Implementing Velocity Boundary

AS_Aero · February 14, 2018, 02:07

Hi All

I am trying to do a basic study on the building flow. Wind flow along the obstruction, for example when there is wind flowing towards a tall building there is a increase in velocity at the tip of the building roof , and this is experimentally proved that it has some 110% increase in the velocity at that region, But I want to try to simulate that in CFX, the question is , is it possible to give some velocity function for this ? I have attached an image which explains the same. Kindly help me how I can achieve this.

Cheers!!

Gert-Jan · February 14, 2018, 03:10

You can write a CEL expression where you can define something like vel_u=a*Y^2+b*Y+c.
Very simple. I do not know the variable for Y by heart. It could be Y coordinate.
When defining an expression, just use your right mouse button to find possible expressions, locators and variables.
Make sure that the units are correct. Meaning, every term should be in [m/s]. So, since Y in in [m], in my (silly) example c is in [m/s], b is in [1/s] and a in [1/(ms)]

AS_Aero · February 14, 2018, 03:42

Thanks for the reply.. I know the normal fuction for velocity profile. But this case as I explained how will we know the velocity profile will be like this function ? And whether this function I have to give it from the base face or the side face ? The real building is 10m but here am using only 2m, so there should be some difference right ?

Gert-Jan · February 14, 2018, 04:11

I guess you need far field information in a near field problem.
I don't know what a logical function would be.

Why not perform a far field calculation with a building of 10m? Then extract the velocity profile on planes that coincide with the inlet boundaries of your near field simulation?

Alternatively include the whole building in your near field calculation with a few large elements. That might be more accurate than a CEL-function.

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February 14, 2018, 03:10		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,885 Rep Power: 27	You can write a CEL expression where you can define something like vel_u=aY^2+bY+c. Very simple. I do not know the variable for Y by heart. It could be Y coordinate. When defining an expression, just use your right mouse button to find possible expressions, locators and variables. Make sure that the units are correct. Meaning, every term should be in [m/s]. So, since Y in in [m], in my (silly) example c is in [m/s], b is in [1/s] and a in [1/(ms)]

February 14, 2018, 03:42		#3
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Thanks for the reply.. I know the normal fuction for velocity profile. But this case as I explained how will we know the velocity profile will be like this function ? And whether this function I have to give it from the base face or the side face ? The real building is 10m but here am using only 2m, so there should be some difference right ?

February 14, 2018, 04:11		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,885 Rep Power: 27	I guess you need far field information in a near field problem. I don't know what a logical function would be. Why not perform a far field calculation with a building of 10m? Then extract the velocity profile on planes that coincide with the inlet boundaries of your near field simulation? Alternatively include the whole building in your near field calculation with a few large elements. That might be more accurate than a CEL-function.