Reynolds-number effects in flow around more-or-less bluff bodies

This paper reviews the results of experiments carried out over a wide range of Reynolds numbers (104<Re<107) on 2-D sections in the high-pressure wind tunnel in Gِttingen. The bodies are a circular cylinder, a sharp-edged, trapezoidal-shaped bridge element, and a thick airfoil at a high attack angle. In all cases, appropriately shifting the value of Re led to dramatic changes in the force coefficients and St. number. Changes in the topological structure of the separated flow cause the observed phenomena: oil-flow pictures were used to make the phenomena visible. These events, which are triggered by the laminar/turbulent transition and its location, have characteristics that appear to be universally valid because they occur in similar form in flow over the three different bodies. The important results are: (i) flow around 2-D bodies can be highly three-dimensional; (ii) there are 2-D sections that behave as a bluff- or streamlined body, depending on Re; and (iii) slender bodies with sharp-edged cross-sections may also suffer pronounced Reynolds-number effects.