Answer:

Open channel flow refers to the flow of water in channels where the surface of the water is exposed to the atmosphere. It is commonly seen in rivers, canals, and streams. Measuring the velocity of open channel flow is important for various engineering and environmental applications. Among the options provided, the correct answer is d) Current meter.

Explanation:

1. Pitot tube:
A Pitot tube is a device used to measure the velocity of a fluid flow, typically in closed pipes. It works based on the principle of Bernoulli's equation, which relates the pressure difference between the static and dynamic pressure to the fluid velocity. However, Pitot tubes are not commonly used for measuring velocity in open channel flow.

2. Bend meter:
A bend meter, also known as a bend orifice, is a device used to measure the flow rate in open channels. It consists of a bend in the channel with a pressure tap located at the throat of the bend. By measuring the pressure difference across the bend, the flow rate can be determined. However, it does not directly measure the velocity of the flow.

3. Venturimeter:
A Venturimeter is another device used to measure the flow rate in open channels. It consists of a converging-diverging section in the channel, which causes a pressure difference. By measuring the pressure difference, the flow rate can be calculated. Similar to the bend meter, it does not directly measure the velocity of the flow.

4. Current meter:
A current meter is a device specifically designed to measure the velocity of flow in open channels. It is commonly used in hydrology and hydraulic engineering to determine the flow characteristics of rivers, streams, and canals. The current meter typically consists of a propeller or impeller that rotates when subjected to the flow. The rotation is then converted into a velocity measurement using calibration factors. The current meter provides a direct measurement of the flow velocity, making it suitable for open channel flow measurements.

In conclusion, among the options provided, the current meter is the most appropriate device for measuring velocity in open channel flow. It provides accurate and direct measurements of flow velocity, making it a valuable tool for hydraulic engineering and hydrological studies.