On the turbulence characteristics of axisymmetric pipe flow across varying cross section: An experimental Investigation

Development of flow along axisymmetric pipes, encountering contractions and expansions are of great engineering significance. Sudden changes in cross sectional area of pipe flow can cause pressure losses, as well as sedimentation in the case of particle laden flows. In the current study, a refractive index matched facility with concentrated aqueous solution of Sodium Iodide is used to study the turbulent nature of the flow near the vicinity of the varying cross-sectional area. Time resolved Tomographic Particle Image Velocimetry (PIV) and 2D high resolution PIV are performed for five different geometries (±90^o, ±45^o,0^o). Refractive index matching facilitates unobstructed optical access through the curved walls of the axisymmetric acrylic test section. The area ratios studied are 2.56 for contraction and 0.391 for expansion. The 2D PIV data along the center axial plane is used to measure the boundary layer profile as well as estimate turbulence statistics for various Reynolds numbers (1.8x10⁴– 1.1x10⁵). The results are compared for various cross section, including the forward-facing step (-90^o) and backward facing step (90^o) cases, and the growth and development of the recirculation region is analyzed. The 3D instantaneous flow structures are studied to understand the dominant features in the central flow as well as the near-wall region.