Open channel flow - AbsoluteAstronomy.com

Open-Channel Flow, a branch of Hydraulics

Hydraulics

Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...

, is a type of liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

flow within a conduit with a free surface, known as a channel

Stream channel

Stream channel is the physical confine of a stream consisting of a bed and stream banks.Stream channels exist in a variety of geometries. The stream channel development is controlled by both water and sediment movement. There is a difference between low gradient streams and high gradient streams...

. The other type of flow within a conduit is pipe flow

Pipe flow

Pipe flow, a branch of Hydraulics and Fluid Mechanics, is a type of liquid flow within a closed conduit . The other type of flow within a conduit being open channel flow....

. These two types of flow are similar in many ways, but differ in one important respect; the free surface. Open-channel flow has a free surface

Free surface

In physics, a free surface is the surface of a fluid that is subject to constant perpendicular normal stress and zero parallel shear stress,such as the boundary between two homogenous fluids,for example liquid water and the air in the Earth's atmosphere...

, whereas pipe flow does not.

Classifications of Flow

Open-channel flow can be classified and described in various ways based on the change in flow depth with respect to time and space. The fundamental type of flow dealt with in open-channel hydraulics.

Time as the Criterion

Steady Flow – The depth of flow does not change over time, or if it can be assumed to be constant during the time interval under consideration.

Unsteady Flow - The depth of flow does change with time.

Space as the Criterion

Uniform Flow – The depth of flow is the same at every section of the channel. Uniform flow can be steady or unsteady, depending on whether or not the depth changes with time, (although unsteady uniform flow is rare).

Varied Flow - The depth of flow changes along the length of the channel. Varied flow technically may be either steady or unsteady (although unsteady varied flow is a practically impossible condition).

Varied flow can be further classified as either rapidly or gradually varied.

Rapidly varied flow - The depth changes abruptly over a comparatively short distance. Rapidly varied flow is known as local phenomenon. Examples are the hydraulic jump

Hydraulic jump

A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface...

and the hydraulic drop.

Gradually varied flow – The depth changes over a long distance.

Continuous flow - The discharge is constant throughout the reach

Reach (geography)

A reach in geography can mean several things. Most generally, a reach is any length of a stream between any two points. The points may be selected for any reason, such as gauging stations, river miles, natural features, and topography. They may be arbitrary or vague.A reach may also be an expanse,...

of the channel under consideration. This is often the case with steady flow. This flow is considered continuous and therefore can be described using the continuity equation for continuous steady flow.

Spatially varied or discontinuous flow - The discharge of a steady flow is nonuniform along a channel. This happens when water enters and/or leaves the channel along the course of flow. An example of flow entering a channel would be a road side gutter. An example of flow leaving a channel would be an irrigation channel. This flow can be described using the continuity equation for continuous unsteady flow requires the consideration of the time effect and includes a time element as a variable.

Summary of the Classification of Open-Channel Flow

Steady Flow

Uniform Flow

Varied Flow

Gradually Varied flow

Rapidly varied flow

Unsteady flow

Unsteady Uniform Flow (rare)

Unsteady Flow (“unsteady varied flow” should be here, but it is a practically impossible condition, so it is known simply as unsteady flow)

Gradually Varied Unsteady flow

Rapidly varied Unsteady flow

States of Flow

The behavior of open channel flow is governed by the effects of viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

and gravity relative to the inertial

Inertia

Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

forces of the flow. Surface tension has a minor contribution, but does not play a significant enough role in most circumstances to be a governing factor.
Depending on the effect of viscosity relative to inertia, as represented by the Reynolds number, the flow can be either laminar

Laminar flow

Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...

, turbulent, or transitional

Laminar-turbulent transition

The process of a laminar boundary layer becoming turbulent is known as boundary layer transition. This process is an extraordinarily complicated process which at present is not fully understood...

Classifications of Flow

Summary of the Classification of Open-Channel Flow

States of Flow

See Also