Publications

Sensing flow gradients is necessary for learning autonomous underwater navigation

Published in Nature Communications, 2025

1. Egocentric underwater navigation (using body-frame flow and target measurements) performs as well as geocentric sensing (using inertial-frame measurements) when supplemented with local flow-gradient information, either transverse or tangential flow gradients.
2. Egocentric policies inherently obey rotational symmetry, maintaining high performance under arbitrary rotations of the flow field, whereas geocentric policies rapidly degrade when the flow alignment shifts.
3. When generalizing to different flows (including from reduced order modle to actual flow), both egocentric and egocentric policies are robust in naviagting through vortical wakes.
4. Analysis of the learned policies as vector fields reveals “preferred directions” (stable equilibria) that explain differing success rates upstream vs. downstream and the role of sensory ambiguities in decision-making.

Recommended citation: Jiao, Y., Hang, H., Merel, J. and Kanso, E. (2025). Sensing flow gradients is necessary for learning autonomous underwater navigation. https://doi.org/10.1038/s41467-025-58125-6

Mapping Spatial Patterns to Energetic Benefits in Groups of Flow-coupled Swimmers

Published in eLife, 2024

1. Through fluid structure interation, flapping swimmers self-organize into stable emrgent formations without sensing and control. Across all fluid models and experimental data, the spacing between swimmers scales linearly with their flapping phase difference, revealing a universal passive matching rule.
2. In side‑by‑side in‑phase formations the energy savings are shared nearly equally, whereas side‑by‑side antiphase costs more power; in contrast, in‑line and diagonal formations give almost all the benefit to the follower (up to about a 60 % drop in its cost of transport).
3. We developed a diagnostic tools “flow agreement parameter”, which predicts where followers will stably position and how much energy they’ll save based on leader’s wake, without needing two‑way coupled simulations.
4. Inline schools remain cohesive only up to three swimmers; the fourth (and any beyond) break away with no hydrodynamic gain. Side‑by‑side formations, however, stay together formuch more swimmers (up to 10 in our simulation), with almost equal energy savings.

Recommended citation: Heydari, S., Hang, H., and Kanso, E. (2024). Mapping Spatial Patterns to Energetic Benefits in Groups of Flow-coupled Swimmers. https://doi.org/10.7554/eLife.96129.2

Flow sensing and feedback control for maintaining school cohesion in uncoordinated flapping swimmers

Published in American Control Conference, 2024

1. We devised local flow sensing model and feedback controller that stabilize a pair of frequency-uncoordinated swimmers into a cohesive formation.

Recommended citation: Hang, H., Heydari, S., and Kanso, E. (2024). Flow sensing and feedback control for maintaining school cohesion in uncoordinated flapping swimmers. https://10.23919/ACC60939.2024.10644662

Reynolds-number scaling analysis on lift generation of a flapping and passive rotating wing with an inhomogeneous mass distribution

Published in Chinese Journal of Aeronautics, 2023

1. A coordinative relationship between active stroking and passive rotating motion is demonstrated in flapping wing system. Moreover, the phase shift between wing rotation and stroke reversal depends on active stroking motion. And advanced rotation is obtained when driving Reynolds number smaller than 100.
2. The mean lift coefficient decreases with the increase in Reynolds number, and increases with the increase in additional torque.
3. The additional torque can be used as a control parameter for modulating wing roation and lift generation.

Recommended citation: Qin, S., Hang H., Xiang, Y., and Liu, H. (2023). Reynolds-number scaling analysis on lift generation of a flapping and passive rotating wing with an inhomogeneous mass distribution. https://doi.org/10.1016/j.cja.2023.09.030

Active tail flexion in concert with passive hydrodynamic forces improves swimming speed and efficiency

Published in Journal of Fluid Mechanics, 2021

1. Passive bending is more efficient but could reduce swimming speed compared to rigid flapping, but the addition of active bending could enhance both speed and efficiency;
2. The phase difference between the posterior and anterior sections of the body is an important kinematic factor that influences performance;
3. Active antiphase flexion, consistent with the passive flexion phase, can simultaneously enhance speed and efficiency in a region of the design space that overlaps with biological observations.

Recommended citation: Hang, H., Heydari, S., Costello, J. H., and Kanso, E. (2022). Active tail flexion in concert with passive hydrodynamic forces improves swimming speed and efficiency. https://doi.org/10.1017/jfm.2021.984

Scaling analysis of the circulation growth of leading-edge vortex in flapping flight

Published in Acta Mechanica Sinica, 2021

1. Formation time of Leading edgs vortex (LEV)) in flapping wings experiment stays in the range of 2.5–5.5, agrees with the scaling formation number predicted by other vortices;
2. The mode of wing rotation plays a controllable role in the formation number of LEV by modulating the characteristic length scale that feeds the formation of LEV. After reaching the formation number, the LEVs stably remain attached on the flapping wing and even further grow at some spanwise locations because of vorticity transport;
3. Before the leading ledge vortex reach pinch off, the formation time calculated by the circulation of the LEV has a linear relationship with the formation time predicted by the kinematics of flapping wings, which implies the circulation growth of LEV can be predicted based on wing kinematics.

Recommended citation: Xiang, Y., Hang, H., Qin, S., and Liu, H. Scaling analysis of the circulation growth of leading-edge vortex in flapping flight. Acta Mech. Sin. (2021). https://doi.org/10.1007/s10409-021-01134-7

An objective-adaptive refinement criterion based on modified ridge extraction method for finite-time Lyapunov exponent (FTLE) calculation

Published in Journal of Visualization, 2019

In the calculation of inite-time Lyapunov exponent (FTLE), using a physics-based refine criterion is able enhance computation efficiency faster than a universal refine criterion based on error or gradient calculation.

Recommended citation: Hang, H., Yu, B., Xiang, Y., Zhang, B., and Liu, H. (2020). An objective-adaptive refinement criterion based on modified ridge extraction method for finite-time Lyapunov exponent (FTLE) calculation. Journal of Visualization, 23(1), 81-95. https://doi.org/10.1007/s12650-019-00605-1