How Do the World’s Largest Flags Actually Fly?

Flags are designed to display a message.

by Dr. Eberhard Haug
flags

Do you ever see an enormously large flag and think to yourself, “how does that flag actually fly?”. Ok, maybe you don’t – but I bet you do ask yourself how you can save money and time on building lightweight structures.

Discover eight of the world’s tallest flags and how wind velocity exposure affects flying performance. It is essential that the flag, pole and attachments are designed to ensure optimum display and safe performance in varying natural winds. These studies, performed by SL Rasch, can help save time, energy and cost when deciding how to build similar extraordinary and lightweight structures in the future.

SL Rasch GmbH, a German architecture firm that specializes in extraordinary constructions and lightweight structures, explored the performance of large national flags hoisted on very tall flag poles flying in the natural winds of their respective locations.

A list of the world’s tallest flags:

flagsmap
World’s Tallest Flags

Ever-increasingly large flags require even heavier, more tear-resistant fabrics, but nevertheless should present as consistently flying even under moderate winds.

In a pilot study, SL-Rasch chose to simulate the behavior of a tall flag at scale 1:3 to determine the design requirements for the fabric´s tensile strength and weight, its attachments, and the flag pole dimensions to guarantee the effective and safe performance of such tall flags both at low and high average wind speeds.

The challenge was for SL Rasch to represent numerically the sustained motion of the flexible fabric, waving in the air under different average horizontal wind velocities, and over sufficiently large time intervals for statistical relevance.

Fluid-Structure Interaction (FSI) simulations – coupling Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD), make it possible to track the complex waving motions of the flexible fabric in the winds and the stresses in the fabric, as well as the flag pole’s bending moments caused by the horizontal drag forces from the flag.

The flag’s CSD model was placed in a 600x400x400m CFD domain.

cfddomain
SLR Flying Flag in CFD domain at height 150mts

The safe average wind speed before bringing the flag down was assumed to be 30m/s at a height of 150m. Using ESI’s Virtual Performance Solution and CFD codes in coupled FSI mode, produced the time snapshots at 30m/s average wind speed shown below.

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SLR Flying Flag snapshots at 30m/s wind speed at 150m

The lowest average wind speed under which the flag still displays its message was determined in subsequent simulations under decreasing wind velocities.

To this end, the average high wind velocity was suddenly reduced from 30m/s to 15m/s, 3.77m/s and 1.25m/s.

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Ashgabat 133m Flag and calculated SLR Flying Flag snapshots at winds decreasing to 15m/s, 3.77m/s and 1.25m/s

The results suggest that the flag will no longer display its message at wind speeds below about 5m/s.

About SL Rasch

SL Rasch specializes in extraordinary constructions and Lightweight Structures, integrating architecture and engineering. The company harbors an interdisciplinary team of professionals, composed of architects, structural engineers, mechanical engineers and computer specialists, together with in-house departments for project management, graphic design and model making.

For more information visit: SL RASCH collaborates with ESI to achieve architectural excellence in Mecca and Medina


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