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In a hydroelectric project, settling basin is to be provided after intake for settlement of objectional sizes of sediments usually taken as 0.2 mm and above to prevent cavitation damage of the turbines. In conventional design of settling basin, the horizontal and vertical transitions are provided to connect the basin with intake concurrently resulting in separation and jet type flow resulting in highly non-uniform distribution of velocity within the basin. Discusser has suggested modification in the design of inlet expanding transition by providing wide-angle horizontal expansion in plan followed by vertical transition so that the inflow is diffused to the full width of the basin by using adverse slope of transition floor. [ABSTRACT FROM AUTHOR]

The verticality of a dynamically installed anchor (DIA) during free fall in water is vital for successful installation of the anchor within the seabed. Two lines, a deployment line and a mooring chain, are usually used to install a DIA. The mooring chain is prereleased to the seabed surface, presenting a catenary profile in the seawater. When the anchor is released and falls freely in the seawater, the verticality of the anchor is affected by the tension on the pad eye resulting from the connected mooring chain. Therefore, a vertically hanging arrangement method for the mooring chain is proposed, which is effective in reducing the tilt angle of the anchor during free fall in water compared with the traditional catenary arrangement method. Model tests are subsequently designed to justify the effectivity of the vertical-hanging arrangement of the mooring chain. Finally, a theoretical framework is established to predict not only the fall velocity but also the tilt angle of the DIA during free fall in water, with the deployment line and mooring chain taken into consideration. [ABSTRACT FROM AUTHOR]

Continuous measurements of rainfall parameters are required for radio planning and flood management among others. However, due to the dearth of requisite infrastructure and expertise within the region, it is usually necessary to use satellite and reanalysis data. In this study, the performance of ECMWF ERA5 data in providing fall velocity (W) and liquid water content (LWC) estimates at different altitudes was compared with measurements from a ground-based Micro Rain Radar in a tropical location. Results obtained showed correlation values of 0.60–0.94 and 0.16–0.93 for fall velocity and liquid water content, respectively. The performance of ERA5 was found to decrease with altitude. In general, the results show the sufficient reliability of the ERA-5 reanalysis data for the two parameters on vertical scales. Therefore, it can be inferred that the ERA-5 data are reliable in estimating the average LWC and W, especially on a vertical scale for both hydrological and radio wave propagation studies. [ABSTRACT FROM AUTHOR]

Characteristics of raindrop size distribution during summer are studied by using the data from six Parsivel disdrometers located in the northeastern Tibetan Plateau. The analysis focuses on convective and stratiform precipitation at high altitudes from 2434 m to 4202 m. The results show that the contribution of stratiform and convective precipitation with rain rate between 1 ⩽R<5 mm h−1 to the total precipitation increases with altitude, and the raindrop scale and number concentration of convective precipitation is larger than that for stratiform precipitation. The droplet size spectra of both stratiform and convective precipitation shows a single peak with a peak particle size between 0.31–0.50 mm, and they have essentially the same peak particle size and number concentration at the same altitude. The maximum spectral widths of stratiform clouds are between 4 mm and 5 mm, while those of convective clouds range from 4 mm to 8 mm. The Gamma distribution is more suitable than the Marshall-Palmer distribution in terms of the actual raindrop spectrum distribution. The stratiform precipitation particles are smaller with higher number concentration, while the opposite is true for the convective precipitation particles. The convective precipitation particles drop faster than stratiform precipitation particles when the particle size exceeds 2 mm, and the falling velocity of raindrops after standard curve fitting is underestimated during the observation period. Moreover, conventional radar estimation methods would underestimate the precipitation in the Northeastern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The fall velocity of sediment particles plays a key role in sediment transport studies. Researchers have attempted to determine the terminal fall velocity, and most of the studies in this regard have been based on experimental, quasi-experimental, and in-situ measurements. The present study aimed to use a numerical model to estimate the fall velocity of a single sediment particle in distilled and motionless water. We used spherical quartz particles with the diameters of 0.77, 1.09, 2.18, and 4.36 millimeters and density of 2,650 kg/m3. The Flow-3D software was applied to estimate the fall velocity based on the environment of experiment by Ferguson and Church (2004) using the void of flow method. The main objective of this research was to demonstrate the power of the numerical model to simulate the fall velocity of sediment particles. To validate the results of the model, they were compared with the experimental results and 26 well-known publications during 1933-2016 using the root-square-mean and mean-absolute-percentage errors. The results showed good agreement between the experimental and numerical data. Therefore, the proposed numerical model could be used to determine the fall velocity of sediment particles with a wide range of diameters in the proposed environment and particle types.

In the present work, particle arrangement and their packing in the sediment layer of zirconia suspension were studied. To evaluate the particle settling, aqueous suspensions of zirconia nanoparticles were prepared in different dispersion states. In one state, Dolapix CE64 was used as a dispersant to provide electrosteric mechanism. In another state, pH of the suspension was adjusted at 4 to provide electrostatic mechanism. The other state was the combination of dispersant and pH adjustment which resulted in the most stable suspension. First of all, the stability of all dispersion states was evaluated by zeta potential, sediment volume (SV) and height, viscosity, and packing density (PD). Then, the sediment layers of all suspensions were characterized. Incorporation of electrostatic mechanism was resulted in a main decrease in viscosity with high surface charges, while electrosteric mechanism caused lower sedimentation of particles. Fall velocities of particles/agglomerates were estimated, and the influences of dispersion states on the particles fall velocities were characterized. The microstructural observation revealed homogeneous packing of particles in the sediment layer of the stable suspension demonstrating the proper dispersion of particles. Dolapix CE64 and pH adjustment resulted in a uniform arrangement of particles without agglomeration and spherical and regular granules with a uniform shape. [ABSTRACT FROM AUTHOR]

Through the collision and freezing of supercooled droplets on the surface of snow particles, riming can change the density, fall velocity and aspect ratio of snow particles. Riming plays an important role in the formation of precipitation in cold clouds. To our knowledge, the impact of riming on snow microphysics is yet well understood, and there is a lack of observations obtained in China addressing this issue. For the first time in East China, the connection between riming and snow microphysical properties was investigated quantitatively during snow events in two winters. To quantify the degree of riming, the rime mass fraction (FR) is derived using the combination of a 2‐D video disdrometer (2DVD) and a weighing gauge. FR is added to the density‐diameter and fall velocity‐diameter relations, and a quantitative relation between riming and shapes of snowflakes is established based on the in situ observation of the 2DVD. The results show that riming can well explain the variability in the density and fall velocity of snowflakes. The changes in the shape of snowflakes can be divided into two distinct stages with increasing riming: in the initial stage (FR < 0.5), the aspect ratio increases very slowly, while in the later stage (FR > 0.5), the aspect ratio increases rapidly. Direct observations of the continuous changes in the shapes of snowflakes with riming are in good agreement with the retrieval results of radar. Key Points: The impact of riming on snow microphysical properties is reported for the first time in East ChinaA quantitative relation between riming and the shapes of snowflakes is established based on in situ observations with a 2‐D video disdrometerThe changes in the shapes of snowflakes can be divided into two distinct stages as riming progresses [ABSTRACT FROM AUTHOR]

In reservoir of dams, especially near the dam body, the cohesive fine sediments are deposited mainly. The mechanical behavior of these sediments is largely controlled by the interparticle attraction caused by electrostatic and physiochemical forces. these properties cause the stickiness and accumulation of clay particles and formation of dense masses called flocs, which is sometimes referred to as flocculation. flocculation is influenced by several factors including salinity, flow regime, sediment concentration and organic matter. Flocculation is the most important factor that makes the settling, fall velocity and transfer of cohesive sediment considerably more complex and dynamic than non-cohesive sediments. In order to determine the relations governing cohesive sediments, the physical characteristics and behavior of these sediments should be identified. The terminal settling velocity of sedimentary particles in liquids, called particle fall velocity, is one of the most important properties in determining the physical properties of sediments caused transfer, deposition and consolidation. The fall velocity of cohesive sediments is influenced by many factors, including salinity, initial particle size, turbulence, temperature of water, and suspended sediment concentration. McLaughlin (1959) provided a method for measuring the fall velocity of particle (ω) in stillwater, using a settling cylinder with 10 cm diameter and less than 1 meter height and established a differential formula based on its research as follows: d(wC)/dz+dC/dt=0 (1) Fathi Moghadam et al (2011) studied the settling properties of the cohesive sediments in the Dez Dam reservoir. They concluded that particles for all concentrated samples and in all depths reached to their maximum fall velocity approximately at the same time (15 minutes after starting the test). The lower concentration samples appeared to have higher maximum fall velocities than the higher concentration samples, but for a shorter duration.

The determination of the fall velocity of non-cohesive particles (sand) was measured experimentally by using a technique of filming with a commercial camera. The diameters of the sediment ranged from 0.075 mm to 2 mm, the experimental values were compared with the ones calculated by empirical equations, four equations only consider the diameter to calculate the falling speed, the fifth equation considers diameter and form factor (cfs), and sixth equation considers diameter, form factor, and roundness (P). The results showed that the filming technique allows measuring the falling speed adequately and a sediment diameter range from 0.075 mm to 1.2 mm, the equation that includes the form factors and roundness had one of the best performances. [ABSTRACT FROM AUTHOR]

The study of the fall velocity variation of fine sediment in estuarine areas plays an important role in determining how various factors affect the flocculation process. Previous experimental studies have focused solely on the relation between the median fall velocity and influencing factors, while in the current study, the variation of the fall velocity in quiescent water also was examined. The experimental results showed that the vertical distribution of sediment concentration was more uneven, and larger variations occurred earlier during the settling process under higher salinity and/or sediment concentration conditions. The fall velocity initially increased then decreased over time, peaking at ∼20 min after settling began, and stabilizing at a value similar to that in freshwater, regardless of the initial sediment concentration and salinity combinations. Along the water depth, the fall velocity increased monotonically with a gradually decreasing gradient. The median fall velocity increased then decreased with increased salinity. The salinity at which the peak fall velocity occurred depended on the initial sediment concentration. The relation between the median fall velocity and initial sediment concentration displayed an obvious two-stage pattern (i.e., accelerated flocculation and decelerated, hindered settling) at higher salinities; whereas the maximum median fall velocity was observed at two consecutive sediment concentration values under lower salinity conditions. Finally, an empirical equation estimating the median fall velocity of cohesive fine sediment was formulated, incorporating the effects of both salinity and sediment concentration. [ABSTRACT FROM AUTHOR]

The Weather Research and Forecasting (WRF) Double-Moment 7-Class (WDM7) cloud microphysics scheme was developed to parameterize cloud and precipitation processes explicitly for mesoscale phenomena in the Korean Integrated Model system. However, the WDM7 scheme has not been evaluated for any precipitating convection system over the Korean peninsula. This study modified WDM7 and evaluated simulated convection during summer and winter. The suggested modifications included the integration of the new fall velocity–diameter relationship of raindrops and mass-weighted terminal velocity of solid-phase precipitable hydrometeors (the latter is for representing mixed-phase particles). The mass-weighted terminal velocity for snow and graupel has been suggested by Dudhia et al. (2008) to allow for a more realistic representation of partially rimed particles. The WDM7 scheme having an additional hail category does not apply this terminal velocity only for hail. Additionally, the impact of enhanced collision-coalescence (C-C) efficiency was investigated. An experiment with enhanced C-C efficiency overall improved the precipitation skill scores, such as probability of detection, equitable threat score, and spatial pattern correlation, compared with those of the control experiment for the summer and winter cases. With application of the new mass-weighted terminal velocity of solid-phase hydrometeors, the hail mixing ratio at the surface was considerably reduced, and rain shafts slowed down low-level winds for the winter convective system. Consequently, the simulated hydrometeors were consistent with observations retrieved via remote sensing. The fall velocity–diameter relationship of raindrops further reduced the cloud ice amount. The proposed modifications in our study improved the simulated precipitation and hydrometeor profiles, especially for the selected winter convection case.

In Nanjing, China, a comparative measurement of rainfall by using a self-developed Precipitation Microphysical Characteristics Sensor (PMCS), a 2D Video Disdrometer (2DVD), a OTT PARSIVEL Disdrometer (OTT), and a tipping bucket rain gauge (Gauge) is presented to quantitatively evaluate their performances. The rain rate, rainfall duration, rainfall accumulation, raindrop size distribution (DSD), Z -R relationship, μ-λ relationship, fall velocity, axis ratio, and their features are compared and discussed in detail. The PMCS, 2DVD, OTT and Gauge can measure the rainfall accumulation accurately, but the Gauge tends to underestimate the rainfall duration and rain rate significantly. The optical-based PMCS, 2DVD, and OTT can monitor rainfall with a higher precision in a wide range of rain rate. The PMCS, 2DVD, and OTT agree well between 0.8 and 3.25 mm. The PMCS is able to measure raindrops of at least 0.1 mm, and both the 2DVD and OTT can measure more large raindrops (> 3.5 mm). For fall velocity and axis ratio, both the PMCS and 2DVD basically agree well with the Atlas relationship, and the PMCS can describe the fall velocity with a lower deviation. It is interesting that 2.5% raindrops larger than 3 mm are found to be spherical, it may be attributed to the periodic oscillation of raindrops. This study reveals that a combination of the PMCS and 2DVD can compensate for their shortcomings and promote the DSD measurement in the entire drop size spectrum, and it's highly suggested that the reliability of DSD measurements needs careful cross-assessment after quality control. • PMCS, 2DVD, and OTT can monitor rainfall with a higher precision than rain gauge in a wide range of rain rate. • PMCS and 2DVD have the best correlation in convective rain, PMCS and OTT have the worst correlation in convective rain. • PMCS can describe the fall velocity of raindrops with a lower deviation. • A combination of PMCS and 2DVD can promote the DSD measurement in the entire drop size spectrum. [ABSTRACT FROM AUTHOR]

In the present study, we analyzed the particle size distribution (PSD) of falling volcanic ash particles measured using optical disdrometers during six explosive eruptions of the Sakurajima volcano in Kagoshima Prefecture, Japan. Assuming the gamma PSD model, which is commonly used in radar meteorology, we examined the relationships between each of the gamma PSD parameters (the intercept parameter, the slope parameter, and the shape parameter) calculated by the complete moment method. It was shown that there were good correlations between each of the gamma PSD parameters, which might be one of the characteristics of falling volcanic ash particles. We found from the normalized gamma PSD analysis that the normalized intercept parameter and mass-weighted mean diameter are suitable for estimating the ash fall rate. We also derived empirical power law relationships between pairs of integrated PSD parameters: the ash fall rate, the volcanic ash mass concentration, the reflectivity factor, and the total number of ash particles per unit volume. The results of the present study provide essential information for studying microphysical processes in volcanic ash clouds, developing a method for quantitative ash fall estimation using weather radar, and improving ash transport and sedimentation models.

Abstract In this study, the structure and dynamics of tropical cirrus clouds were characterized based on their microphysical properties. The altitude and temperature dependence of the microphysical properties, their interdependence and the most probable shape of the crystals in cirrus clouds were investigated. Studies on the effective size distribution of ice particles, which decides the lifetime of the cirrus clouds is important to understand the radiative properties of the clouds. The small sized crystals having low fall velocities undergo homogeneous nucleation processes resulting in cirrus with longer life time. The microphysical properties of these tropical cirrus, and the role of fall velocity in radiative transfer are discussed from the data obtained using the ground based lidar system over the tropical site Gadanki [13.5° N, 79.2°E], India over a period of 5 years from 2006 to 2010. The CALIPSO satellite based CALIOP lidar observations are used to fortify the ground based observation. It is noted that the life time of the cirrus is enhanced due to the decrease in cloud temperature. Highlights • The relation between the ice crystal morphology and radiation have been studied with the data obtained from the ground based lidar system installed at NARL, Gadanki (13.5° N, 79.2° E) and observation has been further verified with the available night time observations from the CALIOP on board the CALIPSO satellite. • Ice crystal morphology has been derived by obtaining lidar ratio, depolarisation ratio, optical depth, mid cloud temperature and extinction coefficient from both the ground based and satellite data. • The fall-velocity of the ice crystals increases with temperature, indicating the influence of particle growth in cirrus coverage and their temperature dependence. The linear relation between particle size and fall velocity has been time-honored with the above data. • From the present study, it is observed that C IR is less than 9 W/m2 for sub-visual cirrus clouds and its ranges from 9 to 80 W/m2 for the observed thin cirrus clouds and C IR is much higher for optically thick clouds (>80 W/m2). • The lower values of ice crystal fall velocity in cirrus clouds observation indicate slow cirrus clouds dissipation over tropics and resulting significant increase in the atmospheric heating process. [ABSTRACT FROM AUTHOR]

New experimental measurements of the falling velocity, size and orientation of individual hexagonal plate‐like ice crystals are reported. The measurements were conducted at three different temperatures: −13, −16 and −20 °C. The diameter of the ice crystals measured in the experiments were between 50 μm and 250 μm, a size range which is in agreement with the size found in natural clouds. In this range, ice crystals show a random orientation during free fall and a falling velocity which increases with size. Results show that the fall velocity is insensitive to the temperature at which the ice crystals grow for the temperatures used in this study. An empirical power‐law between the Best and Reynolds numbers is presented using the capacitance as characteristic length and an estimation of the ice crystal mass. Despite the dispersion of the experimental data, the Best–Reynolds relationship found seems to be similar to the relationship for falling spheres in Stokes flow using the capacitance as the hydrodynamic radius. The fall velocities of hexagonal and columnar ice crystals were compared. The columnar ice crystals show a velocity larger than that of hexagonal ice crystals with the same value of capacitance. However, both crystalline habits show a unique empirical Be–Re relationship. The sedimentation of ice crystals has a relevant role in several cloud process and in the Earth–Sun radiative budget. New experimental measurements of the fall velocity, size and falling orientation of hexagonal planar ice crystals are presented. The fall velocity of ice crystals increases with size and the ice crystals show a random orientation during free fall. A unique empirical power‐law relationship between the Best and Reynolds numbers for hexagonal and columnar ice crystals is reported. [ABSTRACT FROM AUTHOR]

Abstract: Turbidity currents run out over 100 km in lakes and reservoirs, and over 1,000 km in the ocean. They do so without dissipating themselves via excess entrainment of ambient water. Existing layer‐averaged formulations cannot capture this. We use a numerical model to describe the temporal evolution of a turbidity current toward steady state under condition of zero net sediment flux at the bed. The flow self‐partitions itself into two layers. The lower “driving layer” approaches an invariant flow thickness, velocity profile, and suspended sediment concentration profile that sequesters nearly all of the suspended sediment. This layer can continue indefinitely at steady state over a constant bed slope. The upper “driven layer” contains a small fraction of the suspended sediment. The devolution of the flow into these two layers likely allows the driving layer to run out long distances. [ABSTRACT FROM AUTHOR]

Determination of the fall velocity of cohesive sediment particles is very complex because of the adhesion properties of cohesive sediments and flocculation phenomenon. Therefore the deposition of fine sediments was investigated using an annular flume and in this work the hydraulic parameters was measured using Acoustic Doppler Velocimeter (ADV). Based on the experimental results the fall velocity of cohesive sediment was determined for different shear stresses values and different concentrations based on some empirical equations. The obtained results showed that a good agreement was observed between the Krone's equation and Burban's relationship. The obtained result from Lou and Krishnappan's method was inversely comparison the aforementioned equation. Finally, the results showed that the fall velocity is depended on the flow shear stress as well as the sediment concentration.

Fall velocity of sediment grains affect sediment transport phenomenon significantly. In the present study some experiments were conducted in a circular flume of the Hydraulic laboratory of Shahrekord University, Iran, with 1.9 m of outer diameter. Cohesive sediment samples were taken from the North Karun watershed. Modified Krons's equation has been applied for calculating of the cohesive sediments fall velocity. In order to determine the equation's parameters, the velocity and the shear stress profile and sediment concentration were measured at various times for each experiment. The results showed that increasing in concentration causes the fall velocity of the sediments to be decreased. It denotes inverse correlation between sediment concentration and fall velocity. Finally, based on the obtained results of this study an equation was obtained to calculate the cohesive sediments fall velocity.

Fall velocity (FV) is such a fundamental parameter for sediment researchers that its accurate determination has been regarded as a top priority in improving numerical modelling and conceptual understanding of fine sediment dynamics. With their cohesive nature, fine sediments are prone to aggregation and form flocculation network structures (flocs). The rheological behaviour of fluid may complicate this problem. By means of a new apparatus, FV of fine Yangtze Estuary sediment can be studied in the laboratory. The experimental data show that (1) suspended sediment concentration (SSC), salinity and temperature all affect FV, but to different extents; (2) the relationships between the FV of estuarine fine sediments and its determinants are highly dependent upon specific environmental conditions; (3) the dependencies of various determinants (SSC, salinity and temperature) on FV in different flocculation stages are varied; and (4) for Yangtze estuarine mud, the FV peaks when the SSC is in the range 3–8 g/l, and the salinities for maximum flocculation settling are approximately 7 and 10 PSU in dry and wet seasons, respectively. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of raindrop size distributions (RSDs) and polarimetric radar parameters retrieved by T-matrix for stratiform and convective precipitation in Beijing and Zhangbei (northern China), and Yangjiang (southern China) are studied and compared based on RSD data observed with PARSIVEL disdrometers in these three different climatic regions. The effects of observed and fitted RSD on scattering simulation are also discussed. The conclusions further confirm the obvious variation of RSDs in different climatic regions and rain types. There is significant regional difference in rainfall microphysical parameters for convective precipitation, and small regional difference for stratiform precipitation, instead. Convective precipitations from Beijing and Yangjiang both have higher mass-weighted mean diameter D m and log10 N w (N w : normalized intercept parameter) values than stratiform precipitations. The averaged RSDs from both rain types in Beijing and Yangjiang are in good agreement with gamma distribution while those in Zhangbei cannot be well fitted either by gamma or M–P (Marshall–Palmer) distribution. It is essential to take into account the effect of air density on raindrop fall velocity in highlands far away from sea level, such as Zhangbei. The μ–Λ relation varies with location. For a given Λ value, the fits to the data in the three regions have higher μ values than Florida relation (Zhang et al., 2003). It is robust to retrieve polarimetric radar parameters by T-matrix. There is an exponential relationship between differential reflectivity Z DR and radar reflectivity factor Z H , as well as the relation between specific differential phase K DP and Z H . The variation of the relations in different climate regions and rain types results from RSD's sensitivity to climatic regions and rain types. Observed RSD is superior to the fitted one in retrieving polarimetric radar parameters. [Copyright &y& Elsevier]

Farrell, E.J. and Sherman, D.J., 2013. Estimates of the Schmidt Number for vertical flux distributions of wind-blown sand. In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.) From studies of suspended sediments in water or dust in air it is recognized that the Rouse profile represents a theoretically sound, first approximation of characteristic sediment concentration gradients. Rouse (1938) combined the influence of grain size and shear velocity changes into a universal equation. The Rouse number relates sediment size (in the form of settling velocity, w 0) to shear velocity, the von Kármán constant (0.4) and the Schmidt Number, typically assumed to be equal to 1.0 but with much larger values reported. The shape of the Rouse concentration profile is controlled by the Rouse number exponent. We applied the Rouse profile model to 14 vertical flux profiles of wind-blown sand measured during a field experiment in Jericoacoara, Brazil in 2008. These data were supplemented with 96 vertical flux profiles obtained from fourteen wind tunnel and field experiments reported in the literature, for a total of 110 profiles. The analyses show that the performance of the Rouse model is not sensitive to changes in the range of variability we can expect to observe in values of fall velocity, shear velocity and the von Kármán constant but is very sensitive to changes in the values of the Schmidt number. First, the Rouse model was applied to vertical flux profiles using the common Schmidt number value of 1.0 in the Rouse number exponent. The model performed poorly in predicting the shape and magnitude of the vertical flux distributions (38% and 18% of the observed transport rate in field and wind tunnels, respectively). Alternative values were derived by adapting the profiles to equivalent concentration Rouse-type ratios and obtaining the slope values of the fitted power functions. With this approach, in field and wind tunnel experiments, the values of the Schmidt number ranged from 4.46 - 19.10 and 0.68 - 23.24, respectively, and predicted, on average, 82% and 90% of the observed transport rate. We tested a third method that does not require measuring vertical flux profiles. We found a strong relationship between the Schmidt number and a shear velocity - fall velocity ratio ( r 2= 0.65), with values ranging from 6.11 - 14.80 and 0.86 - 17.83, for field and wind tunnel experiments, respectively. These modified values of the Rouse exponents also resulted in vertical profiles that predicted similar distributions to the observed flux data and provide good predictions of the total transport rates (86% and 81% in field and wind tunnels experiments, respectively). To our knowledge, these are the first values of the Schmidt number derived for aeolian sand transport and the first successful application of the Rouse model to vertical flux profiles of saltation. [ABSTRACT FROM AUTHOR]

In small hydropower (SHP) projects, desilting tank is an important component, which protects the mechanical equipment specially turbine from the silt carried by the water conductor system. Desilting tanks are used in water treatment plants and hydropower channels to remove objectionable sediment of a specified size and quantity. Design of desilting tank requires evaluation of sediment removal efficiency over the range of sediment particle sizes. In the design of desilting tank, various combinations of width, depth, and length of the tank are possible to achieve desired silt removal efficiency. In the present study, an attempt has been made to develop correlations for sizing of desilting tank and estimation of quantities of various materials required in the construction of desilting tank. The results obtained were verified with the data collected on desilting tanks of recently developed SHP projects. The results are found within maximum error of ±12%. The correlation developed can be used for sizing and cost estimation of the desilting tank. [ABSTRACT FROM PUBLISHER]

Abstract: The study of the settling behaviour of particles in viscoplastic fluids is closely related to the study of rheology. In this paper, a thorough examination of the flow behaviour of viscoplastic fluids, in the form of aqueous polyacrylamide solutions, has been presented. The results of this study suggest that the experimental fluids exhibit time-dependent flow characteristics, where the apparent viscosity of the solutions depends highly on their shear history. This time dependency has been attributed towards the processes of destruction and rejuvenation in the ‘structural network’ of the fluids (due to the presence of hydrogen bonding between polyacrylamide and water molecules), as they are subjected to changing rates of shear. A new fluid model was thus developed to capture this flow behaviour. This model, termed as ‘semi-viscoplastic’, features temporary yield stress characteristics that tend to dissipate once the structural network of the fluid is destroyed due to the application of shear. The time dependency of the fluid viscous parameters becomes apparent in the settling sphere experiment, where it has been demonstrated that a sphere that is following the flow path of another sphere tends to attain a fall velocity that is significantly higher than the preceding sphere. Based on this finding, a new generalised correlation has been developed, through which predictions of the fall velocity of spherical particles settling through viscoplastic fluids, of various shear history, can be made. [ABSTRACT FROM AUTHOR]

Abstract: In a cloud of blowing sand, collisions between falling sand grains and the sand bed are an important process that is the main source of additional saltating sand grains, because the collision process transfers momentum to the sand bed and initiates the liftoff of new sand grains. The initial liftoff and fall velocities of saltating sand grains are key parameters in the collision process, but little information exists on the subsequent fall velocities of the saltating sand grains. One important reason for this lack is that the collision process is unclear. Based on experimental data from high-speed multi-flash photographic images obtained in a wind tunnel and on a motion model of saltating sand grains, this paper discusses the fall velocities of saltating sand grains and the corresponding velocity frequency distributions. The results demonstrate that fall angles are small (less than 20°), and decrease with increasing frictional wind velocity. The vertical component of fall velocity increases with increasing frictional wind velocity, and is consistent with the initial vertical liftoff velocity. The fall angle and vertical velocity both follow a gamma distribution. The horizontal fall velocity mainly determines the resultant fall velocity, and both increase with increasing frictional wind velocity. The horizontal and resultant fall velocities follow a Pearson IV distribution. These results improve our understanding of the collision process between falling sand grains and sand beds and will also help us to modify the splash function that connects the parameters of falling saltating sand grains and their initial liftoff parameters. [Copyright &y& Elsevier]

The shape parameter is important for the theoretical description of the sandy coastal profiles. This parameter has previously been defined as a function of the sediment-settling velocity. However, the settling velocity cannot be characterized over a wide range of sediment grains. This, in turn, limits the calculation of the shape parameter over a wide range. This paper provides a simpler and faster analytical equation to describe the shape parameter. The validity of the equation has been tested and compared with the previously estimated values given in both graphical and tabular forms. The results of this study indicate that the analytical solutions of the shape parameter improved the usability of profile better than graphical solutions, predicting better results both at the surf zone and offshore. [ABSTRACT FROM AUTHOR]

Abstract: New methods are proposed for predicting the hindered settling conditions encountered by concentrated suspensions containing mixtures of sand particles and mud flocs. These methods, based on two-fraction formulations, are developed by consideration of the settling characteristics of monodisperse and polydisperse solid particle suspensions applied to cohesive/non-cohesive mixtures of mud flocs and sand particles. The behaviour of these predictive methods is evaluated over a wide range of mixture conditions and compared with existing formulations, with their parametric dependence on the relative volumetric concentrations and floc/particle sizes for the mud and sand constituents established. The results indicate that consideration of the full return flow effects generated by both fractions provides the best modelling framework for predicting the hindered settling conditions over a wide range of sand–mud mixtures. [Copyright &y& Elsevier]

Abstract: One of the factors contributing to the uncertainties involved in the estimation of particle settling velocity in viscoplastic fluids is the time-dependent effect where the viscous parameters of the fluid change as a particle flows through and shears the medium. These changes, particularly at low shear Reynolds numbers, are reflected in the settling velocity of a following sphere that is released some time after an initial one, with the following sphere having a significantly greater velocity. This study found that changes in both fall velocity and equivalent viscosity can be correlated satisfactorily by a power law equation to the dimensionless form of the time interval between releases, and the rheogram shape factor for the fluid. A collision of particles occurs in cases where the time interval between releases is small, after which the particles combine and travel at a terminal velocity. A new variable, β, which takes into account the different surficial stress of the combined spheres, was introduced to the correlation of Wilson et al. [Wilson, K.C., Horsley, R.R., Kealy, T., Reizes, J.A., Horsley, M.R., 2003. Direct prediction of fall velocities in non-Newtonian materials. Int. J. Miner. Process. 71, 17–30] β was found to depend on the rheogram shape factor for the fluid and the shear Reynolds number for the particle. The validity of this approach was supported by experimental data. [Copyright &y& Elsevier]

This paper examines the transport of calcareous sand in unidirectional flow and its prediction through existing sediment transport models. A flume experiment of four sand samples collected on Oahu, Hawaii, provides 29 sets of sediment transport data in the bed-form and suspended transport stages. The measured transport data are compared with direct predictions from four energy-based transport models developed for siliceous particles. Corrections for the grain-size, fall velocity, and critical velocity of calcareous sand based on recent research are applied to the models and the results are compared with the direct calculations and measured data. The comparison illustrates the important role particle shape plays in the transport of calcareous sand. All four sediment transport models give consistent predictions and good agreement with the majority of the measured data. Two of the models respond positively to the corrections in both the bed-form and suspended transport stages indicating that such an approach may provide an interim solution for the transport of calcareous sand. [ABSTRACT FROM AUTHOR]

Viscoplastic drilling fluids discharge excavated solid particles in soft soil tunnelling. Deposition of particles in pipes and boreholes should be kept to a minimum. To quantify the fall velocity of particles in shear flow different combinations of fluid properties and particle sizes were tested. It is shown that co-rotation of the particles with the flow and vertical equilibrium of forces determine the fall velocity in these fluids. The use of a Stokes type formula for fall velocity in shear flow of tunnelling fluids is justified. [Copyright &y& Elsevier]

Abstract: Allochthonous fine organic particles (0.45μm–1mm, FPOM) are a significant natural source for biotic production of alkalinity in acidic, post-mining waters. Hence, the depositional characteristics and the nature of FPOM are of interest to remediation. Our objectives were firstly, to determine interactions between allochthonous FPOM and water affected by mine drainage and secondly, to verify the consequence on particle fall velocity, a major factor in particle distribution in streams and lakes. In laboratory experiments we studied interactions between particles (shredded leaves) and two types of post-mining water (very acidic, pH 3.7; extremely acidic, pH 2.9). Particle fall velocity was assessed by gravity sedimentation analysis. Suspended particles released minerals (1mass%) and organic compounds (10mass%) within 25h. Fe3+ was quickly reduced to Fe2+ in aerobic conditions by organic reductants. Most of the iron attached to the particles (88% in very acidic and 71% in extremely acidic water) when the particle concentration was high. The median particle fall velocity was in acidic, iron-rich water up to 3 times higher than in distilled water. We assume particle aggregation in the highly mineralised water to be the reason for the increase in particle fall velocity. [Copyright &y& Elsevier]

New data on the settling velocity of artificial sediments and natural sands at high concentrations are presented. The data are compared with a widely used semiempirical Richardson and Zaki equation (Trans. Inst. Chem. Eng. 32 (1954) 35), which gives an accurate measure of the reduction in velocity as a function of concentration and an experimentally determined empirical power n. Here, a simple method of determining n is presented using standard equations for the clear water settling velocity and the seepage flow within fixed sediment beds. The resulting values for n are compared against values derived from new and existing laboratory data for beach and filter sands. For sands, the appropriate values of n are found to differ significantly from those suggested by Richardson and Zaki for spheres, and are typically larger, corresponding to a greater reduction in settling velocity at high concentrations. For fine and medium sands at concentrations of order 0.4, the hindered settling velocity reduces to about 70% of that expected using values of n derived for spheres. At concentrations of order 0.15, the hindered settling velocity reduces to less than half of the settling velocity in clear water. These reduced settling velocities have important implications for sediment transport modelling close to, and within, sheet flow layers and in the swash zone. [Copyright &y& Elsevier]

The estimation of particle fall velocities in viscoplastic materials is still full of uncertainties, despite the number of experimental studies and data correlations performed over the years by various authors. In addition to increasing the corpus of experimental data, the present paper develops a new method for direct (non-iterative) prediction of these velocities. Existing techniques employ the customary curve of drag coefficient versus Reynolds number, which has the major disadvantage that the required fall velocity occurs in both co-ordinates. A new curve (which can be applied to both Newtonian and non-Newtonian fluids) has been devised to overcome this difficulty, and thus eliminate the need for iteration. For non-Newtonian materials the new curve is used with an ‘equivalent Newtonian viscosity’, which has been calibrated against available experimental data. It is shown how this viscosity can be determined directly from the rheogram, without the need for a specific fit equation. [Copyright &y& Elsevier]

The Weather Research and Forecasting (WRF) Double-Moment 7-Class (WDM7) cloud microphysics scheme was developed to parameterize cloud and precipitation processes explicitly for mesoscale phenomena in the Korean Integrated Model system. However, the WDM7 scheme has not been evaluated for any precipitating convection system over the Korean peninsula. This study modified WDM7 and evaluated simulated convection during summer and winter. The suggested modifications included the integration of the new fall velocity–diameter relationship of raindrops and mass-weighted terminal velocity of solid-phase precipitable hydrometeors (the latter is for representing mixed-phase particles). The mass-weighted terminal velocity for snow and graupel has been suggested by Dudhia et al. (2008) to allow for a more realistic representation of partially rimed particles. The WDM7 scheme having an additional hail category does not apply this terminal velocity only for hail. Additionally, the impact of enhanced collision-coalescence (C-C) efficiency was investigated. An experiment with enhanced C-C efficiency overall improved the precipitation skill scores, such as probability of detection, equitable threat score, and spatial pattern correlation, compared with those of the control experiment for the summer and winter cases. With application of the new mass-weighted terminal velocity of solid-phase hydrometeors, the hail mixing ratio at the surface was considerably reduced, and rain shafts slowed down low-level winds for the winter convective system. Consequently, the simulated hydrometeors were consistent with observations retrieved via remote sensing. The fall velocity–diameter relationship of raindrops further reduced the cloud ice amount. The proposed modifications in our study improved the simulated precipitation and hydrometeor profiles, especially for the selected winter convection case. [ABSTRACT FROM AUTHOR]

A large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 μm in diameter. The majority of these particles become incorporated into a wide range of aggregate types, the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory. In the absence of such data, theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities. Larger volcanic ejecta often consists of pumice of lower than bulk density. Experimental data are presented for the fall velocities of porous aggregates and single particles, determined in systems analogous to that of ejecta falling from a volcanic plume. It is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity, and a size corresponding to an enclosing sphere. Particles incorporated into aggregates attain a substantially higher fall velocity than single particles. This is due to the larger physical dimensions of the aggregate, which overcomes the effect of lower aggregate density. Additionally, the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity. The increase in fall velocity of particles incorporated into aggregates, rather than falling individually, results in the enhanced removal of fine material from volcanic plumes. [ABSTRACT FROM AUTHOR]

Snow falling into a melting layer will eventually consist of a fraction of meltwater and hence change its characteristics in terms of size, shape, density and fall speed. Most microphysical parameterizations in numerical weather prediction models typically only represent purely solid or liquid hydrometeors. Generally, this has been an acceptable compromise since the melting layer is typically very shallow and adding a mixed solid/liquid particle type would result in increased computational time. This research shows how improvements were made to the treatment of melting snow in a microphysical parameterization within the Weather Research and Forecasting (WRF) model by implementing an approximation of snowflake melted fraction together with a physically-based expression for melting particle terminal velocity. In addition, the more appropriate definition of melting level defined by the wet-bulb temperature was consistently used in various process rates, all while not adding additional prognostic variables that would add computational cost. Multiple events observed during the 2015–2016 Olympic Mountain Experiment (OLYMPEX) were used to compare with the WRF model results. The modified scheme is able to represent disdrometer observations of joint particle size and fall velocity during wet snow events, as well as fall velocity profiles through the melting layer derived from a vertically-pointing radar. The improved scheme removes 'bulls' eyes' of snow accumulation in lee-side areas within the melting zone, and should result in better predictions of surface precipitation phase and amount. • NWP model lee-side melting zone snow overprediction problem is investigated. • Melting snow fall velocity in a microphysics scheme is improved. • The modified scheme compares well with disdrometer and MRR measurements. • The 'bulls-eye' snow accumulations are gone, and snow is advected to nearby areas. [ABSTRACT FROM AUTHOR]