Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "liquid sheet"

When fluid dynamicists get into the ALS ice bucket challenge, they give it a good fluidsy twist. Here are some selections, including lots of high speed video and an infrared video. Check out all those liquid sheets breaking up. Links to the full videos are below. (Image credits: Ewoldt Research Group, source videoTAMU NAL, source video; BYU Splash Lab, source videos 1, 2, 3, 4)

Photographers Cassandra Warner and Jeremy Floto produced the "Clourant" series of high-speed photographs of colorful liquid splashes. The artists took special care to disguise the origin of splashes, making them appear like frozen sculptures. The photos are beautiful examples of making fluid effects and instabilities. Many of them feature thin liquid sheets with thicker rims just developing ligaments. In other spots, surface tension has been wholly overcome by momentum’s effects and what was once ligaments has exploded into a spray of droplets. (Photo credit: C. Warner and J. Floto; submitted by jshoer; via Colossal)

When a droplet strikes a pool, a thin, fast-moving sheet of liquid expands outward from the region of contact. These ejecta sheets come in many forms depending on surface tension, viscosity, air pressure, and droplet momentum. When the ejecta sheet curls downward to touch the pool, it can spray microdroplets outward or trap a layer of air underneath the droplet. For more, see this video by S. Nagel et al., and the papers Thoroddsen (2002) and Thoroddsen et al. (2008).  (Photo credits: S. Thoroddsen et al.; GIF from this video by S. Thoroddsen et al.)

Artist Fabian Oefner enjoys capturing both art and science in his work. In his latest series, “Orchid”, the blossom-like images are the result of splashes. He layered multiple colors of paint, ending with a top layer of black or white, then dropped a sphere into the paint. The images show how the colors mix and rebound, a delicate splash crown seen from above. The liquid sheet thickens at the rim and breaks up into ligaments from the instability of the crown’s edge. It makes for a remarkable demonstration of the effects of momentum and surface tension. Several of Oefner’s previous collections have appeared on FYFD (1, 2, 3). (Photo credit: F. Oefner)

Water sculptures—a marriage of liquids, photography, and timing—are spectacular form of fluid dynamics as art. Artist Markus Reugels is a master of the form. This video captures the life and death of such water sculptures at 2,000 fps, beginning with the fall of the initial blue droplet. The droplet’s impact causes a rebounding Worthington jet, which reaches its pinnacle just as a second droplet strikes. The impact spreads into an umbrella-like skirt consisting of a thin, expanding liquid sheet with a thicker rim. The rim itself is unstable, breaking into regularly spaced filaments and tiny satellite droplets that shoot outward before the entire structure collapses into the pool. One especially cool aspect of watching this in video is seeing how the blue dye from each droplet spreads as the water splashes and rebounds. You can see the set-up Reugels uses for his photography here. (Video credit: M. Reugels and L. Lehner)

When two liquid jets collide, they can form an array of shapes ranging from a chain-like stream or a liquid sheet to a fishbone-type structure of periodic droplets. This series of images show the collision of two viscoelastic jets—in which polymer additives give the fluids elasticity properties unlike those of familiar Newtonian fluids like water. The jet velocities increase with each image, changing the behavior from a fluid chain (a and b); to a fishbone structure (c and d); to a smooth liquid sheet (e); to a fluttering sheet (f and g); to a disintegrating ruffled sheet (h), and finally a violently flapping sheet (i and j). The behavior of such jets is of particular interest in problems of atomization, where it can be desirable to break an incoming stream of liquid up into droplets as quickly as possible. (Photo credit: S. Jung et al.)

Here fluid is ejected as two flat plates collide, creating a sheet of silicone oil. The initially smooth sheet forms a thicker ligament about the edge. Gravity causes the sheet to bend downward like a curtain, and growing instabilities along the ligament cause the development of a wavy edge. The points of the waves develop droplets that eject outward. Not long after this photograph, the entire liquid sheet will collapse into ligaments and flying droplets. (Photo credit: B. Chang, B. Slama, and S. Jung)

In fluid dynamics, we like to classify flows as laminar—smooth and orderly—or turbulent—chaotic and seemingly random—but rarely is any given flow one or the other. Many flows start out laminar and then transition to turbulence. Often this is due to the introduction of a tiny perturbation which grows due to the flow’s instability and ultimately provokes transition. An instability can typically take more than one form in a given flow, based on the characteristic lengths, velocities, etc. of the flow, and we classify these as instability modes. In the case of the vertical rotating viscous liquid jet shown above, the rotation rate separates one mode (n) from another.  As the mode and rotation rate increase, the shape assumed by the rotating liquid becomes more complicated. Within each of these columns, though, we can also observe the transition process. Key features are labeled in the still photograph of the n=4 mode shown below. Initially, the column is smooth and uniform, then small vertical striations appear, developing into sheets that wrap around the jet. But this shape is also unstable and a secondary instability forms on the liquid rim, which causes the formation of droplets that stretch outward on ligaments. Ultimately, these droplets will overcome the surface tension holding them to the jet and the flow will atomize. (Video and photo credits: J. P. Kubitschek and P. D. Weidman)


High speed video of of spray starch from a can. Once the initial transients die down, a cone-shaped annular sheet forms.  Disturbances propagate in the sheet, tearing it into filaments that break down into droplets. Beautiful complexity hidden in a simple everyday device. (Video credit: John Savage)

When two jets of liquid collide, they form a sheet of fluid.  As the speeds of the jets change, the sheet can become unstable, forming a set of liquid ligaments and droplets that look like a fish’s bones. This is shown in the video above. For purposes of orienting yourself, flow in the video is moving right to left and the video has been rotated 90-degrees clockwise (i.e. the two out-of-frame jets forming the flow seen are falling due to gravity). (Video credit: Sungjune Jung, University of Cambridge)