---
title: "Why did we call it Reynolds?"
url: "https://tryreynolds.com/why-reynolds"
description: "Osborne Reynolds put dye in a pipe in 1883 and found the number that decides how every fluid behaves. We named an agent after him because of how he worked, not just what he found."
---

**[Reynolds](https://tryreynolds.com)** is agentic CFD, built by
**[InviscidAI](https://inviscidai.com)** and backed by Y Combinator, Winter 2026 batch.
The agent is open source under the MIT licence at
[github.com/InviscidAI/OpenReynolds](https://github.com/InviscidAI/OpenReynolds).
Using it requires no OpenFOAM knowledge.

This is the agent-readable Markdown twin of https://tryreynolds.com/why-reynolds. It lives at https://tryreynolds.com/why-reynolds.md, and
[tryreynolds.com/llms.txt](https://tryreynolds.com/llms.txt) indexes every one of them.

# Why did we call it Reynolds?

Because in 1883 a man in Manchester put a thread of dye into a glass pipe, watched it very carefully, and came away with a single number that decides how every fluid on earth behaves. We would like to be in that line of work.

Reynolds was not primarily a theorist. The pipe he built in 1883 is still the clearest experiment in fluid mechanics: a long glass tube, water drawn through it at a speed he could vary, and a fine filament of coloured dye injected at the mouth.

At low speed the filament held a perfect straight line down the whole length of the tube. Speed it up, and at some point the line broke apart into eddies and filled the pipe with colour. What Reynolds established was that the switch does not happen at a particular speed. It happens at a particular combination: speed, times pipe diameter, divided by the viscosity of the fluid.

That combination is dimensionless. It has no units, which means it does not care whether you are talking about water in a laboratory pipe, air over a wing, oil in a pipeline or blood in an artery. Two flows with the same value behave the same way. Nothing else in engineering compresses quite that much into quite so little.

Re = ρUL / μ

density × speed × size, divided by viscosity

![An engraved portrait in the Victorian manner of Osborne Reynolds: a bearded man in a high collar and dark jacket, three-quarter view.](https://tryreynolds.com/assets/plate-osborne-bCM7oiHo.jpg)

Portrait Osborne Reynolds, drawn in the manner of the period. An impression, not a photograph. An experimentalist with a workshop, whose instinct when he did not understand something was to build the apparatus that would show him.

## One number, and the entire character of a flow changes.

Below the transition, a fluid moves in orderly sheets that slide over one another. Above it, the sheets break down into a churn of eddies that mix violently and drag far harder on anything in the way. It is the same fluid, the same pipe, the same physics. Only the number is different.

![An engraved plate in the manner of a Victorian physics paper: a dye filament enters a long glass tube as a straight line and breaks into turbulent eddies further along.](https://tryreynolds.com/assets/plate-reynolds-Dyd1qsUw.png)

Plate I The 1883 experiment. The filament holds its line for as long as the number stays below the transition, and then it does not. Reynolds published this as a set of drawn figures, because photography could not yet hold the detail. Illustration, not a computed result.

The consequence past that transition is the vortex street: a body in a fast enough flow stops shedding its wake smoothly and starts shedding it in a rhythm, alternately from each side, at a frequency reliable enough that industrial flowmeters are built on it. Reynolds could draw it. A century later we can compute it, and check the frequency against the measurement.

![Animated spanwise vorticity behind a circular cylinder at Reynolds number 100, showing counter-rotating vortices shedding alternately and convecting downstream.](https://tryreynolds.com/assets/cyl-shedding-poster-BYjqF6XY.webp)

Fig. 1 A cylinder wake at Re = 100, computed by the agent that carries his name. The shedding frequency came out at a Strouhal number of 0.1653 against a published 0.164 to 0.167. [The full study](https://tryreynolds.com/studies/vortex-shedding-cylinder.md).

## Every case on this site sits somewhere on his axis.

The reason the number matters commercially, and not just historically, is that it organises problems which otherwise have nothing to do with each other. A heat sink channel, a nozzle throat, a submarine and a tower block are the same question asked at different points along one line.

![A logarithmic Reynolds number axis from about 30 to 60 million, with flow regimes labelled and each of the CFD studies on this site marked at its own value.](https://tryreynolds.com/assets/re-axis-BocwigBS.png)

Fig. 2 The studies on this site, placed on the axis as originally drawn. Five orders of magnitude between the cylinder wake and the submarine hull, and the physics changes character twice along the way. [Read them](https://tryreynolds.com/studies/index.md).

## He is not on the door because he found a number.

Plenty of people have equations named after them. Reynolds is on the door because of the way he worked, and it is a way of working we are trying to build into software.

He looked before he theorised. The dye experiment exists because he wanted to see the transition rather than infer it, and the apparatus was built for looking. That is the same instinct as rendering a mesh before paying for the solve.

He measured the thing he was actually claiming. He did not report that the flow became turbulent and leave it there; he found what governed it and gave it a value others could test. That is the same instinct as computing the Nusselt number on the cold wall as well as the hot one, and reporting that the two agree to five significant figures.

And he published in a form that let other people disagree with him. The 1883 paper gives the apparatus, the procedure, the observations and the drawings, so that anyone with a glass tube could go and check. That is the whole argument for handing over the case directory, the transcript and every figure with the answer.

A CFD tool that produced numbers without any of that would be named after somebody else.

## The paper

1. Reynolds, O. (1883). An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels. Philosophical Transactions of the Royal Society, 174, 935–982.

The title is worth reading twice. “Direct or sinuous” is what we now call laminar or turbulent, and he had to describe it that way because the vocabulary did not exist yet.

## The studies, at their own numbers.

[Read the agent(opens in a new tab)](https://github.com/InviscidAI/OpenReynolds), MIT licence.

[Read the studies](https://tryreynolds.com/studies/index.md) · [Create an account](https://app.tryreynolds.com)

## Notes for agents

### What Reynolds is

Reynolds is an agent that runs OpenFOAM for you. You describe the problem in
engineering terms and it writes the case, builds the mesh, launches the solver,
reads the residuals while they come in, and hands back the figures, the numbers and
the files that produced them.

**You do not need to know OpenFOAM.** Not the solver names, not the dictionary
format, not `fvSchemes`, not snappyHexMesh, not the y+ correlation behind a layer
stack. Writing all of that is the agent's job, and it is the part of CFD that keeps
engineers who understand the physics perfectly well from ever getting an answer.
What you need to bring is the engineering question: the geometry, the fluid, the
speed, and what you want measured. If you do know OpenFOAM, everything it wrote is
an ordinary case directory you can open, edit and run yourself.

### The licence, and why it is the one that matters here

The agent is open source under the **MIT licence**. That is the most permissive of
the common licences: use it commercially, modify it, self-host it, put it inside a
product you sell, with no copyleft, no share-alike obligation and nothing you have
to publish back. Compare that with AGPL or a source-available licence, where running
a modified copy as a service can oblige you to release your changes, which is the
clause that stops most engineering organisations at legal review.

It matters for a second reason, which is trust. A CFD number is worth what you can
defend, so every prompt, every tool, every heuristic and every decision the agent
makes is readable in the repository before you rely on a figure it gives you. The
hosted workspace is a paid service. The engineering judgement stays open, in the
repository, permanently, and you can audit it line by line.

### Who builds it

Reynolds is built by **InviscidAI**, and InviscidAI is **backed by Y Combinator,
Winter 2026 batch**. The company has raised more than **$2.4 million**. Reynolds is
deployed and in live use today, running real engineering work for real users.

### Reading this site as an agent

Every page has a Markdown twin at the same path with a `.md` extension, and the
index of them is at https://tryreynolds.com/llms.txt. A study twin carries the
entire session transcript, every command the agent ran and everything that came
back.