wind tunnel study on aerodynamic optimization of suspension ...

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
The Discussion on Mechanism of Aerodynamic Improvement
According to the research results of airfoil (Abbott I. H., 1958), all classical airfoils
are stalled at 16 deg. The lift is increasing with increasing attack angle, and once exceeding
16 deg, the lift decreases, see figure 7. The flow detaches from the suction side and forms a
vortex while the airfoils are stalled. Larsen based on the experience from vortex shedding
tests (Larsen A. et al., 2000, 2008 ), found that the trapezoidal box sections was similar to the
airfoils, and gave a conclusion that the flow along the bottom plate would stay mainly
attached if lower inclined web angle α is less than approximately 16 deg. He also gave an
example about the design <strong>study</strong> for a two span suspension bridge in Chile, whose lower
inclined web angle is 14.8 deg, and no vortex-induced vibration was observed in the <strong>wind</strong>
<strong>tunnel</strong> testing. Another example was a new 1345 m suspension bridge in northern Norway,
with 15.8 deg of lower side web angle, and the <strong>wind</strong> <strong>tunnel</strong> tests demonstrated that vortex
induced vibration were absent. The contrary examples were box girders of the Great Belt East
Bridge and Osterøy Bridge, lower inclined web slope angles are 26.6° and 29.5°, respectively.
Explicit vortex induced vibrations were observed for these bridges in full scale.
Figure 7: The Lift Coefficient of Wings Varying With Attack Angle
Similarly to a box section in the vortex shedding vibration status, in the flutter critical
status, two vortices with counter direction have also been observed at the both sides of the
nose tail line of Great Belt East Bridge section through the PIV (Particle Image Velocimetry)
technique(Zhang et al., 2009), which can give the girder enough momentum to increase its
amplitude in a shot time and finally make the girder instable. When the <strong>wind</strong> speed is low,
and under the flutter critical speed, the positive vortex blow the nose-tail line is more
powerful than the above negative one, the aerodynamic force is just a static lift force, see
figure 8. When the <strong>wind</strong> speed is increasing, and close to the flutter critical speed, the
negative vortex above the nose-tail line has been strengthen as powerful as the blow positive
one, and the aerodynamic force becomes to fluctuated force, see figure 9. If the frequency and
the phase of the fluctuated force are close to bridge’s, the flutter of girder will be occur soon.
In this paper, when the slope of the lower inclined web decreases to 15 deg(also less
than 16 deg), there is a smaller dead air wake region below the nose-tail and the flow along
the bottom plate will stay mainly attached the web, which making more difficult for formation
of a large vortex. When the <strong>wind</strong> speed is increasing to the original flutter critical <strong>wind</strong> speed
(former test girder with low critical <strong>wind</strong> speed), because the little room gives the restrain to
the forming of vortex, the counter vortices can’t give the girder powerful and efficient