Hyperbolic Relaxation system (related to multiphase flow)
 

Hello. I'm currently reading on introductory concepts on multiphase flow and I've been focusing on hyperbolic relaxation systems. I am having a hard time understanding the concept of relaxation. How is it different than just using the local equilibrium assumption? Do we use it in CFD?

Also, if you have recommended reading materials on homogenous mixture models and multiphase flow in general, please leave a comment. Thank you.