Vortex Shedding

Vortex Shedding: Vortex shedding causes wind induced vibration and it occurs close to the resonance wind velocity of the structure. It causes the structure to resonate resulting in large forces and deflections which can result in fatigue failure. Vortex shedding is the process by which vortices formed continuously by the aerodynamic conditions associated with a solid body in gas or air are carried downstream by the flow in the form of a vortex street. A vortex street is a regular stream of vortices or parallel streams of vortices carried downstream by the flow of a fluid over a body. These are sometimes made visible by vapour condensation as in the vortex trails from the wing tips of an aeroplane.

When the wind blows across a slender prismatic or cylindrical structure, rhythmic vortices are shed alternatively from opposite sides causing alternating low pressure zones. Consequently the structure is acted upon by a load perpendicular to its length and perpendicular to the wind direction. These alternating low pressure zones can cause a structure to move towards the low pressure zone causing movement perpendicular to the direction of the wind. When the critical wind speed is reached, these forces can cause the structure to resonate causing large forces and deflections to occur if the structure is not heavily damped. This occurs at wind velocities close to the resonance wind velocity which is calculated using the dimensions of the structure and the Strouhal number.

Crane Boom Experiencing Vortex Shedding	Monitoring Instrumentation

Accelerometers Attached to Brace	Accelerometers Sealed with Silicon

Experimental Work	Wind Vane and Cup Anemometer Installed on Crane’s ‘A’ Frame
Vortex Shedding Investigation on Offshore Crane



Transducers Positioned on Crane Boom Braces for Vibration Measurements