Lift reversal from vortex-surface phase coupling in a heaving foil near a free surface
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| Format: | Preprint |
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2025
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| _version_ | 1866918248351531008 |
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| author | Feng, Qimin Han, Tianjun Zhong, Qiang |
| author_facet | Feng, Qimin Han, Tianjun Zhong, Qiang |
| contents | Classical descriptions of flapping propulsion near a free surface emphasize the energetic penalties of wave generation, treating the interface primarily as an energy sink. Here, we show that the same deformable boundary can also act as a phase-dependent kinematic constraint on vertical force generation. Using force measurements, particle image velocimetry and potential-flow simulations, we characterize how a free surface reorganizes vortex shedding for a heaving hydrofoil at moderate Reynolds number (O(10^4)). For moderate to deep submergence, the cycle-averaged lift undergoes a systematic transition from repulsion to suction as the unsteady number increases. The reversal occurs within a narrow band of unsteady numbers, where the phase-shifted surface motion generates vertical advection that alters the pairing of trailing-edge vortices and redirects the wake momentum flux. A force decomposition shows that the reversal arises from a coordinated change in quasi-steady pressure loading and wake-induced force. These results identify the phase of the free-surface response, organized by unsteady number, as a key parameter governing near-surface lift and illustrate how deformable boundaries can reconfigure unsteady loading through vortex-surface phase coupling. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2512_12485 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Lift reversal from vortex-surface phase coupling in a heaving foil near a free surface Feng, Qimin Han, Tianjun Zhong, Qiang Fluid Dynamics Classical descriptions of flapping propulsion near a free surface emphasize the energetic penalties of wave generation, treating the interface primarily as an energy sink. Here, we show that the same deformable boundary can also act as a phase-dependent kinematic constraint on vertical force generation. Using force measurements, particle image velocimetry and potential-flow simulations, we characterize how a free surface reorganizes vortex shedding for a heaving hydrofoil at moderate Reynolds number (O(10^4)). For moderate to deep submergence, the cycle-averaged lift undergoes a systematic transition from repulsion to suction as the unsteady number increases. The reversal occurs within a narrow band of unsteady numbers, where the phase-shifted surface motion generates vertical advection that alters the pairing of trailing-edge vortices and redirects the wake momentum flux. A force decomposition shows that the reversal arises from a coordinated change in quasi-steady pressure loading and wake-induced force. These results identify the phase of the free-surface response, organized by unsteady number, as a key parameter governing near-surface lift and illustrate how deformable boundaries can reconfigure unsteady loading through vortex-surface phase coupling. |
| title | Lift reversal from vortex-surface phase coupling in a heaving foil near a free surface |
| topic | Fluid Dynamics |
| url | https://arxiv.org/abs/2512.12485 |