Wind Sway and the Tokyo ANSYS Convergence Meeting

In June, I had the pleasure and privilege to present at the Tokyo ANSYS Convergence User Group meeting. Presenting highlights of the ANSYS fluid dynamics solution to more than 1,200 attendees was exhilarating! But since many of you may have attended these events, I won’t do a repeat of the presentation here. Instead, i will share my visit to the

Tokyo SkyTree tower, that city’s new TV tower. The old Tokyo Tower was too small to transmit digital TV signals, since many high rises obstructed its line of sight. Therefore, the SkyTree tower was built and measures 634 meters high, which is 301 meters higher than the old Tokyo Tower. It is also the tallest tower in the world and the second tallest structure, just behind the Burj Khalifa.

While visiting, I overheard a couple asking each other if a violent wind could sway the tower. The answer, thanks to clever architecture, is that it will not. (I could have told them this answer, but who would trust me!) How could a 650 meters thin tower not sway in the wind? The answer is in the core design.

I’ll start by reviewing what could make the cylindrical tower sway. Is it the pressure of the wind? No! It’s actually the forces created by the vortices on the downwind sides of the cylinder tower, which builds and breaks up at the same frequency, but at different phases. If you were to look at those vortices, you would see a Von Karman Street structure as shown on this video.

Each of those vortices, when building up or breaking up, induces different pressure on the side of the building. As they are out of phase on each side, this makes a long cylinder sway in the direction perpendicular to the wind direction.

You might think that this explanation so far is not helping, since I told you that a tall building will sway in the wind! No panic, there is a good news!

The frequency of those vortices changes as a function of the shape of the structure and the radius of the cylinder (if the structure is cylindrical). Therefore, the architects who designed the Tokyo SkyTree tower made sure to use this trick by building a tower where the base is large and triangular and, as the height increases, transitions to a cylindrical structure of always-changing diameter.

The vortices will always exist, but they will have different frequency and behavior at different heights, thus making sure that none of the associated pressure forces will be able to build in a coherent manner to sway the building. Of course, the architects performed some CFD to make sure the design was correct!

Want to learn more about the aerodynamics of buildings and how CFD can be used? Visit the Construction/Wind Engineering page on ansys.com.

Now that we know about wind sway, what about earthquakes? Upon further investigation I learned that the seismic proofing ensures that 50% of the seismic energy would be absorbed by the tower. But as I was standing at 450 meters above the ground, I thought — what about the other 50%! Oh oh …

Still, if you ever are in Tokyo, don’t hesitate to visit the SkyTree tower to see amazing views of the city!

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