Experimental and numerical investigation of laminar-to-transitional pipe flow past a sudden expansion

The mechanisms controlling the pressure recovery characteristics of separated flow past a sudden-expansion in a pipe were investigated, for inflow Reynolds numbers spanning the laminar to early transition flow regimes. Experiments and computations were performed for fully developed flow past a geometrical model featuring a fixed expansion area ratio of 1:4. Pressure distribution characteristics indicated that flow similarity exists below Re ≈ 1000, marking the range for laminar flow applicability. Above this Reynolds number the separated shear layer undergoes strong localized transition. Initially the effected streamwise portion of the separated shear layer is near the region of reattachment, but a larger upstream section of the separated flow is encompassed as the inflow Reynolds number increases. The local static pressure distribution captures the growth of this process, depicting abrupt pressure gradient changes, which are used to evaluate the laminar extent of the separated flow region.