Miscible jets in complex environments: Roles of ambient rheology and geometric constraints

We experimentally study miscible jet dynamics in which a Newtonian fluid is injected horizontally through a circular nozzle into a domain filled with a non-Newtonian (viscoplastic) ambient fluid. Motivated by jet-based cleaning in plug and abandonment operations of oil and gas wells, a perforated wall introduces a geometric constraint that partitions the domain into two main zones. The jet and ambient fluids are density-matched (neutrally buoyant) and miscible. A combination of non-intrusive techniques—high-speed imaging (HSI), laser-induced fluorescence (LIF), and time-resolved tomographic particle image velocimetry (Tomo PIV)—is employed to characterize the jet behavior and to quantify how the governing dimensionless parameters, namely the Reynolds number ( R e ), Bingham number ( B N ), and the geometric parameter ( D p ), control jet characteristics, including jet radius, impingement time, back-flow velocity, concentration profiles, and mixing indices. We find that stronger restriction (increasing B N and/or decreasing D p ) reduces the jet radius and increases the impingement time. The post-impingement back-flow is quantified by a characteristic mean back-flow velocity, V ̄ ˆ b , which scales as V ̄ ˆ b ≃ 0 . 014 V ˆ j , 0 , where V ˆ j , 0 is the mean injection velocity, and progressively weakens with increasing B N . Using an inertial–yield stress balance, we identify a threshold of B N / R e ≳ 5 . 9 × 1 0 − 4 , beyond which back-flow is no longer observed. We compare the main identified regimes (mixing, mushroom, fingering, and fracturing) and show that variations in D p systematically modify the regime-transition boundaries. Turbulence measurements further demonstrate that, in the mixing regime, the normalized peak turbulence intensity remains nearly x -independent with I u , max ∗ ≈ 0 . 23 , whereas increasing B N progressively suppresses shear-layer activity, enhances intermittency, and increases anisotropy, ultimately localizing the flow into narrow yielded pathways (fingers). Our results also suggest that the onset of strong suppression occurs when the turbulent stresses become comparable to the resisting yield stress scale. While motivated by jet cleaning applications, these results are expected to inform a broader class of jet-based processes, including additive manufacturing, bioprinting, needle-free drug delivery, and industrial mixing. • Ambient rheology and perforation size control miscible jet regimes. • Four regimes were identified: mixing, mushroom, fingering, fracturing. • Back-flow disappears when yield stress overcomes jet impact stresses. • Smaller perforations reduce momentum transfer and delay impingement. • Yield stress suppresses turbulence and localizes flow into fingers.