The time step critical elements in the SWMM 5 report text output file tells you which elements were controlling the time step during the simulation. If no element was controlling the time step then the program will just use the maximum time step. For example, if the maximum time step was 10 seconds and the average time step was 9.8 then only a few time steps were set by a link or node that needed a smaller than maximum time step (Figure 1). If the maximum time step for the same simulation was 30 seconds then many links and nodes will set the time step (Figure 2).
Figure 1. Most of the time the simulation used the maximum time step of 10 seconds so only a few links were time step critical.
Figure 2. Most of the time the simulation used less than the maximum time step of 30 seconds so many links were time step critical.
Comments
I have attached my file in a new post.
Hope you would have got that.
Can you send me your model so I can take a look at it?
I am getting flow instability in my model.
I have already used lesser flow routing time step, ignore inertial terms option and variable time step option, but still numerical stability at some of the conduits are consistent.
How to reduce it...
is there any upper limit for maximum flow instability index in conduits or it must be zero.
The listing of the time step critical element is for your information, it does not indicate a problem, merely what link the program is using to determine variable time step to use during the simulation.
Here is an example, the link is only 3 percent time step critical because only 3 percent of the time was a time step less than 10 seconds needed
Should we use time step = 1 s ? if it is all about giving the smaller time step.
As in my case
When I used 1 s instead of 5 s and 10 s I got no link as a time step critical element in status report.
The time step critical elements are the links with the smallest CFL condition during all of the time steps. If you link has a value of 4o percent it means 4o percent of the time that particular link was the time step critical link. If you lower the time step then it makes sense that this number goes down. It just means that a you lower the time step the needed time step is ABOVE your new maximum time step so no link is time step critical.
SWMM 5 Link Time Step Calculations
It you select the variable time step option in SWMM 5 then the program will compute the CFL time step for each link based on the ending system variables in the last time step based on the following steps. The smallest value of t is used for each time step but often the same small set of links will be the controlling time for the whole simulation. In the example shown below link 1570c is controlling the time step 83 percent of the time. The link time step is usually the controlling time step.
Hi Bob
When we use a smaller time step for routing say 3 s instead of 10 s, number of time step critical elements get decreased.
Whether we really need to worry for the time step critical elements, if yes, then how it affects the drainage system physically.
for example
Time step critical element
Link 4 (48 %) when routing time step is 10 s
and its decreased to (10 %) when routing time step is 3 sec
it may further decrease if we reduce the routing time step to minimum.
Please help me in understanding this.
#thanx