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Forum: Main CFD Forum

January 11, 2012, 13:17

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

Since you are dealing with a separation bubble at...

Since you are dealing with a separation bubble at the top and bottom of your square, which is driven by the sharp leading edge, I'm not really sure I know how y+=1 translates into a real grid spacing...

Forum: Main CFD Forum

January 11, 2012, 12:13

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

On aspect of this question is how much does one...

On aspect of this question is how much does one believe in 3D URANS? I don't do a lot of unsteady calculations so I can't really comment. However, IMO, RANS equations throw so much eddy viscosity...

Forum: Main CFD Forum

January 9, 2012, 20:17

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

The interesting part is that you are actually...

The interesting part is that you are actually trying to model a bridge deck not a rectangular cylinder. There may be cars, girders, guard rails, etc. All this adds turbulence and may cause the flow...

Forum: Main CFD Forum

January 9, 2012, 19:00

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

I don't think Strouhal number is necessarily...

I don't think Strouhal number is necessarily linked to CD. But I'm arm waving here, so I'll let others speak up on this.

In regards to mean CD, it is very much sensitive to it. Well, broadly...

Forum: Main CFD Forum

January 9, 2012, 17:36

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

Maybe another misunderstanding. What I really...

Maybe another misunderstanding. What I really suggest doing is comparing 2D and 3D versions of rectangular cylinders. A square cylinders is a unique case of a rectangular cylinder where...

Forum: Main CFD Forum

January 9, 2012, 17:21

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

Maybe I miss understood the problem from the...

Maybe I miss understood the problem from the beginning. For me, a square cylinder is a circular cylinder where the diameter is equal to the length. Ooops.

For your case, which I assume is at low...

Forum: Main CFD Forum

January 9, 2012, 15:48

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

In general, for production subsonic runs I'll use...

In general, for production subsonic runs I'll use an outer boundary 50x-100x of max length for 3D cases and 100x-150x of max length for 2D cases. For 2D, the outer boundary can be less but I'm very...

Forum: Main CFD Forum

January 9, 2012, 14:18

Replies: 35

2D vs 3D

Views: 15,877

Posted By Martin Hegedus

In regards to drag, I always recommend Hoerner's...

In regards to drag, I always recommend Hoerner's book "Fluid-Dynamic Drag" It is a treasure chest of information.

In general, 3D shapes have lower drag than 2D shapes. However, it sounds like...

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