Scientists have created an enclosed turbulent ball in a water tank that may help answer a series of long-standing questions. Turbulence is all around us. Turbulence is everywhere, from the swirl of coffee and milk in a latte, to the aerodynamic forces on airplane wings and the sides of cars, and even to the flow of blood in the heart after a valve closes. However, we still don't fully understand all of them.
One stumbling block is the traditional approach of physicists, who often like to study phenomena in isolation from external factors. But when it comes to turbulence, just like stirring a glass of liquid, the spoon is still an integral part of the process, influencing the behavior of the fluid. To date, methods to isolate turbulence as an independent variable have been elusive.
However, a team of scientists at the University of Chicago has pioneered a method of creating contained turbulence in a water tank. They used annular jets to spray the annular flow until an isolated "ball" of turbulence formed and persisted.
"This was a surprise to us," said physicist Takumi Matsuzawa, first author of a study describing the discovery published in Nature Physics. Professor William Irvine, the corresponding author of the study, said: "It's like sitting calmly in a field having a picnic and watching a storm rage 50 feet away."
They hope this breakthrough will open up a new avenue of research to better understand turbulence.
"Turbulence - the chaotic flow of matter in a heterogeneous mixture - is an old problem," Owen said. "It's often cited as one of the great unsolved problems in physics."
Over the past few decades, scientists have made progress in describing the behavior of "idealized" turbulent conditions. In other words, turbulence has no confounding variables such as boundaries, nor changes in intensity and time. However, there is much more to know about turbulence in the real world.
"Turbulence is all around us, but it has been elusive in what physicists consider a satisfactory description," Owen said. "For example, if you ask, can I predict what will happen next if I puncture this turbulent region? The answer is no. Not even with supercomputers."
One big problem is the presence of confounding variables in experiments. You can create turbulence by rapidly spraying water through a pipe or stirring paddles in a tank, but turbulence always rubs against the walls of the container and the stirrer, affecting the results of the experiment.
Matsuzawa, Owen and their collaborators have been conducting "vortex ring" experiments in water tanks - like smoke rings, but in water. When they try to combine them to create turbulence, the energy usually bounces back and then dissipates.
But once they discovered a special configuration—a box with eight corners, each with a vortex ring generator—something strange happened. As they repeatedly fired the rings that met at the center, they saw a self-contained ball of turbulence forming, away from the box walls.
This was a breakthrough in itself: "No one thought this was possible before, turbulence is very good at mixing liquids; if you mix milk into coffee, you only stir it once or twice before it's completely mixed, but we can keep it in place, which is very surprising."
A stand-alone turbulence sphere would allow scientists to track its parameters more precisely using lasers and multiple fast cameras. This includes its energy and helicity (a measure of how tangled or "knotted" the loop is), as well as impulse and angular impulse (equivalent to the fluid's momentum and angular momentum).
What's more, they can play with it by changing the parameters. They can change whether the loop fed in is a clockwise or counterclockwise spiral. They could change the input energy, or stop adding rings and watch the turbulence dissipate, or change the helicity of the rings and watch how the turbulence evolves over time.
"How does turbulence dissipate? How does it expand? What does it 'remember'? How does energy travel across scales? Are there different types of turbulence? We can ask all kinds of questions, and this is a unique environment in which to ask them," Owen said. "I really hope this helps us break new ground in this area.