Analysis of velocity profile measurements from wind-tunnel experiments with saltation

Investigations of wind-field modification due to the presence of saltating sediments have relied heavily on wind tunnels, which are known to impose geometric constraints on full boundary layer development. There remains great uncertainty as to which portion of the vertical wind-speed profile to analyze when deriving estimates of shear velocity or surface roughness length because the lower sections are modified to varying degree by saltation, whereas the upper segments may be altered by artificially induced wake-like effects. Thus, it is not obvious which of several alternative velocity-profile parameterizations (e.g., Law of the Wall, Velocity Defect Law, Wake Law) should be employed under such circumstances.A series of experimental wind-tunnel runs was conducted across a range of wind speed using fine- and coarse-grained sand to collect high-quality, fine-resolution data within and above the saltation layer using thermal anemometry and ruggedized probes. After each run, the rippled bottom was fixed with fine mist, and the experiment repeated without saltation. The measured wind-speed profiles were analyzed using six different approaches to derive estimates of shear velocity and roughness length. The results were compared to parameter estimates derived directly from sediment transport rate measurements, and on this basis, it is suggested that one of the six approaches is more robust than the others. Specifically, the best estimate of shear velocity during saltation is provided by the logarithmic law applied to the profile data within about 0.05 m of the bottom, despite the fact that this near-surface region is where profile modification by saltating sediments is most pronounced. Uncertainty remains as to whether this conclusion can be generalized to field situations because progressive downwind adjustments in the interrelationship between the saltation layer and the wind field are anticipated in wind tunnels, thereby confounding most analyses based on equilibrium assumptions.     

Barrier response to sea level rise and storms

This commentary brings together, as a Virtual Special Issue, several recent papers in Earth Surface Processes and Landforms that improve our understanding of coastal barrier response to relative sea level rise and a change in the frequency and/or magnitude of storm events. The ability to predict barrier response depends on the ability to quantify the spatial and temporal scales of sediment exchange amongst the nearshore, beach and dune. This exchange controls the height of the dune, which in turn determines the transfer of sediment to the backbarrier through washover and/or blowouts. The papers in this issue provide new insight on beach–dune interaction and the importance of this interaction to long‐term barrier evolution across a range of sites and scales, and how active management can influence this interaction and alter barrier response. Copyright © 2018 John Wiley & Sons, Ltd.     

Theory and implementation of switch‐based hybrid simulation technology for earthquake engineering applications

Hybrid simulation (HS) is a novel technique to combine analytical and experimental sub‐assemblies to examine the dynamic responses of a structure during an earthquake shaking. Traditionally, HS uses displacement‐based control where the finite element program calculates trial displacements and applies them to both the analytical and experimental sub‐assemblies. Displacement‐based HS (DHS) has been proven to work well for most structural sub‐assemblies. However, for specimens with high stiffness, traditional DHS does not work because it is difficult to precisely control hydraulic actuators in small displacement. A small control error in displacement will result in large force response fluctuations for stiff specimens. This paper resolves this challenge by proposing a force‐based HS (FHS) algorithm that directly calculates trial forces instead of trial displacements. The proposed FHS is finite element based and applicable to both linear and nonlinear systems. For specimens with drastic changes in stiffness, such as yielding, a switch‐based HS (SHS) algorithm is proposed. A stiffness‐based switching criterion between the DHS and FHS algorithms is presented in this paper. All the developed algorithms are applied to a simple one‐story one‐bay concentrically braced moment frame. The result shows that SHS outperforms DHS and FHS. SHS is then utilized to validate the seismic performance of an innovative earthquake resilient fused structure. The result shows that SHS works in switching between the DHS and FHS modes for a highly nonlinear and highly indeterminate structural system. Copyright © 2017 John Wiley & Sons, Ltd.     

Comparison of methods used for computing the impact of sound on the marine environment

Concern about the impact of sound on marine mammals has increased over the last decade, causing governments to take a more rigorous look at the potential impact of activities that introduce sound into the ocean. Environmental Impact Statements (EIS's) can be prepared using differing analysis methods to estimate the impact on marine mammals. To assess consistency in assessment methods, differences in the base assumptions were investigated; in particular, differences that arise between assumptions of dynamic marine mammals (animat method) and static distributions of marine mammals (static distribution method). Using several ocean environment scenarios and species, it is demonstrated that differences consistently arise between the two methods. The static distribution method underestimates the number of behavioral harassments compared with the animat method. Repeating many simulations with the animat method provides a robust risk assessment, provides a measure of variability, and allows the probability of "spurious events" to be estimated.     

Ferroelastic phase transition in cryolite,Na3AlF6, a mixed fluoride perovskite: High temperature single crystal X-ray diffraction study and symmetry analysis of the transition mechanism

Cryolite, Na₃AlF₆[ = 2Na⁺(Na_0.5 ⁺Al_0.5 ³⁺)F₃] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF₆] and [AlF₆] octahedra and the cavities are occupied by Na⁺ ions. At 295 K, it is monoclinic (α phase), space group P2 ₁/n with a = 5.4139 (7), b = 5.6012 (5) and c = 7.7769 (8) Å and β = 90.183 (3)°, Z = 2. A high temperature single crystal X-ray diffraction study in the range 295–900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at T ₀ ～ 885 K, in contrast to a previous report that it becomes cubic at ～823 K. The space group of the high temperature β phase is Immm with a = 5.632 (4), b = 5.627 (3) and c = 7.958 (4) Å, Z = 2 at 890 K. Above T ₀, the coordination number of the Na⁺ ion in the cavity increases from eight to twelve and the zigzag Na1 — Al octahedral chains parallel to c become straight with the Na1-F-Al angle = 180 °. The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [AlF₆] group and transforms according to the Γ ₄ ⁺ irreducible representation of Immm. Coupled to the [AlF₆] rotation is a second primary order parameter corresponding to the displacement of the Na2⁺ ion in the cavity from its equilibrium position. This order parameter transforms according to the X ₃ ⁺ irreducible representation of Immm. Following Immm → P2 ₁ /n phase transition, four equivalent domains of P2 ₁/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral Al-F and Na-F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above T ₀ indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The β phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the a phase. This view is consistent with the entropy of phase transition, ΔS_trans (11.43 JK^?1 mol^?1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK^?1 mol^?1), where 4 is the number of domains formed during the phase transition. The dynamic nature of the β phase is independently confirmed from a considerable narrowing of the ²⁷Al nuclear magnetic resonance (NMR) line-shape above T ₀ (Stebbins et al. 1992).     

Mineral resources of China: Apparent controls on distribution

This paper attempts to define apparent mineral distribution patterns of China according to tectono-stratigraphic terrane geology. Associations between specific mineral and hydrocarbon deposits and specific tectono-stratigraphic terranes in China suggest that deposits of certain types are spatially constrained by terrane geology. The definition of specific tectono-stratigraphic terranes of China is a valuable initial step in interpreting the geologic history of the nation, although terrane analysis of China is at a very early stage. The recognition that specific deposit types are associated with specific tectono-stratigraphic terranes may make it possible to select appropriate analogues from elsewhere in the world to evaluate China's mineral and hydrocarbon potential. Additionally, understanding the associations between mineral and hydrocarbon deposits and tectono-stratigraphic terranes in China may serve as an aid to future exploration in the nation.     

Deterministically Driven Avalanche Models of Solar Flares

We develop and discuss the properties of a new class of lattice-based avalanche models of solar flares. These models are readily amenable to a relatively unambiguous physical interpretation in terms of slow twisting of a coronal loop. They share similarities with other avalanche models, such as the classical stick–slip self-organized critical model of earthquakes, in that they are driven globally by a fully deterministic energy-loading process. The model design leads to a systematic deficit of small-scale avalanches. In some portions of model space, mid-size and large avalanching behavior is scale-free, being characterized by event size distributions that have the form of power-laws with index values, which, in some parameter regimes, compare favorably to those inferred from solar EUV and X-ray flare data. For models using conservative or near-conservative redistribution rules, a population of large, quasiperiodic avalanches can also appear. Although without direct counterparts in the observational global statistics of flare energy release, this latter behavior may be relevant to recurrent flaring in individual coronal loops. This class of models could provide a basis for the prediction of large solar flares.