Rapid flow oscillation between hydraulically close tanks

Product(s):WaterGEMS, WaterCAD
Version(s):10.XX.XX.XX, 08.11.XX.XX


How do I model hydraulically close tanks?

Flow oscillates rapidly between two nearby tanks.


In general, tanks that are hydraulically close should typically be modeled as a single equivalent tank, particularly if they are close in proximity. Oscillations can be caused by having one tank in a model at a higher HGL than the other, in one time step. This would cause the flow to go towards the lower tank. Then in the next time step, the low HGL tank is at a higher HGL (from the filling in the previous timestep), so the flow reverses the other direction, and this process repeats.


Option 1 - Combine tanks into a hydraulically equivalent composite tank

If the tanks are spatially close to one another, they could be combined into a single composite tank with a diameter that provides the same volume as the individual tanks. This would avoid oscillating flows that may occur in cases where the tanks are modeled individually.


Option 2 - Use a smaller hydraulic time step

Using a smaller hydraulic time step, in the Calculation Options, may improve the results. The default value is 1 hour. Try setting this to 1 minute, and if results are improved, it can be increased to find an appropriate value that provides acceptable results and a fast computation time.