When computing a transient simulation in HAMMER the calculation fails and the following red user notification appears:
"Invalid results. Please check the input data for the connected elements."
This is a generic message that can occur in a number of different situations in which the transient results become so unstable that the calculation cannot continue.
First, review your model data input for mistakes that could cause rapid changes and instability. Next, determine roughly which timestep the calculation fails and change the simulation duration calculation option to be slightly less than this. This will enable you to complete the calculation and review results (such as a profile animation) for clues on the source of the instability.
You can also try adjusting the maximum trials and convergence accuracy per this article which may help, but it is better to find the root cause of the issue and address that instead.
Rapidly changing check valves
One potential cause is rapidly changing check valves due to their configuration or placement. If you are using the check valve node element, ensure that you have it set either to model an instant closure, or use the slow-closing option with non-zero open time, close time and pressure threshold values.
If you have multiple check valves adjacent to pipes with zero initial flow (for example next to pumps that are initially off), it may be best to exclude them from the model by morphing them into junctions for example.
Overflowing surge tanks
Another potential cause of instability is if you have multiple surge tanks in close proximity and/or with overflow or empty conditions for most of the simulation. For example if a surge tank is used in a situation where there is a high inflow (with respect to the tank size) during the initial conditions, it may instantly overflow during the transient simulation, causing it to use the weir equation to determine the overflow rate. If there are multiple such tanks nearby, it may be hydraulically challenging for the numerical solver to balance flow and the overtopping water surface elevation. In such cases if the tanks actually represent a constantly hydraulic grade, it may be best to simplify the approach and use reservoir elevations set to the same respective initial elevation.
Bad D2A Rating Table
If the discharge to atmosphere element property "Discharge Rating Type" is set to "Rating Curve" then make sure the Pressure Head vs. Flow data contains a sufficient range of values.For example, the image below shows an additional point that was inserted into the table, which ensured that the range would not be exceeded during the simulation.
Modeling Reference - Discharge To Atmosphere [TN]