Ansys Fluent can model the effects of open channel flow (for example, rivers, dams, and surface-piercing structures in unbounded stream) using the VOF formulation and the open channel boundary condition. These flows involve the existence of a free surface between the flowing fluid and fluid above it (generally the atmosphere). In such cases, the wave propagation and free surface behavior becomes important. Flow is generally governed by the forces of gravity and inertia. This feature is mostly applicable to marine applications and the analysis of flows through drainage systems.
Open channel flows are characterized by the dimensionless Froude Number, which is defined as the ratio of inertia force and hydrostatic force.
 | (14–36) |
where 
is the velocity magnitude, 
is gravity, and 
is a length scale, in this
case, the distance from the bottom of the channel to the free surface.
The denominator in Equation 14–36 is the
propagation speed of the wave. The wave speed as seen by the fixed
observer is defined as
 | (14–37) |
Based on the Froude number, open channel flows can be classified in the following three categories:
When
, that is, 
<
(therefore 
< 0 or 
> 0),
the flow is known to be subcritical where disturbances
can travel upstream as well as downstream. In this case, downstream
conditions might affect the flow upstream.When
(therefore 
= 0), the
flow is known to be critical, where upstream propagating waves remain
stationary. In this case, the character of the flow changes.When
, that is, 
>
(therefore 
> 0), the flow is known to be supercritical where disturbances cannot travel upstream. In this case, downstream
conditions do not affect the flow upstream.
There are two options available for the upstream boundary condition for open channel flows:
pressure inlet
mass flow rate
The total pressure 
at the inlet
can be given as
 | (14–38) |
where 
and 
are the position vectors of the face centroid
and any point on the free surface, respectively, Here, the free surface
is assumed to be horizontal and normal to the direction of gravity. 
is the gravity vector, 
is the gravity magnitude, 
is the unit vector of gravity, 
is the velocity magnitude, 
is
the density of the mixture in the cell, and 
is the reference density.
From this, the dynamic pressure 
is
 | (14–39) |
and the static pressure 
is
 | (14–40) |
which can be further expanded to
 | (14–41) |
where the distance from the free surface to the reference position, 
, is
 | (14–42) |
In open channel flows, Ansys Fluent internally calculates the volume fraction based on the input parameters specified in the boundary conditions dialog box, therefore this option has been disabled.
For subcritical inlet flows (Fr < 1), Ansys Fluent reconstructs the volume fraction values on the boundary by using the values from the neighboring cells. This can be accomplished using the following procedure:
Calculate the node values of volume fraction at the boundary using the cell values.
Calculate the volume fraction at the each face of boundary using the interpolated node values.
For supercritical inlet flows (Fr > 1), the volume fraction value on the boundary can be calculated using the fixed height of the free surface from the bottom.
Determining the static pressure is dependent on the pressure specification method:
Free Surface Level: The static pressure is dictated by Equation 14–40 and Equation 14–42.
For subcritical outlet flows (Fr <1), the static pressure is taken from the pressure profile specified over the boundary, otherwise the pressure is taken from the neighboring cell. For supercritical flows (Fr > 1), the pressure is always taken from the neighboring cell.
From Neighboring Cell: The static pressure is always taken from the neighboring cell.
Gauge Pressure: The static pressure is a user-specified value.
Outflow boundary conditions can be used at the outlet of open channel flows to model flow exits where the details of the flow velocity and pressure are not known prior to solving the flow problem. If the conditions are unknown at the outflow boundaries, then Ansys Fluent will extrapolate the required information from the interior.
It is important, however, to understand the limitations of this boundary type:
You can only use single outflow boundaries at the outlet, which is achieved by setting the flow rate weighting to
1. In other words, outflow splitting is not permitted in open channel flows with outflow boundaries.There should be an initial flow field in the simulation to avoid convergence issues due to flow reversal at the outflow, which will result in an unreliable solution.
An outflow boundary condition can only be used with mass-flow inlets. It is not compatible with pressure inlets and pressure outlets. For example, if you choose the inlet as pressure-inlet, then you can only use pressure-outlet at the outlet. If you choose the inlet as mass-flow-inlet, then you can use either outflow or pressure-outlet boundary conditions at the outlet. Note that this only holds true for open channel flow.
Note that the outflow boundary condition assumes that flow is fully developed in the direction perpendicular to the outflow boundary surface. Therefore, such surfaces should be placed accordingly.
In certain applications, it is desirable to suppress the numerical reflection caused by an outlet boundary for passing waves. To avoid wave reflection, a damping sink term is added in the momentum equation for the cell zone in the vicinity of the pressure outlet boundary [505], [742].
 | (14–44) |
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The scaling factors in the 
and 
directions
are defined by Equation 14–45 and Equation 14–46, respectively:
 | (14–45) |
 | (14–46) |
where the damping functions in the 
and 
directions,
respectively, are
 | (14–47) |
 | (14–48) |
In Equation 14–45, 
and 
are the start and end points of the damping
zone in the 
direction.
In Equation 14–46, 
and 
are the free surface and bottom level along the 
direction.
To include numerical beach in your simulation, see Numerical Beach Treatment for Open Channels.