| 1. |
- Bhatnagar, Akshay, et al.
(författare)
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Heavy inertial particles in turbulent flows gain energy slowly but lose it rapidly
- 2018
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Ingår i: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 97:3
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Tidskriftsartikel (refereegranskat)abstract
- We present an extensive numerical study of the time irreversibility of the dynamics of heavy inertial particles in three-dimensional, statistically homogeneous, and isotropic turbulent flows. We show that the probability density function (PDF) of the increment, W(tau), of a particle's energy over a time scale tau is non-Gaussian, and skewed toward negative values. This implies that, on average, particles gain energy over a period of time that is longer than the duration over which they lose energy. We call this slow gain and fast loss. We find that the third moment of W(tau) scales as tau(3) for small values of tau. We show that the PDF of power-input p is negatively skewed too; we use this skewness Ir as a measure of the time irreversibility and we demonstrate that it increases sharply with the Stokes number St for small St; this increase slows down at St similar or equal to 1. Furthermore, we obtain the PDFs of t(+) and t(-), the times over which p has, respectively, positive or negative signs, i.e., the particle gains or loses energy. We obtain from these PDFs a direct and natural quantification of the slow gain and fast loss of the energy of the particles, because these PDFs possess exponential tails from which we infer the characteristic loss and gain times t(loss) and t(gain), respectively, and we obtain t(loss) < t(gain) for all the cases we have considered. Finally, we show that the fast loss of energy occurs with greater probability in the strain-dominated region than in the vortical one; in contrast, the slow gain in the energy of the particles is equally likely in vortical or strain-dominated regions of the flow.
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| 2. |
- Bhatnagar, Akshay, et al.
(författare)
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How long do particles spend in vortical regions in turbulent flows?
- 2016
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Ingår i: Physical Review E. - 2470-0045. ; 94:5
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Tidskriftsartikel (refereegranskat)abstract
- We obtain the probability distribution functions (PDFs) of the time that a Lagrangian tracer or a heavy inertial particle spends in vortical or strain-dominated regions of a turbulent flow, by carrying out direct numerical simulations of such particles advected by statistically steady, homogeneous, and isotropic turbulence in the forced, three-dimensional, incompressible Navier-Stokes equation. We use the two invariants, Q and R, of the velocity-gradient tensor to distinguish between vortical and strain-dominated regions of the flow and partition the Q-R plane into four different regions depending on the topology of the flow; out of these four regions two correspond to vorticity-dominated regions of the flow and two correspond to strain-dominated ones. We obtain Q and R along the trajectories of tracers and heavy inertial particles and find out the time t(pers) for which they remain in one of the four regions of the Q-R plane. We find that the PDFs of tpers display exponentially decaying tails for all four regions for tracers and heavy inertial particles. From these PDFs we extract characteristic time scales, which help us to quantify the time that such particles spend in vortical or strain-dominated regions of the flow.
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| 3. |
- De, Sadhitro, et al.
(författare)
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Dynamic multiscaling in stochastically forced Burgers turbulence
- 2023
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- We carry out a detailed study of dynamic multiscaling in the turbulent nonequilibrium, but statistically steady, state of the stochastically forced one-dimensional Burgers equation. We introduce the concept of interval collapse time, which we define as the time taken for a spatial interval, demarcated by a pair of Lagrangian tracers, to collapse at a shock. By calculating the dynamic scaling exponents of the moments of various orders of these interval collapse times, we show that (a) there is not one but an infinity of characteristic time scales and (b) the probability distribution function of the interval collapse times is non-Gaussian and has a power-law tail. Our study is based on (a) a theoretical framework that allows us to obtain dynamic-multiscaling exponents analytically, (b) extensive direct numerical simulations, and (c) a careful comparison of the results of (a) and (b). We discuss possible generalizations of our work to higher dimensions, for the stochastically forced Burgers equation, and to other compressible flows that exhibit turbulence with shocks.
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| 4. |
- De, Sadhitro, et al.
(författare)
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Uncovering the multifractality of Lagrangian pair dispersion in shock-dominated turbulence
- 2024
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- Lagrangian pair dispersion provides insights into mixing in turbulent flows. By direct numerical simulations (DNSs) we show that the statistics of pair dispersion in the randomly forced two-dimensional Burgers equation, which is a typical model of shock-dominated turbulence, is very different from its incompressible counterpart because Lagrangian particles get trapped in shocks. We develop a heuristic theoretical framework that accounts for this - a generalization of the multifractal model - whose prediction of the scaling of Lagrangian exit times agrees well with our DNS.
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| 5. |
- Donker, Erik, et al.
(författare)
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European List of Essential Medicines for Medical Education : a protocol for a modified Delphi study
- 2021
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Ingår i: BMJ Open. - : BMJ Publishing Group Ltd. - 2044-6055. ; 11:5
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Tidskriftsartikel (refereegranskat)abstract
- Introduction Junior doctors are responsible for a substantial number of prescribing errors, and final-year medical students lack sufficient prescribing knowledge and skills just before they graduate. Various national and international projects have been initiated to reform the teaching of clinical pharmacology and therapeutics (CP&T) during undergraduate medical training. However, there is as yet no list of commonly prescribed and available medicines that European doctors should be able to independently prescribe safely and effectively without direct supervision. Such a list could form the basis for a European Prescribing Exam and would harmonise European CP&T education. Therefore, the aim of this study is to reach consensus on a list of widely prescribed medicines, available in most European countries, that European junior doctors should be able to independently prescribe safely and effectively without direct supervision: the European List of Essential Medicines for Medical Education. Methods and analysis This modified Delphi study will recruit European CP&T teachers (expert group). Two Delphi rounds will be carried out to enable a list to be drawn up of medicines that are available in >= 80% of European countries, which are considered standard prescribing practice, and which junior doctors should be able to prescribe safely and effectively without supervision. Ethics and dissemination The study has been approved by the Medical Ethics Review Committee of VU University Medical Center (no. 2020.335) and by the Ethical Review Board of the Netherlands Association for Medical Education (approved project no. NVMO-ERB 2020.4.8). The European List of Essential Medicines for Medical Education will be presented at national and international conferences and will be submitted to international peer-reviewed journals. It will also be used to develop and implement the European Prescribing Exam.
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| 6. |
- Donker, Erik M., et al.
(författare)
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The European List of Key Medicines for Medical Education: A Modified Delphi Study
- 2024
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Ingår i: Clinical Pharmacology and Therapeutics. - : WILEY. - 0009-9236 .- 1532-6535. ; 115:3, s. 515-524
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Tidskriftsartikel (refereegranskat)abstract
- Rational prescribing is essential for the quality of health care. However, many final-year medical students and junior doctors lack prescribing competence to perform this task. The availability of a list of medicines that a junior doctor working in Europe should be able to independently prescribe safely and effectively without supervision could support and harmonize teaching and training in clinical pharmacology and therapeutics (CPT) in Europe. Therefore, our aim was to achieve consensus on such a list of medicines that are widely accessible in Europe. For this, we used a modified Delphi study method consisting of three parts. In part one, we created an initial list based on a literature search. In part two, a group of 64 coordinators in CPT education, selected via the Network of Teachers in Pharmacotherapy of the European Association for Clinical Pharmacology and Therapeutics, evaluated the accessibility of each medicine in his or her country, and provided a diverse group of experts willing to participate in the Delphi part. In part three, 463 experts from 24 European countries were invited to participate in a 2-round Delphi study. In total, 187 experts (40%) from 24 countries completed both rounds and evaluated 416 medicines, 98 of which were included in the final list. The top three Anatomical Therapeutic Chemical code groups were (1) cardiovascular system (n = 23), (2) anti-infective (n = 21), and (3) musculoskeletal system (n = 11). This European List of Key Medicines for Medical Education could be a starting point for country-specific lists and could be used for the training and assessment of CPT.
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| 7. |
- Shukla, Sanjay, et al.
(författare)
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Inertial particles in superfluid turbulence : Coflow and counterflow
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:1, s. 015153-
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Tidskriftsartikel (refereegranskat)abstract
- We use pseudospectral direct numerical simulations to solve the three-dimensional (3D) Hall-Vinen-Bekharevich-Khalatnikov (HVBK) model of superfluid helium. We then explore the statistical properties of inertial particles, in both coflow and counterflow superfluid turbulence (ST) in the 3D HVBK system; particle motion is governed by a generalization of the Maxey-Riley-Gatignol equations. We first characterize the anisotropy of counterflow ST by showing that there exist large vortical columns. The light particles show confined motion as they are attracted toward these columns, and they form large clusters; by contrast, heavy particles are expelled from these vortical regions. We characterize the statistics of such inertial particles in 3D HVBK ST: (1) The mean angle (SIC)(tau) between particle positions, separated by the time lag r, exhibits two different scaling regions in (a) dissipation and (b) inertial ranges, for different values of the parameters in our model; in particular, the value of (SIC)(tau), at large r, depends on the magnitude of U-ns. (2) The irreversibility of 3D HVBK turbulence is quantified by computing the statistics of energy increments for inertial particles. (3) The probability distribution function (PDF) of energy increments is of direct relevance to recent experimental studies of irreversibility in superfluid turbulence; we find, in agreement with these experiments, that, for counterflow ST, the skewness of this PDF is less pronounced than its counterparts for coflow ST or for classical fluid turbulence.
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| 8. |
- Verma, Akhilesh Kumar, et al.
(författare)
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First-passage-time problem for tracers in turbulent flows applied to virus spreading
- 2020
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- We study the spreading of viruses, such as SARS-CoV-2, by airborne aerosols, via a first-passage-time problem for Lagrangian tracers that are advected by a turbulent flow: By direct numerical simulations of the three-dimensional (3D) incompressible Navier-Stokes equation, we obtain the time t(R) at which a tracer, initially at the origin of a sphere of radius R, crosses the surface of the sphere for the first time. We obtain the probability distribution function P(R, t(R)) and show that it displays two qualitatively different behaviors: (a) for R << L-I, P(R, t(R)) has a power-law tail similar to t(R)(-alpha), with the exponent alpha = 4 and L-I the integral scale of the turbulent flow; (b) for L-I less than or similar to R, the tail of P(R, t(R)) decays exponentially. We develop models that allow us to obtain these asymptotic behaviors analytically. We show how to use P(R, t(R)) to develop social-distancing guidelines for the mitigation of the spreading of airborne aerosols with viruses such as SARS-CoV-2.
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