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Rayleigh Benard 3D coding with finite differences

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Old   February 17, 2016, 01:16
Post Rayleigh Benard 3D coding with finite differences
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Madhup
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I am developing a simple fortran solver in finite difference. My domain is 2pi*2pi*1 as taken by S. Chandrasekhar. My equations are having non dimensional parameters as Reynolds Number, Rayleigh Number and Prandtl No.
The code is running well but w- velocity contours(heating at bottom and cold at top) are coming only positive and negative alternatively and i am not getting rolls as expected. Can someone explain it.? Thanks in advance
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Old   February 17, 2016, 04:18
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Originally Posted by cfdprojects View Post
I am developing a simple fortran solver in finite difference. My domain is 2pi*2pi*1 as taken by S. Chandrasekhar. My equations are having non dimensional parameters as Reynolds Number, Rayleigh Number and Prandtl No.
The code is running well but w- velocity contours(heating at bottom and cold at top) are coming only positive and negative alternatively and i am not getting rolls as expected. Can someone explain it.? Thanks in advance

please, provide much more details about your code such as formulation, type of discretization, grid arrangement, values of the non-dimensional parameters, etc
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Old   February 17, 2016, 06:07
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I am implementing Pressure- velocity method, and using pressure poisson equation for divergence free flow. Ra=1800, Re=1.0, Pr=1. Grid is uniform in x and y while clustered with cos function in z direction(gravity). discreatisation is done utilizing fourth order Central differencing for diffusion terms, while third order Kawamura and K. Kuwahara upwinding for convective terms. thanks for reply.
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Old   February 17, 2016, 06:11
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I am implementing Pressure- velocity method, and using pressure poisson equation for divergence free flow. Ra=1800, Re=1.0, Pr=1. Grid is uniform in x and y while clustered with cos function in z direction(gravity). thanks for reply.

if you use a second order discretization and a colocated arrangement, you could have experienced a decoupling-mode solution in the pressure equation.
Could you post some figures of velocity and pressure fields?
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Old   February 17, 2016, 06:21
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if you use a second order discretization and a colocated arrangement, you could have experienced a decoupling-mode solution in the pressure equation.
Could you post some figures of velocity and pressure fields?

Here are the figures encl :
Attached Images
File Type: jpg Temp.jpg (31.3 KB, 15 views)
File Type: jpg W-velocity.jpg (35.6 KB, 14 views)
File Type: jpg w-velocity in z-direction.jpg (46.1 KB, 7 views)
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Old   February 17, 2016, 06:42
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it is a decoupling along the gravity axis, you can see the pressure field oscillating ... why don't you try using a staggered grid?
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Old   February 17, 2016, 06:52
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it is a decoupling along the gravity axis, you can see the pressure field oscillating ... why don't you try using a staggered grid?
Yes i am using collocated grid till now. Would you please suggest me some writeup on staggered grid implementation in view of this problem.
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Old   February 17, 2016, 07:01
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Yes i am using collocated grid till now. Would you please suggest me some writeup on staggered grid implementation in view of this problem.

a second order discretization on staggered grid is a very old and well suited formulation, you can find that in any CFD textbook. For example, see the book of Peric & Ferziger
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Old   February 17, 2016, 07:15
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a second order discretization on staggered grid is a very old and well suited formulation, you can find that in any CFD textbook. For example, see the book of Peric & Ferziger

Thankyou. I will apply it and come back.
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Old   February 22, 2016, 01:17
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Now i applied staggered grid, please see w-velocity profile , i can see rolls still. Will they apper after long time ?
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Old   February 22, 2016, 05:32
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Now i applied staggered grid, please see w-velocity profile , i can see rolls still. Will they apper after long time ?

from this plot I see still oscillations
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finite difference method, fortran code, prandtl number, rayleigh-benard

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