Civil Engineering Reference
In-Depth Information
The aerodynamic derivatives for a flat plate that emerge from the buffeting theory
(i.e. quasi-static values) are obtained from Eq. 5.26 by the introduction of
⎡
CC
CC
CC
′
⎤
00
02
02
⎡
⎤
DD
⎢
⎥
⎢
⎥
′
=
π
(5.29)
⎢
⎥
⎢
L
L
⎥
⎢
⎥
⎢
′
⎥
π
⎣
⎦
⎣
⎦
MM
Thus, for a flat plate the non-zero quasi-static aerodynamic derivatives are given by
π
⎡
⎤
ˆ
ˆ
2
VV
−
π
−
*
*
⎢
⎥
⎡
HA
HA
⎤
i
i
2
1
1
⎢
⎥
⎢
=
⎢
⎥
(5.30)
*
*
π
⎥
⎢
⎥
ˆ
ˆ
2
2
⎣
⎦
⎢
2
VV
3
3
π
i
i
⎥
2
⎣
⎦
5.3 Vortex shedding
Fig. 5.4
Relevant displacement components and vortex shedding forces
When the air flow is met by a solid bridge or tower type of structure flow separation will
occur on the surface of the structure causing vortices to be shed alternately on either side
of the structure.
Assuming that along wind load effects may be disregarded, these vortices give rise to
fluctuating across wind forces
q
and cross sectional torsion moment
q
θ
, accompanied
by fluctuating displacements
r
or
r
θ
, as shown in Fig. 5.4. Harmful vortex induced
vibrations may particularly occur in cases of resonance.
