virtual flow sewerGems

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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This conduit that I attached graph, is far from sewer network and near to treatment plan. it means practically for about 2~3 hours this conduit must be empty. but we saw wastewater is flowing equal about 50 l/s and its velocity is 3 m/s. I suppose this flow and velocity related to virtual flow. is it true?
thanks in advance.

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I uploaded my project file. I didn't success in setting calculation parameters :( because in some conduits there are problems. as you see, sanitary load for manhole 28969 is 3.71 l/s but when model run,  flowing in conduit 31112 in initial time come bout 45 l/s. in many case the same problem is occurred.

what's your opinion about virtual flow and calculation parameters?

thanks.

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

I forgot to mention that I used a Routing Time Step of 5 seconds in the SWMM solver calculation options.

Do you mean explicit SWMM solver (dynamic wave) as confident as implicit Dynamic wave?

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

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