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sanchezz November 18, 2009 01:36

Radiator replacement in racecar sidepod
Hi there,

I am completely new to CFX and my knowledge is only based on the first 10 tutorials of CFX.

I want to design a new geometry for a racecar sidepod. The sidepod consists basically of a tube with a radiator inside.
In order to safe calculation time, I do not want to model the whole radiator consisting of a ton of fins and tubes, but instead model it by its pressure drop.
So basically what I need is a plane representing the radiator, after which the pressure has dropped by a certain amount. What is the easiest way to go about the task?

I would be great if you guys would hve any advice for me... Thanks.


ghorrocks November 18, 2009 06:35

Two options, one 2D and one 3D:

In 2D you can put an interface in with a pressure drop or a pressure drop as a function of flow rate. This is useful if the radiator is a thin plane. (See interfaces for how to apply these)

In 3D you can use a porous region, with a resistance profile chosen to give the correct pressure versus flow relation. This is useful if the radiator is thick or has internal cross-flow. (See sub domains for how to apply these)

Dimeflow November 18, 2009 12:40

hi sanchezz,

if you have no pressure drop data to hand and dont fancy using a free area approximation, you could (although a bit timely) explicitly model a local section of the fins in a 'mock' duct. This would allow you to calc a pressure drop coeff by hand which you could then use for your porous material domain. obviously, you will have to do the exact same test for the different fin orientation(s), if of different lengths.

You'd also need to do a similar local explicit test to evaluate your porous material's conductivity .... this time though you would need to include the heating element(s). Using monitor points up and down stream of the fins you can get the temp drop. By constructing a dimensionally equivalent 'mock' test, but this time with the fins modelled as a porous region (remember you will now have the pressure drop coeffs to use), you can start with an arbitrary porous conductivity. By using the same monitor point positions you can then adjust the conductivity until the difference between both models temp difference is acceptable.

you then have the pressure drops and conductivity to use in your large model.

Ive used this approach with similar apps, but there could be far more efficient methods so please ..... anyone jump in with ideas!


sanchezz December 14, 2009 23:47

Hi, thanks for your answers. I have used a subdomain with a porous loss as suggested, but I am not completely clear on the adjustments I have to make. All I know of my real radiator is the pressure drop coefficient (I guess that's what it is) of about 0.21, i.e. the total pressure on the outlet side is always at about 79% of the inlet side.

How can I model this realistically in CFX?
According to the help file, I assumed that directional loss is the way to go as the radiator does not allow crossflow due to the fins.
Since I do only have the total loss coefficient, I'll probably have to recalculate that to a realtive coefficient in relation to the radiator thickness (e.g. 0.21/5cm=4.2/m). Is that correct?

What I also don't get is what I should choose for the permeability and the transverse loss. Since I specifically selected directional loss, why is there even an option for transverse loss?

Thanks for all the help and sorry for my probably stupid questions...

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