Ardalan Javadi | Searcharxiv

arXiv2026

Laminar-to-Turbulent Transition of Yield-Stress Fluids in Pipe and Channel Flows

We present direct numerical simulations (DNS) of laminar to turbulent transition in Herschel-Bulkley (HB) yield-stress fluids flowing through pipes and rectangular channels. The simulations employ a Herschel-Bulkley formulation that captures the yield-stress-driven plug, its breakdown, and the emergence of near-wall turbulent structures, enabling direct resolution of the transition mechanisms. The DNS cover a broad range of generalized Reynolds numbers, Re_G = 378 to 5300, allowing us to resolve plug formation, transition onset, and fully turbulent regimes. In pipe flow, the simulations reproduce the characteristic transition sequence, which includes a strong plug and negligible turbulence at low Re_G, a sharp rise in turbulence intensity and u'rms within a narrow transitional window (Re_G ~ 2000 to 3000), and wall-dominated turbulence with a weakened core at higher Re_G. Transition occurs only when local Reynolds stresses exceed the yield stress. The resulting regime boundaries (Re_G < 1735 laminar, 1735 < Re_G < 2920 transitional, and Re_G > 2920 turbulent) align with trends reported for Carbopol fluids. This work provides the first DNS resolving the complete laminar to turbulent transition in HB fluids for both pipe and channel configurations, offering unified insight into plug breakdown, turbulence localization, and the role of yield stress in transition mechanisms. Experimental validation using a 3.6 m acrylic channel with particle image velocimetry (PIV) is planned to further assess the DNS predictions and quantify geometry-dependent transition thresholds.

physics.flu-dyn↗

arXiv2025

Bubble coalescence dynamics in a high-Reynolds number decaying turbulent flow

This study experimentally investigates bubble size evolution and void fraction redistribution in an unexplored, coalescence-dominated regime of decaying turbulent bubbly flow. The flow is generated downstream of a regenerative pump in a duct, with Taylor-scale Reynolds number (Re_λ~10^3), but turbulence decays rapidly along the duct. Shadowgraph imaging and particle shadow velocimetry are used for measurements. High-speed imaging and statistical analysis reveal that bubble coalescence dominates over breakup across most of the domain, leading to monotonic growth in Sauter mean diameter (d_{32}) and progressive broadening of the bubble size distribution. The normalised extreme-to-mean diameter ratio increases axially and asymptotically saturates at~2.2, indicating the emergence of a quasi-self-similar bubble size distribution. The probability density function of bubble diameter exhibits a dual power law tail with exponents $-10/3$ and -3/2 near the duct inlet, where the flow is coalescence-dominated. However, after a few hydraulic diameters, a single~-3/2 power law scaling emerges, indicating a regime of pure coalescence in which all bubbles are smaller than the Hinze scale. The cumulative distribution with d/d_{32} exponent (~1.3) emerges only after the size distribution stabilises. Although classical Hinze scaling gives d_H ~ L^0.9, our theory for d_{32} and~d_{99.8}(99.8th percentile bubble diameter) in a pure-coalescence regime predicts the slower law~ L^0.5, which our experimental results confirm, indicating negligible breakup and sub-Hinze growth. In contrast to current models, transient void fraction profiles evolve from nearly uniform to sharply core-peaked Gaussian distributions in the developing regime, with increasing centerline values and decreasing near-wall values, due to lift-force reversal.

physics.flu-dyn↗