To Study the Flow Characteristics Over the Hump or Weir in a Rectangular Channel.

Objectives:

1.     To study the variation in the flow with the introduction of different types of humps in the flow.

2.     To draw the water surface profile over the hump.

Apparatus:

—  S6 glass sided Tilting lab flume with slope adjusting scale.

—  Point gauge (For measuring depth of channel)

—  Broad crested weir (Round corner and Sharp corner)

Related theory

Wier: 

            It is the streamlined wall or structure constructed across a river or a stream at a suitable location. It is commonly used to raise the water level at a river or stream to divert the required amount of water into an off taking canal. Weirs can be gated or un-gated. Gated weir is called as BARRAGE.

Function of a weir:

i)                    A barrier used to alter its flow characteristics (level on u/s & d/s) and it can alter floods.

ii)                  It is also used to measure discharge.  

      

Hump:

            It is a streamline construction provided at the bed of channel. It is a local rise given to the channel bed. When the water or fluid is flowing over the hump the behavior of free surface is sharply different according to whether the approach flow is sub-critical or super-critical.

Effect of the hump height over the flow conditions:  

            Height of hump is less than critical hump height then there will be sub critical flow over the hump, downstream of the hump and upstream of the hump. Depth of flow over the hump will decrease by a certain amount as there is a slight depression in the water.

Further increase in the height of hump will create more depression of water surface over the hump until finally the depth becomes equals to the critical depth. When the hump height will be equal to the critical depth then there will be critical flow over the hump, sub critical on the upstream side and super critical just downstream of the hump.

If the hump is made still higher, critical depth will maintain over the hump and depth on upstream side will be increased. This phenomenon is referred to as damming action.

Critical Hump Height: 

It is the minimum hump height that can cause the critical depth over the hump is called as critical hump height.

CASE 1     When Z <<< Zc and y2 >>> yc

•               The flow conditions will be sub critical

•               Upstream level increases

•               Over hump y2 > yc

•               At downstream depth is recovered after a long distance.

Figure 3.1: Hump Height <<< Critical Hump Height

CASE 2     When Z < Zc and y2 > yc

•               The flow conditions will be sub critical

•               Upstream level increases

•               Over hump y2 > yc

•               At downstream depth is recovered after a long distance than case-1

Figure 3.2: Hump Height < Critical Hump Height

CASE 3     When Z = Zc

•               Upstream level increases

•               Over hump y2 = yc

Figure 3.3: Hump Height = Critical Hump Height

CASE 4     When Z > Zc

•               Afflux on upstream side (damming action)

•               y1 > y3 and y2 = yc

Figure 3.4: Hump Height > Critical Hump Height

Damming Action:

            It is the sudden increase of the water depth at upstream side due to increase in hump height.

Procedure:

1.      Fix the slope of the flume

2.      Introduce a round corner wide crested weir in the flume at certain location

3.      Set the discharge in the flume having certain value.

4.      Note depth of flow at upstream side of hump, over the hump and downstream side of hump at certain point.

5.      Repeat steps 2-4 for the other discharges

6.      Repeat the same procedure for sharp cornered wide crest weir

7.      Predict the type of flow at every section

8.      Compare depths with critical depth for every discharge value and report the type of flow.

9.      Draw flow profile over the hump for both types of humps.

Precautions:

·         Take discharge readings only when flow is steady.

·         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

Type of weir	Length(mm)	Width(mm)	Height(mm)
Round corner
Sharp corner
Wedged Shaped

Width of channel = _______________

Length of Channel = ______________

Table of Calculations

Surface Profiles

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