Antennas and Open-Frame Structures: Aerodynamic Interference and Wind Loads, Slides of Environmental Law and Policy

Download Antennas and Open-Frame Structures: Aerodynamic Interference and Wind Loads and more Slides Environmental Law and Policy in PDF only on Docsity!

Antennas and open-frame

structures

• Single frames
• Multiple frames
• Antennas - isolated structures - radio telescopes and microwave antennas
• Lattice towers
• Antennas - attached to towers - aerodynamic interference

Common feature : aerodynamic interference between various elements - e.g. antennas and supporting tower or other antennas, members of a frame

• Radio telescope
• Paraboloid dish

Normal to dish surface intersects axis at 2  focal length

2f

Approximate center of aerodynamic forces 2

e 2f  d

e

d

C

• Radio telescope
• Paraboloid dish

Wind

FX

FY

e

b



d

Fy force generates significant moments about dish supports

• Microwave dish antenna
• Impermeable dish

Small effect of turbulence

0 20 40 60 80 100 120 140 160 180

1% turbulence 10% turbulence

 (degrees)

b 21 a^2 A^ 

D (^) ρ U C ( ) D(^ ^ )

A = (b^2 /4) (projected area)

• Microwave dish antenna
• Interference factor WIND

WIND

Da

Dt WIND

De

Da

De Dt Ki

 

De  Dt  Ki.Da

• Cell-phone antenna
• isolated panels

b

Cd 1.

120 O

Cd (ref.b)  0.

• Cell-phone antenna
• grouped panels

~2b

combined Cd (ref.b)  1.

combined Cd (ref.b)  0.

grouping gives large reduction in total drag

• Open frames (^) • Single frame. Two-dimensional. Normal wind
  • sharp-edged members

reference area for drag coefficient = ‘solid’ area of frame

0 0.5 1. Solidity ratio, 

CD

solidity = ‘solid’ area of frame/total enclosed area

drag coefficient relatively independent of details of member arrangement

• Open frames (^) • Single frame. Two-dimensional. Normal wind

at high solidity, frame acts as a solid plate (Lecture 8)

at low solidity, members act as individual elements

intermediate solidity : aerodynamic interference between members CD  1.

0 0.5 1. Solidity ratio, 

CD

• Open frames
• 3 frames in series. Solidity = 0.

X(a) = force normal to frame

ρ U A

C ( ) X(^ ) 2 21 a

N X α ^ α

angle of attack, a

spacing/width = 1.

spacing/width = 0.

15 75

A = projected area of one frame at 0 o^ angle of attack

• Open frames
• 3 frames in series. Solidity = 0.

A = projected area of one frame at 0 o^ angle of attack

Maximum CXN^ at 30o^ to 45o angle of attack, a

spacing/width = 1.

spacing/width = 0.

15 75

ρ U A

C ( ) X(^ ) 2 21 a

N X α ^ α

• Open frames
• 10 frames in series. Solidity = 0.

A = projected area of one frame at 0 o^ angle of attack

Maximum CXN^ at 30o^ to 45o angle of attack, a

spacing/width = 1.

spacing/width = 0.

15 75

ρ U A

C ( ) X(^ ) 2 21 a

N X α ^ α

• Open frames
• Design method :

‘Wind loads and anchor bolt design for petrochemical facilities’ (ASCE)

Needs more wind tunnel studies for pipe racks etc.