Partikkelikuvamenetelmä (PIV): Sovellus portaan ohi tapahtuvalle virtaukselle

A digital Particle Image Velocimetry (PIV) technique is used to study the flow over a backward-facing step. Detailed measurements within the laminar flow regime are presented for two different three-dimensional inflow conditions, corresponding to different modifications of the experimental set-up. It is found that the flow is very sensitive to variations in inflow conditions. In particular, for one of the inflow conditions, no recirculation is observed in the upper wall region. For the other inflow condition, a recirculation region is present in the upper wall, and the measurements show excellent agreement with two-dimensional simulations up to Reynolds number Re = 500. However, for Reynolds number increasing from Re = 663 to Re = 889, it is found that the reattachment length decreases. Recently, it has been reported that a 'wall-Jet' induces three-dimensionality in the flow even at low Reynolds numbers, the effect being more pronounced for increasing Reynolds numbers (Williams & Baker 1997). Thus, it is assumed that the observed 'reversed effect' of decreasing reattachment length at increasing Reynolds number most likely is due to the non-ideal inflow condition 'feeding' the three-dimensionality of the flow. For Re < 500 the excellent agreement of our experimental results with literature data might be explained with the strength of the wall-jet increasing with Reynolds number, making the flow less susceptible to three-dimensional effects at lower Reynolds numbers