TY - RPRT
T1 - Stochastic oscillations induced by vortex shedding in wind
T2 - Report based on Ph.D.-thesis (supervisor: Ove Ditlevsen)
AU - Christensen, Claus
PY - 1997
Y1 - 1997
N2 - 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.
AB - 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.
M3 - Report
BT - Stochastic oscillations induced by vortex shedding in wind
ER -