Optical turbulence caused by refractive index fluctuations poses a major challenge for imaging, free-space communication, and directed-energy systems. Rayleigh-B & eacute;nard (RB) convection offers a controlled laboratory analog for studying buoyancy-driven turbulence and its optical effects. Building on theoretical predictions that link turbulence strength to heat flux, we experimentally determine the scaling constant gamma by simultaneously measuring the refractive index structure constant (Cn2) and heat flux in an RB environment. Using a variable turbulence generator (VTG), we validate RB conditions through Nusselt-Rayleigh scaling and direct numerical simulations (DNS). Three independent optical diagnostics were employed to estimate Cn2 (scintillation, beam wander, and long-term beam spot size), while embedded sensors captured heat flux. This scaling constant validation is confined to the experimental conditions described. Results confirm the predicted Cn2 proportional to Q4/3 relationship, with gamma = 8.79 +/- 0.61 closely matching simulations (8.65). This strong agreement demonstrates the robustness of the heat-flux-based scaling relationship and establishes RB systems as effective testbeds for turbulence characterization. These findings provide a practical framework for predicting optical performance in complex environments and advancing turbulence mitigation strategies. (c) 2026 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Coherent vortical structures are an important feature of near-wall turbulence, and understanding how particles and bubbles move with respect to these structures can be useful for many applications of measurement and flow control. While many studies have characterized bubbles and particles in terms of preferential concentration within turbulent flows, few studies have examined this relationship within the context of targeting near-wall vortices for flow control applications. The current study examines the entrapment of bubbles and particles by these near-wall turbulent vortices by simulating 10(6) small bubbles and heavy particles in a turbulent channel flow using a one-way coupled point particle approximation with direct numerical simulations for Reynolds numbers up to Re-tau=395. Their entrapment relative to tracer particles shows a clear preference for bubbles to move into vortex cores and heavy particles to move away from vortex cores, and special limiting flow cases demonstrate the effect of flow time scales on overall entrapment. Our findings suggest that the entrapment (or lack thereof) saturates on a time scale of t(+)approximate to 100 across all simulated Reynolds numbers for both particles and bubbles, and we show that bubble entrapment is limited by the lifetimes of local structures in near-wall turbulence. Therefore, the current work provides a characterization of an important constraint for potential particle-based flow control strategies.

The interaction of vortical structures with boundaries has been extensively studied in Newtonian fluids, where conditions such as no slip walls, free surfaces, or contaminated surfaces dictate whether vortices rebound, dissipate, or generate secondary structures. In this work, we investigate a related but fundamentally different problem: the interaction of a vortex pair with a finite, non uniform layer of polymeric fluid. Numerical simulations employing the finitely extensible nonlinear elastic Peterlin model are used to examine the effects of polymer concentration, relaxation time, polymer layer thickness, and maximum polymer extension on the evolution of kinetic energy and enstrophy. The results show that, while the polymeric fluid dissipates vortical motion, vortex polymer layer interactions can also generate new coherent structures. In particular, the formation of secondary and tertiary vortices coincides with transient increases in kinetic energy, a behavior absent in the Newtonian case. Unlike classical vortex boundary interactions, where the primary vortex survives, we find that under certain conditions it completely dissipates upon interaction with the polymer layer. These findings emphasize that fluids with non-uniform polymer concentrations, act not only as dissipative agents but also as sources of vorticity, extending the traditional view of polymer induced drag reduction and providing new insight into vortex polymer interactions.

Direct numerical simulations are performed to study the effects of spatially non-uniform polymer concentration on a Lamb–Oseen vortex. This simplified problem relates to turbulent drag reduction using polymer additives and is a step toward optimizing polymer drag reduction strategies. Previous work has examined spatially uniform polymer concentrations; however, this study investigates how the localized distribution of polymer affects a single vortex. The goal of the study is to determine the optimal polymer placement relative to the vortex core and to relate this positioning to identifiable flow features that maximize the enstrophy dissipation. The simulations reveal a strong correlation between regions of high strain rate around the initial vortex and the most effective polymer placement. Key results indicate that placing the polymer approximately 39% beyond the vortex core radius yields the greatest enstrophy dissipation, aligning closely with regions of maximum strain rate at the initial conditions.

The effectiveness of polymer drag reduction by targeted injection is studied in comparison with that of a uniform concentration (or polymer ocean) in a turbulent channel flow. Direct numerical simulations are performed using a pseudo-spectral code to solve the coupled equations of a viscoelastic fluid using the finitely extensible nonlinear elastic dumbbell model with the Peterlin approximation. Light and heavy particles are used to carry the polymer in some cases, and polymer is selectively injected into specific flow regions in the other cases. Drag reduction is computed for a polymer ocean at a viscosity ratio of $\beta = 0.9$ for simulation validation, and then various methods of polymer addition at $\beta = 0.95$ are compared for their drag-reduction performance and general effect on the flow. It was found that injecting polymer directly into regions of high axial strain inside and around coherent vortical structures was the most effective at reducing drag, while injecting polymer very close to the walls was the least effective. The targeting methods achieved up to 2.5 % higher drag reduction than an equivalent polymer ocean, offering a moderate performance boost in the low drag-reduction regime.

A model for the structure function constant associated with index of refraction fluctuations in Rayleigh-Benard turbulence is developed. The model is based upon the following assumptions: (1) the turbulence is homogeneous and isotropic at or near the mid-plane, (2) the rate of production is in balance with the rate of dissipation, (3) an inertial region exists, and (4) estimates for the rate of dissipation of temperature fluctuations and of turbulent kinetic energy can be made by assuming that the large-scale turbulence is dissipated in one eddy turnover time. From these assumptions, the dependence of the structure function on the geometry, heat flux, and the properties of the fluid is obtained. The model predicts that the normalized structure function constant is independent of the Rayleigh number. To verify the model, numerical simulations of Rayleigh-Benard turbulence were performed using two different approaches: an in-house code based on a pseudo-spectral method, and a finite volume code which employs a model for the smallest scales of the turbulence. The model was found to agree with the results of the simulations, thereby lending support for the assumptions underlying the theory.

It is shown that laminar vortex rings can be generated by impulsive body forces having particular spatial and temporal characteristics. The method produces vortex rings in a fluid initially at rest, and once generated, the flow field automatically satisfies the boundary conditions and is divergence-free. Numerical simulations and analytical models show that the strength of these rings can be accurately predicted by considering diffusion alone, despite the nonlinear nature of the generation process. A particularly simple model, which approximates the source of vorticity within vertical slabs, is proposed. This model predicts the ring circulation almost as accurately as a model which uses the exact geometry of the source of vorticity. It is found that when the duration of the force is less than a time scale based on the force radius and fluid viscosity, the ring circulation can be predicted accurately using an inviscid model.

We simulate Rayleigh Benard natural convection in air and propagate Gaussian Laguerre beams with varying topological charge through it. We present the impact of the optical turbulence on the phase and intensity of the beams.

Optical propagation through turbulence remains a topic of active research and is critically important to the development of novel optical communication systems in both air and water. A widely used tool to study propagation through turbulence are laboratory tanks where optically active turbulence is generated through heating and cooling of the horizontal tank walls, akin to classic Rayleigh-Bénard convection. An important complement to the laboratory setup are numerical simulations that can supplement the sparser laboratory measurements through full fields of temperature and velocity. Such simulations can also provide phase screens for modeling of optical propagation through turbulence. We performed numerical simulations of different configurations of Rayleigh-Bénard turbulence tanks for comparison to other physical and numerical convective tanks. Results then provided the basis for optical modeling and the description of beam wander due to optical turbulence.

In a high viscosity, polymeric fluid initially at rest, the release of elastic energy produces vorticity in the form of coherent motions (vortex rings). Such behavior may enhance mixing in the low Reynolds number flows encountered in microfluidic applications. In this work, we develop a theory for such flows by linearizing the governing equations of motion. The linear theory predicts that when elastic energy is released in a symmetric manner, a wave of vorticity is produced with two distinct periods of wave motion: (1) a period of wave expansion and growth extending over a transition time scale, followed by (2) a period of wave translation and viscous decay. The vortex wave speeds are predicted to be proportional to the square root of the initial fluid tension, and the fluid tension itself scales as the viscosity. Besides verifying the predictions of the linearized theory, numerical solutions of the equations of motion for the velocity field, obtained using a pseudo-spectral method, show that the flow is composed of right- and left-traveling columnar vortex pairs, called vortex waves for short. Wave speeds obtained from the numerical simulations are within 1.5% of those from the linear theory when the assumption of linearity holds. Vortex waves are found to decay on a time scale of the order of the vortex size divided by the solution viscosity, in reasonable agreement with the analytical solution of the linearized model for damped vortex waves. When the viscoelastic fluid is governed by a nonlinear spring model, as represented by the Peterlin function, wave speeds are found to be larger than the predictions of the linear theory for small polymer extension lengths.

Nicholas V. Scott, Jack McCarthy, Tian-Jian Hsu Riverside Research, Open Innovation Center, Dayton Research Center 2640 Hibiscus Way, Beavercreek, Ohio, 45431, USA nscott@riversideresearch.org; jack.mccarthy@duke.edu Duke University, Department of Statistical Science 214 Old Chemistry, Box 90251, Durham, NC, 27708, USA thsu@udel.edu University of Delaware, Center for Applied Coastal Research 259 Academy Street, Newark, DE, 19716, USA

Numerical simulations of a Rayleigh-Bénard turbulent convective flow are examined to determine the optical and mechanical turbulence properties and resulting index of refraction and temperature structure function fields with the goal of understanding the propagation characteristics of a laser beam carrying orbital angular momentum. Beams carrying orbital angular momentum are a topic of interest for secure high data density free-space communications systems in both the atmosphere and underwater environment. The choice of Rayleigh-Bénard convection provides a highly controllable configuration for studying optical turbulence and once the flow reaches a steady state, it may be treated as homogeneous. With a well characterized turbulent state provided by the simulations, attention is focused on the mechanics of beam propagation through the turbulence. Simulations are performed using the open source computational fluid dynamics package OpenFoam, a finite volume solver, and an in-house developed code that uses spectral methods. In the case of each solver, the Boussinesq approximation is used to model buoyancy and both the Navier-Stokes equations and the thermal energy equation are simultaneously solved. The outcome from the two computational schemes will be cross compared for result fidelity, spatial resolution, and computation time. The initial effort will examine air as the working medium in a domain with dimensions of 0.5 m on a side and a height of 0.1 m.

Bubble trajectories in the presence of a decaying Lamb–Oseen vortex are calculated using a modified Maxey–Riley equation. Some bubbles are shown to get trapped within the vortex in quasi-equilibrium states. All the trapped bubbles exit the vortex at a time that is only a function of the Galilei number and the vortex Reynolds number. The set of initial bubble locations that lead to entrapment is numerically determined to show the capturing potential of a single vortex. The results provide insight into the likelihood of bubble entrapment within vortical structures in turbulent flows.

The remarkable ability of viscoelastic fluids to augment local and global surface heat transfer characteristics is demonstrated by new experimental results, which are provided for a rotating Couette flow (RCF) environment (also referred to as von Karman swirling flow) with convective heat transfer. Included are Nusselt number variations, flow visualization results, and spectral analysis of flow static temperature fluctuations. Augmented surface Nusselt numbers are measured for sucrose-based, viscoelastic solutions with polyacrylamide (as they are subject to different magnitudes of flow strain), relative to Newtonian flows and relative to fluids with zero shear rate. The resulting Nusselt number enhancements are related to the experimental conditions that are believed to be associated with the onset and development of elastic instabilities. Resulting comparisons show that the Weissenberg number is the parameter that best correlates and characterizes Nusselt number augmentations, which are strongly correlated with pronounced redistributions of fluorescein FWT red tracer dye fronts, obtained from flow visualizations. As such, the present investigation provides new insight into thermal transport of viscoelastic fluids, and new experimental data that illustrate the utility of different analytic and numerical models for predicting experimental conditions associated with significant Nusselt number augmentations.

Viscoelastic fluids have been shown to undergo instabilities even at very low Reynolds numbers, and these instabilities can give rise to a phenomenon called elastic turbulence. This phenomenon, observed experimentally in viscoelastic polymer solutions, is driven by the strong coupling between the fluid velocity and the elasticity of the flow. To explore the emergence of these instabilities in a viscoelastic flow, we have chosen to explore, by means of direct numerical simulations, a particular case called von Karman swirling flow. The simulations employ the finitely extensible nonlinear Peterlin model to represent the dynamics of a dilute polymer solution. Notably, a log-conformation technique is used to solve the governing equations. This method is useful in overcoming the high Weissenberg number problem. The results obtained from the simulations were generally in good agreement with experiments. The torque on the top plate was decomposed into Newtonian and polymeric components, and it was found that the polymeric component was dominant. In addition, flow visualizations revealed that a toroidal vortex was strongly correlated with the distribution of the stresses on the rotating plate.

Geo-intelligence remote sensing platforms situated over spatially diverse areas are often tasked with geo-intelligence surveillance and adversarial monitoring for military organizations. Limited resources disallow continuous sampling of local areas at the same time, necessitating a need for smart sensing of diverse environments according to a rational evidence-based rule. Such algorithms should not only provide insight into which local region should be focused on, but should also facilitate decisions as to which environmental features should be measured over time once a local site has been selected. Multicomponent optimal learning observational arrays are demonstrated using numerically simulated data of turbulent flow to show not only the feasibility of how individual observational platforms should be chosen in a Bayesian sense, but also how goal state directed sampling of complex systems or turbulent processes over local regions can be accomplished. A Bayesian amalgamation algorithm guides which observational arrays perform knowledge gradient policy based optimal learning to smartly sample observations in local regions. Machine learning and operations research algorithms function as data agnostic, Bayesian processors demonstrating how geo-intelligence information can be efficiently captured to help solve data-driven problems.

All-atom molecular dynamics is used to investigate the structural, energetic, and dynamical properties of polyacrylamide (PAM) oligomers of different lengths solvated in pure glycerol, a 90:10 glycerol–water mixture, and pure water. We predict that the oligomers’ globular structure is obtained only when the modeling strategy considers the solvent as a continuous background. Meanwhile, for all-atom modeled solvents, the glycerol solutions display a strong tendency of trapping the oligomers in instantaneous elongated random coiled structures that remain locked-in over tens of nanoseconds. In pure water, the oligomers acquire considerably shorter random coiled structures of increased flexibility. The all-atom force field, generalized amber force field, is modified by including restrained electrostatic potential atomic charges for both glycerol and PAM. Three PAM oligomer lengths containing 10, 20, and 30 monomers are considered in detail by monitoring the radius of gyration, end-to-end distance, intra-potential energy, and solvent–oligomer interaction energies for decades of nanoseconds. The density and radial distribution function of glycerol solutions are calculated when modeled with the modified atomic charges, showing a very good agreement with the experimental results at temperatures around 300 K. Glycerol has multiple applications, including its use in gel formation for PAM gel electrophoresis. Our findings are relevant for the design of sensors based on microfluidics and tailored pharmaceutical buffer solutions.

Poly-lactic-co-glycolic acid (PLGA) is a biodegradable co-polymer with common use in nanoparticle drug encapsulation. Although well studied experimentally, the mechanical behavior of PLGA is not well understood at the atomic level. Here, we develop atomic charges for the all-atom Generalized Amber Force Field (GAFF) and conduct all-atom molecular dynamics simulations of PLGA with a 50:50 ratio between its two constituent monomers for five samples of the polymer condensed phases that span 1579 u to 20183 u in molecular weight. We predict several PLGA properties that will improve the knowledge of its atomistic organization in the glassy solid, rubber, and liquid states. We report the impact of molecular weight on cohesive energy, solubility, thermodynamic response properties, structural properties related to chain entanglement, and glass transition temperatures. Properties are compared against known experimental values when available. We find that the restrained electrostatic potential atomic charges are better for simulating the caloric curve leading to the glass transition temperature, which agrees very well with experiments.

Abstract It is well known that the mixing of two or more species in flows at low Reynolds numbers cannot be easily achieved since inertial effects are essentially absent and molecular diffusion is slow. To achieve mixing in Newtonian fluids under these circumstances requires innovative new ideas such as the use of external body forces (eg, electromagnetic mixers) or the stretching and folding of fluid elements (eg, chaotic advection). For non‐Newtonian fluids with elasticity, mixing can be achieved by enabling the emergence of elastic instabilities that results in chaotic flows in which mixing is significantly enhanced. In this work, our goal is to demonstrate that clearly identifiable vortical structures (eg, vortex rings) can be generated in a viscoelastic fluid initially at rest by the release of elastic stresses. In turn, these vortex motions promote bulk mixing by transporting fluid elements from one location to another more efficiently than diffusion alone. We demonstrate this first theoretically by using the finitely extensible nonlinear elastic Peterlin (FENE‐P) model to show that elastic forces can generate torque. Using this model, we derive an expression for the time rate of change of vorticity in an elastic fluid initially at rest caused by a sudden release of stored elastic stress. This process can be thought of as the release of elastic energy from a stretched rubber band that is suddenly cut at its center. We confirm this ansatz by performing a series of direct numerical simulations based on an in‐house pseudo‐spectral code that couples the FENE‐P model to the equations of motion for an incompressible fluid. The simulations reveal that a pair of vortex rings traveling in opposite directions, with Reynolds numbers on the order of one, is generated from the sudden release of elastic stresses. Secondary vortical structures are also generated. In the concluding section of this work, we address the potential for vortex motions generated by elastic stresses to promote mixing in microflows, and we describe a possible experiment that may demonstrate this effect.