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-   -   how to deal with turbulance in FLOTRAN (http://www.cfd-online.com/Forums/cfx/19712-how-deal-turbulance-flotran.html)

 Avijit June 21, 2003 08:22

how to deal with turbulance in FLOTRAN

hi all!!

im using ANSYS FLOTRAN(ver 7) to get standard graph between drag coefficient (Cd) and reynolds no. (Ry), for both cylindrical as well as spherical objects. im facing following problems:

(1). im getting different answers for 2-d axisymmetrical sphere and 3-d model. results for 3-d model (only laminar condition) are matching with the standard results. i have used INTSFR command to find the drag force due to pressure and shear.

(2). im not getting the results for turbulant condition. im getting perfectly correct plot of Cd vs Ry for 3-d sphere in laminar condition, but im not getting the sudden fall in the plot at turbulant conditoin. (pl. refer to any standard fluids text book for the plot between Cd and Ry). i have tried both standard k-e model and new k-e model to analyse the problem.

looking forward to any replies....

avijit

 derrek June 24, 2003 20:29

Re: how to deal with turbulance in FLOTRAN

Avijit:

I would be interested to hear the specifics to your model? Axisymmetric problems can hide be tricky sometimes. What are the BCs in your 3D model and axisymm model? Are you using a mapped mesh in both cases? What does your mesh look like in each case?

cheers,

derrek

 avijit marwaha June 26, 2003 09:57

Re: how to deal with turbulance in FLOTRAN

hi derrek !

thanks fr the prompt reply.

to see the mesh plz go to http://photos.yahoo.com/avijit_fem

the boundary condition for 3-d sphere are :

1. Vx = Vy = Vz = 0 on the surface of the sphere. 2. velocities at entrance Vx = Vy = 0, Vz = v. 3. pressure at exit = 0 (relative to 1.031 e5). 4. vel at the vertical walls enclosing the sphere, Vx = Vy = 0, Vz = v. since i have to simulate a freely falling sphere in a fluid under the force of gravity, so by doing that i have obivated the edge-wall effect on the sphere.

boundary condition for 2-d axisymmetric sphere:

1. Vx = Vy = 0, at the surface of the sphere. 2. velocities at entrance Vx = 0, Vy = v. 3. pressure at exit = 0 (relative to 1.031 e5). 4. velocities at the axis of symmetry (i.e. y-axis), Vy = 0. 5. vel at the vertical walls enclosing the sphere, Vx = 0, Vy = v. for the same reasons stated above.

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