AN INFORMATIVE BULLETIN PROVIDED BY
Warren Environment, Inc.
VORTEX SHEDDING
When wind flows around a structure a positive pressure is exerted on the windward side of the structure and a negative pressure is created on the leeward side of the structure. This pressure differential will cause air to “swirl” around the structure creating a vortex. These vortices alternately create zones of negative pressure which can pull a stack from side to side. This action is similar to the lift created by an airplane wing.
Like a huge tuning fork, each stack has it’s own natural frequency. This is the frequency at which the stack will vibrate on its own. In addition to a natural frequency, each stack has its own ability to arrest, or dampen, movement. Steel stacks have little inherent structural damping. Therefore, the displacements associated with vibration due to vortex shedding can grow quite large when their frequency coincides with the stack’s natural frequency.
Steel stacks frequently have self-induced cross wind movement due to vortex shedding. Since steel stacks are relatively light weight structures, this phenomenon occurs at common wind speeds. When the vortex shedding frequency equals the natural frequency of the stack, the stack has little natural resistance to the dynamic forces. The wind speed at which the vortex shedding frequency matches the stack’s natural frequency is referred to as the critical wind velocity. It is possible for this action to cause eventual failure of a steel stack due to fatigue as it oscillates.
In addition to self-induced cross wind movement, stacks can be affected by vortices shed by other nearby structures. These motions are caused by the rhythmic compressed air pressure zones and are not dissipated by aerodynamic means. This phenomenon is referred to as buffeting. When buffeting occurs in a series of similar stacks, they are said to gallop.
Many techniques have been developed to combat the detrimental effects of vortex shedding on steel stacks. Refer to Warren Environment bulletins HELICAL SPOILERS and TUNED MASS DAMPER for more information.
