arXiv subjects

Prasoon Suchandra

Publications and source records attributed to Prasoon Suchandra.

6 recordsLinked to original sources

arXiv2026

On the scaling of bubble interactions in dynamic turbulence: theoretical, numerical, and experimental study

This study investigates dilute bubbly decaying homogeneous isotropic turbulence at high Reynolds number using theory, direct numerical simulation, and experiments. The turbulent kinetic energy and dissipation rate follow power-law decay, while the bubble population reorganizes relative to the evolving Hinze scale. When the dissipation decays sufficiently rapidly, the Hinze scale grows faster than the characteristic bubble diameter, driving the population from super-Hinze toward sub-Hinze sizes. The system passes through a mixed regime in which coalescence dominates but breakup remains active, followed by a pure-coalescence regime. Residual breakup in the mixed regime increases the number of small bubbles and enhances coalescence, leading to faster growth of the characteristic bubble size. DNS of dilute bubble-laden turbulence shows decay exponents close to single-phase turbulence and a bubble-size distribution that shifts toward smaller diameter relative to the Hinze scale. Before the transition, the distribution exhibits two power-law ranges associated with capillary effects and inertial breakup; after the transition, it approaches a single capillary-dominated scaling. Theory and DNS predict distinct growth laws for bubble diameter in the mixed and pure-coalescence regimes, together with corresponding scalings for number density, interfacial area, and coalescence rate. These predictions are further assessed in a spatially developing pump-driven bubbly duct flow at higher Reynolds number. Despite confinement, inhomogeneity, and wall production, the measured trends agree with the theoretical and DNS-based scalings. The results identify Hinze-scale drift as the organizing mechanism for bubble interactions in both idealized and practical decaying turbulent flows, and provide guidance for population-balance and interfacial-area-transport models.

physics.flu-dyn↗

arXiv2026

Mass-Transfer Control With Microbubbles in Highly Turbulent Decaying Flows

We hypothesize that combining extreme turbulence with a minute reduction in surface tension $\sigma$ (surface tension of the liquid) using surfactant provides a simple and scalable route for controlling micron scale bubble size in gas--liquid systems. To test this, we generate high-intensity turbulence using a multiphase pump [turbulent intensity $\ge 40\%$; Taylor Reynolds number $Re_\lambda=\mathcal{O}(10^3)$; bulk Reynolds number $Re=\mathcal{O}(10^5)$] feeding a straight duct, which produces a decaying turbulent flow where, without additives, bubble coalescence dominates and causes monotonic downstream growth in the mean diameter $d_\mathrm{avg}$ of the bubbles. This growth is governed by the turbulent dissipation rate $\varepsilon$. High-speed imaging, back-lit shadowgraph and particle shadow velocimetry (PSV) quantify bubble statistics ($d_\mathrm{avg}$, and the bubble-size distribution) and turbulence metrics (turbulent kinetic energy $k$, turbulence intensity $\mathcal{I}$, and dissipation rate $\varepsilon$). We then introduce a minute amount ($\sim 0.01\%$ critical micelle concentration) of additive that produces a slight reduction in $\sigma$, used here only as an interfacial tuning knob because the same change in surface tension can be achieved with non surface active agents. This small decrease in $\sigma$ enhances breakup, slightly suppresses coalescence, and makes smaller bubbles more breakup prone, resulting in reduced $d_\mathrm{avg}$ and a narrower bubble-size distribution. Turbulence statistics remain unchanged within experimental uncertainty, indicating that the effect arises entirely from interface rather than hydrodynamic changes. Overall, combining extreme turbulence with a minute reduction in surface tension offers a low complexity and tunable lever for setting bubble-size distributions and intensifying mass transfer in industrial multiphase flows.

physics.flu-dyn↗

arXiv2026

Impact of the formation angle on the drag of bio-inspired $\pmb \vee$-formations

Bio-inspired $\pmb \vee$ flight formation is a well known technique for energy saving among groups of fixed-wing aircraft, and as of recently, for groups of quad-rotors. Here, we study the effect of the formation angle on the performance of each of the members of a 5-member $\pmb \vee$-formation in terms of the flow field, and drag force. We employ axisymmetric cylinders, which are non-lifting in solo condition to reduce/eliminate the effect of the lift (lateral force) on the group performance, and use time-resolved, multi-illumination, consecutive-overlapping particle image velocimetry (PIV) to capture the velocity field around and in-between the members. Over a range of $\pmb \vee$-formation angles, we see various degree of drag reduction, with the highest drag reduction ($\sim 80\%$) for the interior members of the tightest formation (formation with the smallest $\pmb \vee$-angle and the most overlap in frontal views). All formation members experience some levels of drag reduction up for $\pmb \vee$-angle of around $50^{\circ}$ and in formation with $\pmb \vee$-angle greater than $50^{\circ}$, only the leading member experiences observable drag reduction. We explore the complex flow dynamics between the formation members in terms of wake-body and wake-wake interactions, and the bleeding (gap) flow. We present the mean and fluctuating quantities, as well as the dynamics of the vortex shedding and circulation in the wakes of the members, and discuss how these flow characteristics relate to the drag of each member, both as a function of their position within the $\pmb \vee$ and the angle of the formation. This current study serves as a baseline for further explorations of wake-body and wake-wake interactions of flow past groups of bodies, and demonstrates how changing formation angle can help achieve a desired group performance (like minimum drag).

physics.flu-dyn↗

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_\lambda~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↗

arXiv2023

Impact of bio-inspired V-formation on flow past arrangements of non-lifting objects

Inspired by the energy-saving character of group motion, great interest is directed toward the design of efficient swarming strategies for groups of unmanned aerial/underwater vehicles. While most of the current research on drone swarms addresses controls, communication, and mission planning, less effort is put toward understanding the physics of the flow around the members of the group. Currently, a large variety of drones and underwater vehicles consist of non-lifting frames for which the available formation flight strategies based on lift-induced upwash are not readily applicable. Here, we explore the V-formations of non-lifting objects and discuss how such a configuration alters the flow field around each member of the array compared to a solo flyer and how these changes in flow physics affect the drag force experienced by each member. Our measurements are made in a water tunnel using a multi-illumination particle image velocimetry technique where we find that in formations with an overlap in streamwise projections of the members, all the members experience a significant reduction in drag, with some members seeing as much as 45% drag reduction. These findings are instrumental in developing generalized energy-saving swarming strategies for aerial and underwater vehicles irrespective of the body shapes.

physics.flu-dyn↗