Exercise B- Introduction to flow through closed conduits

Exercise C- Using a Pitot tube to measure point velocity

Mohamed Amine Bouzaghrane

Howard University

College of Engineering, Architecture, and Computer Sciences

CIEG 311 Fluid Mechanics Lab, Fall 2013

Instructor: Dr. Ahlam Shalaby

October, 10th, 2013

Exercise B- Introduction to flow through closed conduits

Introduction to Laboratory Experiment

The objective of this exercise is to configure the S16 for teaching different laboratory

exercises involving water flowing through a closed conduit (i.e. Closed Pipe). The S16 equipment

will be configured in this manner every time before proceeding with any exercise that involves water

flowing through a closed conduit. It also introduces the topic of flow in closed conduits. It

demonstrates visually the different phenomena and properties associated with the flow through closed

conduits. The following exercises will go more in depth in this matter by demonstrating application

of the Bernoulli equation.

Background and Theory on Laboratory Experiment

This lab exercise demonstrates how changes in fluid velocity in a closed conduit affect both

the total head and the static head in the whole system. The changes in the static and total head are due

to the frictional losses in the equipment. The following exercises examine this phenomena and

evaluate these changes in more detail.

Laboratory Experiment Equipment

Armfield S16 Hydraulic Flow Demonstrator (S16-10 or S16-11)

Armfield F1-10 Hydraulics Bench

Exercise A (Preparing S16 to demonstrate flow through closed conduits) performed before

starting Exercise B.

Data Collection/Data

Manometers Readings(mm)

Position of the bed 1 2 3 4 5 6

110mm 191 191 180 198 178 177

75mm 172 172 171 173 172 174

0mm 179 179 179 179 179 179

Table 1- Manometers readings for different positions of bed

Data Analysis

From the data available from the conduction of the experiment, we can observe that when the

bed is at 110mm, the static head at throat is below the levels in inlet and outlet tanks. The total

head at the contraction is almost the same as the one upstream.

Due to frictional losses, there is a small loss in total head between the throat and the outlet of

working section.

When the bed is at its lowest position it can be observed that the static head and total head are

approximately the same for all the three positions in the working station.

Discussion of Results

The results obtained seem to be logical following the experiment. Since the continuity

equation says that as long as the cross section of the pipe decreases, the velocity of the fluid

increases and therefore the static head increases.

Conclusions: Implications of Results

All the results obtained from the exercise show that changes velocity (due to changes in bed

height) cause the static head and total head to change. Friction also causes the static head to

fall slightly.

Recommendations

One of the recommendations will be to verify the height of the bed and the height of the pitot

tubes whenever we want to do readings. It would also be beneficial if we leave the flow to

stabilize and the manometers levels to settle before performing the readings.

References

- Finnemore E. John, Franzini B. Joseph,(2002). Fluid Mechanics with Engineering

Applications, 10th

edition. New York: McGraw-Hill.

- S16 Instruction Manual.(2011)

Exercise C- Using a Pitot tube to measure point velocity

Introduction to Laboratory Experiment

This experiment has a main objective of demonstrating how a Pitot tube can be used to find

the total head at a certain point in a fluid. Therefore, the local velocity will be calculated from the

measurements of the total head and piezometric head at that certain point.

This will be done by changing the flow in the closed conduit, and then finding the total head

and piezometric head corresponding to each point in order to calculate the fluid velocity at any point

in the closed conduit.

Background and Theory on Laboratory Experiment

There are some holes on the bed of the closed conduit. These holes serve to measure the static

head that corresponds to the pressure in that point in the closed conduit. There is a manometer tube

connected to the whole. The manometer will indicate the static head defined by

. In order to

find the static pressure at any other depth, it is necessary to deduct the height relative to the bed.

Therefore, .

The Pitot tube is a small bore tube that is parallel with the flow and bent through 90 degrees.

The tip of the Pitot tube is called the stagnation point. At this point, the pressure is called the

stagnation pressure. The manometer that is connected to the Pitot tube will show the total head

corresponding to the stagnation pressure (the sum of the dynamic pressure and the static pressure).

From the Bernoulli equation, we have:

or

Since , we will find

=

. Therefore, we

can find the velocity head

.and

Therefore the velocity head can be seen as the difference between the readings of the

manometers.

The piezometric head that is measured by the manometer tubes is equal to the sum of the pressure and

potential heads. The velocity and total head are not constant across a certain section of the closed

conduit.

By moving the pitot tube vertically, we can calculate the velocities at different point of one

cross section of the closed conduit and therefore extract the velocity profile.

The following table shows the different variables of this exercise:

Laboratory Experiment Equipment

Armfield S16 Hydraulic Flow Demonstrator (S16-10 or S16-11)

Armfield F1-10 Hydraulics Bench

Exercise A (Preparing S16 to demonstrate flow through closed conduits) performed

before starting Exercise B.

Data Collection/Data

Following the equipment set up and the procedure of the laboratory experiment, we end up

getting the following results

Manometers Readings(mm)

Position of the bed 1 2 3 4 5 6

0mm 179 179 179 179 179 179

100mm 184 184 176 186 174 173

130mm 263 263 202 262 174 173

Table 2- Manometers readings for different positions of bed

For the reading at the lowest position of the bed (0mm), the three pitot tubes were at mid height in the

conduit.

For the readings at 100mm, the middle Pitot tube was at 125mm, which means that .

For the readings at 130mm, the middle Pitot tube was at 140mm, which means that .

Calculations for velocity:

Velocity when: Bed is at 110mm and Pitot tube is at 125mm

Velocity when: Bed is at 130mm and Pitot tube is at 140mm

Data Analysis

We can see that the values of the reading of the manometers are the same when the bed is at

the 0mm position. The values of the readings from the upstream and downstream manometers are the

same. The values of the readings from the pitot tube and the bed tapping in the middle position are

different. The difference is used to compute the velocity at that certain point.

Discussion of Results

From the calculations performed above, we can see that the velocity increases as we raise the

position of the bed. This means that the cross sectional area o the closed conduit decreases (Q=vA).

For the other positions, we can see that the values of the pitot tube and the piezometer readings are

the same, which makes the velocity equal the zero. The Pitot tubes should have been increased to a

certain height to see the change in manometers readings.

Conclusions: Implications of Results

From the calculations above, it is clear that the velocity at a certain point ina fluid can be

calculated from the measurements of total head and static head at that certain point in the stream. The

difference between the readings obtained from the Pitot tube and bed tapping.

Recommendations

The best recommendation would be to change the position of the Pitot tube so we can get

different readings for the total head and therefore be able to find the velocity at those points.

References

- Finnemore E. John, Franzini B. Joseph,(2002). Fluid Mechanics with Engineering

Applications, 10th

edition. New York: McGraw-Hill.

- S16 Instruction Manual.(2011).