Stochastic oscillations induced by vortex shedding in wind: Report based on Ph.D.-thesis (supervisor: Ove Ditlevsen)

Claus Christensen

    Research output: Book/ReportReportpeer-review

    Abstract

    As a fluid flows past a circular cylinder,vortices are shed alternately from each side at most values of the Reynolds number. Over a certain range of windspeeds, the periodicity in the wake is synchronized or captured by the mechanical system. The shedding abruptly deviates from the linear Strouhal dependence and stays constant at the mechanical natural frequency. This coupling between the velocity field and the motion of the mechanical system is referred to as the lock-in phenomenon. The lock-in phenomenon has importance in structural engineering for slightly damped slender structures exposed to wind, current etc., because the phenomenon can occur at low flow velocities that are common and therefore may induce fatigue damage at "hot spots". The phenomenon itself can rarely cause failure directly. Wind tunnel experiments show that the position and the width of the interval depend on the structural damping and on whether the wind velocity increases or decreases. Also the experiments show that the presence of even a low degree of turbulence in the wind causes the interval bounds to be rather uncertain. A stochastic model for the length and position of the lock-in interval and different load models for the vortex shedding load during lock-in are obtained and discussed. Due to the complexity of the problem, the models give an idealized phenomenological description of the "lock-in" phenomenon, but for engineering analysis, especially a fatigue analysis, such simple model may be sufficient. All the results are supported by experimental wind tunnel investigations. Department of Structural Engineering and Materials, DTUSeries R No 23 1997As a fluid flows past a circular cylinder, vortices are shed alternately from each side at most values of the Reynolds number. Over a certain range of windspeeds, the periodicity in the wake is synchronized or captured by the mechanical system. The shedding abruptly deviates from the linear Strouhal dependence and stays constant at the mechanical natural frequency. This coupling between the velocity field and the motion of the mechanical system is referred to as the lock-in phenomenon. The lock-in phenomenon has importance in structural engineering for slightly damped slender structures exposed to wind, current etc., because the phenomenon can occur at low flow velocities that are common and therefore may induce fatigue damage at "hot spots". The phenomenon itself can rarely cause failure directly. Wind tunnel experiments show that the position and the width of the interval depend on the structural damping and on whether the wind velocity increases or decreases. Also the experiments show that the presence of even a low degree of turbulence in the wind causes the interval bounds to be rather uncertain. A stochastic model for the length and position of the lock-in interval and different load models for the vortex shedding load during lock-in are obtained and discussed. Due to the complexity of the problem, the models give an idealized phenomenological description of the "lock-in" phenomenon, but for engineering analysis, especially a fatigue analysis, such simple model may be sufficient. All the results are supported by experimental wind tunnel investigations.
    Original languageEnglish
    Number of pages118
    Publication statusPublished - 1997

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