Balanced/Imbalanced wind loads on Parabolic Dish

[mike cangelos]

Friends,

I have a solid (not mesh) parabolic dish mounted on a 2-axis tracking mechanism. This assembly is often mounted atop buildings or open scapes and is subject to direct and indirect wind loading. I am trying to recall my fluids and aero schooling but am not sure of some assumptions. This piece is physically similar to satellite dishes mounted outdoors if this helps those of the left brain.

My concern and request for advisory here is on worst case scenarios based on positioning of the unit within winds at about 50mph. The mounted dishes are interchangeable and are roughly 48" and 64".

1: I envision there being significant loading when the dish is in the park position (pointing straight looking like a bird bath) and the wind loading is side loading the assembly.

2: Dish pointing 90deg to post and there being
A: uniform balanced airflow head on
B: imbalanced force on one side of dish; perhaps possible if mounted nearby structure causes laminar flow and whips airstream only partially loading dish face.

Ultimately I would like to quantify the rough Torque values on the post where the drive train resides. In order to do so first making sure the assumptions with wind loading are in order. I will gladly elaborate more information regarding this system but do not want to risk disconcerting you with a lengthy first post.

Best Regards,

MIKE


+++by the way is Andy Resnick the Andy Resnick of UNC Ch-Hill Physics Dept?

[minger]

Quick Google gave me this:

http://ocw.mit.edu/NR/rdonlyres/Civil-and-Environmental-Engineering/1-051Fall2003/B56746CA-BF5C-4E25-AF42-418CF0C59202/0/rec1wind_eqloads.pdf

I guess there are ASCE standards for calculating wind loads. An accurate answer for the load and consequentally the moment applied will be requesite on you getting and accurate loading condition/magnitude.

As a quick back-of-paper guess, you could just use the drag equation:

F_d = \frac{1}{2} c_d \rho v^2 A

A table of drag coefficients in the engineering toolbox:

http://www.engineeringtoolbox.com/drag-coefficient-d_627.html

Will give you a value somewhere between 1.7 and 1.1. Get a high-low go from there.

[schroder]

[QUOTE=mike cangelos;1865930]Friends,

I have a solid (not mesh) parabolic dish mounted on a 2-axis tracking mechanism. This assembly is often mounted atop buildings or open scapes and is subject to direct and indirect wind loading. I am trying to recall my fluids and aero schooling but am not sure of some assumptions. This piece is physically similar to satellite dishes mounted outdoors if this helps those of the left brain.

My concern and request for advisory here is on worst case scenarios based on positioning of the unit within winds at about 50mph. The mounted dishes are interchangeable and are roughly 48" and 64".

1: I envision there being significant loading when the dish is in the park position (pointing straight looking like a bird bath) and the wind loading is side loading the assembly.

2: Dish pointing 90deg to post and there being
A: uniform balanced airflow head on
B: imbalanced force on one side of dish; perhaps possible if mounted nearby structure causes laminar flow and whips airstream only partially loading dish face.

Ultimately I would like to quantify the rough Torque values on the post where the drive train resides. In order to do so first making sure the assumptions with wind loading are in order. I will gladly elaborate more information regarding this system but do not want to risk disconcerting you with a lengthy first post.

Best Regards,

MIKE


The first place I would go for the wind loading information is the antenna manufacturer. Most manufacturers, such as Andrew Antenna, have done extensive testing using wind tunnels and can provide you with detailed profiles. In general, the best position is straight up (like a bird bath) which is refered to as the 90 Degree stow position. The wind load contour is typically close to zero along the edge in this position. The zero degree position, with the antenna facing directly into the wind will take the most wind load, with contours typically around 1.65 in the lower center ranging to 1.5 at the edge. I’m going from memory here. The 180 Degree position, facing away from the wind is almost as high loading as the zero Deg. with contours up around 1.4. Now, you want to be careful at the 60 Deg and 120 Deg positions as there you get some real bad flutter effects around the edges that can cause severe vibration and damage. It is often the vibration which tears tha antenna off its mounts rather than static loading. Generally anything above 80 Degrees is considered to be at stow and the dish should be able to take up to 100 mph there. But you really need to get this info from the manufacturer especially if you are looking to get permits to put these up in the city.