When vibrated, fluid surfaces can exhibit standing waves known as Faraday waves. In this experiment, increased forcing of these standing waves causes the formation of a jet. Under the right conditions, as the standing wave collapses, a singularity forms on the fluid surface when velocity and surface curvature diverge. The narrow jet column forms as a result of the fluid’s kinetic energy getting focused by the collapse. For more, see this letter to Nature. #
Month: March 2011
Reynolds on Transition
For although only the disciplined motion is recognized in military tactics, troops have another manner of motion when anything disturbs their order. And this is precisely how it is with water: it will move in a perfectly direct disciplined manner under some circumstances, while under others it becomes a mass of eddies and cross streams, which may be well likened to the motion of a whirling, struggling mob where each individual particle is obstructing the others. The larger the army, and the more rapid the evolutions, the greater the chance of disorder; so with fluid, the larger the channel, and the greater the velocity, the more chance of eddies.
Volcanic Shock Waves
This footage of last year’s eruption of Eyjafjallajökull in Iceland shows shockwaves emanating from the mouth of the volcano as hot ash and gases explode from underground.
Laminar Flow Control
On Wednesday, March 30, 2011 at 3:00 EDT NASA engineers are holding an online chat about a current project to achieve laminar flow control on business jet-class airplanes. Keeping flow over an airplane’s wings laminar could decrease the total drag on an airplane by as much as 15%. In particular, this project involves placing tiny hockey-puck-shaped discrete roughness elements (DREs) along the front of the wing. These DREs are positioned such that they perturb the mean-flow over the wing at a higher frequency than the naturally most unstable frequency; as a result, flow actually remains laminar over a greater extent of the wing than would normally be the case. For more on the technical ideas, see this NASA blog post or feel free to ask questions in the comments. #
Full disclosure: This project is being conducted in joint with professors with whom I work, and the subject matter is related to my own research.
Computational Vortex Rings
Computational fluid dynamics (CFD) sometimes gets a bad rep as “colorful fluid dynamics”, but as computers get faster and faster, more complicated and physically accurate simulations are possible. Shown here are simulations of vortex rings and wingtip vortices in stunningly gorgeous detail. Understanding the evolution of these vortices from a fundamental level helps fluid mechanicians design better methods of controlling them. As mentioned in the video, wingtip vortices are a particularly hazardous everyday example; the time it takes for one plane’s wingtip vortices to disperse determines how quickly the next airplane can take-off or land on that same runway. Being able to break down these vortices faster would allow more frequent use of existing facilities.
Solar Fluid Dynamics
The sun is a wild place fluid dynamically. The surface is riddled with convection cells the size of the Earth, and prominences of plasma (ionized gas) erupt from the surface following the sun’s magnetic field lines. Violent, but beautiful. #
Underwater Explosions
As powerful as explosions can be above ground, they are even more dangerous underwater. Since water, unlike air, is incompressible, the pressure wave at the front of an underwater explosion is not damped to the extent it would be in air. A high-pressure, high-temperature bubble of gas also forms in the explosion, and, as with cavitation, if the bubble collapses near metal, the damage can be extensive. (via Gizmodo)
Smoke Visualization on an F-16
Flow around an F-16XL Scamp model is visualized using smoke illuminated by laser sheets. Lasers are common equipment in fluids laboratories; they’re useful for flow visualization and for many velocimetry techniques.
Droplet Impact on Superhydrophobic Surfaces
High-speed video of water droplets impacting on superhydrophobic surfaces demonstrates the impressive elasticity and surface tension of the droplets. Impacts vibrate and reflect through the droplet, but only a drop from the largest height actually causes breakup.
Wind Tunnel Testing
This photo shows a prototype of the X-48C blended wing body aircraft being tested in NASA Langley’s 12-Foot Low-Speed Tunnel. Blended wing bodies have many advantages over conventional tube-and-wing designs: the entire surface of the craft can generate lift; the usable cargo/passenger area of the craft is increased; and, structurally, the craft is easier to manufacture. Flight tests of a remote-controlled version of the craft have also taken place.
