Tag Archives: turbulence

Why do stars twinkle, or do they twinkle at all? For astronomers, this is important

When we look at the sky, we see different types of objects. Some are man-made (like the International Space Station), some are from our solar system (like Venus or Saturn), but many are twinkling, shiny objects — of course, stars from outside our solar system.

Stars have fascinated humans since time immemorial, especially because sometimes, they seem to twinkle. Stars don’t actually twinkle per se — the twinkling we observe here has more to do with the atmosphere on Earth rather than the stars themselves. There are three main factors that influence how stars “twinkle”, and to truly understand them, we need to take a short dive into some atmospheric physics.

A view of the stars photographed at Klein Flintbek. Behind the tree with the red lights is the Kiel telecommunications tower. The light pollution (Kiel) is also easy to see. Credits: Fabian Horst.

Turbulence

The first physical phenomenon that makes stars appear to twinkle is turbulence.

We observe stars that are far away because the light that they emit reaches our eyes (or telescopes). But in order to do that, it must first pass through the atmosphere. That means that light is indirectly subjected to phenomena that affect the Earth’s atmosphere.

Turbulence is a phenomenon that often happens on smaller scales. In the atmosphere, we have large-scale phenomena like cold fronts or hurricanes happening every day, but inside these events, turbulence is significant on a small scale. So cold fronts bring large thunderstorms, the clouds within the front can make the sky turbulent, and that’s when the airplane pilot tells you “Ladies and gentlemen, we’re experiencing some turbulence.”

Image credits: AEES.gov.

There are several types of turbulence, including one called thermal turbulence — which happens when there is a mix between hotter and colder air. This could happen whether the sky is cloudy or not. When a mass of air in the atmosphere is hotter than its surroundings, it starts to rise, creating convective currents. Basically, you end up with moving columns or pockets of heated air that arise from warmer surfaces of the earth.

These moving pockets of air can create turbulence, and in the process, they also distort light that passes through them.

When it comes to stars, twinkling is caused by the passing of light through different layers of the turbulent atmosphere. This is more pronounced near the horizon than directly overhead since light rays near the horizon pass through denser layers of the atmosphere, but twinkling (technically called scintillation) can be observed on all parts of the sky.

But there’s more to this story.

Scintillation

Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a layer of turbulent mixing in the atmosphere. The vertical scale of the wavefronts plotted is highly exaggerated.

When light passes through any medium (including the Earth’s atmosphere), some of it is reflected back, while some passes through the atmosphere, but at a different angle — something called refraction. When the atmosphere is turbulent in a region, the refraction angle is not constant, so light can change path quickly. 

Altering the refractive index changes the apparent position of objects, just like the straw in a glass of water experiment, it looks bent. So the turbulent sky, constantly changing the refractive index makes stars appear to be moving, so they twinkle, or scintillate

The different refraction index in water (versus air) makes objects appear bent. If this is happening quickly and in multiple places, it can make objects appear twinkling.

Due to scale differences, if an astronomical object is large enough compared to the turbulence, it won’t affect the way we see it. But the light of a smaller object (or one that’s farther away) will be affected as it crosses the turbulent air. That’s the reason why planets twinkle less (or almost don’t twinkle at all) — they are closer and it makes them ‘bigger’ compared to the turbulence.

Fortunately, atmospheric scientists developed a way to monitor changes in the refractive index of the atmosphere due to turbulence. They use instruments to measure the turbulence and use it to try to estimate a future outcome.

Different skies

For astronomers, twinkling can be quite problematic. So they look for the “best sky” to avoid the phenomenon. Usually, this means an environment whose climate is very dry. When that’s not possible, they try to find the dryness by placing the instruments at a high altitude. Whenever is possible to combine altitude and mostly dry weather, they have a good spot for a telescope.

In the images above we see the difference very clearly: both skies were clear when the images were taken, but one (on the left) was less turbulent than the other (on the right). On the left, we see a video of a star recorded on Mount Fuji in Japan — the star appears to be bouncing chaotically due to a turbulent sky. On the right, we see a recording of the same star taken on the Andes Mountains in Chile, a very dry, high-altitude area; the star bounces, but much less than in the Japanese images.

A map of all ground-based telescopes that the MST have procured to observe during K2C9. Credits: 10.1088/1538-3873/128/970/124401

So stars don’t exactly twinkle, but they do appear to twinkle from here on Earth. For astronomers, though, making sure they eliminate the “twinkling” is important.

Of course, if you set your telescopes in space, you don’t have these problems because your observation point is above the atmosphere. But even here on Earth, astronomers are careful to pick the best locations for placing large optical telescopes. They typically look for the driest areas, at the highest altitude possible, without any light pollution. There’s another consideration: because the air is usually flowing from west to east because of Earth’s rotation, a way to avoid pollution is placing telescopes on west coasts or in ilands in the middle of the ocean. This rules out the vast majority of places on Earth, which is why astronomers are so particular about where they place their telescopes.

Airplane.

Warmer climate will make for more air turbulence, bumpier flights

Climate heating will, unexpectedly, make your flights much bumpier in the future.

Airplane.

Image via Pixabay.

Researchers at the University of Reading report that the jet stream is becoming more turbulent in the upper atmosphere over the North Atlantic. Since satellites began observing it in 1979, they explain, jet stream shearing has increased by 15%.

Jet, streamed, sheared

“Over the last four decades, temperatures have risen most rapidly over the Arctic, whilst in the stratosphere — around 12 km above the surface — they have cooled,” says lead author Simon Lee, Ph.D. student in Meteorology at the University of Reading.

“This has created a tug-of-war effect, where surface temperature changes act to slow the jet down, while temperature changes higher up act to speed it up.”

Wind shear is the variation in wind velocity at right angles to the wind’s direction. It sounds pretty complex but, in essence, wind shearing is when bodies of air move perpendicular to the direction the wind is blowing, and generate a turning force.

Vertical wind shearing, the increase in wind speed at higher altitudes, is particularly dangerous as it creates clear-air (invisible) turbulence, potentially with enough force to throw passengers out of their seats.

Tens of thousands of planes encounter severe turbulence every year, causing hundreds of injuries — both from passengers and flight attendants. Overall, the estimated cost of clear-air turbulence for the global aviation sector is estimated to be around a billion dollars annually, through a combination of flight delays, injuries to cabin crew and passengers, and structural damage to aircraft.

The new study is the first one to show that, while man-made climate heating is closing the temperature difference gap between Earth’s poles and the equator at ground level, the opposite is happening at around 34,000 feet, a typical airplane cruising altitude. 

The jet stream, like all wind, is powered by these differences in temperature. Growing differences in high-altitude temperatures are strengthening the stream, driving an increase in turbulence-generating wind shear at cruising altitudes that has gone unnoticed up until now, the team reports. They also add that their findings support previous research at Reading indicating that human-induced climate change will make severe turbulence up to three times more common by 2050-80.

“Our study shows these opposing effects currently balance out, meaning the speed of the jet stream has not changed. However, we looked for the first time at the wind shear, where significant change has previously gone unnoticed,” Lee explains.

“This strengthens previous projections for increased clear-air turbulence, as we can see an increase in one of the driving forces has happened already.

He explains that the upper-level element of that “tug-of-war” mentioned earlier will eventually win out, and that the jet stream will accelerate. “This has serious implications for airlines, as passengers and crew would face a bigger risk of injury,” Lee adds. It’s also likely that this change in the jet stream will increase flight times from Europe towards the US and speed up flights in the other direction.

The study’s lead researcher, Professor Paul Williams from the University of Reading’s Department of Meteorology, first linked increased turbulence to climate change. Prof. Williams is currently collaborating with the aircraft industry to design the next generation of planes — one that is better fit for a warmer and bumpier airspace.

The paper “Increased shear in the North Atlantic upper-level jet stream over the past four decades” has been published in the journal Nature.

Credit: Flickr, Kuster & Wildhaber Photography.

Climate change will make flights worse too. Severe turbulences may increase by 150%, study finds

The first study to examine how climate change will affect turbulences found that the random up-and-down motions will become far more common in the future. According to the new research conducted by researchers from the University of Reading, UK, all kinds of turbulences from light to moderate to severe will considerably become more frequent.

Dr. Paul Williams and colleagues modeled what would happen to atmospheric currents at an altitude of around 12 km (39,000 feet) if there were 560 parts per million of CO2 in the atmosphere. Right now, global average CO2 levels stand at about 410 ppm, the highest they’ve been in three million years. A previous study featured on ZME Science this week found that in a business as usual scenario CO2 in the atmosphere could rise to 600ppm by the mid-century, which is unprecedented in 50 million years since the hot days of the Eocene.

Climate change caused by anthropogenic global warming will cause stronger wind shears within the jet stream. These wind shears are a major cause of turbulence when they become unstable. The researchers found light turbulence will increase by 59%, light to moderate turbulence by 75%, moderate by 94%, moderate to severe by 127%, and severe by 149%, as reported in Advances in Atmospheric Science

Don’t panic

While this may sound distressing, turbulences can’t crash an airplane especially today when modern aircraft are fitted with high-tech stabilizing technology. But that’s not to say flights won’t become a lot bumpier in the coming decades. Injuries because of turbulences are also destined to become more frequent. Sometimes, a passenger who doesn’t wear a seatbelt might be thrown about the airplane or get injured by falling overhead luggage. Every year, turbulences cause hundreds of injuries from minor to severe.

“For most passengers, light turbulence is nothing more than an annoying inconvenience that reduces their comfort levels, but for nervous fliers even light turbulence can be distressing,” Williams said in a statement.

“However, even the most seasoned frequent fliers may be alarmed at the prospect of a 149% increase in severe turbulence, which frequently hospitalises air travellers and flight attendants around the world.”

To tackle these new challenges, aircraft will have to become even ‘smarter’ than they are today. Wired reports that a French company called Thales demonstrated an onboard LIDAR system that can spot turbulences 18 miles ahead of the airplane. However, this technology adds a lot of weight to the aircraft and can be extremely expensive. For the moment, airlines and manufacturers aren’t interested in such an investment but they might change their minds sooner than they care to think.

Overall, climate change will make flying more uncomfortable — and it’s not just because of the more frequent turbulence. A report issued last year by the International Civil Aviation Organisation (ICAO) found that, besides increased turbulence, climate change will intensify take-off difficulties, icing incidents as well as dust storms that might threaten the engines.

“Aviation is an extremely risk averse business. Climate change poses a new set of risks that airports need to assess properly. The last decades have provided a glimpse of the future climate, but the main effects will be more evident three or four decades from now, and onwards,” the report reads.

“There is thus no reason to panic, but much of the airport infrastructure erected today will be there in the new climate.”

“The robustness of aircraft and indeed the robustness of the entire aviation system should be monitored carefully, as the sector will have to prepare for the more extreme meteorological conditions that are expected in the future as the climate continues to change,” the report’s authors cautioned.

Williams and colleagues will focus next on investigating other flight routes from around the world. They will examine how altitude and seasonal changes influence flights under various warming scenarios.

These snapshots at various times during the simulation of a stimulated turbulence in hot plasma show the energy density. In the bright regions, energy and temperature are the greatest in each case. © David Radice / Luciano Rezzolla (AEI)

Relativistic computation brings us one step closer to accurately describing turbulence

This might strike some of you as a surprise, but turbulence, a phenomenon we all encounter on a daily basis  – be it while mixing coffee, starting your car (fuel and air mixture) or of course while flying – and which has been first scientifically described some 600 years ago, remains today one of the greatest unsolved problems in science. A recent study by researchers Max Planck Institute for Gravitational Physics (Albert Einstein Institute / AEI) in Potsdam offers new insights that may help solve this problem after findings show that a side of a widely accepted theory for turbulence requires revision.

Turbulence was first studied scientifically by Leonardo da Vinci way back in the 15th century, while the most basic description of turbulence cam in the 18th century when renowned scientists  Claude Navier and George Stokes formulated equations that described the motion of fluids and gases – to this day these are known as the “Navier-Stokes equations”. Once  the world became more and more industrialized, technological demand required scientists to come up with a better understanding of turbulence which governs a slew of mechanical processes. Fuel mixture quality, which is dependent on turbulent vortices greatly influence efficiency, for instance.  The breakthrough came during WWII when Russian mathematician Andrey Kolmogorov wrote a phenomenological theory for turbulence that is still valid today.

These snapshots at various times during the simulation of a stimulated turbulence in hot plasma show the energy density. In the bright regions, energy and temperature are the greatest in each case. © David Radice / Luciano Rezzolla (AEI)

These snapshots at various times during the simulation of a stimulated turbulence in hot plasma show the energy density. In the bright regions, energy and temperature are the greatest in each case. © David Radice / Luciano Rezzolla (AEI)

His theory offers a way to predict how turbulence may behave,  however a fundamental mathematical theory of turbulence is still lacking and to this day – despite a $1 million prize has been put up by the  Clay Mathematics Institute in Cambridge/Massachusetts to whomever can come up with a solution since the year 2000 – remains unsolved.

David Radice and Luciano Rezzolla  researched turbulence in relativistic conditions of speed and energies, such as those expected near a black hole or in the early universe – what’s important to note is that the speed of the fluids nears the speed of light. These conditions were simulated in a virtual environment, powered by supercomputers at AEI and the Garching-based Computing Centre, and employed to solve nonlinear differential equations of relativistic hydrodynamics.

“Our studies showed that Kolmogorov’s basic predictions for relativistic phenomena must be modified, because we are observing anomalies and new effects,” says Rezzola. “Interestingly, however, the most important prediction of Kolmogorov’s theory appears to be still valid”, notes Rezzolla when referring to the so-called -5/3 Kolmogorov law, which describes how the energy of a system is transferred from large to small vortices.

What their findings represent is a very big step forward in our attempt to finally come up with a fundamental mathematical model for turbulence. Their computations haven’t solved the problem, maybe it hasn’t even scratched the surface, but what it demonstrates is that that the previous theory has to be modified and how this should be done. Wrong assumptions won’t ever rend positive results, but now we’ll hopefully be able to further our knowledge of basic properties of relativistic turbulence.

Findings were reported in the journal Science.

 

A massive coronal mass ejection. (c) Wikimedia Commons

Turbulence in space confirmed and measured for the first time

A massive coronal mass ejection. (c) Wikimedia Commons

A massive coronal mass ejection. (c) Wikimedia Commons

A rather frustrating issue for astronomers and astrophysicists is space turbulence. Like in the air, when for instance an airplane meets unfavorable jets of wind, so too a spacecraft or satellite can be jolted a bit by the high energy of gusty winds in space. What’s aggravating however is that while in the first case turbulence is an undeniable scientific fact, the later could not be confirmed until only recently. Imagine getting whacked in the head by an annoying glove, only the glove is invisible to both eyes, nose or touch. You’re positive it’s there, but can’t prove it. Scientists finally overcame this hurdle after they found a way to measure this hard-to-prove cosmic turbulence.

“Turbulence is not restricted to environments here on Earth, but also arises pervasively throughout the solar system and beyond, driving chaotic motions in the ionized gas, or plasma, that fills the universe,” says lead author Gregory Howes, assistant professor of physics and astronomy at the University of Iowa.

Turbulence in space is believed to have played a role in shaping our universe.

“It is thought to play a key role in heating the atmosphere of the sun, the solar corona, to temperatures of a million degrees Celsius, nearly a thousand times hotter than the surface of the sun,” says lead author of the study Gregory Howes, an assistant professor of physics and astronomy at the University of Iowa.

“Turbulence also regulates the formation of the stars throughout the galaxy, determines the radiation emitted from the super massive black hole at the center of our galaxy and mediates the effects that space weather has on the Earth.”

The main source of these cosmic turbulence are highly charge particle emissions spewed into space by the sun known as coronal mass ejections.  Plasma waves can measure millions of degrees in temperature, last for several hours and grow to sized several times that of the Earth. In space, turbulence is caused by Alfven waves, which are moving disturbances of the plasma and magnetic field.   Nonlinear interactions between Alfven waves traveling up and down the magnetic field – such as two magnetic waves colliding to create a third wave – are basically the building blocks that lead to cosmic turbulence and  Howes along with colleagues have proved this for the first time.

“We have presented the first experimental measurement in a laboratory plasma of the nonlinear interaction between counter-propagating Alfven waves, the fundamental building block of astrophysical turbulence,” Mr. Howes notes.

The study’s findings were published today in the online edition of the journal Physical Review Letters.


Flying owl

Owl wings may inspire stealthier aircraft

Flying owl

While the owl is commonly associated with wisdom, make no mistake  – it’s a vicious predator of the night. It’s main weapon is its stealth, as it silently dashes through pitch black catching pray off-guard. This remarkable ability  of noiseless flight has intrigued scientists who are looking to develop aircraft inspired by the owl.

The owl has a special plumage that allows is to fly on sly. When air airfoils travel through a shape, be it the wing of a bird or a plane, turbulence is created. The turbulence is significantly amplified towards the trailing edge of the wing, which also causes noise to occur.

Aircraft wings have hard and relatively rigid trailing edges, which coupled with extremely high speeds, causes loud noise to be generated. Researchers are now studying the wing structure of the owl to better understand how it mitigates noise so they can apply that information to the design of aircraft.

“Many owl species have developed specialized plumage to effectively eliminate the aerodynamic noise from their wings, which allows them to hunt and capture their prey using their ears alone,” Cambridge researcher Justin Jaworski said.

“No one knows exactly how owls achieve this acoustic stealth, and the reasons for this feat are largely speculative based on comparisons of owl feathers and physiology to other not-so-quiet birds such as pigeons.”

So far, the Cambridge researchers have identified three aspects that have been liked with owl silent flight: a comb of stiff feathers along the leading edge of the wing; a soft downy material on top of the wing; and a flexible fringe at the trailing edge of the wing.

With these preliminary findings, the researchers created a computer model of wing trailing edge with elastic and porous properties and learned, indeed, that this kind of design would allow for a better noise mitigation.

“This implied that the dominant noise source for conventional wings could be eliminated,” researcher Nigel Peake said.

The findings were presented Sunday at a meeting of the American Physical Society‘s Division of Fluid Dynamics in San Diego.