Discussion of the Osborne Reynolds Number Experiment
Check Your Reynolds Number
Enter your experimental data and calculate the Reynolds number manually, then check your answer:
Given constants: ρ = 1000 kg/m³, D = 16 mm, μ = 0.001 Pa·s
1. When you observed the dye stream at very low flow rates, what characteristic feature indicated laminar flow?
Answer: In laminar flow, fluid particles move in parallel layers with no mixing between them, so the dye maintains a clear, straight path.
2. During the transition from laminar to turbulent flow, what was the first sign of instability you observed?
Answer: Transition begins with small instabilities (waves) in the dye stream before fully breaking down into chaotic turbulent motion.
3. The Reynolds number is a dimensionless parameter. Why do engineers prefer using dimensionless numbers rather than just flow velocity?
Answer: Dimensionless parameters like Re allow results from small-scale experiments to predict behavior in full-scale systems, regardless of fluid type or system size.
4. The critical Reynolds number for pipe flow is approximately 2300. What does this value represent?
Answer: Below Re ≈ 2300, flow tends to remain laminar; above Re ≈ 4000, flow is typically turbulent. Between these values is the transition region.
5. In the Reynolds number Re = ρVD/μ, what happens to the ratio of inertial to viscous forces as velocity increases?
Answer: As velocity increases, Re increases, meaning inertial forces (which promote turbulence) become more significant compared to viscous forces (which damp disturbances).
6. How did the dye stream behave at different Reynolds numbers? Did you observe laminar, transitional, and turbulent flows as indicated in the following pictures?
Laminar
Turbulent
7. Did your observations agree with Reynolds calculated? In other words, below Re 2300 did you observe laminar, and above Re 4000 did you observe turbulent?
Explanation: If you observed transitional or turbulent flow earlier than expected, it could be due to a high initial flow rate. When the speed is reduced with the valve, residual turbulence in the equipment may take time to dissipate, delaying the return to laminar flow. Another possible cause is using too much ink, which can artificially enhance turbulence and affect the observation.
8. What is the physical mechanism that causes the flow to change from laminar to turbulent flow?
Answer: The transition from laminar to turbulent flow depends on the balance between viscous and inertial forces. Viscous forces smooth out disturbances, keeping the flow orderly, while inertial forces amplify disruptions, leading to turbulence. At low Reynolds numbers, viscous forces dominate, maintaining laminar flow. As Re increases, inertial forces grow stronger, making the flow unstable.
Boundary layer control can help to delay transition, maintaining a more favorable laminar flow and reducing drag.
9. What sources of error might have affected your determination of the Reynolds number in this experiment? How could these errors be minimized in future experiments?
Common errors include:
- Incorrect amount of ink
- Time error
- Volume error
- Vibrations due to the pump
These errors can be reduced by:
- Taking a longer time for the reading
- Carefully adding the ink
- Isolating the vibration of the pump
- Using a less noisy room
© 2025 Nidiana Rosado Hau, University of Sheffield