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-   -   Need Help on Fluent Modelling Laminar on BluffBody (http://www.cfd-online.com/Forums/main/9187-need-help-fluent-modelling-laminar-bluffbody.html)

 ary May 18, 2005 06:07

Need Help on Fluent Modelling Laminar on BluffBody

Hello Everyone..

I am studying how to predict CD,CL,CM on Bridge deck (suspension) using Fluent. Since there is no one expert on this external flow in my university, i 'd like to ask you all expert.

I have read some report from selvam, frandsen, etc. They are working directly in turbulent. Since i am just a rookie in CFD, i began to study from laminar and modelling bridge as airfoil like cornell's tutorial notes.

My Bluff body (bridge deck) is a local bridge :14m wide and 2 m deep in sectional design (like Great East Belt section deck).

Oke start to my problem: The section was modelled thru gambit. 1. First problem: Do i need to scale the bridge 14m wide to 1 (unity) like airfoil ? 2. If the bridge wide is B. So i have modelled an windtunnel with 12.5B in front of section model with 12.5B clearance to top and to the bottom wall from section model and 25B outlet from the model.

The model in internet always modelled as front as velocity inlet, and back as pressure outlet (0 pascal). Which the best situation of top and bottom tunnel boundary condition: as a WALL or VELOCITY INLET.

3. I have modelled in laminar segregated 2d steady problem. The energy solver is turned off. I confused about the pressure. Since my bridge will operate in subsonic so it is incompressible right? Fluent said in the manual that i need to define mean pressure in operating condition? What it is? I leaved it on 101325Pa. Where the reference x,y i must put after section or before section ?

4. The materials is air: Density and viscosity constant. Where is the REYNOLD NUMBER input??

5. My model has solution control pressure standard, viscosity coupling: simple , and momentum: quick. With under relaxation pressure 0.1, density 1, body forces 1, and momentum 0.3. Do you agree with me?

6. The main problem is the speed scaling. How the speed scaling where the problem is laminar? Do i need to speed scaling since the Cd, Cl, Cm force monitor on airfoil sometimes > 1. The speed i used is 10 20 30 40ms.

 ary May 19, 2005 05:59

Re: Need Help on Fluent Modelling Laminar on Bluff

I know later that we must define reference value in depth, area and length in reference value to get the correct cl,cd,cm.

What depth , area , and length in this meaning?? Does the same meaning with http://www.grc.nasa.gov/WWW/K-12/airplane/geom.html

Length in here is 1 (assumed plane strain) or the length of the chordal (in my bridge = 14m)

Where the area is chord x length (in here assumed plane strain = 1) right?

What the depth mean anyway?

In incompressible flows on laminar we must use mean pressure,so I have the wind tunnel test, but where is it usually they put in the value?

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