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23.10.2 Modeling Open Channel Flows

Using the VOF formulation, open channel flows can be modeled in FLUENT. To start using the open channel flow boundary condition, perform the following:

1.   Turn on gravity.

(a)   Open the Operating Conditions panel.

Define $\rightarrow$ Operating Conditions...

(b)   Turn on Gravity and set the gravitational acceleration fields.

2.   Enable the volume of fluid model.

(a)   Open the Multiphase Model panel.

Define $\rightarrow$ Models $\rightarrow$ Multiphase...

(b)   Under Model, turn on Volume of Fluid.

(c)   Under VOF Scheme, select either Implicit, Explicit.

3.   Under VOF Parameters, select Open Channel Flow.

In order to set specific parameters for a particular boundary for open channel flows, turn on the Open Channel Flow option in the corresponding boundary condition panel. Table  23.10.1 summarizes the types of boundaries available to the open channel flow boundary condition, and the additional parameters needed to model open channel flow. For more information on setting boundary condition parameters, see Chapter  7.


Table 23.10.1: Open Channel Boundary Parameters for the VOF Model
Boundary Type Parameter
pressure inlet Inlet Group ID;
Secondary Phase for Inlet;
Flow Specification Method;
Free Surface Level, Bottom Level;
Velocity Magnitude
pressure outlet Outlet Group ID;
Pressure Specification Method;
Free Surface Level; Bottom Level
mass flow inlet Inlet Group ID;
Secondary Phase for Inlet;
Free Surface Level;
Bottom Level
outflow Flow Rate Weighting



Defining Inlet Groups


Open channel systems involve the flowing fluid (the secondary phase) and the fluid above it (the primary phase).

If both phases enter through the separate inlets (e.g., inlet-phase2 and inlet-phase1), these two inlets form an inlet group. This inlet group is recognized by the parameter Inlet Group ID, which will be same for both the inlets that make up the inlet group. On the other hand, if both the phases enter through the same inlet (e.g., inlet-combined), then the inlet itself represents the inlet group.

figure   

In three-phase flows, only one secondary phase is allowed to pass through one inlet group.



Defining Outlet Groups


Outlet-groups can be defined in the same manner as the inlet groups.

figure   

In three-phase flows, the outlet should represent the outlet group, i.e., separate outlets for each phase are not recommended in three-phase flows.



Setting the Inlet Group


For pressure inlets and mass flow inlets, the Inlet Group ID is used to identify the different inlets that are part of the same inlet group. For instance, when both phases enter through the same inlet (single face zone), then those phases are part of one inlet group and you would set the Inlet Group ID to 1 for that inlet (or inlet group).

In the case where the same inlet group has separate inlets (different face zones) for each phase, then the Inlet Group ID will be the same for each inlet of that group.

When specifying the inlet group, use the following guidelines:



Setting the Outlet Group


For pressure outlet boundaries, the Outlet Group ID is used to identify the different outlets that are part of the same outlet group. For instance, when both phases enter through the same outlet (single face zone), then those phases are part of one outlet group and you would set the Outlet Group ID to 1 for that outlet (or outlet group).

In the case where the same outlet group has separate outlets (different face zones) for each phase, then the Outlet Group ID will be the same for each outlet of that group.

When specifying the outlet group, use the following guidelines:

figure   

For three-phase flows, when all the phases are leaving through the same outlet, the outlet should consist only of a single face zone.



Determining the Free Surface Level


For the appropriate boundary, you need to specify the Free Surface Level value. This parameter is available for all relevant boundaries, including pressure outlet, mass flow inlet, and pressure inlet. The Free Surface Level, is represented by $y_{\rm local}$ in Equation  23.3-25.


 y_{\rm local} = - (\overrightarrow{a} \cdot \hat{g}) (23.10-1)

where $\overrightarrow{a}$ is the position vector of any point on the free surface, and $\hat{g}$ is the unit vector in the direction of the force of gravity. Here we assume a horizontal free surface that is normal to the direction of gravity.

We can simply calculate the free surface level in two steps:

1.   Determine the absolute value of height from the free surface to the origin in the direction of gravity.

2.   Apply the correct sign based on whether the free surface level is above or below the origin.

If the liquid's free surface level lies above the origin, then the Free Surface Level is positive (see Figure  23.10.1). Likewise, if the liquid's free surface level lies below the origin, then the Free Surface Level is negative.



Determining the Bottom Level


For the appropriate boundary, you need to specify the Bottom Level value. This parameter is available for all relevant boundaries, including pressure outlet, mass flow inlet, and pressure inlet. The Bottom Level, is represented by a relation similar to Equation  23.3-25.


 y_{\rm bottom} = - (\overrightarrow{b} \cdot \hat{g}) (23.10-2)

where $\overrightarrow{b}$ is the position vector of any point on the bottom of the channel, and $\hat{g}$ is the unit vector of gravity. Here we assume a horizontal free surface that is normal to the direction of gravity.

We can simply calculate the bottom level in two steps:

1.   Determine the absolute value of depth from the bottom level to the origin in the direction of gravity.

2.   Apply the correct sign based on whether the bottom level is above or below the origin.

If the channel's bottom lies above the origin, then the Bottom Level is positive (see Figure  23.10.1). Likewise, if the channel's bottom lies below the origin, then the Bottom Level is negative.

Figure 23.10.1: Determining the Free Surface Level and the Bottom Level
figure



Specifying the Total Height


The total height, along with the velocity, is used as an option for describing the flow. The total height is given as


 y_{\rm tot} = y_{\rm local} + \frac{V^2}{2g} (23.10-3)

where $V$ is the velocity magnitude and $g$ is the gravity magnitude.



Determining the Velocity Magnitude


Pressure inlet boundaries require the Velocity Magnitude for calculating the dynamic pressure at the boundary. This is to be specified as the magnitude of the upstream inlet velocity in the flow.



Determining the Secondary Phase for the Inlet


For pressure inlets and mass flow inlets, the Secondary Phase for Inlet field is significant in cases of three-phase flows.

figure   

Note that only one secondary phase is allowed to pass through one inlet group.

Consider a problem involving a three-phase flow consisting of air as the primary phase, and oil and water as the secondary phases. Consider also that there are two inlet groups:

For the former inlet group, you would choose water as the secondary phase. For the latter inlet group, you would choose oil as the secondary phase.



Choosing the Pressure Specification Method


For a pressure outlet boundary, the outlet pressure can be specified in one of two ways:

figure   

This option is not available in the case of three-phase flows since the pressure on the boundary is taken from the neighboring cell.



Limitations


The following list summarizes some issues and limitations associated with the open channel boundary condition.



Recommendations for Setting Up an Open Channel Flow Problem


The following list represents a list of recommendations for solving problems using the open channel flow boundary condition:


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