Predicting air turbulence is almost like catching the wind

Clear air turbulence has long been a nuisance and a hazard to aircraft and passengers. It is invisible, unpredictable, and tends to strike when it is least welcome - during your in-flight lunch, for example.

But turbulence also occurs closer to the ground, and a better understanding of how it works would be extremely helpful to the aviation and other industries.

My research group at the Ecole Polytechnique Federale de Lausanne (EPFL) focuses on wind engineering and renewable energy, and understanding atmospheric turbulence is vital.

Wind may flow smoothly into a wind turbine, but the rotating blades that slice through the moving air stir it up into a chaotic collage of whorls - turbulence.

Since it takes time for this turbulence to die down, a second turbine further downwind will be driven by wind that hasn't yet had time to fully settle.

This process can repeat itself throughout an entire wind farm, cutting the output of downwind turbines by up to 40 per cent.

But turbulence is hard to measure. In wind farms, researchers usually rely on sensors mounted on poles or towers. But you just can't set up enough sensors at enough locations within and surrounding a wind farm to get an accurate picture of where the main whorls tend to be, how far they extend, and how long they persist.

Working with EPFL Middle East, we have developed a way to solve this conundrum using beams of bundled light - lasers.

By shooting a laser beam into the air and measuring the light reflected back by humidity or tiny particles in the atmosphere, a lidar - the laser equivalent of a radar - can provide a detailed profile of the air it traverses, almost in real time.

Last year, for the first time, we used multiple lidars to detect the three-dimensional structure of turbulence behind a real wind turbine.

These unique field measurements can be used to confirm the predictions of computer simulations of wind blowing through wind farms, and provide data that is otherwise difficult or impossible to obtain.

Turbulent phenomena are complex, and it often takes more than one single experimental method to identify and understand them.

In wind tunnel experiments we recently found that the turbulence created by wind turbines settles faster when the surrounding air is itself turbulent - such as on a hot day when the warm ground heats the air just above it, setting it in motion.

Our computer simulations under these so-called convective conditions show the same behaviour. Now we plan on studying the turbulence behind the wind turbines in Valais, Switzerland, on a hot day to see if we can confirm this phenomenon outdoors, and understand why it occurs.

In the long run, we expect a better understanding of these phenomena to lead to improved design of wind farms, taking into account not only the arrangement of the individual turbines within them, but also the surrounding topography, climate, and dominant winds, ultimately leading to more efficient exploitation of these expensive infrastructure projects.

Our ability to visualise turbulence has found applications in other fields, too.

Solar Impulse is a lightweight solar aircraft on a mission to achieve what seems impossible: to fly around the planet without a drop of fuel.

The challenge at the moment is that the plane, being developed in Switzerland, is so light that it gets bounced around by turbulence like a bird in a storm.

At high altitudes this risk can be managed, but nearer the ground, especially during take-off and landing, a bounce in the wrong direction could be dangerous.

Addressing this risk, we collaborated with Solar Impulse to develop a portable lidar-based solution to scan the air around the airfield for dangerous whorls.

Once the system gives a pilot the green light, he or she can be assured that the plane will safely descend towards the runway for landing.

Prof Fernando Porte-Agel is the director of the Wind Engineering and Renewable Energy Laboratory at EPFL in Lausanne, Switzerland