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Sumer B.M.,Technical University of Denmark | Sen M.B.,Technical University of Denmark | Sen M.B.,Grontmij A S | Karagali I.,Technical University of Denmark | And 7 more authors.
Journal of Geophysical Research: Oceans | Year: 2011

Two parallel experiments involving the evolution and runup of plunging solitary waves on a sloping bed were conducted: (1) a rigid-bed experiment, allowing direct (hot film) measurements of bed shear stresses and (2) a sediment-bed experiment, allowing for the measurement of pore water pressures and for observation of the morphological changes. The two experimental conditions were kept as similar as possible. The experiments showed that the complete sequence of the plunging solitary wave involves the following processes: shoaling and wave breaking; runup; rundown and hydraulic jump; and trailing wave. The bed shear stress measurements showed that the mean bed shear stress increases tremendously (with respect to that in the approaching wave boundary layer), by as much as a factor of 8, in the runup and rundown stages, and that the RMS value of the fluctuating component of the bed shear stress is also affected, by as much as a factor of 2, in the runup and hydraulic jump stages. The pore water pressure measurements showed that the sediment at (or near) the surface of the bed experiences upward directed pressure gradient forces during the down-rush phase. The magnitude of this force can reach values as much as approximately 30% of the submerged weight of the sediment. The experiments further showed that the sediment transport occurs in the sheet flow regime for a substantial portion of the beach covering the area where the entire sequence of the wave breaking takes place. The bed morphology is explained qualitatively in terms of the measured bed shear stress and the pressure gradient forces. Copyright 2011 by the American Geophysical Union. Source


Mishin O.V.,Riso National Laboratory for Sustainable Energy | Mishin O.V.,Technical University of Denmark | Segal V.M.,Engineered Performance Materials Company, LLC | Ferrasse S.,Honeywell
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012

A detailed quantitative analysis of the microstructure has been performed in three orthogonal planes of 15-mm-thick aluminum plates heavily deformed via two equal channel angular extrusion (ECAE) routes. One route was a conventional route A with no rotation between passes. Another route involved sequential 90 deg rotations about the normal direction (ND) between passes. The microstructure in the center of these plates, and especially the extent of microstructural heterogeneity, has been characterized quantitatively and compared with that in bar samples extruded via either route A or route Bc with 90 deg rotations about the longitudinal axis. Statistically robust data were obtained in this work using gallium enhanced microscopy and EBSD mapping of large sample areas. For the plate processed using route A, the fraction of high-angle boundaries was found to strongly depend on the inspection plane, being smallest in the plane perpendicular to the ND (plane Z), where the largest subgrain size and most profound microstructural heterogeneities were also revealed. In comparison, the plate extruded with 90 deg rotations about the ND was less heterogeneous and contained smaller subgrains in plane Z. Comparing the plate and bar samples, the most refined and least heterogeneous microstructure was observed in the route Bc bar sample. The differences in the microstructure are reflected in the hardness data; the hardness is lowest after ECAE via route A and greatest in the bar sample processed using route Bc. © 2012 The Minerals, Metals & Materials Society and ASM International. Source


Borden Z.,University of California at Santa Barbara | Koblitz T.,University of California at Santa Barbara | Koblitz T.,Riso National Laboratory for Sustainable Energy | Meiburg E.,University of California at Santa Barbara
Physics of Fluids | Year: 2012

Bores, or hydraulic jumps, appear in many natural settings and are useful in many industrial applications. If the densities of the two fluids between which a bore propagates are very different (i.e., water and air), the less dense fluid can be neglected when modeling a bore analytically-a single-layer hydraulic model will accurately predict a bore's speed of propagation. A two-layer model is required, however, if the densities are more similar. Mass is conserved separately in each layer and momentum is conserved globally, but the model requires for closure an assumption about the loss of energy across a bore. In the Boussinesq limit, it is known that there is a decrease of the total energy flux across a bore, but in the expanding layer, turbulent mixing at the interface entrains high speed fluid from the contracting layer, resulting in an increase in the flux of kinetic energy across the expanding layer of a bore. But it is unclear if this finding will extend to non-Boussinesq bores. We directly examine the flux of energy within non-Boussinesq bores using two-dimensional direct numerical simulations and find that a gain of energy across the expanding layer only occurs for bores where the density ratio, defined as the ratio of the density of the lighter fluid to the heavier fluid, is greater than approximately one half. For smaller density ratios, undular waves generated at the bore's front dominate over the effects of turbulent mixing, and the expanding layer loses energy across the bore. Based on our results, we show that if one can predict the amount of energy radiated by bores through undular waves, it is possible to derive an accurate model for the propagation of non-Boussinesq bores. © 2012 American Institute of Physics. Source


Toft H.S.,University of Aalborg | Sorensen J.D.,University of Aalborg | Sorensen J.D.,Riso National Laboratory for Sustainable Energy
Structural Safety | Year: 2011

Reliability-based design of wind turbine blades requires identification of the important failure modes/limit states along with stochastic models for the uncertainties and methods for estimating the reliability. In the present paper it is described how reliability-based design can be applied to wind turbine blades. For wind turbine blades, tests with the basic composite materials and a few full-scale blades are normally performed during the design process. By adopting a reliability-based design approach, information from these tests can be taken into account in a rational way during the design process. In the present paper, a probabilistic framework for design of wind turbine blades are presented and it is demonstrated how information from tests can be taken into account using the Maximum-Likelihood method and Bayesian statistics. In a numerical example, the reliability is estimated for a wind turbine blade in both ultimate and fatigue limit states. Information from tests is used to formulate the stochastic models used in the limit state equations. Partial safety factors for use in traditional deterministic design are estimated using the stochastic models. © 2011 Elsevier Ltd. Source


Hogberg P.,Swedish University of Agricultural Sciences | Johannisson C.,Swedish University of Agricultural Sciences | Yarwood S.,Oregon State University | Callesen I.,Riso National Laboratory for Sustainable Energy | And 3 more authors.
New Phytologist | Year: 2011

Trees reduce their carbon (C) allocation to roots and mycorrhizal fungi in response to high nitrogen (N) additions, which should reduce the N retention capacity of forests. The time needed for recovery of mycorrhizas after termination of N loading remains unknown. Here, we report the long-term impact of N loading and the recovery of ectomycorrhiza after high N loading on a Pinus sylvestris forest. We analysed the N% and abundance of the stable isotope 15N in tree needles and soil, soil microbial fatty acid biomarkers and fungal DNA. Needles in N-loaded plots became enriched in 15N, reflecting decreased N retention by mycorrhizal fungi and isotopic discrimination against 15N during loss of N. Meanwhile, needles in N-limited (control) plots became depleted in 15N, reflecting high retention of 15N by mycorrhizal fungi. N loading was terminated after 20 yr. The δ15N and N% of the needles decreased 6 yr after N loading had been terminated, and approached values in control plots after 15 yr. This decrease, and the larger contributions compared with N-loaded plots of a fungal fatty acid biomarker and ectomycorrhizal sequences, suggest recovery of ectomycorrhiza. High N loading rapidly decreased the functional role of ectomycorrhiza in the forest N cycle, but significant recovery occurred within 6-15 yr after termination of N loading. © The Authors (2010). Journal compilation © New Phytologist Trust (2010). Source

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