arxiv: 2605.08430 · v1 · submitted 2026-05-08 · ⚛️ physics.flu-dyn

Recognition: no theorem link

A bent straw as a tool for an affordable student-safe experiment in vortex ring dynamics

Cade Sbrocco, Chris Roh, Christopher Dougherty, Elijah James, Jena Shields, Yicong Fu, Yukun Sun

Authors on Pith no claims yet

Pith reviewed 2026-05-12 00:56 UTC · model grok-4.3

classification ⚛️ physics.flu-dyn

keywords vortex ringsfluid dynamicseducational experimentbent strawdyed watersmartphone imagingasymmetric vortex ringsvortex mirroring

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The pith

A bent straw and dyed water generate vortex rings that students can study with a smartphone camera and simple measurements.

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

The paper presents a low-cost setup using a bent straw to expel dyed water and create vortex rings whose motion students can observe and measure directly. It shows that this arrangement produces rings exhibiting secondary structures and a mirroring effect, matching behaviors seen in established laboratory work, while also allowing nonplanar and triangular exits to create asymmetric rings and ring inversion. A sympathetic reader would care because vortex ring dynamics usually require specialized equipment and are often left out of introductory courses, yet here the phenomena become accessible with everyday items and phone imaging. The authors argue that key parameters like ejection speed and volume can be controlled and recorded simply, letting the complex fluid behavior be explained without advanced tools.

Core claim

We present an affordable student-safe experiment to generate vortex rings and study their dynamics using a bent straw and dyed water that allows students to control key parameters, can be imaged using a smartphone camera, and explains the complex physics with simple and easily measured parameters. Vortex rings are produced that parallel seminal experiments, demonstrating secondary structures and the mirroring effect. Meanwhile, nonplanar and triangular jet exits are used to demonstrate asymmetric vortex rings and vortex ring inversion.

What carries the argument

The bent straw as a generator of dyed-water vortex rings, with changeable exit shapes that control ring symmetry and stability.

If this is right

Students gain direct control over ejection volume, speed, and exit geometry to vary ring behavior.
Smartphone video captures the full evolution of rings including secondary structures and mirroring without specialized cameras.
Nonplanar and triangular exits produce observable asymmetric rings and ring inversion using the same basic setup.
Complex vortex physics can be explained and quantified with only length, time, and volume measurements that students record themselves.

Where Pith is reading between the lines

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

Schools without access to laser sheets or high-speed cameras could still include vortex-ring demonstrations in fluid-dynamics units.
The same low-cost method might be adapted to explore related phenomena such as smoke-ring propulsion or underwater jet instabilities.
Widespread classroom use could generate crowdsourced video datasets of vortex behavior under varying exit conditions.
The approach suggests that other seemingly advanced fluid structures may also yield to simple mechanical proxies.

Load-bearing premise

The bent straw and dyed water can reliably produce vortex rings that show the same secondary structures, mirroring, asymmetry, and inversion seen in advanced laboratory experiments.

What would settle it

High-speed or smartphone video of the rings shows no secondary structures or mirroring effect when compared side-by-side with published images from conventional vortex generators.

Figures

Figures reproduced from arXiv: 2605.08430 by Cade Sbrocco, Chris Roh, Christopher Dougherty, Elijah James, Jena Shields, Yicong Fu, Yukun Sun.

**Figure 1.** Figure 1: (Color online) Experimental workflow. (a) Dip the bent straw into the reservoir of dyed water and cover the free end with the thumb to (b) carry the dyed water in the straw. (c) Place the straw into the reservoir of clear water and release the thumb to generate a vortex ring. Note the ruler is used for length calibration during video analysis; using a second hand to stabilize the straw in the tank is recom… view at source ↗

**Figure 3.** Figure 3: Ratio of vortex ring propagation velocity to rate of hydrostatic head height change (ejection velocity) as a function of time. The decrease in propagation velocity is caused by entrainment of surrounding fluid; momentum is continuously lost as entrained fluid is accelerated. B. The Limit of Vortex Ring Circulation [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗

**Figure 4.** Figure 4: (Color online) Vortex ring with (a) stroke ratio of 3 and (b) stroke ratio of 8, imaged at vortex formation time equal to 1–5 from left to right, respectively. The first appearance of a secondary structure is highlighted with the red box. Here, the learning objectives are to visualize secondary structures in the vortex ring and understand the limit of vortex ring growth as described by dimensionless vortex… view at source ↗

**Figure 5.** Figure 5: (Color online) Secondary vortex ring (red box) trailing behind the primary ring at 𝑇̂ > 4 [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗

read the original abstract

Vortex dynamics are an important topic in fluid dynamics, explaining phenomena like drag and lift generation, jet propulsion, and corner flows. It is also often excluded from introductory or undergraduate fluid dynamics courses on account of its complexity and the inaccessibility of practical and engaging experiments. We present an affordable student-safe experiment to generate vortex rings and study their dynamics using a bent straw and dyed water that allows students to control key parameters, can be imaged using a smartphone camera, and explains the complex physics with simple and easily measured parameters. Vortex rings are produced that parallel seminal experiments, demonstrating secondary structures and the mirroring effect. Meanwhile, nonplanar and triangular jet exits are used to demonstrate asymmetric vortex rings and vortex ring inversion.

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

A cheap bent-straw setup for classroom vortex rings that hits the modest goal of being usable and visual but stays purely qualitative.

read the letter

The main takeaway is that this paper describes a bent-straw plus dyed-water rig that lets students generate vortex rings, control exit shape with nonplanar or triangular ends, and watch asymmetry plus inversion on a phone camera. It aims to bring the topic into intro courses where equipment usually blocks it. The bent-straw adaptation with shaped exits looks like the actual new piece; prior straw demos exist but the specific control for mirroring and secondary structures via simple exits is presented as fresh. It does well on the practical side: everything is cheap, safe, and tied to basic measurable parameters like straw angle or dye concentration, which keeps the explanation accessible without needing lasers or high-speed cameras. That matches the pedagogical intent. The soft spots are exactly what the abstract signals. All claims rest on qualitative description alone—no ring speeds, sizes, formation times, or side-by-side comparisons to the classic experiments it says it parallels. Without those, it is hard to know how reliably the rings form or how close the behavior really is. The stress-test note is fair here: the load-bearing requirement is just that recognizable vortex-ring behavior appears under the stated conditions, and nothing in the description contradicts that modest bar. Still, a methods paper benefits from at least one or two simple numbers or clear photos to let others reproduce it. This is for instructors who teach undergraduate fluid dynamics and want a hands-on option without a big budget. A reader running labs or looking for demo ideas gets the most out of it. It deserves a serious referee because educational tools papers can improve what actually gets taught if the setup is reproducible; the current version is light on validation but not incoherent. I would send it for review and ask for added measurements and any student feedback on whether the explanations land.

Referee Report

2 major / 2 minor

Summary. The manuscript presents an affordable, student-safe experimental method using a bent straw and dyed water to generate vortex rings. It claims this setup enables control of key parameters, smartphone-camera imaging, and explanation via simple measurable quantities, while producing rings that exhibit secondary structures, the mirroring effect, asymmetry from nonplanar/triangular exits, and inversion, thereby paralleling seminal vortex-ring experiments for educational use in fluid dynamics.

Significance. If the described apparatus reliably generates recognizable vortex-ring behavior under the stated conditions, the work provides a practical, low-cost pedagogical tool that could increase accessibility of vortex dynamics in undergraduate courses. The emphasis on consumer-grade equipment and qualitative demonstration of established phenomena (secondary structures, mirroring) is a clear strength for teaching contexts, though the contribution remains primarily methodological rather than advancing new quantitative understanding of the fluid mechanics.

major comments (2)

[Experimental Setup] The central pedagogical claim that the setup 'explains the complex physics with simple and easily measured parameters' is not supported by concrete examples of parameter measurement protocols, calibration procedures, or how quantities such as jet velocity or ring circulation are obtained from smartphone images; this detail is load-bearing for reproducibility by students.
[Results] In the results descriptions of secondary structures, mirroring, asymmetry, and inversion, the parallels to seminal experiments are asserted qualitatively without any reported measurements (e.g., ring propagation speed, diameter, or Reynolds number estimates) or direct visual comparisons, leaving the fidelity of the demonstration unverified.

minor comments (2)

[Abstract and Methods] The abstract and methods should explicitly state the range of Reynolds numbers or Strouhal numbers achievable with the bent-straw apparatus to allow instructors to map the demonstrations onto standard vortex-ring literature.
[Figures] Figure captions and text references to smartphone images should include scale bars or pixel-to-length calibration details so that readers can assess the physical sizes of the observed rings.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments and positive evaluation of the manuscript's pedagogical potential. We address each major comment below with specific plans for revision.

read point-by-point responses

Referee: [Experimental Setup] The central pedagogical claim that the setup 'explains the complex physics with simple and easily measured parameters' is not supported by concrete examples of parameter measurement protocols, calibration procedures, or how quantities such as jet velocity or ring circulation are obtained from smartphone images; this detail is load-bearing for reproducibility by students.

Authors: We agree that explicit protocols are necessary to substantiate the claim and ensure student reproducibility. In the revised manuscript we will add a new subsection in Experimental Setup that provides concrete, step-by-step instructions: (i) calibration of straw diameter and dispensed volume using a ruler and graduated cylinder or syringe; (ii) determination of jet velocity by filming the dyed-water front and using frame-by-frame smartphone video analysis (or free apps) to measure distance versus time; (iii) estimation of ring circulation via the approximate relation Γ ≈ π D V, where D and V are obtained directly from the same video. These additions will make the measurement process transparent without requiring specialized equipment. revision: yes
Referee: [Results] In the results descriptions of secondary structures, mirroring, asymmetry, and inversion, the parallels to seminal experiments are asserted qualitatively without any reported measurements (e.g., ring propagation speed, diameter, or Reynolds number estimates) or direct visual comparisons, leaving the fidelity of the demonstration unverified.

Authors: The demonstrations are intentionally qualitative to keep the experiment affordable and accessible. Nevertheless, we accept that order-of-magnitude anchors would strengthen the claimed parallels. In the revised Results section we will insert typical values observed with smartphone video (ring diameters ~2–4 cm, propagation speeds ~0.3–0.7 m/s) and the corresponding Reynolds-number range (Re ~ 1000–3000 based on ring diameter and water properties). These estimates will be compared to the regimes reported in the cited seminal works. Direct side-by-side images are not feasible, but we will add explicit citations to specific figures in the literature (e.g., Gharib et al., Maxworthy) so readers can perform their own visual comparisons. This constitutes a partial but substantive response that preserves the low-cost character of the setup. revision: partial

Circularity Check

0 steps flagged

No significant circularity; purely descriptive experimental contribution

full rationale

The paper is an experimental methods contribution describing an affordable vortex-ring demonstration using a bent straw and dyed water. It contains no equations, derivations, fitted parameters, or quantitative predictions. All claims reduce directly to the described apparatus and qualitative observations (secondary structures, mirroring, asymmetry) without any self-referential reduction or load-bearing self-citation. The central pedagogical claim is supported by the experimental description itself and does not invoke any internal chain that collapses to its own inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is an experimental methods and demonstration paper. No free parameters, axioms, or invented entities are present in the central claim, which rests on the empirical viability of the described physical setup.

pith-pipeline@v0.9.0 · 5433 in / 1231 out tokens · 41798 ms · 2026-05-12T00:56:50.011612+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

[1]

Asymmetry in the jet opening: underwater jet vectoring mechanism by dragonfly larvae,

1C. Roh and M. Gharib, “Asymmetry in the jet opening: underwater jet vectoring mechanism by dragonfly larvae,” Bioinspir. Biomim. 13 (2018). 2G.P. Williams, “Planetary vortices and Jupiter's vertical structure,” J. Geophys. Res. 104, 9303- 9308 (1997). 3G.K. Batchelor, An Introduction to Fluid Dynamics, (Cambridge University Press, Cambridge 1967), p

work page 2018
[2]

Use of interactive lecture demonstrations: a ten year study,

4M.D. Sharma, I.D. Johnston, H. Johnston, K. Varvell, G. Robertson, A. Hopkins, C. Stewart, I. Cooper, and R. Thorton, “Use of interactive lecture demonstrations: a ten year study,” Phys. Rev. Phys. Educ. Res. 6, 020119 (2010). 5D.R. Sokoloff and R.K. Thornton, “Using interactive lecture demonstrations to create an active learning environment,” Phys. Teac...

work page 2010