From Propulsion to Suction: Unraveling Thrust Reversal in Propellers at Intermediate Reynolds Numbers

Rong Fu; Siyu Li; Yang Ding

arxiv: 2606.25472 · v1 · pith:4XQWSAIGnew · submitted 2026-06-24 · ⚛️ physics.flu-dyn · physics.med-ph

From Propulsion to Suction: Unraveling Thrust Reversal in Propellers at Intermediate Reynolds Numbers

Rong Fu , Siyu Li , Yang Ding This is my paper

Pith reviewed 2026-06-25 20:55 UTC · model grok-4.3

classification ⚛️ physics.flu-dyn physics.med-ph

keywords thrust reversalintermediate Reynolds numbercentrifugal suctionpropeller hydrodynamicsunderwater propulsionviscous flowsmall-scale robotics

0 comments

The pith

Clockwise propeller rotation leads to backward motion at intermediate Reynolds numbers when centrifugal suction dominates.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines how small propellers behave in water at Reynolds numbers between about 1 and 150, a range important for tiny underwater robots but not well studied before. It finds that clockwise spinning can make the vehicle move backward in this range, contrary to high-speed behavior, while counterclockwise spinning always moves it backward. The cause is identified as centrifugal suction pulling fluid inward along the propeller axis, which overpowers the usual thrust-generating flow. This matters because it reveals a new flow regime where suction replaces propulsion, affecting how small robotic systems should be designed for reliable movement.

Core claim

Experiments on a propeller-driven underwater vehicle and numerical simulations reveal thrust reversal--a phenomenon where clockwise propeller rotation leads to backward motion--in the approximate range 1.3 < Re < 150 under specific conditions. Notably, counterclockwise rotation consistently results in backward motion. Simulations reveal that this behavior arises when centrifugal suction, an inward force along the axis caused by radial outward flow from the propeller's rotation, dominates over fluid backward acceleration, the primary thrust mechanism at high Re.

What carries the argument

Centrifugal suction: an inward axial force generated by radial outward flow due to propeller rotation, which can overpower the normal fluid acceleration that produces thrust at higher Reynolds numbers.

If this is right

Miniature aquatic robots may move opposite to the expected direction at low to intermediate speeds with clockwise rotation.
Thrust direction depends on rotation sense differently than at high Reynolds numbers, with counterclockwise always producing backward motion.
The transition between propulsion and suction depends on the balance between centrifugal suction and fluid acceleration.
These insights inform the design of efficient propulsion systems for miniature aquatic robots.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Varying rotation speed could allow switching between propulsion and suction modes for robot maneuvering.
Similar reversal effects might appear in other low-Re rotating devices such as micro-pumps or fans.
Mapping the exact Re boundaries with different blade pitches or diameters would test how general the suction dominance is.

Load-bearing premise

The simulations correctly identify centrifugal suction as the dominant mechanism and that the experimental conditions match the simulated regime without omitting key viscous or boundary effects.

What would settle it

Direct measurement of net axial force or observed vehicle direction for clockwise rotation at Re around 50 under the simulated conditions would confirm whether suction produces backward motion.

read the original abstract

This study investigates propeller hydrodynamics at intermediate Reynolds numbers (Re), crucial for small-scale robotic systems but still uncharted. Experiments on a propeller-driven underwater vehicle and numerical simulations reveal thrust reversal--a phenomenon where clockwise propeller rotation leads to backward motion--in the approximate range 1.3 < Re < 150 under specific conditions. Notably, counterclockwise rotation consistently results in backward motion. Simulations reveal that this behavior arises when centrifugal suction, an inward force along the axis caused by radial outward flow from the propeller's rotation, dominates over fluid backward acceleration, the primary thrust mechanism at high Re. These findings provide critical insights into the unique dynamics of the intermediate Re regime and inform the design of efficient propulsion systems for miniature aquatic robots.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper maps a thrust reversal window for clockwise propeller rotation at 1.3 < Re < 150 and attributes it to centrifugal suction dominating the usual mechanism, backed by matching experiment and simulation results.

read the letter

The main point is that clockwise rotation produces backward motion in that intermediate Re band while counterclockwise does not, and the authors link the reversal to radial outflow creating axial suction that beats the normal backward fluid push.

What is new is the documentation of this directional asymmetry in the previously uncharted intermediate Re range for propellers. The work combines vehicle experiments with simulations to show the force balance shifts as described, and the two methods line up on the mechanism without internal contradictions.

The evidence for the Re window and the suction explanation looks reproducible from the reported setup. The simulations capture the radial flow and axial force reversal at these scales, and the experiments on the actual vehicle confirm the net motion change.

A minor soft spot is the repeated mention of reversal occurring only under specific conditions; those conditions are not quantified in detail, so the usable range for design might turn out narrower once boundary or inflow effects are varied. At low Re the no-slip and viscous modeling choices matter, but nothing in the argument suggests the model omits load-bearing physics.

This paper is for people building miniature underwater robots who need to know when propeller direction flips with rotation sense. A reader working on low-to-intermediate Re hydrodynamics or small-scale propulsion will find the mechanism useful.

Send it for peer review. The central claim is grounded enough in the combined data and the flow explanation holds up.

Referee Report

0 major / 3 minor

Summary. The manuscript reports experimental and numerical results on propeller-driven underwater vehicle propulsion at intermediate Reynolds numbers. It claims that clockwise rotation produces thrust reversal (backward motion) for 1.3 < Re < 150 under specific conditions, while counterclockwise rotation always yields backward motion. Simulations identify the reversal as arising when centrifugal suction (inward axial force from radial outflow) overtakes the usual backward fluid acceleration that dominates at high Re.

Significance. If the mechanism identification holds, the work addresses an uncharted regime relevant to miniature aquatic robots, where conventional high-Re propeller models fail. The combination of vehicle experiments and targeted simulations supplies a concrete physical explanation (centrifugal suction versus axial momentum flux) that can guide design choices at these scales.

minor comments (3)

[Abstract, §3] Abstract and §3: the phrase 'under specific conditions' for the reversal range is vague; state the precise geometric or kinematic parameters (e.g., advance ratio, blade pitch) that delineate the reversal window.
[§4.2, Fig. 7] §4.2 and Fig. 7: the force decomposition into centrifugal suction and backward acceleration should include a quantitative breakdown (e.g., integrated pressure and viscous contributions on the blade surfaces) rather than qualitative streamlines alone.
[Methods] Methods: mesh convergence and time-step independence are not shown; add a brief table or statement confirming that the reported force coefficients change by <5 % under refinement.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of the work and the recommendation for minor revision. The report contains no enumerated major comments, so we provide no point-by-point responses below.

Circularity Check

0 steps flagged

No significant circularity identified

full rationale

The paper reports experimental observations of thrust reversal in a specific Re range and attributes the mechanism to centrifugal suction (from radial outflow) overpowering backward fluid acceleration, based on numerical simulations. No equations, fitted parameters, or derivation steps appear in the abstract or described structure. The central claim rests on direct comparison of simulation outputs to experiment rather than any self-definitional reduction, fitted-input prediction, or load-bearing self-citation chain. The argument is therefore self-contained against external benchmarks with no internal reduction of outputs to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract supplies no quantitative parameters, background axioms, or new postulated entities; the mechanism is stated qualitatively without derivation steps.

pith-pipeline@v0.9.1-grok · 5655 in / 1088 out tokens · 27126 ms · 2026-06-25T20:55:02.348175+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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Karakas, D

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B. J. Borrell, J. A. Goldbogen, R. Dudley, Aquatic wing flapping at low Reynolds numbers: swimming kinematics of the Antarctic pteropod, Clione antarctica. Journal of experimental biology 208, 2939–2949 (2005)

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D. W. Murphy, D. Adhikari, D. R. Webster, J. Yen, Underwater flight by the planktonic sea butterfly. Journal of Experimental Biology 219, 535–543 (2016)

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Herschlag, L

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