Abstract
LDV measurements in a 90 degrees elbow which curvature radius coincides with the diameter have been examined for the case of a deflected inflow that became faster at the inside. Comparison with a result of an inflow from a short pipe that measures 4.9D shows that the deflected inflow reinforced a convex velocity distribution occurring near the curvature inside in the downstream region. This result concludes that the deflected inflow promotes the secondary flow of Prandtl's first kind in the elbow. Frequency analyses of axial flow fluctuations show that the dominant frequency can be seen in the downstream of the elbow even in the deflected inflow case. However its Strouhal number increases to 0.6 from 0.5, compared with the short pipe case. Results of frequency analyses are also shown for other cases, i.e. an inflow from a long pipe (having an axial length of 10D) and a swirl generator. Dominant Strouhal numbers in most of the cases become 0.5, except for 0.6 in cases of the inflow from the long pipe and deflector. This frequency shift might be related with the boundary layer size and the local flow velocity, since the corresponding fluctuation is caused by vortex shedding from the boundary layer at the elbow inside.
