HYDRAUL-EXPT#9

8/12/2019 HYDRAUL-EXPT#9

1/6

EXPT.No.9 1

HYDHYDRAULICS LABORATORY

Experiment No.9Measurement of Velocity using Pitot Tube

A Pitot-static tube can measure the fluid flow velocity by converting the kinetic energy inthe fluid flow into potential energy.

The principle is based onthe Bernoulli Equation where each term can be interpreted asa form of pressure

p + 1/2 v2+ h = constant along a streamline (1)wherep =static pressure (relative to the moving fluid) (Pa) =density (kg/m3) =specific weight (kN/m3)

v = flow velocity (m/s)g =acceleration of gravity (m/s2)h = elevation height (m)

Each term of this equation has the dimension force per unit area - N/m2 or in imperialunits psi, lb/ft2.

Static PressureThe first term - p - is the static pressure. It is static relative to the moving fluid and canbe measured through a flat opening in parallel to the flow.

Dynamic PressureThe second term - 1/2 v2- is called thedynamic pressure.

Hydrostatic PressureThe third term - h- is called the hydrostatic pressure. It represents the pressure due tochange in elevation.

Stagnation Pressure
http://www.engineeringtoolbox.com/bernouilli-equation-d_183.htmlhttp://www.engineeringtoolbox.com/static-pressure-head-d_610.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/accelaration-gravity-d_340.htmlhttp://www.engineeringtoolbox.com/dynamic-absolute-kinematic-viscosity-d_412.htmlhttp://www.engineeringtoolbox.com/dynamic-absolute-kinematic-viscosity-d_412.htmlhttp://www.engineeringtoolbox.com/accelaration-gravity-d_340.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/static-pressure-head-d_610.htmlhttp://www.engineeringtoolbox.com/bernouilli-equation-d_183.html


2/6

EXPT.No.9 2


Since the Bernoulli Equation states that the energy along the streamline is constant, (1)can be modified to

p1+ 1/2 v12+ h1= p2+ 1/2 v2

2+ h2= constant along the streamline (2)

wheresuffix 1is a point the free flow upstreamsuffix 2is the stagnation point where the velocity in the flow is zero

Flow VelocityIn a measuring point we regard the hydrostatic pressure as a constant, h1= h2and thispart can be eliminated. Since v2is zero, (2) can be modified to:

p1+ 1/2 v12= p2 (3)or

v1= [ 2 (p2- p1) / ]1/2 (4)

wherep2- p1= dp (differential pressure)

With (4) it's possible to calculate the flow velocity in point 1 - the free flow upstream - ifwe know the differential pressure difference dp = p2- p1and the density of the fluid.

The pitot tube is a simple and convenient instrument to measure the difference betweenstatic, total and dynamic pressure (or head).

The head - h - (or pressure difference - dp) can be measured and calculated with u-tubemanometers,electronic pressure transmitters or similar instrumentation.
http://www.engineeringtoolbox.com/u-tube-manometer-d_611.htmlhttp://www.engineeringtoolbox.com/u-tube-manometer-d_611.htmlhttp://www.engineeringtoolbox.com/u-tube-manometer-d_611.htmlhttp://www.engineeringtoolbox.com/u-tube-manometer-d_611.html


3/6

EXPT.No.9 3


I. Experiment Objectives

The objective of this experiment is to show how a pitot tube can be used to measureflow velocity and to compare the measured velocity profile in a closed conduit with theprofile assumed for an ideal fluid.

II. Materials and Apparatuses Required

The apparatuses necessary to perform this experiment are the following:

- Flume/SHEM- Piezometer- Pitot tube

III. Procedures

1. Using the crank, elevate the base of the channel to a height of 4 inches. Insurethat the sluice gate is all the way in its upward position.2. Start the pump. Adjust the inlet and outlet valves so that the inlet reservoir is at a

height of 9.5 inches and the level of the outlet reservoir is at about 9 inches. Donot readjust heights once you have started to take measurements.

3. Measure the flow rate once the system is in equilibrium.4. Measure the water level in the piezometer and the pitot tubes.5. Keeping the flow rate constant, repeat Step 3 for the following bed levels:

a) 3 inches, b) 2 inches, c) 1 inch.Make sure that the horizontal portion of the pitot tube is midway between the topand the bottom of the channel for each bed level. (As the base of the channel isadjusted, the level in the inlet and outlet reservoirs will decrease since the cross-section of the flow area is increasing at the throat.)

6. Reset the bed to 4 inches. Repeat Step 3 with the horizontal portion of the pitottube at the following levels:

a) 4.1 inches, b) 4.3 inches, c) 4.7 inches, d) 5.0 inches, e) 5.2 inches f)5.5 inches, g) 5.7 inches, h) 6.0 inches, i) 6.2 inches

7. Change the flow rate by adjusting the inlet and outlet valves. Make sure thedifference between the levels of the inlet and outlet reservoirs is about 0.5 inch.

8. Repeat Steps 5 and 6 for one additional flow rate. (Be sure to measure the flowrate in each case.)


4/6

EXPT.No.9 4


IV. Laboratory Data

BED LEVELS(in.)

Flow Rate

Height

Piezometer Pitot tube

3

2

1

4.1

4.3

4.7

5

5.2

5.5

5.7

6.0

6.2


5/6

EXPT.No.9 5


V. Observations

_________________________________________________________________________________________________________________________________________________________________________________________________________

_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

VI. Conclusions

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

VII. Review Questions


6/6

EXPT.No.9 6


1. Plot the average velocity vs. the centerline velocity determined by the pitot tube(see Note). This graph should contain 8 data points, 4 for each flow rate. Thedata points for each flow rate should be distinguished by using different symbols.

2. Compute the flow rate from the measured velocity distribution.

3. Discuss the difference between the centerline velocity measured by the pitottube and the average velocity calculated from the flow rate. The graph shoulderfect agreement so that the

comparison can be made.4. Plot the velocity profile for each flow.5. Comment on the shape of the profiles. Remember that the conduit is

rectangular in shape and that the flow is probably turbulent so that the profile willgenerally not be parabolic.

HYDRAUL-EXPT#9

Documents

Transcript of HYDRAUL-EXPT#9