I am working with antenna modeling, and besides directivity or gain , I am interested in wind load. The formula says wind load is proportional to Area , the drag coefficient and the length/width ratio. Could you point to a OpenFOAM or Ansys model / document about how to get that coefficient. DO I need to get the force on the antenna due to the wind and then obtain directly the coefficient, or there is another trick. Do you know a way to automatically generate many models on those softwares? (I need to simulated hundreds of antennas, for example Yagis with different dimensions of elements and/or separation between elements.
I used to work in this field a couple of decades ago. At that time we just looked up drag coeficients from tables (CP3 part V, Wind Loading on Buildings) and applied the resulting uniformly distributed loads directly to different parts of the structure assuming one or two worst case loading conditions. We didn't bother with shielding effects (one element shielding another from the full wind velocity) as we knew this simplification was a conservative one.
This paper may help you: http://www.ngtsummitapac.com/media/whitepapers/Andrew-Solutions0_NGTAPAC.pdf
Alternatively you could try contacting the author.
hi Eric, the max evenly distributed load will be F=1/2 x rho x A x V^2. If you run a CFD model and get the wind direction drag force, say Fcfd then the Drag coefficient will be
Cd = Fcfd / F
If you want to chat about the modelling I would be happy try and help.
Hi Eric
CFD is the last resort! You could find drag coefficient in most of the wind codes. For instance, Australian wind code AS/ANZ 1170.2 has all you need. It also includes specific analysis methodology for towers which best matches with your case of antenna.
Amir
Thanks to Eric and Allan. In a antenna like this: http://en.wikipedia.org/wiki/Evolved_antenna , how should I calculate Area A?
Amir, I want to approximate wind load in strange looking antennas (example: fractal antennas, or evolved antennas like this: http://en.wikipedia.org/wiki/Evolved_antenna); those antennas are not listed in any table or code.
Dear friend
Each structure is consisted of simple elements, i.e. rods, beams, plates, etc, which have simple geometrical shapes. The drag coefficient for these shapes are given in codes and standards. The links you provided above does not give any information such that I can suggest what should be done. But if you could decompose your antenna into simple geometric shapes, then you can obtain drag coefficient for each of them from a code, calculate wind pressure on each member and add them up to get to full wind load. I assume you are not a structural engineer, you can get help from one as calculating wind loads is a routine in their everyday work. They may even offer you software that automatically or semi-automatically calculates the force.
Amir
Dear Amir, It s true, I am not structural eng., I am a computer / electronics guy. I appreciate your answer. Like you said, my antennas are mostly beams joined in strange arrangements, but beams anyway. One question, if I get the drag coefficient for the beams but they are not all neither in the same plane nor on perpendicular plane, how must I proceed? I must use the complete length of the all beams, including "not on the same plane" beam, or only the length of its projection on the plane facing the wind?
Regards
Dear Glen, this is an interesting area of research. It would always be the projected area that you use to calculate the drag force. Fractals have quite a few interesting effects on the drag force and the resulting wake. Prof. C. Vassilicos at Imperial has done some interesting work in this area. The antenna in your link does resemble the stems of many plants. I think if you move onto fractals it would make sense to include some testing combined with CFD. Regards Allan.
Glen
Since you are not a structural engineer, I assume that your interest on wind loading is not from a research stand point on this topic. It should be something minor to your main topic of research. If it is right, then you don't need to enter the state of the art wind research. All you need, probably is some coefficients and some book or standard which describes the analysis method for you. For the actual analysis, I advise you seek assistance from a structural engineer using standard software. For your information, I extracted some tables of drag coefficients from some book I had. I strongly advise you to read Australian standard AS 1170.2. In Appendix A it includes staff that most probably fits your purpose. For a brief description of wind loading and analysis method, you may consult NORSOK Standard N-03. You may find free pdf copies in the net. Hope this information helps you.
Amir
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