Oblate sink particles / Geopotential model

[danieljprice]

Type of PR: new physics

Description:

• Added external force for orbits around an oblate central object with J2 moment (https://en.wikipedia.org/wiki/Geopotential_model)
• Sink particles can now carry J2 moment indicating equatorial oblateness
• Oblate sink particles require specifying the true body radius (stored in xyzmh_ptmasss(iReff,i)) and the initial spin angular momentum of the sink particle
• Spin of sink particles can now evolve with a ds/dt term, in the Geopotential model this is driven by the gravity of orbiting material back onto the oblate body. This is in addition to any spin changes induced by accretion onto the sink particle
• Implemented user-friendly setup for oblate planets as part of SETUP=disc, can specify J2, rotation period in hours, planet radius, a concentration parameter and an obliquity angle that are used to set up the initial spin angular momentum

Testing:

• New unit tests added to test_ptmass to check that linear and angular momentum are conserved
• routines to compute the Geopotential force tested as part of test_externf.f90, e.g. testing that acceleration is the gradient of the potential
• SETUP=binary routine can be used to test orbits of individual particles (e.g. single sink particle in external potential, or one sink particle orbiting another sink particle with oblateness)

Did you run the bots? no