virtual flow sewerGems

Question

Hi to all, 
 In my project, because of dry condition I suppose virtual flow depth, LPI Coefficient, NR weighting coefficient, Relaxation Weighting Coefficient, NR Iterations, Computation Distance and Calculation Time Step in order equal 0.004 m, 1, 1, 15.2 m and 0.025.I attached graphs. I don't know exactly where is the boundary between the virtual and real water in flow graph?

Sushma Choure · Answer

The amount of virtual flow that the numerical model applies in the dry flow condition is very small and is invisible with no practical impact on the computational results. It adds a small base flow to every conduit which has a depth < the virtual depth value that you enter in the calculation options. 
 Here is the link to the support solution about virtual depth. 
 http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/22527.virtual-flow-depth-solution-500000063442 
 If this doesn't help then , you may want to upload a copy of your model for our review. 
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Jesse Dringoli · Answer

Hello Amir, 
 It looks like you're using an older version of SewerGEMS (though I cannot tell which version since only the .SQLITE file was provided). I tried computing the model as-is with the Implicit solver, but it was taking an extremely long time, so I cancelled and tried the SWMM solver. Performance with the SWMM solver was much better and seemed to give good results (low continuity error and low percentage of non-converged timesteps) . Here's a screenshot of flow in the pipe in question. 
 
 Generally speaking, the beginning of the simulation with a dynamic solver tends to have a higher chance of having instability, as the dry pipes first fill with water. Often you can ignore this, or attempt to use calculation option adjustments to prevent it. The Start Type in this case may have an effect, as well as the Virtual Flow Depth if the flow in question is very low. Here's an article on Virtual Flow Depth: 
 Purpose of the Virtual Flow Depth Calculation Option

Scott Kampa · Answer

Hello Amir, 
 
Yes, there can be cases where the SWMM solver will work better for a given system. If you see issues with the Implicit solver, it can be beneficial to try the SWMM solver to see how the results look. 
 
The following link has information on the different solvers available in SewerGEMS: communities.bentley.com/.../11852.sewercad-gvf-convex-solver-vs-sewergemscivilstorm-implicit-and-explicit-dynamic-solvers-vs-stormcad-gvf-rational-solver 
 
Regards, 
Scott