# Latest papers in fluid mechanics

### Oscillating grid generating turbulence near gas-liquid interfaces in shear-thinning dilute polymer solutions

Author(s): T. Lacassagne, S. Simoëns, M. EL Hajem, and J.-Y. Champagne

A first experimental characterization of low Reynolds number, oscillating grid generated, and near-surface turbulence in shear-thinning dilute polymer solutions is presented. Energy transfer and horizontal damping mechanisms are evidenced. The evolution of the viscous sublayer depth can be explained by both viscous and shear-thinning effects.

[Phys. Rev. Fluids 5, 033301] Published Tue Mar 31, 2020

### Differential diffusion effects on density-driven instability of reactive flows in porous media

Author(s): Timan Lei and Kai H. Luo

In diverse applications such as enhanced oil recovery and carbon sequestration, solute A in fluid 1 diffuses into fluid 2 where it reacts with solute B following A + B → C. Pore-scale simulations based on the lattice Boltzmann method reveal intricate density-driven instability and differential diffusion effects. New fingering regimes are observed under certain combinations of density ratios, diffusivity ratios, and Rayleigh number ratios among the participating fluids and species.

[Phys. Rev. Fluids 5, 033903] Published Tue Mar 31, 2020

### Equilibrium measurement method of slip length based on fluctuating hydrodynamics

Author(s): Hiroyoshi Nakano and Shin-ichi Sasa

We perform equilibrium molecular dynamics simulations for nanoscale fluids confined between two parallel walls and investigate how the autocorrelation function of force acting on one wall is related to the slip length. We demonstrate that for atomically smooth surfaces, the autocorrelation function ...

[Phys. Rev. E 101, 033109] Published Mon Mar 30, 2020

### Generalized Shields criterion for weakly cohesive granular materials

Author(s): Florian Brunier-Coulin, Pablo Cuéllar, and Pierre Philippe

The erosion onset and kinetics of weakly cohesive granular materials based on experimental data from optically adapted jet erosion tests is investigated. The scouring kinetics are examined qualitatively for samples with an intergranular cohesion induced by either liquid or solid bonds. From a quantitative perspective, the erosion onset for the case of solid bond cohesion is here described by a generalized form of the usual Shields criterion including a cohesion number defined from yield tensile values considering both the micro- and macroscales.

[Phys. Rev. Fluids 5, 034308] Published Mon Mar 30, 2020

### Capillary bulldozing of sedimented granular material confined in a millifluidic tube

Author(s): Guillaume Dumazer, Bjørnar Sandnes, Knut Jørgen Måløy, and Eirik G. Flekkøy

When granular matter is involved in a confined two-phase flow, three ingredients that do not usually occur together must be considered in parallel: viscosity, capillarity, and solid friction. The displacement regimes of a gas-liquid interface along a horizontal millifluidic tube containing a sedimented granular phase are documented in detail. Experimental observations are made with a simple setup, and an associated theoretical interpretation that can describe a spectrum of capillary bulldozing regimes is developed.

[Phys. Rev. Fluids 5, 034309] Published Mon Mar 30, 2020

### Structure of coherent columnar vortices in three-dimensional rotating turbulent flow

Author(s): I. V. Kolokolov, L. L. Ogorodnikov, and S. S. Vergeles

The turbulence in a fast rotating fluid becomes effectively two-dimensional. The inverse energy cascade leads to formation of coherent columnar vortices. An analytical theory describing interaction of such a vortex with turbulent pulsations is developed. The radial velocity profile of the vortex is established.

[Phys. Rev. Fluids 5, 034604] Published Mon Mar 30, 2020

### Numerical aero-thermal study of high-pressure turbine nozzle guide vane: Effects of inflow conditions

Accurate predictability of high-pressure turbine nozzle guide vane aero-thermal performance is highly desired in the development campaign due to the exposure of the component to a frequent and high heat load. In this paper, the representative vane profile in modern aero-engines is numerically studied. Aerodynamics and aero-thermal validations of the blade profile have been performed in comparison with the available experimental data. It has been shown that a satisfactory agreement could be achieved with the use of the transitional turbulence model shear stress transport γ–θ due to its superiority in capturing the laminar–turbulent transition. Sensitivity studies on the increase in the inlet turbulence intensity, inlet endwall boundary layer thickness, and inlet total temperature profile have been performed to understand the impact of inflow conditions’ uncertainty on the aero-thermal predictability. Increasing the inlet turbulence intensity increases the pressure surface heat transfer coefficient and induces an earlier transition onset on the suction surface. Due to the rapid decay of turbulence intensity in the numerical model, the use of an artificially high inlet turbulence intensity has been shown to be effective in the prediction improvement. On the other hand, the change in the inlet boundary layer thickness influences the formation and strength of the secondary flow, namely, horseshoe vortex and passage vortex. These secondary flow phenomena affect the local blade surface heat transfer coefficient in the near-endwall region although the most significant rise in heat transfer is found on the endwall. The temperature distortion amplitude of a hot streak and its relative clocking position with the vane significantly affect the heat flux distribution. In contrast, the heat transfer coefficient is less sensitive to the change in hot streak conditions. However, it has been shown that increasing the temperature distortion amplitude could induce a larger difference among different clocking configurations. In addition, decreasing the difference between the fluid and wall temperature would delay the transition onset and stabilize the boundary layer. Further analysis of the unsteady effects has been carried out by comparing the steady and time-averaged flow solutions. It has been observed that the discrepancy between these solutions is attributed to the flow field nonlinearity. Thus, a significant discrepancy can be found in the laminar–turbulent transition as well as in the trailing edge region. However, since the contribution of these regions on the total area-averaged heat transfer is small, their influence on the total vane heat transfer is limited.

### Vortex-induced vibrations of a confined circular cylinder for efficient flow power extraction

The effects of confinement on vortex-induced vibration (VIV) of a circular cylinder (diameter D) and its flow power extraction capability are presented. A two-dimensional numerical study is performed on VIV of a circular cylinder inside a parallel plate channel of height H at Reynolds numbers 100 and 150. The blockage ratio (b = D/H) is varied from 1/4 to 1/2. The cylinder is elastically mounted with a spring such that it can only vibrate transversely to the flow. It has a fixed non-dimensional mass (m*) of 10. The energy extraction process is modeled as a damper (damping ratio ζ) attached to the cylinder. With increasing blockage, the vibration amplitude of the cylinder decreases, the lock-in happens at a larger non-dimensional natural frequency of the cylinder, and the initial branch of the vibration response of the cylinder shrinks. There is a minimum value of the channel height and Reynolds number for the existence of the initial branch. The extracted power is found to increase rapidly with the blockage. For the maximum blockage (b = 1/2), the flow power extracted by the cylinder is an order of magnitude larger as compared to what it would extract in an open domain with free stream velocity equal to the channel mean velocity. The optimal mass-damping (α = m*ζ) for extracting the maximum power is found to lie between 0.2 and 0.3. An expression is derived to predict the maximum extracted power from the undamped response of a confined/unconfined cylinder.

### Energy dissipation analysis based on velocity gradient tensor decomposition

A velocity gradient tensor decomposition method based on a normal frame is introduced in this paper. The velocity gradient tensor is decomposed into a compression–stretching tensor, pure rotation tensor, and pure shear tensor. The analysis shows that both the strain rate tensor and vorticity tensor in Helmholtz velocity decomposition contain shear tensor components, and the total pure shear tensor is the combination of shear components in the two tensors. Based on this decomposition and the physical meaning of each tensor term, the energy dissipation of the channel flow with or without a pressure gradient and a turbine passage flow are analyzed. The results show that the energy dissipation is caused by shear deformation and expansion and contraction deformation of the motion fluid, and pure rotation does not cause energy dissipation. In particular, the pure shear is the primary factor of energy dissipation. Shear accounts for 99.9% of energy dissipation in the fully developed turbulence of zero-pressure gradient channel flow, 99% of the energy dissipation in the separated boundary layer flow is caused by the pure shear, and 81% of the energy dissipation in the turbine stage flow is caused by pure shear.

### From modulational instability to focusing dam breaks in water waves

Author(s): Félicien Bonnefoy, Alexey Tikan, François Copie, Pierre Suret, Guillaume Ducrozet, Gaurav Prabhudesai, Guillaume Michel, Annette Cazaubiel, Eric Falcon, Gennady El, and Stéphane Randoux

The Benjamin-Feir, or modulational, instability has been regarded as the main mechanism of spontaneous disintegration of broad weakly nonlinear wavepackets in deep water waves. We report water wave experiments with another result. For a range of initial parameters, a nearly rectangular initial wavepacket develops a strongly nonlinear modulation with two dispersive shock waves expanding from the edges of the wavepacket towards the center at finite speed. This is supported by theory based on the semiclassical limit of the one-dimensional focusing nonlinear Schrodinger equation.

[Phys. Rev. Fluids 5, 034802] Published Fri Mar 27, 2020

### Investigation on the flow-induced noise propagation mechanism of centrifugal pump based on flow and sound fields synergy concept

This paper explores the flow-induced noise propagation mechanism of centrifugal pump from the view of flow and sound field synergy concept. First, the unsteady synergetic relationship between flow and sound fields is deduced, and the synergy angle is defined to describe the synergy degree. It is shown that the domain-averaged synergy angle (θave) changes little with flow time, which implies that the synergy degree is basically unchanged with flow time. With increasing rotational speed or flow rate, the time-averaged θave (θtave) in the impeller and the volute moves far away from 90° gradually, i.e., the synergy degree increases. Meanwhile, the noise outside the pump increases, and the variation of both the noise outside the pump and θtave tends to be gradual. The results manifested that the flow-induced noise propagation mechanism of the centrifugal pump can be well described by the change in synergy degree and the increase in synergy degree can cause the noise tending to propagate outside. In addition, the impact of the blade outlet angle on the noise propagation characteristics is investigated. Considering the synergy degree in the impeller and the volute comprehensively, the deviation of θtave from 90° decreases from 6.48° to 4.74° as the angle increases from 15° to 35°, i.e., θtave tends to approach 90°, and the synergy degree decreases gradually, indicating that increasing the blade outlet angle can weaken the tendency of noise propagating outside by decreasing the synergy degree. These conclusions can guide noise control research and engineering design.

### Transition effects on flow characteristics around a static two-dimensional airfoil

Flows past a static NACA0015 airfoil are numerically investigated via Reynolds-averaged Navier–Stokes simulations at the Reynolds number 1.95 × 106, the Mach number 0.291, and the angle of attack (AoA) from 0° to 18°. Specifically, a one-equation local correlation-based transition model (γ model) coupled with Menter’s k–ω shear stress transport (SST) model (SST–γ model) is employed to approximate the unclosed Reynolds quantities in the governing equations. Distributions of mean velocity and Reynolds stresses as well as typical integral quantities, such as the drag coefficient, lift coefficient, and moment coefficient, are calculated and compared with previously reported experimental data and present numerical data based on Menter’s original k–ω SST model. It turns out that the SST–γ model enables the capture of a laminar separation bubble (LSB) near the leading edge of the airfoil and shows significant advantages over the traditional “fully turbulent” models for the prediction of static stall. As the AoA varies from 0° to 18°, the flow regime is affected by different processes, i.e., flow transition, flow separation, and interaction between the LSB and the trailing-edge separation bubble, which, respectively, correspond to the linear-lift stage, light-stall stage, and deep-stall stage.

### Heat transfer enhancement and reduction in low-Rayleigh number natural convection flow with polymer additives

The effects of viscoelasticity, here caused by polymer additives, on Rayleigh Bénard convection flows are investigated via direct numerical simulations at a marginally turbulent Rayleigh number. Simulations with a range of polymer length and relaxation time scales show heat transfer enhancement (HTE) and reduction (HTR). The selection of HTE and HTR depends strongly on the maximum extensional viscosity of the solution, whereas the magnitude of heat transfer modification is a function of both the maximum extensional viscosity and relaxation time of the polymer solution. The underlying physics of HTE and HTR are explored, and a mechanism of the interaction between convection cells and polymers is proposed. The findings are extrapolated to high Ra to shed some new light onto experimental observations of HTR.

### Electroosmosis of a viscoelastic fluid over non-uniformly charged surfaces: Effect of fluid relaxation and retardation time

We investigate the electroosmotic flow of a quasi-linear viscoelastic fluid over a surface having charge modulation in narrow confinements. We obtain analytical solutions using a combination of regular and matched asymptotic expansions in order to describe the viscoelastic flow field and apparent slip velocity besides pinpointing variations of the flow rate and ionic currents due to the surface charge modulation. We demonstrate excellent agreement between the asymptotic analytical solution for the flow field and the full numerical solution in the limiting condition of a thin electrical double layer and weakly viscoelastic fluid. For a wide range of flow governing parameters, we analyze the flow velocity, vortex dynamics, flow rates, and streaming current. We demonstrate that the magnitude of the observed electroosmotic slip velocity is more sensitive to the thickness of the electrical double layer rather than the viscoelasticity of the fluid. We have observed that the contribution of fluid elasticity is prominent in breaking the axial symmetry in the electroosmotic flow with the presence of periodic charge distributions, which is in contrast to the symmetric electroosmotic flow field of a Newtonian fluid over the same charge modulated walls. The results hold the key toward understanding the flow of biological fluids in microfluidic flows by leveraging electrokinetic transport over charge modulated surfaces. We believe that the results of net throughput, streaming current, and vortex dynamics will aid our understanding of the complex fluid behavior and microfluidic mixers.

### Large eddy simulation of the separated flow transition on the suction surface of a high subsonic compressor airfoil

A large eddy simulation (LES) was conducted to investigate the separated flow transition on the suction surface of a high subsonic compressor airfoil at two Reynolds number (Re) conditions (1.5 × 105 and 0.8 × 105). The detailed vortex evolution in the separated shear layer was revealed. The instability amplification in the transition process and the associated loss mechanism were clarified. At Re = 1.5 × 105, the two-dimensional spanwise vortices shed periodically and were further distorted with the interaction of the streamwise evolving vortices, and then, small vortices were generated in the streamwise pairing of the neighboring spanwise vortices. Finally, three-dimensional hairpin vortices broke down into small-scale turbulent structures near the reattachment, along with the “ejection-sweeping” process near the wall. When the Reynolds number decreased to 0.8 × 105, the initial vortex shedding was not periodic, but the subsequent vortex evolution process was very similar to the case of Re = 1.5 × 105. The results have demonstrated the importance of the Tollmien–Schlichting (T–S) mechanism for the initial growth of disturbances in the attached boundary layer, but the transition process that occurred in the separated shear layer was dominated by the inviscid Kelvin–Helmholtz (K–H) instability. Moreover, a secondary instability observed in the vortex pairing process was supposed to have a great impact on the onset of transition. With the decrease in Re, the shear layer instability declined to a lower level, leading to a delayed transition. In addition, the deformation works associated with the Reynolds shear stress was found to be mainly responsible for the loss generation in the transitional flow. Compared with the traditional Reynolds average Navier–Stokes method, the LES was more accurate in predicting the profile loss at a low Re.

### Effect of temperature on gelation and cross-linking of gelatin methacryloyl for biomedical applications

Hydrogels with or without chemical cross-linking have been studied and used for biomedical applications, such as tissue repair, surgical sealants, and three dimensional biofabrication. These materials often undergo a physical sol–gel or gel–sol transition between room and body temperatures and can also be chemically cross-linked at these temperatures to give dimensional stability. However, few studies have clearly shown the effect of heating/cooling rates on such transitions. Moreover, only a little is known about the effect of cross-linking temperature or the state on the modulus after cross-linking. We have established rheological methods to study these effects, an approach to determine transition temperatures, and a method to prevent sample drying during measurements. All the rheological measurements were performed minimizing the normal stress build-up to compensate for the shrinking and expansion due to temperature and phase changes. We chemically modified gelatin to give gelatin methacryloyl and determined the degree of methacryloylation by proton nuclear magnetic resonance. Using the gelatin methacryloyl as an example, we have found that the gel state or lower temperature can give more rigid gelatin-based polymers by cross-linking under visible light than the sol state or higher temperature. These methods and results can guide researchers to perform appropriate studies on material design and map applications, such as the optimal operating temperature of hydrogels for biomedical applications. We have also found that gelation temperatures strongly depend on the cooling rate, while solation temperatures are independent of the heating rate.

### Secondary instability of stationary Görtler vortices originating from first/second Mack mode

This work investigates the origination of the secondary instability in Görtler vortices using the linear stability theory, BiGlobal analysis, three-dimensional linear parabolized stability equations (3DLPSEs), and direct numerical simulation (DNS). The flow over a concave wall suffering from the Görtler instability and first/second Mack mode instability is selected. Furthermore, this work simulates the evolution of infinitesimal Mack mode disturbance in a flow perturbed by finite-amplitude Görtler vortices by using DNS and 3DLPSE methods. The 3DLPSE approach accurately predicts the process of Mack mode disturbance evolving into the secondary instability of Görtler vortices, and a perfect agreement with results by DNS is obtained. The results indicate that the secondary instability of stationary Görtler vortices can originate from the first/second Mack mode. The evolutions of first/second Mack mode with different spanwise wavenumbers are performed based on 3DLPSE and compared against the BiGlobal method. The results show that the shape functions and growth rates of disturbances always tend to the results of dominant modes obtained by the BiGlobal method. Because the dominant mode might shift from one to another, the overall evolution cannot be predicted only by the BiGlobal method based on a fixed mode. According to our computations, it is deduced that the Mack modes with the same frequency and symmetric characteristics would finally develop into the secondary instability with similar shapes.

### Interaction network analysis in shear thickening suspensions

Author(s): Marcio Gameiro, Abhinendra Singh, Lou Kondic, Konstantin Mischaikow, and Jeffrey F. Morris

Dense frictional particulate suspensions in a viscous liquid respond to applied shear in a complex manner that includes continuous or discontinuous shear thickening. This change in rheology as a function of particle packing fraction and/or applied stress is associated with a formation of complex interaction networks that develop on a mesoscopic scale. The properties of these networks, analyzed by the newly developed tools based on persistent homology in both two and three spatial dimensions, are found to be closely related to the macroscopic system response.

[Phys. Rev. Fluids 5, 034307] Published Mon Mar 23, 2020

### Universal moments of accelerations in two-dimensional turbulence

Author(s): Igor V. Kolokolov and Maria M. Kostenko

We consider two-dimensional turbulence in the presence of a condensate. The nondiagonal correlation functions of the Lagrangian accelerations are calculated, and it is shown that they have the same universality properties as the nondiagonal correlation functions of the velocity fluctuations.

[Phys. Rev. E 101, 033108] Published Fri Mar 20, 2020

### Local analysis of the clustering, velocities, and accelerations of particles settling in turbulence

Author(s): Mohammadreza Momenifar and Andrew D. Bragg

We use three-dimensional Voronoi tessellation to perform a local analysis of particle motion in isotropic turbulence while independently varying the flow Reynolds and Froude number, and particle Stokes number. Among other interesting findings, our local analysis shows that in flow regions where particles are clustered, fluid accelerations at the particle positions are significant. These results call into question the validity of the “sweep-stick” mechanism for particle clustering in turbulence, which states that inertial particles cluster in flow regions where the fluid acceleration is zero.

[Phys. Rev. Fluids 5, 034306] Published Fri Mar 20, 2020