Part of the beauty of numerical simulation is its ability to explore the physics of a situation that would difficult or impossible to create experimentally. Here the Rayleigh-Taylor instability–which occurs when a heavier fluid sits atop a lighter fluid–is simulated in two-dimensions. Viscosity and diffusion are set extremely low in the simulation; this is why we see intricate fractal-like structures at many scales rather than the simulation quickly fading into gray. (The low diffusion is also what causes the numerical instabilities in the last couple seconds of video.) The final result is both physics and art. (Video credit: Mark Stock)
Tag: fluid dynamics
How to Escape a Whitewater Hole
One of the perils of whitewater sports is getting stuck in what paddlers call a “hole” or a “hydraulic”. This river feature forms just downstream of large obstacles like rocks or low-level dams. As water pours over the obstacle and into its shadow, the flow forms a recirculating vortex-like zone. Immediately next to the obstacle, water is pulled upstream toward the obstacle and then down toward the bottom of the river. This makes the hydraulic very dangerous and hard to escape. Note in the video how the raft is held in place by the upstream motion of the water at the surface of the hydraulic. The rafters are preventing their craft from flipping over by weighing down the side experiencing the upward flow of the vortex. Escaping a hydraulic usually requires getting near its edge, where its current is weaker. If swimming, the best way to escape is to swim toward the bottom of the river and then downstream with the current of the hydraulic rather than against it at the surface.
Flapping Flags
The flapping of flexible objects like flags have long fascinated mankind. The figure above from Shelley and Zhang 2011 shows several possible flapping states. In (a) a thread immersed in a running soap film displays the standard von Karman vortex street of shed vortices in its wake. Parts (b) and © show the thread in coherent flapping motion; (b) shows an snapshot of the flapping thread in the soap film whereas © is a timelapse of the thread showing its full range of motion. Image (d) shows the effects of a higher flow speed–the flapping motion becomes aperiodic. Image (e) shows a stiff metal wire bent into the shape of a flapping filament; note the strong boundary layer separation around the wire compared to the thread in Image (b). As one might expect, the drag on the unflapping wire is significantly greater than the drag on the flapping thread. (Image credit: M. Shelley and J. Zhang, Shelley and Zhang 2011)
Astro Puffs
Microgravity continues to be a fascinating playground for observing surface tension effects on the macroscale without pesky gravity getting in the way. Here astronaut Don Pettit has created a sphere of water, which he then strikes with a jet of air from a syringe. Initially, the momentum from the jet of air creates a sharp cavity in the water, which rebounds into a jet of water that ejects one or more satellite drops. Surface waves and inertial waves (inside the water sphere) reflect back and forth until the fluid comes to rest as a sphere once more. Note how similar the behavior is to the pinch-off of a water column. Both effects are dominated by surface tension, but on Earth we can only see this behavior with extremely small droplets and high-speed cameras! (Video credit: Don Pettit, Science Off the Sphere)
Pinch-Off
This high-speed video reveals a fascinating bit of kitchen sink physics. When a water droplet pinches off from the nozzle, the thin filament of fluid that connected the droplet to the water on the nozzle often breaks off as well. Surface tension snaps the filament together into a sphere, causing wild oscillations and even ejection of microjets in the tiny satellite droplet. (Video from S. Thoroddsen et al. 2008’s Annual Review)
Vortex Cross-Sections
The photos above show cross-sections through the leading edge vortices on a highly swept delta wing at angle of attack. Flow in the photos is from the upper left to lower right. Notice how the vortices grow and develop waviness as they move downstream. When perturbations enter the vortex–for example, due to the shear between the vortex fluid and the freestream–some will grow and eventually cause a break down to turbulence, as in the lower picture. (Photo credits: R. Nelson and A. Pelletier)
Soap Film Loops
Here’s a fun demonstration of the effects of surface tension. If a loop of thread is dropped onto a soap film, as shown above, popping the soap film inside the thread will pull the thread into a circle. This is because the surface tension of the soap film outside the thread is reacting to the sudden loss of the balancing force exerted by surface tension inside the thread loop. Surface tension arises from intermolecular forces in a fluid. Because those forces are in balance except along the interface of a fluid–where the fluid molecules are not completely surrounded by identical molecules–there is a net force exerted at the surface.
Martian Landing Physics
A little over a week ago, NASA’s Curiosity rover landed on Mars, the culmination of years of engineering. The mission’s landing, in particular, was the subject of intense scrutiny as Curiosity’s size necessitated some new techniques in the final segments of the landing sequence. As it hit the Martian atmosphere at 13,000 mph, the compression of the carbon dioxide behind the capsule’s shock wave slowed the descent. At roughly 1,000 mph–speeds still large enough to be supersonic–Curiosity deployed its parachute. Shown above are the parachute in numerical simulation (from Karagiozis et al. 2011), wind tunnel testing at NASA Ames, and during descent thanks to the Mars Reconnaissance Orbiter. The simulation shows contours of streamwise velocity at different configurations; note the bow shock off the capsule and the additional shocks off the parachute. These help generate the drag needed to slow the capsule. For an interesting behind-the-scenes look at the wind tunnel testing for Curiosity’s parachute check out JPL’s four–part video series. Congratulations to all the scientists and engineers who’ve made the rover a success. We look forward to your discoveries! (Photo credits: K. Karagiozis et al., NASA JPL, NASA MRO)
Bubble Art
Photographer Janet Waters uses liquids and bubbles to create her fascinating abstract macro art. Check out this interview with the artist and her portfolio for more. (Photo credits: Janet Waters)
London 2012: Discus Physics
Like the javelin, the discus throw is an athletic event dating back to the ancient Olympics. Competitors are limited to a 2.5 m circle from which they throw, leading to the sometimes elaborate forms used by athletes to generate a large velocity and angular momentum upon release. The flight of the discus is significantly dependent on aerodynamics, as the discus flies at an angle of attack. Spin helps stabilize its flight both dynamically and by creating a turbulent boundary layer along the surface which helps prevent separation and stall. Unlike many other events, a headwind is actually advantageous in the discus throw because it increases the relative velocity between the airflow and the discus, thereby increasing lift. The headwind also increases the drag force on the discus, but research shows the benefits of the increased lift outweigh the effects of increased drag, so much so that a discus flies further in air than it would in a vacuum. (Photo credits: P Kopczynski, Wiki Commons, EPA/K Okten)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sports. Check out our previous posts, including why corner kicks swerve, what makes a pool fast, how an arrow flies, and how divers avoid splash.
