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Antennas and open-frame structures. Wind loading and structural response Lecture 23 Dr. J.D. Holmes. Antennas and open-frame structures. Antennas - isolated structures - radio telescopes and microwave antennas. Antennas - attached to towers - aerodynamic interference. Single frames.

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antennas and open frame structures

Antennas and open-frame structures

Wind loading and structural response

Lecture 23 Dr. J.D. Holmes

antennas and open frame structures1
Antennas and open-frame structures
  • Antennas - isolated structures - radio telescopes and microwave antennas
  • Antennas - attached to towers - aerodynamic interference
  • Single frames
  • Multiple frames
  • Lattice towers

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

antennas and open frame structures2

f

Focus

Antennas and open-frame structures
  • Radio telescope
  • Paraboloid dish
antennas and open frame structures3

2f

Approximate center of aerodynamic forces

d

C

e

Antennas and open-frame structures
  • Radio telescope
  • Paraboloid dish

Normal to dish surface intersects axis at 2  focal length

antennas and open frame structures4

FY

d

FX

e

b

Wind

Antennas and open-frame structures
  • Radio telescope
  • Paraboloid dish

Fy force generates significant moments about dish supports

antennas and open frame structures5

Focus

0.10

0.08

0.06

0.04

0.02

0

Total moment

Azimuth angle, 135o

Effect of boundary

layer profile

Zenith angle b

Altitude moment

CM

Azimuth moment

Wind

Altitude axis

Azimuth angle

0 20 40 60 80 90

Zenith angle, degrees

a

PLAN VIEW

Antennas and open-frame structures
  • Radio telescope
  • Paraboloid dish
antennas and open frame structures6

2.0

1.5

b

1.0

0.5

1% turbulence

10% turbulence

0.0

120

140

160

0

40

60

180

80

100

20

 (degrees)

Antennas and open-frame structures
  • Microwave dish antenna
  • Impermeable dish

A = (b2/4) (projected area)

Small effect of turbulence

antennas and open frame structures7

Da

WIND

Dt

WIND

De

WIND

Antennas and open-frame structures
  • Microwave dish antenna
  • Interference factor
antennas and open frame structures8

Experimental data

Equation with t=0.5

1.5

1

Interference factor

0.5

0

0

45

90

135

180

Wind direction (degrees)

Antennas and open-frame structures
  • Microwave dish antenna
  • Interference factor

Ki = exp [-k(CD)2]. [(1+t) + t cos 2( - d - 90)]

antennas and open frame structures9

Cd (ref.b)  0.8

Cd1.1

120O

b

Antennas and open-frame structures
  • Cell-phone antenna
  • isolated panels
antennas and open frame structures10

combined

Cd (ref.b)  1.1

combined

Cd (ref.b)  0.9

~2b

Antennas and open-frame structures
  • Cell-phone antenna
  • grouped panels

grouping gives large reduction in total drag

antennas and open frame structures11

60o

0o

Antennas and open-frame structures
  • Cell-phone antenna
  • grouped panels

total drag of group : about 30% less than sum of individual elements

antennas and open frame structures12

2.0

CD

1.0

0

0.5

1.0

Solidity ratio,

Antennas and open-frame structures
  • Open frames
  • Single frame. Two-dimensional. Normal wind
  • sharp-edged members

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

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

drag coefficient relatively independent of details of member arrangement

antennas and open frame structures13

2.0

CD

1.0

0

0.5

1.0

Solidity ratio, 

Antennas and open-frame structures
  • Open frames
  • Single frame. Two-dimensional. Normal wind

at low solidity, members act as individual elements

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

intermediate solidity : aerodynamic interference between members CD 1.6

antennas and open frame structures14

b

s

approximately, 1  1,

Antennas and open-frame structures
  • Open frames
  • Pairs of frames. Two-dimensional. Normal wind

CD(2) = CD(1) [ 1 + 2]

1 CD(1) is drag coefficient of upstream frame (downstream frame influences upstream frame)

2 CD(1) is drag coefficient of downstream frame

0 <  < 0.5

For circular members, equivalent solidity to calculate 2 , e  1.2 1.75

antennas and open frame structures15

spacing/width = 1.0

spacing/width = 0.1

15

75

angle of attack, 

Antennas and open-frame structures
  • Open frames
  • 3 frames in series. Solidity = 0.1

X() = force normal to frame

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

antennas and open frame structures16

spacing/width = 1.0

spacing/width = 0.1

15

75

angle of attack, 

Antennas and open-frame structures
  • Open frames
  • 3 frames in series. Solidity = 0.5

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

Maximum CXN at 30o to 45o

antennas and open frame structures17

spacing/width = 1.0

spacing/width = 0.1

15

75

angle of attack, 

Antennas and open-frame structures
  • Open frames
  • 10 frames in series. Solidity = 0.1

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

slide19

spacing/width = 1.0

spacing/width = 0.1

15

75

angle of attack, 

Antennas and open-frame structures

  • Open frames
  • 10 frames in series. Solidity = 0.5

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

Maximum CXN at 30o to 45o

antennas and open frame structures18
Antennas and open-frame structures
  • Open frames
  • Design method :
  • ‘Wind loads and anchor bolt design for petrochemical facilities’ (ASCE)

Needs more wind tunnel studies for pipe racks etc.

antennas and open frame structures19

4.0

3.5

3.0

2.5

2.0

1.5

Drag coefficient CD (q=0O)

Australian Standards

0.0 0.2 0.4 0.6 0.8 1.0

Solidity Ratiod

Antennas and open-frame structures
  • Drag coefficients for lattice tower (Lecture 21)

Square cross section with flat-sided members (wind normal to face)

CD = 4.2 - 7 (for 0.1< < 0.2)

CD = 3.5 - 3.5 (for 0.2< < 0.5)

(ASCE-7 : CD = 4.02 – 5.9 +4.0 )

 = solidity of one face = area of members  total enclosed area

includes interference and shielding effects between members

antennas and open frame structures20
Antennas and open-frame structures
  • Drag coefficients for lattice tower

Triangular cross section with flat-sided members

CD = 3.5 - 4 (for 0.1< < 0.3)

CD = 2.9 – 2 (for 0.3<  < 0.5)

(ASCE-7 : CD = 3.42 – 4.7 +3.4 )

antennas and open frame structures21
Antennas and open-frame structures
  • Drag coefficients for lattice tower

Cross section with circular members

depends on Reynolds Number

for super-critical flow - Cd for cross section ~ 0.5 times that for equivalent sharp-edged tower with same solidity

some members may be in super-critical flow - others in sub-critical flow