Reynolds number: laminar and turbulent flow in a pipe (dye experiment)
Inject dye into water flowing in a pipe. At a low speed the dye moves in straight layers; raise the speed, the pipe size or use a thinner liquid and it breaks into eddies. Read Re = ρvd/η and the critical value of about 2000.
Also searched as: reynolds number class 11 · reynolds number class 11 · laminar and turbulent flow · reynolds number critical value · critical velocity · streamline and turbulent flow
About this experiment
| Subject | Physics |
|---|---|
| Class | 11 |
| Chapter | 9. Mechanical Properties of Fluids |
| NCERT section | 9.6–9.7 |
| Level | NCERT level |
| Time | about 15 minutes |
| Access | Included in a PractiSci plan |
Aim
To see the change from laminar to turbulent flow in a pipe, and to use the Reynolds number to predict it.
Theory
In laminar (streamline) flow the fluid moves in smooth layers that do not mix: a thread of dye stays a thin straight line. In turbulent flow the motion is irregular, with eddies, and the dye mixes with the water at once.
Osborne Reynolds found that the type of flow depends on one number, the Reynolds number Re = ρ v d / η, where ρ is the density, v the mean speed, d the diameter of the pipe and η the viscosity. It is the ratio of inertial forces to viscous forces. For a pipe, the flow is laminar for Re < 2000 and turbulent for Re > 4000; between the two it is unstable.
So the flow becomes turbulent when the speed is high, the pipe is wide, the fluid is dense or the fluid has a small viscosity (water and air, but not glycerine or honey). The critical velocity is v_c = Re_c η / (ρ d).
Formulas used
- Re = ρ v d / η
- laminar: Re < 2000 turbulent: Re > 4000
- critical speed v_c = Re_c η / (ρ d) (Re_c ≈ 2000)
- laminar flow in a pipe: u(r) = 2 v (1 − r²/R²)
Procedure: what to do in the virtual lab
- Water, 20 mm pipe, 0.05 m/s: read Re (about 1000) and note the straight dye lines.
- Raise the speed until Re passes 2000 and then 4000. Watch the dye break up.
- Change to glycerine at the same speed: why is the flow laminar now?
- Choose air in a wide pipe and find the critical speed.
- Use the graph to find the critical speed for each fluid.
Observations to make
- The change from laminar to turbulent happens near Re ≈ 2000–4000 whatever the fluid.
- A viscous liquid stays laminar at a speed at which water is turbulent.
- In laminar flow the central layer moves twice as fast as the mean speed.
What you can change in this experiment
- Fluid: Water (ρ = 1000 kg/m³, η = 0.001 Pa·s) / Glycerine (ρ = 1260 kg/m³, η = 1.4 Pa·s) / Air (ρ = 1.2 kg/m³, η = 0.000018 Pa·s) / Light oil (ρ = 850 kg/m³, η = 0.08 Pa·s)
- Mean speed of the flow (0.005 to 3 m/s)
- Diameter of the pipe (5 to 60 mm)
Viva questions on Reynolds number
Questions an examiner may ask in the Class 11 physics practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.
- What is the Reynolds number and what does it mean?
- At what value does flow in a pipe change from laminar to turbulent?
- Why does a thin stream of honey flow smoothly but water from a tap can be turbulent?
- What is the critical velocity?
Frequently asked questions
What is the aim of the Reynolds number experiment?
To see the change from laminar to turbulent flow in a pipe, and to use the Reynolds number to predict it.
Which formulas are used in the Reynolds number experiment?
Re = ρ v d / η; laminar: Re < 2000 turbulent: Re > 4000; critical speed v_c = Re_c η / (ρ d) (Re_c ≈ 2000); laminar flow in a pipe: u(r) = 2 v (1 − r²/R²)
What viva questions are asked on Reynolds number?
What is the Reynolds number and what does it mean? At what value does flow in a pipe change from laminar to turbulent? Why does a thin stream of honey flow smoothly but water from a tap can be turbulent? What is the critical velocity?
How do I do the reynolds number class 11 experiment for Class 11 online?
Open the Reynolds number virtual lab on PractiSci, follow the procedure above, change the values and read the live graph. It matches the Class 11 physics practical, with the aim, theory, observations and viva questions on this page.
What is the Reynolds number?
Re = ρ v d / η, the ratio of inertial to viscous forces. It decides whether the flow is laminar or turbulent.
What are the critical values of the Reynolds number for pipe flow?
Below about 2000 the flow is laminar, above about 4000 it is turbulent, and between them it is unstable.
Why does the dye thread break up at high speed?
At high Re, eddies mix the layers of the liquid, so the dye spreads across the pipe instead of staying in a thin straight line.
Can I do the Reynolds number experiment online?
Yes. Open it as a 3D virtual lab in your browser or on your phone, change the values and read the results from live graphs. It is part of a PractiSci plan.
Syllabus: Class XI Physics, Chapter 9 — Mechanical Properties of Fluids, §9.6 (viscosity), §9.7 (Reynolds number)
What you do in the lab
Change the controls and watch the physics behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.
Topics: Reynolds number · laminar flow · turbulent flow · viscosity · critical velocity · streamline flow · fluids
Read the full guide: Mechanical properties of fluids.
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