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Taylor–Couette flow between independently rotating cylinders with a relatively small aspect ratio Γ = 2.4 has been investigated numerically. The Navier–Stokes equations with the uncoupled convection-diffusion-reaction equation are solved using a spectral development in one direction combined together with a finite element discretization in the two remaining directions. The study includes the effects of the gravitational and the centrifugal potentials on the stability of incompressible Taylor–Couette flow. When the cylinders start rotating from rest, it is shown that the counter rotation promotes the appearance of the abnormal mode. Furthermore, for a strong counter rotation new small vortices appeared between the outer cylinder and the normal cells. In addition, the effect of the counter-rotation of the cylinders on the mass transfer is pointed out showing a distortion on the Sherwood number and an increase on its amplitude. [ABSTRACT FROM AUTHOR]

The quality of graphene sheets significantly depends on the degree of oxidation of graphite and the methods used for synthesis. Therefore, selecting an eco-friendly and cost-effective process is an important step in order to increase the oxidation level. The latest studies show that Taylor–Couette reactors are one of the best options to improve the oxidation level of graphite. Graphene suspensions show shear-thinning behavior, and the emergent flow structures in TC flows significantly influence the oxidation degree. In this study, we investigated the flow patterns of shear-thinning fluids in a TC reactor. The effect of radius ratio, power-law index and the rotating direction of the cylinders on the flow patterns and their critical values is studied experimentally in a Taylor–Couette flow that occurred between concentric cylinders. The Reynolds numbers defined with the wall shear viscosities (Rei and Reo) are used for evaluating the critical conditions of various flow structures. The results demonstrate that fluid properties and radius ratio may have significant destabilization effects in forming non-axisymmetric flow patterns and change their critical values. The characteristics of various flow regimes are altered substantially with increasing inner cylinder speed. A strong influence of the rotation direction of the outer cylinder on flow structures and their critical Reynolds numbers has also been revealed in this study. The obtained results also provide a deeper understanding of fluid–suspension interactions in TC reactors. These new findings will help in designing and developing more efficient TC reactors to be used in synthesizing high-quality graphene products. [ABSTRACT FROM AUTHOR]

Decades ago S. Lundquist, S. Chandrasekhar, P. H. Roberts and R. J. Tayler first posed questions about the stability of Taylor–Couette flows of conducting material under the influence of large-scale magnetic fields. These and many new questions can now be answered numerically where the nonlinear simulations even provide the instability-induced values of several transport coefficients. The cylindrical containers are axially unbounded and penetrated by magnetic background fields with axial and/or azimuthal components. The influence of the magnetic Prandtl number P m on the onset of the instabilities is shown to be substantial. The potential flow subject to axial fields becomes unstable against axisymmetric perturbations for a certain supercritical value of the averaged Reynolds number R m ¯ = R e ⋅ R m (with R e the Reynolds number of rotation, R m its magnetic Reynolds number). Rotation profiles as flat as the quasi-Keplerian rotation law scale similarly but only for P m ≫ 1 while for P m ≪ 1 the instability instead sets in for supercritical R m at an optimal value of the magnetic field. Among the considered instabilities of azimuthal fields , those of the Chandrasekhar-type, where the background field and the background flow have identical radial profiles, are particularly interesting. They are unstable against nonaxisymmetric perturbations if at least one of the diffusivities is non-zero. For P m ≪ 1 the onset of the instability scales with R e while it scales with R m ¯ for P m ≫ 1 . Even superrotation can be destabilized by azimuthal and current-free magnetic fields; this recently discovered nonaxisymmetric instability is of a double-diffusive character, thus excluding P m = 1 . It scales with R e for P m → 0 and with R m for P m → ∞ . The presented results allow the construction of several new experiments with liquid metals as the conducting fluid. Some of them are described here and their results will be discussed together with relevant diversifications of the magnetic instability theory including nonlinear numerical studies of the kinetic and magnetic energies, the azimuthal spectra and the influence of the Hall effect. [ABSTRACT FROM AUTHOR]

Periodic and symmetric two-dimensional textures with various cross-sectional profiles have been employed to improve and optimize the physical response of the surfaces such as drag force, superhydrophobicity, and adhesion. While the effect of the height and spacing of the textures have been extensively studied, the effect of the shape of the textures has only been considered qualitatively. Here, a polynomial framework is proposed to mathematically define the cross-sectional profiles of the textures and offer a quantitative measure for comparing the physical response of the textured surfaces with various shapes. As a case study, textured surfaces designed with this framework are tested for their hydrodynamic frictional response in the cylindrical Couette flow regime in Taylor–Couette flows. With the reduction in torque as the objective, experimental and numerical results confirm that textures with height-to-half-spacing ratios of lower and equal to unity with concave profiles offer a lower torque compared to both smooth surfaces and triangular textures. In addition, across multiple polynomial orders, textures defined by second-order polynomials offer a wide range of responses, eliminating the need for considering polynomials of higher orders and complexity. While the case study here is focused on the laminar flow regime and the frictional torque, the same type of analysis can be applied to other surface properties and physical responses as well. • 2D textures with curved profiles extend the range of flow responses of a surface. • Compared to V-grooves, concave profiles reduce and convex ones increase the drag. • Concave textures defined by 2nd order polynomials offer the lowest drag among all. [ABSTRACT FROM AUTHOR]

Local mixing dynamics was recently investigated experimentally in Taylor–Couette single-phase flow, thanks to simultaneous Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) techniques. The results highlighted the influence of the successive flow bifurcations and the role of azimuthal wave states on the dispersion of dye injected in Taylor–Couette flows. The present work extends this study to two-phase configurations with spherical solid particles. The respective effect of particle size and concentration on the vortices size and transition thresholds between the various flow regimes has been examined thanks to flow visualizations and PIV measurements. These hydrodynamic features have been complemented with PLIF experiments, that revealed a drastic enhancement of mixing due to the presence of particles regardless of the flow regime, highlighting the existence of significant particle-induced mixing in Taylor–Couette flows. [ABSTRACT FROM AUTHOR]

• Explored the effect of spatially varying thermal buoyancy forces on flow structures and turbulence in the rotating domain. • From statistical observations, there is a substantial increase/decrease in the values of the peaks near the walls suggesting enhanced/mitigated turbulence. • Vortical structures are observed only in half the circumferential domain due to the spatial stable or unstable stratification in the azimuthal direction. • Near-wall coherent structures was found to be highly sensitive to thermal stratification. In the present work, counter-rotating turbulent Taylor-Couette flows with radial temperature gradient have been studied as an extension of previously studied simple-rotating turbulent Taylor-Couette flows. The rotation axis is orthogonal to the gravity vector. Direct Numerical Simulations (DNS) have been carried out for Reynolds number ranging from 1000 to 5000 and Richardson number varying from 0 to 0.4. The effect of variation of Reynolds number and Richardson number on the flow statistics, flow dynamics, and near-wall coherent structures are studied. For neutrally buoyant cases, near-wall coherent structures exist near the inner and the outer wall, with the core being almost vortex-free. With an increase in Richardson number, more dense and finer vortical structures spread out in the core in half the circumferential domain only. This behavior is due to the spatial stable or unstable stratification in the azimuthal direction, leading to the generation of turbulence in the lower half of the domain and mitigation of turbulence in the upper half of the domain. Further, the turbulent kinetic energy (TKE) budget analysis reveals an increase in turbulence on heating the outer cylinder wall due to an increase in the production term. Heating the outer cylinder wall leads to a slight decrease in skin friction for the inner cylinder. [ABSTRACT FROM AUTHOR]

Abstract: Smoothed particle hydrodynamics (SPH) is a relatively young meshless particle method used in fluid simulation. The method has not yet reached a mature state and still is in the need of rigorous evaluation tests that contribute to its consolidation as a reliable numerical method. With this need in mind, the first portion of this article is dedicated to presenting a new set of validation tests. The tests consist of different types of planar Taylor–Couette flows which will be equipped with their corresponding Navier–Stokes equations’ analytical solutions. Analytical solutions have been found for compressible and weakly-compressible (WC) regimes. The second part of this article is dedicated to using the obtained solutions as evaluation tests of a WC-SPH variant based on the divergence form of the Navier–Stokes equations and on the simultaneous use of different kernels. The proposed variant is compared to other previous WC-SPH variants commonly used in the literature. Velocities as well as pressure profiles can be compared with the provided analytical solutions. The performed numerical experiments show that, although all the tested WC-SPH variants match quite correctly the theoretical velocity profile, the previous variants present highly noisy pressure profiles or even give erroneous pressure solutions. Among the evaluated WC-SPH formulations, the proposed variant does a better job in matching, both, velocity profiles and pressure profiles with much lower levels of noise. [Copyright &y& Elsevier]