Reynolds Number Experiment — Apparatus, Procedure and Observation

Aim of the Experiment

To study the nature of fluid flow (laminar, transitional, or turbulent) in a pipe and to determine the Reynolds number at which flow transitions occur.

Apparatus Required

• Reynolds number apparatus (transparent pipe with dye injection)
• Overhead dye reservoir (with food colouring or fluorescent dye)
• Flow control valve
• Measuring tank and stopwatch
• Thermometer

Theory

The Reynolds number (Re) is a dimensionless parameter that predicts the flow regime in a pipe. It is defined as:

Re = ρVD / μ = VD / ν

Where: ρ = fluid density (kg/m³), V = mean flow velocity (m/s), D = pipe internal diameter (m), μ = dynamic viscosity (Pa·s), ν = kinematic viscosity (m²/s).

Flow regimes: Re < 2000 → Laminar | 2000 < Re < 4000 → Transitional | Re > 4000 → Turbulent

Procedure

1. Set up the apparatus and fill the overhead tank with water. Allow flow to stabilise.
2. Open the dye injection valve slightly so a thin thread of dye enters the pipe.
3. Start with a very low flow rate. Observe the dye thread — a straight, unbroken thread indicates laminar flow.
4. Gradually increase the flow rate using the control valve.
5. Note the flow rate at which the dye thread begins to waver (transitional flow).
6. Continue increasing flow until the dye disperses completely across the pipe cross-section (turbulent flow).
7. For each flow condition, collect water in the measuring tank over a known time and calculate discharge Q.
8. Calculate velocity V = Q/A, then Re = VD/ν using the kinematic viscosity of water at the measured temperature.

Observation Table

Result

Laminar flow observed up to Re = _______ | Transition begins at Re ≈ 2000 | Turbulent flow confirmed at Re > 4000. The experimental observations match the theoretical classification of flow regimes.

Precautions

• Use a very small quantity of dye to avoid disturbing the flow.
• Allow the flow to fully stabilise before taking observations.
• Record water temperature to obtain accurate kinematic viscosity values.

Internal Links

Related resources: Fluid Mechanics Lab Equipment | Pipe Friction Experiment | Bernoulli’s Theorem Experiment | Civil Engineering Lab Equipment List

Reynolds Number Experiment — Detailed Procedure and Observation Table

Step-by-Step Procedure

1. Fill the Osborne Reynolds apparatus tank with water. Allow it to settle for 10–15 minutes to remove turbulence.
2. Open the inlet valve slowly to allow water to flow through the glass visualisation tube at very low velocity.
3. Open the dye injector valve to introduce a thin stream of coloured dye into the flow.
4. Observe the dye filament: at very low flow, the dye forms a straight, undisturbed line — this is laminar flow.
5. Gradually increase the flow rate by opening the outlet valve.
6. At a certain flow rate, the dye filament begins to waver — this is the transition zone.
7. At higher flow rates, the dye mixes completely with the water — this is turbulent flow.
8. For each observation, collect water in a measuring cylinder over a timed period to calculate flow rate Q.
9. Calculate velocity V = Q/A, then Re = ρVD/μ.

Observation Table

Sample Calculation

Pipe diameter D = 0.025 m, A = π×D²/4 = 4.91×10⁻⁴ m²

Water density ρ = 997 kg/m³ at 25°C

Dynamic viscosity μ = 8.9×10⁻⁴ Pa·s at 25°C

Collected volume = 1500 mL in 30 s → Q = 1.5×10⁻³/30 = 5×10⁻⁵ m³/s

V = Q/A = 5×10⁻⁵ / 4.91×10⁻⁴ = 0.102 m/s

Re = ρVD/μ = 997 × 0.102 × 0.025 / 8.9×10⁻⁴ = 2,856 (transitional flow)

Critical Reynolds Numbers

• Re < 2000: Laminar flow — dye stream remains straight and stable
• 2000 < Re < 4000: Transitional flow — unstable, fluctuating between laminar and turbulent
• Re > 4000: Turbulent flow — dye mixes completely, flow chaotic

Significance in Engineering

The Reynolds number governs pipe flow design, heat exchanger performance, pump and turbine selection, aerodynamic drag, and chemical reactor mixing. For pipe flow, the friction factor (needed for pressure drop calculation) depends on Re and the Moody chart.

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