How to calculate Wind Loading on a Rectangular Antenna 2

[SolSurfer1]

I need to calculate the wind-loading force (lbf or N) that will be applied to my rectangular antenna enclosure (face area of 2.6 sq ft /0.24 sq m) in a wind speed of 60mph (97km/h). The estimated drag coefficient (based on a flat face with rounded edges) is 1.9.
Can someone provide me with the correct formula to use, or point me in a direction to solving this?
Thank you,
Solsurfer1

 

[btrueblood]

F = (Cd)(Area)(0.5)(rho)(V^2)

...oh, and don't double-post.

 

[SolSurfer1]

Thank you btrueblood for both the equation and the forum policy; I appreciate it.

 

[rb1957]

that is the drag load ... you might consider the antenna as a lifting surface, Cl instead of Cd and acting in the normal direction ... assume a few degress of sideslip (creating AoA for the antenna).

you might also want to consider a "ground handling" load, something like 200 lbs.

 

[SolSurfer1]

rb1957,
Can you elaborate? My apologies, but I don't quite understand; too many years in another field has evaporated my aerodynamics studies.

 

[rb1957]

drag load on an antenna tends to be very small.

you could imagine the antenna (i'm assuming a "large" VHF type of blade antenna) could work like a small lifting surface (generating lift) ... a bigger load, working the attachments and the supporting structure somewhat more, but still typically not very large.

in my experience, if you assume a ground handling load of 200 lbs acting normal to the blade (torquing the attachments) you exercise the attachments a reasonable amount and this causes you to add a reasonable amount of structure to support the antenna so it almost certainly (never in my experimence) never vibrates. typically you'll need a U-channel to support the antenna, clips to attach to adjacent frames and a dblr to hide the rivet CSKs.

and a damage tolerance analysis to define the new inspection program.

 

[rb1957]

seeing your dbl post ... is the antenna on a plane or a building ?

 

[SolSurfer1]

rb1957,
Sorry about the dbl post...wont happen again.
The antenna is mounted to a tower or pole. I'm trying to understand what the wind load on my antenna will have on its mounts and the tower/pole interface.

 

[rb1957]

i think you'll get wind speeds from the building codes. maybe they give you force as well ... i'd expect that they'd define so many psi rather than a wind speed of so many fps. so then it'd be just p*A

 

[gerritgroot]

Just use the drag tables from Hoerner's drag book titled "Fluid Dynamic Drag" from 1958, that will do.

Neglecting deflection (preventing that you have to calculate eventual "lift") will be conservative and realistic.

 

[FeX32]

Draw a CAD model, then model it in a finite element software capable of solving the Navier Stokes equations.
This will then verify your empirical calcs.


Fe

 

[gerritgroot]

@FeX32

No, this will verify your Navier-Stokes solver, not your empirical model. In stalled flow, it's more a coincidence than wisdom if your CFD coincides with measurements.

CFD is not able to predict this in general, the solution is strongly dependent on the turbulence model you use. Even though some companies tuned their solvers endlessly, and their results will be reasonable, you can never, and should never, use CFD in general for these purposes (unless you're doing DNS or so which is still unfeasible nowadays).