Lab assignment fluid mechanics

Fluid Mechanics Laboratory

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Experiment 2: An Introduction to Hydrostatic Pressure

Introduction

Part I: Barometer Measurements

Theory

Barometers are used to determine the atmospheric pressure at their given location. These

devices typically consist of a glass tube filled with mercury (or some other fluid), with one end

closed and the other open to the atmosphere. Above the fluid, an excellent vacuum is needed.

The surface height at which the fluid rests above the fluid reservoir indicates atmospheric

pressure according to the scale on the tube.

Apparatus

The setup of this experiment consists of the world’s tallest barometer.

Procedure

1. Record the measurements indicated by the lab (mercury) barometer.

2. Guided Discovery: To make efficient use of lab time, the group will go through a guided

discovery process to see the various unusual features of the barometer.

3. Record the measurement indicated by the Bennett Barometer.

The purpose of this experiment is to explore the concept of hydrostatic pressure. The lab is separated into two parts. First students will explore how a barometer functions using the concept of hydrostatic pressure. Next they will use a Mariotte Bottle to understand how the surrounding environment of a fluid affects the hydrostatic pressure it imparts on other objects.

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Part II: Mariotte Bottle

Theory

The Marriotte Bottle, invented around 1670, is a device capable of providing a stream of water

with constant pressure and flow rate using the hydrostatic pressure developed between the air

vent, and the fluid outlet. This deceptively simple apparatus combines water head pressure and

atmospheric pressure into a practical and instructive device. Note that the orifice flow rate

depends on the pressure head, not the height of the free water surface.

Apparatus

Students will use a modified Mariotte Bottle with multiple openings and the

central air tube and external scale acting as a manometer to investigate the

transient behavior of the device.

Procedure

1. Guided Discovery: To make efficient use of lab time, the group will go

through a guided discovery process to see the various unusual features

of the Mariotte Bottle.

2. Next break up into small groups. Make predictions of how the Mariotte

Bottle will behave with regards to the worksheet questions among your

lab partners.

3. Repeat the guided discovery using a meter stick or attached scale to

answer the worksheet questions.

Figure 1. Mariotte

Bottle Apparatus

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Lab 2A Worksheet

Please write answers to the following questions in the space below. Attach any figures or calculations to the end. Turn in the completed packet to your lab TA one week following the experiment at the start of lab class.

Mariotte Bottle

1. Complete the attached Table 1 (or prepare your own and attach it) noting what happens

when the stopper above, below, and at the same height of the vent tube is removed. Use

the manometer measurements to clearly explain this behavior.

2. Complete the attached Table 2 (or prepare your own and attach it) to calculate the flow

rate when a stopper below the vent tube is removed. Is it constant? Perform at least three

trials. How would the theoretical flow rate be determined (see your text and describe using

the features of the Mariotte bottle)?

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3. Complete the attached Table 3 (or prepare your own and attach it) to illustrate what

happens when you move the vent tube up or down. What trend do you observe? Explain

why this occurs.

4. Explain how the Mariotte Bottle works making use of the following terms: Atmospheric

Pressure, Absolute Pressure, Partial Vacuum, and Water Head Pressure.

5. What are the height limitations of the Mariotte Bottle? How is this affected by elevation?

6. Explain why the Mariotte Bottle is used to fertilize agricultural crops and with the Double

Ring Soil Infiltrometer.

Fluid Mechanics Laboratory

Barometer Measurements

7. Using the measurements of both barometers and the known density of mercury, calculate

the density of the fluid in the Bennett Barometer.

8. Draw a figure that illustrates the pressure distribution (gauge pressure) within the Bennett

Barometer. Clearly indicate the pressure at the top and bottom of the tube and formulate

an equation that describes the pressure as a function of height above the bottom of the

tube. What is the hydrostatic pressure (roughly) just at the bottom of the tube?

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Table 1: Effect of air vent height relative to stopper height in a Mariotte Bottle.

Position of Vent Tube

Fluid Outlet Height (cm)

Air Vent Height (cm)

Water Behavior Observations

Manometer Observations

Above stopper hole

At stopper hole

Below stopper hole

Table 2: Flow rate of water from Mariotte Bottle with constant pressure head.

Trial Volume of Water

(mL) Time

(s) Flow Rate

(mL/s)

Table 3: Flow rate of water from Mariotte Bottle with varying pressure head.

Pressure Head Height (cm)

Volume of Water (mL)

Time (s)

Flow Rate (mL/s)