Changes In Drop Size Distribution Research Articles

Corrections of Precipitation Particle Size Distribution Measured by a Parsivel OTT2 Disdrometer under Windy Conditions in the Antisana Massif, Ecuador

Monitoring precipitation in mountainous areas using traditional tipping-bucket rain gauges (TPB) has become challenging in sites with strong variations of air temperature and wind speed (Ws). The drop size distributions (DSD), amount, and precipitation-type of a Parsivel OTT2 disdrometer installed at 4730 m above sea level (close to the 0 °C isotherm) in the glacier foreland of the Antisana volcano in Ecuador are used to analyze the precipitation type. To correct the DSDs, we removed spurious particles and shifted fall velocities such that the mean value matches with the fall velocity–diameter relationship of rain, snow, graupel, and hail. Solid (SP) and liquid precipitation (LP) were identified through −1 and 3 °C thresholds and then grouped into low, medium, and high Ws categories by k-means approach. Changes in DSDs were tracked using concentration spectra and particle’s contribution by diameter and fall velocity. Thus, variations of concentration/dispersion and removed hydrometeors were linked with Ws changes. Corrected precipitation, assuming constant density (1 g cm−3), gives reliable results for LP with respect to measurements at TPB and overestimates SP measured in disdrometer. Therefore, corrected precipitation varying density models achieved fewer differences. These results are the first insight toward the understating of precipitation microphysics in a high-altitude site of the tropical Andes.

Assessing Dual-Polarization Radar Estimates of Extreme Rainfall during Hurricane Harvey

AbstractHurricane Harvey hit the Texas Gulf Coast as a major hurricane on 25 August 2017 before exiting the state as a tropical storm on 29 August 2017. Left in its wake was historic flooding, with some locations measuring more than 60 in. (150 cm) of rain over a 5-day period. The WSR-88D radar (KHGX) maintained operations for the entirety of the event. Rain gauge data from the Harris County Flood Warning System (HCFWS) was used for validation with the full radar dataset to retrieve daily and event-total precipitation estimates for the period 25–29 August 2017. The KHGX precipitation estimates were then compared with the HCFWS gauges. Three different hybrid polarimetric rainfall retrievals were used, along with attenuation-based retrieval that employs the radar-observed differential propagation. An advantage of using a attenuation-based retrieval is its immunity to partial beam blockage and calibration errors in reflectivity and differential reflectivity. All of the retrievals are susceptible to changes in the observed drop size distribution (DSD). No in situ DSD data were available over the study area, so changes in the DSD were interpreted by examining the observed radar data. We examined the parameter space of two key values in the attenuation retrieval to test the sensitivity of the rain retrieval. Selecting a value of α = 0.015 and β = 0.600 provided the best overall results, relative to the gauges, but more work needs to be done to develop an automated technique to account for changes in the ambient DSD.

A Prototype Quantitative Precipitation Estimation Algorithm for Operational S-Band Polarimetric Radar Utilizing Specific Attenuation and Specific Differential Phase. Part II: Performance Verification and Case Study Analysis

Abstract The quantitative precipitation estimate (QPE) algorithm developed and described in Part I was validated using data collected from 33 Weather Surveillance Radar 1988-Doppler (WSR-88D) radars on 37 calendar days east of the Rocky Mountains. A key physical parameter to the algorithm is the parameter alpha α, defined as the ratio of specific attenuation A to specific differential phase KDP. Examination of a significant sample of tropical and continental precipitation events indicated that α was sensitive to changes in drop size distribution and exhibited lower (higher) values when there were lower (higher) concentrations of larger (smaller) rain drops. As part of the performance assessment, the prototype algorithm generated QPEs utilizing a real-time estimated and a fixed α were created and evaluated. The results clearly indicated ~26% lower errors and a 26% better bias ratio with the QPE utilizing a real-time estimated α as opposed to using a fixed value as was done in previous studies. Comparisons between the QPE utilizing a real-time estimated α and the operational dual-polarization (dual-pol) QPE used on the WSR-88D radar network showed the former exhibited ~22% lower errors, 7% less bias, and 5% higher correlation coefficient when compared to quality controlled gauge totals. The new QPE also provided much better estimates for moderate to heavy precipitation events and performed better in regions of partial beam blockage than the operational dual-pol QPE.

Creation of well-defined particle stabilized oil-in-water nanoemulsions

The preparation of oil-in-water (o/w) nanoemulsions stabilized with silica nanoparticle sols has been investigated. The emulsification was performed using a high shear homogenizer (Microfluidizer TM processor, Microfluidics, USA). The effect of different processing conditions on the droplet size distribution and stability was investigated in emulsions prepared using different types of oils, oil concentration and particle/oil ratios. It was the ability of the particles to attach to, and stabilize the newly created interface, rather than their ability to lower the interfacial tension, what proved important for the drop size of the resulting emulsions. Changes in drop size distribution with time, attributed to Ostwald ripening effects, were observed for the more soluble oils, while stable nanoemulsions with droplet size of ∼100–200nm could be produced using a virtually water-insoluble oil such as squalene.

The influence of density ratio on the primary atomization of a turbulent liquid jet in crossflow

In this paper, we study the impact of density ratio on turbulent liquid jet in crossflow penetration and atomization if all other characteristic parameters, i.e., momentum flux ratio, jet and crossflow Weber and Reynolds numbers, are maintained constant. We perform detailed simulations of the primary atomization region using the refined level set grid method to track the motion of the liquid/gas phase interface. We employ a balanced force, interface projected curvature method to ensure high accuracy of the surface tension forces, use a multi-scale approach to transfer broken-off, small scale nearly spherical drops into a Lagrangian point particle description allowing for full two-way coupling and continued secondary atomization, and employ a dynamic Smagorinsky large eddy simulation approach in the single phase regions of the flow to describe turbulence. We compare simulation results obtained previously using a liquid to gas density ratio of 10 for a momentum flux ratio 6.6, Weber number 330, and Reynolds number 14,000 liquid jet injected into a Reynolds number 740,000 gaseous crossflow to those at a density ratio of 100, a value typical for gas turbine combustors. The results show that the increase in density ratio results in a noticeable increase in jet penetration, change in drop size distribution resulting from primary atomization, change in drop velocities generated by primary atomization in the crossflow and jet direction, but virtually no change in drop velocities in the transverse direction.

Chemically Induced Destabilization of Water-in-Crude Oil Emulsions

The effect of systematic changes in the structure of chemicals commonly used in demulsification operations (alkylphenol polyalkoxylated resins and polyurethanes) on the stability and properties of brine-in-crude oil emulsions was assessed experimentally. The relative rates of water separation were characterized via bottle tests and rheometry. Equilibrium interfacial tensions were also measured. Transient changes in drop size distributions were quantified using nuclear magnetic resonance. The phenolic resins promoted coalescence of droplets. Optimum performance was obtained with resins exhibiting intermediate hydrophilicity in a manner consistent with the condition of least emulsion stability for conventional oil−water−surfactant systems. In contrast, polyurethanes promoted flocculation but only slow coalescence. The performance of polyurethanes improved with increase of molecular weight. Phenolic resins and polyurethanes acted synergistically when added simultaneously, rendering water separation rates sig...

Radar reflectivity calibration using differential propagation phase measurement

A method for calibrating radar reflectivity using polarization radar measurements in rain is described. Accurate calibration of radar reflectivity is essential for obtaining reliable rain rate estimation. In the case of polarization radar, rain rate can be independently estimated using power and phase measurements. Thus phase measurement can be estimated from power measurements. Comparison of the direct estimate of propagation phase (Φm) measurement, which is unaffected by absolute calibration of the radar system, with the estimated propagation phase (Φe) from power measurements is the basis for the calibration method. Polarization measurements such as reflectivity (Z), differential reflectivity (ZDR), and differential propagation phase (ΦDP) are sensitive to drop size distribution (DSD) and mean drop shape. It is important to devise a calibration technique relatively unperturbed by changes in DSD and drop shape. Statistical fluctuation in Φe is derived to estimate the accuracy of the calibration procedure, raindrop shape, and attenuation. The proposed method is applied for calibrating reflectivity measurements of the National Center for Atmospheric Research (NCAR) S‐band polarization radar (S‐Pol) in midlatitude, subtropical, and tropical rain events.

Transient Behavior of Polydisperse Emulsions Undergoing Mass Transfer

Numerical simulations of transport in polydisperse oil-in-water emulsions were carried out for three situations in which the aqueous phase contained surfactant micelles: solubilization of a pure oil; Ostwald ripening of a pure oil; and compositional ripening of a mixed emulsion initially containing drops of two pure oils, only one of which was solubilized to a significant extent. Transport was assumed to be controlled by interfacial effects, not by diffusion of micelles in the aqueous phase. The results were compared with published data for changes in drop size distribution in hydrocarbon-in-oil emulsions stabilized by nonionic surfactants for each of the three phenomena. In some cases the published data were supplemented by measurements of interfacial tensions and solubilization rates of single drops of the same oils in solutions of the same nonionic surfactants. Excellent agreement between simulation results and the published data was found for the solubilization and compositional ripening cases with no adjustable parameters. Moreover, accounting for polydispersity was found to be important, i.e., agreement was sometimes not as good when the same transport mechanism was assumed but with a monodisperse distribution corresponding to average drop size. Although results for the Ostwald ripening case showed that interfacial effects and not diffusion controlled transport, confirming that the same mechanism of transport applied in all three cases, numerical agreement between simulations and experiment was not as good, probably because of additional transport between flocculated drops not accounted for in the theory.

Coalescence in Surfactant-Stabilized Emulsions Subjected to Shear Flow

Experiments were carried out to study the coalescence dynamics of a neutrally buoyant liquid−liquid emulsion subjected to a simple shear flow in a Couette device. The effect of shear rate and dispersed phase holdup were studied. The surfactant-stabilized emulsions were prepared in a stirred tank at high shear rates which were applied for long times so as to obtain reproducible equilibrium drop size distributions. Low shear rates were used in the Couette device to prevent drop breakup. Sufficiently large drops and density matching ensured that coalescence due to Brownian motion and creaming was negligible and thus the change in drop size distribution with time was entirely due to shear-induced coalescence. The evolving volume density distributions were measured using optical microscopy and image analysis. A population balance model based on Smoluchowski's result for the rate of shear-induced coalescence and an empirical form for the coalescence efficiency was used to describe the system. Shearing results in stabilization of the emulsion, and the coalescence rate reduces with increasing shear rates. The stabilizing effect due to shearing is greater for higher holdups. The drop size distribution for high values of the holdup becomes bimodal at long times. The coalescence efficiency, back-calculated from experimental data, is independent of drop size for low holdups but is size dependent for higher holdups. This work demonstrates a simple experimental method for evaluating the coalescence efficiency for emulsions with high holdups where theoretical models are not available.

Flow and Stress Induced Cavitation in a Journal Bearing With Axial Throughput

The problem of predicting flow between rotating eccentric cylinders with axial throughput is studied. The system models a device used to test the stability of emulsions against changes in drop size distribution. The analysis looks for the major variation in flow properties which could put an emulsion at risk due to coalescence or breakage and finds the most likely candidate in the pressure gradient defined as the ratio of the difference between the maximum and minimum pressure to the arc length between the difference. The axial throughput is modeled by flow driven by a constant pressure gradient. The flow is calculated from the Navier-Stokes equation using the code SIMPLER (Patankar 1980). The effects of inertia at values typical for the device are studied. Several eccentricities and different rotational speeds are computed to sample the changes in flow and stress parameters in the idealized device for typical conditions. The numerical analysis is validated against the lubrication approximation in the low Reynolds number case. Conditions for stress induced cavitation are evaluated. The flow is completely determined by a Reynolds number, an eccentricity ratio and a dimensionless pressure gradient and all computed results are either presented or can be easily expressed in terms of these dimensionless parameters. The effect of inertia is to shift the eddy or re-circulation zone which develops in the more open region of the gap toward the region of low relative pressure; the zero of the relative pressure migrates away from the center and the distribution breaks the skew symmetry of the Stokes flow solution. The state of stress in the journal bearing is analyzed and a cavitation criterion based on the maximum tensile stress is compared with the traditional criterion based on pressure.

A microphysics‐based investigation of the radiative effects of aerosol‐cloud interactions for two MAST Experiment case studies

We use a size‐ and composition‐resolved externally mixed aerosol microphysical model and a delta‐Eddington exponential‐sum‐fit radiation algorithm to examine the interactions between aerosol particles and cloud drops, and their influence on solar radiation. Both the aerosol model and the radiation code are designed to explicitly handle external and internal aerosol particle and cloud drop mixtures. Using observations from the Monterey Area Ship Track (MAST) Experiment, we model changes in aerosol and cloud drop size distributions for a clean marine cloud and ship track and a continentally influenced marine cloud and ship track. Linking these results to the radiation algorithm with a Mie‐scattering subroutine, we investigate the corresponding changes in cloud albedo, cloud absorption, and transmission. The differences in 0.3–3.0 μm albedo and transmission between the clouds and ship tracks as a result of the changes in drop size distribution and composition are found to be substantial, and the composition of the cloud drops is found to be important particularly in the continentally influenced case. Both the clouds and ship tracks enhance atmospheric absorption with respect to a clear sky, with a cloud forcing ratio ranging from 1.15 to 1.37, where the clear sky is defined to be cloud‐ and aerosol‐free. Sensitivity studies are performed with respect to the updraft velocity, the updraft area fraction, dilution of the ship emissions, and the composition of supermicron continental particles. The radiation results are also compared with Meteorological Research Flight (MRF) C‐130 in situ aircraft measurements and with parameterizations of the “Twomey effect.”

Diagnostics for fuel sprays as applied to emulsified fuels

Measurements of drop-size distributions and vaporized fuel concentrations have been combined with a spray model in the comparative study of the evaporation of sprays of mulsified and non-emulsified (neat) fuels at elevated temperatures and pressures. The fuel spray was generated by a pressure jet atomizer and was polydisperse; the air stream was approximately fully-developed turbulent flow in a constant area duct. Diagnostic techniques have been adapted or developed which allow rapid measurements of overall drop-size distributions in the presence of evaporation in high-temperature high-pressure air, and also the fuel vapor concentration distribution within sprays. It was concluded that even in conditions of turbulence and high Reynolds number for drop motion relative to the air, some emulsified fuel drops apparently undergo microexplosions. Further, it was determined that the initial atomization quality depends on the air conditions, with better atomization at higher air densities. This implies that spray drop-size measurements performed at atmospheric conditions need to be corrected for air density and temperature effects to predict atomization quality for altitude relight or high-pressure combustion. A spray model has been developed which represents the spray being studied and provides data in a form comparable to the experimental diagnostics. Comparisons are provided between the predicted and measured drop-size distribution and fuel-vapor concentrations during the evaporation process. Trends for changes in drop-size distribution (both average size and width of the distribution) with distance from the nozzle are predicted fairly well by the model except close (