Powder Avalanche Research Articles

Powder flowability characterisation at preheating temperature in additive manufacturing

ABSTRACT The powder spreading process is usually performed at preheating temperature in powder-bed-based additive manufacturing (AM). Thus, the powder flowability characterisation at preheating temperature is important for powder spreading processes. However, devices for traditional powder flowability characterising methods are mainly designed for specific conditions at room temperature and cannot consider the effect of temperature on powder flowability. In this work, an experimental platform with a heated rotating drum was set up and a high-speed camera was used to record powder avalanche processes in a heated rotating drum for the powder flowability characterisation at preheating temperature. Nylon and stainless steel powder flowability at different temperatures was assessed by the statistical analysis of avalanche angle, avalanche time, arithmetic mean deviation and surface linearity of powder surface profile. Four parameters provide a good characterisation of powder flowability. The results can provide guidance for the powder flowability characterisation method at preheating temperature in AM.

Powder snow avalanche impact on hanging cables

In this paper we perform a stress wave analysis to determine the forces in hanging cables under the action of powder snow avalanches. Powder avalanches are a serious threat to communities and infrastructure in mountainous regions. They endanger power transmission lines which transport electricity over avalanche prone terrain. Powder avalanches produce an elevated air-blast that induces above ground forces in hanging cables, leading to failure and often the over-turning of the supporting towers. We solve a nonlinear system of three wave equations with appropriate boundary conditions that describes the internal stress state and the cable deformation in three directions. The impact pressures are estimated by simulating powder avalanche motion in three-dimensional terrain. The stress wave model is tested numerically, the obtained results are in good accordance with experimental measurements. We also investigate a possibility of usage of hanging cables as loading sensors to reconstruct powder cloud pressures. For that we formulate an inverse problem, solve it analytically in time-independent case and study numerically the obtained solution.

Introduction of the energy to break an avalanche as a promising parameter for powder flowability prediction

Dynamic avalanche testing is a newer method useful even with cohesive powder samples. To evaluate flow properties of pharmaceutical powder excipients, break energy (BE) is introduced as a new promising avalanche parameter. Conventional pharmacopoeial methods and an annular shear cell were utilized to characterize 23 pharmaceutical powder samples including binary mixtures. Subsequently, rotating drum avalanching was studied to better understand behaviour of powders. A good linear regression between the energy to break a powder avalanche and powder cohesion with R2 = 0.823 was detected. The relationship between BE and the flow function value, estimated by shear testing, allowed to identify two samples with excellent flow functions: Cellets 100 and Neusilin US2. The latter sample also had very low BE although the mass flow rate was the poorest observed out of 23 samples. Therefore, the BE from avalanching testing seems to be a sensitive measure of cohesion and flow function behaviour.

Seismo-acoustic energy partitioning of a powder snow avalanche

Abstract. While flowing downhill, a snow avalanche radiates seismic waves in the ground and infrasonic waves in the atmosphere. Seismic energy is radiated by the dense basal layer flowing above the ground, while infrasound energy is likely radiated by the powder front. However, the mutual energy partitioning is not fully understood. We present infrasonic and seismic array data of a powder snow avalanche, which was released on 5 February 2016, in the Dischma valley above Davos, Switzerland. A five-element infrasound array, sensitive above 0.1 Hz, and a seven-element seismic array, sensitive above 4.5 Hz, were deployed at a short distance (<500 m) from each other and close (<1500 m) to the avalanche path. The avalanche dynamics were modelled by using RAMMS (rapid mass movement simulation) and characterized in terms of front velocity and flow height. The use of arrays rather than single sensors allowed us to increase the signal-to-noise ratio and to identify the event in terms of back-azimuth angle and apparent velocity of the recorded wave fields. Wave parameters, derived from array processing, were used to identify the avalanche path and highlight the areas, along the path, where seismic and infrasound energy radiation occurred. The analysis showed that seismic energy is radiated all along the avalanche path, from the initiation to the deposition area, while infrasound is radiated only from a limited sector, where the flow is accelerated and the powder cloud develops. The recorded seismic signal is characterized by scattered back-azimuth angle, suggesting that seismic energy is likely radiated by multiple sources acting at once. On the contrary, the infrasound signal is characterized by a clear variation of back-azimuth angle and apparent velocity. This indicates that infrasound energy radiation is dominated by a moving point source, likely consistent with the powder cloud. Thanks to such clear wave parameters, infrasound is revealed to be particularly efficient for avalanche detection and path identification. While the infrasound apparent velocity decreases as the flow moves downhill, the seismic apparent velocity is quite scattered but decreases to sound velocity during the phase of maximum infrasound radiation. This indicates an efficient process of infrasound to seismic energy transition, which, in our case, increases the recorded seismic amplitude by ∼20 %, at least in our frequency band of analysis. Such an effect can be accounted for when the avalanche magnitude is estimated from seismic amplitude. Presented results clearly indicate how the process of seismo-acoustic energy radiation by a powder avalanche is very complex and likely controlled by the powder cloud formation and dynamics, and the process is hence affected by the path geometry and snow characteristics.

The Avalanche of Les Fonts d’Arinsal (Andorra): An Example of a Pure Powder, Dry Snow Avalanche

On 8th February 1996, in the north-western part of Andorra in the Pyrenees, the Les Fonts d’Arinsal (LFd’A) pure powder avalanche was triggered, descending some 1200 m to the bottom of the Arinsal valley and continuing up the opposite slope for about 200 m. This size 4–5 avalanche reached velocities of up to 80 ms−1, devastated 18 ha of forest, involved a minimum volume of up to 1.8 × 106 m−3 and caused major damage to eight buildings. Fortunately, no one was injured thanks to an evacuation, but 322 people lost their properties. This study describes the physical characteristics of the LFd’A avalanche path and provides data on earlier avalanches, the meteorological synoptic situation and snowpack conditions that generated the avalanche episode, the warning and preventive actions carried out, the effects and evidence of the large avalanche, and the defence system implemented afterwards. A discussion of the avalanche dynamics based on observations and damage, including the role of snow entrainment, the total lack of characteristic dense flow deposits, as well as the evidence of a two-phase flow (fluidisation and suspension), is presented. This case study is an example of a paradigmatic large, pure powder, dry-snow avalanche, which will be useful for model calibration.

Dynamic magnification factors for tree blow-down by powder snow avalanche air blasts

Abstract. We study how short duration powder avalanche blasts can break and overturn tall trees. Tree blow-down is often used to back-calculate avalanche pressure and therefore constrain avalanche flow velocity and motion. We find that tall trees are susceptible to avalanche air blasts because the duration of the air blast is near to the period of vibration of tall trees, both in bending and root-plate overturning. Dynamic magnification factors for bending and overturning failures should therefore be considered when back-calculating avalanche impact pressures.

Granular dynamics of cohesive powders in a rotating drum as revealed by speckle visibility spectroscopy and synchronous measurement of forces due to avalanching

We have used speckle visibility spectroscopy (SVS) and synchronized force measurements to compare the granular dynamics of two cohesive lactose powders, with Sauter mean diameters of ~29 and ~151μm, in a rotating drum. A load cell (LC) was used to measure forces on the drum mounting frame and enable monitoring of bulk powder motion; SVS is a dynamic light scattering technique particularly suited for studying dynamics in dense, non-ergodic granular systems. Our results reveal that surface slumping and intermittent collisional dynamics in the bulk of the bed are correlated, especially for the fine more cohesive particles (Geldart group C/A boundary), but not as much for the less cohesive larger particles (Geldart group A/B boundary). The specific dissipation energy of the particles in the drum is similar for both powders, and increases linearly with increasing drum speed. However, the dependencies of the load cell and SVS signals on rotation speed have opposing trends for these two powders, indicating different dissipation mechanisms for the different Geldart Groups; collisional dissipation is more important for the Geldart C/A powder, while for the Geldart A/B powder avalanche dissipation is dominant.

Powder flowability characterisation methodology for powder-bed-based metal additive manufacturing

Key in powder-bed-based additive manufac- turing is the use of appropriate powder materials that fit to the process conditions. There are many parameters affect- ing the build process and the corresponding quality of the parts being built. Therefore, an accurate assessment of the powders becomes important. Such an assessment involves, besides others, the powder flowability, which should be sufficient in order to create good-quality powder layers. The current study aims at the development of suitable parameters and values for the qualification of metal pow- ders for selective laser melting (SLM) with regard to their flowability. The powder flowability is assessed by the statistical analysis of the measured powder avalanche angles and the powder surface fractal, which give valuable information about the significance of inter-particle forces. A set of 21 different Fe- and Ni-based powders has been analysed and good correlations between the powder ava- lanche angles, the surface fractal and the particle shape with the optically evaluated flowability could be derived. The method allows a quantitative powder flowability assessment, which correlates with the experiences for powders for powder-bed-based additive manufacturing, especially for SLM.

Investigation of Avalanche Time and Carr’s Index of Poultry Litter Powder as Flowability Indices

Abstract The world’s dependence on chemical fertilizer as the primary source for enriching agricultural fields is continually increasing that cause nature pollution. This has led researchers to aggressively investigate renewable fertilizer resources, biomass, to produce organic crops and reduced wastage. Poultry litter is a bulk solid and biomass feed stocks. Flow behavior of bulk solid is a critical factor in designing and developing suitable equipments (e.g. pelletizing machine). The bulk density, tap density, Carr’s index and powder avalanche time technique were applied to evaluate the flow properties of poultry litter. The experiments were carried out at moisture content (10, 20 and 30% w.b.), particle size (0.3, 0.6 and 1.18 mm) for the bulk and tap densities as well as Carr’s index. In addition to the moisture content (10, 20 and 30 %w.b.) and particle size (0.3, 0.6 and 1.18 mm) the rotational speed of drum (0.5, 1 and1.5 rpm) were also investigated for the avalanche time. The results showed that with increasing moisture content Carr’s index increased significantly (P<0.01) in the ranges of 16.2% to 18.5% and with increasing particle size the Carr’s index decreased from 20.35% to 14.78%. The litter powder avalanche time (AT) increased significantly (P<0.01) with increasing moisture content and decreasing rotational speed and particle size. The bulk and tap densities of the litter powder was decreased with increasing moisture content and increasing the particle size. The bulk and tap densities of the driest and finest poultry litter sample were higher than other ones.

Return period calculation and passive structure design at the Taconnaz avalanche path, France

AbstractThis paper aims to show how recent knowledge developed in the field of avalanche research can be used for a real case study, the Taconnaz avalanche path, where passive structures already existed but had to be improved. First a morphological analysis of the site is done and historical data are analysed. Second, each recorded event is back-calculated using a numerical model of dense-flow avalanches. For each surveyed avalanche, parameters at the entry of the runout zone upstream of the defence structures are defined. Third, a statistical analysis of these parameters allows characterization of 100 year return period events. Fourth, physical and numerical models of dense avalanches interacting with defence structures are combined in order to design the most effective passive structure able to contain the reference scenarios. Finally, physical and numerical modelling of the interaction between the powder avalanche and the designed defence structure is performed, to show that the proposed improvements do not increase the residual risk due to the powder part in areas downstream of the defence structures.

Numerical and Physical Modelling of the Effect of a Dam on Powder Avalanche Motion: Comparison with Previous Approaches

As part of the European project CADZIE, the interactionbetween powder avalanches and dams has been studied. First, we simulated a powder avalancheusing a heavy salt solution in a water tank. These experiments showed that (1) the ratio betweenthe maximum velocity parallel to the slope and the front velocity is greater than 1 (up to 1.5),and that (2) the velocity normal to the slope can rise to 74% of the front velocity. The presence ofa dam leads to a reduction of front velocity. The ratio between the maximum horizontal velocityand the front velocity increases with the dam height and reaches a maximum value of 1.9. Aswe did not have access to the density distribution in our experimental set-up, we calibrateda numerical model thanks to the experiments and we deduced the dynamic pressure. Our resultsshowed that, for an avalanche without obstacle, AVAER (Rapin, 1995) and Beghin and Closet'shypothesis (1990) lead to an overestimation of the dynamic pressure in the lower partof the flow as well as in the upper part, and that the use of front velocity for estimating the lossof kinetic energy leads to an overestimation of the dam's effectiveness.

Avalanche of 18 January 1997 on Brenva glacier, Mont Blanc Group, Western Italian Alps: an unusual process of formation

AbstractAt 1455 h on 18 January 1997, an airborne powder avalanche with a volume of about 3 × 106 m3 flowed along the tongue of Brenva glacier, high Aosta valley, on the southern flank of Val Veny, Italy It descended 2000 m and covered a distance of 5.5 km, with a rate of movement on the intermediate stretch of > 70 m s–1. It killed two skiers following a valley-bottom piste, damaged a hotel, destroyed a wide belt of woodland on the opposite slope and caused other minor damage. The avalanche occurred immediately after a large rockfall on the southern slope of the Sperone della Brenva (3567 m a.s.L). However, the relationship between the two events can only be considered indirect, since the main mass of the rockfall stopped at the base of the scar, on a large plateau on the glacier surface. The rockfall and subsequent shock caused the contemporaneous detachment of masses of ice and/or firn from hanging glaciers at the head of a nearby cirque, leading to the formation of the avalanche. The complex mechanism of detachment hypothesized was induced from available documentation and from in situ investigations. Its phases also coincide with seismograms recorded at a station 45 km from the avalanche area. Finally, a mathematical model of the avalanche was used to reconstruct its dynamics and path.

Two-phase numerical model of powder avalanche theory and application

In this paper the powder snow avalanche is considered as a two-phase flow (air and snow particles). The equations governing this flow are the fluid mechanics conservation laws. The mass and the momentum conservation are considered for each phase. The interaction between the two phases takes into account the drag force between the particle and the air. Owing to high turbulence in the powder flow, a closure model was used based on a modified k - e model in order to take into account the reduction of turbulence energy by the particles. The dense avalanche is modeled using the shallow water equations. The formation and the development of the powder avalanche is modeled using a mass and momentum exchanges between the powder flow and the dense flow. The flow area is digitized horizontally and vertically using a finite elements mesh. The numerical scheme is obtained by integrating the equations on each cell. The model thus built was calibrated using laboratory measurements of density current carried out in a flume. The model was successfully applied to reproduce many avalanches observed in France. At the end of this paper, an application of this model to an engineering case study is presented. It concerns the Uzengili path where an avalanche occurred in 1993. In this paper we use the integrated dense/powder avalanche model to define the effect of a powder avalanche flow in this path. Different simulations allow display of maps of the exposed zones for different available snow depths in the starting zone. The results were mapped in terms of dynamic pressure field and recommendations are proposed to the local authorities.

Simulation of a destructive avalanche at Maseguchi, Japan

In 1986, a large avalanche destroyed 11 houses and killed 13 people at Maseguchi, Japan. Previous attempts to model the avalanche were based on the assumption that it was a powder avalanche consisting of snow particles suspended by air turbulence. In this paper, the avalanche is modelled as a dry flowing avalanche with a dense core of flowing material at the base. It is suggested that for a comprehensive explanation of the observed damage and the characteristics of the avalanche deposit, the assumption that the avalanche was a flowing avalanche is more appropriate. The comparison of model results from a flowing versus a powder avalanche is of general interest for avalanche zoning and design of structures in avalanche-threatened areas.

Simulation of a destructive avalanche at Maseguchi, Japan

In 1986, a large avalanche destroyed 11 houses and killed 13 people at Maseguchi, Japan. Previous attempts to model the avalanche were based on the assumption that it was a powder avalanche consisting of snow particles suspended by air turbulence. In this paper, the avalanche is modelled as a dry flowing avalanche with a dense core of flowing material at the base. It is suggested that for a comprehensive explanation of the observed damage and the characteristics of the avalanche deposit, the assumption that the avalanche was a flowing avalanche is more appropriate. The comparison of model results from a flowing versus a powder avalanche is of general interest for avalanche zoning and design of structures in avalanche-threatened areas.

Determination of stresses behind a shock wave front in snow powder avalanche with allowance for the firmness of the solid phase

Determination of stresses behind a shock wave front in snow powder avalanche with allowance for the firmness of the solid phase

DYNAMICS OF SNOW AVALANCHE (WITH ESTIMATION FOR FORCE ON A BRIDGE)

Abstract Critical conditions for the occurrence of snow avalanches are first reviewed and summarized. Physical descriptions of slab avalanche, powder avalanche and mature powder avalanche are given. A two-dimensional dynamic model based on a fluid mechanics principle to describe snow avalanche is introduced. The vertical distribution of pressure forces above ground are estimated from this model based on some of the empirical relationships determined by A. Voellmy [18]. Finally, the estimations of the anticipated forces by a snow avalanche on a proposed bridge are given.

AN EXAMPLE OF DAMAGE FROM A POWDER AVALANCHE

Abstract A powder avalanche moved a 3,200 kg truck 19,8 m horizontally and dropped it 15 2 m into a gully without serious damage. Data from the site are used to compute the velocity of snow-free air and of a snow-air mixture necessary to cause this event. These velocities are found to be within the velocity of the avalanche as estimated from the equations published by A. Voellmy.