Developments in modelling of backward erosion piping

One of the failure mechanisms that can affect the safety of a dyke or another water-retaining structure is backward erosion piping, a phenomenon that results in the formation of shallow pipes at the interface of a sandyor silty foundation and a cohesive cover layer. Themodels available for predicting the critical head at which the pipe progresses to the upstreamside have been validated and adapted on the basis of experiments with two-dimensional (2D) configurations. However, the experimental base for backward erosion in three-dimensional (3D) configurations in which the flow concentrates towards one point, a situation that is commonly encountered in the field, is limited. This paper presents additional 3D configuration experiments at two scales with a range of sand types. The critical gradients, the formed pipes and the erosion mechanism were analysed for the available experiments, indicating that the erosion mechanism is more complex than previously assumed, as both erosion at the tip of the pipe (primary erosion) and in the pipe (secondary erosion) are relevant. In addition, a 3D configuration was found to result in significantly lower critical gradients than those predicted by an accepted calculation model calibrated on the basis of 2D experiments, a finding that is essential for the application of the model in the field

3D character of backward erosion piping

Backward erosion piping is an important failure mechanism for cohesive water-retaining structures which are founded on a sandy aquifer. Nowadays, piping research and safety assessments are often based on experimental or numerical modelling using arbitrary model widths or even two-dimensional (2D) assumptions. This technical note shows the influence of this limitation through a series of small-scale experiments with varying model widths. The flow pattern proves to be highly three-dimensional (3D), influencing both the pipe geometry and critical gradients leading to piping failure. A 2D model is unable to capture the important aspects of the erosion mechanism and a correction factor needs to be applied if the minimum width for correctly simulating a 3D situation is not accomplished

Magnetar-like X-ray Bursts from an Anomalous X-ray Pulsar

Anomalous X-ray Pulsars (AXPs) are a class of rare X-ray pulsars whose energy source has been perplexing for some 20 years. Unlike other, better understood X-ray pulsars, AXPs cannot be powered by rotation or by accretion from a binary companion, hence the designation ``anomalous.'' AXP rotational and radiative properties are strikingly similar to those of another class of exotic objects, the Soft Gamma Repeaters (SGRs). However, the defining property of SGRs, namely their low-energy gamma-ray and X-ray bursts, have heretofore not been seen in AXPs. SGRs are thought to be ``magnetars,'' young neutron stars powered by the decay of an ultra-high magnetic field. The suggestion that AXPs are magnetars has been controversial. Here we report the discovery, from the direction of AXP 1E 1048-5937, of two X-ray bursts that have many properties similar to those of SGR bursts. These events imply a close relationship between AXPs and SGRs, with both being magnetars.Comment: 14 pages, 2 figures, accepted for publication in Nature. Note: The content of this paper is embargoed until 1900 hrs London time / 1400 US Eastern Time on Sept 1

3D character of backward erosion piping: small-scale experiments

Backward erosion piping is an important failure mechanism for cohesive water retaining structures which are founded on a sandy aquifer. At present, the prediction models for safety assessment are often based on 2D assumptions. In this work, the 3D character of the phenomenon is demonstrated on the basis of small-scale experiments. Our approach reveals the correlation between the occurrence of piping initiation and progression and the width of the physical model, which is a measure for the inclusion of the third dimension (to be regarded in a real dike situation as the influence zone of a crater). In addition, it was found that the model width has an impact on pipe characteristics and pipe development. Therefore, our results enable a better understanding of the complex physical mechanism related to backward erosion piping and thus can lead to a significant improvement in the safety assessment of water retaining structures

Atom--Molecule Coherence in a Bose-Einstein Condensate

Coherent coupling between atoms and molecules in a Bose-Einstein condensate (BEC) has been observed. Oscillations between atomic and molecular states were excited by sudden changes in the magnetic field near a Feshbach resonance and persisted for many periods of the oscillation. The oscillation frequency was measured over a large range of magnetic fields and is in excellent quantitative agreement with the energy difference between the colliding atom threshold energy and the energy of the bound molecular state. This agreement indicates that we have created a quantum superposition of atoms and diatomic molecules, which are chemically different species.Comment: 7 pages, 6 figure

Iron, silicate, and light co-limitation of three Southern Ocean diatom species

The effect of combined iron, silicate, and light co-limitation was investigated in the three diatom species Actinocyclus sp. Ehrenberg, Chaetoceros dichaeta Ehrenberg, and Chaetoceros debilis Cleve, isolated from the Southern Ocean (SO). Growth of all species was co-limited by iron and silicate, reflected in a significant increase in the number of cell divisions compared to the control. Lowest relative Si uptake and drastic frustule malformation was found under iron and silicate co-limitation in C. dichaeta, while Si limitation in general caused cell elongation in both Chaetoceros species. Higher light intensities similar to SO surface conditions showed a negative impact on growth of C. dichaeta and Actinocyclus sp. and no effect on C. debilis. This is in contrast to the assumed light limitation of SO diatoms due to deep wind driven mixing. Our results suggest that growth and species composition of Southern Ocean diatoms is influenced by a sensitive interaction of the abiotic factors, iron, silicate, and light

Taylor approximations of operator functions

This survey on approximations of perturbed operator functions addresses recent advances and some of the successful methods.Comment: 12 page

Measurement of WZ and ZZ production in pp collisions at [Formula: see text] in final states with b-tagged jets.

Measurements are reported of the WZ and ZZ production cross sections in proton-proton collisions at [Formula: see text][Formula: see text] in final states where one Z boson decays to b-tagged jets. The other gauge boson, either W or Z, is detected through its leptonic decay (either [Formula: see text], [Formula: see text] or [Formula: see text], [Formula: see text], or [Formula: see text]). The results are based on data corresponding to an integrated luminosity of 18.9 fb[Formula: see text] collected with the CMS detector at the Large Hadron Collider. The measured cross sections, [Formula: see text] and [Formula: see text], are consistent with next-to-leading order quantum chromodynamics calculations