Turbulent air-flow measurement with the aid of 3-D particle tracking velocimetry in a curved square bend

A three-dimensional particle tracking velocimeter (3-D PTV) was applied to air-flow measurement in a strongly curved U-bend of a square cross-section. He-filled neutral-buoyant soap bubbles were employed as a flow tracer, and turbulent statistics including all Reynolds stress components were measured. The pressure-induced secondary flow, of which magnitude reached about 30% of the bulk mean velocity, was observed. The present experimental result is mostly in good agreement with the LDA data at higher bulk-mean Reynolds number taken by Chang et al. The effect of the secondary flow on the production mechanism of turbulent kinetic energy as well as on the distributions of the invariants of stress anisotropy tensor was examined in detail.