Experimental and numerical investigations of the torsional flutter oscillations of a 4:1 rectangular cylinder

The torsional flutter oscillations of a 4:1 rectangular cylinder around its
pitching axis are investigated through wind tunnel experiments and numerical
simulations. The rectangle’s responses to different initial conditions and
turbulence excitations at various wind tunnel airspeeds are recorded. Timeresolved
Particle Image Velocimetry measurements are taken at two different
airspeeds, when the rectangle undergoes Limit Cycle Oscillations. Aeroelastic
simulations are carried out using the Discrete Vortex Method and the
resulting responses are compared to the experimental measurements.
The Common-base Proper Orthogonal Decomposition method is used to
analyse and compare the measured and simulated unsteady flow fields around
the rectangle. A discussion of the participation of each mode in the different
states of the flow-field is presented, at two different amplitudes of oscillation.
The Motion Induced Vortex (MIV) is identified as the fundamental cause
of the torsional flutter phenomenon and its role over a complete cycle is
studied. MIV-induced oscillations can be started either by a suitable initial
disturbance or by a second, nearly linear self-excited instability that causes negative aerodynamic damping. The combination of these two instabilities
results in a complete description of the torsional flutter of the rectangle.