Linear Material Is Broken (and also no test cases currently exists for it)

[JRao-rgb]

Description

There exists no tests cases for the linear material (does anyone use this anyways?). But it is broken, as seen below when I try to run a test on it and it returns zero pressure gradient.

Reproduction

pull the solver; build the solver. Change the material type of any of the test cases to "linear" and replace k1 with 1.8e10. Run the tests and you will see that it fails.

Expected behavior

The linear material should behave identically with the Olufsen material at high vessel stiffness, which we are considering here. Instead, the linear material returns zero pressure drop along the centerline.

Additional context

No response

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