In the picture you see a person walking a slackline. A slackline is a tensioned flatband of polyester. Typical tensions are between 1 kN to 15 kN depending on the length of the line. The lines are very thin (about 1 mm - 2 mm I would say), but the width is usually an inch (2.5 cm). There are other materials than polyester in use, but the properties are very similar. Under the high tension the lines are very "bouncy", so they stretch a lot and have relatively high efficiency in returning the bounce energy.
My question is about the oscillations you can see on the rope. For simplicity let us assume that no person is walking on the line, the oscillations still appear then, I just couldn't find a better photo.
The oscillations are cause by the wind. There are two kinds, transverse waves and rotational/twist waves which are visible since the cross-section of the line is not circularly symmetric. I know that the driving force for the transverse waves can be explained by a Kármán vortex street (see also this question and answer about the Tacoma bridge). I was wondering how that relates to the driving force for the rotational waves?
- Is the principal the same as for the transverse waves? There is a few reasons why I can't really see that working: the rotational waves in this case are sometimes so large that the flat part goes vertical (i.e. against the wind) or even rotates by multiple revolutions.
- The most important one: Why is the wavelength of the rotation waves (i.e. the separation of the nodes as can be seen in the picture) so much lower than for the transverse waves? This is an observation that is not obvious from the picture, but usually there are only like 3 or 4 oscillations nodes for the transverse ones and like 30 for the rotational ones.
- The question will get too broad if I ask more questions about this, so let's focus on the above. I am generally interested in this phenomenon and there are other questions I can't quite answer (e.g. when it is not very windy there aren't any oscillations and sometimes when the wind is weak oscillations come and go. So why is there a "critical wind speed" at which they start resonating up?).