Delft, The Netherlands, 12 Jul 2021

A hidden bias on bubbly flows

A recent study by an international team from ETH Zurich, IHE Delft and UNSW identified a significant bias in bubbly flow measurements and developed a correction scheme. The paper was published in Nature Communications.

This news article originates from the UNSW website.

Gas-liquid flows occur in a wide range of natural systems and technical applications. Flow velocities in gas-liquid flows are typically measured with so-called phase detection intrusive probes, comprising two needles which pierce bubbles and droplets. A recent study by an international team from ETH Zurich, IHE Delft and UNSW identified a significant bias in these measurements and developed a correction scheme.

Accurate measurement of flow velocities is of importance for many engineering applications as well as our coastal, estuarine and riverine environments. In many natural environments such as breaking waves, river rapids and waterfalls, strong turbulence leads to aerated flows (white waters). In human-made systems, air-water flows can be found in water treatment systems, nuclear reactors and flow conveyance infrastructure. Under these conditions, proven standard flow measurements do not work and since the 1960s, researchers and engineers have used specific intrusive air-water flow probes. Using these probes allowed to improve our understanding of flow processes as well as engineering design. However, while most research looked at the air bubbles inside the flow, this study looked at the probe itself.

This study, led by Benjamin Hohermuth from ETH Zurich, may have implications on engineering design. “Intrusive bubble sampling may include an important bias in past bubbly flow measurements. This means that spillway flows, for instance, might be flowing faster than expected during design”, says Daniel Valero of IHE Delft. Spillways need to safely pass floods with return periods of 1,000 years, or even larger. Problems during operation can result in catastrophic consequences.

Experiments conducted by the international team showed that previous measurements underestimated interfacial velocities in many applications. In this research, published in Nature Communications, the team compared typical air-water flow measurements by state-of-the-art phase-detection intrusive probes with a laser instrument, showing that velocity estimates with intrusive probes were systematically lower compared to the ground truth. Untangling all forces during the interactions of the phase-detection probe tips with the flowing air-water entities, the researchers showed that particle-probe interactions resulted in a deceleration of the recorded interfaces and a systematic bias in velocity estimates.

The paper is open access here

Hohermuth, B., Kramer, M., Felder, S., Valero, D. Velocity bias in intrusive gas-liquid flow measurements. Nat Commun 12, 4123 (2021). https://doi.org/10.1038/s41467-021-24231-4

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