Discharge Over Notches

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Discharge Over Notches is a fundamental experiment in fluid mechanics laboratories to measure the rate of flow of liquid over sharp-edged openings in open channels. This apparatus is commonly used in civil, mechanical, and hydraulic engineering labs to study the relationship between water head and discharge.

The study of discharge over notches is essential for understanding open channel hydraulics, designing irrigation channels, controlling water supply, and analyzing flow behavior. This experiment bridges the gap between theoretical fluid mechanics and practical engineering applications.

What is a Notch?

A notch is a carefully calibrated opening in a thin plate placed in a tank or channel. Water flows over the notch, and the flow rate is determined by measuring the head of water above the notch crest. The discharge depends on the shape and size of the notch.

Common types of notches include:

• Rectangular Notch: Suitable for large flow rates. Discharge depends on the width of the notch and head of water.

• V-Notch (Triangular Notch): Sensitive for small flow rates and accurate measurement.

• Trapezoidal Notch (Cipolletti Notch): Designed to reduce end contraction losses.

The Discharge Over Notches apparatus typically includes interchangeable notch plates to allow multiple experiments.

Principle of Discharge Measurement

The experiment is based on Bernoulli’s theorem, continuity equation, and energy conservation in open channel flow. When water flows over a notch, the velocity is related to the head of water above the crest, and the discharge can be calculated using empirical equations.

The general discharge formula is:

$$Q = C_d \times \text{Theoretical Discharge}$$

Where:

• $Q$ = Actual discharge

• $C_d$ = Coefficient of discharge

By measuring the head and calculating the theoretical discharge, students can determine the coefficient of discharge experimentally.

Types of Notches Used

1. Rectangular Notch

• Measures large flow rates

• Discharge depends on width and head

2. V-Notch

• Used for small flow rates

• High sensitivity

• Discharge is proportional to $H^{5/2}$, where $H$ is the head

3. Cipolletti Notch

• Trapezoidal shape

• Eliminates contraction effects

• Provides accurate discharge for moderate flows

The apparatus allows easy substitution of notch plates for comprehensive learning.

Construction of the Apparatus

A typical Discharge Over Notches apparatus is constructed for durability, accuracy, and ease of use:

1. Notch Tank

A tank made of stainless steel or acrylic with transparent sides for clear observation.

2. Notch Plates

Precision-machined V-notch, rectangular, and trapezoidal plates.

3. Point Gauge

Used to measure the head of water above the crest accurately.

4. Water Supply System

Includes inlet pipe, control valve, and outlet drain for steady flow.

5. Measuring Tank

Collects water to measure actual discharge using the volumetric method.

6. Supporting Frame

A robust frame ensures stability and proper alignment.

The design is user-friendly and suitable for repeated laboratory use.

Working Procedure

The experiment follows a systematic procedure:

1. Fill the notch tank with water.

2. Adjust the flow rate using the control valve.

3. Allow water to flow over the notch steadily.

4. Measure the head of water above the notch crest using the point gauge.

5. Collect the discharged water in a measuring tank to determine actual discharge.

6. Calculate theoretical discharge using standard equations.

7. Determine the coefficient of discharge $C_d$.

8. Repeat for different heads and notch types to study variations.

This method helps students correlate theoretical and experimental results effectively.

Experimental Calculations

Discharge equations vary with notch type:

Rectangular Notch

$$Q = \frac{2}{3} C_d b \sqrt{2g} H^{3/2}$$

V-Notch

$$Q = \frac{8}{15} C_d \sqrt{2g} \tan(\theta/2) H^{5/2}$$

Where:

• $b$ = Width of rectangular notch

• $H$ = Head of water above notch crest

• $g$ = Acceleration due to gravity

• $\theta$ = Angle of V-notch

These calculations reinforce theoretical learning with practical observations.

Applications

Educational Applications

• Civil and mechanical engineering labs

• Fluid mechanics teaching experiments

• Open channel flow demonstrations

• Determination of coefficient of discharge

Practical Applications

• Irrigation channel design

• Dams and reservoirs discharge calculation

• Water supply systems

• Wastewater treatment channels

The experiment provides real-world insights into open channel flow measurement.

Advantages

• Simple and robust construction

• Accurate discharge measurement for small and medium flow rates

• Easy observation of flow patterns

• User-friendly and cost-effective

• Suitable for repeated laboratory experiments

• Helps understand practical open channel hydraulics

• Reliable and reproducible results

These advantages make it a preferred apparatus in engineering labs.

Limitations

• Not suitable for very large flow rates

• Requires steady and uniform flow

• Sensitive to upstream disturbances

• Accurate head measurement is critical

Despite limitations, the apparatus is highly effective for laboratory education and training.

Importance in Engineering Education

The Discharge Over Notches apparatus enhances conceptual understanding of open channel flow. Students learn to:

• Correlate head with discharge

• Calculate coefficient of discharge

• Understand open channel hydraulics

• Analyze experimental errors

• Develop practical engineering skills

It is a foundational experiment before advancing to complex hydraulic structures and irrigation systems.

Quality and Reliability

High-quality apparatus is made of precision-machined notch plates, corrosion-resistant materials, and accurate point gauges. Leak-proof construction ensures long service life and reliable experimental results.

Trusted suppliers provide fully tested equipment suitable for engineering laboratories, colleges, and research institutions. The apparatus meets international standards for accuracy and durability.