45,417 research outputs found
On the saltation of fresh snow in a wind tunnel: profile characterization and single particle statistics
No full textWe present experimental results on the snow drift in a turbulent boundary layer over a flat fresh snow-covered surface. Vertical profiles of mass flux and of the distribution of particle diameters were obtained by means of a pair of Snow Particle Counters parallel with measurements of the stream-wise velocity profile. The aim of the paper is to discuss current parameterizations of the vertical mass flux profile for fresh snow and to investigate the range of timescales involved in a developing saltation layer occurring in a turbulent boundary layer. The novelty of the work consists of using an intact fresh snow cover as an erodible surface able to provide realistic snow crystals as drifting particles. Results show that (1) the parameters scaling the vertical mass flux profiles of fresh snow can significantly differ from those given in the literature for ice or compacted snow particles; (2) though drifting snow covers an extremely wide range of temporal scales, the mean time interval between saltating particles ??t ? is the key timescale of the saltation process; (3) ??t ? allows for the optimal reconstruction of the mass flux as a continuous signal and for neglecting the effects related to the heterogeneous distribution of particle size on the mass flux. Implications on the modeling of snow drift and on the processing of field observations are discusse
Micrometeorological processes driving snow ablation in an Alpine catchment
No full textMountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micrometeorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological model (ARPS) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modelled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances except for very strong wind conditions. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is below a critical value. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover. Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.<br/
Statistical properties of fresh snow roughness
No full textWe present results from a series of experiments in which fresh snow roughness was measured by means of digital photography and analyzed using the random field approach. The aim of the paper is to investigate the scaling properties of fresh-snow-covered surfaces and to capture key roughness length scales which can characterize the surface geometry and the size of the snow crystals. Results from our experiments show the following: (1) fresh snow roughness exhibits two distinguished scaling regimes, one at scales comparable with the crystals size and another one at larger scales; (2) we confirm that the large scales are built up during snowfall and their scaling behavior is consistent with that of Ballistic Deposition (BD) processes; and (3) we suggest that the crossover length scale separating the two scaling regimes effectively defines a representative length scale of the aggregated snow crystals on the surface. The definition of this length scale is independent of the difficulties associated with measuring snow grain sizes by means of standard microscopic analysis of disaggregated crystals. Furthermore it can be obtained from a low-cost and quick experimental procedure. Results from this study provide a plausible justification for the wide scatter of aerodynamic roughness length values encountered in the literature for fresh snow. Moreover, they provide insight on the key roughness length scales which should be used for the modeling of this parameter
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On the significance of form induced stress in rough wall turbulent boundary layers
No full textThis paper presents a review of recent experimental and numerical studies which deal with the analysis of form-induced stress in rough wall turbulent boundary layers. The aim of the paper is to assess the importance of this stress for various rough wall geometries and flow conditions. Analysis of the significance of form-induced stress is first performed by comparing its magnitude with the magnitude of Reynolds stress for each data set available in literature. Then, by selecting a special set of data, we analyze the comparison between the gradients of both stresses. We point out that the comparison of stress gradients gives a different perspective on the role of form-induced stress in rough wall boundary layers
Numerical modeling of the tool-rock penetration process using FEM coupled with SPH technique
No full textThe numerical simulation of penetration into rock is an important tool to gain insights into rock drilling mechanisms, since it can be exploited as an alternative to the expensive field testing. This research aims to present an innovative computer simulation of rock penetration process on the basis of the finite element method (FEM) coupled with smoothed particle hydrodynamics (SPH). An advanced material model, namely the Karagozian and Case Concrete (KCC) model, was employed for this purpose. The Punch Penetration test (PPT) was carried out on a medium strength sandstone for validating the numerical method. The comparison of the numerical and experimental results obtained concluded that the FEM coupled with SPH method in conjunction with the fully calibrated KCC material model is a reliable method for the study of rock penetration due to its ability to deal with large deformations and its realistic constitutive modeling. The modeling approach was finally applied to estimate the required force to penetrate an offshore reservoir rock block under the in-situ confining pressure with a double conical tool up to 5 mm depth. The effective stresses in sedimentary basins of Agosta and Dosso Campus at a depth of 3000 m below the seabed are considered as the confining pressures of this study
Comment on “Pore water flow due to near-bed turbulence and associated solute transfer in a stream or lake sediment bed" by M. Higashino et al.
No full textCalibration of a constitutive material model for sub-sea pipelines
No full textThe bulk transport of oil and gas are today carried out by pipelines, and the maximum economical and feasible transport distance and installation environment is constantly extended. This has challenged the need for advanced technology for the whole life cycle of pipelines. New design challenges have to be dealt with concerning arctic environmental loads, but also accidental loads such as anchoring, dropped objects, fishing activities, vessel impact, and of course ice gouging. Thus, considering the actual and increasingly high demands for energy and the challenges with respect to inspection, monitoring and repairing, reliable design criteria are fundamental. Here, numerical simulations are important tools, but the reliability of such simulations of pipelines subjected to different load scenarios rely on a proper modelling of the pipe material. Hence, the aim of this paper is the calibration of a suitable constitutive model for a typical material used in a sub-sea pipeline. Starting from a large experimental test matrix, executed on specimens cut directly from a pipe, the calibration of a material model with an anisotropic yield function and a ductile failure criterion is presented in some detail. In particular the choice of a suitable yield surface to take into account the strong anisotropic flow properties found in the experimental tests is underlined
Quasi-static and low-velocity impact biaxial flexural fracture of aluminosilicate glass — An experimental and numerical study
No full textIn this study, the quasi-static biaxial fracture behavior of aluminosilicate glass is investigated via Ball-On-Ring (BOR) and Ring-On-Ring (ROR) tests aided by a three-dimensional Digital Image Correlation (3D-DIC) technique. In-plane strain field and out of plane deformation distribution during the loading process can be obtained by this test system, overcoming the problem of inaccurate displacement data provided by the loading machine. During the loading process, a uniform strain distribution field is formed below the load ring of the ROR specimen while a gradient strain distribution is built for the BOR specimens. For dynamic loading conditions, low-velocity impact BOR tests are conducted at velocities of 2 m/s and 4 m/s showing a dynamic strengthening effect and high-speed cameras provide detailed fracture and failure sequences of the glass plates. Finite Element Method (FEM) simulations with the JH-2 material model are carried out showing good consistency with experimental results for the quasi-static loading conditions. FEM coupled to Smooth Particle Hydrodynamics (FEM-SPH) technique is utilized for dynamic simulations to solve the element distortion and non-physical problems. The influence of the tensile strain rate effect on low-velocity impact behavior of aluminosilicate glass is analyzed in detail. The original JH-2 model does not provide a reasonable prediction of the dynamic tensile response of brittle materials and the model requires further modifications to describe the dynamic tensile fracture behavior. By introducing rate-dependence maximum hydrostatic tensile strength σt,max to the updated model, better predictions for the contact force and projectile residual velocity histories of low-velocity impact tests can be provided
Digital filtering of acceleration data acquired during the intervention of a lift safety gears
No full textIn this paper two digital filters are applied to elaborate acceleration data acquired from two accelerometers fixed in a lift frame during safety gears tests. The experimental tests consist
in free falls of a test lift car and the subsequent gripping phases initiated by the safety gears activation due to over-speed condition. Unfortunately, the reliability of acceleration data is
heavily impaired by measurement noise, which needs to be suppressed to the largest extent possible in order to use the data for comfort evaluation. To this end, two classes of digital filters are tested: the Savitzky–Golay and the Butterworth filters. This choice is motivated by the key features of these filters, including their computational simplicity and high suitability to represent time varying features in acceleration measurements. A
tuning procedure is proposed for these filters, such that the measurement noise corrupting the experimental data is maximally dumped
Spigolature sulla natura fiduciaria della causa della cessione di crediti in garanzia
No full textIl contributo affronta il tema della causa del contratto di cessione del credito con causa di garanzia, analizzandolo nel prisma delle teorie sul negozio fiduciario
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