Fluid Dynamicists Hear About Oil Spills, Wind on Jupiter, and More
Jupiter’s Shrinking Red Spot! The best map of wind speeds on Jupiter ever produced proves that the massive weather system known as the Great Red Spot has shrunken over the past dozen years. Understanding cloud patterns on distant planetary surfaces, such as those at Saturn or Jupiter, is potentially confusing because clouds deform over time. Using sophisticated software, scientists at the University of California at Berkeley have been able to take the deformations into account (and the much easier-to-deal-with factor of the planet’s rotation) and have calculated the best velocity maps yet for the surface of Jupiter.
Using data recorded by the Galileo and Cassini spacecraft, views of Jupiter’s surface have been made that essentially factor out the planet’s rotation, simplifying the view of what is happening to the cloud decks. For mid-latitudes, the velocity resolution for this mapping procedure is 3 m/s. For higher latitudes, the resolution is 3 m/s to 6 m/s. The maps consist of tens of millions of velocity measurements.
According to Berkeley scientist Xylar Asay-Davis, these maps represent the highest resolution and highest accuracy full-planet map ever produced. With this approach, such meteorological features as the Great Red Spot or the Red Oval can be monitored more carefully than before. The high-precision velocity measurements show definitively that the Great Red Spot has shrunk over the past dozen years, says Asay-Davis.
Spill, Baby, Spill. Oil spills are a major environmental problem because they often occur at sea and in remote, ecologically-sensitive areas where their impact on birds, sea mammals and subsurface life may last for years. The best way to mitigate this damage is to clean up spills immediately, and typically this starts with skimming off as much oil as possible. Such cleanups may leave large areas covered with a thin slick of spilled oil, which is often dispersed by spraying the spill area with chemical “surfactants” that break the film into small oil droplets that are consumed by bacteria, dissolved, evaporated, or attached to small solid particles and sink to the bottom of the ocean.
When dispersants are sprayed over a spill in the open sea, the turbulent mixing forced by ocean currents and the wind actually helps in the cleanup process, but how much such turbulence contributes is not completely understood scientifically. Up to now, the breakup of oil mixed with dispersants has not been thoroughly studied in the laboratory, and there is little information on how wind, weather, and other local conditions contribute to the effectiveness of a cleanup process.
Now Johns Hopkins graduate student Balaji Gopalan and his mentor Professor Joseph Katz have imaged the dispersion of tens of thousands of oil droplets in carefully controlled laboratory settings and observed the effect of local turbulence on this process. Pre-mixing the oil with the commercial dispersant COREXIT 9527, they observed how it breaks into numerous tiny droplets smaller than the period at the end of this sentence. Following each droplet in three dimensions, they observed how tails or thread-like structures grew from its surface, the thickness of the tails being less than 17 micron in size, and the breakup of which could produce even smaller droplets.
This better understanding of the basic physics of the dispersion process should allow environmental engineers to better predict how well dispersants will work in the field, says Gopalan, which should help inform decision makers during major oil spills. The work is part of a large collaboration among biologists, ecologists, physical oceanographers, computer modelers, and engineers, primarily associated with the Coastal Response Research Center, that aims to model and predict the fate of oil after it spills, taking into account the properties of the oil, dispersant, weather conditions, and ecological data. In the future, an improved “response model” based on this larger collaboration may suggest the optimal approach to cleaning up any specific oil spill.
Trapping Greenhouse Gases. Of all the possible ways of reducing future greenhouse gas emissions, one of the most immediately feasible is carbon dioxide “sequestration,” which involves compressing the gas into a liquid and piping it deep underground instead of releasing it into the atmosphere. Earth has abundant geological formations known as saline aquifers that would seem to be ideal storage bins for such sequestered carbon.
However, says Jerome Neufeld of the University of Cambridge in England, if carbon sequestration is to play a major role in reducing greenhouse gas emissions, the process needs to be deployed on a global scale, and new tools will be needed to monitor the long-term stability and fate of trapped gas.
The principle of sequestration is simple. Saline aquifers are basically porous regions of rock soaked with brackish fluids. The density of carbon dioxide is much less than that of the brine, so gas pumped into the aquifer will rise through the porous rock until it hits an impermeable “cap” rock. Over very long time scales, trapped carbon dioxide will saturate the brine and become mineralized. But what happens in the short term? If you pump carbon dioxide into saline aquifers, will it stay put and mineralize or leak away completely?
Neufeld and his colleagues have created a simple tool to predict the fate of carbon dioxide “plumes” rising though aquifers after being pumped underground. Their model shows how the shape of rising plumes is influenced by the structure of the surrounding rock, and it suggests that there are advantages to injecting carbon dioxide into reservoirs that are like geological layer cakes, with alternating stacks of porous and seal rock. When a plume reaches an impermeable boundary, it spreads until it can rise again, filling out a shape that looks like an inverted Christmas tree. As the plume pools it mixes with the brine, ultimately resulting in a more stable long-term sequestration.
Those Flexible, Flappable Flying Machines. Modern aircraft have been fabulously successful with rigid wings and rotors. But just imagine the flying machines that would be possible if we could understand and harness the most efficient and acrobatic airfoils in nature: the flexible wings of the bat.
The aerodynamics of “compliant” structures, such as bat wings, are very complicated because both the structure and airflow change and adapt to each other in a highly nonlinear way. Bats’ wing bones are even flexible, unlike those of birds, which gives the mammals added control but is an additional challenge for scientists trying to understand them. Kenny Breuer’s research group at Brown University is designing a series of fundamental experiments that will allow scientists to isolate, observe and analyze a variety of specific flow-structure interactions that are important in understanding bat flight and, in general, the aerodynamics of compliant structures. Ultimately, Breuer expects that experiments like these will yield insights enabling new generations of flying machines that are impossible to consider today.
Arnold Song, who is one member of this research group, described the basic motions–and their aerodynamic implications–that he and his colleagues at Brown have discovered so far by measuring how paddles and stretched ribbons of sailcloth vibrate in manmade breezes in a wind tunnel. As the airflow increases, for example, a paddle on a post first twists and then flaps, like a stop sign being pummeled by hurricane-force winds. The ribbon’s behavior is more complicated, but also essential for understanding how bat wings or other compliant structures generate lift so efficiently.
How Germs Spread in Airplanes. Airliner ventilation systems are designed to limit passengers’ exposure to airborne particles–from ill travelers’ contagious germs to terrorists’ aerosol biohazards. Vents in a plane’s center ceiling direct air out and down toward the floor below the windows, creating a swirling flow pattern within each row of seats that effectively confines contaminants to a single row or, at worst, its next-row neighbors.
But new research at Purdue University has shown that anyone–a flight attendant or a passenger, for instance–merely walking down an airliner’s aisle will disrupt this carefully designed flow pattern by creating a wake of eddies that can spread contaminants as far as 10 rows away. Moreover, lead scientist Michael Plesniak says, the eddies’ interaction with the ventilation system’s swirling flow creates a stagnant zone “at just the wrong place.” The height of the stagnant zone is exactly where seated passengers breathe. Future research aims to devise ways for breaking up this stagnant zone and reducing the ability of wakes from people moving around the cabin to disperse contaminants.
Plesniak described his team’s research and how it is also helping to create a computational turbulence simulation software tool that airplane manufacturers and safety regulators could use to model complex, realistic cabin ventilation scenarios more rapidly and economically than is now possible. The tool could also be used to design ventilation systems for interiors of buildings, subways and tunnels.
Improving Jet Engine Performance. Actions similar to those of a pulsating water-massage shower head may lead to more effective control of rocket engines and cleaner, more efficient jet aircraft engines, UCLA scientists have discovered. Juliett Davitian described new research into the behavior of “transverse” jets, which consist of gas or liquid injected into a crossflow of a similar fluid. Engine manufacturers use transverse jets to introduce gases into jet engines for reducing emissions or cooling the turbine blades. These jets are also used to control the thrust of rocket engines.
In some applications, rapid and thorough mixing of the jet and crossflow fluids is needed. In other cases, deep penetration of the jet into the crossflow is desired. Sometimes both characteristics are required. By studying the fluid-mechanical interactions between transverse jets and the crossflow under a wide range of controlled conditions, the UCLA scientists learned that pulsing the transverse jet fluid in sinusoidal or square-wave patterns, depending on the conditions and desired outcome, can greatly enhance mixing, penetration or both. Continuing research will explore the behavior of transverse jets of different densities, which have a wide range of practical applications in energy-generation devices, such as stationary power plants and utility burners.
Mysterious Sand Ripples on Mars. When the Mars Exploration Rover Opportunity landed on fresh Martian sand ripples in 2004, its on-board microscope showed the grains there to be much finer than predicted, revealing a major mystery to be solved. As on Earth, Mars’ famous dust storms loft the finest particles high into the atmosphere, while coarser particles bounce along the surface, forming ripples and dunes. Well-established theories developed for Earth ripples in air and water and extended to Martian surface conditions predicted that the transitional particle diameter between these behaviors on Mars would be four times that of Earth’s. Yet they were essentially the same. Why was the established theory wrong for Mars?
Numerical simulations performed by a team at Cornell University now suggest a plausible answer. It turns out that the combination of the Martian atmosphere’s low density–100 times less than Earth’s–and the higher wind speeds necessary to move grains of any size on Mars conspire to make Martian winds less effective than Earth’s in lifting particles high into the air. The simulations showed why: Particles react more slowly to an upward turbulent eddy on Mars (due to low atmospheric density) and the eddies themselves pass by so much faster (due to the high wind speeds) that they don’t have the combination of power and time to elevate the larger particles, even in Mars’ lower gravity.
David Korda described the team’s simulation. He and his colleagues are preparing physical experiments to use in a NASA-Ames wind tunnel that can imitate Martian atmospheric conditions to see if the simulation’s prediction is accurate.
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