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-   -   Rayleigh Number in Natural Convection along a vertical plate (https://www.cfd-online.com/Forums/openfoam-pre-processing/218156-rayleigh-number-natural-convection-along-vertical-plate.html)

Mondal131211 June 10, 2019 20:23
Rayleigh Number in Natural Convection along a vertical plate
 
Hi Foamers,

I am a bit confused about the varying of Rayleigh number during the simulation. Can anyone please explain to me,
What is the simplest parameter to vary Rayleigh number in natural convection?

Note that: I do not want to change my geometry, so there is no chance of changing the length of the plate.

Ra=gB(DletaT)L^3/Nu.alpha

I know all parameter will change Ra number. I also know there is the simplest way to change DeltaT to change Ra number. For that case how can I control B for an ideal gas? In case of an ideal gas, B=1/Tf where Tf is the film temperature. If I change DeltaT each time, do I need to change Tf every time?

Please clear me about this confusion.

Cheers
Razon

agustinvo June 11, 2019 02:58
Hi Razon,
which kind of solver do you plan to use?


If you go for buoyantBoussinesqSimpleFoam or *PimpleFoam, changing your DeltaT should not be a problem. If you take a look into the code, the kinematic density only depends on DeltaT, not on their values. Then, the buoyant force in the momentum equation takes into account the derivative of such density, so you could still use the same beta coefficient.


In my case (I'm finishing my PhD on LES in such natural convection boundary layers) I found easier to modify the thermal diffusivity. But as far as you respect the dimensionless number, you should be fine.

jherb June 24, 2019 10:56
Are you sure, Delta T is the time step size? I think it is the temperature difference. See e.g. https://en.wikipedia.org/wiki/Rayleigh_number




Quote:
Originally Posted by Mondal131211 (Post 735898)
Hi Foamers,

I am a bit confused about the varying of Rayleigh number during the simulation. Can anyone please explain to me,
What is the simplest parameter to vary Rayleigh number in natural convection?

Note that: I do not want to change my geometry, so there is no chance of changing the length of the plate.

Ra=gB(DletaT)L^3/Nu.alpha

I know all parameter will change Ra number. I also know there is the simplest way to change DeltaT to change Ra number. For that case how can I control B for an ideal gas? In case of an ideal gas, B=1/Tf where Tf is the film temperature. If I change DeltaT each time, do I need to change Tf every time?

Please clear me about this confusion.

Cheers
Razon

Mondal131211 June 24, 2019 11:02
Quote:
Originally Posted by jherb (Post 737106)
Are you sure, Delta T is the time step size? I think it is the temperature difference. See e.g. https://en.wikipedia.org/wiki/Rayleigh_number
Hi Jherb,

I didn't say DeltaT is time step. I know its temperature difference. Probably, there is misunderstading here. I wanted to say if I change DeltaT every time, do I need to change Beta every time?

Cheers
Razon

praphul June 26, 2019 06:55
Hi Mondal,

The simplest parameter that you can change to vary the Rayleigh number, Ra is by changing the acceleration due to gravity, g. By this, you can retain all other fluid parameters which is evaluated at a suitable temperature and there's no need to change the temperature difference, delT .

In my opinion, as long as the flow is set at a particular Rayleigh number, the flow physics remain the same.

Regards,
Praphul


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