We hypothesize that combining extreme turbulence with a minute reduction in surface tension σ (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 ≥ 40%; Taylor Reynolds number Re_λ=𝒪(10^3); bulk Reynolds number Re=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_avg of the bubbles. This growth is governed by the turbulent dissipation rate ε. High-speed imaging, back-lit shadowgraph and particle shadow velocimetry (PSV) quantify bubble statistics (d_avg, and the bubble-size distribution) and turbulence metrics (turbulent kinetic energy k, turbulence intensity ℐ, and dissipation rate ε). We then introduce a minute amount (∼ 0.01% critical micelle concentration) of additive that produces a slight reduction in σ, 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 σ enhances breakup, slightly suppresses coalescence, and makes smaller bubbles more breakup prone, resulting in reduced d_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.

Bio-inspired ∨ 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 ∨-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 ∨-formation angles, we see various degree of drag reduction, with the highest drag reduction (∼ 80%) for the interior members of the tightest formation (formation with the smallest ∨-angle and the most overlap in frontal views). All formation members experience some levels of drag reduction up for ∨-angle of around 50^∘ and in formation with ∨-angle greater than 50^∘, 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 ∨ 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).

A novel approach is presented to simultaneously measure volume fraction (using two-tracer planar laser induced fluorescence (PLIF)) and velocity (using particle image velocimetry (PIV)) to study three-component mixing in the multilayer Rayleigh–Taylor instability, constituting of three layers in a blow-down gas tunnel. This approach enables non-intrusive, planar measurements of each of the three components in the mixture. By using acetone and anisole as PLIF tracers and a three-camera system, each tracer’s fluorescence is captured alongside scattered light from PIV particles. Key considerations for two-tracer PLIF with acetone and anisole are discussed, including spectral conflicts as well as the independence of anisole fluorescence from the presence of acetone, and vice-versa. The diagnostic is first validated and then applied to the Rayleigh–Taylor instability in a three-layer configuration, yielding previously unattainable results. This includes simultaneous volume fraction profiles, the covariances between the volume fraction fluctuations of different layers, as well as each layer’s covariance between their respective volume fraction and vertical velocity fluctuations. Such quantities are crucial for the development and validation of variable density turbulence models. The presented diagnostic is extendable to other three-component gas mixing experiments with small pressure variations, provided that the tracer volume fractions remain small.

This study experimentally investigates bubble size evolution and void fraction redistribution in an unexplored, coalescence-dominated regime of a decaying turbulent bubbly flow. The flow is generated downstream of a regenerative pump in a duct, with bulk Reynolds number (Re) $\sim \mathcal{O}(10<^>5)$ , Taylor-scale Reynolds number (Re $_\lambda$ ) $\sim \mathcal{O}(10<^>3)$ and void fraction ( $\phi$ ) $\sim \mathcal{O}(1\,\%)$ , where the inlet turbulence is extremely intense (turbulence intensity $\gt 30\,\%$ ) but decays rapidly along the duct. Shadowgraph imaging and particle shadow velocimetry are used for measurements. The experimentally obtained turbulent dissipation in the duct flow decays as $\varepsilon \sim \mathcal{L}<^>{-2}$ , where $\mathcal{L}$ is the axial position, in close agreement with the homogeneous isotropic turbulence prediction of $\varepsilon \sim \mathcal{L}<^>{-2.2}$ . High-speed imaging and statistical analysis reveal that bubble coalescence dominates over breakup across most of the domain, leading to monotonic growth in the Sauter mean diameter ( $d_{32}$ ) and progressive broadening of the bubble size distribution. The normalised extreme-to-mean diameter ratio ( $\mathcal{D}$ ) increases axially and asymptotically from ${\sim} 1.9$ (breakup regime) and saturates at ${\sim} 2.2$ (coalescence regime), indicating the emergence of a quasi-self-similar bubble size distribution. The probability density function of the bubble diameter exhibits a dual power-law tail with exponents $-10/3$ and $-3/2$ near the duct inlet. 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 plotted against $d/d_{32}$ shows that the slope decreases and the distribution width increases with both axial position and void fraction $(\phi )$ . Although classical Hinze scaling gives $d_{ extit{H}} \propto \mathcal{L}<^>{0.9}$ , our theory for $d_{32}$ and $d_{99.8}$ (99.8th percentile bubble diameter) in a pure-coalescence regime predicts the slower law $\propto \mathcal{L}<^>{0.5}$ , which our experimental results confirm - indicating negligible breakup and sub-Hinze growth. Concurrently, in contrast to current models, transient $\phi$ profiles evolve from nearly uniform to sharply core-peaked Gaussian distributions in the developing regime, with increasing centreline values and decreasing near-wall values, due to lift-force reversal. These results provide the first spatially resolved characterisation of coalescence-dominated bubbly flows at high Re, advancing the design of industrial systems as in nuclear cooling and multiphase forming processes (e.g. paper manufacturing, chemical reactors).

Turbulent/non-turbulent interfaces (TNTI) are evaluated for flows subject to the Rayleigh-Taylor instability (RTI). Experiments are conducted in a gas tunnel facility with air as heavy fluid and helium+nitrogen+air mixture as light fluid giving a low Atwood number of $ \approx 0.1 $ approximate to 0.1. Simultaneous velocity-density measurements are taken via particle image velocimetry and laser induced fluorescence. The nature of the TNTI on bubble front, as well as the change in mean quantities and turbulence statistics across this TNTI are investigated. The molecular mixing is studied relative to the TNTI. The TNTI shows a complex conditionally averaged volume fraction profile in its vicinity. In the external layer, the fluid is mostly pure heavy fluid, leading to no concentration gradients and a nearly zero measurement of the scalar dissipation. At the interface, there is a very large magnitude of scalar dissipation. In the adjustment layer, the scalar dissipation is nearly constant. These results challenge the conventional shape of profiles of turbulence statistics in RTI flows which are typically assumed parabolic. An alternate way to interpret the variation of turbulence statistics across the mixing width is to assume them to be nearly uniform in the core of the flow and be modulated by the location of the TNTI.

Results are presented for the explosive blast-driven instability of a stratified interface in a diverging cylindrical geometry using high-speed optical diagnostics. Specifically, the perturbation growth was studied using the qualitative Mie scattering technique, and the velocity, mixing transition, and vorticity characteristics are studied to investigate the behavior of the interface using planar particle image velocimetry (PIV). The role of density contrast is studied using CO2-air (low Atwood number, A) and SF6-air (high-A) interfaces. The perturbation growth h t theta was studied, where the decay parameter theta was found for A 0.22 and A 0.68 to be 0.34 and 0.50, respectively. Using velocity information from PIV, it was found that the initial vorticity deposition from the pressure impulse can accurately be estimated using known models previously developed for Richtmyer-Meshkov instability. There is a significant addition to the interface circulation by the long pressure decay phase of the blast wave, which gives rise to a variable acceleration Rayleigh-Taylor instability contribution. The high-A case showed a prolonged increase in circulation, plausibly due to higher inertia. This trend was also supported by variations in peak turbulence intensity, where a sharp increase at early times quickly decays with increased mixing between the two fluids. Finally, the interface Reynolds number of the higher A was found to exceed all criteria for the mixing transition (also indicated by the qualitative observations of the mixing in spike-structures in the Mie scattering images).

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.

This paper investigates the multilayer Rayleigh–Taylor instability (RTI) using statistically stationary experiments conducted in a gas tunnel. Employing diagnostics such as particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF), we make simultaneous velocity–density measurements to study how dynamics and mixing are linked in this variable density flow. Experiments are conducted in a newly built, blow-down three-layer gas tunnel facility. Mixing between three gas streams is studied, where the top and bottom streams are comprised of air, and the middle stream is an air–helium mixture. Shear is minimized between these streams by matching their inlet velocities. The four experimental conditions investigated here consist of two different density ratios (Atwood numbers 0.3 and 0.6), each investigated at two instability development times (or equivalently, two streamwise locations), and all experiments are with the same middle stream thickness of 3 cm. The growth of the middle layer is measured using laser-based planar Mie scattering visualization. The mixing width is found to grow linearly with time at late times. Various quantitative measures of molecular mixing indicate a very high degree of molecular mixing at late times in the multilayer RTI flow. The vertical turbulent mass flux $a_y$ is calculated. In addition to mostly negative values of $a_y$ , typical of buoyancy-dominated flows due to negative correlation between velocity and density fluctuations, positive regions are also observed in profiles of $a_y$ due to entrainment and erosion at the lower edge of the mixing region. Global energy budgets are calculated for the multilayer RTI flow at late times and it is found that the majority of potential energy released has been dissipated due to viscous effects, and a large value of mixing efficiency ( $\sim$ 60 %) is observed.

The dynamics of molecular mixing and the energy transfer process in the Rayleigh-Taylor instability (RTI) are studied through the collection of simultaneous velocity-density measurements using particle image velocimetry (PIV) and laser induced fluorescence (LIF). Statistically stationary experiments are performed in a convective-type gas tunnel facility which allows long experimental times and enables collection of statistically important turbulence data. The data and analyses presented in this paper are expected to help validate variable-density turbulence models and further our understanding of instability-driven flows.

In the present work, effects of compressibility on the dynamic stall of NACA 0012 airfoil, pitching sinusoidally from 5.03° to 24.79°, are investigated computationally using implicit large eddy simulations in a finite difference framework. Simulations of two-dimensional (2D), high Reynolds number, compressible flows are carried out without any transition or turbulence model to capture the physics of the dynamic stall process. The problem is formulated in a body-fixed, rotating, non-inertial frame. High accuracy, dispersion relation preserving optimized upwind compact scheme is used to compute convective flux derivatives, and an optimized three-stage Runge-Kutta method is used for time integration. Results are presented for free stream Mach number M∞ = 0.283, 0.4, and 0.5, where the Mach number is varied independent of the Reynolds number. The computations have been quite successful in capturing the essential features of the dynamic stall mechanism. It is observed that dynamic moment and lift stalls occur at smaller angles of attack as the Mach number increases. Reduction in the size of airload hysteresis loops and maximum attainable load coefficients are observed with increasing Mach number. Weak shock waves are observed near the leading edge (LE) at M∞ = 0.4, and lambda-shock is formed near the LE for M∞ = 0.5. It is observed that with increasing Mach number, the impact of dynamic stall on the aerodynamic loads (Cl, Cd, and Cm) becomes less dramatic as the maximum value attained by these aerodynamic loads decreases with an increase in the Mach number. An increase in positive damping area in the hysteresis loop is observed with an increase in the Mach number, inhibiting possible vulnerability to stall flutter.

The dynamics of the coupled Kelvin–Helmholtz (KH) and Rayleigh–Taylor (RT) instability (referred to as KHRT instability or KHRTI) is studied using statistically steady experiments performed in a multi-layer gas tunnel. Experiments are performed at four density ratios ranging in Atwood number$A_{t}$from 0.035 to 0.159, with varying amounts of shear and$\unicode[STIX]{x0394}U/U$ranging from 0 to 0.48, where$\unicode[STIX]{x0394}U$is the speed difference between the two flow streams being investigated and$U$is the mean velocity of these two streams. Three types of diagnostics – back-lit visualization, hot-wire anemometry and particle image velocimetry (PIV) – are employed to obtain the mixing widths, velocity field and density field. The flow is found to be governed by KH dynamics at early times and RT dynamics at late times. This transition from KH-instability-like to RT-instability-like behaviour is quantified using the Richardson number. Transitional Richardson number magnitudes obtained for the present KHRT flows are found to be in the range 0.17–0.56 similar to the critical Richardson numbers for stably stratified free shear flows. Comparing the evolution of density and velocity mixing widths, the density mixing layer is found to be approximately two times as thick as the velocity mixing layer. Scaling of velocity fluctuations is attempted using combinations of KH and RT scales. It is found that the proposed KHRT velocity scale, obtained using the combined mixing-layer growth equation, is appropriate for intermediate stages of the flow when both KH and RT dynamics are comparable. Probability density functions (p.d.f.s) for different fluctuating quantities are presented. Multiple peaks in p.d.f.s are qualitatively explained from the development of coherent KH roll-ups and their subsequent transition into turbulent pockets. The evolution of energy spectra indicates that density fluctuations start to show an inertial subrange from earlier times compared to velocity fluctuations. The spectra exhibit a slightly steeper slope than the Kolmogorov–Obukhov five-thirds law.

elds, with S-PIV employed for the rst time for such experimental conditions. Velocity and density statistics, and their correlations (u’, v’, w’, ′ ; ′ v ′ ) are presented. Calculations of probability density functions (p.d.f.s) and energy spectra are made to provide further insight into the ow physics. Energy budget of the ow is also discussed. Reference: AKULA, B. & RANJAN, D. 2016 Dynamics of buoyancy-driven ows at moderately high Atwood numbers. Journal

Various physical mechanisms of transition to turbulence in zero pressure gradient (ZPG) flow past a flat plate are reported with the help of theoretical and computational studies. Spatio-temporal wave-fronts (STWF) have been reported in [Phys. Rev. Lett., 107, 154501 (2011); Phys. Rev. E., 89, 043018 (2014)], from the solution of two-(2D) and three-dimensional (3D) Navier-Stokes equation (NSE), as the precursor of transition to turbulence. This causes a shift in our understanding of transition showing centrality of STWF, as opposed to the view-point that the growth of so-called Tollmien-Schlichting waves under favourable condition eventually leads to turbulence. Having shown the commonality of STWF's role in causing transition by 2D and 3D routes, here we investigate the role of STWF for different routes of transition from the accurate direct numerical simulation of 2D NSE. Present work explains the computational requirements for accurately capturing and studying STWF.