Spray Droplet Generation in Breaking Water Waves

Figure 1

The investigation of sea spray generation is a topic of significant oceanographic and climatological interest. Droplet evaporation plays an integral role in the transfer of heat to the atmosphere and the formation and strengthening of tropical cyclones.(1,2) As such, the mechanism of droplet production is investigated in the Hydrodynamics Laboratory using a tank and mechanically generated waves.




Fig. 1:  Image of crest breaking just prior to the impact of the jet with the front face of the wave.  The wave has been illuminated by Fluorescein dye exposed to blue laser light from a 7W Argon-Ion laser.


Fig. 2: Schematic of the wave tank where spray measurements are taken. High speed cameras are located at varying positions within the measurement window where the wave consistently breaks.


The experiments are carried out in a 14.8m long by 1.2m wide wave tank (See Figure 2). On one end, a mechanical wave maker generates waves using the dispersive focusing technique. These waves consistently break at the same position in the tank where two high speed Phantom v640 cameras with microscopic lenses are located. The images are backlit with a diffuse Nd:YAG PIV laser beam. A still image from the high speed camera showing spray droplets can been seen in Figure 3 to the right.




Fig. 3:  2560x1600 (25.6x16.0 mm) Pixel still image taken with Phantom v640 camera operated at 650 frames per second.  This configuration is capable of resolving droplet diameters down to approximately 40 microns.


Figure 3



Figure 4

The high speed movies taken from just above the surface of the waves are processed to determine spray droplet diameters and velocities.  The fits for the diameters are corrected later for focus and depth-of-field.  An image with diameter fits superimposed over the droplets is shown in Figure 4.  After a number of movies have been processed at multiple positions throughout the wave, statistics on droplet diameter and velocity can be determined.  Figure 5 shows the spray droplet diameter distributions from two different waves.




Fig. 4:  Still image of droplets showing diameter fits before correction for focus.  Ultimately, all droplets in the image that have not yet been counted are measured for diameter and velocity.



Fig. 5: Normalized probability density for spray droplet diameters in two different breaking waves. Wave on the left has a greater amplitude than the wave on the left. Distribution is for all droplets observed over the course of 10 runs of each wave.


1Wang, Y., Kepert, J., and Holland, G. (2001). The effect of sea spray evaporation on tropical cyclone boundary layer structure and intensity. Monthly weather review, 129(10):24812500. 2 Andreas, Edgar L. "A new sea spray generation function for wind speeds up to 32 m s-1." Journal of Physical Oceanography 28, no. 11 (1998): 2175-2184

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