CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   FLUENT (http://www.cfd-online.com/Forums/fluent/)
-   -   First Order to Higher Order Blending Factor (http://www.cfd-online.com/Forums/fluent/106693-first-order-higher-order-blending-factor.html)

NormalVector September 5, 2012 13:47

First Order to Higher Order Blending Factor
 
I'm trying to understand the discretization blending factor Fluent has available (accessible through TUI solve/set/numerics). From what I see in the Fluent users' manual, a value of 0 will use a first order discretization while a value of 1 will "recover higher order discretization." It notes that in order to use it effectively, one of the higher order schemes must be selected for the desirable variables. Does that mean that I have to use at least a second order discretization for density, momentum, etc. in order for the blending to work properly?

I'm having convergence troubles at second order upwind so I'm trying to squeeze all the accuracy I can out of first order. :rolleyes:

asal April 1, 2013 17:57

Hi!

I have the same question. Do you find any answer?
Thanks.

NormalVector April 1, 2013 18:56

Nope, never did. I just pushed through second order by changing the relaxation factors.

oj.bulmer April 3, 2013 15:43

Yes you do need to have a second (or higher order) solution in order to be able to use the blending factor. In very simplistic terms, blending factor B can be defined as:

\phi  =  \phi_{first-order} +  B*(\phi_{higher-order}-\phi_{first-order})

Here \phi is the quantity like velocity, pressure etc.

Now, if you have a look at the equation, if B=0, we have results of first order and if B=1, we have results of higher order. But if you don't have a higher order solution, then \phi_{higher-order} becomes \phi_{first-order}, and for any value of B, you get the solution the same as \phi_{first-order}.

Hence it is necessary to have a second or higher order solution in order to use blending factor.

It is healthy to go gradually from first order to different blending factors (0.4, 0.7) etc depending on your unstability and then switch to second order, if you are having problems with direct second order solutions.

OJ


All times are GMT -4. The time now is 08:09.