The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution

The stability properties of one-dimensional radiative shocks with a power-law cooling function of the form $\Lambda \propto \rho^2T^\alpha$ are the main subject of this work. The linear analysis originally presented by Chevalier & Imamura, is thoroughfully reviewed for several values of the cooling index $\alpha$ and higher overtone modes. Consistently with previous results, it is shown that the spectrum of the linear operator consists in a series of modes with increasing oscillation frequency. For each mode a critical value of the cooling index, $\alpha_\textrm{c}$, can be defined so that modes with $\alpha < \alpha_\textrm{c}$ are unstable, while modes with $\alpha > \alpha_\textrm{c}$ are stable. The perturbative analysis is complemented by several numerical simulations to follow the time-dependent evolution of the system for different values of $\alpha$. Particular attention is given to the comparison between numerical and analytical results (during the early phases of the evolution) and to the role played by different boundary conditions. It is shown that an appropriate treatment of the lower boundary yields results that closely follow the predicted linear behavior. During the nonlinear regime, the shock oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle. The modes of oscillations during this phase do not necessarily coincide with those predicted by linear theory, but may be accounted for by mode-mode coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical Journa

A Cut Finite Element Method for the Bernoulli Free Boundary Value Problem

We develop a cut finite element method for the Bernoulli free boundary problem. The free boundary, represented by an approximate signed distance function on a fixed background mesh, is allowed to intersect elements in an arbitrary fashion. This leads to so called cut elements in the vicinity of the boundary. To obtain a stable method, stabilization terms is added in the vicinity of the cut elements penalizing the gradient jumps across element sides. The stabilization also ensures good conditioning of the resulting discrete system. We develop a method for shape optimization based on moving the distance function along a velocity field which is computed as the $H^1$ Riesz representation of the shape derivative. We show that the velocity field is the solution to an interface problem and we prove an a priori error estimate of optimal order, given the limited regularity of the velocity field across the interface, for the the velocity field in the $H^1$ norm. Finally, we present illustrating numerical results

A Cut Finite Element Method for the Bernoulli Free Boundary Value Problem

We develop a cut finite element method for the Bernoulli free boundary problem. The free boundary, represented by an approximate signed distance function on a fixed background mesh, is allowed to intersect elements in an arbitrary fashion. This leads to so called cut elements in the vicinity of the boundary. To obtain a stable method, stabilization terms is added in the vicinity of the cut elements penalizing the gradient jumps across element sides. The stabilization also ensures good conditioning of the resulting discrete system. We develop a method for shape optimization based on moving the distance function along a velocity field which is computed as the $H^1$ Riesz representation of the shape derivative. We show that the velocity field is the solution to an interface problem and we prove an a priori error estimate of optimal order, given the limited regularity of the velocity field across the interface, for the the velocity field in the $H^1$ norm. Finally, we present illustrating numerical results

Long-Term Multiwavelength Studies of High-Redshift Blazar 0836+710

Aims. The observation of gamma -ray flares from blazar 0836+710 in 2011, following a period of quiescence, offered an opportunity to study correlated activity at different wavelengths for a high-redshift (z=2.218) active galactic nucleus. Methods. Optical and radio monitoring, plus Fermi-LAT gamma-ray monitoring provided 2008-2012 coverage, while Swift offered auxiliary optical, ultraviolet, and X-ray information. Other contemporaneous observations were used to construct a broad-band spectral energy distribution. Results. There is evidence of correlation but not a measurable lag between the optical and gamma-ray flaring emission. On the contrary, there is no clear correlation between radio and gamma-ray activity, indicating radio emission regions that are unrelated to the parts of the jet that produce the gamma-rays. The gamma-ray energy spectrum is unusual in showing a change of shape from a power law to a curved spectrum when going from the quiescent state to the active state.Comment: 11 pages, 10 figures, Accepted for publication in A&

Determinants of guideline use in primary care physical therapy: a cross-sectional survey of attitudes, knowledge, and behavior

Background Understanding of attitudes, knowledge, and behavior related to evidence-based practice (EBP) and use of evidence-based clinical practice guidelines in primary care physical therapy is limited. Objectives The objectives of this study were: (1) to investigate self-reported attitudes, knowledge, behavior, prerequisites, and barriers related to EBP and guideline use among physical therapists in primary care and (2) to explore associations of self-reported use of guidelines with these social cognitive factors along with demographic and workplace characteristics. Design This was a cross-sectional survey. Methods A web-based survey of 419 physical therapists in primary care in western Sweden was performed. Multiple logistic regression analysis was performed to examine factors associated with guideline use. Results The response rate was 64.7%. Most respondents had positive attitudes toward EBP and guidelines: 90% considered EBP necessary, and 96% considered guidelines important. Approximately two thirds reported confidence in finding and using evidence. One third reported being aware of guidelines. Thirteen percent knew where to find guidelines, and only 9% reported having easy access to guidelines. Fewer than half reported using guidelines frequently. The most important barriers to using guidelines were lack of time, poor availability, and limited access to guidelines. Young age and brief work experience were associated with positive attitudes toward EBP. A postgraduate degree was associated with higher application of EBP. Positive attitudes, awareness of guidelines, considering guidelines to facilitate practice, and knowing how to integrate patient preferences with guideline use were associated with frequent use of guidelines. Limitations Data were self-reported, which may have increased the risk of social desirability bias. Conclusions Use of guidelines was not as frequent as could be expected in view of the positive attitudes toward EBP and guidelines among physical therapists. Awareness of and perceived access to guidelines were limited. The identified determinants can be addressed when developing guideline implementation strategies

Cloudy operating systems

This article discusses the notion of cloud operating systems, the operation of the cloud system and the specifics of its use. Types of cloud operating systems, as well as advantages and disadvantages of using cloud operating systems