Hydrodynamic Signatures Research Articles

Hydrodynamic signatures in thermal transport in devices based on two-dimensional materials: An ab initio study

We investigate the features arising from hydrodynamic effects in graphene and phosphorene devices with finite heat sources, using ab initio calculations to go beyond Callaway's model and inform a full linearized scattering operator, and solving the phonon Boltzmann transport equation through energy-based deviational Monte Carlo methods. We explain the mechanisms that create those hydrodynamic features, showing that boundary scattering and the relation of sample dimensions to the nonlocal length $\ensuremath{\ell}$ are the determinant factors, regardless of the relative importance of normal versus resistive scattering. From this point of view, the nonlocal length $\ensuremath{\ell}$ reflects the ability of scattering to randomize the heat flux, and we show that approximations made on the scattering operator may have, through the value of $\ensuremath{\ell}$, qualitative consequences on the signatures of hydrodynamic behavior.

Collective dynamics and rheology of confined phoretic suspensions

Similarly to their biological counterparts, suspensions of chemically active autophoretic swimmers exhibit a non-trivial dynamics involving self-organisation processes as a result of inter-particle interactions. Using a kinetic model for a dilute suspension of autochemotactic Janus particles, we analyse the effect of a confined pressure-driven flow on these collective behaviours and the impact of chemotactic aggregation on the effective viscosity of the active fluid. Four dynamic regimes are identified when increasing the strength of the imposed pressure-driven flow, each associated with a different collective behaviour resulting from the competition of flow- and chemically induced reorientation of the swimmers together with the constraints of confinement. Interestingly, we observe that the effect of the pusher (respectively puller) hydrodynamic signature, which is known to reduce (respectively increase) the effective viscosity of a sheared suspension, is inverted upon the emergence of autochemotactic aggregation. Our results provide new insights into the role of the collective dynamics in complex environments, which are relevant to synthetic as well as biological systems.

R(refracted)-PIV measurements of water film flow: application to flow under a rolling tire

An analysis of the water flow generated by a tire rolling over a water film has been carried out on an experimental level. The present investigation is focused on a commercial tire with a winter V-shaped complex sculpture. Flow measurements were performed using an optical technique derived from the standard 2D-PIV, the so-called r(refracted)-PIV. Some specific image processing tools have been developed in order to take into account the jitter of the tire location inside the camera field of view when analyzing a set of independent runs. Considering the analysis of the flow either inside the grooves or in front of the tire, different approaches are proposed. On one hand, a geometrical segmentation of the grooves allows a flow analysis inside these tread elements. A characteristic water velocity deduced from measurements performed over a set of independent runs appears to correlate with the position of the segment under consideration along the contact patch area. It has been considered as the hydrodynamic signature of the present tread design. On the other hand, inside the water-bank in front of the tire, an ensemble averaging process of the local velocity is performed to show the influence of groove presence at small scale and the impact of tread design at large scale.Graphical abstract

How inertial lift affects the dynamics of a microswimmer in Poiseuille flow

The transport of motile microorganisms is strongly influenced by fluid flows that are ubiquitous in biological environments. Here we demonstrate the impact of fluid inertia. We analyze the dynamics of a microswimmer in pressure-driven Poiseuille flow, where fluid inertia is small but non-negligible. Using perturbation theory and the reciprocal theorem, we show that in addition to the classical inertial lift of passive particles, the active nature generates a ‘swimming lift’, which we evaluate for neutral and pusher/puller-type swimmers. Accounting for fluid inertia engenders a rich spectrum of complex dynamics including bistable states, where tumbling coexists with stable centerline swimming or swinging. The dynamics is sensitive to the swimmer’s hydrodynamic signature and goes well beyond the findings at vanishing fluid inertia. Our work will have non-trivial implications on the transport and dispersion of active suspensions in microchannels.

Riverbank Filtration – A Potential Water Source Exploitation for the Red River Delta Region

Riverbank filtration technology has been widely applied worldwide because of its high-capacitycollection and good water quality throughout natural purification processes. Infiltration water can beextracted from Holocene (qh) layer or the Pleistocene deep layer (qp), replenished with water from theriver through hydrogeological windows. Hydrodynamic and isotopic signatures were employed todetermine water seepage capacity. The results show that infiltrated water is found in the sand layers alongthe rivers. However, the seepage rate shows a heterogeneously spatial variation ranging from 30 m3/d inthe Dinh Dao river to 33,600 m3/d. Km along the shoreline in the Red River (RRD). Also, the exploitationcapacity of seepage water differs widely in order of large (> 3,000 m3/d), medium (1,000-3,000 m3/d),small (500-1,000 m3/d), and very small capacity (200-500 m3/d). This study indicated that RRD couldapply riverbank filtration techniques to overcome freshwater scarcity in the delta due to increasing surfacepollution and discharge reduction.

Bluff body uses deep-reinforcement-learning trained active flow control to achieve hydrodynamic stealth

We propose a novel active-flow-control strategy for bluff bodies to hide their hydrodynamic traces, i.e., strong shears and periodically shed vortices, from predators. A group of windward-suction-leeward-blowing (WSLB) actuators are adopted to control the wake of a circular cylinder submerged in a uniform flow. An array of velocity sensors is deployed in the near wake to provide feedback signals. Through the data-driven deep reinforcement learning, effective control strategies are trained for the WSLB actuation to mitigate the cylinder's hydrodynamic signatures. Only a 0.29% deficit in streamwise velocity is detected, which is a 99.5% reduction from the uncontrolled value. The same control strategy is found also to be effective when the cylinder undergoes transverse vortex-induced vibration. The findings from this study can shed some light on the design and operation of underwater structures and robotics to achieve hydrodynamic stealth.

Effect of characteristic size on the collective phonon transport in crystalline GeTe

We study the effect of characteristic size variation on the phonon thermal transport in crystalline GeTe for a wide range of temperatures using the first-principles density-functional method coupled with the kinetic collective model approach. The characteristic size dependence of phonon thermal transport reveals an intriguing collective phonon transport regime, located in between the ballistic and the diffusive transport regimes. Therefore, systematic investigations have been carried out to describe the signatures of phonon hydrodynamics via the competitive effects between grain size and temperature. A characteristic nonlocal length associated with phonon hydrodynamics and a heat wave propagation length has been extracted. The connections between phonon hydrodynamics and these length scales are discussed in terms of the Knudsen number. Further, the scaling relation of thermal conductivity as a function of characteristic size in the intermediate size range emerges as a crucial indicator of the strength of the hydrodynamic behavior. A ratio concerning normal and resistive scattering rates has been employed to understand these different scaling relations, which seems to control the strength and prominent visibility of the collective phonon transport in GeTe. This systematic investigation emphasizes the importance of the competitive effects between temperature and characteristic size on phonon hydrodynamics in GeTe, which can lead to a better understanding of the generic behavior and consequences of the phonon hydrodynamics and its controlling parameters in low-thermal conductivity materials.

Learning hydrodynamic signatures through proprioceptive sensing by bioinspired swimmers

Objects moving in water or stationary objects in streams create a vortex wake. Such vortex wakes encode information about the objects and the flow conditions. Underwater robots that often function with constrained sensing capabilities can benefit from extracting this information from vortex wakes. Many species of fish do exactly this, by sensing flow features using their lateral lines as part of their multimodal sensing. To replicate such capabilities in robots, significant research has been devoted to developing artificial lateral line sensors that can be placed on the surface of a robot to detect pressure and velocity gradients. We advance an alternative view of embodied sensing in this paper; the kinematics of a swimmer’s body in response to the hydrodynamic forcing by the vortex wake can encode information about the vortex wake. Here we show that using artificial neural networks that take the angular velocity of the body as input, fish-like swimmers can be trained to label vortex wakes which are hydrodynamic signatures of other moving bodies and thus acquire a capability to ‘blindly’ identify them.

Hydrochemical interactions in dilute phoretic suspensions: From individual particle properties to collective organization

Janus phoretic colloids (JPs) self-propel as a result of self-generated chemical gradients and exhibit spontaneous nontrivial dynamics within phoretic suspensions, on length scales much larger than the microscopic swimmer size. Such collective dynamics arise from the competition of (i) the self-propulsion velocity of the particles, (ii) the attractive/repulsive chemically-mediated interactions between particles and (iii) the flow disturbance they introduce in the surrounding medium. These three ingredients are directly determined by the shape and physico-chemical properties of the colloids' surface. Owing to such link, we adapt a recent and popular kinetic model for dilute suspensions of chemically-active JPs where the particles' far-field hydrodynamic and chemical signatures are intrinsically linked and explicitly determined by the design properties. Using linear stability analysis, we show that self-propulsion can induce a wave-selective mechanism for certain particles' configurations consistent with experimental observations. Numerical simulations of the complete kinetic model are further performed to analyze the relative importance of chemical and hydrodynamic interactions in the nonlinear dynamics. Our results show that regular patterns in the particle density are promoted by chemical signaling but prevented by the strong fluid flows generated collectively by the polarized particles, regardless of their chemotactic or antichemotactic nature (i.e. for both puller and pusher swimmers).

Acoustic velocimetry imaging

We review a technique that we developed that combines shadowgraph or schlieren imaging with image processing tools to study the sound produced by turbulent shear flows. These flows are challenging acoustically because they are characterized by a distributed cloud of correlated sources that produce sound through multiple mechanisms. In the far field, 100 s of characteristic lengths from the source region, the pressure waves are purely acoustic, the apparent the source location shrinks to a point, and the common assumption is that the propagation behavior is spherical. In the near field, pressure signals from both acoustic and hydrodynamic (or evanescent) signatures are difficult to separate, especially when the convective Mach number of the sound-producing events is supersonic. Single-point measurements such as condenser-type microphones are blind to the direction of the approaching/departing pressure waves unless they are combined in arrays. Non-intrusive optical-based methods offer some benefit in this scenario and can be tailored in new and different ways. Examples include acoustic laser Doppler velocimetry and quantitative schlieren methods. Our technique combines modern digital camera technology (with high-resolution, high-speed capabilities) with tomographical image processing techniques images to yield a “picture” of the sound field that can provide quantitative assessment of the source distribution and source velocities. This optical technique is a strong complement for single-point or array microphone measurements.

Active cloaking in Stokes flows via reinforcement learning

Abstract

FLUVIO-DELTAIC DEPOSITIONAL COMPLEX OF THE CAMPANO-MAASTRICHTIAN GOMBE FORMATION OF THE GONGOLA SUB-BASIN, NORTHERN BENUE TROUGH, N.E. NIGERIA

Evaluation of the stratigraphic architecture of the Gombe Formation of the Gongola Sub-basin in the Northern Benue Trough indicated a build-up from six facies assemblage that consist of trough crossbedded sandstone, planar crossbedded sandstone, massive bedded sandstone, ripple laminated sandstone, parallel laminated sandstone and mudstones. These units were packaged into three facies association that constitutes of bedded sandstone facies association (FAgI), interbedded sandstone and mudstone facies association (FAgII) and amalgamated trough crossbedded sandstone facies association (FAgIII). These reflects mouth-bars, pro-delta and delta slope deposits and upper delta plain respectively. This deltaic prism is characterized by unidirectional current system devoid of signatures of tide and waves hydrodynamics in the subaqueous delta regions, thus indicating that the delta system of the Gombe Formation is a fluvial dominated delta.

Effects of the intrinsic curvature of elastic filaments on the propulsion of a flagellated microrobot

Understanding the propulsion mechanism of swimming microorganisms will facilitate the development of synthetic microswimmers for active cargo deliveries. Herein, we studied, theoretically and numerically, inertialess locomotion of a microswimmer—a spherical body propelled by two symmetrically actuated elastic filaments in the shape of a circular arc at rest, focusing on the effects of their uniform intrinsic curvature κ¯c. Combining the resistive force theory for viscous flow and Euler–Bernoulli beam theory for elastic filaments, the elasto-hydrodynamics was solved asymptotically. Our theory was verified by simulations using regularized Stokeslets posed on the filament centerlines, with and without considering hydrodynamic interactions (HIs) between the body and filaments. The asymptotic and numerical results showed qualitative agreement. Reasonable quantitative agreement between the asymptotic results and the numerical predictions neglecting body–filament HIs was observed, especially for small |κ¯c|. However, they deviated quantitatively from the numerical results with body–filament HIs, especially at a large κ¯c when the HIs became important owing to the short body–filament distance. The propulsive force generated by two arc-shaped filaments significantly depend on their uniform intrinsic curvature κ¯c. An appreciable increase in the thrust can be achieved by adjusting κ¯c, which qualitatively confirms and explains the experimentally reported propulsive enhancement facilitated by intrinsically curved appendages [Z. Ye, S. Régnier, and M. Sitti, “Rotating magnetic miniature swimming robots with multiple flexible flagella,” IEEE Trans. Rob. 30, 3–13 (2014)]. The increase in κ¯c can even change the sign of the thrust, leading to counter-intuitive, backward propulsion. The flow field reveals the hydrodynamic signature of the swimmer that shifts with time between a neutral swimmer, a pusher, and a puller.

Shape Classification Using Hydrodynamic Detection via a Sparse Large-Scale 2D-Sensitive Artificial Lateral Line

Artificial lateral lines are fluid flow sensor arrays, bio-inspired by the fish lateral line organ, that measure a local hydrodynamic environment. These arrays are used to detect objects in water, without relying on light, sound, or on an active beacon. This passive sensing method, called hydrodynamic imaging, is complementary to sonar and vision systems and is suitable for collision avoidance and near-field covert sensing. This sensing method has so far been demonstrated on a biological scale from several to tens of centimeters. Here, we present measurements using a large-scale artificial lateral line of 3.5 meters, consisting of eight all-optical 2D-sensitive flow sensors. We measure the fluid flow as produced by the motion of five different objects, towed across a swimming pool. This results in repeatable stimuli, whose measurements demonstrate a complementary aspect of 2D-sensing. These measurements are both used for constructing temporal hydrodynamic signatures, which reflect the object’s shape, and for flow-feature based near-field object classification. For the latter, we present a location-invariant feature extraction method which, using an Extreme Learning Machine neural network, results in a classification F1-score up to 98.6% with selected flow features. We find that, compared to the traditional sensing dimension parallel to the sensor array, the novel transverse fluid velocity component bears more information about the object shape. The classification of objects via hydrodynamic imaging thus benefits from 2D-sensing and can be scaled up to a supra biological scale of several meters.

Scalable Flux Metrics at the Channel‐Floodplain Interface as Indicators of Lateral Surface Connectivity During Flood Events

AbstractLateral surface connectivity (LSC), defined here as the surficial component of hydrologic exchange flows perpendicular to longitudinal flow, is vital to biogeochemical, geomorphological, and ecological processes on floodplains. Because rivers throughout much of the world have been subjected to anthropogenic manipulation, LSC has been greatly altered. While qualitative descriptions of LSC exist, only a limited number of studies quantify this process. The objective of this study was to quantify mass and momentum flux as proxies for LSC. This objective was met through the analysis of hydrodynamic model outputs along a 10‐km reach of the Middle Rio Grande near Albuquerque, New Mexico, USA. Mass and momentum fluxes were quantified for two synthetic floods across three spatial scales: geomorphic sub‐units, units, and reaches. In addition, three floodplain surface elevations were defined: inset, restored, and historical. Mass and momentum fluxes displayed distinct hydrodynamic signatures at each floodplain surface elevation. Inset floodplains were subjected to the greatest and most widely distributed flux magnitudes, an indication of the anthropogenic alteration along the reach. Flux heterogeneity on inset surfaces was also greatest, and increased flood magnitude did not result in comparable heterogeneity at higher floodplain elevations. Unsteady results showed that peak flux magnitudes can occur on the rising and falling limbs of a flood hydrograph, providing evidence of hysteresis in channel‐floodplain connections. Ultimately, the results are indicative of complexities associated with LSC processes along geomorphologically altered channel‐floodplain systems. The scalable methods are applicable to a wide variety of river‐floodplain systems and are complementary to empirical flux measurements.

Detecting exotic wakes with hydrodynamic sensors

Wake sensing for bioinspired robotic swimmers has been the focus of much investigation owing to its relevance to locomotion control, especially in the context of schooling and target following. Many successful wake sensing strategies have been devised based on models of von Karman-type wakes; however, such wake sensing technologies are invalid in the context of exotic wake types that commonly arise in swimming locomotion. Indeed, exotic wakes can exhibit markedly different dynamics, and so must be modeled and sensed accordingly. Here, we propose a general wake detection protocol for distinguishing between wake types from measured hydrodynamic signals alone. An ideal-flow model is formulated and used to demonstrate the general wake detection framework in a proof-of-concept study. We show that wakes with different underlying dynamics impart distinct signatures on a fish-like body, which can be observed in time-series measurements at a single location on the body surface. These hydrodynamic wake signatures are used to construct a wake classification library that is then used to classify unknown wakes from hydrodynamic signal measurements. Under ideal settings, the wake detection protocol is found to have an accuracy rate of over 95% in the majority of performance studies conducted. Further, proper tuning can lead to accuracy rates of 80% or better in low signal-to-noise environments. Thus, exotic wake detection is shown to be a viable concept, suggesting that such technologies have the potential to become key enablers of multiple-model sensing and locomotion control strategies in the future.

The interaction between suction feeding performance and prey escape response determines feeding success in larval fish.

The survival of larval marine fishes during early development depends on their ability to feed before depleting their yolk reserves. Most larval fish capture prey by expanding their mouth, generating a 'suction flow' that draws the prey into it. These larvae dwell in a hydrodynamic environment that impedes their ability to capture even non-evasive prey; however, the marine environment is characterized by an abundance of evasive prey, predominantly copepods. Copepods sense the hydrodynamic disturbance created by approaching predators and perform high-acceleration escape maneuvers. Using a 3D high-speed video system, we characterized the interaction between Sparus aurata larvae and prey from a natural zooplankton assemblage that contained evasive prey, and assessed the factors that determine the outcome of these interactions. At 8-33 days post hatching, larvae preferentially attacked large prey that was moving prior to the initialization of the strike; however, feeding success was lower for larger, more evasive prey. Thus, larvae were challenged in capturing their preferred prey. Larval feeding success increased with increasing Reynolds numbers, but decreased sharply when the prey performed an escape maneuver. The kinematics of successful strikes resulted in a shorter response time but higher hydrodynamic signature available for the prey, suggesting that strike success in our experiments was determined by brevity rather than stealth: executing a fast strike eliminated a potential escape response by the prey. Our observations of prey selectivity reveal that larval performance, rather than preferences, determines their diet during early development.

Electronic hydrodynamics and the breakdown of the Wiedemann-Franz and Mott laws in interacting metals

We present the theory of thermoelectric transport in metals with long-lived quasiparticles, carefully addressing the interplay of electron-electron scattering as well as electron-impurity scattering, but neglecting electron-phonon scattering. In Fermi liquids with a large Fermi surface and weak electron-impurity scattering, we provide universal and simple formulas for the behavior of the thermoelectric conductivities across the ballistic-to-hydrodynamic crossover. In this regime, the electrical conductivity is relatively unchanged by hydrodynamic effects. In contrast, the thermal conductivity can be parametrically smaller than predicted by the Wiedemann-Franz law. A less severe violation of the Mott law arises. We quantitatively compare the violations of the Wiedemann-Franz law arising from (i) momentum-conserving electron-electron scattering in the collision integral, (ii) hydrodynamic modifications of the electron-impurity scattering rate, and (iii) thermal broadening of the Fermi surface, and show that (i) is the largest. We present simple formulas for electrical and thermal magnetoconductivity across the ballistic-to-hydrodynamic limit, along with a more complicated formula for the thermoelectric magnetoconductivity. In a finite magnetic field, the Lorenz number may be smaller or larger than predicted by the Wiedemann-Franz law, and the crossover between these behaviors is a clear prediction for experiments. The arbitrarily strong violation of the Wiedemann-Franz law found in our work arises entirely from electron-electron interaction effects within the Fermi liquid paradigm, and does not imply any non-Fermi liquid behavior. We predict clear experimental signatures of bulk hydrodynamics in high-mobility 2D GaAs semiconductor structures, where failure of the Wiedemann-Franz law should persist down to very low temperatures in high-quality, low-density samples.

Man-made flows from a fish’s perspective: autonomous classification of turbulent fishway flows with field data collected using an artificial lateral line

The lateral line system provides fish with advanced mechanoreception over a wide range of flow conditions. Inspired by the abilities of their biological counterparts, artificial lateral lines have been developed and tested exclusively under laboratory settings. Motivated by the lack of flow measurements taken in the field which consider fluid-body interactions, we built a fish-shaped lateral line probe. The device is outfitted with 11 high-speed (2.5 kHz) time-synchronized pressure transducers, and designed to capture and classify flows in fish passage structures. A total of 252 field measurements, each with a sample size of 132 000 discrete sensor readings were recorded in the slots and across the pools of vertical slot fishways. These data were used to estimate the time-averaged flow velocity (R2 = 0.952), which represents the most common metric to assess fishway flows. The significant contribution of this work is the creation and application of hydrodynamic signatures generated by the spatial distribution of pressure fluctuations on the fish-shaped body. The signatures are based on the collection of the pressure fluctuations’ probability distributions, and it is shown that they can be used to automatically classify distinct flow regions within the pools of three different vertical slot fishways. For the first time, field data from operational fishway measurements are sampled and classified using an artificial lateral line, providing a completely new source of bioinspired flow information.

(Invited) Tracking Hydrodynamic Signatures of Metal Nucleation Events Via Lateral Molecular Force Microscopy

The direct visualization of local events leading to the formation of a critical nucleus remains as one of the most formidable challenges in electrodeposition. Recently, state-of-the-art video-rate scanning tunnelling microscopy has been able to follow lateral growth of Bi deposits with exquisite atomic resolution [1]. However, strong local potential bias between tip and substrate often leads to ‘shielding effects’, significantly decreasing local nucleation rates. Substrate-tip interactions in conventional atomic force microscopy also have a strong effect in local nucleation rate. In this contribution, we shall demonstrate a new approach to monitor individual nucleation events based on tracking local perturbations of hydration shells employing high-speed lateral molecular force microscopy (HS-LMFM). A unique optical feedback mechanism lies at the center of the operating mode of HS-LMFM [2,3]. Under this configuration, the position of a vertically oriented probe (VOP) is tracked by measuring the scattering of the evanescent wave of a laser impinging under total internal reflection from the back of an optically transparent electrode. As the tip approaches the surface oscillating close to its resonant frequency, the oscillation frequency is affected by the visco-elastic properties of the various hydration layers. Changes in the VOP oscillations are optically detected by the scattering evanescent field. This approach enables monitoring interaction with individual hydration layers simultaneously with, but independent of, the optical feedback. In these experiments, the tip is positioned at a distance in which shear force interactions with hydration layers of an In-doped SnO2 electrode (approximately 2.9 nm roughness level) become negligible [4]. As nucleation events are triggered, the hydration layers around individual nuclei move into a detectable range of the VOP, leading to changes in the oscillation frequency. Shear force maps uncover a highly dynamic landscape prior to the formation of stable nucleus. At constant potential, the stochastic formation and dissolution of nuclei can be observed in the sub-second time scale. The growth of a stable nucleus is also followed in real-time, illustrating the potential of this technique for probing complex phase formation phenomena at electrode surfaces. References Matsushima, H. et al. Faraday Discuss. 193, 171-185 (2016).Fletcher, J. M. et al. Science 340, 595-599 (2013).Antognozzi, M. et al. Nat. Phys. 12, 731-735 (2016).Harniman, R.L. et al. Nat. Commun. 8, 971 (2017)