Aerodynamische Wirkung schnell bewegter bodennaher Körper auf ruhende Objekte

Kurzfassung

In an unsteady flow field pressure variations lead to aerodynamic loads on objects like spheres or disks. Under the assumption of inviscid flow, the potential flow theory is a suitable method to predict aerodynamic loads on those objects. In an unsteady flow field of a passing train aerodynamic loads are able to destabilize waiting passengers on a platform or trackside workers on open track. In literature neither real train geometries nor the object geometry were considered so far. In this thesis a more flexible and more precise analytical model is derived. This model is able to predict the aerodynamic loads on an object in unsteady flow induced by the pressure pulse of a passing train head for model-scale experiments as well as for full-scale measurements adequately. In order to validate the accuracy of the analytical model model-scale experiments are performed in the Tunnel-Simulation Facility Gottingen (TSG). Additional experiments in the unsteady flow field of a huge loudspeaker are performed in order to investigate the physical effects causing the loads on spheres. The validity of the equation by Basset, Boussinesq, and Oseen with the modifications by Odar and Hamilton is examined. Only small Reynolds numbers up to Re=100 were investigated so far in literature. In this thesis a Reynolds number range from Re=200 until Re=1800 is considered for the first time. The so called added-mass force dominates the force on the spheres in all measured cases. Neither drag forces nor the history force plays a major role. Only the factor CA of the added-mass force influences the loads on the sphere and depends on the so-called acceleration number which can be expressed as the ratio between the displacement of the fluid and the diameter of the sphere. For a potential flow it is CA=2. For small acceleration numbers CA approaches a threshold value which is CA=2.3. Since CA is independent of the Reynolds number it can be set CA=2.3 for model-scale experiments as well as for full-scale measurements. A comparison with the results obtained from the TSG measurements shows that the deviations between experiment and theory can be reduced significantly by setting CA=2.3 instead of CA=2. Therefore, aerodynamic loads on spheres induced by the pressure pulse of a passing train can be predicted more precisely.