By Sierra Castillo, for Get Wet 2016.
Suspended splash distorts beautiful mountain background in Phelps Lake in Grand Teton National Park, WY.
Get Wet report
By Sierra Castillo for Flow Visualization, MCEN 4151
Introduction and Background
Flow Visualization is dedicated to producing images and videos of fluid phenomenon. The purpose of the Get Wet image, as our first assignment, is to experiment with different flow phenomena and to get practice producing images of fluids. For my submission, I wanted to demonstrate the physics of water when a splash is induced. Using a GoPro, I swam out into a lake and filmed myself splashing my hand/arms in the water.
For this image, there was no technical apparatus used to produce the image. Quite simply, I swam into the lake and began treading water. I positioned my camera close to the water and in line with my body. Once it was positioned as square and level as possible, I swung my arm over my head and into the view of the camera. Once it hit the water, the splash commenced and finished within the span of a few seconds. Later, I went through the video frame by frame, and exported a still image of the splash. It would be impossible to exactly reproduce this image, but capturing a similar one would be fairly simple with a waterproof camera that has video capabilities. A sketch of the positioning of the camera and my arm is shown below in Figures 1 and 2.
Figure 1: Top View of position of my body in the lake.
Figure 2: Front View of position of my body in the lake.
Analysis of Image
Though the process is quite simple, the physics of the image are quite interesting. The image was captured at the end of the splash. If this were a single drop of water, this would be the moment of the Worthington Jet. By this, I mean that the air cavity created when my arm hit the surface of the water was refilled rapidly by water. This in turn, propelled a quantity of water into the air. Due to the surface tension acting on these propelled particles, the sheet of water was created. There is a spot on this sheet that appears to have been hit by a stray particle producing ripples within the sheet. This is an interesting addition to the image and gives some insight into the chaos that commenced after splashing the water. Another interesting aspect of the image is the fingers that form on the edges of the flow.
The camera used to capture the image is the GoPro Hero 4. Unfortunately, the GoPro Hero 4 lineup does not have metadata available, so I can only give information about the settings I changed. The ISO was set to 1600. The white balance was set to 5500K. Finally, the EV compensation was set to 0. The rest of the settings are automatically changed by the camera. The sheet of water is about a foot in length. The distance from the lens to the splash is about 1.5’. The focal length is about 21.9mm for this camera (medium FOV). For post processing, I simply increased the contrast of the image and darkened the shadows a little bit in Photoshop. My intent in these edits was to emphasize the details in the various nooks and crannies in the flow that were lost in the original image. The original image can be seen in Figure 3, below . The edited version can be seen above.
Figure 3: Original, Unedited Still
I like that the image is chaotic and fuzzy in one corner, while the other corner is calm and sharp. The sheet of water is very clear. I like that there is one droplet that bounced off of this sheet and has little ripples in it. It is an interesting “blemish” on the sheet. I also like the various fingers that extend from the sheet of water. Finally, I really enjoyed capturing the image. It isn’t often that a technical engineering course gives you the opportunity to play in the water. It was fun messing around with different splash techniques and going through the footage to see what was actually happening. I wish that I could have captured more of the background so that the distortion of the background has some context. It would have been nice to see the difference between the water-distorted and actual mountain background. I also wish that the lighting was a little better. I think if I had been able to catch the image at a later time in the day when the sun was more behind me, the reflections could have been really interesting. It would also have helped if the clouds weren’t blocking the sun as much as they were. Overall, I am happy with the outcome, especially since it wasn’t a “controlled” experiment so much as it was a test of the spontaneity in the splashing water.
Gekle, Stephan, and Jose Manuel Gordillo. “Generation and Breakup of Worthington Jets After Cavity Collapse.” Journal of Fluid Mechanics (2010): 1-3.Research Gate. Web. 27 Sept. 2016.