16.1.3. Indian Institute of Technology-Guwahati

Today let us discuss on open channel flow. This is the last class on open channel flow. As we discuss about the specific energy and today we will solve a few problems as well as we will discuss about hydraulic jump and the best hydraulic cross sections what is required for designing a canal structures.

In this introduction, the focus is on 'open channel flow', which is the flow of fluid with a free surface, as opposed to flow in pipes. The speaker notes that this session will cover specific energy, solve problems, and discuss hydraulic jump and design of canal structures.

Think of a river or a stream; the water flows freely above the ground and is affected by gravity. Similar principles apply when we study how we can design canals, sluice gates, and spillways to manage the flow of water.

The basic concept what we use is we will talk about that the conservations of mass and energy equations. So these two equations as we consider for the one-dimensional flow that is what we have simplified it one-dimensional incompressible okay steady flow.

When studying fluid mechanics, especially in open channel flow, it's crucial to understand that we rely heavily on the principles of conservation of mass (continuity) and conservation of energy (Bernoulli's equation). In this context, the flow is simplified to be one-dimensional, meaning we mainly consider changes along the direction of flow and assume there are no changes in density (incompressible) or energy over time (steady flow).

Imagine a garden hose. When you pinch the hose (constraining it), water flows out faster from the end. This is a simple illustration of mass conservation—if the mass of water entering one section of the hose is restricted, the water must speed up to exit the narrowed section.

The basic idea to know it, the flow depth variations in open channels, the velocity variations, how the velocity changes it, how much of energy losses, okay. losses is happening it because of the flow and mostly it is governed by the gravity forces and the frictional forces as I discussed earlier.

Specific energy refers to the energy per unit weight of fluid, considering both the flow depth and velocity. In open channels, as flow depth (y) changes, the flow velocity (v) also varies due to mass conservation. As the depth decreases, the velocity increases, a relationship that also affects energy losses, often due to friction with the channel bottom and sides.

Consider how a stream gets faster and shallower as it flows over rocks; the changes in depth speed up the flow. This effectively illustrates the balance of energy as the stream tries to navigate its path; less depth means less volume, leading to faster speeds.

So I am to repeat it that subcritical flow okay as we discuss more details subcritical flow that means when you have a flow proud number lesser than 1. ... There will be a conditions will come it that this gate will come with a critical flow okay that is what is critical conditions will prevail it beyond that it will go sub critical super critical level.

Flow can be categorized into three main types based on the Froude number, which compares the inertia of the flow to the gravitational forces. Subcritical flow (F < 1) is calm and dominated by gravitational effects; critical flow (F = 1) occurs when the flow speed matches surface wave speed; supercritical flow (F > 1) is fast and more chaotic.

Think of a flowing river during heavy rain. When the river is calm and deep (subcritical), you can easily float a boat. As the rain makes the river rise and flow faster (supercritical), the waters may become turbulent and unmanageable. The difference in flow types illustrates how water reacts to changing conditions.

So this is to come back to the hydraulic charms how does it forms it. ... So when the flow passes through the supercritical to subcritical with a very limited ranges then there are a lot of turbulent structures created.

A hydraulic jump occurs when flow transitions from supercritical to subcritical, causing a sudden increase in flow depth and turbulence. This jump results in energy loss, often seen downstream of dams and weirs where fast-moving water slams into slower water, creating turbulence and increasing depth.

Imagine water cascading over a waterfall. As it plummets, it speeds up and becomes more turbulent. When it hits a calm pool below, the water's energy causes a dramatic splash and turbulence—a real-life example of a hydraulic jump.

Energy losses happen it when flow goes through the supercritical please remember supercritical to subcritical. ... So if I want to draw energy lines or energy gradient line.

In any hydraulic system, energy loss is an important parameter because it impacts the performance and efficiency of water conveyance structures. Energy gradients display how energy decreases due to losses, mainly occurring in hydraulic jumps, illustrating the relationship between flow depth and velocity at both the upstream and downstream sections.

Think about draining water from a bathtub. As the water rushes down the drain (supercritical flow), energy is lost due to turbulence and friction against the tub's surface, much like how energy is dissipated during hydraulic jumps.

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