laboratory experiments - · pdf filethe flow within the hydraulic jump is extremely turbulent,...

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2011 – 2012

CAIRO UNIVERSITY

Faculty of Engineering

Irrigation and Hydraulics Department

3rd Year Civil

OPEN CHANNEL HYDRAULICS

LABORATORY EXPERIMENTS

2011 – 2012


Experiment (1) Roughness of Open Channel

I- Introduction: An open channel is a waterway, canal or conduit in which a liquid flows with a free surface. Open channel flow describes the fluid motion in an open channel. In most applications, the liquid is water and the air above the flow is usually at rest and at standard atmospheric pressure. In the absence of any other channel control, the flow is controlled only by friction with the bed and the sides of the channel. In this case, water flows with a uniform depth (normal depth) at which the weight component in the direction of the flow balances with the friction force induced by the flow resistance with the bed and sides. Many equations were developed to relate the bed roughness with the flow parameters in open channels. Among these equations is the manning equation which is widely used to relate the flow velocity and the cross sectional parameters with the manning coefficient (n) which is a function in the bed material roughness. Another less common equation is the Chezy equation where the bed roughness is being expressed by chezy coefficient (c). II- Objectives: The main objective of this experiment is to determine an average value of both manning (n) and chezy (c) coefficients for the ArmField laboratory flume. The sides of the flume are made of glass while the bed is made of steel. Changing the water depth changes the contribution of the sides in the computed average roughness while the bed contribution remains the same. It’s required to compute an average value of (n) and (c) for different water depths and discharges. III- Anticipated Results: The students should be able to:

a. Control and measure the bed slope, water depth, and discharge in the laboratory flume.

b. Use both Manning and Chezy equations and get the manning and chezy coefficients knowing all the other flow parameters.

c. Notice how the roughness coefficients are affected by the average water depth inside the flume.

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2011 – 2012 IV- Experimental Setup and Tools used: The students are going to use the ArmField flume setup in the eastern laboratory inside the Irrigation and Hydraulics Department. The Flume is about 10m long with a rectangular cross section of 30 cm width and 70 cm maximum depth. The flume has a slope adjusting mechanism to adjust the bed slope ranging from 1:40 (positive slope) to 1:200 adverse slope. The tail water gate at the end of the flume can be used to control the water level inside the channel. A point gage can be used to measure the water depth at any section in the fume. The point gage can be moved both laterally (along the cross section) and longitudinally (with the flow). Water is pumped into the flume from a ground reservoir tank and then collected at the channel end back to the tank again. The pumping system is supplied with a gate valve to control the discharge and a 90 mm orifice meter with a differential manometer to measure the discharge.

Tail Water Gate

Orifice meterBed Slope Wheel

Point Gauge

Pumping System


Point Gage

Stilling Device Tail Gate

Ground Tank

Pumping System

Flow Direction

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 V- Experimental Procedure: 1. Open the valve to get the suitable discharge and calculate Q using the deflection

of the differential manometer connected with the Orifice-meter and the calibrated chart.

2. Adjust the tail water gate to have an average water level of 25 cm inside the flume

(low position). Do not use very low water levels. 3. Using the point gauge measure the water level (W.L) and the bed level (B.L.) at

two sections 3 - 4 m apart. We should wait at least 15 minutes before measuring to reach the steady state.

4. For the same discharge set before, raise the tail water gate to increase the water

level inside the flume (medium then high positions) and measure the W.L and B.L at the same sections. Care should be taken when raising the tail gate to avoid over flooding the flume.

5. Change the valve opening to get another different discharge in the flume and

repeat the previous steps. VI- Equations Used:

o ASRn

AScRQ ee21

32

21

21 1

==

o y1 = (W.L)1 – (B.L.)1, y2 = (W.L)2 – (B.L.)2

o 2

21

1 ,byQV

byQV = =

o 0

22

2

21

1 22S

Lg

Vyg

VySe +

⎟⎟⎠

⎞⎜⎜⎝

⎛+−⎟⎟

⎠

⎞⎜⎜⎝

⎛+

=

o 2

21 yyyavg+

=

o

avg

avg

avg

avg

ybby

pAR

ybp

byA

2

2

,

+==

+=

=

Where: Q = Flow rate (m3/sec) V = Average flow velocity (m/s) Se = Slope of total energy line

(m/m) n = Manning coefficient c = Chezy coefficient R = Hydraulic Radius (m) P = Wetted perimeter (m) A = Cross sectional area (m2) W.L = Water level (m) B.L = Bed Level (m) y1 = Water depth at section 1 (m) y2 = Water depth at section 2 (m) yavg = Average water depth (m)

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 VII- Results: 1. Calculate the manning and chezy coefficients for all the previous runs. 2. Plot the variation in manning coefficient versus the water depth (tail gate

position) for each flow rate on the same plot. Also plot the variations with chezy coefficient.

3. Do manning and chezy coefficients vary with water depth (at the same

discharge)? 4. Do manning and chezy coefficients vary with discharge (at the same water

depth)? VIII- Suggested Datasheet Headings:

Tail Gate

Position Section W.L B.L y

(m) V

(m/s) Se yav (m)

A (m2)

P (m)

R (m) c n

1 Low 2

1 Med 2

1

Q1

High 2

1 Low 2

1 Med 2

1

Q2

High 2


Experiment (2)

Velocity Distribution

I- Introduction: When a fluid flows past a solid boundary, the velocity at all points adjacent to the surface is zero (in case of no slip condition) and it increases as we move away from the boundary. In open channels, the measured velocity will always vary across the channel section due to friction along the boundaries. This velocity distribution is not ax-symmetric as in case of pipes due to the presence of the free surface. The point of maximum velocity is not at the surface (expected location of zero shear stress) but is found just below it. This is due to the presence of secondary currents circulating from the boundaries towards the section center and the resistance at the air/water interface. Also, when the velocity distribution across a channel cross section is known, we can simply find the flowing discharge. The above method of discharge measurement is known as the velocity distribution method, and is commonly used in natural or irregular cross sections. II- Objective: The objective of this experiment is to get the velocity distribution across the open channel section. The discharge can be calculated by averaging the velocity of each point along the area of its neighborhood and compare the calculated discharge with the discharge obtained from the orifice meter. III- Anticipated Results: The student should be able to:

1. Use the electromagnetic current meter to measure the velocity at a point 2. Integrate the measured velocity over the cross section area to get the flowing

discharge and compare it with other measuring devices and find the percent error.

3. Draw velocity contours for the channel cross section and find the point of

maximum velocity.

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 IV- Experimental Setup and Tools used: The same ArmField laboratory flume described in Experiment-1 will be used in this experiment. The “820 Dual axis EM Flow Meter” will be installed on the flume to measure the velocity at any point. The flow meter can be moved both laterally and vertically across the cross section and can be moved longitudinally along the flume.

820 Dual Axis EM Flow Meter

Point Gage

Stilling Device Tail Gate

Pumping System

Flow Direction

Flow Meter

Ground Tank

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 V- Experiment Procedures:

1. Open the inlet valve to get a suitable discharge flowing into the flume. Measure the discharge using the orifice meter.

2. Adjust the tail water gate to have an average water depth of 35cm.

3. Wait to reach the steady state, and then measure the water depth at a section near

the middle of the flume using the point gage. Place the current meter at the measured section.

4. Divide the section into 3 columns each column is divided into 4 strips. Note the

area of each strip. Place the current meter’s measuring disk in the middle of each strip and record the velocity from the screen after the 25 sec count is finished.

VI- results:

1. Plot on the same plot the vertical velocity distribution for each column. 2. Plot on the same plot the horizontal velocity distribution for each row. 3. Make and plot the velocity contours for the cross section (to scale). 4. Calculate the discharge in each strip (equals the velocity at the middle of the strip

multiplied by the strip’s area), and then add all strips together to get the total discharge flowing through the section.

5. Compare the calculated discharge using the current meter with the one measured

with the orifice meter.

y

30 cm


Experiment (3) Rapidly and Gradually Varied Flow

I- Introduction: In most practical cases, the cross section, depth and velocity in a channel vary along the channel and the uniform flow conditions are not often reached. Non-uniform flow usually forms when there is an obstruction in the direction of flow. If the resulting change in depth is abrupt, then the case is rapidly varied flow otherwise it is gradually varied flow. According to the specific energy principle, when a control structure like a gate or a weir is introduced into the open channel flow, water accumulates upstream the structure to save enough energy to pass through the resulting constriction. In this case, the water surface profile upstream the control is a non uniform gradually varied flow. The increased water depth in the channel upstream the structure should be considered when designing the channel cross section or when planning to feed secondary channels. There are many methods to calculate the free surface backwater profile, among these methods are the step by step method, graphical method, and direct integration method. An open channel flow can change from subcritical to supercritical in a relatively ‘low loss’ manner. On the other hand, the transition from supercritical flow to subcritical flow is characterized by a strong dissipative mechanism called the hydraulic jump. The flow within the hydraulic jump is extremely turbulent, it’s characterized by the development of large-scale turbulence, surface waves and spray, energy dissipation and air entrainment. The basic equations use the momentum principle to evaluate the flow properties of the hydraulic jump (Belanger equations). II- Objective: The objective of this experiment is to create gradually and rapidly varied flow inside the laboratory flume, measure, calculate, and draw the water surface profile of the formed backwater curve. Also, understand the characteristics of the hydraulic jump and how they are related to the upstream Froude number. The various assumptions used in the basic equations describing the hydraulic jump can be validated from actual measurements. Also the effect of both the sluice gate opening and the tail water gate level on the location and strength (size) of the hydraulic jump will be studied.

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 III- Anticipated Results: The student should be able to:

a. Measure the backwater curve formed upstream of a sluice gate for different discharge values and compare it with the computed profile using the step by step and the approximate methods.

b. Measure the hydraulic jump length, upstream and downstream depths and compare them with those calculated using Belanger equations.

c. Calculate the energy lost through the hydraulic jump as a percentage of the original flow energy before the jump.

d. Study the effect of both the sluice gate opening and the tail water gate level on the hydraulic jump location and size.

e. Differentiate between subcritical and supercritical flow. IV- Experimental Setup and Tools used: The same ArmField flume used in Experiment-1 will be used in this experiment. A sluice gate will be installed along the flume to develop the water profile shown in the figure below.

Tail Water Gate d Cc.d

M3

M1 Sluice Gate

y1m

y2m So


VI- Experimental Procedure:

1. Adjust the bed slope of the flume to be (1/200) 2. Open the inlet valve to allow the flow to pass through the experimental flume

and measure the discharge by the orifice meter.

3. Use sluice gate and tail gate at the end of the flume to create a free hydraulic jump and the water surface profiles within the flume.

4. Measure the water depths y1m and y2m (the two conjugate depths) and the

hydraulic jump length (Ljm).

5. M1, is created before the gate, M3 is created after the gate and before the jump.

6. For each water surface profile, measure from three to five points (depths and distances) and then draw each water surface profile to scale

7. Raise the tail water gate at the downstream end of the flume and observe the

position and size of the jump.

8. Reduce the gate opening and observe the location and size of the hydraulic jump.

Supercritical flow Subcritical flow

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 VII- Results:

1. Calculate the normal depth (yo) (n= 0.011 metric), and the critical depth (yc).

2. Calculate Froude number at the upstream depth (y1m) and the downstream depth (y2m) of the jump.

3. Compare the measured (y2m) with the theoretical y2 calculated from:

4. Calculate the head lost in the jump:

⎟⎟⎠

⎞⎜⎜⎝

⎛+−⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

gv

yg

vyh mmL 22

22

2

21

1

5. Compare the measured length of the jump to the length given by the empirical

formula:

Lj = (5 to 7)(y2m – y1m) 6. M1 is calculated starting from downstream (start with the water depth just

upstream the gate), while M3 is calculated starting from upstream (start with the water depth equals Cc*d). Use the step by step method.

7. Compare the calculated and the measured water surface profiles

8. At each measurement point, calculate the total energy and then draw the total

energy line.

9. Do the hydraulic jump size and location change by changing the tail water gate level?

10. Do the hydraulic jump size and location change by changing the gate opening?

11. Why the water surface after the jump contains more surface waves and is

unstable compared to the flow before the gate?

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛++−=

3

1

12 811

2 m

cm

yyyy


Experiment (4) Discharge Measurement

I- Introduction: The need for measuring the discharge in streams and rivers arises from the value of water to community. Accurate flow measurements are important for any water resources project. The fixed relationship between depth and discharge that marks the critical flow makes this type of flow a convenient basis for discharge measurement. Based on that principle, various devices for flow measurement have been developed. In such devices, the critical depth is usually created either by constructing a hump on the channel bottom such as a weir or by producing a contraction in the cross section, such as a critical flow flume. The fixed depth-discharge relationship in such devices depends on the physical nature of the structure. Once it’s known, it’s easy to measure the flowing discharge upon measuring the water depth at the defined section. II- Objective: The objective of this experiment is to measure the flow rate in an open channel using different open channel flow measuring structures. III- Anticipated Results: The students should be able to:

1. Setup critical flow in an open channel using a weir and a venturi flume. 2. Measure the discharge in the open channel using the given equations for each

structure and compare it with the measured discharge from the orifice meter to get the percent error for various flow rates.

3. Find a better value for the coefficient of discharge in each equation to better fit

the data.

CAIRO UNIVERSITY HYDRAULICS Faculty of Engineering 3rd year Civil Eng. Irrigation & Hydraulics Department 2010 – 2011 A- Weir: A structure used for measuring the discharge by making use of a combination of the phenomena of critical depth and hydraulic jump. The discharge is given by:

Q= 2.05 B H1.5

IV- Experimental Procedure: 1. Adjust the bed slope of the flume to be (1/200). 2. Open the inlet valve to allow the flow to pass through the experimental flume and

measure the discharge by the orifice meter. 3. Measure the head upstream the weir (H) and the weir length (B). 4. Calculate the discharge from: Q = 2.05 B H1.5 5. Compare the measured discharge with the calculated discharge. 6. Repeat for two other values of (Q) and find the constant in the weir equation. 7. Raise the tail water level gate till the weir becomes submerged and try to measure

the discharge. B- Venturi Flume: It consists of a bell-mouthed entry; throat, and down stream diverging portion. There are two conditions of flow through the Venturi Flume; the submerged condition, and the free condition. Free Condition: Transition from sub critical to supercritical flow occurs at the throat, followed by a hydraulic jump down stream of the throat. The General discharge equation reads:

Q = 1.7 b H1.5

H Q H.J

So


IV- Experimental Procedure:

1. Adjust the bed slope of the flume to be (1/400). 2. Open the inlet valve to allow the flow to pass through the experimental flume

and measure the discharge by the orifice meter.

3. Measure the head upstream the contraction (H) and the minimum width (b).

4. Calculate the discharge from: Q = 1.7 b H1.5.

5. Compare the measured discharge with the calculated discharge.

H

So

b B

Elevation

Plan


APPENDIX

Flow Rate Chart

For 90 mm Orifice meter

laboratory experiments - · pdf filethe flow within the hydraulic jump is extremely turbulent,...

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