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Model Slope

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Message 1 of 11
Berkan_vent
728 Views, 10 Replies

Model Slope

Hi CFD users,

 

I really need to know about CFD 2014 consider "slope" for models? I mean I have a tunnel model and it has 2 degrees slope, I want to simulate tunnel fire, as is known tunnel slope effects behavior of smoke movement in the tunnel. So I need this important information.

 

Thanks in advance,

 

Berkan.

10 REPLIES 10
Message 2 of 11
Royce_adsk
in reply to: Berkan_vent

Berkan,

 

The slope is going to be defined by your gravity vector. You will set this up in your physics tab when you go to define the solver.



Royce.Abel
Technical Support Manager

Message 3 of 11
Berkan_vent
in reply to: Royce_adsk

Hi Royce,

 

Thank you for your quick reply.  I examined wiki help but i couldnt find how to calculate gravity vector. Im confused little bit sorry. my tunnel model's slope is 2 degrees. 

 

So according to my calculation my gravity vector should be like this:

 

a= 2 degrees

 

Gravity vector = -9.81*(sin(a),0,cos(a))

 

x= -8,920208

y= 0

z= 4,0824

 

if this is not right, please help me how to calculate gravity vector?

 

Thanks in advance,

Berkan

 

Message 4 of 11
Jon.Wilde
in reply to: Berkan_vent

You can use pythagoras:

 

The 9.81 is on the hypotenuse

 

This gives you the vertical: 9.81 x cos 2 = 9.801

And the horizontal: 9.81 x sin 2 = 0.342

Message 5 of 11
Berkan_vent
in reply to: Jon.Wilde

Hi Jon,

 

I calculated with excel, i think that's why im wrong because of radian degrees. Thank you for correction. 

In physics tab, i defined gravity vector like this: 

 

Tunnel slope (2 degrees)

 

x=0

y=0

z=0.342

 

but i had to choose gravity components. Otherwise, i mean when i choose earth, divergence occurs and i got high air velocity. Is this correct?

solve screen.png

 

Thank you

 

Berkan,

 

 

Message 6 of 11
Jon.Wilde
in reply to: Berkan_vent

Hi Berkan,

 

You should be able to use components and just set them as I gave you.

 

Set 9.801 in the near vertical downwards direction

Set 0.342 in the direction of the slope - I do not know which way your model slopes vs the axes so you will need to decide on the direction (positive or negative) of this value.

 

If you can share an image of your model with the axes and which way the slope travels this might be easier.

 

(Note: With earth we assume 9.81 as a unit so you would just use 1,0,0 for instance to define a single gravity direction).

 

Best regards,

Jon

Message 7 of 11
Berkan_vent
in reply to: Jon.Wilde

Hi Jon,

 

Im able to use components and set them as you gave but divergence occured.

 

*Tunnel slope is + 2 degrees on X axis.

 

I put the image of my model and attached real model .cfz file so you can see it clearly.

 

model_with axis.png

 

And also some of my jet fans doesnt work. i assigned mesh regions for every jet fans. But i dont know why they dont work.

 

 

Thank you,

 

Berkan.

Message 8 of 11
Jon.Wilde
in reply to: Berkan_vent

Hi Berkan,

 

Thank you for sharing the model. I am a little confused about the goal of this analysis though.

 

Can I ask why you are running with gravity when you have no thermal conditions to cause any air density changes?

Right now I am not sure whtat to run with. I did see that radiation was on but there were no solar gain settings, should there be? 

 

Also, you have air set to fixed, which would not allow it to change density.

 

I suggest that you need a finer mesh - those regions need to be refined at least 3 more graduations, right now the mesh across the fans is too coarse.

I also propose some refinement to the tunnel - check out the mesh - maybe something using half the element size might be more representative?

 

Tunnel Mesh.png

 

Kind regards,

Jon

Message 9 of 11
Berkan_vent
in reply to: Jon.Wilde

Hi Jon,


Sorry for late reply because i had to finalized my model. I want to simulate tunnel fire. As i mentioned before, my tunnel model has a slope that's why i want to use gravity method (component).


For my first model (as i attached before) i just wanted to see proper meshing for jet fans. Because some of my jet fans doesn't work. So i didnt pay attention on thermal conditions, air density changes or radiation situation. But you are right i must be careful 🙂
According to my final model i want to simulate car fire in the tunnel and it's heat release rate is 50 MW (50.000.000 w) i used resistance for fire and i assigned some gaps beneath the fire for air movement.
But i didnt understand how to set the fire properties.

 

resistance_material.png

 

*First of all how should i assing 50 MW heat release rate to resistance material?


*And also i didnt understand flow direction, normal direction 1 and normal direction 2? How should i use these direction. im a little confused about parameters of resistance material.

 

property_settings.png

 

* Also my tunnel has +2 degree slope, so how should i decide to assign gravity method because my model vs the axes is little bit different? (For gravity method;  you said "Set 9.801 in the near vertical downwards direction" and " Set 0.342 in the direction of the slope)

 


I attached my model so you can understand me clearly.
Thanks in advance Jon,
Berkan.

Message 10 of 11
Jon.Wilde
in reply to: Berkan_vent

Hi Berkan,

 

The fire material is OK, as each direction has the same resistance the direction you set does not matter.

 

You have assigned the heat load correctly - although there should be a solid 'ring' around the outside of the 'fire' so that only the top and bottom of the resistance are open to the air.

 

You cannot have scalar and temp on both ends of the tunnel, you would really need to decide which is an inlet (with all the BC's) and which is an outlet, with just a P=0.

 

As for the gravity, it would look something like this, although you may need to switch the x to negative depending on which way the slope goes. The gravity would act downhill.

 

Component Gravity.png

 

Kind regards

Jon

Message 11 of 11
Berkan_vent
in reply to: Jon.Wilde

Jon thank you for your fast reply,

 

Could you explain more about the solid ring? Because i designed the car as rectangular prism. Should i put the solid ring in the bottom of rectangular prism?

 

By the way i just made up the numbers of "Through Flow-K", "Normal Direction 1K, Normal Direction 2K and Conductivity " and  dont have any idea what is these for? 

 

I assigned scalar (0) and temperature to tunnel portals now. But this tunnel is bi directional so i dont know which is inlet or which is outlet. Should i assign P=0 for both portals? Or Should i assign Unknown?

 

My tunnel is uphill so i will set up the gravity component like this: x=-0,342 y=0 z= 9,801.

 

Thank you,

 

Berkan,

 

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