arXiv subjects

Ron Shnapp

Publications and source records attributed to Ron Shnapp.

15 recordsLinked to original sources

arXiv2026

Peristaltic pumping in short annular geometries: An experimental approach for studying Glymphatic flow

Peristaltic pumping is hypothesized to drive fluid transport in several physiological systems, including cerebrospinal fluid flow through cerebral perivascular spaces (PVSs). Cerebral PVSs are unique in the context of peristaltic pumping because they have annular geometry and are orders of magnitude shorter than the peristaltic wavelength. Due to these features, questions were raised as to whether peristaltic pumping is possible under such conditions, and experimental tests for this concept are lacking. This work presents a novel experimental setup that enables direct, detailed measurements of peristaltic flow in short annular channels formed between a compliant inner tube and a rigid outer tube. A propagating pulse wave along the inner tube generates back and forth fluid motion in the annular gap, which we measure using particle tracking velocimetry in a refractive-index matched setup. Despite the instantaneous back and forth motion, net axial fluid transport in the direction of wave propagation is observed, and the resulting net velocity profiles collapse across a range of wall deformation amplitudes. These results provide experimental evidence for net transport induced by long wave length peristaltic deformations in a physiologically relevant flow regime.

physics.flu-dyn↗

arXiv2026

Lagrangian Proper Orthogonal Decomposition

We introduce a modal representation for Lagrangian trajectories in turbulence, termed Lagrangian Proper Orthogonal Decomposition (LPOD). An ensemble of particle trajectories is used to construct velocity time series, which are normalized independently for each trajectory to isolate fluctuations. Principal Component Analysis is then applied to the resulting dataset, with temporal instances defining the feature space. The method is tested on trajectories from both direct numerical simulations of homogeneous isotropic turbulence and three-dimensional particle-tracking experiments, showing that the leading modes exhibit similar structures and energy distributions in both cases. Truncated reconstructions are obtained by combining modes and coefficients, rescaling the fluctuations, and integrating in time. For trajectories of the order of the integral time scale, single-particle dispersion and curvature statistics are accurately reproduced using a limited number of modes (c.a. 10), whereas capturing the tails of acceleration distributions requires a larger set (c.a. 30-60). Longer trajectories require progressively more modes for accurate reconstruction. These results suggest a possible route to data-driven generation of synthetic particle trajectories via stochastic sampling of the modal Lagrangian dynamics.

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arXiv2025

The Lagrangian kinetic energy cascade in Rayleigh-B\'{e}nard convection

Rayleigh-B\'{e}nard convection at high Rayleigh number exhibits turbulence superimposed on large-scale circulation. While buoyancy forces drive the flow at certain scales, how kinetic energy is transfers across the scales is not understood. Here, utilizing a Lagrangian description of the kinetic energy flux, we present experimental evidence of a split cascade where energy flows downwscale at small scales and upscale at large scales. The flow topology of these energy transfer events differ profoundly, and the transition between them occurs gradually, over a broad range of scales.

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arXiv2025

Velocity alignment explains Lagrangian irreversibility in turbulence

Lagrangian particles in turbulence separate away from each other faster in the backward in time direction as compared to forward in time. In this work, we show that time irreversibility is kinematically rooted in the fact that, when viewed backward in time, the alignment of particles' relative velocities is better than when viewed forward in time.

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arXiv2023

Bubble-induced convection and flow-instability in a soft reactor

Buoyancy-driven bubbly flows play pivotal roles in various scenarios, such as the oxygenation and mixing in the upper ocean and the reaction kinetics in chemical and bio-reactors. This work focuses on the convective flow induced by the localized release of large air bubbles ($D_b=3.7$ mm, $\mathrm{Re}_b=950$) in a water tank, exploring the resulting flow and the transition from laminar to disturbed states as a function of the Rayleigh number in the range $3\times10^3 \div 2\times10^5$. At low $\mathrm{Ra}$ the flow is smooth and laminar with weak temporal oscillations, while a highly disturbed state appears above a critical value $\mathrm{Ra}_c$. A theoretical analysis is presented that links the mean flow circulation to the Rayleigh number. Through an experimental investigation, utilizing 3D-particle tracking velocimetry and flow visualization, we confirm the theory presented, and characterize the laminar to disturbed transition in the system. The study offers insights into the convective flow dynamics generated by bubbles, with implications for applications such as bio-reactor soft mixer design.

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arXiv2023

On local isotropy and scale dependence of pair dispersion in turbulent canopy flows

Canopy flows in the atmospheric surface layer play important economic and ecological roles, governing the dispersion of passive scalars in the environment. The interaction of high-velocity fluid and large-scale surface-mounted obstacles in canopy flows produces drag and causes intense, inhomogeneous, and anisotropic turbulence. In this work, we focus on the turbulent dispersion of passive scalars by studying the ``pair dispersion'' - a statistical measure of relative motion between particles. We analyze the results of a 3D-PTV experiment in a wind tunnel canopy flow, focusing on small scales. We confirm the existence of local isotropy of pair dispersion at scales smaller than a characteristic shear length scale $L_\Gamma=(\epsilon/\Gamma^3)^{1/2}$, where $\epsilon$ and $\Gamma$ are the mean dissipation rate and shear rate, respectively. Furthermore, we show that pair dispersion in this locally isotropic regime is a scale-dependent super-diffusive process, similar to what occurs in homogeneous isotropic turbulent flows. In addition, we measure the pair relative velocity correlation function, showing that its de-correlation occurs in the locally isotropic regime, and discuss the implications of this observation for modeling pair dispersion. Thus, our study extends the fundamental understanding of turbulent pair dispersion to the anisotropic, inhomogeneous, turbulent canopy flow, bringing valuable information for modeling scalar dispersion in the atmospheric surface layer.

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arXiv2022

Universal alignment in turbulent pair dispersion

Countless processes in nature and industry, from rain droplet nucleation to plankton interaction in the ocean, are intimately related to turbulent fluctuations of local concentrations of advected matter. These fluctuations can be described by considering the change of the separation between particle pairs, known as pair dispersion, which is believed to obey a cubic in time growth according to Richardson's theory. Our work reveals a universal, scale-invariant alignment between the relative velocity and position vectors of dispersing particles at a mean angle that we show to be a universal constant of turbulence. We connect the value of this mean angle to Richardson's traditional theory and find agreement with data from a numerical simulation and a laboratory experiment. While the Richardson's cubic regime has been observed for small initial particle separations only, the constancy of the mean angle manifests throughout the entire inertial range of turbulence. Thus, our work reveals the universal nature of turbulent pair dispersion through a geometrical paradigm whose validity goes beyond the classical theory, and provides a novel framework for understanding and modeling transport and mixing processes.

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arXiv2022

Clustering through pair interactions in swimming zooplankton

This work focuses on the formation of mating aggregates in zooplankton. In particular, sexual encounters are behaviourally supported by males actively swimming in search for females, and approaching them for mating once they are found. While the random search leads to a diffusive flux of individuals, the approaching for encounter supports attraction. Thus, we ask whether these competing mechanisms of diffusion and attraction can support aggregation and lead to the formation of mating clusters. To answer our question we formulate a model in which particles performing random walks can briefly make contact with other particles if they are found within a particular distance from each other. Our analysis shows that this model supports clustering in a way analogous to the process of colloid aggregation. Following that, we analyze a dataset of 3D trajectories of swimming copepods and show that the results compare well with our model. These results support the hypothesis that pair-interactions promote mating aggregates in zooplankton and are sufficient to overcome the diffusive nature of their mate searching behavior. Our results are useful for understanding small-scale clustering of zooplankton, which is crucial for predicting encounter rates and reproduction rates in the ocean.

q-bio.PE↗

arXiv2022

Splitting of localized disturbances in viscoelastic channel flow

We examine the response of an inertia-less viscoelastic channel flow to localized perturbations. A simplified model shows that the non-linear interaction between the velocity and elastic stress fields can split an initial pulsed disturbance into two separate pulses if the initial disturbance and the base elastic stress are sufficiently high. In accordance, we demonstrate that a transition to a pulse-splitting regime can be achieved experimentally. These results suggest a possible new direction for studying the elastic instability of viscoelastic channel flows at high elasticity through the growth of localized perturbations.

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arXiv2021

Non-modal elastic instability and elastic waves in weakly perturbed channel flow

In this paper, we present experimental results and reveal that strong perturbations are not necessary for elastic instability to occur in straight-channel, inertialess, visco-elastic flows at high elasticity. We show that a non-normal mode bifurcation is followed by chaotic fluctuations, self-organized as stream-wise streaks, and elastic waves due to weak disturbances generated by a small cavity at the center of the top channel wall. The chaotic flow persists in the transition, elastic turbulence, and drag reduction regimes, in agreement with previous observations for the case of strong perturbations at the inlet. Furthermore, the elastic waves we observe propagate in the span-wise direction, which allows to confirm the elastic waves linear dispersion relation directly for the first time. In addition, the span-wise propagating elastic wave's velocity depends on $\mathrm{Wi}$ with the same scaling that was previously observed for stream-wise propagating waves, although their velocity magnitude is significantly smaller than what was previously observed for the stream-wise ones.

physics.flu-dyn↗

arXiv2020

On small-scale and large-scale intermittency of Lagrangian statistics in canopy flow

The interaction of fluids with surface-mounted obstacles in canopy flows leads to strong turbulence that dominates dispersion and mixing in the neutrally stable atmospheric surface layer. This work focuses on intermittency in the Lagrangian velocity statistics in a canopy flow, which is observed in two distinct forms. The first, small scale intermittency, is expressed by non-Gaussian and not self-similar statistics of the velocity increments. The analysis shows an agreement in comparison with previous results from homogeneous isotropic turbulence (HIT) using the multifractal model, extended self-similarity, and acceleration autocorrelations. These observations suggest that the picture of small-scale Lagrangian intermittency in canopy flows is similar to that in HIT, and therefore, they extend the idea of universal Lagrangian intermittency to certain inhomogeneous and anisotropic flows. Second, it is observed that the RMS of energy increments along Lagrangian trajectories depend on the direction of the trajectories' time-averaged turbulent velocity. Subsequent analysis suggests that the flow is attenuated by the canopy drag while leaving the structure function's scaling unchanged. This observation implies the existence of large-scale intermittency in Lagrangian statistics. Thus, this work presents a first empirical evidence of intermittent Lagrangian velocity statistics in a canopy flow that exists in two distinct senses and occurs due to different mechanisms.

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arXiv2019

Turbulence -- Obstacle Interactions in the Lagrangian Framework: Applications for Stochastic Modeling in Canopy Flows

Lagrangian stochastic models are widely used to predict and analyze turbulent dispersion in complex environments, such as in various terrestrial and marine canopy flows. However, due to a lack of empirical data, it is still not understood how particular features of highly inhomogeneous canopy flows affect the Lagrangian statistics. In this work, we study Lagrangian short time statistics by analyzing empirical Lagrangian trajectories in sub-volumes of space that are small in comparison with the canopy height. For the analysis we used 3D Lagrangian trajectories measured in a dense canopy flow model in a wind-tunnel, using an extended version of real-time 3D particle tracking velocimetry (3D-PTV). One of our key results is that the random turbulent fluctuations due to the intense dissipation were more dominant than the flow's inhomogeneity in affecting the short-time Lagrangian statistics. This amounts to a so-called quasi-homogeneous regime of Lagrangian statistics at small scales. Using the Lagrangian dataset we calculate the Lagrangian autocorrelation function and the second-order Lagrangian structure-function, and extract associated parameters, namely a Lagrangian velocity decorrelation timescale, $T_i$, and the Kolmogorov constant, $C_0$. We demonstrate that in the quasi-homogeneous regime, both these functions are well represented using a second-order Lagrangian stochastic model that was designed for homogeneous flows. Furthermore, we show that the spatial variations of the Lagrangian separation of scales, $T_i/\tau_\eta$, and the Kolmogorov constant, $C_0$, cannot be explained by the variation of the Reynolds number, $Re_\lambda$, in space, and that $T_i/\tau_\eta$ was small as compared with homogeneous turbulence predictions at similar $Re_\lambda$. We thus hypothesize that this occurred due to the so-called "wake production", and show empirical results supporting our hypothesis.

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arXiv2018

Extended 3D-PTV for direct measurements of Lagrangian statistics of canopy turbulence in a wind tunnel

Direct estimation of Lagrangian turbulence statistics is essential for the proper modeling of dispersion and transport in highly obstructed canopy flows. However, Lagrangian flow measurements demand very high rates of data acquisition, resulting in bottlenecks that prevented the estimation of Lagrangian statistics in canopy flows hitherto. We report on a new extension to the 3D Particle Tracking Velocimetry (3D-PTV) method, featuring real-time particle segmentation that outputs centroids and sizes of tracer particles and performed on dedicated hardware during high-speed digital video acquisition from multiple cameras. The proposed extension results in four orders of magnitude reduction in data transfer rate that enables to perform substantially longer experimental runs, facilitating measurements of convergent statistics. The extended method is demonstrated through an experimental wind tunnel investigation of the Lagrangian statistics in a heterogeneous canopy flow. We observe that acceleration statistics are affected by the mean shear at the top of the canopy layer and that Lagrangian particle dispersion at small scales is dominated by turbulence in the wake of the roughness elements. This approach enables to overcome major shortcomings from Eulerian-based measurements which rely on assumptions such as the Taylor's frozen turbulence hypothesis, which is known to fail in highly turbulent flows.

physics.flu-dyn↗

arXiv2017

Generalization of Turbulent Pair Dispersion to Large Initial Separations

We present a generalization of turbulent pair dispersion to large initial separations ($\eta < r_0 < L$), by introducing a new time scale, $\tau_{v_0}$, that reflects the persistence of initial conditions at time $\tau=0$. Results of 3D Lagrangian tracking experiments at moderate Reynolds numbers show that pairs, for which the new time scale is shorter than the eddy turnover time scale, separate as in the Richardson superdiffusive regime, $\langle \Delta r^2 \rangle \propto \tau^3$. The analysis of delay times (time interval to cross $\Delta r = \rho \, r_0$) of these conditionally sampled pairs exhibit $\rho^{2/5}$ scaling.

physics.flu-dyn↗

arXiv2014

Detailed comparative study and a mechanistic model of resuspension of spherical particles from rough and smooth surfaces

Resuspension of solid particles by a tornado-like vortex from surfaces of different roughness is studied using a three-dimensional particle tracking velocimetry (3D-PTV) method. By utilizing the three-dimensional information on particle positions, velocities and accelerations before, during and after the resuspension (lift-off) event, we demonstrate that the resuspension efficiency is significantly higher from the rough surface, and propose a mechanistic model of this peculiar effect. The results indicate that for all Reynolds numbers tested, the resuspension rate, as well as particle velocities and accelerations, are higher over the rough surface, as compared to the smooth counterpart. The results and the model can help to improve modeling and analysis of resuspension rates in engineering and environmental applications.

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