Transient wave reflection time

Sin, the characteristic time is more of a physical property of the network since the length and wave speed are essentially fixed physical parameters. A transient wave during a transient simulation in HAMMER will travel based on the wave speed. Note that the wave speed or length can be adjusted by HAMMER if your timestep is too large and/or pipe lengths are too short. Check the wave speed or length adjustments in the pipe Flextable. See more here: Understanding length/wave speed adjustments and their impact on results

As for the timing you are seeing in the results and the reason for the change, it might be better to visualize them with a profile animation, covering the complete pipe length. Animate the profile results to get a better understanding of the wave propagation. Depending on what is happening in your system, the interactions may be more complex. Additional boundary conditions could be forming such as a vapor or air pocket, or more complex interactions with reflections from branched connections.

If this does not help, please provide a copy of the model and reply here with details steps to reproduce and observe the specific results you are looking at.

Hi,

Sure. I had uploaded two scenarios for your reference. Normal closure and slam shut case for Pelton turbine.

Thanks for sending the model.

If you animate the profile path while keeping the HGL time history graph open at the same time, you will see that the characteristic time is reflected not between the HGL peaks but rather from the peak to the low point. For example for the s1a "normal closure" model if you start animating at 141 seconds, you will see the wave travels to the end of the system by about 145 seconds and then reflects back and reaches the origin at around 148 seconds (at a low point in the HGL graph) for a total travel time of 7 seconds as expected (again, based on the pipe length and wave speed). 

Waves will reflect with a different sign depending on what they reflect off of. See: HAMMER Transient wave reflection behavior for different boundary conditions

In the "s4b" model with the instant closure, the conditions are more chaotic (as expected) but if you animate the profile you can again observe the wave taking 7 seconds to travel from the transient origin to the end of the system and back (between 100 and 107 seconds). 

Here are some other observations:

• You are using a slightly older version, 10.03.05.05. I recommend upgrading to 10.04.00.108 since it has a major improvement to performance and results granularity in the Transient Results Viewer. See this for more.
• There is an excessive amount of junctions, causing the pipe lengths to be short, resulting in the need for a small timestep (and long run time) in order to avoid excessive adjustment to the wave speed as described in this article. There are some changes in elevation along the profile length but not large enough to warrant so many extra junctions. In my experience I have seen users do this because of their past experience with other transient modeling software where the results are only calculated at the nodes, but that is not the case with HAMMER. HAMMER uses the Method of Characteristics and calculates at intermediate points along the pipe length, so it is best to only use junctions where needed (significant elevation changes for example) and keep the pipe lengths long to avoid having to use a tiny timestep. See more here: Method of Characteristics vs. Wave Plan Method
• Using two separate model files is not necessary, as the powerful scenario management feature can be used instead to model both situations in the same model file. Meaning, you could simply create a new scenario in one model, for the fast closure vs. slow closure. With different valve operating rules and different initial status, you would want to use a new child transient alternative and new child initial settings alternative in your other scenario. See more here: 
  • Scenario and Alternative Management
  • Viewing HAMMER results for multiple scenarios

Hi Jesse,

Thanks a lot for the comments and recommendations.