Two years ago on a flight from Amsterdam to Dar Es Salaam, I was a passenger on a 777. On the nearly 10-hour flight, I had plenty of opportunity to consider just how an aircraft that large could be held aloft by the aerodynamic forces that enabled those (albeit very large) wings to make the seemingly impossible, possible.
I studied fluid dynamics in engineering school. I have a private pilot’s license, so understand the concepts of lift, thrust, drag and airflow over a wing. In spite of all this, I’m still impressed by the results of the proper application of these engineering principles.
Please forgive me for going all “Big Bang Theory” on you for a second, but fluid mechanics, specifically Bernoulli’s Principle, explains how wings create lift. The principle states that when the speed of airflow increases, the pressure decreases. Airflow must travel farther, and hence faster, over the top of a typical wing than the air going underneath the wing. So the pressure on top of the wing is lower, and hence, the wing creates lift.
I also learned about the concept of vortex shedding. This occurs when air flows around a non-streamlined body: wires, tubular steel members, pipes and so forth. In this scenario, vortices are created on the downwind side of the object, and they detach periodically from alternating sides of the shape. These create alternating low-pressure areas and can cause the object to move in the direction of the alternating low-pressure areas. The structure can sometimes begin to resonate…vibrate and oscillate in the wind.
Those of us in the electric utility industry actually have occasions where the principles of wings and airflow must be managed to properly design various electric grid components.
Years ago, a two-pole, double-circuit 345 kV tubular steel structure skirted the rural eastern border of Lincoln, Neb. It had been in service for a number of years, and there was a tubular steel arm attached to the structure that was to accommodate a future 115 kV circuit. We were working on a project to do just that; design and attach the additional wires to the existing structures. It seemed pretty simple. Wrong.
While preparing to attach the wires, the contractor noticed that there were rust-stained areas on the painted steel arms. Closer examination revealed that the welds on the tubular arms had corroded and had begun to rust. New arms were quickly ordered and installed, and the project was completed.
A similar issue happened on a project in southeast Iowa. A contractor was building a new line. It was a single-pole, galvanized tubular steel 345 kV system that used davit arms for conductor support. The structures were installed and the contractor began stringing the conductors. Because it was the early days of T2 type conductors, there were issues during stringing that required replacing a number of reels. It only took a few short weeks to get replacement conductors, but by the time the contractor installed the new wires, the arms had begun to crack at the welds.
In each of these cases, vortex shedding and the resulting member vibration caused fatigue failure in the welds on the arms. Where these conditions occur, or are anticipated, solutions can include resizing the members and/or installing damping devices. Sometimes the solution is as simple as hanging insulators or weights on the arms until the conductor is installed. In all cases the structure designer should be involved in the selection of the protection scheme.
In the Midwest, it is not uncommon to have a layer of ice deposit on conductors during the winter months. Most of us who live in this part of the country have probably seen power line conductors dancing, or galloping, during windy, icy weather. Ice can build up on the conductors and essentially turn a circular conductor into a wing shape. If the wind speed and direction are just right, the conductor starts to ”fly,” and you see the galloping.
One solution to prevent galloping is to use twisted pairs of conductors. These provide a constantly variable cross-section on which the wind/ice combination has little to no effect. This would be similar to what would happen if you twisted the wings on a 777; the flight would be very short!
As I write this, it is sunny, breezy and 80+ lovely degrees in Iowa. Yet, I feel compelled to get outside and inspect for galloping conductors…especially those that are near a particular local golf course. I’ll let you know what I find.