Publications
Journal articles, listed from newest to oldest.
See my Google Scholar profile for current citation counts.
2026
- Tensor-geometry-activated spectral energy transfer at Reynolds numbers of order unity in two-dimensional flowsZiyue Yu, Xinyu Si, and Lei FangJournal of Fluid Mechanics, 1044, A32 , Oct 2026
We demonstrate that enhanced inter-scale energy transfer – a defining hallmark of turbulence – can be actively induced in Navier–Stokes flows even at Reynolds numbers of order unity. By reinterpreting the spectral energy flux as mechanical work resulting from the alignment between turbulent stress and large-scale rate of strain tensors, we show that, by actively enforcing geometric alignment between these tensors and amplifying the perturbation-induced stress, the flow exhibits scale-to-scale energy flux magnitudes comparable to those reported in the weakly turbulent regime of two-dimensional flows, even in the absence of dominant inertia. This study indicates that the mechanisms underpinning spectral energy flux are not exclusive to high-Reynolds-number turbulence but can extend into regimes previously deemed dynamically trivial. Our result also reveals a generic mechanism for directly accessing the same stress–strain-rate energy-flux pathway that underlies classical turbulence, but without the need for inertial dominance.
@article{yu2026tensor, title = {Tensor-geometry-activated spectral energy transfer at Reynolds numbers of order unity in two-dimensional flows}, author = {Yu, Ziyue and Si, Xinyu and Fang, Lei}, journal = {Journal of Fluid Mechanics}, volume = {1044}, pages = {A32}, year = {2026}, month = oct, day = {8}, publisher = {Cambridge University Press}, doi = {10.1017/jfm.2026.12071}, url = {https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/tensorgeometryactivated-spectral-energy-transfer-at-reynolds-numbers-of-order-unity-in-twodimensional-flows/8A114D1CE7A6748D458A5E8E3E61BE7A}, }
2025
- Manipulating the direction of turbulent energy flux via tensor geometry in a two-dimensional flowScience Advances, 11(30), eadv0956 , Jul 2025
In turbulent flows, energy flux, the cornerstone of turbulence theory, refers to the transfer of kinetic energy across different scales of motion. The direction of net energy flux is prescribed by the dimensionality of the fluid system: Energy cascades to smaller scales in three-dimensional flows but to larger scales in two-dimensional (2D) flows. Manipulating energy flux is a formidable task because the energy at any scale is not localized in the physical space. Here, we report a theoretical framework that enables control over energy flux direction. On the basis of this framework, we conducted experiments and direct numerical simulations, producing a 2D turbulence with forward energy flux, contrary to classical expectations. Beyond theory, we discuss how our theoretical framework can have profound applications and implications in natural and engineered systems across length scale ranges from 10^-3 to 10^6 meters, including enhanced mixing of microfluidic devices, biologically generated turbulence, breaking persistent coastal transport barriers, and ocean energy budget.
@article{si2025manipulating, title = {Manipulating the direction of turbulent energy flux via tensor geometry in a two-dimensional flow}, author = {Si, Xinyu and De Lillo, Filippo and Boffetta, Guido and Fang, Lei}, journal = {Science Advances}, volume = {11}, number = {30}, pages = {eadv0956}, year = {2025}, month = jul, day = {25}, publisher = {American Association for the Advancement of Science}, doi = {10.1126/sciadv.adv0956}, url = {https://doi.org/10.1126/sciadv.adv0956}, }
2024
- Interaction between swarming active matter and flow: The impact on Lagrangian coherent structuresXinyu Si and Lei FangPhysical Review Fluids, 9(3), 033101 , Mar 2024
Selected as an Editors’ Suggestion and included in the Physical Review Fluids Top Papers of 2024 collection. University of Pittsburgh news.
We conducted experiments using a typical centimeter swimmer, A. salina, and an electromagnetically driven quasi-two-dimensional flow to study the interaction between active matter and flow. The flow spanned from time-independent cellular flow to weakly time-dependent regimes. We focused on the impact of swarming active matter on hyperbolic Lagrangian coherent structures (LCSs) that mark the most straining regions in the flow. There is one decade of scale separation between active matter agents and the length scale of LCSs. We illustrated that the impact of active matter on LCSs was much more significant compared to localized random noise with similar energy input. In addition, we revealed that the perturbation generated by active matter could couple with the background flow and further deform the LCSs. In addition to the impact on the most straining hyperbolic regions of flow, we also revealed that the rotational elliptical region of the flow was much more susceptible to active matter perturbation in a Lagrangian perspective. We further described how the influence of active matter changed with their number densities and background flow intensities. We revealed that the LCSs could be decently altered even at a small number density of active matter. Through this work, we aim to provide valuable insights and draw attention to the problem regarding the interaction between active matter and external flow structures.
@article{si2024interaction, title = {Interaction between swarming active matter and flow: The impact on Lagrangian coherent structures}, author = {Si, Xinyu and Fang, Lei}, journal = {Physical Review Fluids}, volume = {9}, number = {3}, pages = {033101}, year = {2024}, month = mar, day = {8}, publisher = {American Physical Society}, doi = {10.1103/PhysRevFluids.9.033101}, url = {https://doi.org/10.1103/PhysRevFluids.9.033101}, } - Biologically generated turbulent energy flux in shear flow depends on tensor geometryXinyu Si and Lei FangPNAS Nexus, 3(2), pgae056 , Feb 2024
It has been proposed that biologically generated turbulence plays an important role in material transport and ocean mixing. Both experimental and numerical studies have reported evidence of the nonnegligible mixing by moderate Reynolds number swimmers, such as zooplankton, in quiescent water, especially at aggregation scales. However, the interaction between biologically generated agitation and the background flow, as a key factor in biologically generated turbulence that could reshape our previous knowledge of biologically generated turbulence, has long been ignored. Here, we show that the geometry between the biologically generated agitation and the background hydrodynamic shear can determine both the intensity and direction of biologically generated turbulent energy flux. Measuring the migration of a centimeter-scale swimmer—as represented by the brine shrimp Artemia salina—in a shear flow and verifying through an analog experiment with an artificial jet revealed that different geometries between the biologically generated agitation and the background shear can result in spectral energy transferring toward larger or smaller scales, which consequently intensifies or attenuates the large-scale hydrodynamic shear. Our results suggest that the long ignored geometry between the biologically generated agitation and the background flow field is an important factor that should be taken into consideration in future studies of biologically generated turbulence.
@article{si2024biologically, title = {Biologically generated turbulent energy flux in shear flow depends on tensor geometry}, author = {Si, Xinyu and Fang, Lei}, journal = {PNAS Nexus}, volume = {3}, number = {2}, pages = {pgae056}, year = {2024}, month = feb, day = {8}, publisher = {Oxford University Press}, doi = {10.1093/pnasnexus/pgae056}, url = {https://doi.org/10.1093/pnasnexus/pgae056}, }
2022
- Preferential transport of swimmers in heterogeneous two-dimensional turbulent flowXinyu Si and Lei FangPhysical Review Fluids, 7(9), 094501 , Sep 2022
We investigate the performance of active swimmers in a strongly heterogeneous two-dimensional weakly turbulent flow. The flow is heterogeneous in Reynolds number along one direction. Using a hybrid experimental-numerical model, we demonstrate that there are three regimes of preferential transport for rodlike swimmers as the swimmers’ intrinsic speed increases. Using Lagrangian statistics along swimmers’ trajectories, we reveal that the three regimes are due to the relative strengths of three different effects: the intrinsic speed of the swimmers, the reorientation ability of the shear layer at the interface of two flow regions, and the attracting Lagrangian coherent structures of the flow field. Our results elucidate the mechanism of preferential transport for swimmers in heterogeneous flow. We hope to raise researchers’ attention to the dynamics of swimmers in strongly heterogeneous flow environments.
@article{si2022preferential, title = {Preferential transport of swimmers in heterogeneous two-dimensional turbulent flow}, author = {Si, Xinyu and Fang, Lei}, journal = {Physical Review Fluids}, volume = {7}, number = {9}, pages = {094501}, year = {2022}, month = sep, day = {27}, publisher = {American Physical Society}, doi = {10.1103/PhysRevFluids.7.094501}, url = {https://doi.org/10.1103/PhysRevFluids.7.094501}, }
2021
- A novel social distance model reveals the sidewall effect at bottlenecksXinyu Si and Lei FangScientific Reports, 11(1), 20982 , Oct 2021
Intermittent and periodic outbreaks of infectious diseases have had profound and lasting effects on societies throughout human history. During the global spread of SARS-CoV-2 and the resulting coronavirus disease (COVID-19), social distance has been imposed worldwide to limit the spread of the virus. An additional deliberate intention of keeping a minimum safety distance from neighbors can fundamentally alter the “social force” between individuals. Here, we introduce a new “social distance” term inspired by gas molecular dynamics and integrate it into an existing agent-based social force model to describe the dynamics of crowds under social-distanced conditions. The advantage of this “social distance” term over the simple increasing of the repulsive range of other alternatives is that the fundamental crowd properties are precisely described by our model parameters. We compare the new model with the Helbing and Molnar’s classical model and experimental data, and show that this new model is superior in reproducing experimental data. We demonstrate the usability of this model with a bottleneck motion base case. The new model shows that the bottleneck effect can be significantly alleviated through small wall modifications. Lastly, we explain the mechanism of this improvement and conclude that this improvement is due to spatial asymmetry.
@article{si2021social, title = {A novel social distance model reveals the sidewall effect at bottlenecks}, author = {Si, Xinyu and Fang, Lei}, journal = {Scientific Reports}, volume = {11}, number = {1}, pages = {20982}, year = {2021}, month = oct, day = {25}, publisher = {Springer Nature}, doi = {10.1038/s41598-021-00486-1}, url = {https://doi.org/10.1038/s41598-021-00486-1}, } - Preferential alignment and heterogeneous distribution of active non-spherical swimmers near Lagrangian coherent structuresXinyu Si and Lei FangPhysics of Fluids, 33(7), 073303 , Jul 2021
Selected as an Editor’s Pick in Physics of Fluids.
We report the interaction between active non-spherical swimmers and a long-standing flow structure, Lagrangian coherent structures (LCSs), in a weakly turbulent two-dimensional flow. Using a hybrid experimental–numerical model, we show that rod-like swimmers have a much stronger and more robust preferential alignment with attracting LCSs than with repelling LCSs. Tracing the swimmers’ Lagrangian trajectories, we reveal that the preferential alignment is the consequence of the competition between the intrinsic mobility of the swimmers and the reorientation ability of the strain rate near the attracting LCSs. The strong preferential alignment with attracting LCSs further leads to a strong accumulation near the attracting LCSs. Moreover, we show the self-similarity of this accumulation, which reduces the intricate interaction to only one control parameter. Our results generically elucidate the interaction between active and non-spherical swimmers with LCSs and, thus, can be widely applied to many natural and engineered fluids.
@article{si2021preferential, title = {Preferential alignment and heterogeneous distribution of active non-spherical swimmers near Lagrangian coherent structures}, author = {Si, Xinyu and Fang, Lei}, journal = {Physics of Fluids}, volume = {33}, number = {7}, pages = {073303}, year = {2021}, month = jul, day = {6}, publisher = {AIP Publishing}, doi = {10.1063/5.0055607}, url = {https://doi.org/10.1063/5.0055607}, }
