# question about streamwise direction in the porosity

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 June 5, 2013, 22:06 question about streamwise direction in the porosity #1 New Member   shaodong Join Date: Jun 2013 Posts: 4 Rep Power: 5 Hi, everyone! I have encountered a question when I set the streamwise direction in the porosity domain. The porosity domain is of revolution, and consquently the streamwise direction varies with theta, which is shown in the pic. However, the option in setting porosity just provides a unquine direction, which can't vary with theta. Is anyone helping me?

 June 5, 2013, 23:26 #2 Senior Member   Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 388 Rep Power: 8 You can specify the direction in cylindrical coordinates rather than Cartesian. Looks like that would work for your case.

June 6, 2013, 00:25
#3
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shaodong
Join Date: Jun 2013
Posts: 4
Rep Power: 5
Quote:
 Originally Posted by cdegroot You can specify the direction in cylindrical coordinates rather than Cartesian. Looks like that would work for your case.
I just specified the direction in cylindrical coordinates.
what confused me is the option only set one direction.
When theta=0 degree, the direction is 45 degree and in cylindrical coordinates r=1, z=1, which is shown in the left of the pic.
When theta=180 degree, the direction is 135 degree and in cylindrical coordinates r=1, z=1.
In the option , the theta is just a value, and I can't let it equal from 0 to 360.
What shall I do?
http://ww4.sinaimg.cn/bmiddle/65a92d...20g105hq3f.jpg

 June 6, 2013, 07:52 #4 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 12 Why do you insist on using a thicker porous model? Why not use the porous interface? Since the flow is normal to the inlet surface of your porous zone and proceeds (relatively) normally, interface makes sense. You can deduce the pressure drop as a function of dot product of local surface normal of inlet surface and the local velocity. OJ

June 6, 2013, 10:39
#5
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Chris DeGroot
Join Date: Nov 2011
Posts: 388
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Quote:
 Originally Posted by starhunter521 Thank you for your suggestion. I just specified the direction in cylindrical coordinates. what confused me is the option only set one direction. When theta=0 degree, the direction is 45 degree and in cylindrical coordinates r=1, z=1, which is shown in the left of the pic. When theta=180 degree, the direction is 135 degree and in cylindrical coordinates r=1, z=1. In the option , the theta is just a value, and I can't let it equal from 0 to 360. What shall I do? http://ww4.sinaimg.cn/bmiddle/65a92d...20g105hq3f.jpg
The theta value you are entering would be the component of the direction vector in the theta direction (not a value of theta), which in your case is zero. The direction vector has only components in the r and z directions. Since the vector is pointing inwards (towards the rotation axis) it seems like your r component would be -1 and your z component would be z=1 to have a 45 degree angle from the horizontal. This assumes the rotation axis is at the centre of your volume of revolution.

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