The onset of Kármán-vortex shedding is studied experimentally in the wake of different two-dimensional bluff bodies, namely an oblong cylinder, circular cylinders and plates of rectangular cross-section. Different control measures, such as wake heating, transverse body oscillations and base bleed are investigated. As the steady-periodic Kármán shedding has previously been identified as a limit-cycle, i.e. as self-excited oscillations, the experiments are interpreted in the framework of the Stuart–Landau model. The coefficients of the Stuart–Landau equation for the characteristic vortex shedding amplitude, i.e. the linear temporal growth rate, linear frequency and the Landau constant, are fully determined for the two cylinders and in part for the plate. For this purpose transients are generated by suddenly switching transverse body oscillations or base bleed on or off. The analysis of these transients by a refined method based on complex demodulation provides reliable estimates of the model coefficients and yields an experimental validation 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.
The laminar Kármán vortex flow in the wake of a cylinder can be suppressed in a certain Reynolds number range, if the cylinder is stimulated to small oscillations in a direction normal to the mean flow either externally by high-frequency fluctuations, or internally by negative feedback of a hot-wire signal picked up in the wake. If suppression is obtained, the transverse mean velocity profiles become deeper in comparison with those of vortex shedding, however the drag is of the same order in both cases. On the other hand the laminar vortex flow can be stabilized and extended to both lower and higher Reynolds numbers, by exciting the cylinder with frequencies, which synchronize the wake fluctuations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.