Rohini Bhalwankar

Spontaneous breakup of charged and uncharged water drops freely suspended in a wind tunnel

Time for which charged or uncharged water drops of different sizes can be suspended over a vertical wind tunnel before their spontaneous breakup and the size distribution of droplets resulting from their breakup have been determined. Probability of spontaneous breakup of a drop has been found to increase with the size and charge of the drop. It has been observed that water drops carrying a charge of 5×10−10 C breakup immediately after their suspension if their diameter > 8 mm. Total number of droplets produced on spontaneous breakup of a drop increases with the size of the drop, and if the drop size is > 6.6 mm, the total number of droplets is more when the drop is uncharged than that when it is charged. However, the number of droplets larger than a critical size is more if the parent drop is charged and the number of droplets smaller than that critical size is more if the parent drop is uncharged. It has been attempted to qualitatively explain the experimental results as the result of enhanced surface charge density around the waist of the drop during its oscillation. Charge on the drop has been suggested to cause an increase in width of the base of suspended drop.

The onset of disintegration and corona in water drops falling at terminal velocity in horizontal electric fields

The onset of disintegration and corona in water drops falling at their terminal velocity in a vertical wind tunnel and exposed to horizontal electric fields has been investigated. Contrary to previous observations, the drops elongate in horizontal direction and distort into the shape of a concavo-convex lens with a convex bottom and a sharp-edged rim facing upward. Drops of diameter < 6.6 mm which do not break up in absence of electric field in this wind tunnel, break up in presence of the horizontal electric field. The values of horizontal electric field required for instability of the drops are much lower than those either predicted by Taylors criterion of instability or observed in previous experimental studies. The criterion for instability of water drops freely suspended in presence of horizontal electric fields can be expressed as FH (ro/σ)½ = 0.98 ± 0.03 where FH is the horizontal electric field in esu, ro is the drop radius in centimeters and σ is the surface tension in dynes per centimeter. Most of the drops produce corona just before their breakup. Among various drops that are freely suspended in the wind tunnel, one by one, the number of drops that produce corona and/or breakup increases with increase in the electric field and/or drop size. While all drops of diameter ≥ 7.1 mm produce corona in a horizontal electric field of 500 kV/m, only a small fraction of very large drops of 8.0 mm diameter produce corona when the electric field is equal to 200 kV/m. Comparatively, very low values of instability field observed in our experiment are qualitatively explained because of long exposure of the freely suspended drops to the horizontal electric fields. The drops become unstable and produce corona when the drops oscillation amplitude overshoots its equilibrium value and the plane of the drop oscillation coincides with the direction of electric field. From the results, it seems likely that horizontal electric fields in the bases of thunderclouds may cause disintegration of large raindrops and the occurrence of corona from their surfaces may trigger a lightning discharge.

A wind tunnel investigation of the deformation of water drops in the vertical and horizontal electric fields

the ratio of minor-to-major axis of the 2.6-mm diameter drop by 3%. An examination of the frequency distribution of the drop’s axis ratio shows that during its oscillations, oblateness decreases more often when subjected to vertical electric field and increases more often when subjected to horizontal electric field. Extreme values of distortion increase and are attained more frequently when drop is oscillating in electric field. It is concluded that the drop size distribution will be wider and thus the rate of drop’s growth faster in those regions of cloud where the electric field direction is vertical rather than horizontal.

The evaporation of the charged and uncharged water drops suspended in a wind tunnel

A laboratory experiment has been performed to study the effect of ventilation on the rate of evaporation of the millimeter sized charged and uncharged water drops suspended in a vertical wind tunnel. The linear relationship,fu = 0.907 + 0.282X, observed between the mean ventilation coefficient, fu, and a non-dimensional parameterX, (X =NSc,v1/3NRe1/2whereNSc,uis Schmidt number andNReis Reynold’s number) is in agreement with the results of earlier investigations for uncharged water drops. However, in case of charged drops carrying 10-10C of charge, this relationship gets modified tofu = 0.4877 + 0.149X. Thus, the rate of evaporation of charged drops is slower than that of uncharged drops of the same size. Oscillations of the drop and the change in airflow around drops are suggested to contribute to lowering of the ventilation coefficients for charged drops. Applicability of the results to a small fraction of highly charged raindrops falling through the sub-cloud layer below thunderstorm is discussed. The relaxation time required for a ventilated drop to reach its equilibrium temperature increases with the drop size and is higher for the charged than for the uncharged drops. It is concluded that in a given distance, charged drops will evaporate less than that of uncharged drops.

Breakup Modes of the Drops Suspended in a Vertical Wind Tunnel in Presence of the Horizontal Electric Field

The influence of strong horizontal electric field (EH) on different stages of deformation and eventual breakup of the large water drops of 6.6, 7.0 and 7.25 mm diameter has been observed in a vertical wind tunnel using high-speed photography. Dumbbell, filament and bag modes of drop breakup are observed when EH = 0. However, drops elongate in horizontal direction, mostly develop sharp curvature at their ends, eject a fine jet spray of tiny droplets and ultimately break up into several droplets in EH = 500 kVm-1. Extreme elongation upto 29 mm is observed for a 7.0 mm diameter drop. Results show that the breakup time, i.e. the time from the drops extreme prolate shape to its breakup in its final oscillation, ranges from 13 - 41 ms when EH = 0 and 57 – 105 ms when EH = 500 kVm-1. So, although the lifetime of the drop since its suspension to breakup is reduced, its elongation and breakup time increase in EH. It suggests that the effect of EH in final oscillation before breakup overcomes the effect of hydrodynamic and aerodynamic forces in elongating the drop. Also, no breakup of bag type is observed in EH = 500 kVm-1. Moreover, the fragments formed after the drop breakup and tiny droplets ejected by their fragments, carry electrical charges of polarity determined by the induced charge on the parent drop in EH. The significance of the results is discussed in modifying the drop growth and the radar echo– precipitation relationships in thunderclouds.