Unlimited Concentration

Hello Sim,

Unlimited Concentration would mean there is no upper limit for concentration of a constituent. Constituent analyses are usually computed for a relatively long period, such as over a course of a number of days. For instance, with a model run time of 228 hours, you should see the concentration will be capped at the Concentration Limit you enter. You may also want to use a smaller hydraulic time step, as this can help with the water quality results.

Keep in mind that your input should make sense based on the constituent you are modeling, as this will impact the results. Bad data into the model will mean bad results. Also the limit is meant to be limit on the concentration over time. So a reasonable run time and input are needed. The limit will obviously be some reasonable value larger than the initial input. If you add 1 mg/L of chlorine at a reservoir, a limit of 1 mg/L as well would not be reasonable. A limit of 10 mg/L might be more reasonable.

Regards,
Scott

Scott

Still I can not comprehend the function of 'unlimited Concentration'. There is a simple sample attached. I set 1 and other values for it, but nothing happened. What is the function of this item?

7418.Sample.rar

Sim

Hello Sim,

You have an initial concentration of 1.2 mg/L and a concentration limit of 1.0 mg/L. This by itself is not intuitive. The concentration limit is meant to stop an exponential growth of the constituent and should be set to a reasonable value. The following forum post has some information on this: communities.bentley.com/.../9019.

That said, the concentration at the downstream junction is less than 1.0 mg/L anyway, regardless of the limit. This is because of the other factors in the Constituent manager (bulk reaction rate and first order wall reaction rate). That might be one reason there is no change: it is already below the limit of 1.0 mg/L. But again, a smaller value for the concentration limit really doesn't make much sense in this case, since it would still be smaller than the initial concentration.

Basically, the concentration limit should be set to a reasonable value to inhibit exponential growth. The value you will obviously be related to your system.

Regards,
Scott

Scott
I did it (setting 1.0 mg/l for Concentration limit and 1.2 mg/l for initial concentration) deliberately, because I needed to know what happens if concentration of junction becomes greater that concentration limit. I thought limitation prevents Concentration of junction to exceed concentration limit. But it did not happened and according to your explanations, I know the reason of it.
But two points:
1) I set 0.5 mg/l as concentration limit and nothing happened again (I mean concentration of junction became greater that 0.5 mg/l). Does not concentration limit (0.5 mg/l) have to prevent concentration (of junction) growing from 0.5 mg/l?
2) How does an exponential growth occur? If you set 1 mg/l as concentration limit and set 0.5 mg/l as initial concentration, then concentration of downstream nodes NEVER exceeds initial or injection concentration (0.5 mg/l here). Am I right? because of Bulk and Wall reaction rates, some of constituents consumes in links and concentration of nodes MUST be less that concentration initially.
Sim

Hello Sim,

1) This is likely occurring because you have concentration limit set to a value less than initial concentration. This is not really a valid setting. A future release of WaterGEMS will have a user notification generate when something like this occurs.

2) The results are highly dependent on only on the input (diffusivity, bulk reaction rate, first order wall reaction rate), but the setup of the model too. A larger model with high demands, tanks, etc., may see different results. As mentioned in the forum link I included with my earlier email, the user was see a large concentration in his model. The concentration limit is meant for cases like this.

Regards,
Scott