Pipe Flow & Reynolds Number Simulator
Pick a fluid and a pipe, set the flow rate, and watch whether it flows in smooth layers or chaotic turbulence.
Set the fluid, pipe diameter, length, roughness and flow rate. Watch the particles: a smooth parabola means laminar flow, chaotic mixing means turbulent flow.
About the Pipe Flow & Reynolds Number Simulator
Free pipe flow & reynolds number simulator. Pick a fluid and a pipe, set the flow rate, and watch whether it flows in smooth layers or chaotic turbulence. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for engineering, the pipe flow & reynolds number simulator runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Pick a fluid and a pipe, set the flow rate, and watch whether it flows in smooth layers or chaotic turbulence. Use it to explore engineering ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Pipe Flow & Reynolds Number Simulator
- Use the controls to change Fluid, Pipe material (roughness), Flow rate Q, Total pipe length, Pipe segments, and more. The simulation reacts instantly.
- Pick an option such as Water, Air, Motor oil, Glycerin to switch modes or load an example.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Set the fluid to water and slowly raise the flow rate from near zero: watch the particle pattern break from a smooth parabola into chaotic mixing right around Re = 2300-4000.
- Switch the fluid to glycerin without changing anything else, and see the flow drop back to laminar even at a flow rate that made water turbulent.
- Turn on 3 segments and make the middle one much narrower than the other two. Check the table: Q stays fixed but V and Re both jump up in the narrow segment.
- Compare smooth PVC against rough concrete at the same turbulent flow rate and see how much the pressure drop readout increases from roughness alone.
- Try air at a very low flow rate: because air's viscosity is tiny, even a gentle breeze-like flow in a small pipe can already be turbulent.
Key ideas you can learn
- The Reynolds number Re = ρVD/μ compares inertial forces to viscous forces inside a moving fluid, and it alone decides whether a flow is laminar or turbulent.
- Laminar flow (Re < 2300) moves in smooth parallel layers with a parabolic velocity profile: fastest at the pipe's centre, zero at the wall.
- Turbulent flow (Re > 4000) mixes chaotically and has a much flatter velocity profile across most of the pipe, with a thin slow layer right at the wall.
- Between 2300 and 4000 the flow is transitional: it wavers unpredictably between laminar and turbulent behaviour.
- A thick, syrupy fluid like glycerin stays laminar at speeds where thin fluids like water or air are already turbulent, because viscosity resists the chaotic mixing.
- Friction factor f feeds directly into the Darcy-Weisbach equation h_f = f·(L/D)·(V²/2g), which tells you how much pressure a real pipe loses to friction over its length.
- Rougher pipe walls raise the friction factor in turbulent flow (through the ε/D term) but barely affect it in laminar flow, where friction only depends on Re.
- By continuity, the same fluid volume per second (Q) must pass through every segment of a pipe in series, so a narrower segment forces a higher velocity and a higher Reynolds number.
The formulas behind the simulation
Reynolds number: Re = ρVD/μ — ρ is fluid density, V is average velocity, D is pipe diameter and μ is dynamic viscosity. Re compares inertial forces to viscous forces inside the fluid.
Regime thresholds: Re < 2300 laminar (smooth layers), 2300–4000 transitional (wavers between the two), Re > 4000 turbulent (chaotic mixing).
Friction factor f: laminar flow uses f = 64/Re. Turbulent flow uses the Haaland approximation to the Colebrook equation, 1/√f = -1.8·log10[(ε/D/3.7)^1.11 + 6.9/Re], where ε is the pipe's absolute roughness. This simulation blends the two formulas across the transitional band.
Darcy-Weisbach head loss: h_f = f·(L/D)·(V²/2g), giving a pressure drop of ΔP = ρ·g·h_f.
Pump head: switching the pump on adds a fixed head H_p (in metres) to the system, the same units as head loss, so it plugs straight into the same equation: net pressure at the outlet is ρ·g·(H_p − h_f,total). If the pump head is smaller than the total friction head loss, the net result is negative — the pump cannot push the fluid through that pipe at that flow rate.
Moody chart: the small log-log chart plots friction factor f against Reynolds number Re — the straight laminar line is f = 64/Re, the curved turbulent lines are the Haaland equation at a few fixed roughness ratios ε/D, and the dot marks where the current settings actually sit.
This is not the same idea as Bernoulli's principle. Bernoulli's principle (see the separate Bernoulli simulation) trades velocity for pressure in a frictionless, narrowing pipe. This simulation is about friction: how a fluid's viscosity and speed decide whether it flows in smooth layers or chaotic eddies, and how much pressure is lost to that friction along a real pipe with rough walls.
Where this is used in the real world
Plumbers and HVAC engineers size pipes and ducts using these same laminar/turbulent thresholds; oil and gas pipelines are designed around Reynolds number and Darcy-Weisbach head loss to choose pump power; blood flow in large arteries is normally laminar but can turn turbulent past a narrowing or aneurysm, which doctors listen for as a bruit; and aircraft fuel-line and hydraulic-line sizing all rely on exactly this friction-factor and pressure-drop calculation.
Who is this simulation for?
Engineering and technology students, makers, robotics clubs and teachers of design and technology. It gives a hands-on feel for how machines behave before you build a real one.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
How is this different from Bernoulli's principle?
Bernoulli's principle (see the separate Bernoulli's Principle simulation) is about frictionless energy trade-offs between velocity and pressure in a narrowing pipe. This simulation is about friction and turbulence: it uses the Reynolds number to decide whether the fluid moves in smooth layers or chaotic eddies, and uses the Darcy-Weisbach equation to work out how much pressure the pipe loses to that friction. Bernoulli's equation on its own has no friction term at all.
Why use the Haaland equation instead of solving Colebrook directly?
The Colebrook equation for turbulent friction factor is implicit: f appears on both sides and it needs iteration to solve. The Haaland equation is an explicit algebraic approximation that is accurate to within about 1-2% of Colebrook across the normal engineering range, so it can be calculated instantly and is a standard textbook shortcut.
Why does the same pipe carry water as turbulent but glycerin as laminar at the same speed?
Reynolds number is inversely proportional to viscosity μ. Glycerin is roughly 1,500 times more viscous than water, so for the same velocity and diameter its Reynolds number is about 1,500 times smaller, which is easily enough to keep it laminar where water is already deep into the turbulent regime.
What happens exactly at Re = 2300 or Re = 4000?
Those numbers are widely used textbook guideposts, not sharp physical walls. Real pipe flow can stay laminar a little past 2300 if it is very undisturbed, or trip into turbulence a little early if there is roughness or vibration. This simulator blends the friction factor smoothly across the 2300-4000 transitional band to reflect that uncertainty.
Is the Pipe Flow & Reynolds Number Simulator free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Pipe Flow & Reynolds Number Simulator work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.