Fluid Instabilities Research Articles

Modeling of Separation in a Binary Mixture with Negative Soret Effect in a Cylindrical Thermogravitational Column

Abstract A numerical simulation of convective instability of a binary fluid with negative Soret effect in a cylindrical thermogravitational column is performed. The general problem statement, including equations of motion and heat/mass transfer with boundary conditions, are written in cylindrical coordinates. This is implemented in order to take into account the impact of the column geometry on the separation process and stability of the convective flow. The calculations for two cylindrical columns are performed in Ansys Fluent 14.5. The used parameters and values of applied temperature differences between the walls correspond to the reported experimental data. The considered binary fluid is an ethanol–water mixture at a concentration ratio of 0.2204/0.7796. At such a composition the mixture exhibits a negative Soret effect (the lighter component, ethanol, is enriched in the cold region). The results of simulation show that the convective flow in the column with a smaller gap between the walls is unstable for all applied temperature differences, while it remains stable in the column with a larger gap.

Thermal instability of a viscoelastic fluid in a fluid-porous system with a plane Poiseuille flow

The thermal convection of a Jeffreys fluid subjected to a plane Poiseuille flow in a fluid-porous system composed of a fluid layer and a porous layer is studied in the paper. A linear stability analysis and a Chebyshev τ-QZ algorithm are employed to solve the thermal mixed convection. Unlike the case in a single layer, the neutral curves of the two-layer system may be bi-modal in the proper depth ratio of the two layers. We find that the longitudinal rolls (LRs) only depend on the depth ratio. With the existence of the shear flow, the effects of the depth ratio, the Reynolds number, the Prandtl number, the stress relaxation, and strain retardation times on the transverse rolls (TRs) are also studied. Additionally, the thermal instability of the viscoelastic fluid is found to be more unstable than that of the Newtonian fluid in a two-layer system. In contrast to the case for Newtonian fluids, the TRs rather than the LRs may be the preferred mode for the viscoelastic fluids in some cases.

Interaction of a cold cloud with a hot wind: the regimes of cloud growth and destruction and the impact of magnetic fields

ABSTRACT Multiphase galaxy winds, the accretion of cold gas through galaxy haloes, and gas stripping from jellyfish galaxies are examples of interactions between cold and hot gaseous phases. There are two important regimes in such systems. A sufficiently small cold cloud is destroyed by the hot wind as a result of Kelvin–Helmholtz instabilities, which shatter the cloud into small pieces that eventually mix and dissolve in the hot wind. In contrast, stripped cold gas from a large cloud mixes with the hot wind to intermediate temperatures, and then becomes thermally unstable and cools, causing a net accretion of hot gas to the cold tail. Using the magneto-hydrodynamical code arepo, we perform cloud crushing simulations and test analytical criteria for the transition between the growth and destruction regimes to clarify a current debate in the literature. We find that the hot-wind cooling time sets the transition radius and not the cooling time of the mixed phase. Magnetic fields modify the wind–cloud interaction. Draping of wind magnetic field enhances the field upstream of the cloud, and fluid instabilities are suppressed by a turbulently magnetized wind beyond what is seen for a wind with a uniform magnetic field. We furthermore predict jellyfish galaxies to have ordered magnetic fields aligned with their tails. We finally discuss how the results of idealized simulations can be used to provide input to subgrid models in cosmological (magneto-)hydrodynamical simulations, which cannot resolve the detailed small-scale structure of cold gas clouds in the circumgalactic medium.

Intense whistler-frequency emissions at the pedestal collapse in KSTAR H-mode plasmas

The edge confinement barrier of high-confinement mode (H-mode) plasma involves a variety of plasma waves alongside with fluid instabilities and collective particle transport during the barrier collapse. We demonstrate a new method of resolving the plasma waves by measuring the modulations embedded in the second harmonic electron cyclotron emission (ECE). Utilizing mm-wave heterodyne detection and fast digitization technologies on the KSTAR tokamak, we resolve not only the frequency spectrum but the wavenumber as well. At the plasma boundary during the barrier collapse, we observe multiple bursts of broadband whistler-frequency (<8 GHz) waves, together with intense narrowband emissions. The narrowband emissions exhibit rapid rising and falling tones within a few microseconds. Bispectral analyses reveal the existence of nonlinear interactions between the broadband and the narrowband waves, confirming their concurrence in the barrier zone. We estimate that the vertical wavenumber of the narrowband waves is comparable to that of a whistler wave, utilizing multiple mm-wave mixer channels. Our work opens a new experimental way to study wave–wave and wave–particle interactions in magnetically confined plasmas.

Experimental and numerical investigation of the Rayleigh-Taylor instability of the Newtonian and dilatant fluid system

The present work is devoted to the Rayleigh–Taylor instability of dilatant and Newtonian fluids. The main aim of this work was to carry out experiment, numerical simulation of mixing of two media, and making a comparison between gained data. A series of experiments was carried out to study the Rayleigh–Taylor instability of a dilatant liquid, which is a mixture of potato starch and water. The initial perturbation of the liquid-air interface is given by the shape of the metal profile separating the two media. The difference in the development of this instability in water-air and dilatant fluid-air systems is shown.

Instability mitigation by integrating twin Tesla type valves in supercritical carbon dioxide based natural circulation loop

Flow instability in supercritical fluid based natural circulation loop (NCL) is still an investigation aspect of physical and mathematical problems to comprehend. Therefore, NCLs require precise design assessment that focuses on the interaction of all the transient responses of buoyancy and friction forces which can ensure a stable zone of operation. To promote the uni-directional circulatory movement of loop fluid and to decrease the magnitude of instability, this research emphasizes the development of NCL integrated with two modified Tesla type valves. In this article, numerical simulations have been carried out for a range of supercritical pressures (80–100 bar) and heat inputs (500–2000 W) to do the comparative investigation of instability phenomenon in supercritical carbon dioxide based regular natural circulation loop and a new modified twin Tesla NCL. Results show that the use of modified Tesla valves leads to better stabilization for all supercritical pressures and heat inputs considered in the study. It is also found that the proposed Tesla NCL mitigates the temperature and velocity oscillations with a marginal drop of ⩽3% in the heat transfer performance. Using asymmetrical flow resistance to stimulate directional circulation is an efficient technique to combat this instability issue. Obtained results are validated with the existing correlations, and a good agreement is obtained.

Explosive electrical wire failures behave differently depending on the voltage applied

High-speed imaging of electrical wire failure reveals regions of fluid instability, bubble growth and rapid change from fluid breakup to phase explosion.

A unified model to study the effects of elasticity, viscosity, and magnetic fields on linear Richtmyer–Meshkov instability

A unified analytical approach to study the effects of elasticity, viscosity, and magnetic fields on the Richtmyer–Meshkov (RM) instability by using the impulsively accelerated model is presented. This model clarifies the discontinuity in the oscillation periods and yields the asymptotic decaying rate in elastic solids. It reveals that the complex eigenvalues produce better results compared with the numerical simulations for RM instability in viscous fluids and resolves the standing controversy between the analytical theory and numerical simulations at a vacuum/fluid interface. At last, it easily retrieves the results when the normal or tangential magnetic field is present. Those good agreements, between numerical simulations and theoretical analysis, would enable the model to be valuable in more complex situations such as in the elastic–plastic slabs with or without the presence of magnetic fields, as well as in the nonlinear regime.

A new LES approach to trans-critical mixing and combustion processes in high-pressure liquid-injectant engines

The understanding of flame holding mechanism is very important for combustor design. To elucidate the flame holding mechanism at the injector exit in liquid rocket engines, it is indispensable to know the underlying physics of trans-critical mixing process, because the flame holding location is exactly where trans-critical mixing process, which is still veiled, takes place. In the present paper, a new Large Eddy Simulation (LES) method is developed by gaining deeper physical insights into possible sub-grid-scale (SGS) thermodynamic and fluid dynamic instabilities excitable at thin large-density-gradient-magnitude portions to describe trans-critical processes. The presence of thin large-density-gradient-magnitude portions in trans-critical mixing process is troublesome for numerical stability of supercritical fluid flow LES from various points of view. Our idea is to utilize an SGS model functioning at the crossover between the liquid-like and gas-like supercritical fluids in a hybrid scheme of Lagrangian particle tracking and two-phase flow LES. Reduced dynamic viscosity at high pressure may cause SGS thermodynamic and hydrodynamic instabilities in the trans-critical mixing flow field. They are (1) phase separation due to thermodynamic instability, (2) Landau's hydrodynamic instability occurring at the pseudo-boiling location within a heated injector, and (3) Rayleigh–Taylor instability at the pseudo-boiling location, which is induced by Kelvin–Helmholtz instability vortices. These are analyzed and organized into new SGS models characterizing the trans-critical jet mixing process.

Interfacial Instability of Thixotropic Fluids in Triple-Layered Channel Flow

Interfacial Instability of Thixotropic Fluids in Triple-Layered Channel Flow

A Priori Validation of Subgrid-scale Models for Astrophysical Turbulence

Abstract We perform a priori validation tests of subgrid-scale (SGS) models for the turbulent transport of momentum, energy, and passive scalars. To this end, we conduct two sets of high-resolution hydrodynamical simulations with a Lagrangian code: an isothermal turbulent box with rms Mach numbers of 0.3, 2, and 8, and the classical wind tunnel where a cold cloud traveling through a hot medium gradually dissolves due to fluid instabilities. Two SGS models are examined: the eddy diffusivity (ED) model widely adopted in astrophysical simulations and the “gradient model” due to Clark et al. We find that both models predict the magnitude of the SGS terms equally well (correlation coefficient &gt;0.8). However, the gradient model provides excellent predictions for the orientation and shape of the SGS terms while the ED model predicts both poorly, indicating that isotropic diffusion is a poor approximation to the instantaneous turbulent transport. The best-fit coefficient of the gradient model is in the range of [0.16, 0.21] for the momentum transport, and the turbulent Schmidt number and Prandtl number are both close to unity, in the range of [0.92, 1.15].

Marangoni instability in the linear Jeffreys fluid with a deformable surface

Advantages of thermocapillary techniques over other approaches to polymer patterning have been recently recognized and will promote them to further expansion and development. In this perspective, a study of Marangoni instability in a layer of a viscoelastic liquid with a deformable interface is of great importance. Its linear stability analysis shows the presence of (and competition among) the long-wave stationary, short-wave stationary, and oscillatory modes, which leads to the emergence of codimension-two and codimension-three points in the parameter space.

The structure of the blue whirl revealed.

The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a previously unknown potential way for low-emission combustion. Here, we use numerical simulations to present the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames-a diffusion flame and premixed rich and lean flames-all of which meet in a fourth structure, a triple flame that appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability that occurs in swirling flows. These simulations are a critical step forward in understanding how to use this previously unknown form of clean combustion.

Physical models of streaming instabilities in protoplanetary discs

ABSTRACT We develop simple, physically motivated models for drag-induced dust–gas streaming instabilities, which are thought to be crucial for clumping grains to form planetesimals in protoplanetary discs. The models explain, based on the physics of gaseous epicyclic motion and dust–gas drag forces, the most important features of the streaming instability and its simple generalization, the disc settling instability. Some of the key properties explained by our models include the sudden change in the growth rate of the streaming instability when the dust-to-gas mass ratio surpasses one, the slow growth rate of the streaming instability compared to the settling instability for smaller grains, and the main physical processes underlying the growth of the most unstable modes in different regimes. As well as providing helpful simplified pictures for understanding the operation of an interesting and fundamental astrophysical fluid instability, our models may prove useful for analysing simulations and developing non-linear theories of planetesimal growth in discs.

On the Rayleigh–Taylor instability in compressible viscoelastic fluids under [formula omitted]-norm

In Wang and Zhao (2018), the authors investigated the Rayleigh–Taylor (abbr. RT) instability in the compressible viscoelastic fluid driven by the gravity in a bounded domain Ω based on Oldroyd-B model. In particular, the authors proved that the steady-state is nonlinearly unstable in the sense of H3(Ω)-norm to the viscoelastic RT (abbr. VRT) problem. In this paper, by switching our analysis to Lagrangian coordinates, we can show the nonlinear instability result in the sense of L1(Ω)-norm based on a bootstrap instability method. By an inverse transformation of Lagrangian coordinates, we can further get the nonlinear instability result for the VRT problem in the sense of L1(Ω)-norm in Eulerian coordinates, and thus improve the instability result in Wang and Zhao (2018).

Structured nanoscale metallic glass fibres with extreme aspect ratios.

Micro- and nanoscale metallic glasses offer exciting opportunities for both fundamental research and applications in healthcare, micro-engineering, optics and electronics. The scientific and technological challenges associated with the fabrication and utilization of nanoscale metallic glasses, however, remain unresolved. Here, we present a simple and scalable approach for the fabrication of metallic glass fibres with nanoscale architectures based on their thermal co-drawing within a polymer matrix with matched rheological properties. Our method yields well-ordered and uniform metallic glasses with controllable feature sizes down to a few tens of nanometres, and aspect ratios greater than 1010. We combine fluid dynamics and advanced in situ transmission electron microscopy analysis to elucidate the interplay between fluid instability and crystallization kinetics that determines the achievable feature sizes. Our approach yields complex fibre architectures that, combined with other functional materials, enable new advanced all-in-fibre devices. We demonstrate in particular an implantable metallic glass-based fibre probe tested in vivo for a stable brain-machine interface that paves the way towards innovative high-performance and multifunctional neuro-probes.

The Electromagnetic Signature of Outward Propagating Ion-scale Waves

Abstract First results from the Parker Solar Probe (PSP) mission have revealed ubiquitous coherent ion-scale waves in the inner heliosphere, which are signatures of kinetic wave–particle interactions and fluid instabilities. However, initial studies of the circularly polarized ion-scale waves observed by PSP have only thoroughly analyzed magnetic field signatures, precluding a determination of solar wind frame propagation direction and intrinsic wave polarization. A comprehensive determination of wave properties requires measurements of both electric and magnetic fields. Here, we use full capabilities of the PSP/FIELDS instrument suite to measure both the electric and magnetic components of circularly polarized waves. Comparing spacecraft frame magnetic field measurements with the Doppler-shifted cold plasma dispersion relation for parallel transverse waves constrains allowable plasma frame polarizations and wavevectors. We demonstrate that the Doppler-shifted cold plasma dispersion has a maximum spacecraft frequency for which intrinsically right-handed fast-magnetosonic waves propagating sunwards can appear left-handed in the spacecraft frame. Observations of left-handed waves with are uniquely explained by intrinsically left-handed, ion-cyclotron waves (ICWs). We demonstrate that electric field measurements for waves with are consistent with ICWs propagating away from the Sun, verifying the measured electric field. Applying the verified electric field measurements to the full distribution of waves suggests that, in the solar wind frame, the vast majority of waves propagate away from the Sun, indicating that the observed population of coherent ion-scale waves contains both intrinsically left- and right-hand polarized modes.

High-resolution patterning of organic semiconductor single crystal arrays for high-integration organic field-effect transistors

The pursuit of low-cost, high-performance electronic applications with solution-processible organic semiconductors drives the development of efficient methods to pattern organic semiconductor single crystals (OSSCs). However, fluid instabilities and a complex evaporation process have limited patterning of OSSCs with high resolution. Here, we present a solvent-free patterning approach, capillary force-driven molecule flow (CFDMF), to achieve highly aligned 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single crystal patterns with sub-micron resolution and high fidelity. The position as well as pattern shape and resolution of the C8-BTBT single crystal arrays can be predetermined through photolithography. Using this method, we have demonstrated a high-integration circuit comprising over 169 organic field-effect transistors (OFETs) with a high resolution of 310 dpi. The resultant OFETs show good field-effect properties with an average mobility of 4.44 cm2 V−1 s−1. This patterning technique constitutes a major step toward the use of the high-mobility OSSCs for integrated device applications.

Ribbing instability of Newtonian fluid coated on a topographic surface

Periodic wavy patterns emerge and align in a transverse direction once a viscous fluid is coated onto a surface moving above a critical speed. In this study, we examine how this flow instability, which is usually referred to as ribbing, is triggered or damped on a moving topographic surface. To examine the transient flow behavior, the liquid first came into contact with a smooth surface, followed by a downstream surface with small-scale V-shaped grooves. Flow visualizations revealed a topography-induced flow stabilization and a matching of rib frequencies with the underlying groove patterns, depending on the groove geometry and capillary number.

A Simple, Entropy-based Dissipation Trigger for SPH

Abstract Smoothed particle hydrodynamics (SPH) schemes need to be enhanced by dissipation mechanisms to handle shocks. Most SPH formulations rely on artificial viscosity and, while this works well in pure shocks, attention must be paid to avoid dissipation where it is not wanted. Commonly used approaches include limiters and time-dependent dissipation parameters. The former try to distinguish shocks from other types of flows that do not require dissipation while in the latter approach the dissipation parameters are steered by some source term (“trigger”) and, if not triggered, they decay to a predescribed floor value. The commonly used source terms trigger on either compression, , or its time derivative. Here we explore a novel way to trigger SPH-dissipation: since an ideal fluid conserves entropy exactly, its numerical nonconservation can be used to identify “troubled particles” that need dissipation because they either pass through a shock or become noisy for other reasons. Our new scheme is implemented into the Lagrangian hydrodynamics code MAGMA2 and is scrutinized in a number of shock and fluid instability tests. We find excellent results in shocks and only a very moderate (and desired) switch-on in instability tests. The new scheme is robust, trivial to implement into existing SPH codes, and does not add any computational overhead.