Difference in Results Depending on the Journal Velocity Setting in Multibody Dynamics–Hydrodynamic Bearing Coupling

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I am modeling a rotating shaft using Multibody Dynamics (MBD) and a Hydrodynamic Bearing (HDB).

In MBD, I applied Prescribed Rotational Motion to the shaft and specified a constant angular velocity, omega.

I then defined the journal bearing surface using the HDB interface. I tested two different settings for the Journal Velocity:

  1. Multiphysics-coupled velocity: The Journal Velocity is automatically obtained from the MBD interface through the Multiphysics coupling.
  2. Manually specified velocity: The Journal Velocity is directly set to the same constant angular velocity, omega.

Since the shaft is prescribed to rotate at the same angular velocity omega in both cases, I expected the HDB results to be identical. However, I am obtaining different results between the two cases, such as differences in pressure distribution and other bearing characteristics.

Why can these two Journal Velocity settings produce different HDB results even though the shaft is rotating at the same prescribed angular velocity omega in both cases?

I would appreciate any explanation of what velocity is actually being passed to the HDB interface through the Multiphysics coupling.



1 Reply Last Post 2026年8月20日 GMT+8 19:39
Soumya S S COMSOL Employee

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Posted: 2 hours ago 2026年8月20日 GMT+8 19:39
Updated: 2 hours ago 2026年8月20日 GMT+8 19:39

Dear Sangmin,

The two settings can produce different results because, in the coupled case, the Hydrodynamic Bearing interface uses not only the prescribed angular velocity, omega, but also the journal motion imported from the Multibody Dynamics model. In other words, the bearing response is based on the combined effect of shaft rotation and journal motion, rather than on the spin rate alone. Therefore, even if the same omega is specified in both cases, the effective journal velocity seen by the bearing can differ, which may lead to differences in pressure distribution and other bearing characteristics.

-Soumya

Dear Sangmin, The two settings can produce different results because, in the coupled case, the Hydrodynamic Bearing interface uses not only the prescribed angular velocity, *omega*, but also the journal motion imported from the Multibody Dynamics model. In other words, the bearing response is based on the combined effect of shaft rotation and journal motion, rather than on the spin rate alone. Therefore, even if the same *omega* is specified in both cases, the effective journal velocity seen by the bearing can differ, which may lead to differences in pressure distribution and other bearing characteristics. -Soumya

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