Many machines we rely on every day—from industrial mixers and cooling systems to power transmission devices—use rotating parts to move liquids.
In many real-world systems, these liquids are mixed with gas, creating what scientists call gas–liquid two-phase flows.
While these systems are common, they often lose large amounts of energy, especially at certain rotational speeds.
Until now, it has not been clear why these losses peak at specific speeds rather than simply increasing as machines spin faster.
A new study by researchers from the University of Osaka, the University of Tokyo, and RIKEN has uncovered the key reasons behind this puzzling behavior.
Using Japan’s powerful “SQUID” supercomputer, the team ran highly detailed simulations to examine how energy is lost when rotors spin inside mixtures of gas and liquid.
Their findings were published in the journal Multiphase Science and Technology.
The researchers focused on torque, which is a measure of how much force is needed to keep a rotor spinning.
Higher torque means more energy loss. They discovered that torque does not increase smoothly with speed. Instead, it reaches a peak at a particular rotational speed, where energy loss is maximized.
One major reason for this peak is a phenomenon known as resonance. When a rotor spins, it causes waves to form on the surface between the liquid and the gas, much like sloshing water in a container.
At certain speeds, the rotation matches the natural frequency of these waves. When this happens, the waves grow stronger and more unstable, leading to intense fluid motion.
The study revealed that energy loss is not caused only by the rotor physically hitting the liquid surface, as previously assumed. While these collisions do play a role, the simulations showed that pressure imbalances around the rotor are just as important.
When the system enters a resonant state, liquid is drawn strongly toward the front of the rotor, creating high pressure there.
At the same time, unstable flow forms behind the rotor, producing low-pressure regions. This pressure difference significantly increases the resistance against the rotating motion, driving torque and energy loss to their peak.
By combining experimental data with large-scale simulations, the researchers were able to see these pressure changes and flow patterns in detail for the first time.
This deeper understanding helps explain why machines can suddenly become less efficient or experience excessive mechanical stress at certain operating speeds.
The findings have important practical implications. By avoiding operating speeds that trigger resonance, engineers can reduce energy waste and improve efficiency.
Understanding these mechanisms also helps reduce mechanical wear, lowering the risk of failure and extending the lifespan of equipment. In addition, the insights provide new guidance for designing rotor shapes and system layouts that minimize harmful pressure imbalances.
Lead author Mayu Kawamura emphasized that although rotor-driven gas–liquid flows are all around us, their inner workings have been poorly understood.
This research marks a step toward replacing trial-and-error design with solid theory, helping create quieter, safer, and more energy-efficient machines for the future.
