Look beneath the waves on a beach or in a bay, and you’ll find ripples in the sand. Passing waves shape these sandforms and can even build them to heights that require dredging to keep waterways passable to large ships. To better understand how the sand interacts with the flow, researchers build computer models that couple the flow of the water with the behavior of individual sand grains. One recent study found that sand grains experienced the most shear stress as the flow first accelerates and then again when a vortex forms near the crest of the ripple. (Image credit: D. Hall; research credit: S. DeVoe et al.; via Eos)
Search results for: “waves”
Roll Waves in Debris Flows
When a fluid flows downslope, small disturbances in the underlying surface can trigger roll waves, seen above. Rather than moving downstream at the normal wave speed, roll waves surge forward — much like a shock wave — and gobble up every wave in their way.
Such roll waves are fairly innocuous when flowing down a drainage ditch but far more problematic in the muddy debris flows of a landslide. Debris flows are harder to predict, too, thanks to their combined ingredients of water, small grains, and large debris.
A new numerical model has shed some light on such debris flows, after showing good agreement with a documented landslide in Switzerland. The model suggests that roll waves get triggered in muddy flows at a higher flow speed than in a dry granular flow but a lower flow speed than is needed in pure water.
For a great overview of roll waves, complete with videos, check out this post by Mirjam Glessner. (Image credit: M. Malaska; research credit: X. Meng et al.; see also M. Glessmer; via APS)
Fediverse Reactions
-
Capturing River Waves
Rainfall, ice jams, and dam breaks create surges of high flow that make their way down a river in a wave that stretches tens to thousands of kilometers in length. Traditionally, scientists monitor such flow waves using river gauges, which measure river height at specific locations. But gauges are few and far between on many rivers, so a group of researchers are supplementing that data with the SWOT (Surface Water and Ocean Topography) spacecraft. SWOT bounces microwaves off the water to precisely measure the water’s height, giving researchers a glimpse of the flow wave’s shape along the entire river.
In their paper, the team identify and describe flow waves on three different rivers — the Yellowstone, Colorado, and Ocmulgee rivers — ranging in height up to 9 meters and stretching up to 400 kilometers. (Image credit: CNES; research credit: H. Thurman et al.; via Gizmodo)
Fediverse Reactions
-
“Skimming the Waves”
Common terns are gregarious sea birds that cruise low over the water to fish. When they spot prey, they will dip down to grab a fish from the surface, or they will fold their wings to plunge-dive to depths of half a meter. Compared to gannets and boobies, these are slower, shallower dives that involve less impact risk. Presumably the birds’ choice of dive height reflects the typical swim depth of their preferred fish. (Image credit: N. Kovo/WPOTY; via Colossal)
Fediverse Reactions
-
Revealing Gravity Waves
Severe weather — like thunderstorms, tornadoes, and hurricanes — can push air upward into a higher layer of the atmosphere and trigger gravity waves. Aboard the International Space Station (ISS), the Atmospheric Waves Experiment (AWE) instrument captures these waves by looking for variations in the brightness of Earth’s airglow (above). Recently, when Hurricane Helene hit the southeastern United States, AWE caught a series of gravity waves some 55 miles up, pushed by the storm (below). It’s incredible to see these long-ranging ripples spreading far beyond the heart of the storm. (Video credits: NASA Goddard and Utah State University)
Waves Break Up Floating Rafts
Small particles can float on a liquid, held together as a raft through capillary action. But those rafts — like the tea skin below — break up when waves jostle them. In this study, researchers looked at how standing waves broke up a raft of graphite powder. Although the raft’s break-up resembles fields of sea ice breaking apart, the researchers found that different mechanisms were responsible. In their experiment, waves pushed and pulled horizontally at the raft, causing it to fracture. But that push-and-pull is negligible in sea ice, where sheets instead break from the up-and-down motion of waves vertically bending the ice. Nevertheless, the new insights are valuable for various biofilms and some ice floes. (Image and research credit: L. Saddier et al.; via APS Physics)
The skin atop a cup of tea breaks up into polygons after stirring with a spoon. Although the effect resembles sea ice breakup, the specific wave mechanism differs. 
The Crashing Waves of French Polynesia
Surfer and photographer Tim McKenna lives in the village of Teahupo’o on Tahiti’s southeastern coast. The area’s shallow coral reef system creates some of the world’s biggest barreling waves, which attract surfers from around the world. McKenna captures the majestic power of these surges in these black-and-white photographs; you can find more of his work on his website and Instagram. (Image credit: T. McKenna; via Colossal)
Waves Lap on Titan’s Shores
Titan, one of Saturn’s moons, is the only other planetary body known to have liquid lakes, rivers, and seas at its surface. Whether those bodies — made up of hydrocarbons rather than water, like here on Earth — have waves is a matter of ongoing debate. What data we have from visiting spacecraft is inconclusive. So a group of researchers decided to look for the effects of wave action instead.
Beginning with a model of flooded areas similar to Titan’s, the team simulated a coastline’s erosion assuming three different situations: 1) no coastal erosion, 2) erosion from waves, and 3) uniform erosion through dissolution. Each set of conditions resulted in a very different final coastline. But, of the three, the wave-eroded coast was most similar to those seen on Titan. That’s a good indicator that, even if our spacecraft couldn’t see waves on Titan, they’re likely there. (Image credit: ESA; research credit: R. Palermo et al.; via Gizmodo)
Star-Birthing Shock Waves
Although the space between stars is empty by terrestrial standards, it’s not devoid of matter. There’s a scattering of cold gas and dust, pocked by areas known as prestellar cores with densities of a few thousand particles per cubic centimeter. This is just enough matter to help gravity eventually win its tug of war with the forces that would drive molecules apart.
When shock waves pass through these regions — whether thrown off a dying star or a newly birthed one — they compress the material, kickstarting the process of stellar formation. Passing shock waves can also shake loose molecules stuck to the dust, providing key tracer elements that astronomers can use to visualize shock waves and the areas they affect. To learn more, see this article over at Physics Today. (Image credit: NASA/ESA/CSA/STSCI/K. Pontoppidan/A. Pagan; see also Physics Today)
Calming the Waves
Wave action can be a major source of erosion along riverbanks and shorelines. But in a recent study, scientists were able to perfectly absorb incoming waves to create a downstream region with calm, wave-free waters.
Experimental data shows that waves approaching from the left interact with the resonant chambers and get perfectly absorbed, leaving the water on the right side still. The group began with a narrow channel that waves could move down. They added two small, side-by-side cavities perpendicular to the channel; as waves travel down the channel, they resonate with the cavities, which reflect and transmit their own waves back into the channel. With the right tuning to the size and spacing of the cavities, the team was able to make the cavities’ waves perfectly cancel the channel’s waves. The group demonstrated this absorption theoretically, numerically, and experimentally.
Currently, they’ve only managed perfect absorption with a single wave frequency, but an array of cavities should be able to absorb a range of incoming waves. The authors hope their work will one day help protect coastal structures and prevent erosion by countering incoming waves. (Image and research credit: L-P. Euvé et al.; via APS Physics)
