[en] Despite their well-known limitations, Reynolds-Averaged Navier–Stokes (RANS) models are still the workhorse tools for turbulent flow simulations in today's engineering analysis, design and optimization. While the predictive capability of RANS models depends on many factors, for many practical flows the turbulence models are by far the largest source of uncertainty. As RANS models are used in the design and safety evaluation of many mission-critical systems such as airplanes and nuclear power plants, quantifying their model-form uncertainties has significant implications in enabling risk-informed decision-making. In this work we develop a data-driven, physics-informed Bayesian framework for quantifying model-form uncertainties in RANS simulations. Uncertainties are introduced directly to the Reynolds stresses and are represented with compact parameterization accounting for empirical prior knowledge and physical constraints (e.g., realizability, smoothness, and symmetry). An iterative ensemble Kalman method is used to assimilate the prior knowledge and observation data in a Bayesian framework, and to propagate them to posterior distributions of velocities and other Quantities of Interest (QoIs). We use two representative cases, the flow over periodic hills and the flow in a square duct, to evaluate the performance of the proposed framework. Both cases are challenging for standard RANS turbulence models. Simulation results suggest that, even with very sparse observations, the obtained posterior mean velocities and other QoIs have significantly better agreement with the benchmark data compared to the baseline results. At most locations the posterior distribution adequately captures the true model error within the developed model form uncertainty bounds. The framework is a major improvement over existing black-box, physics-neutral methods for model-form uncertainty quantification, where prior knowledge and details of the models are not exploited. This approach has potential implications in many fields in which the governing equations are well understood but the model uncertainty comes from unresolved physical processes. - Highlights: • Proposed a physics–informed framework to quantify uncertainty in RANS simulations. • Framework incorporates physical prior knowledge and observation data. • Based on a rigorous Bayesian framework yet fully utilizes physical model. • Applicable for many complex physical systems beyond turbulent flows.

[en] The development of a sagittally focusing double-multilayer monochromator is reported, which produces a spatially extended wide-bandpass X-ray beam from an intense synchrotron bending-magnet source at the Advanced Photon Source, for ultrafast X-ray radiography and tomography applications. This monochromator consists of two W/B4C multilayers with a 25 (angstrom) period coated on Si single-crystal substrates. The second multilayer is mounted on a sagittally focusing bender, which can dynamically change the bending radius of the multilayer in order to condense and focus the beam to various points along the beamline. With this new apparatus, it becomes possible to adjust the X-ray beam size to best match the area detector size and the object size to facilitate more efficient data collection using ultrafast X-ray radiography and tomography

[en] The effects of several transition-metallic ions on the technetium-99m labelling reaction of the anticancer drug, Pingyangmycin (PYM), were studied. The results showed that the ions from chromium, manganese, zinc and cadmium had no influence on the yield of Tc-99m-PYM but Cu⁺, Cu²⁺ and Fe³⁺ compete with Tc-99m for PYM complexing. (UK)

[en] The ubiquitousness of the solar inter-network horizontal magnetic field has been revealed by space-borne observations with high spatial resolution and polarization sensitivity. However, no consensus has been achieved on the origin of the horizontal field among solar physicists. For a better understanding, in this study, we analyze the cyclic variation of the inter-network horizontal field by using the spectro-polarimeter observations provided by the Solar Optical Telescope on board Hinode, covering the interval from 2008 April to 2015 February. The method of wavelength integration is adopted to achieve a high signal-to-noise ratio. It is found that from 2008 to 2015 the inter-network horizontal field does not vary when solar activity increases, and the average flux density of the inter-network horizontal field is 87 ± 1 G, In addition, the imbalance between horizontal and vertical fields also keeps invariant within the scope of deviation, i.e., 8.7 ± 0.5, from the solar minimum to maximum of solar cycle 24. This result confirms that the inter-network horizontal field is independent of the sunspot cycle. The revelation favors the idea that a local dynamo is creating and maintaining the solar inter-network horizontal field

[en] The Sudbury Neutrino Observatory contains 1000 t of ultra-pure heavy water and 7000 t of very pure light water. In order to achieve the goal of determining accurately the total number of neutrinos emitted by the sun regardless of type, the radioactivity in the water must be maintained with a radiopurity of less than 10^-14 g/g of ²³²Th and ²³⁸U in secular equilibrium with their daughters. This paper describes the design and application of an electrostatic radon detector which determines the amount of radon emanating from a column containing MnO₂ used to extract radium from the water. This electrostatic radon detector has a total efficiency of 35% for detecting ²²²Rn and 22% for ²²⁰Rn. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)

[en] Study of microflares on the quiet Sun is extremely important in learning the physics of both solar flare and atmospheric heating. Here, for the first time, we report the detailed observations of two homologous microflares from Atmospheric Imager Assembly (AIA) images and Heliospheric Magnetic Imager magnetograms in a very quiet region. The two microflares are observed clearly in the extreme ultraviolet and faintly in the ultraviolet in AIA images. They have an area of ∼59 Mm² and ∼46 Mm², and a duration of 25 minutes and 22 minutes. The magnetic cancellation continuously takes place during the microflares, and the magnetic flux decrease is apparent, with a value of ∼4 × 10¹⁸ Mx. The obvious mass ejections are observed during the microflares, and dimming occurs during and after the microflares. The velocity of mass ejection is up to 160 km s⁻¹. For the two microflares, the time of peak intensity in λ 30.4 nm precedes that of the coronal peak emission, up to 2.4–3.3 minutes. Their thermal energy is estimated to be 1.3 × 10²⁷ erg and 2.5 × 10²⁶ erg, respectively, which heats the corona up to 5.8 MK and 2.8 MK. There are many similarities of microflares on the quiet Sun to major flares in the active regions.

[en] We investigate the effects of slight Dy/Mn off-stoichiometry on the multiferroic behaviors of DyMnO₃ (Dy_1−xMn_1+xO₃). It is revealed that the distorted lattice structure and ferroelectric property exhibit higher stability against the Mn deficiency than the Dy deficiency. Since the electric polarization includes the contributions from the Mn-Mn and Dy-Mn interactions, the measured polarization exhibits different dependence on the Mn deficiency from that on the Dy deficiency. The present work suggests that the Dy/Mn off-stoichiometry is detrimental to the ferroelectricity, owing to the modulated spin interactions and reshuffled spin frustration

[en] The multiferroic RMn₂O₅ family, where R is rare-earth ion or Y, exhibits rich physics of multiferroicity which has not yet well understood. DyMn₂O₅ is a representative member of this family. The ferroelectric polarization of DyMn₂O₅ is claimed to be magnetically relevant and have more than one component. Therefore, the polarization reversal upon the sequent magnetic transitions is expected. We investigate the evolution of the ferroelectric polarization upon a partial substitution of Mn³⁺ by nonmagnetic Al³⁺ in order to tailor the Mn³⁺-Mn⁴⁺ interactions and then to modulate the polarization in DyMn_2−x/2Al_x/2O₅. It is revealed that the polarization can be successfully reversed by Al-substitution via substantially suppressing the Mn³⁺-Mn⁴⁺ interactions, while the Dy³⁺-Mn⁴⁺ interactions can sustain against the substitution until a level as high as x = 0.2. In addition, the independent Dy spin ordering is shifted remarkably down to an extremely low temperature due to the Al³⁺ substitution. The present work unveils the possibility of tailoring the Mn³⁺-Mn⁴⁺ and Dy³⁺-Mn⁴⁺ interactions independently, and thus reversing the ferroelectric polarization

[en] We investigate the feasibility of probing anomalous top-quark couplings of Wtb, Ztt-bar, and γtt-bar in terms of an effective Lagrangian with dimension-six operators at future e⁺e^- linear colliders with a c. m. energy √s ∼ 500 - 800 GeV. We first examine the constraints on these anomalous couplings from the Z → bb-bar data at LEP I and from unitarity considerations. We then consider in detail the effects of anomalous couplings on tt-bar spin correlations in the top-pair production and decay with three spin bases: the helicity, beamline and off-diagonal bases. Our results show that the polarized beams are more suitable for exploring the effects of different new operators. For polarized beams, the helicity basis yields the best sensitivity. (author)

[en] The cavitation erosion behavior of Ni foam/polyurethane (PU) co-continuous composites under different experimental conditions was investigated using an ultrasonic vibration cavitation erosion device. Experimental results showed that the Ni foam metal skeleton structure had severe effects on the cavitation resistance of the composites. The composites with thick metal arris and large pore size exhibited good resistance to cavitation erosion. Stress waves had greater influence on the cavitation erosion behavior of composites than micro-jets. In a synergistic way of buffering stress wave in the resin phase and absorbing stress waves in the metal phase, the composites could exhibit better cavitation resistance than pure PU. (paper)