In the summer months, a breeze can set long grasses waving in an impressive display. Similar behaviors are seen in aquatic plants during tides. Researchers simulate the behavior in two-dimensions using a flowing soap film and nylon filaments. Flow visualization reveals the strong differences between flow above and between the grass. Vortices recirculate between the filaments at speeds much slower than the flow overhead. The instantaneous interaction of the high-speed freestream, the unsteady vortices, and the resistance of the grass results in familiar synchronous waves of grain. (Video credit: R. Singh et al.)
Search results for: “waves”
Reader Question: Wave Vortex
Reader unquietcode asks:
I saw this post recently and it made me wonder what’s going on. If you look in the upper right of the frame as the camera submerges, you can see a little vortex of water whirring about. Even with the awesome power of the wave rolling forward a little tornado of water seems able to stably form. Any idea what causes this phenomenon?
This awesome clip was taken from John John Florence’s “& Again” surf video. What you’re seeing is the vortex motion of a plunging breaking wave. As ocean waves approach the shore, the water depth decreases, which amplifies the wave’s height. When the wave reaches a critical height, it breaks and begins to lose its energy to turbulence. There are multiple kinds of breaking waves, but plungers are the classic surfer’s wave. These waves become steep enough that the top of the wave overturns and plunges into the water ahead of the wave. This generates the vortex-like tube you see in the animation. Such waves can produce complicated three-dimensional vortex structures like those seen in this video by Clark Little. Any initial variation in the main vortex gets stretched as the wave rolls on, and this spins up and strengthens the rib vortices seen wrapped around the primary vortex. (Source video: B. Kueny and J. Florence)
Measuring Wind Speed by Satellite
Weather modeling and forecasting in recent decades have benefited enormously from the availability of more data. For example, satellites now measure wind speeds over the open ocean, instead of data simply coming from isolated ships and buoys. The satellites do this by measuring the roughness of the ocean using radar or GPS signals bounced off the ocean surface. From this researchers can construct a map of wave height and direction like the one in the animation above. For a large body of water, waves are primarily generated by wind shearing the water at the interface. The waves we see are a result of the Kelvin-Helmholtz instability between the wind and ocean. Because this is a well-known behavior, it is possible to connect the waves we observe with the wind conditions that must have generated them. (Image credit: ESA; animation credit: Wired; submitted by jshoer)
Mach Diamonds
Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)
Wave Tank
A new wave tank facility opening at the University of Edinburgh promises new capabilities to simulate ocean wave behavior. The circular 25m diameter wave tank is lined with 168 wave makers and is equipped with 28 submerged flow-drive units. Together, these allow the tank to simultaneously simulate nearly any wave type as well as tidal currents up to 1.6 m/s. The facility is intended for 1/20th scale modeling; projected to full-size, this means that the tank is capable of making waves representative of 28 m high ocean waves and tidal currents in excess of 12 knots. It’s expected to be particularly valuable in the development and testing of wave and tidal motion generators for clean energy. For more, see BBC News and FloWave’s own website. (Image credit: Brightspace/BBC News; submitted by srikard)
Kelvin-Helmholtz in the Lab
The Kelvin-Helmholtz instability looks like a series of overturning ocean waves and occurs between layers of fluids undergoing shear. This video has a great lab demo of the phenomenon, including the set-up prior to execution. When the tank is tilted, the denser dyed salt water flows left while the fresh water flows to the right. These opposing flow directions shear the interface between the two fluids, which, once a certain velocity is surpassed, generates an instability in the interface. Initially, this disturbance is much too small to be seen, but it grows at an exponential rate. This is why nothing appears to happen for many seconds after the tilt before the interface suddenly deforms, overturns, and mixes. In actuality, the unstable perturbation is present almost immediately after the tilt, but it takes time for the tiny disturbance to grow. The Kelvin-Helmholtz instability is often seen in clouds, both on Earth and on other planets, and it is also responsible for the shape of ocean waves. (Video credit: M. Hallworth and G. Worster)
Space Balls (of Water!)
The microgravity environment of space is an excellent place to investigate fluid properties. In particular, surface tension and capillary action appear more dramatic in space because gravitational effects are not around to overwhelm them. In this animation, astronaut Don Petit injects a jet of air into a large sphere of water. Some of the water’s reaction is similar to what occurs on Earth when a drop falls into a pool; the jet of air creates a cavity in the water, which quickly inverts into an outward-moving jet of water. In this case, the jet is energetic enough to eject a large droplet. Meanwhile, the momentum, or inertia, from the air jet and subsequent ejection causes a series of waves to jostle the water sphere back and forth. Surface tension is strong enough to keep the water sphere intact, and eventually surface tension and viscosity inside the sphere will damp out the oscillations. You can see the video in full here. (Image credit: Don Petit/Science off the Sphere)
Rubens’ Table
Veritasium’s new video has an awesome demonstration featuring acoustics, standing waves, and combustion. It’s a two-dimensional take on the classic Rubens’ tube concept in which flammable gas is introduced into a chamber with a series of holes drilled across the top. Igniting the gas produces an array of flames, which is not especially interesting in itself, until a sound is added. When a note is played in the tube, the gas inside vibrates and, with the right geometry and frequency, can resonate, forming standing waves. The motion of the gas and the shape of the acoustic waves is visible in the flames. Extended into two-dimensions, this creates some very cool effects. (Video credit: Veritasium; via Ryan A.; submitted by jshoer)
Rebounding
A water droplet can rebound completely without spreading from a superhydrophobic surface. The photo above is a long exposure image showing the trajectory of such a droplet as it bounces. In the initial bounces, the droplet leaves the surface fully, following a parabolic path with each rebound. The droplet’s kinetic energy is sapped with each rebound by surface deformation and vibration, making each bounce smaller than the last. Viscosity damps the drop’s vibrations, and the droplet eventually comes to rest after twenty or so rebounds. (Image credit: D. Richard and D. Quere)
“Becoming Harmonious”
Much as I try to keep from getting repetitious, this was just too neat to pass up. This new music video for The Glitch Mob’s “Becoming Harmonious” is built around the standing Faraday waves that form on a water-filled subwoofer. The vibration patterns, along with judicious use of strobe lighting, produce some fantastic and kaleidoscopic effects. (Video credit: The Glitch Mob/Susi Sie; submitted by @krekr)