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We investigate the behaviour of a flexible stem completely submerged under a surface gravity wave of finite amplitude using fully resolved direct numerical simulations. By varying the rigidity of the stem over ten orders of magnitude, we explore its motion in the drag-dominated regime with realistic air and water properties. Our findings reveal two distinct structural responses of the stem depending on the ratio between its natural frequency (f_nat) and the wave frequency (f_wave). For f_nat/f_wave >> 1, the stem maintains on average a straight configuration and exhibits streamwise oscillations in phase-opposition with the wave, moving symmetrically with respect to the vertical direction. Conversely, for f_nat/f_wave << 1, the stem reconfigures under the influence of the Stokes drift, bending forward and breaking the symmetry, and exhibits oscillations that are more coherent with the surrounding flow field. Resonance is observed when f_nat is approximatively f_wave. These results provide new insights into the dynamics of slender vegetation and man-made structures in wave fields, offering valuable implications for marine biology and engineering., Comment: 12 pages, 7 figures

The addition of polymers fundamentally alters the dynamics of turbulent flows in a way that defies Kolmogorov predictions. However, we now present a formalism that reconciles our understanding of polymeric turbulence with the classical Kolmogorov phenomenology. This is achieved by relying on an appropriate form of the K\'{a}rm\'{a}n-Howarth-Monin-Hill relation, which motivates the definition of extended velocity increments and the associated structure functions, by accounting for the influence of the polymers on the flow. We show, both analytically and numerically, that the ${\rm p}$th-order extended structure functions exhibit a power-law behaviour in the elasto-inertial range of scales, with exponents deviating from the analytically predicted value of ${\rm p}/3$. These deviations are readily accounted for by considering local averages of the total dissipation, rather than global averages, in analogy with the refined similarity hypotheses of Kolmogorov for classical Newtonian turbulence. We also demonstrate the scale-invariance of multiplier statistics of extended velocity increments, whose distributions collapse well for a wide range of scales.

Elastoviscoplastic (EVP) fluids, which exhibit both solid-like and liquid-like behavior depending on the applied stress, are critical in industrial processes involving complex geometries such as porous media and wavy channels. In this study, we investigate how flow characteristics and channel design affect EVP fluid flow through a wavy channel, using numerical simulations supported by microfluidic experiments. Our results reveal that elasticity significantly influences flow dynamics, reducing pressure drops and expanding unyielded regions. Notably, we find that even minimal elasticity can shift the flow from steady to time-dependent regimes, a transition less pronounced in viscoelastic fluids. Additionally, we show that the development of stagnation regions can be prevented when using a modified EVP fluid with enhanced elasticity, thus providing a full global yielding of the material. This study elucidates the role of elasticity in modifying flow patterns and stress distribution within EVP fluids, offering insights into the optimization of industrial applications, such as the displacement of yield stress fluids in enhanced oil recovery, gas extraction, cementing, and other processes where flow efficiency is critical., Comment: 15 pages, 19 figures

We investigate the fluid-solid interaction of suspensions of Kolmogorov-size spherical particles moving in homogeneous isotropic turbulence at a microscale Reynolds number of $Re_\lambda \approx 140$. Two volume fractions are considered, $10^{-5}$ and $10^{-3}$, and the solid-to-fluid density ratio is set to $5$ and $100$. We present a comparison between interface-resolved (PR-DNS) and one-way-coupled point-particle (PP-DNS) direct numerical simulations. We find that the modulated energy spectrum shows the classical $-5/3$ Kolmogorov scaling in the inertial range of scales and a $-4$ scaling at smaller scales, with the latter resulting from a balance between the energy injected by the particles and the viscous dissipation, in an otherwise smooth flow. An analysis of the small-scale flow topology shows that the particles mainly favour events with axial strain and vortex compression. The dynamics of the particles and their collective motion studied for PR-DNS are used to assess the validity of the PP-DNS. We find that the PP-DNS predicts fairly well both the Lagrangian and Eulerian statistics of the particles motion for the low-density case, while some discrepancies are observed for the high-density case. Also, the PP-DNS is found to underpredict the level of clustering of the suspension compared to the PR-DNS, with a larger difference for the high-density case.

We present numerical analysis of the lateral movement of spherical capsule in the steady and pulsatile channel flow of a Newtonian fluid, for a wide range of oscillatory frequency. Each capsule membrane satisfying strain-hardening characteristic is simulated for different Reynolds numbers Re and capillary numbers Ca. Our numerical results showed that capsules with high Ca exhibit axial focusing at finite Re similarly to the inertialess case. We observe that the speed of the axial focusing can be substantially accelerated by making the driving pressure gradient oscillating in time. We also confirm the existence of an optimal frequency which maximises the speed of axial focusing, that remains the same found in the absence of inertia. For relatively low Ca, on the other hand, the capsule exhibits off-centre focusing, resulting in various equilibrium radial positions depending on Re. Our numerical results further clarifies the existence of a specific Re for which the effect of the flow pulsation to the equilibrium radial position is maximum. The roles of channel size on the lateral movements of the capsule are also addressed. Throughout our analyses, we have quantified the radial position of the capsule in a tube based on an empirical expression. Given that the speed of inertial focusing can be controlled by the oscillatory frequency, the results obtained here can be utilised for label-free cell alignment/sorting/separation techniques, e.g., for circulating tumor cells in cancer patients or precious hematopoietic cells such as colony-forming cells.

The dimensional transition in turbulent jets of a shear-thinning fluid is studied via direct numerical simulations. Our findings reveal that under vertical confinement, the flow exhibits a unique mixed-dimensional (or 2.5D) state, where large-scale two-dimensional and small-scale three-dimensional structures coexist. This transition from three-dimensional turbulence near the inlet to two-dimensional dynamics downstream is dictated by the level of confinement: weak confinement guarantees turbulence to remain three-dimensional, whereas strong confinement forces the transition to two-dimensions; the mixed-dimensional state is observed for moderate confinement and it emerges as soon as flow scales are larger than the vertical length. In this scenario, we observed that the mixed-dimensional state is an overall more energetic state and it shows a multi-cascade process, where the direct cascade of energy at small scales and the direct cascade of enstrophy at large scales coexist. The results provide insights into the complex dynamics of confined turbulent flows, relevant in both natural and industrial settings.

In this study, we delve into the intricacies of elastoviscoplastic (EVP) fluids, particularly focusing on how polymer additives influence their extensional behavior. Our findings reveal that polymer additives significantly alter the extensional properties of the EVP fluids, such as relaxation time and extensional stresses, while having negligible impact on the shear rheology. Interestingly, the modified fluids exhibit a transition from yield stress-like behavior to viscoelastic-like behavior under high extensional rates, ultimately leading to destabilization under extreme deformation. This research enhances the fundamental understanding of EVP fluids and highlights potential advancements in applications, especially in precision-demanding fields like 3D printing., Comment: 20 pages, 15 figures

We investigate by direct numerical simulations the fluid-solid interaction of non-dilute suspensions of spherical particles moving in triperiodic turbulence, at the relatively large Reynolds number of $Re_\lambda \approx 400$. The solid-to-fluid density ratio is varied between $1.3$ and $100$, the particle diameter $D$ ranges between $16 \le D/\eta \le 123$ ($\eta$ is the Kolmogorov scale), and the volume fraction of the suspension is $0.079$. Turbulence is sustained using the Arnold-Beltrami-Childress cellular-flow forcing. The influence of the solid phase on the largest and energetic scales of the flow changes with the size and density of the particles. Light and large particles modulate all scales in a isotropic way, while heavier and smaller particles modulate the largest scales of the flow towards an anisotropic state. Smaller scales are isotropic and homogeneous for all cases. The mechanism driving the energy transfer across scales changes with the size and the density of the particles. For large and light particles the energy transfer is only marginally influenced by the fluid-solid interaction. For small and heavy particles, instead, the classical energy cascade is subdominant at all scales, and the energy transfer is essentially driven by the fluid-solid coupling. The influence of the solid phase on the flow intermittency is also discussed. Besides, the collective motion of the particles and their preferential location in relation with properties of the carrier flow are analysed. The solid phase exhibits moderate clustering; for large particles the level of clustering decreases with their density, while for small particles it is maximum for intermediate values.

We perform numerical simulations of planar jets of elastoviscoplastic (EVP) fluid (\citet{saramito2007new} model) at low Reynolds number. Three different configurations are considered: $(i)$ EVP jet in EVP ambient fluid, $(ii)$ EVP jet in Newtonian ambient fluid (miscible), and $(iii)$ EVP jet in Newtonian ambient fluid (immiscible). We investigate the effect of the Bingham number, \textit{i.e.} of the dimensionless yield stress of the EVP fluid, on the jet dynamics, and find a good agreement with the scaling for laminar, Newtonian jets for the centerline velocity $u_c/u_0\propto (x/h)^{-1/3}$ and for the jet thickness $\delta_{mom}/h\propto (x/h)^{2/3}$ at small Bingham number. This is lost once substantial regions of the fluid become unyielded, where we find that the spreading rate of the jet and the decay rate of the centerline velocity increase with the Bingham number, due to the regions of unyielded fluid inducing a blockage effect on the jet. The most striking difference among the three configurations we considered is the extent and position of the regions of unyielded fluid: large portions of ambient and jet fluids (in particular, away from the inlet) are unyielded for the EVP jet in EVP ambient fluid, whereas for the other two configurations, the regions of unyielded fluid are limited to the jet, as expected. We derive a power law scaling for the centerline yield variable and confirm it with the results from our numerical simulations. The yield variable determines the transition from the viscoelastic Oldroyd-B fluid (yielded) to the viscoelastic Kelvin-Voigt material (unyielded).

This study explores the dynamics of finite-size fibers suspended freely in a viscoelastic turbulent flow. For a fiber suspended in Newtonian flows, two different flapping regimes were identified previously by Rosti et al (2018). Here we explore, how the fiber dynamics is modified by the elasticity of the carrier fluid by performing Direct Numerical Simulations of a two-way coupled fiber-fluid system in a parametric space spanning different Deborah numbers, fiber bending stiffness and the linear density difference between fiber and fluid. We examine how these parameters influence various fiber characteristics such as the frequency of flapping, curvature, and alignment with the fluid strain and polymer stretching directions. Results reveal that the neutrally-bouyant fibers, depending on their flexibility, oscillate with large and small time scales transpiring from the flow, but the smaller time-scales are suppressed as the polymer elasticity increases. Polymer stretching is uncommunicative to denser-than-fluid fibers, which flap with large time scales from the flow when flexible and with their natural frequency when rigid. Thus, the characteristic elastic time scale has a subdominant effect when the fibers are neutrally-bouyant, while its effect is absent when the fibers become more inertial. Additionally, we see that the inertial fibers have larger curvatures and are less responsive to the polymer presence, whereas the neutrally-bouyant fibers show quantitative changes. Also, the neutrally-bouyant fibers show a higher alignment with the polymer stretching directions compared to the denser ones. In a nutshell, the polymers exert a larger influence on neutrally-bouyant fibers compared to the denser ones. The study comprehensively addresses the interplay between polymer elasticity and the fiber structural properties in determining its response behaviour in an elasto-inertial turbulent flow.

Flexible canopy flows are often encountered in natural scenarios, e.g., when crops sway in the wind or when submerged kelp forests are agitated by marine currents. Here, we provide a detailed characterisation of the turbulent flow developed above and between the flexible filaments of a fully submerged dense canopy and we describe their dynamical response to the turbulent forcing. We investigate a wide range of flexibilities, encompassing the case in which the filaments are completely rigid and standing upright as well as that where they are fully compliant to the flow and deflected in the streamwise direction. We are thus able to isolate the effect of the canopy flexibility on the drag and on the inner-outer flow interactions, as well as the two flapping regimes of the filaments already identified for a single fiber. Furthermore, we offer a detailed description of the Reynolds stresses throughout the wall-normal direction resorting to the Lumley triangle formalism, and we show the multi-layer nature of turbulence inside and above the canopy. The relevance of our investigation is thus twofold: the fundamental physical understanding developed here paves the way towards the investigation of more complex and realistic scenarios, while we also provide a thorough characterisation of the turbulent state that can prove useful in the development of accurate turbulence models for RANS and LES., Comment: 33 pages, 23 figures

We characterize the dynamical behavior of a clamped flexible fiber immersed in wall turbulence over a wide range of natural frequencies by means of direct numerical simulations. Only two flapping states are possible: one where the fiber oscillates at the characteristic frequency of the largest turbulent eddies and another where the natural structural response dominates. The former is obtained in the more flexible cases, while the latter in the more rigid ones. We observe here for the first time that in the turbulence dominated regime the fiber always sways at a frequency proportional to the largest scale of the flow, regardless of its structural parameters. The hindrance of the clamp to the wall prevents the synchronization of the fiber with turbulent eddies of comparable size., Comment: 6+4 pages, 6+3 figures

We investigate the influence of dispersed solid spherical particles on the largest scales of the turbulent Arnold-Beltrami-Childress (ABC) flow. The ABC flow is an ideal instance of a complex flow: it does not have solid boundaries, but possesses an inhomogeneous and three-dimensional mean shear. By tuning the parameters of the suspension, we show that particles deviate towards quasi-straight trajectories and exhibit anomalous transport. In doing this, they enhance the mean-flow energy and modulate the largest scales of the flow towards an anisotropic and quasi two-dimensional state.

We perform fully coupled numerical simulations using immersed boundary methods of finite-size spheres and fibres suspended in a turbulent flow for a range of Taylor Reynolds numbers $12.8

The chaotic flow of elastic fluids at low Reynolds number (Re) is typically distinguished into elasto-inertial and elastic turbulence (EIT/ET). However, the clear separation among these two turbulent regimes in parallel flows with a gradual Re decrease remains elusive. Here, spanning Re over four orders of magnitude, we investigate the statistical and structural nature of the flows computed by fully resolved 3D numerical simulations, revealing that as inertia becomes sub-dominant all flows share similar dynamical properties. Supported by the results, we thus propose a unified view of fully developed ET and EIT in parallel flows., Comment: 7+6 pages, 3+4 figures

Addition of polymers modifies a turbulent flow in a manner that depends non-trivially on the interplay of fluid inertia, quantified by the Reynolds number $Re$, and the elasticity of the dissolved polymers, given by the Deborah number $De$. We use direct numerical simulations to study polymeric flows at different $Re$ and $De$ numbers, and uncover various features of their dynamics. Polymeric flows exhibit a multiscaling energy spectrum that is a function of $Re$ and $De$, owing to different dominant contributions to the total energy flux across scales. This behaviour is also manifested in the real space scaling of structure functions. We also shed light on how the addition of polymers results in slowing down the fluid non-linear cascade resulting in a depleted flux, as velocity fluctuations with less energy persist for longer times in polymeric flows. These velocity fluctuations exhibit intermittent, large deviations similar to that in a Newtonian flow at large $Re$, but differ more and more as $Re$ becomes smaller. This observation is further supported by the statistics of fluid energy dissipation in polymeric flows, whose distributions collapse on to the Newtonian at large $Re$, but increasingly differ from it as $Re$ decreases. We also show that polymer dissipation is significantly less intermittent compared to fluid dissipation, and even less so when elasticity becomes large. Polymers, on an average, dissipate more energy when they are stretched more, which happens in extensional regions of the flow. However, owing to vortex stretching, regions with large rotation rates also correlate with large polymer extensions, albeit to a relatively less degree than extensional regions.

Elastic turbulence is the chaotic fluid motion resulting from elastic instabilities due to the addition of polymers in small concentrations at very small Reynolds ($\mbox{Re}$) numbers. Our direct numerical simulations show that elastic turbulence, though a low $\mbox{Re}$ phenomenon, has more in common with classical, Newtonian turbulence than previously thought. In particular, we find power-law spectra for kinetic energy $E(k) \sim k^{-4}$ and polymeric energy $E_{\rm p}(k) \sim k^{-3/2}$, independent of the Deborah ($\mbox{De}$) number. This is further supported by calculation of scale-by-scale energy budget which shows a balance between the viscous term and the polymeric term in the momentum equation. In real space, as expected, the velocity field is smooth, i.e., the velocity difference across a length scale $r$, $\delta u \sim r$ but, crucially, with a non-trivial sub-leading contribution $r^{3/2}$ which we extract by using the second difference of velocity. The structure functions of second difference of velocity up to order $6$ show clear evidence of intermittency/multifractality. We provide additional evidence in support of this intermittent nature by calculating moments of rate of dissipation of kinetic energy averaged over a ball of radius $r$, $\varepsilon_{r}$, from which we compute the multifractal spectrum.

We perform direct numerical simulations of surfactant-laden droplets in homogeneous isotropic turbulence with Taylor Reynolds number of 180. The droplets are modelled using the volume of fluid method, and the soluble surfactant is transported using an advection-diffusion equation. Effects of surfactant on the droplet and local flow statistics are well approximated using a lower, averaged value of surface tension, allowing us to extend the framework developed by Hinze (1955) and Kolmogorov (1949) for surfactant-free bubbles. The Kolmogorov-Hinze scale $d_H$ is indeed found to be a pivotal length scale in the droplets' dynamics, separating the coalescence-dominated (droplets smaller than $d_H$) and the breakage-dominated (droplets larger than $d_H$) regimes in the droplet size distribution. We find that droplets smaller than $d_H$ have compact, regular, spheroid-like shapes, whereas droplets larger than $d_H$ have long, convoluted, filamentous shapes with a diameter equal to $d_H$. This results in two different scaling laws for the interfacial area of the droplet. The normalised area, $A/d_H^2$, of droplets smaller than $d_H$ is proportional to $d^2$, while the area of droplets larger than $d_H$ is proportional to $d^3$, where $d$ is the droplet size. We further characterise the large filamentous droplets by computing the number of handles (loops of the dispersed phase extending into the carrier phase) and voids (regions of the carrier fluid entirely enclosed by the dispersed phase) on each droplet. The number of handles per unit length of filament scales inversely with surface tension. The number of voids is proportional to the droplet size and independent of surface tension. Handles are indeed an unstable feature of the interface and are destroyed by the restoring effect of surface tension, whereas voids can move freely in the interior of the droplets, unaffected by surface tension., Comment: 33 pages, 13 figures

We show that the energy required by a turbulent flow to displace a given amount of fluid through a straight duct in a given time interval can be reduced by modulating in time the pumping power. The control strategy is hybrid: it is passive, as it requires neither a control system nor control energy, but it manipulates how pumping energy is delivered to the system (as in active techniques) to increase the pumping efficiency. Our control employs a temporally periodic pumping pattern, where a short and intense acceleration (in which the pumping system is on) followed by a longer deceleration (in which the pumping system is off) makes the flow alternately visit a quasi-laminar and a turbulent state. The computational study is for a plane channel flow, and employs direct numerical simulations, which present specific computational challenges, for example the highly varying instantaneous value of the Reynolds number, and the importance of discretisation effects. Particular care is devoted to a meaningful definition of drag reduction in the present context. The ability of the forcing to yield significant savings is demonstrated. Since only a small portion of the parameter space is investigated, the best performance of the control technique remains to be assessed., Comment: Submitted to J. Fluid Mech

Flexible filamentous beds interacting with a turbulent flow represent a fundamental setting for many environmental phenomena, e.g., aquatic canopies in marine current. Exploiting direct numerical simulations at high Reynolds number where the canopy stems are modelled individually, we provide evidence on the essential features of the honami/monami collective motion experienced by hairy surfaces over a range of different flexibilities, i.e., Cauchy number. Our findings clearly confirm that the collective motion is essentially driven by fluid flow turbulence, with the canopy having in this respect a fully-passive behavior. Instead, some features pertaining to the structural response turn out to manifest in the motion of the individual canopy elements when focusing, in particular, on the spanwise oscillation and/or on sufficiently small Cauchy numbers.

We present numerical analysis of the lateral movement of a deformable spherical capsule in a pulsatile channel flow, with a Newtonian fluid in almost inertialess condition and at a small confinement ratio $a_0/R = 0.4$, where $R$ and $a$ are the channel and capsule radius. We find that the speed of the axial migration of the capsule can be accelerated by the flow pulsation at a specific frequency. The migration speed increases with the oscillatory amplitude, while the most effective frequency remains basically unchanged and independent of the amplitude. Our numerical results form a fundamental basis for further studies on cellular flow mechanics, since pulsatile flows are physiologically relevant in human circulation, potentially affecting the dynamics of deformable particles and red blood cells (RBCs), and can also be potentially exploited in cell focusing techniques.

Viscous dissipation causes significant energy losses in fluid flows; in ducts, laminar flows provide the minimum resistance to the motion, whereas turbulent currents substantially increase the friction at the wall and the energy requirement for pumping. Great effort is currently being devoted to find new ways of reducing the energy losses induced by turbulence. Here we propose a simple and novel drag-reduction technique which achieves substantial energy savings in internal flows, even after the energy cost of the control is accounted for. Our approach consists in driving the flow with a pulsed pumping, unlike the common practice of a constant pumping. We alternate "pump on" phases where the flow accelerates, and "pump off" phases where the flow decays freely. The flow cyclically enters a quasi-laminar state during the acceleration, and transitions to a more classic turbulent state during the deceleration. Our numerical results demonstrate that important net energy savings can be achieved by simply modulating the power injection in the system over time. The physical understanding we have achieved can help the industry in reducing the waste of energy, making it cheaper and preserving the environment by reducing harmful emissions., Comment: 12 pages, 3 figures

Non-Newtonian fluids have a viscosity that varies with applied stress. Elastoviscoplastic fluids, the elastic, viscous and plastic properties of which are interconnected in a non-trivial way, belong to this category. We have performed numerical simulations to investigate turbulence in elastoviscoplastic fluids at very high Reynolds-number values, as found in landslides and lava flows, focusing on the effect of plasticity. We find that the range of active scales in the energy spectrum reduces when increasing the fluid plasticity; when plastic effects dominate, a new scaling range emerges between the inertial range and the dissipative scales. An extended self-similarity analysis of the structure functions reveals that intermittency is present and grows with the fluid plasticity. The enhanced intermittency is caused by the non-Newtonian dissipation rate, which also exhibits an intermittent behaviour. These findings have relevance to catastrophic events in natural flows, such as landslides and lava flows, where the enhanced intermittency results in stronger extreme events, which are thus more destructive and difficult to predict., Comment: 12 pages and 10 figures, including supplemental material

We study the statistics of free-surface turbulence at large Reynolds numbers produced by direct numerical simulations in a fluid layer at different thickness with fixed characteristic forcing scale. We observe the production of a transient inverse cascade, with a duration which depends on the thickness of the layer, followed by a transition to three-dimensional turbulence initially produced close to the bottom, no-slip boundary. By switching off the forcing, we study the decaying turbulent regime and we find that it cannot be described by an exponential law. Our results show that boundary conditions play a fundamental role in the nature of turbulence produced in thin layers and give limits on the conditions to produce a two-dimensional phenomenology., Comment: 9 pages, 8 figures

We study the role of the capillary number, $Ca$ and of the surface wettability on the dynamics of the interface between an invading and a defending phase in a porous medium by means of numerical simulations. We employ a hybrid phase field-immersed boundary approach to successfully model the contact line dynamics over the solid objects. Using a phase-field method which naturally incorporates dynamic wetting we eliminate the need for empirical contact line models to address contact line singularity. We map the two dominant modes governing the motion of the interface, namely, capillary fingering, and stable penetration, in the ($Ca$-$\theta$) plane, with $\theta$ the static contact angle prescribed at the solid pores. Capillary fingering dominates at lower values of $Ca$ and pores hydrophobic to the invading phase, while a stable penetration is observed on hydrophillic surfaces. We present new measurements and analyses, including curvature probability density functions (pdf) and average curvature. We also show that the pressure needed for the invading phase to advance at constant flow rate decreases with the capillary number, and increases with the contact angle at the capillary numbers considered. The latter is due to a significant increase of the length of the interface in the case of capillary fingering. Finally, we show that it is possible to identify the different interfacial modes by measuring the penetration length and velocity during the medium filling., Comment: 42 pages, 14 figures

Maximum mutual information (MMI) has become one of the two de facto methods for sequence-level training of speech recognition acoustic models. This paper aims to isolate, identify and bring forward the implicit modelling decisions induced by the design implementation of standard finite state transducer (FST) lattice based MMI training framework. The paper particularly investigates the necessity to maintain a preselected numerator alignment and raises the importance of determinizing FST denominator lattices on the fly. The efficacy of employing on the fly FST lattice determinization is mathematically shown to guarantee discrimination at the hypothesis level and is empirically shown through training deep CNN models on a 18K hours Mandarin dataset and on a 2.8K hours English dataset. On assistant and dictation tasks, the approach achieves between 2.3-4.6% relative WER reduction (WERR) over the standard FST lattice based approach., Comment: Presented at Sane Worksop 2022 : https://www.saneworkshop.org/sane2022/

We investigate the modulation of turbulence caused by the presence of finite-size dispersed particles. Bluff (isotropic) spheres vs slender (anisotropic) fibers are considered to understand the influence of the object shape on altering the carrier flow. While at a fixed mass fraction - but different Stokes number - both objects provide a similar bulk effect characterized by a large-scale energy depletion, a scale-by-scale analysis of the energy transfer reveals that the alteration of the whole spectrum is intrinsically different. For bluff objects, the classical energy cascade is shrinked in its extension but unaltered in the energy content and its typical features, while for slender ones we find an alternative energy flux which is essentially mediated by the fluid-solid coupling., Comment: 11 pages, 6 figures

Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-$Z$ gate with $(99.81 \pm 0.02)\%$ fidelity., Comment: 24 pages, 12 figures, Figure 1 updated, Appendix A extended

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) for different volume fractions in a wall-bounded, effectively inertialess, small amplitude oscillatory shear (SAOS) flow for a wide range of applied frequencies. The RBCs are modeled as biconcave capsules, whose membrane is an isotropic and hyperelastic material following the Skalak constitutive law. The frequency-dependent viscoelasticity in the bulk suspension is quantified by the complex viscosity, defined by the amplitude of the particle shear stress and the phase difference between the stress and shear. SAOS flow basically impedes the deformation of individual RBCs as well as the magnitude of fluid-membrane interactions, resulting in a lower specific viscosity and first and second normal stress differences than in steady shear flow. Although it is known that the RBC deformation alone is sufficient to give rise to shear-thinning, our results show that the complex viscosity weakly depends on the frequency-modulated deformations or orientations of individual RBCs, but rather depends on combinations of the frequency-dependent amplitude and phase difference. The effect of the viscosity ratio between the cytoplasm and plasma and of the capillary number are also assessed.

We study the rheological behaviour of bidisperse suspensions in three dimensions under a non-uniform shear flow, made by the superimposition of a linear shear and a sinusoidal disturbance. Our results show that i) only a streamwise disturbance in the shear-plane alters the suspension dynamics by substantially reducing the relative viscosity, ii) with the amplitude of the disturbance determining a threshold value for the effect to kick-in and its wavenumber controlling the amount of reduction and which of the two phases is affected. We show that, iii) the rheological changes are caused by the effective separation of the two phases, with the large or small particles layering in separate regions. We provide a physical explanation of the phase separation process and of the conditions necessary to trigger it. We test the results in the whole flow curve, and we show that the mechanism remains substantially unaltered, with the only difference being the nature of the interactions between particles modified by the phase separation.

We study the rheological behaviour of a dense bidispersed suspension varying the relative size of the two dispersed phases. The main outcome of our analysis is that an enhanced flowability (reduced relative viscosity) of the suspension can be achieved by increasing the dispersion ratio of the phases. We explain the observed result by showing that the presence of large particles increases the packing efficiency of the suspension, leading to a reduction of the contribution of the contacts on the overall viscosity of the suspension in the shear-thickening regime, i.e. where the contacts are the dominating component.

We perform direct numerical simulations of planar jets of non-Newtonian fluids at low Reynolds number, in typical laminar conditions for a Newtonian fluid. We select three different non-Newtonian fluid models characterized by shear-thinning (Carreau), viscoelasticity (Oldroyd-B) and shear-thinning and viscoelasticity together (Giesekus), and perform a thorough analysis of the resulting flow statistics. We observe that, as the Weissenberg number is increased, the jet transitions from a laminar flow at low Weissenberg number, to a turbulent flow at high Weissenberg number. We show that the different non-Newtonian features and their combination give rise to rather different flowing regimes, originating from the competition of viscous, elastic and inertial effects. We observe that both viscoelasticity and shear-thinning can develop the instability and the consequent transition to a turbulent flowing regime; however, the purely viscoelastic Oldroyd-B fluid exhibits the onset of disordered fluid motions at a lower Weissenberg number than what observed for the purely shear-thinning Carreau fluid. When the two effects are both present, an intermediate condition is found, suggesting that, in this case, the shear-thinning feature is acting against the fluid elasticity. Despite the qualitative differences observed in the flowing regime, the bulk statistics, namely the centerline velocity and jet thickness, follow almost the same power-law scalings obtained for laminar and turbulent Newtonian planar jets.

We have performed high-fidelity simulations of turbulent open-channel flows over submerged rigid canopies made of cylindrical filaments of fixed length $l=0.25H$ ($H$ being the domain depth) mounted on the wall with an angle of inclination $\theta$. The inclination is the free parameter that sets the density of the canopy by varying its frontal area. The density of the canopy, based on the solidity parameter $\lambda$, is a widely accepted criterion defining the ongoing canopy flow regime, with low values ($\lambda \ll 0.1$) indicating the sparse regime, and higher values ($\lambda > 0.1$) the dense regime. All the numerical predictions have been obtained considering the same nominal bulk Reynolds number (i.e. $Re_b=U_b H/\nu = 6000$). We consider nine configurations of canopies, with $\theta$ varying symmetrically around $0{\deg}$ in the range $\theta\in [\pm 78.5{\deg}]$, where positive angles define canopies inclined in the flow direction (with the grain) and $\theta=0{\deg}$ corresponds to the wall-normally mounted canopy. The study compares canopies with identical solidity obtained inclining the filaments in opposite angles and assesses the efficacy of the solidity as a representative parameter. It is found that when the canopy is inclined, the actual flow regime differs substantially from the one of a straight canopy that shares the same solidity indicating that criteria solely based on this parameter are not robust. Finally, a new phenomenological model describing the interaction between the coherent structures populating the canopy region and the outer flow is given.