scholarly journals Free-fall experiments of volcanic ash particles using a 2-D video disdrometer

2019 ◽  
Vol 12 (10) ◽  
pp. 5363-5379 ◽  
Author(s):  
Sung-Ho Suh ◽  
Masayuki Maki ◽  
Masato Iguchi ◽  
Dong-In Lee ◽  
Akihiko Yamaji ◽  
...  
Keyword(s):  
Volcanic Ash ◽  
Free Fall ◽  
Oblate Spheroid ◽  
Vertical Axis ◽  
Prolate Spheroid ◽  
Terminal Velocity ◽  
Atmospheric Conditions ◽  
Axis Ratio ◽  
Mean Values ◽  
Ash Fall

Abstract. Information of aerodynamic parameters of volcanic ash particles, such as terminal velocity, axis ratio, and canting angle, are necessary for quantitative ash-fall estimations with weather radar. In this study, free-fall experiments of volcanic ash particles were accomplished using a two-dimensional video disdrometer under controlled conditions. Samples containing a rotating symmetric axis were selected and divided into five types according to shape and orientation: oblate spheroid with horizontal rotating axis (OH), oblate spheroid with vertical axis (OV), prolate spheroid with horizontal rotating axis (PH), prolate spheroid with vertical rotating axis (PV), and sphere (Sp). The horizontally (OH and PH) and vertically (OV and PV) oriented particles were present in proportions of 76 % and 22 %, and oblate and prolate spheroids were in proportions of 76 % and 24 %, respectively. The most common shape type was OH (57 %). The terminal velocities of OH, OV, PH, PV, and Sp were obtained analyzing 2-D video disdrometer data. The terminal velocities of PV were highest compared to those of other particle types. The lowest terminal velocities were found in OH particles. It is interesting that the terminal velocities for OH decreased rapidly in the range 0.5<D<1 mm, corresponding to the decrease in axis ratio (i.e., smaller the particle, the flatter the shape). The axis ratios of all particle types except Sp were found to be converged to 0.94 at D>2 mm. The histogram of canting angles followed unimodal and bimodal distributions with respect to horizontally and vertically oriented particles, respectively. The mean values and the standard deviation of entire particle shape types were close to 0 and 10∘, respectively, under calm atmospheric conditions.

scholarly journals Laboratory analysis of volcanic ash particles using a 2D video disdrometer

2019 ◽  
Author(s):  
Sung-Ho Suh ◽  
Masayuki Maki ◽  
Masato Iguchi ◽  
Dong-In Lee ◽  
Akihiko Yamaji ◽  
...  
Keyword(s):  
Volcanic Ash ◽  
Particle Diameter ◽  
Sharp Decrease ◽  
Oblate Spheroid ◽  
Physical Parameters ◽  
Atmospheric Conditions ◽  
Axis Ratio ◽  
Mean Values ◽  
Wide Range ◽  
Prolate Spheroids

Abstract. Radar variables of volcanic ash clouds are dependent on microphysical processes and can be expressed using physical parameters of volcanic ash particles, such as terminal velocity, axis ratio, and canting angle, which are necessary for quantitative ash-fall estimations. In this study, free-fall experiments of volcanic ash were accomplished using a two-dimensional video disdrometer under controlled conditions. Samples containing a rotating symmetric axis were selected and divided into five types according to shape and orientation, i.e., oblate and prolate spheroids with horizontally and vertically oriented axes and spheres. The horizontally and vertically oriented particles were present in proportions of 75.5 % and 21.6 %, and oblate and prolate spheroids were in proportions of 76.2 % and 23.8 %, respectively. The most common shape type was a horizontally oriented oblate spheroid (57.3 %). The terminal velocities were classified according to shape type. The terminal velocities of prolate spheroids (vertically oriented) particles were higher than those of oblate spheroids (horizontally). Terminal velocities were in the range 0.5 < volume–equivalent spherical particle diameter (D) < 1 mm for OH because of an increase in axis ratio and a sharp decrease in sample size from D < 0.7 mm. The axis ratios fell over a wide range, from 0 to 1.5, at D < 2 mm, but converged to 0.94 at D > 2 mm. The histogram of canting angles followed unimodal and bimodal distributions with respect to horizontally and vertically oriented particles, respectively. The mean values were close to 0° and the standard deviation for the entire particle shape types was close to that of raindrops (10°) under calm atmospheric conditions.

The motion of an ellipsoid in tube flow at low Reynolds numbers

1996 ◽  
Vol 324 ◽  
pp. 287-308 ◽  
Author(s):  
Masako Sugihara-Seki
Keyword(s):  
Initial Conditions ◽  
Flow Direction ◽  
Major Axis ◽  
Oblate Spheroid ◽  
Reynolds Numbers ◽  
Prolate Spheroid ◽  
Stable Configuration ◽  
Axis Ratio ◽  
Low Reynolds Numbers ◽  
Freely Suspended

The motion of a rigid ellipsoidal particle freely suspended in a Poiseuille flow of an incompressible Newtonian fluid through a narrow tube is studied numerically in the zero-Reynolds-number limit. It is assumed that the effect of inertia forces on the motion of the particle and the fluid can be neglected and that no forces or torques act on the particle. The Stokes equation is solved by a finite element method for various positions and orientations of the particle to yield the instantaneous velocity of the particle as well as the flow field around it, and the particle trajectories are determined for different initial configurations. A prolate spheroid is found to either tumble or oscillate in rotation, depending on the particle–tube size ratio, the axis ratio of the particle, and the initial conditions. A large oblate spheroid may approach asymptotically a steady, stable configuration, at which it is located close to the tube centreline, with its major axis slightly tilted from the undisturbed flow direction. The motion of non-axisymmetric ellipsoids is also illustrated and discussed with emphasis on the effect of the particle shape and size.

scholarly journals The Terminal Velocity of Axisymmetric Cloud Drops and Raindrops Evaluated by the Immersed Boundary Method

2021 ◽  
Vol 78 (4) ◽  
pp. 1129-1146
Author(s):  
Chia Rui Ong ◽  
Hiroaki Miura ◽  
Makoto Koike
Keyword(s):  
Density Dependence ◽  
Laboratory Data ◽  
Terminal Velocity ◽  
Immersed Boundary ◽  
Atmospheric Conditions ◽  
Fall Velocity ◽  
Axis Ratio ◽  
Model Calculations ◽  
Air Density ◽  
Free Falling

AbstractThe terminal velocity of cloud drops and raindrops used in numerical model calculations can significantly affect weather predictions. Current formulations rely on laboratory experiments made in the 1940s and 1960s. Because these experiments were performed only at typical environmental conditions of 20°C and 1013 hPa, parameterizations have been introduced to deduce the terminal velocity aloft without rigorous evaluation. In this study, an incompressible two-phase flow direct numerical simulation model is used to calculate the free-falling motion of axisymmetric drops with diameters between 0.025 and 0.5 mm to study the terminal fall velocity. Simulated terminal fall velocities of free-falling drops at 20°C and 1013 hPa agree within 3.2% with the previous empirical parameterization (Beard formula), and 4.5% with existing laboratory data in the diameter range between 0.3 and 0.5 mm. The velocities converge to the analytic Hadamard–Rybczynski solution within 2% for small Reynolds numbers, demonstrating the robustness of our simulations. Simulations under various atmospheric conditions show that existing empirical parameterizations that account for the air density dependence of the terminal velocity have errors up to 11.8% under the conditions examined in this study. We propose a new empirical formula that describes the air density dependence of the terminal velocity. It is also shown that the falling speed of a small drop is not sensitive to shape oscillation, and the terminal velocity decreases by only less than 1.3% when the axis ratio increases by 12% with reduced surface tension. Internal circulation within falling drops is also presented and compared with previous studies.

scholarly journals Microphysical Characteristics of Rainfall Observed by a 2DVD Disdrometer during Different Seasons in Beijing, China

2021 ◽  
Vol 13 (12) ◽  
pp. 2303
Author(s):  
Li Luo ◽  
Jia Guo ◽  
Haonan Chen ◽  
Meilin Yang ◽  
Mingxuan Chen ◽  
...  
Keyword(s):  
Rain Rate ◽  
Heavy Rain ◽  
Occurrence Frequency ◽  
Total Rainfall ◽  
Axis Ratio ◽  
Seasonal Differences ◽  
Mean Values ◽  
Light Rain ◽  
Beijing China ◽  
Maximum Occurrence

The seasonal variations of raindrop size distribution (DSD) and rainfall are investigated using three-year (2016–2018) observations from a two-dimensional video disdrometer (2DVD) located at a suburban station (40.13°N, 116.62°E, ~30 m AMSL) in Beijing, China. The annual distribution of rainfall presents a unimodal distribution with a peak in summer with total rainfall of 966.6 mm, followed by fall. Rain rate (R), mass-weighted mean diameter (Dm), and raindrop concentration (Nt) are stratified into six regimes to study their seasonal variation and relative rainfall contribution to the total seasonal rainfall. Heavy drizzle/light rain (R2: 0.2~2.5 mm h−1) has the maximum occurrence frequency throughout the year, while the total rainfall in summer is primarily from heavy rain (R4: 10~50 mm h−1). The rainfall for all seasons is contributed primarily from small raindrops (Dm2: 1.0~2.0 mm). The distribution of occurrence frequency of Nt and the relative rainfall contribution exhibit similar behavior during four seasons with Nt of 10~1000 m−3 registering the maximum occurrence and rainfall contributions. Rainfall in Beijing is dominated by stratiform rain (SR) throughout the year. There is no convective rainfall (CR) in winter, i.e., it occurs most often during summer. DSD of SR has minor seasonal differences, but varies significantly in CR. The mean values of log10Nw (Nw: mm−1m−3, the generalized intercept parameter) and Dm of CR indicate that the CR during spring and fall in Beijing is neither continental nor maritime, at the same time, the CR in summer is close to the maritime-like cluster. The radar reflectivity (Z) and rain rate (?) relationship (Z = ?R?) showed seasonal differences, but were close to the standard NEXRAD Z-R relationship in summer. The shape of raindrops observed from 2DVD was more spherical than the shape obtained from previous experiments, and the effect of different axis ratio relations on polarimetric radar measurements was investigated through T-matrix-based scattering simulations.

scholarly journals Estimating building vulnerability to volcanic ash fall for insurance and other purposes

2017 ◽  
Vol 6 (1) ◽  
Author(s):  
R. J. Blong ◽  
P. Grasso ◽  
S. F. Jenkins ◽  
C. R. Magill ◽  
T. M. Wilson ◽  
...  
Keyword(s):  
Volcanic Ash ◽  
Building Vulnerability ◽  
Ash Fall

scholarly journals Characterizing the Atmospheric Conditions Leading to Large Error Growth in Volcanic Ash Cloud Forecasts

2018 ◽  
Vol 57 (4) ◽  
pp. 1011-1019 ◽  
Author(s):  
H. F. Dacre ◽  
N. J. Harvey
Keyword(s):  
Flow Separation ◽  
Volcanic Ash ◽  
Dispersion Model ◽  
Forecast Accuracy ◽  
Large Error ◽  
Ensemble Member ◽  
Forecast Errors ◽  
Atmospheric Conditions ◽  
Wind Fields ◽  
Ensemble Forecasts

ABSTRACTVolcanic ash poses an ongoing risk to safety in the airspace worldwide. The accuracy with which volcanic ash dispersion can be forecast depends on the conditions of the atmosphere into which it is emitted. In this study, meteorological ensemble forecasts are used to drive a volcanic ash transport and dispersion model for the 2010 Eyjafjallajökull eruption in Iceland. From analysis of these simulations, the authors determine why the skill of deterministic-meteorological forecasts decreases with increasing ash residence time and identify the atmospheric conditions in which this drop in skill occurs most rapidly. Large forecast errors are more likely when ash particles encounter regions of large horizontal flow separation in the atmosphere. Nearby ash particle trajectories can rapidly diverge, leading to a reduction in the forecast accuracy of deterministic forecasts that do not represent variability in wind fields at the synoptic scale. The flow‐separation diagnostic identifies where and why large ensemble spread may occur. This diagnostic can be used to alert forecasters to situations in which the ensemble mean is not representative of the individual ensemble‐member volcanic ash distributions. Knowledge of potential ensemble outliers can be used to assess confidence in the forecast and to avoid potentially dangerous situations in which forecasts fail to predict harmful levels of volcanic ash.

Inverse Scattering from a Perfectly Conducting Prolate Spheroid in the Quasi-Static Domain

1973 ◽  
Vol 51 (2) ◽  
pp. 219-222
Author(s):  
D. A. Hill
Keyword(s):  
Magnetic Fields ◽  
Magnetic Dipole ◽  
Inverse Scattering ◽  
Oblate Spheroid ◽  
Prolate Spheroid ◽  
Dipole Source ◽  
Intermediate Step

The problem of inverse scattering from a perfectly conducting prolate spheroid in the quasistatic region of a magnetic dipole source is considered. From one observation of the radial and transverse scattered magnetic fields, the parameters which identify the spheroid (interfocal distance and eccentricity) are uniquely determined. The intermediate step requires the determination of the two magnetic polarizabilities. Similar results are also obtained for the oblate spheroid by a transformation.

The gravitational settling of aerosol particles in homogeneous turbulence and random flow fields

1987 ◽  
Vol 174 ◽  
pp. 441-465 ◽  
Author(s):  
M. R. Maxey
Keyword(s):  
Settling Velocity ◽  
High Strain ◽  
Free Fall ◽  
Terminal Velocity ◽  
Homogeneous Turbulence ◽  
Mean Flow ◽  
Particle Inertia ◽  
The Mean ◽  
Random Flow ◽  
Flow Particle

The average settling velocity in homogeneous turbulence of a small rigid spherical particle, subject to a Stokes drag force, is shown to depend on the particle inertia and the free-fall terminal velocity in still fluid. With no inertia the particle settles on average at the same rate as in still fluid, assuming there is no mean flow. Particle inertia produces a bias in each trajectory towards regions of high strain rate or low vorticity, which affects the mean settling velocity. Results from a Gaussian random velocity field show that this produces an increased settling velocity.

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