Fiber-Based Measurement of Turbulent Energy Dissipation

The mean rate at which viscosity converts turbulent kinetic energy into heat is a fundamental quantity in turbulence theory and applications, yet its direct experimental determination remains difficult because it requires measurements of the velocity-gradient tensor at spatial scales comparable to the Kolmogorov length. This seminar discusses how the rotational dynamics of short fibers can provide an alternative route to this quantity. Elongated particles are directly sensitive to the local velocity-gradient tensor, and recent optical techniques make it possible to measure their full rotational motion, including both tumbling and spinning. This enables the mean-square angular velocity of nearly inertia-free fibers to be used as a proxy for the turbulent energy dissipation rate. Experiments in the TU Wien Turbulent Water Channel, complemented by direct numerical simulations, show that fibers with lengths of order the Kolmogorov scale provide an accurate estimate of the mean dissipation rate over most of the channel. The results further show that the relevant rotational timescale is set by eddies of comparable size to the fiber, providing a direct connection between fiber rotation and multiscale turbulent velocity gradients. These findings establish fiber-based dissipation measurement as a robust experimental method for homogeneous and wall-bounded turbulent flows, while also clarifying the conditions under which departures occur close to the wall.

[1] V. Giurgiu, G. Caridi. M. De Paoli, and A Soldati, Phys. Rev. Lett. 133, 054101 (2024).
[2] V. Giurgiu, G. Caridi, M. Alipour, M. De Paoli, and A Soldati Rev. Sci. Instrum. 94, 095101 (2023).
[3] S. Brizzolara, M.E. Rosti, S. Olivieri, L. Brandt, M. Holzner, A. Mazzino, Phys. Rev. X 11, 031060 (2021).
[4] D. Zaza, V. Giurgiu, M. Iovieno, and A Soldati, Phys. Rev. Lett. 136, 054001 (2026).
[5] E. Coliban, D. Zaza, and A Soldati, Scientific Data (136, 054001) (Submitted) (2026).

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Alfredo Soldati is University Professor of Fluid Mechanics at TU Wien, Vienna, Austria, and Professor of Fluid Mechanics at the University of Udine, Italy, where he holds a partial appointment, both since 2016. From 2007 to 2016 he was Full Professor at the University of Udine, where he had previously served as Associate Professor from 2002 to 2006 and as Researcher/Assistant Professor from 1993 to 2002. He was also Research Associate at the University of California, Santa Barbara, from 1991 to 1995, and has held visiting or invited appointments at the Kavli Institute for Theoretical Physics in Santa Barbara, Scuola Superiore Sant’Anna in Pisa, Institut National Polytechnique in Toulouse, and EPFL in Lausanne. He received his Ph.D. in Chemical Engineering from the University of Pisa in 1993 and his Laurea in Nuclear Engineering from the same university in 1989, after completing military/civil service with the Italian National Fire Brigade. His international recognitions include the ASME Fluids Engineering Award (2026), the ASME Freeman Scholar Award and Lecture (2020), election as Fellow of  UROMECH (2020) and of the American Physical Society (2013), the International Panetti–Ferrari Prize and Gold Medal (2018), the ASME Lewis F. Moody Award (2015), and the ASME Robert T. Knapp Award (2007). He is Rector of the  International Centre for Mechanical Sciences (CISM) in Udine, co-President of the ICTAM 2028 conference in Vienna, and previously chaired the 9th International Conference on Multiphase Flow and the Working Party on Multiphase Flow of the European Federation of Chemical Engineering. He is co-Editor-in-Chief of the International Journal of Multiphase Flow and has previously served as Associate Editor of the ASME Journal of Fluids Engineering and Editor of Acta Mechanica.