2012 Image Gallery

Travis Walker, Theresa Hsu, Gerald Fuller
Department of Chemical Engineering
Stanford University

The photograph shows a jet of water (white) rinsing a diluted solution of polyacrylamide (PAM) (blue) off of a surface made of silicon. Dilute solutions of high-molecular weight PAM have highly elastic properties that create interesting flow physics, as seen in this image.

Both the rinsing solution (water) and the solution coating the substrate (PAM) undergo what is called a hydraulic jump, seen at the outer edge of the clear circular area in the center of the picture. At this transition, the flow abruptly rises in height as it changes from a high-velocity laminar flow (in which the streamlines of flow are parallel and do not disturb one another) to a low-velocity turbulent flow (a less-orderly flow with swirls and eddies that promote mixing). The authors took this image in an attempt to view the undercutting of the water as it initially impinges on the solution of PAM; but the vortical mixing and the formation of viscous fingers provided a beautiful image.

The type of flow featured in this image has practical applications as well. The simple addition of a high-molecular weight polymer, such as PAM, to a liquid that coats a surface could be used to more gently and easily remove contaminants from that surface. The coating would work as a soft adhesive that, when rinsed off, would remove particulates from the surface without harming it.

Reporters and Editors
To use this image please contact:

Travis W. Walker
Stanford University
605.840.6253

Associated References
Walker, T.W., T.T. Hsu, C.W. Frank, G.G. Fuller. Role of shear-thinning on the dynamics of rinsing flow by an impinging jet. Phys. Fluids. 24 (9), 2012. DOI:10.1063/1.4752765.

Hsu, T.T., T.W. Walker, C.W. Frank, G.G. Fuller. Role of fluid elasticity on the dynamics of rinsing flow by an impinging jet. Phys. Fluids. 23 (3), 2011. DOI:10.1063/1.3567215.

Brian Chang, Brice Slama, Sunghwan Jung
Department of Engineering Science and Engineering
Virginia Tech

Inspired by children clapping their wet hands, the authors of this image demonstrate how a resting fluid is squeezed by colliding two flat plates. Rapid compression causes the fluid to eject radially, first forming a smooth sheet of liquid with a thicker rim (called a "ligament") at its outer edge, and subsequently generating wavy edges within the fluid sheet and droplets flying outward at a high speed.

This image shows a snapshot of the rim and sheet of a liquid moments before it breaks up into droplets and ligaments. A volume of silicone oil is placed on the bottom of two circular plates. When the top plate collides with the bottom plate, the oil is ejected outwards. One can see the thick rim forming along the outer edge of the fluid, as well as the thin fluid sheet following it.

Instabilities cause the rim to begin to form waves as it expands. Gravity pulls the rim and the sheet down during this expansion, creating the curtain-like shape shown in this image. Moments after this picture was taken, instabilities would have caused the sheet of fluid to break up into ligaments and the rim to form droplets.

Acknowledgment is made to the donors of American Chemical Society Petroleum Research Fund (PRF# 52332-DNI9) for partial support of this research.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "B. Chang, B. Slama and S. Jung at Virginia Tech."

Contact Information
Sunghwan(Sunny) Jung
Assistant Professor, Engineering Science & Mechanics,
228 Norris Hall, Virginia Tech, Blacksburg, VA

Jean Rajchenbach, Alphonse Leroux
Laboratoire de Physique de la Matière Condensée
Université de Nice-Sophia Antipolis
France

Didier Clamond
Laboratoire Jean-Alexandre Dieudonnée
Université de Nice-Sophia Antipolis
France

Waves at the surface of water are described by a set of nonlinear equations. These nonlinearities can induce the emergence of new patterns. In this experiment, a container partly filled with a Newtonian fluid is vibrated vertically. (Newtonian fluids include most common liquids and gases; examples are water and air.) The vibrations give rise to the formation of standing waves at the free surface of the fluid, a phenomenon known as the 'Faraday instability.'

This image shows a surface wave alternating in shape between a pentagon and a star. The order of the symmetry (in this case, five) can be varied according to the frequency and amplitude of the vibrations. Surprisingly, this number does not depend on the container's size or shape. The geometry of the standing wave can be interpreted as resulting from nonlinear resonant couplings between three waves.

This project has been partially supported by CNRS, Société ACRI and Région PACA.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "Jean Rajchenbach, Alphonse Leroux, and Didier Clamond (CNRS and Université de Nice, France)."

Meeting Abstract
G28.00010: Observation of star-shaped surface gravity waves

• J. J. Stoker, Water Waves: The Mathematical Theory with Applications, (Wiley Classics Library, 1992).
• M. Faraday, Phil. Trans. R. Soc. London 121, 299 (1831).
• J. Miles and D. Henderson, Annu. Rev. Fluid Mech. 22,
• 143 (1990).
• J. Rajchenbach, D. Clamond and A. Leroux, submitted to Phys. Rev. Lett.
• (2012).

Contact Information
Jean Rajchenbach
Physique de la Matiere Condensee CNRS UMR 7336
Universite de Nice - Sophia Antipolis
Parc Valrose
28 Avenue Valrose
06108 Nice Cedex 2
Telephone (+33) 4 92 07 67 65
Fax: (+33) 4 92 07 67 54
Homepage: http://www.unice.fr/rajchenbach

Kyle J. Lueptow
Evanston Township High School
Evanston, Illinois

Richard M. Lueptow
Northwestern University
Evanston, Illinois

When a perfectly straight stream of honey hits the surface of the water in a crystal goblet, it begins to spiral. This phenomenon is similar to the coiling effect, called "liquid rope coiling," that occurs when a stream of thick, high-viscosity fluid like honey or syrup traversing through air hits a flat surface like a piece of toast or the bottom of an empty teacup. But in the example depicted above, this coiling instability is triggered by a liquid: the stream of honey impinges on the surface of the water instead of at the bottom of the goblet. As a result, the coiling stream of honey is stretched and deformed as it traverses through the water in the goblet.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "AAPT High School Physics Photo Contest, Kyle James Lueptow, 'Coiling Instability in Honey Poured into Water.'."

Contact Information
Richard M. Lueptow
Professor and Senior Associate Dean
McCormick School of Engineering and Applied Science
Northwestern University
Room L258, Technological Institute
2145 Sheridan Road
Evanston, IL 60208-3111
847-491-2739

Jonathan Sweet, Adrian Avila, Said Shakerin
Department of Mechanical Engineering
University of the Pacific
Stockton, California

Water sheets have attracted the attention of researchers for many years. This image depicts the work of researchers who have used water sheets to develop a new, simple water feature that could be used in yard displays.

Water from a yard faucet enters a specially designed inlet block attached to an acrylic tube about eight inches long. The water spirals up the length of the tube, leaving a core of air in the center of the tube, and exits at the open end, where it forms a rotating water sheet. Eventually, instabilities cause the water sheet to disintegrate into drops.

An adjustable stand was designed to hold the water feature at a given angle (in this example, 15 degrees from the vertical). Also, moving the tube up, down, and sideways produces a playful water sheet and drops, though this is not shown in the image.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "J. Sweet, A. Avila, and S. Shakerin (University of the Pacific, Stockton, CA)."

• G. I. Taylor, "The Dynamics of Thin Sheets of Fluid," Proc. R. Soc., Vol. 253, pp. 289-321, 1959.
• C. Clanet, "Water Bells and Liquid Sheets," Annu. Rev. Fluid Mech., Vol. 39, pp. 469-496, 2007.

Contact Information
Said Shakerin, Professor Department of Mechanical Engineering
University of the Pacific
Stockton, CA 95211

Chris Morton, Serhiy Yarusevych
Department of Mechanical and Mechatronics Engineering
University of Waterloo
Waterloo, Canada

A plethora of vortices (orange) in water, created by a flow of water around a cylinder with two step changes in diameter. The length of the larger cylinder in Figure 1 is three times the size of its diameter, while the length of the larger cylinder in Figure 2 is equal to its diameter.

The images illustrate flow development for a dual-step cylinder, a cylinder with two step changes in diameter. The cylinder model is oriented horizontally at the bottom of each image and the water flow is upwards. The images depict a plethora of vortices (seen in orange) that develop as water flows around the dual-step cylinder.

The images show how the distance between the two step changes in diameter alter the topology of the flow, with other geometric parameters kept constant. In both images, the larger cylinder (bottom center) has twice the diameter of the smaller cylinder (bottom left and right). The length of the larger cylinder in the first image is three times the size of its diameter, while the length of the larger cylinder in the second image is equal to its diameter.

The first image depicts one vortex oriented horizontally, along the span of the cylinder (and called a "spanwise vortex"), as well as smaller-scale vortices emanating from each of the two steps in diameter. The image shows that the spanwise vortex deforms significantly behind the portion of the cylinder with larger diameter. The smaller scale vortices appear as a sequence of orange swirls that grow in size with distance from the cylinder.

The second image shows two spanwise vortices. Here, the distance between the step changes in diameter is reduced, which leads to less significant deformation of spanwise vortices. In addition, no evidence of smaller-scale vortices emanating from the steps can be seen.

The visualization is achieved with a hydrogen bubble technique. A DC voltage is applied to a small-diameter stainless steel wire mounted upstream of the cylinder model. This leads to the formation of hydrogen bubbles on the wire, which are carried away by the water flow. A conical laser beam is used to illuminate the hydrogen bubbles, thereby visualizing flow development.

Reporters and Editors

This image can be freely reproduced with the accompanying credit: "C. Morton and S. Yarusevych (University of Waterloo, Canada)."

Author Talks at the 2012 DFD Meeting

• D14.00006: Reconstructing dominant three-dimensional flow structures in the wakes of cylindrical bodies using planar velocity measurements
• L12.00009: Influence of diameter ratio and aspect ratio on wake development of a dual step cylinder

Contact Information
Chris Morton
Serhiy Yarusevych
Fluid Mechanics Research Laboratory
Mechanical and Mechatronics Engineering
University of Waterloo
Waterloo, Ontario, Canada
Phone: (519) 888-4567 x35442
Fax: (519) 885-5862
Web: http://www.fmrl.uwaterloo.ca

Xiaodong Chen, Vigor Yang
School of Aerospace Engineering
Georgia Institute of Technology
Atlanta, Georgia

Using a sophisticated simulation, engineers have studied the complex evolution of a head-on collision between two high-speed water droplets, each about 400 micrometers in diameter, or about twice the thickness of a human hair. This snapshot from the simulation depicts the droplet dynamics moments after their collision, just as they are beginning to break up into smaller droplets.

Apparent in the image are an expanding "sheet" of liquid caused by the collision, and a thicker "rim" at the outer edge of this sheet. Ligaments of water, formed by the destabilization of the liquid rim, stretch across the radius of the sheet. One can also see small droplets generated by the end-pinching of these ligaments.

The researchers' simulations allowed them to describe and explain the expansion and contraction within the liquid sheet, as well as the fragmentation of the liquid rim and the sizes of the droplets that result from this fragmentation. The rich information obtained from these numerical simulations can be used to establish a physical link between the behavior of the water droplets and their statistical characteristics during an impact.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "Xiaodong Chen and Vigor Yang (School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA)."

Contact Information
Vigor Yang
William R. T. Oates Professor and Chair
Daniel Guggenheim School of Aerospace Engineering
313 Montgomery Knight Building
Georgia Institute of Technology
Atlanta, GA 30332-0150

Sergey A. Suslov
Swinburne University of Technology
Hawthorn, Victoria, Australia

Aleksandra A. Bozhko, Aleksander S. Sidorov, Gennady F. Putin
Perm State National Research University
Perm, Russia

A ferrofluid is a liquid that becomes strongly magnetized in the presence of a magnetic field. The images above show the process of thermomagnetic convection in a ferrofluid, in which both magnetization and temperature play a part in the flow of the liquid.

Thermomagnetic convection arises due to the fact that the fluid's magnetization is sensitive to temperature. Cooler fluid is magnetized stronger, screening the external magnetic field more than warmer fluid. Subsequently, a more strongly magnetized cooler fluid is drawn to warmer regions with a stronger magnetic field.

Figure 1 is a composite of four photographs, showing a series of thermomagnetic convection patterns arising in a homogeneous ferrofluid heated from the front of the photographed surface. The average temperature gradient across the layer of fluid is fixed, but the magnetic field, applied normally to the image plane, increases from image to image. In the leftmost panel, with the weakest magnetic field, a vortical flow appears around the perimeter of the container. An increased magnetic field leads to the stationary thermomagnetic rolls apparent in the middle left pane. Finally, the strongest magnetic field causes the unsteady wave-like patterns in the two rightmost images. Flow visualization is performed using a heat-sensitive liquid crystal film glued on the surface of the layer facing the camera. This film reveals the surface temperature fields associated with the various flow patterns.

Figure 2 is an infrared photograph of two-cell thermomagnetic convection patterns arising in an undivided layer of a stratified ferrofluid that is being cooled from the front of the photographed surface. The vertical stratification is due to a slow gravitational sedimentation of magnetic nanoparticles.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "Aleksandra A. Bozhko, Aleksander S. Sidorov, Gennady F. Putin (Perm State National Research University, Perm, Russia) and Sergey A. Suslov (Swinburne University of Technology, Hawthorn, Victoria, Australia)."

Related Author Talk at the 2012 DFD Meeting
G2.00009: New type of thermal waves in a vertical layer of magneto-polarizable nano-suspension: theory and experiment

Contact Information
Dr. Sergey A. Suslov
Senior Lecturer in Applied Mathematics,
Mathematics H38,
Swinburne University of Technology,
PO Box 218, Hawthorn,
Victoria 3122, Australia
Phone: +61-3-9214 5952, Fax: +61-3-9214 8264
http://www.swin.edu.au/mathematics/staff/Sergey-Suslov

Daniel J. Asselin, Charles H.K. Williamson
Fluid Dynamics Research Laboratories
Sibley School of Mechanical and Aerospace Engineering
Cornell University
Ithaca, New York

The interaction of a pair of counter-rotating vortices with a solid boundary is a problem of fundamental interest to many areas of fluid mechanics, as well as to practical applications in which vortices interact with the ground or with vehicles in air or water. These vortices are subject to both short-wavelength and long-wavelength instabilities. The focus of this study concerns the evolution of vortices initially subject to the long-wavelength instability, which then interact with a wall.

A vortex pair is generated in a water tank using a pair of rotating flaps that span the length of the tank. The interaction of these primary vortices (red) with a solid horizontal wall within the tank leads to the formation of secondary vortices (green). The two sets of images were taken ten seconds apart and show the vortices from the side (top two images in each set) and from below (bottom two images in each set).

Upon interacting with the ground plane, significant flow is directed along the axis of each primary vortex (shown in red). This leads to the periodic concentration of fluid containing vorticity at the peaks of the initially wavy vortex line. In the secondary vortices (shown in green), vortex rings appear to develop from rising vortex loops.

This research was supported by the Office of Naval Research.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "D.J. Asselin & C.H.K. Williamson (Cornell University, Ithaca, NY)."

References

• Crow, S.C.(1970) Stability Theory for a Pair of Trailing Vortices. AIAA J., 8 (12), 2172-2179.
• Tsai, C.-Y. & Widnall, S.E. (1976) The stability of short waves on a straight vortex filament in a weak externally imposed strain field. J. Fluid Mech., 73 (04), 721-733.
• Widnall, S.E., Bliss, D.B., and Zalay, A. (1971) Theoretical and experimental study of the stability of a vortex pair. In: Aircraft Wake Turbulence and Its Detection. New York, Plenum Press, 1971., pp. 305-338.
• Widnall, S.E., Bliss, D.B., and Tsai, Chon-Yin. (1974) The instability of short waves on a vortex ring. J. Fluid Mech., 66 (01), 35-47.

Contact Information
Daniel J. Asselin
Fluid Dynamics Research Laboratories
Sibley School of Mechanical and Aerospace Engineering
Cornell University

T.T. Lim, C.T. Toh
Department of Mechanical Engineering
National University of Singapore
Singapore

M. Cheng, J. Lou
Institute of High Performance Computing
Singapore

A bubble ring is an underwater vortex ring whose inner rotating region or vortex core is filled with air. Scuba divers and free-divers sometimes create them for their amusement by puffing a burst of air through their mouths. In a laboratory, a commonly used technique is to eject a short burst of air through a small opening at the bottom of a water tank.

The ejected air initially forms a bubble, but this subsequently transforms into a donut or ring shape due to the pressure difference between the top and the bottom of the bubble. The force of buoyancy causes the bubble ring to rise, and another force, called the cross-flow lift force, causes its radius to increase and its velocity to decrease.

The photograph above shows a multiple-exposure image of a single bubble ring rising to the top of a 1.5-meter water tank. The photograph is captured with the aid of a stroboscopic light source located at the top of the tank and with the camera aperture fully opened. The "sparkles" on the three uppermost rings are due to reflection of the stroboscopic light from the bubble's surface.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "T.T. Lim & C.T. Toh (National University of Singapore); M. Cheng & J. Lou (Institute of High Performance Computing, Singapore)."

Contact Information
T.T.Lim
Department of Mechanical Engineering
National University of Singapore
9 Engineering Drive 1
Singapore 117576
Tel: (65) 6516-6350,
Fax: (65) 6779-1459

Vicente Fernandez, Orr Shapiro, Melissa Garren, Theresa Santiano-McHatton, Roman Stocker
Department of Civil and Environmental Engineering
Massachusetts Institute of Technology (MIT)

Orr Shapiro, Assaf Vardi
Department of Plant Sciences
Weizmann Institute of Science
Rehovot, Israel

Hair-like cilia are known to cover the entire surface of reef-building corals, but their influence on the boundary layer and local environment of the animal has been overlooked. In this image, the complex cilia-driven flow between Pocillopora damicornis coral polyps is captured by tracks of fluorescent beads. The bundles of arced tracks over the coral surface capture the mixing that is occurring perpendicular to the surface. This ciliary mixing is enhancing mass transport near the coral surface, potentially increasing rates of photosynthesis and carbon fixation by the coral-algal symbiotic system and also affecting invasion of the coral surface by microbial pathogens.

The composite image is generated from 120 video frames taken at 10 frames per second by use of video microscopy. The tracks of the 2-micrometer fluorescent particles are shown superimposed on a single frame to illustrate the relationship between flow and polyp locations. Each polyp is approximately 1 millimeter in diameter, about a thousand times larger than the fluorescent beads. Naturally occurring green fluorescent protein (GFP) gives the coral its green appearance.

Reporters and Editors
This image can be freely reproduced with the accompanying credit: "Stocker Group, Civil and Environmental Engineering, MIT."

Contact Information
Vicente Fernandez
Department of Civil and Environmental Engineering
MIT 48-211
77 Massachusetts Ave.
Cambridge, MA 02139
Tel. : 857 998 1954