Science practice 90

PASSAGE V

Near the end of the 19th century, British engineer Osborne Reynolds ran a set of experiments to observe and predict the transition between laminar (steady) and turbulent flow of a liquid through a pipe. In Reynolds' experiments, dye was forced through a liquid to show visually when the flow changed from laminar to turbulent. Laminar flow is common only in cases in which the flow channel is relatively small, the fluid is moving slowly, and its viscosity (the degree to which a fluid resists flow under an applied force) is relatively high. In turbulent flow, the speed of the fluid at any given point is continuously undergoing changes in both magnitude and direction. Reynolds demonstrated that the transition from laminar to turbulent flow in a pipe depends upon the value of a mathematical quantity equal to the velocity of flow (V) times the diameter of the tube (D) times the mass density (ρ) of the fluid divided by its absolute viscosity (μ). The "Reynolds number," as it is called, is determined by the following equation:

Several students designed similar experiments to observe flow rates of different liquids. To conduct the experiments, the students were given the following apparatus:

o Liquid supply tank with clear test section tube and 'bell mouth' entrance

o 1 Rotameter to measure the velocity of flow (flow rate)

o Tap water

o Motor oil

o 4, 10-ft long smooth pipes of various diameters: 0.25-inch, 0.50-inch, 0.75-inch, 1.0-inch

Figure 1 illustrates an approximation of the set-up of each experiment.

Figure 1

Figure 2 shows approximate viscosities of the water and motor oils used in the experiments.

Figure 2

Experiment 1

In Experiment 1, students began with a pipe of diameter 0.25 inches. The pipe was set first at a 15° angle and tap water was released steadily from the tank into the pipe. The velocity of flow (V) was measured. The pipe was then set at a 30° angle, a 45° angle, and a 60° angle, water was released steadily from the tank into the pipe, and the velocity of flow was measured. The process was then repeated for each diameter of pipe using the same amount of water each time. All data were recorded in Table 1. Temperature of the water was held constant at 20°C.

Experiment 2

In the second experiment, the tap water was replaced by Motor Oil A and the processes were repeated. The results are given in Table 2.

Experiment 3

In a third experiment, the tap water was replaced by Motor Oil B and the processes were repeated.