Effects of three-dimensional disturbances on bluff body near wakes

The three-dimensionality of the near wake of bluff bodies at high Reynolds numbers is studied experimentally. Measurements were carried out in a 0.9Imx 0.91m wind tunnel (for Re=20000 to 60000) and flow visualisation in a 0.6mx 0.6m water flume (for Re=25(X)). The main purpose is to identify inherent three-dimensional features that may also arise in nominally two-dimensional flows. In order to fix the three-dimensional effects in both time and space, a mild, periodic, geometrical disturbance was imposed on the otherwise two-dimensional geometry of a model with a blunt trailing edge. The trailing edge thus followed a sinusoidal pattern, but a straight edge model was also studied for comparison purposes. Quantitative measurements and flow visualisation revealed that a dual shedding frequency characteristic prevails in the wake of the sinusoidal model. Base drag shows a noticeable drop (in comparison to the straight edge model). Most of the activity seems to happen in the region of the peak, where the dual frequency characteristic is more apparent and also the base drag shows its largest variations. Flow visualisation showed different modes of vortex shedding to exist. Vortical structures in the x- and z- directions were observed for both models. Og vortices are present in the near wake. It is believed that the observed vortices are responsible for the intense base pressure fluctuations and gradients, and also for thin "wisps" appearing between Karman vortices in flow visualisation. A model for the dynamics of the formation region is proposed, by considering the interaction of mean, time-averaged quantities. It is suggested that forming vortices have a tendency to straighten-out. A concept is proposed which links the vortex formation length to other wake parameters, most notably wake width and base pressure. Wake similarity arguments are used in order to explain the shedding frequency variations along the span. The dynamics of vortex dislocations are also discussed. A mechanism is proposed which explains the significance of a characteristic dislocation frequency, fd in the near wake dynamics. It is suggested that fd is a result of the geometrical properties of the vortex filaments and that a link exists between the dislocation frequency and fluctuations in base pressure, vortex strengths and spanwise dislocation position.Open Acces