1,320 research outputs found
Mimicking a turbulent signal: sequential multiaffine processes
An efficient method for the construction of a multiaffine process, with
prescribed scaling exponents, is presented. At variance with the previous
proposals, this method is sequential and therefore it is the natural candidate
in numerical computations involving synthetic turbulence. The application to
the realization of a realistic turbulent-like signal is discussed in detail.
The method represents a first step towards the realization of a realistic
spatio-temporal turbulent field.Comment: 4 pages, 3 figures (included), RevTeX 3.
Anomalous scaling of passive scalar in turbulence and in equilibrium
We analyze multi-point correlation functions of a tracer in an incompressible
flow at scales far exceeding the scale at which fluctuations are generated
(quasi-equilibrium domain) and compare them with the correlation functions at
scales smaller than (turbulence domain). We demonstrate that the scale
invariance can be broken in the equilibrium domain and trace this breakdown to
the statistical integrals of motion (zero modes) as has been done before for
turbulence. Employing Kraichnan model of short-correlated velocity we identify
the new type of zero modes, which break scale invariance and determine an
anomalously slow decay of correlations at large scales
Multi-time, multi-scale correlation functions in turbulence and in turbulent models
A multifractal-like representation for multi-time multi-scale velocity
correlation in turbulence and dynamical turbulent models is proposed. The
importance of subleading contributions to time correlations is highlighted. The
fulfillment of the dynamical constraints due to the equations of motion is
thoroughly discussed. The prediction stemming from this representation are
tested within the framework of shell models for turbulence.Comment: 18 pages, 4 eps figure
Acceleration statistics of heavy particles in turbulence
We present the results of direct numerical simulations of heavy particle
transport in homogeneous, isotropic, fully developed turbulence, up to
resolution (). Following the trajectories of up
to 120 million particles with Stokes numbers, , in the range from 0.16 to
3.5 we are able to characterize in full detail the statistics of particle
acceleration. We show that: ({\it i}) The root-mean-squared acceleration
sharply falls off from the fluid tracer value already at quite
small Stokes numbers; ({\it ii}) At a given the normalised acceleration
increases with consistently
with the trend observed for fluid tracers; ({\it iii}) The tails of the
probability density function of the normalised acceleration
decrease with . Two concurrent mechanisms lead to the above results:
preferential concentration of particles, very effective at small , and
filtering induced by the particle response time, that takes over at larger
.Comment: 10 pages, 3 figs, 2 tables. A section with new results has been
added. Revised version accepted for pubblication on Journal of Fluid
Mechanic
Fronts in passive scalar turbulence
The evolution of scalar fields transported by turbulent flow is characterized
by the presence of fronts, which rule the small-scale statistics of scalar
fluctuations. With the aid of numerical simulations, it is shown that: isotropy
is not recovered, in the classical sense, at small scales; scaling exponents
are universal with respect to the scalar injection mechanisms; high-order
exponents saturate to a constant value; non-mature fronts dominate the
statistics of intense fluctuations. Results on the statistics inside the
plateaux, where fluctuations are weak, are also presented. Finally, we analyze
the statistics of scalar dissipation and scalar fluxes.Comment: 18 pages, 27 figure
Reliability of Early Fetal Echocardiography for Congenital Heart Disease Detection: A Preliminary Experience and Outcome Analysis of 102 Fetuses to Demonstrate the Value of a Clinical Flow-Chart Designed for At-Risk Pregnancy Management
Early fetal echocardiography (EFEC) is a fetal cardiac ultrasound analysis performed between the 12th and 16th week of pregnancy (compared with the usual 18-22 weeks). In the last 10 years, the introduction of “aneuploidy sonographic markers” in screening for cardiac defects has led to a shift from late second to end of the first trimester or beginning of the second trimester of pregnancy for specialist fetal echocardiography. In this prospective study, early obstetric screening was performed between January 2014 and October 2015, using “aneuploidy sonographic markers” following SIEOG Guidelines 2014. These parameters were then collected and strategically combined in an evaluation score to select the group of pregnancies for performing EFEC, in accordance with the American Society of Echocardiography guidelines for fetal Echocardiography. All second-level examinations were performed transabdominally using a 3D convex volumetric probe with frequency range of 4-8 MHz (Accuvix – Samsung). The outcome data included transabdominal fetal echocardiography from 18 weeks to term and after birth. Overall, 99 pregnant women in the first trimester underwent EFEC (95 singleton and 4 twin pregnancies). Specifically, 30 fetuses were evaluated for extra-cardiac anomalies evidenced by obstetric screening (30%), 25 for family history of congenital heart diseases (25%), 8 for family history of genetic-linked diseases (8%), 4 for heart diseases suspected by obstetric screening (4%) and 19 by normal screening (19%). Was detected 11 (10.7%) CHD, when EFEC detected CHD, were compared to those performed later in pregnancy (18 weeks GA-term), a high degree of diagnosis correspondence was evidenced. The higher sensitivity value of EFEC vs late-FE, in comparison with the post-natal value, coupled with the high EFEC specificity shown vs both the end points, enabled us to consider it as a really reliable diagnostic technology, at least in perienced hands. The introduction of a key combination of the more sensitive obstetric and cardiologic variables should facilitate the formulation of a possible flow-chart as a guide for CHD at-risk pregnancies
Small scale statistics of viscoelastic turbulence
The small scale statistics of homogeneous isotropic turbulence of dilute
polymer solutions is investigated by means of direct numerical simulations of a
simplified viscoelastic fluid model. It is found that polymers only partially
suppress the turbulent cascade below the Lumley scale, leaving a remnant energy
flux even for large elasticity. As a consequence, fluid acceleration in
viscoelastic flows is reduced with respect to Newtonian turbulence, whereas its
rescaled probability density is left unchanged. At large scales the velocity
field is found to be unaffected by the presence of polymers.Comment: 7 pages, 4 figure
Perceptions of and Satisfaction with Sexual and Reproductive Health Interventions in Pre-Adolescent and Adolescent Students in UE/EEA Countries: A Systematic Review
: The objective of this review is to investigate perceptions of and satisfaction with sexual and reproductive health (SRH) interventions among pre-adolescents and adolescents of all genders and ethnicities in EU/EEA countries. This systematic review was conducted in accordance with PRISMA recommendations. A systematic literature search was conducted on MEDLINE (PubMed), Cochrane Central Register of Controlled Trials (Central), CINAHL (EBSCO), and PsycINFO (EBSCO) up to March 2022 to identify all published articles that included information on perceptions and levels of satisfaction with SRH interventions. The selected studies were independently and blindly evaluated for risk of bias. Finally, only five papers were included in our review, divided according to the implemented intervention design: conducted by research groups (health or non-health professionals) or structured entirely in a peer-to-peer approach. Both types of program designs analyzed in the included studies were well accepted by students, even though satisfaction assessment methods were not standardized. Peer-to-peer conducted SRH programs or interventions with practical components (e.g., exercises, discussion) were more appreciated. We also found higher levels of satisfaction among younger participants. Future SRH educational programs should consider an assessment of participants' perceptions and satisfaction, possibly adopting standardized tools. Following a peer-to-peer structure and delivering programs early could lead to greater participant satisfaction
On the terminal velocity of sedimenting particles in a flowing fluid
The influence of an underlying carrier flow on the terminal velocity of
sedimenting particles is investigated both analytically and numerically. Our
theoretical framework works for a general class of (laminar or turbulent)
velocity fields and, by means of an ordinary perturbation expansion at small
Stokes number, leads to closed partial differential equations (PDE) whose
solutions contain all relevant information on the sedimentation process. The
set of PDE's are solved by means of direct numerical simulations for a class of
2D cellular flows (static and time dependent) and the resulting phenomenology
is analysed and discussed.Comment: 13 pages, 2 figures, submitted to JP
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