Fluent offers different temporal discretization schemes. There are two implicit schemes: Second order and bounded second order. I am wondering what are the advantages and disadvantages of these two schemes. I am working on multiphase problem having Eulerian Multiphase model. Which one should I prefer?
In many multiphase applications, the process can vary spatially as well as temporally. In order to accurately model multiphase flow, both higher-order spatial and time discretization schemes are necessary. In addition to the first-order time scheme in ANSYS FLUENT, the second-order time scheme is available in the Mixture and Eulerian multiphase models, and with the VOF Implicit Scheme.
Dear Hamed Azimi
I am asking about the difference between second order and bounded second order schemes. Pls read the question.
Hi,
the bounded second order scheme (BCD = bounded central difference scheme) is essentially of importance, if the high accuracy of a full 2nd oder scheme is required, but for a complex mesh it cannot be guaranteed that the CFL number is less than 1 everywhere in the mesh. Furthermore boundedness of certain variables is usually of importance. So e.g. you do not like that turbulent kinetic energy could suddenly and unexpectedly become a negative number because of some undershoots of a full 2nd order central difference method. Furthermore in a thermal application you do not like to see, that the temperature becomes for the reason of numerical errors larger than the maximum temperature occuring in your boundary conditions or smaller than BC temperatures, which would be in the absence of other sources and sink terms physically invalid. Using a bounded 2nd order scheme should assure you 2nd order accuracy everywhere where this is numerically possible and boundedness of bounded variables at the same time.
In particular for scale-resolving simulations (LES, DDES, SAS, SBES) the BCD scheme is quite frequently used, since it helps to maximize the timestep of integration for a given mesh and so to make as fast as possible advancement in time integration as possible from a numerical and accuracy point of view, without loosing the capability of resolving turbulent scales in the flow.
Hopefully this explanation helps.
Dr. Th. Frank.
Thanks, Thomas Frank for such a detailed and insightful answer.
So, what I understand is, its always a good idea to use bounded second-order scheme as it provides better stability and doesn't add anything to total computational time. The use of a larger time step may be an added advantage.
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