Introduction to open channel flow - classification of open channels

Summary

In this video, the concept of open channels is explained, highlighting the differences between open channel flow and pipe flow. Open channels are exposed to the atmosphere and flow under gravity, while pipe flow occurs within closed conduits with external pressure. The video also covers classifications of open channels, such as natural vs. artificial, prismatic vs. non-prismatic, and rigid vs. mobile boundary channels. Additionally, it explores channel slopes, with small-slope channels being common in irrigation, while large-slope channels are found in drops and shoots. Viewers are invited to explore further in the next video and access downloadable notes for study.

Takeaways

• 😀 Open channels are defined as channels where the fluid's top surface is exposed to the atmosphere, unlike pipe flow which occurs within a closed system.
• 😀 In pipe flow, the fluid is driven by external pressure, whereas in open channel flow, gravity causes the fluid to move.
• 😀 The maximum velocity in pipe flow occurs at the center of the pipe, whereas in open channel flow, it occurs just below the free water surface.
• 😀 The hydraulic gradient line (HGL) in pipe flow does not coincide with the water surface, but in open channel flow, it always aligns with the free surface.
• 😀 Surface roughness in pipe flow is determined by the pipe material, while in open channel flow, it depends on the depth and geometry of the channel.
• 😀 Pipe flow examples include drainage and sewer systems, while open channels include natural streams, rivers, and irrigation canals.
• 😀 Open channels can be classified into two types: natural channels (formed naturally) and artificial channels (man-made).
• 😀 Prismatic channels have a fixed, constant cross-section, typically seen in artificial channels, while non-prismatic channels have varying cross-sections, commonly seen in natural channels.
• 😀 Rigid boundary channels have immovable boundaries (e.g., concrete-lined canals), while mobile boundary channels have boundaries that are subject to erosion (e.g., unlined earth canals).
• 😀 Small slope channels have a bottom slope of less than 1 in 10, commonly seen in regular canals, while large slope channels have a slope greater than 1 in 10, such as drops and shoots in irrigation systems.

Q & A

What is an open channel?

An open channel is a channel where the top surface of the fluid is open to the atmosphere, such as rivers, streams, or canals.

How is pipe flow different from open channel flow?

Pipe flow occurs in a closed system, where fluid moves due to external pressure. Open channel flow occurs in an open system and is driven by gravity, with the top surface exposed to the atmosphere.

What is the main driving force behind fluid movement in an open channel?

In an open channel, the fluid moves under the influence of gravity, with no external pressure needed.

What is the role of hydraulic pressure in pipe flow?

In pipe flow, the fluid moves due to the difference in hydraulic pressure applied externally, which forces the fluid to move through the closed conduit.

Where is the maximum velocity located in pipe flow compared to open channel flow?

In pipe flow, the maximum velocity occurs at the center of the pipe, while in open channel flow, the maximum velocity occurs slightly below the water surface.

What is the difference in cross-sectional shapes between pipe flow and open channel flow?

Pipe flow typically has a circular cross-section, whereas open channels can have various geometries like trapezoidal, rectangular, or triangular.

What is the hydraulic gradient line (HGL), and how does it differ in pipe flow and open channel flow?

The hydraulic gradient line represents the sum of pressure head and datum head. In pipe flow, it does not coincide with the water surface, while in open channel flow, it always coincides with the free water surface.

What is the impact of surface roughness in pipe flow and open channel flow?

In pipe flow, surface roughness depends on the material of the pipe, whereas in open channel flow, it depends on the geometry and depth of the channel.

What are the main types of open channels based on their formation?

Open channels can be classified into natural channels (formed by nature, like rivers) and artificial channels (constructed by humans, like irrigation canals).

What are prismatic and non-prismatic channels?

Prismatic channels have a fixed, constant cross-section, usually found in artificial channels. Non-prismatic channels have varying cross-sections, typically seen in natural channels.

What defines rigid and mobile boundary channels?

Rigid boundary channels have immovable boundaries, such as lined concrete canals, while mobile boundary channels have boundaries that can erode or change due to the flow, like unlined natural channels.

How are channels classified based on their slope?

Channels are classified as small slope channels if the bottom slope is less than 1 in 10, and large slope channels if the slope is greater than 1 in 10. Small slope channels include regular canals, while large slope channels include steep channels like chutes.