Effect of expansion geometry on turbulence in axisymmetric pipe flows

We investigate the influence of expansion geometry on the flow field and turbulence structure in axisymmetric pipe flows through comparative analysis of abrupt ($90^\circ$) and gradual ($45^\circ$) area expansions with an area ratio of 2.56 at step-height Reynolds numbers of 25000 and 35000. Utilizing refractive index-matched stereo Particle Image Velocimetry, we resolve the three-component velocity fields and extract turbulence statistics with high spatial fidelity. Both configurations exhibit full flow separation, recirculation, and shear layer development; however, the gradual expansion consistently yields elevated turbulence levels, broader shear layers, enhanced Reynolds stress anisotropy, and stronger out-of-plane fluctuations. In contrast, the abrupt expansion generates a secondary vortex that disrupts the return flow, reducing shear layer interaction and turbulent kinetic energy (TKE) production. The governing mechanism is attributed to the geometry-induced modulation of the return flow. In the gradual case, the return flow remains attached to the sloped surface and impinges obliquely on the free-stream, generating a distributed region of high shear and sustained turbulence production leading to intensified TKE and anisotropy in the near-expansion region. The abrupt case confines this interaction, limiting turbulence generation spatially and structurally. These findings reconcile prior observations of increased pressure loss in sloped expansions and reveal the fundamental role of expansion slope in controlling turbulence generation and energy redistribution in separated flows. The observed trends suggest a generalizable mechanism relevant to a broader range of expansion angles and flow conditions.