When the analysis type is Open Channel Flow, an important step in the setup is to indicate which portion of the model is initially filled with fluid (or which portion is filled with gas). This is done with the Initial Fluid Volumes branch in the tree view. Right-click the Initial Fluid Volumes branch and choose Add New Hexahedral Region. This accesses the Initial Fluid Volume (Hexahedral) dialog from which you specified an 8-point region in space which is initially fluid or gas.
On the Initial Fluid Volume (Hexahedral), set the Fluid options drop-down to one of the following options:
Multiple regions can be created. The union of all the regions determine where the fluid and gas starts at time 0. The Enable check box controls whether the region will be used in the computations (checked) or not (not checked).
The initial fluid volume is specified using 8-points that define a hexahedral region (see Figure 1). The points can be specified using either of these methods:
Figure 1: 8-Point Initial Fluid Volume
When the analysis is started, each element is assigned a fluid volume between 0 (no fluid) and 1 (completely filled with fluid) based on the intersection of the element with each of the initial fluid volumes.