Drop Velocity Distributions Research Articles

Evaluation of Multiple Precipitation Sensor Designs for Precipitation Rate and Depth, Drop Size and Velocity Distribution, and Precipitation Type

AbstractObservations of the precipitation rate/depth, drop size distribution, drop velocity distribution, and precipitation type are compared from six in situ precipitation sensor designs over 12 months to assess their performance and provide a benchmark for future design and deployment. The designs considered are tipping bucket (TBR), drop counting (RAL), acoustic (JWD), optical (LPM), single-angle visiometer with capacitor (PWD21), and dual-angle visiometer (PWS100). Precipitation rates are compared for multiple time resolutions over the study period, while drop size and velocity distributions are compared with cases at stable precipitation rates. To examine precipitation type, a new index and a logic algorithm to amalgamate consecutive precipitation type observations consistently is introduced and applied. Overall, the choice of instrument for deployment depends on the usage. For fast response (less than 15 min), the PWD21 and TBR should not be used. As precipitation rate or the duration of a sample increases, the correlation of the TBR with the majority of other instruments increases. However, the PWD21 consistently underestimates precipitation. The RAL, PWS100, and JWD are within ±15% for precipitation depth over 12 months. All instruments are inconsistent in their ability to observe drop size and velocity distributions for differing precipitation rates. There is low agreement between the instruments for precipitation type estimation. The PWD21 and PWS100 rarely report some precipitation types, but the LPM reports more broadly. Meteorological stations should use several instrument designs for redundancy and to more accurately capture precipitation characteristics.

Erosion Potential under Miconia calvescens Stands on the Island of Hawai‘i

AbstractThis study provides evidence that Miconia calvescens has the potential to accelerate surface erosion in stands where it invades by (i) reducing under‐canopy light levels, thereby reducing the establishment of ground cover vegetation, and (ii) producing highly erosive throughfall drops on large leaves in a single‐layer canopy. The throughfall energy in a stand of invasive miconia on the Island of Hawai‘i (USA), assessed by measuring the drop size and drop velocity distributions with a laser disdrometer, was significantly higher than that in a stand of native ‘ōhi‘a (Metrosideros polymorpha) and ambient rainfall. Median throughfall drop size for miconia (3·83 mm) was twice that of ambient rainfall (1·62 mm). Highly erosive throughfall resulted from large drops forming on large miconia leaves and relatively high fall velocities associated with the single‐story miconia canopy. In contrast, multi‐storied natural ‘ōhi‘a had a larger median drop size; however, a lower fall height reduced throughfall effective kinetic energy. Furthermore, the effective kinetic energy for miconia was high because large drops (> 3·8 mm) with high kinetic energy accounted for 60 per cent of the total energy (versus 30–40 per cent for other vegetation types). Consequently, unit kinetic energy of throughfall was 28 J m−2 mm−1 under miconia, compared with <24 J m−2 mm−1 for rainfall and <20 J m−2 mm−1 under ‘ōhi‘a. These data, combined with the observation of limited protective ground cover under miconia, show the potential for accelerated erosion occurring on forest floors in stands of invasive miconia. Copyright © 2013 John Wiley & Sons, Ltd.

CHARACTERISTIC STUDY ON THE LIKE-DOUBLET IMPINGING JETS ATOMIZATION

Experimental studies on like-doublet impinging jets have been conducted by employing a photographic imaging technology and laser diffraction particle sizing device. The injector diameter is first carefully made as small as 0.3 mm, to determine its possible improvement in impinging atomization. Purified tap water is used as test liquid, and the velocity of impinging jets is approximately 50 m/s. Four spray patterns are found, which is somewhat different from the results of Heidmann [2]. Also, drop velocity distribution is bell shaped, and a newly defined shedding angle shows that drops are throw off tangentially from the sheet rim periphery. Shedding angle increases linearly with increasing position angle, with a sharp change near the axial axis, which reveals that drops shed may penetrate through the axial axis from one side to the other. Mean droplet size decreases sharply with increasing jet velocity in the low jet velocity regime, and flattens when the jet velocity increases sufficiently. For all angles of impingement, a unique mean drop size is found when the jet velocity is high enough. Comparison of data with those observed by Dombrowski and Hooper [5] shows an improvement in atomization which is attributed to the fact that the orifice size in this work is smaller than those used by previous authors.