Influence of the Reynolds Numbers on the Unsteady and Steady Behaviour of Flow Around Bridge- and Airfoil Sections

Kurzfassung

Experiments were carried out in the high-pressure wind tunnel in Göttingen, where the Re. No. was varied from 5. 104 up to nearly 107 by means of pressurizing up to 100bar. During the lecture two typical examples will be presented where the variation of the Re. No. leads to the drastic changes in the force coefficients and Strouhal number. The effects are caused by changes in the topology of the separated flow structures, which is shown by oil flow pictures. The forces were measured using a piezoelectric balance which is in particular suitable for aeroelastic investigations. The first example is flow around a 2D-wing at a high angle of attack, where we observed a drastic decay of lift with increasing Re. No. The vortex shedding could be seen in the force signals ( St = 0.2 ). The second example is a sharp-edged bridge section (approach span from the „Great Belt bridge“). Contrary to the common assumption in wind engineering, that flow around bluff and sharp-edged bodies is independent of Re. No., we have found strong Re. No. effects. At high Re. No. near the prototype values (5·106) the Strouhal number is 20% higher than at low Re. No. (5·104). The corresponding drag coefficients show the inverse behaviour, i.e. lower values for increasing Re. No. The physical reasons for the observed phenomena i.e. the Re.No. dependent location of laminar/turbulent transition will be discussed. It is planned to present a simple demonstration experiment of flow induced vibrations of a bridge section.