Turbulent Air Flow Measurement with the Aid of Three-Dimensional Particle Tracking Velocimetry in a Curved Square Duct.

Abstract

A three-dimensional particle tracking velocimeter (3-D PTV) was successfully applied to turbulent air flow measurement in a strongly curved U-bend of square cross-section. He-filled bubbles, of which specific density was very close to that of air were employed as a flow tracer. Distributions of the mean velocities and Reynolds stresses were obtained at 90°downstream of the inlet to the curved section. The secondary flow as strong as 20% of the bulk mean velocity is observed in the cross-stream plane ; it results in a marked trough in the streamwise velocity at a distance of about 0.3 times the channel height from the inner wall. The present experimental result is mostly in good agreement with the LDV data of Chang et al. (1983). Moreover, turbulent statistics including all Reynolds stress components are obtained. The production mechanism of turbulent kinetic energy as well as the distributions of invariants for stress anisotropy tensor are discussed in detail.