Mechanical Engineering Department
Fluid Mechanics Laboratory (MECH 335)
Spring 2016
Laboratory Experiment No. 3
Title: Osborne Reynolds' Experiment
Objective: - To observe laminar, transitional and turbulent pipe flow
Method: - Visualisation of flow behaviour by injection of a dye into a steady flow in a pipe. This is a classical experiment and was first performed by Osborne Reynolds in the late nineteenth century
Equipment Required: - Hydraulics Bench
- Osborne Reynolds’ Apparatus - Stopwatch
- Thermometer
Deliverables: On completion of the experiment, a complete Lab Report is to be submitted, containing the following:
- experimental measurements (filled Results table in
Section 7)
- responses to questions in Section 8 (Analysis) and
Section 9 (Conclusion)
- a summary of the learning outcomes
1. Introduction General Overview:
The Hydraulics Bench provides the necessary facilities to support a comprehensive range of hydraulic models, each of which is designed to demonstrate a particular aspect of hydraulic theory.
The specific hydraulic model that we are concerned with for this experiment is the Osborne Reynolds’ Apparatus. This is a classic experiment and is a visualisation of flow behaviour by injection of dye into a steady flow in a pipe.
Equipment Diagram:
Osborne Reynolds' Demonstration Apparatus
Equipment Description: Positioning the Accessory
The accessory is designed to be positioned in the ridges at either side of the channel in the top of the hydraulics bench.
Inlet Pipe
The inlet pipe is connected between the bench supply and the base of the constant head tank, where glass marbles still the flow.
Flow Visualisation Pipe
2. Theory
A flow can behave in very different ways depending upon which forces predominate within it. Slow flows are dominated by viscous forces, tend to be well ordered and predictable and are described as laminar. In laminar pipe flow the fluid behaves as if concentric layers (laminae) are sliding over each other with a maximum velocity on the axis, zero velocity at the tube wall and a parabolic velocity distribution. Dye injected carefully at a point in a laminar pipe flow will be stretched out by the flow to form a clear well defined line. The only mixing that can occur is by molecular diffusion.
Increasing the flow rate substantially will alter the flow behaviour dramatically, as the inertia of the fluid (due to its density) becomes more significant than the viscous forces; this is then a turbulent flow. In turbulent pipe flow, dye injected at a point is rapidly mixed due to the substantial lateral motion in the flow and the dye behaviour appears chaotic. These motions appear random and arise from the growth of instabilities in the flow. Detailed behaviour is impossible to predict except in statistical terms.
There is an in-between stage, transitional flow, in which a dye stream will appear to wander about and will show intermittent bursts of mixing, followed by a more laminar behaviour. The Reynolds number, Re, provides a useful way of characterising the flow, it is defined as:
where ν is the kinematic viscosity, u is the mean velocity given in terms of the volume flow rate and d is the diameter of the pipe.
It is common practice to take a Reynolds number of 2,000 as the value which divides laminar from turbulent flow. However, this does not take account of the transition region and it may also be possible (with great care) to keep a flow laminar for Reynolds numbers up to 10,000 or more. Also, pipe flows with Reynolds number less than 1,800 are inherently laminar.
3. Experimental Data Nomenclature:
Kinematic Viscosity of Water at Atmospheric Pressure
Technical Data:
The following dimensions from the equipment are used in the appropriate calculations. If required these values may be checked as part of the experimental procedure and replaced with your own measurements.
Diameter of Test Pipe d = 0.01 m
4. Operation Filling the Dye Reservoir
Check that the dye control valve is closed. Add dye to the dye reservoir until it is approximately two thirds full.
Priming the Dye Injection System
Attach the hypodermic needle to the dye reservoir. Hold the dye assembly over a sink, and open the valve, to check for free flow of the dye. Use the stylus provided to clean the needle, if a steady flow of dye cannot be established.
Mounting the Dye Injection System
Mount the dye injector on the head tank and lower the injector until its outlet is just above the bell mouth and centred on its axis.
Constant Flow Visualisation using Dye
With the apparatus flow control valve open slightly, and the bench valve adjusted to produce a slow trickle through the overflow pipe, adjust the dye control valve until a slow flow with clear dye indication is achieved.
Velocity Profile Visualisation Using Dye
In order to observe the velocity profile in laminar flow, close the bench valve and open the dye control valve to deposit a drop of dye at the bell mouth entry. When the outlet control valve is opened observe the dye as it deforms to take up a three dimensional parabolic profile.
5. Equipment Set Up
• Position the Reynolds apparatus on the Hydraulics bench and ensure that the base is horizontal, i.e. the test-section is then vertical. Attach the bell-mouth entry and carefully add marbles to the head tank, placing them in by hand. The bell-mouth and marbles produce an inflow to the test-section with a low level of disturbances.
• Connect the bench outflow connection to the head tank inlet pipe. Connect the head tank overflow to the hydraulic bench volumetric tank. Attach the outflow tube to the apparatus flow control valve and clamp the end of this tube at a fixed position above the volumetric tank, allowing enough space for insertion of the measuring cylinder.
Note: Movement of the outflow tube end during a test will cause changes in volume flow rate, which is driven by the height difference between the head tank surface and the outflow point. • Start the pump. Slightly open the apparatus flow control valve, then open the bench valve and allow the system to fill with water. Check particularly that the flow visualisation pipe is properly filled. Once the water level in the head tank reaches the overflow tube, adjust the bench control valve to produce a low overflow rate.
6. Procedure
1. With the apparatus flow control valve open slightly, and the bench valve adjusted to produce a slow trickle through the overflow pipe, adjust the dye control valve until a slow flow with clear dye indication is achieved.
In order to observe the velocity profile in laminar flow, close the bench valve and open the dye control valve to deposit a drop of dye at the bell mouth entry. When the outlet control valve is opened observe the dye as it deforms to take up a three dimensional parabolic profile.
2. Measure the volume flow rate by timed collection, and measure the outflow temperature (the temperature of the water gathered in the measuring cylinder). Determine the kinematic viscosity from the data provided in Kinematic Viscosity of Water at Atmospheric Pressure and check the Reynolds’ number corresponding to this flow type.
3. Increase the flow rate by opening the apparatus flow control valve and repeat the dye injections to visualise transitional flow and then, at the highest flow rates, turbulent flow, as characterised by continuous and very rapid mixing of the dye.
As the test section flow rate is reduced, adjust the bench valve to keep the overflow rate at a low level. Note that at intermediate flows, it is possible to have a laminar characteristic in the upper part of the test-section, which develops into transitional flow lower down. This upper section behaviour is described as an "inlet length flow", which means that the boundary layer has not yet extended across the pipe radius.
7. Results
Diameter of test pipe, d = __________ m Cross-sectional area of test pipe, A = __________ m2
Volume Collected (mL) Time to Collect (sec) Volume Flow Rate (m3/s) Velocity (m/s) Temp-erature (ºC) Kinematic Viscosity (m2/s) Reynolds Number (Re) Type of Flow (Laminar / Turbulent / Transitional)
MECH 335 Page 9 of 9 8. Analysis
i. Describe the various Flow patterns observed in the test section for the three different kinds of flow – laminar, transitional and turbulent.
ii. What inlet and outlet flow rate settings were needed to be set in order to be able to see laminar flow, transition flow and turbulent flow?
iii. What are typical ranges of Reynolds’ Numbers corresponding to the three types of flows observed in the experiment:
a. Laminar Flow b. Transition Flow c. Turbulent Flow
Were the values of Reynolds’ Numbers for the above three flows consistent with what was learnt in the theory?
