To Study the Flow Characteristics of Hydraulic Jump Developed in Lab Flume.

Objectives:

1.      To physically achieve the hydraulic jump in the lab.

2.      To measure the dimensions of hydraulic jump physically.

3.      To calculate the energy loss due to hydraulic jump.

4.      To plot the hydraulic jump at different Froude no.

Apparatus:

1. —  S6 glass sided Tilting lab flume with slope adjusting scale.
2. —  Point gauge (For measuring depth of channel)
3. —  Broad crested weir

Related theory

Hydraulic Jump:      

            The rise of water level which takes place due to transformation of super critical flow to sub critical flow is termed as hydraulic jump as shown in figure 4.1.

Figure 4.1: Hydraulic Jump

Application of Hydraulic Jump:

•         To dissipate the energy of water flowing over the hydraulic structures and thus preventing scouring (vertical erosion) downstream of structures.
•          To recover head or raise the water level on the downstream of a hydraulic structure and thus to maintain high water level in the channel for irrigation or other water distribution purposes.
• To reduce uplift pressure under the structure by raising water depth.
•          To mix chemicals used for water filtration etc.

Length of Hydraulic Jump:         

            The length between two sections where one section is taken just before the hydraulic jump and second section is taken just after the hydraulic jump is termed as length of hydraulic jump.

Approximate length of hydraulic jump    =      5 -7 times depth of hydraulic jump

Conjugate Depth:

            The depth of flow just before and after the hydraulic jump are called as conjugate depths

 (y1 & y2).

Depth of Hydraulic Jump:

The difference between the depth of flow before and after the hydraulic jump and it is equal to              D.

Y1 = depth of flow just before the formation of hydraulic jump.

Y2 = depth of flow after the end of hydraulic jump.

Energy Loss:

           The loss of energy in hydraulic jump is equal to difference in specific energy before and after the jump.

                  Simplifying

Location of Hydraulic Jump:

Location of hydraulic jump is governed by two factors,

I)    d₂                (Depth of flow just after the hydraulic jump)

II)   y₂               (Normal depth of flow on downstream side of hydraulic structure)

CASE – 01  When d₂ < Y₂

                                                Figure 4.2: Hydraulic Jump when d₂<Y₂

In Case – 01, Hydraulic jump will be formed over the glaces of hydraulic structure as shown in the figure 4.2 and it will be weak jump/Submerged jump.

Hydraulic jump develops at upstream of channel.

It is developed when channel bed slope is steep.

CASE-02   When d₂=Y₂

                                                Figure 4.3: Hydraulic Jump when d₂=Y₂

In Case – 02, Hydraulic jump will be formed on the toe of hydraulic structure as shown in the figure 4.3 and it will be a relatively strong jump than Case - 01.

This is an ideal condition and hydraulic jump develops at toe.

CASE – 03  When d₂ > Y₂

                                                Figure 4.4: Hydraulic Jump when d₂>Y₂

• In Case – 03, Hydraulic jump will be formed ahead of hydraulic structure as shown in the figure 4.4.  And it will be a relatively strong jump as compared to Case – 01 and Case – 02.
• Comparatively, Case – 02 is ideal case with sufficient energy dissipation and structure will also be safe (because jump will be formed at the toe of structure).
• Hydraulic jump develops at downstream side of channels.
• It develops in channel having mild slope.

Instrumentation error

•   Leakage from the flume
• Assumptions of ideal conditions did not prevail:
•   Ideal conditions which prevailed in the theoretical equations were not there and frictional forces also had some effect on the experiment.

Classification of Hydraulic Jump

It can be classified on the base of Froude no. just upstream of the jump which is called as approach Froude no.

1)     Undular Jump __________________ (1 < FR1 < 1.7)

2)     Weak Jump ____________________ (1.7 < FR1 < 2.5)

3)     Oscillating Jump ________________ (2.5 < FR1 < 4.5) When FR1 is within 2.5-4.5 there is an oscillating jet entering the hydraulic jump from the bottom and back again with no prodicity and it produces large waves of irregular period which travels from miles doing damage to the earth bank.

4)     Stable Jump ____________________ (4.5 < FR1 < 9)            Dissipation of energy = 45-70%       Considerable rise in D/S water level

5)     Strong or Rough Jump ____________ (FR1 > 9)           Dissipation of energy = 85%

Procedure:

•    Adjust the S-6 Tilting flume at a slope and check if there is any problem in arrangement or anything residual inside the flume causing obstruction in flow.
•  Setup a specific discharge in the flume.
•   Note down the normal depth and depth of the water surface before, after the hydraulic jump and note corresponding distances.
•   Repeat the above procedure with by increasing discharge.
• Complete the table of observations and calculations and plot the water surface profile for all discharges.

Precautions:

• ·        The reading measurement at the hydraulic jump is difficult, so note the flow carefully and           take the reading at desired point.
• ·        The height should not be measured near the joints or at points where there is turbulence in         flume.
• ·        The height measuring needle must be adjusted precisely.
• ·        The tip of the needle must be just touching the water surface while taking observations.

Observation and Calculation

Width of channel = _______________

Length of Channel = ______________

Table of Calculations

Hydraulic Jump Profiles

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