Reynolds-number-effects in flow around a rectangular cylinder with aspect ratio 1:5

Kurzfassung

The paper reports on experiments carried out over a wide range of Reynolds numbers (2∙104 < Re < 2∙106) in the high pressure windtunnel in Göttingen. The model was a rectangular cylinder with aspect ratio 1:5. It has a sharp-edged generic cross section, which was selected as benchmark problem for bluff bodies in general (www.aniv-iawe.org/barc/BARC). High Reynolds numbers were reached by increasing the pressure in the flow medium up to 70 bar. Steady and unsteady forces were measured with a piezoelectric balance whose major characteristic is its stiffness, which makes the instrument particularly useful for investigating unsteady phenomena. Thus apart from steady i.e. time averaged values including the base pressure coefficient, also power spectra and probability density functions were measured yielding for example Strouhal numbers, higher statistical moments etc. Considering the dependence of the lift coefficient upon the Reynolds number, in particular for α = 4° a significant Reynolds number effect is obvious in the curve, which is double bent and the values are ranging from 0.3 to 0.6. In addition, the trend to lower or to higher Reynolds numbers indicates a further decrease and increase of the lift coefficient, respectively. Also for α = 6° and α = 2° there is a significant increase of the lift in the range Re < 105, at which the rise is shifted to lower for α = 6° and to higher Reynolds numbers in case of 2°. Probably the shape of separation bubbles, that depends on the Reynolds number is responsible for these effects, which are stronger in asymmetric flow (α ≠ 0°) than in the symmetric case(α = 0°), where the effects on the upper and the lower side can compensate each other. Concerning vortex resonance phenomenon, a response diagram was taken for the natural bending motion of the rather slender model, which was fixed at both sides in the stiff balance. Finally, as a supplement the section was also investigated with the bluff i.e. much longer side normal to the flow, a case which can be seen as a test case for the absence of Reynolds number effects.