SELF-EXCITED OSCILLATIONS IN THE WAKE OF 2-DIMENSIONAL BLUFF-BODIES AND THEIR CONTROL

The onset of Karman-vortex shedding is studied experimentally in the w ake of different two-dimensional bluff bodies, namely an oblong cylind er, circular cylinders and plates of rectangular cross-section. Differ ent control measures, such as wake heating, transverse body oscillatio ns and base bleed are investigated. As the steady-periodic Karman shed ding has previously been identified as a limit-cycle, i.e. as self-exc ited oscillations. the experiments are interpreted in the framework of the Stuart-Landau model. The coefficients of the Stuart-Landau equati on for the characteristic vortex shedding amplitude, i.e. the linear t emporal growth rate, linear frequency and the Landau constant, are ful ly determined for the two cylinders and in part for the plate. For thi s purpose transients are generated by suddenly switching transverse bo dy oscillations or base bleed on or off. The analysis of these transie nts by a refined method based on complex demodulation provides reliabl e estimates of the model coefficients and yields an experimental valid ation of the concept that a global instability mode grows or decays as a whole. Also, it is demonstrated that the coefficients of the Stuart Landau equation are independent of the experimental technique used to produce the transients.