LAB 1 - Laminar and Turbulent Flow in Pipes

Introduction

In early experiments with flow in pipes, it was discovered that two different flow regimes exist. At low flow rates, laminar flow exists in a system and the fluid flows in smooth layers. A fluid particle in one layer will remain in that particular layer. The layers of fluid slide by each other without any apparent eddies or swirls. At higher flow rates, turbulent flow will exist. For this flow regime, eddies and vortices mix the fluid by moving particles in very haphazard paths about the cross section.

The existence of the two types of flow regimes is easily visualized by examining results of experiments that were originally conducted by Osborne Reynolds. His apparatus is shown schematically in Figure 1(a). A transparent tube is attached to a constant head tank with water as the fluid. The opposite end of the tube has a valve and flow meter to control and measure the flow rate. Dyed water is injected into the water at the tube inlet, and the resulting flow pattern may be observed. For low flow rates the flow pattern will look similar to Figure 1(b). The dye pattern should be regular and form a single line in the tube. There should be no lateral mixing in any part of the tube. This type of flow is called laminar or viscous flow.

As the flow rate of water is increased beyond a certain point, the dye is observed to disperse. The dye mixes thoroughly with the water as shown in Figure 1(c) as a result of erratic fluid behavior in the pipe. This type of flow is called turbulent flow.

Figure 1. (a) The Reynolds experiment for visualizing flow regimes, (b) dye pattern for laminar flow, (c) dye pattern for turbulent flow.

This experiment could be repeated with several pipes of different diameter; however, a dimensional analysis will indicate that the criterion for distinguishing between these flows is the Reynolds number:

V = average velocity of the pipe flow, D = inside diameter of the tube, r = fluid density, m = fluid dynamic viscosity, n = fluid kinematic viscosity

For straight circular pipes, the flow is always laminar for a Reynolds number less than the critical Reynolds number, which has an accepted value of 2300. The flow is usually turbulent for Reynolds numbers greater than 4000. For the transition regime between these two values, the flow may be either laminar or turbulent, depending on the physical details of the apparatus.

Objective

An apparatus is available in the laboratory that is capable of repeating Reynolds' original experiments. The objective of this experiment is to repeat Reynolds' experiment and to observe and report on the resulting flow regimes as a function of Reynolds number.

Important Data

1. The fluid is water from the city water system.
2. The inside diameter of the clear tube is 5/8 inches.
3. The dye is a water-soluble blue ink (or alternately, liquid food coloring).

A student assistant will be available to operate the apparatus. Flow rates ranging from the completely laminar regime to fully turbulent conditions should be considered. The flow rate can be measured by a rotometer installed at the tube outlet.  Perform a bucket-stop watch measurement of the flow to verify the rated maximum flow rate of 1.48 gpm for the meter.  As the flow rate is varied, your observations of the flow conditions and that of the complete apparatus should be recorded. Keep in mind the questions listed below which must be considered following the operation of the Reynolds apparatus.

Questions

1. Is the quoted maximum flow rate of the meter correct?
2. Report the results of the Reynolds experiment in an appropriate format. How did the flow regime change as the flow rate was increased? What is the critical Reynolds number for this apparatus?
3. Did the results of the test conform with prior empirical results? If not, how did the results differ?
4. Discuss the flow of the dye (or food coloring) in the laminar regime. Does the dye pattern conform to the predicted behavior? If it does not, why? What does this new dye pattern indicate?
5. Briefly describe how some of the physical characteristics of the Reynolds apparatus used in your experiment could cause the fluid flow to behave abnormally (not as predicted). Try to find at least five physical characteristics in this category.
6. Suggest at least three changes which could be made to the apparatus which might produce results that match those reported by Reynolds. Be sure to point out how those changes would improve the test apparatus.