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Internal Boundary Conditions - Piers

Donald Hendon

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I am running version 12.3.4. I have noticed that the velocity no longer goes to zero at the face of the pier. I know that with this last release, Aquaveo got rid of the "warping" effect due to the wet/dry element function, and I was wondering if this is a result of that change? Also, what are the consequences of it not going to zero?

Thanks in advance!

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  This changed as a result of update to SRH-2D 3.2. However, as I recall, the edge of the pier did not get assigned a zero velocity even in the old version.
  SRH-2D would assign values to the nodes from the cell centered values that it computes. The process used a simple averaging of the wet/active cells attached to each node.  So for a pier wall, the velocity from the cell centers was mapped to the edge of the mesh. 
  When 3.2 was released we detected that the node interpolation changed from the 3.1 and the activity mapping appeared to have an error.  We asked the SRH-2D developer to investigate, the response was to use the cell centered data.
  To do this, we added the SRH-Post utility that runs after SRH-2D runs.  SRH-2D creates an XMDFC.h5 file which includes values at cell centers and and cell based activity.  SRH-Post reads this file and performs the interpolation to node based set of data sets.
  The typical interpolation from cell center to cell center includes:
V(node) = Average( v(adjacent_wet_cells))   -   This is done for all datasets ( WSE, Depth, Vx, Vy, Froude, Bed Shear)
  The complications to this approach include:
1- Interpolated water surfaces - Interpolated depths do not equal the nodal Z value.
2- Interpolated water surfaces  at the node could be less than the nodal Z value (particularly when the node is at the edge of the mesh or on a lip of an embankment)
3- Interpolated velocity at the edge of a mesh would be too high because the "dry" cells are just ignored.
  Because of these observations, the old SRH-2D mapping to nodes would modify the basic interpolation to force the water level of a node attached to any active cell to be no lower than the Z value plus a minimum depth.  This solved a "dry embankment lip" issue, but it resulted in what we have called the "warped water surface" issue at the edge of the wet region.
  SRH-Post currently does the following:
a- Assigns activity for the nodes based on the rule that if any cell attached to a node is wet, the node is wet.  This means that isolated "dry" cells will become wet. (In the future we will just use the cell activity directly.) This activity is now mapped to ALL of the datasets created by SRH-Post rather than just the WSE dataset as was computed in SRH-2D.
b- Detects a lip as any node surrounded by all wet cells for which the (interpolated WSE - nodal Z) < (interpolated Depth/3).  For these nodes the interpolated WSE is replaced with a value of (nodal Z + minimum adjacent cell depth)
c- Detects a wet/dry interface as any node with at least one dry cell attached.  For these nodes, the intepolated Depth is replaced with the value of (Interpolated WSE - Z) (Note: this can be negative).  In addition, the velocity is reduced to a minimum of zero) based on the new depth.
I am interested in feedback on this approach.  If I have wrongly described how SRH-2D used to do the interpolation, please let me know.
If you have other thoughts, please let me know.
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Don has reminded me that the default "Wall" boundary does have velocities of zero. 

I will be revising SRH-Post to treat walls as zero-velocity while wet/dry boundaries will still be scaled.

In the future I hope to use both cell centered and edge values to give a more accurate representation of what SRH-2D is doing internally.

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