A numerical study of Richtmyer–Meshkov instability in continuously stratified fluids

Theory and calculations are presented for the evolution of Richtmyer–Meshkov instability in two-dimensional continuously stratified fluid layers. The initial acceleration and subsequent instability of the fluid layer are induced by means of an impulsive pressure distribution. The subsequent dynamics of the fluid layer are then calculated numerically using the incompressible equations of motion. Initial conditions representing single-scale perturbations and multiple-scale random perturbations are considered. It is found that the growth rates for Richtmyer–Meshkov instability of stratified fluid layers are substantially lower than those predicted by Richtmyer for a sharp fluid interface with an equivalent jump in density. A frozen field approximation for the early-time dynamics of the instability is proposed, and shown to approximate the initial behavior of the layer over a time equivalent to the traversal of several layer thicknesses. It is observed that the nonlinear development of the instability results in the formation of plumes of penetrating fluid. Late in the process, the initial momentum deposited by the impulse is primarily used in the internal mixing of the layer rather than in the overall growth of the stratified layer. At intermediate times, some evidence for the existence of scaling behavior in the width of the mixing layer of the instability is observed for the multiple-scale random perturbations, but not for the single-scale perturbations. The time variation of the layer thickness differs from the scaling derived using ideas of self-similarity due to Barenblatt [Non-Linear Dynamics and Turbulence, edited by G. I. Barenblatt, G. Ioos, and D. D. Joseph (Pitman, Boston, 1983), p. 48] even at low Atwood ratio, presumably because of the inhomogeneity and anisotropy due to the excitation of vortical plumes

A Noise-Robust Method with Smoothed \ell_1/\ell_2 Regularization for Sparse Moving-Source Mapping

The method described here performs blind deconvolution of the beamforming output in the frequency domain. To provide accurate blind deconvolution, sparsity priors are introduced with a smooth \ell_1/\ell_2 regularization term. As the mean of the noise in the power spectrum domain is dependent on its variance in the time domain, the proposed method includes a variance estimation step, which allows more robust blind deconvolution. Validation of the method on both simulated and real data, and of its performance, are compared with two well-known methods from the literature: the deconvolution approach for the mapping of acoustic sources, and sound density modeling

Relaxation increases monetary valuations

postprintThe Society for Consumer Psychology (SCP) 2010 Winter Conference, St. Pete Beach, FL., 25-27 February 2010. In Proceedings of the Society for Consumer Psychology 2010 Winter Conference, 2010, p. 173-17

Electrical conductivity measured in atomic carbon chains

The first electrical conductivity measurements of monoatomic carbon chains are reported in this study. The chains were obtained by unraveling carbon atoms from graphene ribbons while an electrical current flowed through the ribbon and, successively, through the chain. The formation of the chains was accompanied by a characteristic drop in the electrical conductivity. The conductivity of carbon chains was much lower than previously predicted for ideal chains. First-principles calculations using both density functional and many-body perturbation theory show that strain in the chains determines the conductivity in a decisive way. Indeed, carbon chains are always under varying non-zero strain that transforms its atomic structure from cumulene to polyyne configuration, thus inducing a tunable band gap. The modified electronic structure and the characteristics of the contact to the graphitic periphery explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure

Light Rays at Optical Black Holes in Moving Media

Light experiences a non-uniformly moving medium as an effective gravitational field, endowed with an effective metric tensor $\tilde{g}^{\mu \nu}=\eta^{\mu \nu}+(n^2-1)u^\mu u^\nu$, $n$ being the refractive index and $u^\mu$ the four-velocity of the medium. Leonhardt and Piwnicki [Phys. Rev. A {\bf 60}, 4301 (1999)] argued that a flowing dielectric fluid of this kind can be used to generate an 'optical black hole'. In the Leonhardt-Piwnicki model, only a vortex flow was considered. It was later pointed out by Visser [Phys. Rev. Lett. {\bf 85}, 5252 (2000)] that in order to form a proper optical black hole containing an event horizon, it becomes necessary to add an inward radial velocity component to the vortex flow. In the present paper we undertake this task: we consider a full spiral flow, consisting of a vortex component plus a radially infalling component. Light propagates in such a dielectric medium in a way similar to that occurring around a rotating black hole. We calculate, and show graphically, the effective potential versus the radial distance from the vortex singularity, and show that the spiral flow can always capture light in both a positive, and a negative, inverse impact parameter interval. The existence of a genuine event horizon is found to depend on the strength of the radial flow, relative to the strength of the azimuthal flow. A limitation of our fluid model is that it is nondispersive.Comment: 30 pages, LaTeX, 4 ps figures. Expanded discussion especially in section 6; 5 new references. Version to appear in Phys. Rev.

The effect of social balance on social fragmentation

With the availability of cell phones, internet, social media etc. the interconnectedness of people within most societies has increased drastically over the past three decades. Across the same timespan, we are observing the phenomenon of increasing levels of fragmentation in society into relatively small and isolated groups that have been termed filter bubbles, or echo chambers. These pose a number of threats to open societies, in particular, a radicalisation in political, social or cultural issues, and a limited access to facts. In this paper we show that these two phenomena might be tightly related. We study a simple stochastic co-evolutionary model of a society of interacting people. People are not only able to update their opinions within their social context, but can also update their social links from collaborative to hostile, and vice versa. The latter is implemented such that social balance is realised. We find that there exists a critical level of interconnectedness, above which society fragments into small sub-communities that are positively linked within and hostile towards other groups. We argue that the existence of a critical communication density is a universal phenomenon in all societies that exhibit social balance. The necessity arises from the underlying mathematical structure of a phase transition phenomenon that is known from the theory of a kind of disordered magnets called spin glasses. We discuss the consequences of this phase transition for social fragmentation in society

Komkommerbontvirus-isolaten en rasgevoeligheid

Op basis van de resultaten van de genetische verwantschappen is duidelijk geworden dat er groepen van oude en nieuwe isolaten te onderscheiden zijn. Dat de virusisolaten die momenteel in de praktijk voorkomen duidelijk genetisch afwijken van hetgeen er voor 2000 voorkwam en ook t.o.v. van buitenlandse isolaten. Er kon echter geen relatie worden gelegd tussen de symptomen en de diverse isolaten. Wel is er een effect gezien van een isolaat uit 1979 en een isolaat uit 2011 op gangbare rassen en rassen waarbij minder symptomen van het virus worden waargenomen. Het isolaat uit 1979 leverde meer symptomen op dan die uit 2011 en dat was onverwachts. Ook de opbrengst was bij de gevoelige rassen lager bij infectie met het oude isolaat

The topological modular forms of RP2\mathbb{R}P^2 and RP2∧CP2\mathbb{R}P^2 \wedge \mathbb{C}P^2

In this paper, we study the elliptic spectral sequence computing $tmf_*(\mathbb{R} P^2)$ and $tmf_* (\mathbb{R} P^2 \wedge \mathbb{C} P^2)$. Specifically, we compute all differentials and resolve exotic extensions by 2, $\eta$, and $\nu$. For $tmf_* (\mathbb{R} P^2 \wedge \mathbb{C} P^2)$, we also compute the effect of the $v_1$-self maps of $\mathbb{R} P^2 \wedge \mathbb{C} P^2$ on $tmf$-homology

Cardiovascular outcomes of type 2 diabetic patients treated with DPP‑4 inhibitors versus sulphonylureas as add-on to metformin in clinical practice

DPP-4 inhibitors (DPP-4i) and sulphonylureas remain the most widely prescribed add-on treatments after metformin. However, there is limited evidence from clinical practice comparing major adverse cardiovascular events (MACE) in patients prescribed these treatments, particularly among those without prior history of MACE and from vulnerable population groups. Using electronic health records from UK primary care, we undertook a retrospective cohort study with people diagnosed type-2 diabetes mellitus, comparing incidence of MACE (myocardial infarction, stroke, major cardiovascular surgery, unstable angina) and all-cause mortality among those prescribed DPP-4i versus sulphonylureas as add-on to metformin. We stratified analysis by history of MACE, age, social deprivation and comorbidities and adjusted for HbA1c, weight, smoking-status, comorbidities and medications. We identified 17,570 patients prescribed sulphonylureas and 6,267 prescribed DPP-4i between 2008–2017. Of these, 16.3% had pre-existing MACE. Primary incidence of MACE was similar in patients prescribed DPP-4i and sulphonylureas (10.3 vs 8.5 events per 1000 person-years; adjusted Hazard Ratio (adjHR): 0.94; 95%CI 0.80–1.14). For those with pre-existing MACE, rates for recurrence were higher overall, but similar between the two groups (21.8 vs 17.2 events per 1000 person-years; adjHR: 0.93; 95%CI 0.69–1.24). For those aged over 75 and with BMI less than 25 kg/m2 there was a protective effect for DPP-I, warranting further investigation. Patients initiating a DPP-4i had similar risk of cardiovascular outcomes to those initiating a sulphonylurea. This indicates the choice should be based on safety and cost, not cardiovascular prognosis, when deciding between a DPP-4i or sulphonylurea as add-on to metformin