Applies To Product(s): Bentley StormCAD, Bentley CivilStorm, Bentley SewerGEMS Version(s): 08.11.03.77+ Environment: N/A Area: Calculations Subarea: Original Author: Jesse Dringoli, Bentley Technical Support Group Problem Description Why do I see different hydraulic results between using a value less than 30 versus a value higher than 30 for the Maximum Network Traversals calculation option? Shouldn't the results be the same as long as they are converging? This applies to StormCAD as well as the GVF Rational numerical solver in CivilStorm and SewerGEMS. Reason Starting the SELECTseries 3 (a patch for 08.11.03.77, automatically included in later builds) a convergence enhancement is included and invoked when setting the Maximum Network Traversals to a value of 30 or greater. For any models where the Max traversals option is set to a value below 30, there should be no change in the results. The main reason why this enhancement was made was because some models had trouble converging with pipes flowing near the crown, in the 'instability zone'. This is the range of depths just below the pipe crown of a circular pipe, where there are two conjugate depths for the same flow. (yn and yn' in the below illustration of normal depth vs flow). As a side note, this problem occurs even with the simplified Capacity analysis option, because of the Actual Uniform Flow Velocity method. The velocity calculated from this method is always based on uniform flow, so the velocity is the same in both the Capacity and Backwater analysis methods (since the flow and physical properties stay the same.) One of our assumptions in this situation was causing an increased likelihood for the solver to have trouble converging. When using the uniform flow velocity method, if the pipe's depth corresponds to a surcharged condition (both start and stop depth above crown), the full flow velocity method is used (V=Q/A). This assumption causes instability in the case of pipes flowing near full, due to the velocity change occuring between the surcharged depth and non-surcharged depth condition. Basically the abrupt change could cause the solver to bounce back and forth between these states (surcharged depth/full flow velocity and non-surcharged depth / uniform flow velocity), leading to convergence issues. To fix this (with traversals set to 30 or higher), we changed the assumption here, so that it aligns with the normal depth assumption with the uniform flow velocity method. We removed the aforementioned depth check, so that the solver will only compare the pipe's flow to the "QMax" (typically about 15% higher than the "full capacity", which is the flow with normal depth equal to the crown) to determine when to use the full flow velocity method (V=Q/A). This is more consistent with the uniform flow assumption and is more stable. In short, the default should be left as 5 traversals (and default flow tolerance of 0.001) and if a model has trouble converging, try increasing the max traversals option. Between a value of 20 and 29 for maxtraversals, a relaxation algorithm will be applied. In difficult cases where pipes are flowing near the top, increasing it to 30 or higher should help, by eliminating the depth check as explained above. See Also My scenario is not converging on a solution. Convergence NOT achieved in StormCAD
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