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-   -   low velocity but high mach number? it is true! (http://www.cfd-online.com/Forums/cfx/26462-low-velocity-but-high-mach-number-true.html)

 frank October 2, 2008 19:52

low velocity but high mach number? it is true!

hi all,

I am modeling a electro-chemical reaction flow in cfx11.0 with velocity about 1.0~2.0 m/s, but the density will change (2~3 times) along the flow channel.

When I solve this problem with normal incompressible N-S equation. The solver will stop with error message "the current mach number is **",which is much more than 1.0.

How to deal with this problem? Any comment will be appreciate! Thanks.

frank

 John S October 3, 2008 11:21

Re: low velocity but high mach number? it is true!

This is a fairly common error message and could be caused by a number of things: poor initial guess, poor boundary conditions, poor quality grid. I would start by examining all of those first. You could also try setting the expert parameter "max continuity loops = 2" but I've had issues with energy imbalance in the past using this.

 CycLone October 3, 2008 16:24

Re: low velocity but high mach number? it is true!

Check you scales. Is the geometry/grid the right size? What about the units on your boundary conditions?

 frank October 3, 2008 21:20

Re: low velocity but high mach number? it is true!

Thanks for John S and CycLone.

Do the scales and size really effect the this problem, CycLone? The flow channel is solid oxide fuel cell flow channel.The geometry and grid are in 'mm' unit(1.5mm x 1.5mm) and the inlet velocity is 'm' unit(1~2m/s).

 pratik mehta October 23, 2008 05:46

Re: low velocity but high mach number? it is true!

you have to estimate your Re number and also put on the ideal gas option ON

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