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How do I model hydraulically close tanks?
I keep getting variations in flow resulting in oscillations when I graph the discharge in a pipe.
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, so the flow reverses the other direction.
There are a couple of options to remedy this situation. First, you can use a smaller hydraulic time step, such as 1 minute (the default settings is 1 hour). This small time step can usually provide a more suitable time step. If a hydraulic time step of 1 minute works, then you can try a higher one, so that the computation time doesn't take as long.
Another solution, particularly if the tanks are spatially close to one another, you would to model the tanks as a single composite tank with the equivalent total surface area of the individual tanks. This process can help to avoid fluctuations or oscillations that may occur in cases where the tanks are modeled individually. This fluctuation is caused by small differences in flow rates to or from the adjacent tanks, which offset the water surface elevations enough over time to become a significant fluctuation. This results in inaccurate hydraulic grades.
Example: Consider a tank connected to a reservoir and time 0, the flow to the tank is 10 cfs and you're making 1 hour time steps. After possibly 10 minutes, the water level is such that the tank level is higher than the reservoir, the model doesn't know this and keeps pushing 10 cfs into the tank so that the tank water level overshoots the correct value. In the next time step, you get 20 cfs flowing from the tank to the reservoir. Then the tank level falls too far.