Standing wave

My route to and from work takes me along an open stretch next to some farm fields. A series of power poles run next to the bike path, and yesterday’s high winds—30 to 40 mph—set the bottom wires into a second mode standing wave pattern. My camera work was a little unsteady because I was trying to keep my bike from blowing over while shooting, but you can clearly see the node at the center and the two alternating antinodes.

Notice that the higher, tighter wires aren’t vibrating much at all. Only the looser wire at the bottom has a low enough natural frequency to be excited so strongly. Other sections of the lower wire, where the spacing of the poles was different, didn’t vibrate like this, either; the change in length between the supports led to a different natural frequencies in those sections.

Because the vibration seems to be horizontal rather than vertical (that is, in the along-wind direction instead of in the across-wind direction), it doesn’t match the usual pattern seen in vortex shedding, the phenomenon that causes flags to flap. Maybe there’s some interaction between the direct horizontal loading of the wind and the vertical loading from vortex shedding. Or maybe the driving force is something else altogether.

Whatever the cause, it was pretty cool looking.