High Reynolds-number flows over bluff bodies

Abstract

Circular cylinders or prisms with square cross-sections in a uniform, steady cross-flow are inherently coupled with large regions of massive, large-scale flow separation, a broad base region containing highly unsteady, recirculating flow and low pressures, and eddy shedding in the near wake. This not only produces a high total drag on the bluff body, but leads also to the well-known Kármán vortex street in its wake. The eddy shedding occurs over a wide range of Reynolds numbers and causes pressure fluctuations on those bluff bodies both parallel and transverse to the oncoming flow. In case they are mounted elastically, these prisms or cylinders can thereupon experience various types of flow-induced vibrations with significant additional increases in the mean drag and the lift fluctuations, with galloping being the most dangerous fluid-elastic instability owing to the increase in the amplitude of the limit cycle oscillation with increasing flow velocity. Modern, high-fidelity Computational Fluid Dynamics (CFD) and reduced-order models already play nowadays a central role in optimising appropriate countermeasures, either passive or active, in the design process to avoid or suppress flow-induced structural excitations. A robust and accurate prediction modelling of the effect of vibration control methods on the (highly) unsteady and complex flow over square-section prisms and circular cylinders, based on sophisticated nonlinear dynamic models, requires even to this day precise statistical validation data that can only be obtained by experiments beforehand. Tests on full-scale structures are costly, not seldom have to be conducted under difficult conditions and time pressure, and are mostly accompanied with a priori unknown and highly dynamic flow conditions, such as impacting waves, atmospheric turbulence, wind gusts, or strong spatial variations in wind shear. Small-scale parametric studies in a laboratory environment, on the other hand, have the advantage of a virtually "unlimited" measurement time and can be performed at well-defined and reproducible boundary conditions. The present monograph deals with the detailed analysis of the impact of various governing and influencing model and flow parameters on the flow over2D prismatic bluff bodies with square cross-sections, arranged either as isolated or as a pair in a tandem configuration in a cross-flow. Eleven experimental measurement campaigns were conducted in the High-Pressure wind tunnel facility Göttingen in a low subsonic flow at Reynolds numbers in the range of 100000 to 10 million. Besides the Reynolds number, the wide range of studied parameters also includes the incidence angle, the lateral edge roundness, the surface roughness, and, in the case of two prisms placed in-line, additionally the spacing between them. Isolated, in pairs, and in combinations of three or more parameters, their influence on the mean and fluctuating aerodynamic forces on the prisms as well as on the eddy shedding frequency is evaluated. Additional mean surface pressure distributions provide information on the locations of boundary layer separation and free shear layer reattachment. In this way, it is assessed to what extent these parameters enable potential valuable countermeasures that can successfully be applied in a passive way to reduce undesired flow-induced excitations.