Wind-Loading question for the Pros

[Xringer]

I'm working on a DIY solar project Solar Tracker Project - EcoRenovator
and, I want to install 4 PV solar panels on a post mount, in a square configuration.

Each panel is approximately 4'x3' and will present a pretty good wind-load.
The 8'x6' square of panels will be mounted at the top of a 6' post.
And will be angled up towards the south at about 42 degrees.

In the event of a strong wind from the north, it seems like panels will be
getting some 'lift', acting as a wing.

So, here's my question:
If I didn't let the panels touch each other, but inserted an open space of 8" between them,
(a '+' gap) for air to pass between them, would the wind-loading be More, or Less??

Wind spec: Up to 50 mph gusts.


~~
Logically, it seems to me that the 8" spacing would make things worse.
Because all the extra edges are exposed and adding drag..
But, something is telling me that air passing through the '+' gaps
will be like air on a screen door.. Less loading.?.

What Say Ye??


Thanks,

[chaster]

Standard Disclaimer: I am not an expert, but aerodynamics was my field of study, so I have some training at least..

As in all flows, the answer is "it depends". It will vary with each gust of wind actually. Aerodynamic force is an interesting beast. Most of what you observe is not due to "frictional" forces but rather from pressure gradient (differences).

So, even though you might increase frictional drag by exposing the "edges", you are also decreasing the pressure gradient force by allowing the flow a more direct path to the low pressure area (downward side of the plate) and at the same time decreasing the amount of cross "wash" from adjacent panels. The thickness of the boundary layer and the turbulence level will have effects the amount of force as well, but like I said, that's going to vary with each and every gust.

Generally, my inclination is that you will encounter less force on the array mount by putting space in between each panel. But that's just my 2 cents.

Eric

[Piwoslaw]

How about wind breaker on the north? Maybe some trees?

[Xringer]

There are trees to the north, (behind the dish is north).

But in the winter time, they don't provide much buffer.
About 70 yards to the northwest, there is an open pond.
The wind comes off the pond, into the small woods (swamp area)
and then into my backyard.

That tree directly behind the dish is going to be where I plan to tie a support cable..

[Bicycle Bob]

SWAG: With the gaps, the loading per unit area goes down, but the area goes up, so overall, the force is greater. This is from an imagined wake survey.

[aerohead]

I'd be inclined to gang them close together,consolidating them into a single 'structure.' Seems like all the moments would be reduced to the minimum given any load,so all the stresses would also be minimized.
One frontal area at Cd 1.11 vs many smaller frontal areas at Cd 1.11,with complete flow separation for both configurations.I suspect that the gaps wouldn't relieve the drag as they'd already be embedded in the separated wake of each panel whose interference drag might escalate the drag over the single structure.
As a guess,you could design for perpendicular flow.

( 1/2 )(0.00237)(1.11)(frontal area)( anticipated worst gust velocity-squared) = Force resolved at center of heliostat.
Your mount, standpipe ,and concrete footing would be required to stand up to that loading.
I wouldn't worry about lift. As others have mentioned,it will be the delta-P across the front/back of the unit which develops the load.
Also,make sure the mount is very strong,as at times there will probably be a Karman vortex street developed behind the heliostat,and this cyclic vortex shedding will want to shake your work apart.

[Xringer]

Thanks for the info gentlemen. I've put a DIY anemometer up on a topped tree
(the big green thing) behind the mount area and so far, I'm seeing very little wind.
Even when the local weather stations are showing 10 & 12 MPH, I'm seeing hardly anything at the solar mount.
So, maybe the trees and the slight ground depression is going to make this a safe area for some solar PV panels.




After about 7" of rain, there is No water in the solar tracker hardware
and it's tracking the sun like a champ!

[chaster]

Originally Posted by Bicycle Bob

SWAG: With the gaps, the loading per unit area goes down, but the area goes up, so overall, the force is greater. This is from an imagined wake survey.

Eh? Area goes up? How so? A gap between panels does not increase the area of the panel as I understand it...

[chaster]

Originally Posted by aerohead

I'd be inclined to gang them close together,consolidating them into a single 'structure.' Seems like all the moments would be reduced to the minimum given any load,so all the stresses would also be minimized.
One frontal area at Cd 1.11 vs many smaller frontal areas at Cd 1.11,with complete flow separation for both configurations.I suspect that the gaps wouldn't relieve the drag as they'd already be embedded in the separated wake of each panel whose interference drag might escalate the drag over the single structure.
As a guess,you could design for perpendicular flow.

( 1/2 )(0.00237)(1.11)(frontal area)( anticipated worst gust velocity-squared) = Force resolved at center of heliostat.
Your mount, standpipe ,and concrete footing would be required to stand up to that loading.
I wouldn't worry about lift. As others have mentioned,it will be the delta-P across the front/back of the unit which develops the load.
Also,make sure the mount is very strong,as at times there will probably be a Karman vortex street developed behind the heliostat,and this cyclic vortex shedding will want to shake your work apart.

Good points. The question in my mind is the magnitude of the interference drag..

[Bicycle Bob]

Originally Posted by chaster

Eh? Area goes up? How so? A gap between panels does not increase the area of the panel as I understand it...

I was taking the overall size, gaps included.

It may be that the real wind hazard in this location is falling trees. You want to think of the kinds of wind that only happen every hundred years. You might want to arrange things so you can haul the panels to your basement as fast as the neighbors can board up their windows.