Discontinuities in the specific internal energy of a point in a system can occur when the direction of flow into or out of a block with storage capacity reverses direction. This includes blocks such as the "Reservoir (2P)" or "Translational Mechanical Converter (2P)" blocks.
A good way to picture this effect is to imagine a reservoir of hot fluid and a reservoir of cold fluid connected by a pipe. If the fluid is flowing from the hot reservoir to the cold reservoir, the fluid in the pipe will be hot. If the flow reverses and the fluid flows from the cold reservoir to the hot reservoir, the fluid in the pipe will be cold. Just by reversing the flow of the fluid, the specific internal energy of the fluid in the pipe switches from that of one reservoir to the other which will show as a jump discontinuity in a plot of specific internal energy.
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