Learning Forum

[DJENDARA]

I RUN TWO SIMULATION WITH TWO ALGHORITM (SIMPLE AND COUPLED ) AND I FOUND DEFFIRENT RESULTS FOR MY UDS POISSON EQUATION, CAN SOME ONE TELL ME WHAT IS THE ADEQUATE ALGHORITHME FOR UDS ?nAND TELL ME IF THERE IS A POSSIBILITY IN FLUENT TO SPECIFY THE COUPLED ALGHORITHME FOR FLUID EQUATUIONS AND SIMPLE FOR UDS EQUATIONS ?n

[YasserSelima]

In all solvers, simple, couple or density based, your UDS is solved separately after solving the momentum and energy equations. n

[DJENDARA]

i am using pressure bassed but when i solve Poisson equation by uds i dont find the same result when i changed the algorithmes i.e ( SIMPLE AND COUPLED ) it gives me different dissipation ?n

[DrAmine]

The UDS equation is using the momentum field which you get from SIMPLE and friends PV-Coupling schemes. If the momentum /velocity field is different means that your solution is still not deeply converged. Moreover if using coupled + uds it also depends on whether you are using pseudo transient option or not. The latter will just add implicit under-relaxation for momentum equations and keep uds equation default solved (local under relaxation).n

[YasserSelima]

nRun a simulation for a short period of time. And Monitor the velocity at some crucial points in your flow field ... Then repeat the same exact simulation with larger number of iterations in the time step ... Then increase it more ... until you get no difference. Now your UDS should give the same result.nThe above assumes that you selected the right time step. Max_Velocity/(2 * minimum_length_scale)n

[DJENDARA]

From respond of DrAmine The UDS equation is using the momentum field which you get from SIMPLE and friends PV-Coupling schemes, can you give me more explanation of this paragraph by equations ?nSecondly I request if it will be possible to choose the computational algorithm to compute UDS separately in the next version of FLUENT ? nBest regards !n

[DrAmine]

What do you mean with separately?nSome of the equations are mentioned in the Fluent Theory Guide. More to be found in papers and Text Books.n

[DJENDARA]

I mean that Fluent allow the possibility to compute momentum+mass conservation + energy with Coupled Algorithm and UDS with Simple algorithm I hope to see that in the next versionn

[YasserSelima]

This is what currently happen. The coupled solver solves the conservation equation simultaneously until conversion, then plug the results into your UDS equation.n

[DrAmine]

UDS Equation is an auxiliary equation and always solved at the end!n

[DJENDARA]

SO WHY I FIND DIFFERENT RESULTS WHEN I CHOOSE ALGORITHM SIMPLE AND COUPLED IN MY UDS ?!n

[YasserSelima]

Simply because you get different values of the flow parameters used to calculate your UDS. nSimple is good to reach overall solution, but if you are interested in transient details, select coupled, decrease the time step and increase the number of iterations in every time step until you reach conversion.n

[DrAmine]

It does depend on whether you are using pseudo-transient for UDS too but in general for a deep converging case you won't see differences in the UDS field.n