integrands with strong IR divergences (convergent only beyond 6 dimensions). Such integrands are not present in (a 1) improvement calculations; there, IR divergent terms are seen to have the same structure as in the (a 0) case, by virtue of parity under integration, and they can thus be handled by well-known techniques. We explain how to correctly extract the full (a 2) dependence; in fact, our method is generalizable to any order in a. The (a 2) corrections to the quark propagator and Green's functions computed in this paper are useful to improve the nonperturbative RI-MOM determination of renormalization constants for quark bilinear operators. Our results depend on a large number of parameters: coupling constant, number of colors, lattice spacing, external momentum, clover parameter, Symanzik coefficients, gauge parameter. To make these results most easily accessible to the reader, we have included them in the distribution package of this paper, as an ASCII file named: Oa2results.m; the file is best perused as Mathematica input

National Category
Other Health Sciences
Research subject
Health Science
Identifiers
urn:nbn:se:ltu:diva-65427 (URN)10.1088/1126-6708/2009/10/064 (DOI)2-s2.0-70350759819 (Scopus ID)
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2017-11-24Bibliographically approved
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-2500-9883

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