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Buoyancy-driven flow and heat transfer characteristics of the solid/hollow cylinders with concave shapes were numerically analyzed in a turbulent regime. The effects of Ra , L / D , and D th / D on heat dissipation have been studied in the range of 1010 to 1012, 2 to 10, and 0.2 to 0.9, respectively. It has been found that the Nu ¯ increases with Ra for solid/hollow cylinders. For a hollow cylinder, Nu ¯ in is always higher than Nu ¯ out except at D th / D = 0.2. The Nu ¯ in of a hollow cylinder increases up to D th / D = 0.6, irrespective of the Ra , and L / D ratios thereafter reduced drastically has been observed. Whereas, at a low D th / D , and high Ra (i.e., Ra ≥ 1011), Nu ¯ out of a hollow cylinder significantly decreases with D th / D . However, it shows a marginal decrement at a high length-to-diameter ratio. For solid and hollow cylinders, the thermo-fluid dynamics have been delineated through the temperature contours, streamline patterns, and vortex generation at the inner as well as outer wall of the hollow cylinder. This numerical study also proposes a correlation for the Nu ¯ of the solid/hollow cylinders to have an accuracy ± 6 % of relevant parameters. [ABSTRACT FROM AUTHOR]

This study examines the flow of the ternary nanofluid over a rotating stretchable cylinder with a torsional motion under the influence of a Cattaneo–Christov heat flux. In order to accomplish the goals of this simulation, a ternary hybrid nanofluid (THNF) that is based on water and incorporates nanoparticles with three distinct morphologies is considered. For assessing the colloidal mixture of spherical, cylindrical, and platelet shaped nanoparticles, the necessary models are used. By using appropriate similarity transformations, the modeled equations are reduced to a set of ordinary differential equations (ODEs). These ODEs are then solved using the finite element approach. The numerical integration's validity and reliability, as well as the newly obtained results, were thoroughly analyzed. Results reveal that the larger values of Reynolds number enhance the system's inertial force, which resists the liquid accelerating force and declines both velocities and heat transport. The rise in values of volume fractions and Reynolds number declines the skin friction coefficients along swirling and radial directions. The increase in values of both the thermal relaxation time parameter and Reynolds number improves the rate of heat transport. [ABSTRACT FROM AUTHOR]

We develop two totally decoupled, linear and second‐order accurate numerical methods that are unconditionally energy stable for solving the Cahn–Hilliard–Darcy equations for two phase flows in porous media or in a Hele‐Shaw cell. The implicit‐explicit Crank–Nicolson leapfrog method is employed for the discretization of the Cahn–Hiliard equation to obtain linear schemes. Furthermore the artificial compression technique and pressure correction methods are utilized, respectively, so that the Cahn–Hiliard equation and the update of the Darcy pressure can be solved independently. We establish unconditionally long time stability of the schemes. Ample numerical experiments are performed to demonstrate the accuracy and robustness of the numerical methods, including simulations of the Rayleigh–Taylor instability, the Saffman–Taylor instability (fingering phenomenon). [ABSTRACT FROM AUTHOR]

We study the stationary Navier–Stokes equations in the region between two rotating concentric cylinders. We first prove that, for a small Reynolds number, if the fluid flow is axisymmetric and if its velocity is sufficiently small in the $L^\infty$ -norm, then it is necessarily the Taylor–Couette–Poiseuille flow. If, in addition, the associated pressure is bounded or periodic in the $z$ axis, then it coincides with the well-known canonical Taylor–Couette flow. We discuss the relation between uniqueness and stability of such a flow in terms of the Taylor number in the case of narrow gap of two cylinders. The investigation in comparison with two Reynolds numbers based on inner and outer cylinder rotational velocities is also conducted. Next, we give a certain bound of the Reynolds number and the $L^\infty$ -norm of the velocity such that the fluid is, indeed, necessarily axisymmetric. As a result, it is clarified that smallness of Reynolds number of the fluid in the two rotating concentric cylinders governs both axisymmetry and the Taylor–Couette–Poiseuille flow with the exact form of the pressure. [ABSTRACT FROM AUTHOR]

• A partitioned fluid-structure interaction (FSI) framework is developed. • The meshless Vector Form Intrinsic Finite Element (VFIFE) is innovatively used as a solid solver. • Various benchmarks involving vortex-induced vibration phenomena are studied with the new algorithm. • Strong adaptability and accuracy of the VFIFE method are proved by numerical results. • An experimental case of flow past flexible curtain in a water tank is investigated. The Vector Form Intrinsic Finite Element (VFIFE) is an advanced meshless Lagrangian structural solver. This study introduces a partitioned framework to integrate Arbitrary Lagrangian-Eulerian and VFIFE methods into fluid-structure interaction (FSI) problems featured by large structural displacements and deformations. The VFIFE method enables the FSI simulations with deformable structures due to the nature of Lagrangian particles, which is challenging for traditional computational fluid mechanics techniques. In addition, the VFIFE can be effectively equipped with the partitioned coupling schemes because of the versatility of governing equations. In this algorithm system, several matrix problems in finite element methods and mapping and interpolation in the fluid-structure interface can be avoided. The significant potential of this algorithm in tackling complex FSI problems is demonstrated through various benchmarks, encompassing scenarios like Taylor-Green vortex, flow over a rigid cylinder (Re = 40 ∼ 200), forced vibration of a rigid cylinder, vortex-induced vibration (VIV) of an elastically mounted cylinder, and VIV of a rigid cylinder with an attached flexible cantilever beam. Finally, an experiment on flow past a curtain in a water tank is conducted to further evaluate the precision and stability of the current coupling strategy in three dimensions, demonstrating the feasibility of such methods in flexible structures. [ABSTRACT FROM AUTHOR]

This article numerically investigates the mixed convection of Bingham fluid in a narrow annulus. The temperature of the inner fixed cylinder ( θ i ) is higher than that of the outer one ( θ o ), which rotates at a constant angular velocity. An OpenFOAM solver, icoFoam, which works based on the finite volume method and the PISO algorithm, is modified to perform the simulations. The modified solver can solve unsteady heat transfer problems for Bingham fluids without using regularization methods. After validating the solver, it solves the problem in different conditions to show the effect of relevant parameters, such as Reynolds number, Bingham number, and aspect ratio, on the flow field and thermal performance of the enclosure for constant amounts of Grashof and Prandtl numbers. The results are presented as streamlines and isotherms, the size and shape of the unyielded regions, the local and mean Nusselt number, and the torque coefficient. The results demonstrate that the flow is steady at Re = 10 for all Bingham numbers, and a decrease in the Reynolds number may result in an unsteady flow depending on the value of the Bingham number. The flow's time history reveals that at a fixed Reynolds number, the flow shifts from periodic to steady-state by enhancing the Bingham number so that the flow becomes entirely steady for a given Bingham number. An interesting phenomenon is observed, the formation, movement, and disappearance of the plug zones (unyielded regions) for the periodic flow at Re = 1 and Bn = 5. [ABSTRACT FROM AUTHOR]

Recently, meshless methods have become popular in numerically solving partial differential equations and have been employed to solve equations governing fluid flows, heat transfer, and species transport. In the present study, a numerical solver is developed employing the meshless framework to efficiently compute the hydrodynamic stability of fluid flows in complex geometries. The developed method is tested on two cases of Taylor–Couette flows. The concentric case represents the parallel flow assumption incorporated in the Orr–Sommerfeld model and the eccentric Taylor–Couette flow incorporates a non-parallel base flow with separation bubbles. The method was validated against earlier works by Marcus ["Simulation of Taylor-Couette flow. Part 1. Numerical methods and comparison with experiment," J. Fluid Mech. 146, 45–64 (1984)], Oikawa et al. ["Stability of flow between eccentric rotating cylinders," J. Phys. Soc. Jpn. 58, 2355–2364 (1989)], Leclercq et al. ["Temporal stability of eccentric Taylor–Couette–Poiseuille flow," J. Fluid Mech. 733, 68–99 (2013)], and Mittal et al. ["A finite element formulation for global linear stability analysis of a nominally two-dimensional base flow," Numer. Methods Fluids 75, 295–312 (2014)]. The results for the two cases and the effectiveness of the method are discussed in detail. The method is then applied to Taylor–Couette flow in an elliptical enclosure and the stability of the flow is investigated. [ABSTRACT FROM AUTHOR]

Gas foil bearings have been utilized in high-speed rotational machinery due to their superiority of load capacity and stability, which are closely associated with the shearing flow induced by variable sections in multiwedge clearance. This study focuses on the evolution pattern of vortices in Taylor–Couette flow with three-lobe multiwedge clearance using large-eddy simulation. As the Taylor number (Ta) increases, the shearing flow transitions from separation flow to Taylor vortex flow, followed by secondary transitions leading to wavy vortex flow. Separation vortices occur around the joint between two lobes, significantly delaying the first transition. Subsequently, Taylor vortices emerge and interact with the separation vortices, resulting in a complex evolution pattern of vortex series with increasing Ta. The presence of separation vortices promotes the formation of angular periodicity in Taylor vortices, accelerating the occurrence of angular periodicity stall and enhancing energy exchange between adjacent Taylor vortices. Taylor vortices are prone to breaking and merging. Additionally, Taylor vortices disrupt the shear layer of separation vortices between the stall region and the mid-radius separation region, leading to the migration of the shear layer and the singularity toward the mid-radius. Consequently, separation vortices move toward the mid-radius and transform from a one main vortex structure to two main vortex structures. Furthermore, spectral analysis of turbulent kinetic energy (TKE) reveals an inverse energy cascade characteristic with a slope of −7/3 at high frequencies, attributed to the TKE transmission between Taylor vortices and separation vortices. Both types of vortices exhibit intermittent and periodic behavior. [ABSTRACT FROM AUTHOR]

This analysis considers the magnetized third-grade fluid stream and microorganisms over a non-linear stretchy cylinder. The radiation impacts are taken into consideration. The effects of the governing flow at the cylinder are represented in the form of PDEs employing boundary layer approximations. The system of the PDEs is further reduced in dimensionless form after applying the similarity transformations. The dimensionless system of non-linear ODEs is solved through the numerical technique bvp4c. The effects of radiation and magnetism on the third-grade liquid over a non-linear extending cylinder are highlighted in graphs and numerically in tabular form. The influence of fluid variables on the velocity curve, such as third-grade parameters, second-grade coefficients, and Reynolds number, is illustrated and explored. Suitable ranges for the parameters |$({1 \le \eta \le 10,\ 0.2 \le {{\alpha }_1} \le 0.5,\ 0 \le {{\alpha }_1} \le 1.5,\ 0.1 \le \beta \le 0.3,\ 0.1 \le \gamma \le 1.6,\ 0.05 \le M \le 0.15,\ 0.5 \le \delta \le 2.0,\ 0.7 \le Pr \le 1.3,\ 0.1 \le Rd \le 0.4,0.1 \le}$|   |${e \le 0.4})$| are chosen depending upon the convergence of the numerical method. The widths of the velocity and momentum boundary layers are revealed to be increasing functions of the curvature parameter. The temperature curve declines when boosting third-grade parameters, thermal stratification, and Hartmann number while boosting up for curvature and radiation parameters. [ABSTRACT FROM AUTHOR]

We combine flow visualisation techniques and particle image velocimetry to experimentally investigate the higher-order transition to elastoinertial turbulence of Boger fluids (El = 0.11-0.34) in Taylor-Couette flows. The observed route to turbulence is associated with the appearance of chaotic inflow jets, termed flame patterns, for increasing inertia, and stable structures of solitons, known as diwhirls, for decreasing inertia. We also report for the first time spatially and temporally resolved flow fields in the meridional plane for the three characteristic viscoelastic flow regimes (diwhirls, flame patterns and elastoinertial turbulence). We observe in all cases coherent structures of dynamically independent solitary vortex pairs. The stability of these coherent structures is jet-dominated and can be mainly ascribed to the high extension of the polymer chains in the inflow boundaries in the r-z plane. Solitary pairs are self-sustained when created through random events and do not split; instead, they merge when moving sufficiently close and annihilate when hoop stresses are not sufficient to sustain them. The highly localised and random events result in highly fluctuating, chaotic flow states. We estimate the decay exponent of spatial power spectral density, illustrating a universal scaling of -2.5 for elastoinertial turbulence. Based on our observations and in an effort to unify and combine precedent theories with our results, we suggest a mechanism for the origins of elastoinertial instabilities, accounting for both the effect of elasticity on the vortex formation and the effect of increasing/decreasing inertia on the flow dynamics. [ABSTRACT FROM AUTHOR]

This work presents a comparison of several high-order numerical methodologies for simulating shock/turbulence interactions based on the supersonic Taylor–Green vortex flow, considering a Reynolds number of 1600 and a Mach number of 1.25. The numerical schemes considered include high-order Finite Difference, Targeted Essentially Non-Oscillatory, Discontinuous Galerkin, and Spectral Difference schemes. The shock capturing methods include high-order filtering, localized artificial diffusivity, non-oscillatory numerical fluxes, and local low-order switching. The ability of the various high-order numerical methodologies to both capture shocks and represent accurately the development of turbulent vortices is assessed. [ABSTRACT FROM AUTHOR]

This paper presents numerical simulations of the Taylor vortex flow under the influence of an externally applied alternating axial mass flux (through-flow) in a Taylor–Couette system with axial periodic boundary conditions. Such an axially modulating flow can lead to a significant variation in the onset of primary instabilities. Depending on the system parameters, the effect can be both stabilizing and destabilizing, i.e., shifting the bifurcation threshold to larger or smaller control parameters, respectively. It is found that the system response around the primary instability is sensitive to and critically influenced by an alternating mass flux, particularly the modulation frequency. We show that such an alternating axial flow represents an easily and, more importantly, precisely controllable key parameter to change the nonlinear system response from subcritical to supercritical behavior and vice versa. Furthermore, we observe different parameter regimes with regular and irregular intermittent flow dynamics. [ABSTRACT FROM AUTHOR]

Curved free shear layers emerge in many engineering problems involving complex flow geometries, such as the flow over a backward-facing step, flows with wall injection in a boundary layer, the flow inside side-dump combustors, or wakes generated by vertical axis wind turbines, among others. Previous studies involving centrifugal instabilities have mainly focused on wall-flows where Taylor instabilities between two rotating concentric cylinders or Görtler vortices in boundary layers are generated. Curved free shear layer flows, however, have not received sufficient attention, especially in the nonlinear regime. The present work investigates the development of centrifugal instabilities in a curved free shear layer flow in the nonlinear compressible regime. The compressible Navier–Stokes equations are reduced to the nonlinear boundary region equations (BREs) in a high Reynolds number asymptotic framework, wherein the streamwise wavelength of the disturbances is assumed to be much larger than the spanwise and wall-normal counterparts. We study the effect of the freestream Mach number M ∞ , the shear layer thickness δ , the amplitude of the incoming disturbance A, and the relative velocity difference across the shear layer Δ V on the development of these centrifugal instabilities. Our parametric study shows that, among other things, the kinetic energy of the curved shear layer flow increases with increasing Δ V and A decreases with increasing d e l t a . It was also found that increasing the disturbance amplitude of the incoming disturbance leads to significant growth in the mushroom-like structure's amplitude and renders the secondary instability structures more prominent, indicating increased mixing for all Mach numbers under consideration. [ABSTRACT FROM AUTHOR]

Stratified Taylor–Couette flow (STCF) undergoes transient growth. Recent studies have shown that there exists transient amplification in the linear regime of counter-rotating STCF. The kinetic budget of the optimal transient perturbation is analysed numerically to simulate the interaction of the shear production (SP), buoyancy flux (BP), and other energy components that contributes to the total optimal transient kinetic energy. These contributions affect the total energy by influencing the perturbation to extract kinetic energy (KE) from the mean flow. The decay of the amplification factor resulted from the positive amplification of both BP and SP, while the growth is attributed to the negative and positive amplification of BP and SP, respectively. The optimal SP is positively amplified, implying that there is the possibility of constant linear growth. These findings agree with the linear growth rate for increasing values of Grashof number. [ABSTRACT FROM AUTHOR]

The current analysis reveals unreported properties of the two-layer flow of dual convective magneto-hydrodynamic viscous nanofluid and Powell–Eyring in flexible concentric cylinders in the existing literature. The transmission of two immiscible fluids involving viscous nanofluid in the peripheral region and Powell–Eyring fluid in the core region is analyzed. The flow is induced by a sinusoidal wave that is propagated on the compliant wall. Over the outer cylinder, concentration and temperature are supposed to be greater than in the inner cylinder. The fluid flow of each phase involves continuity equation, momentum, heat transfer and mass transfer equations with the addition of electromagnetic effect. At interface, the continuity of velocity, stress, temperature are considered. In order to construct the model that includes mass and heat transport for both regions, long wavelength and creeping flow are assumed. The calculations are made against the resultant equations by using homotopy perturbation method. The outcomes of simulation for significant physical parameters are described and illustrated graphically. The graphical analysis shows that the electric and magnetic parameter changes in opposite manner for both temperature and velocity profiles. Both regions exhibit an increase in fluid temperature along with a rise in Brickman number. [ABSTRACT FROM AUTHOR]

We derive the spin Euler equation for ideal flows by applying the spherical Clebsch mapping. This equation is based on the spin vector, a unit vector field encoding vortex lines, instead of the velocity. The spin Euler equation enables a feasible Lagrangian study of fluid dynamics, as the isosurface of a spin-vector component is a vortex surface and material surface in ideal flows. We establish a non-blowup criterion for the spin Euler equation, suggesting that the Laplacian of the spin vector must diverge if the solution forms a singularity at some finite time. The direct numerical simulations (DNS) of three ideal flows – the vortex knot, the vortex link and the modified Taylor–Green flow – are conducted by solving the spin Euler equation. The evolution of the Lagrangian vortex surface illustrates that the regions with large vorticity are rapidly stretched into spiral sheets. The DNS result exhibits a pronounced double-exponential growth of the maximum norm of Laplacian of the spin vector, showing no evidence of the finite-time singularity formation if the double-exponential growth holds at later times. Moreover, the present criterion with Lagrangian nature appears to be more sensitive than the Beale–Kato–Majda criterion in detecting the flows that are incapable of producing finite-time singularities. [ABSTRACT FROM AUTHOR]

It is well known that inertia-free shearing flows of a viscoelastic fluid with curved streamlines, such as the torsional flow between a rotating cone and plate or the flow in a Taylor–Couette geometry, can become unstable to a three-dimensional time-dependent instability at conditions exceeding a critical Weissenberg ($Wi$) number. However, the combined effects of fluid elasticity, shear thinning and finite inertia (as quantified by the Reynolds number $Re$) on the onset of elasto-inertial instabilities are not fully understood. Using a set of cone–plate geometries, we experimentally explore the entire $Wi$ – $Re$ phase space for a series of nonlinear viscoelastic fluids (with the dependence on shear rate $\dot{\gamma}$ quantified using a shear-thinning parameter $\beta _P(\dot {\gamma })$). We tune $\beta _P(\dot {\gamma })$ by varying the dissolved polymer concentration in solution. This progressively reduces shear thinning but leads to finite inertial effects before the onset of elastic instability, and thus naturally results in elasto-inertial coupling. Time-resolved rheometric measurements and flow visualization experiments allow us to investigate the effects of flow geometry, and document the combined effects of varying $Wi, Re$ and $\beta _P(\dot {\gamma })$ on the emergence of secondary motions at the onset of instability. The resulting critical state diagram quantitatively depicts the competition between the stabilizing effects of shear thinning and the destabilizing effects of inertia. We extend the curved streamline instability criterion of Pakdel & McKinley (Phys. Rev. Lett. , vol. 77, no. 12, 1996, p. 2459) for the onset of purely elastic instability in curvilinear geometries by using scaling arguments to incorporate shear thinning and finite inertial effects. The augmented condition facilitates predictions of the onset of instability over a broader range of flow conditions, and thus bridges the gap between purely elastic and elasto-inertial curved streamline instabilities. [ABSTRACT FROM AUTHOR]

Shear significantly influences turbulence in the energy-containing range of shear-dominated flows, and the longitudinal structure functions do not have a universal form as they do in homogeneous isotropic turbulence. Despite this, the relative scaling of structure functions exhibits universal sub-Gaussian behaviour in shear-dominated flows, in particular for turbulent boundary layers, channels and Taylor–Couette flows. Our investigation of a turbulent vertical buoyancy layer at $Pr = 0.71$ using direct numerical simulation shows this universality even in moderate-Reynolds-number buoyancy-driven but shear-dominated boundary layers. It is demonstrated that the universality is related to the energy density of the eddies, which attains a hierarchical equilibrium in the energy-containing range of shear-dominated turbulence. We conjecture that the universal sub-Gaussian behaviour of the energy density of the energy-containing range, which was considered to be non-trivial in prior studies, is related to the universal anomalous scaling exponents of the inertial subrange turbulence. Based on this conjecture, we propose a hypothesis that relates large-scale eddies and the intermittent dissipation field in shear-dominated turbulence, highlighting a relationship between large and small scales. A phenomenological model is also developed to predict the scaling, which is verified using data from a turbulent boundary layer, half-channel and vertical buoyancy layer at friction Reynolds numbers spanning four orders of magnitude. Excellent agreement is observed. [ABSTRACT FROM AUTHOR]

High speed motors are widely used in industrial applications owing to their unique features, such as compact framework, high performance, and high reliability. Based on the finite volume method and numerical heat transfer theory, this study establishes a stator-air gap-rotor model, and the flow and heat transfer in the air gap between the stator and rotor in a high speed motor are investigated with high pressure natural gas as the cooling medium. Meanwhile, the "radial tri-vortex partition, alternating axial distribution" feature of the turbulent Taylor-Couette vortex in the air gap of the motor is determined. Then, the optimal structural parameters which can realize the heat transfer enhancement of motor air gap are obtained. Finally, an optical fiber grating temperature measurement system based on the wavelength division multiplexing technology is utilized to attain the temperature distributions on the stator and rotor surfaces. The simulation results are compared with the experimental data to evaluate the simulation method's precision. [ABSTRACT FROM AUTHOR]

The present study presents a computational investigation into the thermal mixing along with entropy generation throughout the natural convection flow within an arbitrarily eccentric annulus. Salt water is filled inside the eccentric annulus, in which the outer and inner cylinders have T c and T h constant temperatures. The Boussinesq approximation is used to develop the governing equations for the natural convection flow, which are then solved on a structured quadrilateral mesh using the OpenFOAM software package (FOAM-Extend 4.0). The computational simulations are performed for Rayleigh numbers ( R a = 10 3 – 10 5 ), eccentricity ( ϵ = 0 , 0.4 , 0.8 ), angular positions ( φ = 0 ∘ , 45 ∘ , 90 ∘ ), and Prandtl number ( P r = 10 , salt water). The computational results are visualized in terms of streamlines, isotherms, and entropy generation caused by fluid friction and heat transfer. Additionally, a thorough examination of the variations in the average and local Nusselt numbers, circulation intensity with eccentricities, and angular positions is provided. The optimal state of heat transfer is shown to be influenced by the eccentricity, angular positions, uniform temperature sources, and Boussinesq state. Moreover, the rate of thermal mixing and the production of total entropy increase as R a increases. It is found that, compared to a concentric annulus, an eccentric annulus has a higher rate of thermal mixing and entropy generation. The findings show which configurations and types of eccentric annulus are ideal and could be used in any thermal processing activity where a salt fluid ( P r = 10 ) is involved. [ABSTRACT FROM AUTHOR]

In this paper, a new hybrid numerical approach is presented that couples "ghost cell" based immersed boundary (IB) method with the finite volume lattice Boltzmann method (FVLBM). In the implementation process, the grid cells are classified into three types, i.e., " f l u i d c e l l ", " s o l i d c e l l " and " g h o s t c e l l ", where the " g h o s t c e l l " is the first layer of " s o l i d c e l l " near the " f l u i d c e l l ". As the wall boundary condition is reflected in the " g h o s t c e l l ", the high–accuracy reconstruction of variables in it will be the key point of present approach. Then, a FVLBM scheme is used to update the distribution function in the " f l u i d c e l l ", and a least square interpolation based formulation is derived to construct the distribution functions in it. Further, the bounce–back scheme embedded in the streaming–collision lattice Boltzmann method will be employed as the wall boundary condition. Besides, for the class of moving boundary problems, the construction of the distribution function in the " f r e s h c e l l " is also considered. A number of typical benchmarking incompressible viscous fluid flows over both stationary and moving objects are simulated to justify the present method, using obstacles ranging from a stationary and a translating circular cylinders, a rotationally oscillating cylinder, to two more complex flows around an oscillating cylinder in the stationary fluid and in the free–stream. Besides, the temporal and spatial accuracy also are testified by the simulation of Taylor–Couette flow. The obtained accurate simulation results demonstrate the capability of the present hybrid IB–FVLBM for computer simulations of both stationary and moving boundary problems in the incompressible flow regime. [ABSTRACT FROM AUTHOR]

A re-examination of some early classic turbulence literature, mainly of isotropic turbulence, is given in this selective review. Some early studies, including original concepts and points, are reviewed or highlighted. Two earliest studies and six major original concepts are found: (i) Lord Kelvin's pioneering elementary studies of homogeneous, isotropic turbulence; (ii) Kelvin's early introduction of Fourier Principles into turbulence studies; (iii) the Kelvin elementary concept of the direct energy cascade; (iv) the Kelvin early concept of the symmetry of turbulence; (v) the Taylor concept of the coefficient of eddy viscosity; (vi) the Taylor concept of the 'age' of the eddy; (vii) the Taylor–Fage–Townend concept of small eddies or microturbulence or small scale turbulence; and (viii) the Obukhov concept of a function of the inner Reynolds number (i.e., R e dependent coefficient) in both the balance equation and the energy distribution equation (the two-thirds law). Both Kelvin and Taylor should be regarded as the co-founders of the statistical theory of homogeneous, isotropic turbulence. The notion, 'the Maxwell–Reynolds decomposition of turbulent flow velocity', should be used. The Kolmogorov–Obukhov scaling laws are reviewed in detail. Heisenberg's inverse seventh power spectrum is briefly reviewed. The implications or significances of these early studies, original concepts and points are briefly discussed, with special reference to their possible links with modern approaches and theories. [ABSTRACT FROM AUTHOR]

The liquid iron core of the Earth undergoes vigorous convection driven by thermal and compositional buoyancy. The dynamics of convective fluid motions and heat transfer in such conditions are determined by background rotation, geometrical symmetry, and thermal interactions across the boundaries. In this study, rotating thermal convection in a horizontal fluid layer is considered to understand the fluid flow characteristics in the Earth's outer core focusing on the regions close to the rotational axis. The effects of a partial stable stratification on fluid flow and heat transfer are investigated to ascertain the physical significance of thermal core–mantle interaction on geomagnetic field generation driven by core fluid motion. It is found that even with non-linear evolution, convective instabilities retain the fundamental characteristics of linear onset modes. Mildly supercritical regimes lead to near laminar flows with the transition to turbulent convection occurring for strongly driven convection around 50–100 times enhanced buoyancy. Axial symmetry breaking and preferential damping of small-scale vortical structures are the hallmark of penetrative convection. Rapid rotation sustains small-scale helical flows in stable regions, a necessary ingredient for the sustenance of Earthlike dipolar magnetic fields. Coherent flow structures for strongly turbulent convection are obtained using reduced-order modeling. The overall total heat transfer is suppressed (up to 25%) due to the stable stratification although convective efficiency is enhanced (up to 30%) in the unstable regions favored by rapid rotation. Flow suppression is overcome under strong buoyancy forces, a relevant dynamical regime for deep-seated dynamo action in the Earth's core. [ABSTRACT FROM AUTHOR]

We investigate the boundary layer characteristics within annular centrifugal Rayleigh–Bénard convection (ACRBC) considering a Rayleigh number R a ∈ [ 10 8 , 10 11 ] , a Prandtl number Pr = 10.7, and an inverse Rossby number R o − 1 = 16. Our study is based on the temperature and velocity data obtained from direct numerical simulations. Different from the flow over a flat plate, the ACRBC system bifurcates into three regions: the plume-impacting regions, plume-ejecting regions, and plume-sweeping regions, and all three regions are moving with the zonal flow. Our focus is primarily on the temperature dynamics within the plume-sweeping region, where the wind of large-scale circulation shears the boundary. We determine the transient thermal boundary layer thickness over time using the slope method, relying on the temperature curve's orientation relative to the wall. Notably, the probability density function distribution of the thermal boundary layer thickness is reminiscent of traditional RBC systems, albeit with a more extended exponential tail. Employing a dynamic frame based on time resampling, we discern that the temperature boundary layer traits align with the Prandtl–Blasius boundary layer theory. In conclusion, we show that the exponential decay index for the thermal boundary layer thickness harmonizes with the system's heat transfer scaling law. It is found that the ratio between the inner and outer boundary layer thickness remains stable, providing theoretical guidance for the design and control of the internal flow field of high-speed rotating machinery. [ABSTRACT FROM AUTHOR]

We study the flow stability and spatiotemporal spectral dynamics of cellulose nanocrystal (CNC) suspensions in a custom Taylor–Couette flow cell using the intrinsic shear induced birefringence and liquid crystalline properties of CNC suspensions for flow visualizations, for the first time. The analysis is performed at constant ramped speed inputs of the independently rotating cylinders for several cases ranging from only inner or outer rotating cylinders to three counter-rotation cases. All CNC suspensions have measurable elasticity and shear thinning, both increasing with CNC concentration. We show that the flow patterns recorded are essentially Newtonian-like, with non-Newtonian effects ranging from a decrease in wavenumbers to altering the critical parameters for the onset of instability modes. Outer cylinder rotation flow cases are stable for all concentrations whereas inner cylinder rotation flow cases transition to axisymmetric and azimuthally periodic secondary flows. However, counter-rotation cases become unstable to asymmetric spiral modes. With increasing CNC concentration, a counter-rotation case was found where azimuthally periodic wavy patterns transition to asymmetric spiral modes. Based on rheo-SAXS measurements, the shear-thinning region of CNC suspensions is expected to lead to the breakdown of the chiral nematic phase, whose elastic constants constitute the dominant structural elasticity mechanism. Thus, we interpret the Taylor–Couette stability of the CNC suspensions as dominated by their shear-thinning character due to the expected loss of elasticity in nonlinear flow conditions. [ABSTRACT FROM AUTHOR]

We employ the three-way coupled numerical simulations to investigate the flocculation of primary cohesive particles which are locally densely distributed in the Taylor–Green cellular vortex flow. The hydrodynamic and inertial forces as well as the direct contact, lubrication, and cohesion forces between particles during the growth, deformation, and breakup of flocs are captured in detail. The flocculation rate of the primary particles decreases gradually from its maximum value at the initial moment, then levels off during flocculation, yielding the flocculation and equilibrium stages. The flocculation rate is determined by the equilibrium floc size and a flocculation coefficient. A larger equilibrium floc size and a smaller value of the flocculation coefficient yield faster flocculation. An initially dense distribution of cohesive particles accelerates the growth of flocs during flocculation but has minor effects on the equilibrium floc size, compared to an initially dilute distribution. A larger particle-to-fluid density ratio, a smaller size ratio between the particle diameter and the Kolmogorov length scale, and stronger cohesion yield a larger equilibrium floc size and a higher flocculation coefficient. Their influence on the flocculation coefficient becomes more evident when the initial particle distribution becomes more concentrated, while their impact on the maximum flocculation rate is very limited. A simple new model is proposed to describe the flocculation process of unevenly distributed cohesive particles in turbulence. [ABSTRACT FROM AUTHOR]

The results of a numerical study of the flow around a circular rotating cylinder under the subcritical regime of the Reynolds number are presented. A numerical flow simulation was carried out using the Realisable k-ε turbulence model, which effectively visualizes the fallout of vortices. At Reynolds numbers 16,000, 30000, and 50,000 and revolutions 300,500 and 700, the velocity field around the cylinder with the plate and the static pressure distribution field around the cylinder with the plate are obtained. It is determined that with an increase in the rotation frequency of the cylinder with the plate, the vortex formation increases. In the future, there will be an increase in lifting force and drag force due to an increase in flow velocity and, consequently, a decrease in pressure. Due to the rotational motion, there is a change in the pressure field at the ends of the cylinder with the plate. [ABSTRACT FROM AUTHOR]

The properties of multiphase flows are challenging to measure, and yet effective properties are fundamental to modelling and predicting flow behaviour. The current study is motivated by rheometric measurements of a gas-fluidized bed using a coaxial rheometer in which the fluidization rate and the rotational speed can be varied independently. The measured torque displays a range of rheological states: quasistatic, dense granular flow behaviour at low fluidization rates and low-to-moderate shear rates; turbulent toroidal-vortex flow at high shear rates and moderate-to-high fluidization rates; and viscous-like behaviour with rate-dependent torque at high shear rates and low fluidization or at low shear rates and high fluidization. To understand the solid-like to fluid-like transitions, additional experiments were performed in the same rheometer using single-phase liquid and liquid-solid suspensions. The fluidized bed experiments are modelled as a Bingham plastic for low fluidization rates, and as a shear-thinning Carreau liquid at high fluidization rates. The suspensions are modelled using the Krieger-Dougherty effective viscosity. The results demonstrate that, by using the effective properties, the inverse Bingham number marks the transition from solid-like to viscous-flow behaviour; a modified gap Reynolds number based on the thickness of the shear layer specifies the transition from solid-like to turbulent vortical flow; and a gap Reynolds number distinguishes viscous behaviour from turbulent vortical flow. The results further demonstrate that these different multiphase flows undergo analogous flow transitions at similar Bingham or Reynolds numbers and the corresponding dimensionless torques show comparable scaling in response to annular shear. [ABSTRACT FROM AUTHOR]

We illustrate how an alternating magnetic field can restrict and suppress the generation of vortex formation in ferrofluidic Couette flow. Therefore, the initial rotating outer cylinder (inner cylinder at rest) is brought to an abrupt stop, which results in the generation of more complex vortex dynamics in the system, evolving out of the initially fully laminar flow regime. The generated vortex flow structures appear to be axisymmetric Taylor vortices. Different stages during the spin-down process are described and characterised through dynamic quantities, such as the kinetic energy, cross-flow energy, and angular velocity flux. The presence of an alternating magnetic field modifies these dynamics during the spin-down, which is mainly dominated by the modulation amplitude of the alternating field. While moderate modulation amplitudes tend to minimise the vortex formation, i.e. weaken the flow dynamics, large modulation amplitudes suppress any vortex formation. The driving frequency only has a minor effect in general, but may allow to select between different flow pattern within the process. [ABSTRACT FROM AUTHOR]

Approximate streamsurfaces of a three-dimensional velocity field have recently been constructed as isosurfaces of the closest first integral of the velocity field. Such approximate streamsurfaces enable effective and efficient visualization of vortical regions in three-dimensional flows. Here we propose a variational construction of these approximate streamsurfaces to remove the limitation of Fourier series representation of the first integral in earlier work. Specifically, we use finite-element methods to solve a partial differential equation that describes the best approximate first integral for a given velocity field. We use several examples to demonstrate the power of our approach for three-dimensional flows in domains with arbitrary geometries and boundary conditions. These include generalized axisymmetric flows in the domains of a sphere (spherical vortex), a cylinder (cylindrical vortex) and a hollow cylinder (Taylor–Couette flow) as benchmark studies for various computational domains, non-integrable periodic flows (ABC and Euler flows) and Rayleigh–Bénard convection flows. We also illustrate the use of the variational construction in extracting momentum barriers in Rayleigh–Bénard convection. [ABSTRACT FROM AUTHOR]

The steady laminar flow of an incompressible Newtonian fluid in a porous annular duct with accelerated rotating walls is investigated. The flow is located in the polar plane and is driven by suction and injection at the walls. As required by conservation of mass with zero axial velocity, the fluid is injected into one of the cylinders, which becomes the upstream cylinder, and suctioned into the other, which represents the downstream cylinder. Only the case of the downstream cylinder at rest is examined. The duct gap ratio, the velocity ratio comparing the radial and azimuthal velocities, the Reynolds number, and the velocity coefficient that compares the fluid velocity at the upstream and downstream cylinders are the four parameters of the problem. The purpose of this research is to identify the prerequisites for the flow's existence and determine how the aforementioned variables affect the flow velocity and pressure gradients at a fixed Reynolds number. The Navier–Stokes equations are replaced by the polar-plane vorticity equation, which is solved using the similarity-solutions method. The shooting technique, including the fourth-order Runge–Kutta algorithm, is used to produce numerical solutions. From the findings, physical understandings of the flow are derived. More specifically, we discover an unexpected interior zone where the fluid is perpetually at rest even while flow is present. The only solution found corresponds to the case of the inner cylinder upstream and rotating provided that the velocity ratio does not exceed the threshold of 0.1 with a velocity coefficient greater than 1. [ABSTRACT FROM AUTHOR]

The dynamic behavior of the pump-turbine thrust bearing is important to the safety operation of the unit. This paper analyzed the lubrication and energy dissipation mechanism of pump-turbine thrust bearing during load-rejection based on the thermo-hydrodynamic model. The results show that the variation of the axial force and the maximum pad surface pressure is basically consistent with that of the inclination of the pad surface. The change of the friction loss is consistent with the change of rotational speed, while the change of pad surface temperature is affected by the combination of pad inclination and rotational speed. The chaotic flow in the oil tank is accompanied by different forms of vortices, such as Taylor vortices, vortex pairs, and Karman vortices, and results in a significant asymmetry in the pressure distribution. The flow in the bearing pad groove has an effect on the energy dissipation in the oil film. This paper provides a theoretical basis for the design and optimization of thrust bearings, and provides a reference for solving the problems of wear, oil mist, and other related problems of thrust bearings in engineering. [ABSTRACT FROM AUTHOR]

This study investigates the utilization of a trapezoidal cavity with a corrugated bottom wall containing a Carreau hybrid nanofluid composed of water, aluminum, and copper nanoparticles. A heated, rotating cylinder is placed at the center of the enclosure in the presence of an external magnetic field. The impact of Forchheimer and Brinkman porous medium models on the hybrid nanofluid is examined. Three different inlet–outlet placement configurations are considered to investigate their influence on heat transfer. The governing equations for fluid flow and heat transfer are solved numerically. Through simulations, a range of flow-controlling variables is systematically adjusted, including the Darcy number, Reynolds number, Hartmann number, nanoparticle volume fraction, undulation on the hot bottom wall, power law index, and rotational speed of the inner heated cylinder. The results demonstrate that the hybrid nanofluid and rotating cylinder significantly enhance heat transfer within the trapezoidal cavity. Higher values of the Darcy number, Reynolds number, and nanoparticle volume fraction lead to increased heat transfer rates. The placement configuration of the inlet and outlet ports also affects heat transfer performance, with the bottom-top configuration yielding the best results. Furthermore, a comparative analysis of flow profiles and heat distribution is conducted using the multiple expression programing technique. The proposed model accurately predicts the flow and heat transfer characteristics in the trapezoidal cavity, as validated through comparison with provided data sets. [ABSTRACT FROM AUTHOR]

We numerically investigate Taylor–Couette flows within a system consisting of an elliptical outer cylinder and a rotating inner circular cylinder, with particular emphasis on the behavior of Taylor cells. The three-dimensional unsteady Navier–Stokes equations are solved under the assumption of axial periodicity. Also, a scalar transport equation is solved for the heat transfer. Our methodology employs a Fourier-spectral meshless discretization technique, which interpolates variables at scattered points using polyharmonic splines and appended polynomials. A pressure-projection algorithm achieves the time advancement of the flow equations. We present findings for an elliptical enclosure with an aspect ratio of two, examining a range of Reynolds numbers (Re) from subcritical to 300. Our analysis includes streamlines, axial velocity contours, pressure, vorticity, and temperature profiles. The results indicate that the flow remains steady up to R e ≈ 300 before transitioning to an unsteady state at R e ≈ 350. [ABSTRACT FROM AUTHOR]

The windage loss in rotor‐stator gap has an important effect on rotating machinery, especially with higher rotational speed and fluid density. However, the mechanism of axial flow on windage loss in open shaft‐type gap is hardly studied in most literature. To clarify it, the influences of axial Reynolds number Reu and rotational Reynolds number Reω on skin friction coefficient Cf are investigated, and flow characteristics are analyzed with different gap geometry, radius ratio η. First, the results reveal that the Cf remains constant when Reu is less than 2.8 × 104 and increases rapidly as Reu when Reu ≥ 2.8 × 104, which indicates that the effect of axial velocity u on Cf is negligible for low Reu. The positive relative deviation Δ suggests that the axial flow makes windage loss and Cf rise. Besides, a larger number of Taylor vortexes fill with gap when the effect of the centrifugal force is larger than that of the inertial force, but they gradually disappear as Reu. Subsequently, the Cf and Δ increase as η, highlighting that the effect of u on windage loss and Cf is more prominent for larger η. The fact that vorticity near walls is larger than that at the center of gap reveals that windage loss arises from the interaction between walls and fluid rather than the dissipation with fluid itself. Finally, the model of Cf in shaft‐type gap is proposed in different Reω ranges based on numerical results, and the maximum sum of squares error of 1.02 × 10−5 and minimal R2 of 0.969 satisfy the requirement of fitting accuracy and indicate that the fitting model can accurately predict Cf. The conclusions significantly help predict windage loss in open shaft‐type gap with axial flow, and further improve the design for generators of supercritical CO2 turbine‐alternator‐compressor unit. [ABSTRACT FROM AUTHOR]

Purpose: In the present research paper, a dual annular duct (DAD) MR damper is considered, which is having a primary flow duct between the cylinder-piston head and the second one through the piston head. DAD damper is a combined mode model consisting of two annular gaps where the outer one experiences relative shearing motion and the inner one behaves as a fixed wall flow type. Methods: An analytical model is derived for DAD MR Damper. A computational approach has been adopted to compute the dynamic characteristics of the damper. The analytical approach is done by interconnecting CFD and FEA. The magnitude and profile of magnetic flux force have been investigated through magnetostatic analysis in ANSYS. These values are employed in determining the dynamic viscosity of the MR fluid using an empirical model of the MR fluid. Results: The credibility of the results is examined by comparing them with the experimental values of the first case of MR damper. Also, the computational results are verified with analytical results for all models. On examining the results from the simulations of all the models, it can be concluded that higher damping can be achieved by combining more than one mode of operation. [ABSTRACT FROM AUTHOR]

The windage loss in rotor‐stator gap has an important effect on rotating machinery, especially with higher rotational speed and fluid density. However, the mechanism of axial flow on windage loss in open shaft‐type gap is hardly studied in most literature. To clarify it, the influences of axial Reynolds number Reu and rotational Reynolds number Reω on skin friction coefficient Cf are investigated, and flow characteristics are analyzed with different gap geometry, radius ratio η. First, the results reveal that the Cf remains constant when Reu is less than 2.8 × 104 and increases rapidly as Reu when Reu ≥ 2.8 × 104, which indicates that the effect of axial velocity u on Cf is negligible for low Reu. The positive relative deviation Δ suggests that the axial flow makes windage loss and Cf rise. Besides, a larger number of Taylor vortexes fill with gap when the effect of the centrifugal force is larger than that of the inertial force, but they gradually disappear as Reu. Subsequently, the Cf and Δ increase as η, highlighting that the effect of u on windage loss and Cf is more prominent for larger η. The fact that vorticity near walls is larger than that at the center of gap reveals that windage loss arises from the interaction between walls and fluid rather than the dissipation with fluid itself. Finally, the model of Cf in shaft‐type gap is proposed in different Reω ranges based on numerical results, and the maximum sum of squares error of 1.02 × 10−5 and minimal R2 of 0.969 satisfy the requirement of fitting accuracy and indicate that the fitting model can accurately predict Cf. The conclusions significantly help predict windage loss in open shaft‐type gap with axial flow, and further improve the design for generators of supercritical CO2 turbine‐alternator‐compressor unit. [ABSTRACT FROM AUTHOR]

The Hollow Cathode Gas Flow Sputtering (GFS) provides special plasma conditions and is of extensive interest as a more affordable alternative to the high vacuum sputtering techniques. In the case of the tubular cathode a circular outlet symmetry stipulates homogeneity issues for both metallic and reactive deposition regimes. Using the results of Direct Simulation Monte Carlo (DSMC), we propose an external coaxial attachment which is manufactured and examined in a nozzle and a diffuser positioning. The impact on the homogeneity of Ti and  TiO 2  films is examined using profilometry and spectral ellipsometry. Our results demonstrate that the use of the nozzle attachment significantly enhances film homogeneity from about 3  cm 2  to more than 12  cm 2 . It also secures better process control in terms of oxygen stoichiometry and film thickness. Some crucial general issues of the reactive GFS process are discussed. [ABSTRACT FROM AUTHOR]

This study investigated the impact of a cylindrical obstacle on convection in an inclined square cavity filled with water-Al2O3 nanofluid. Using the finite volume method, the problem was resolved by having the inner cylinder rotate adiabatically while other walls were thermally insulated. Additionally, the bottom wall was hotter than the top. The study examined the effects of cylindrical obstacle radius (0.1 ≤ R ≤ 0.2), rotation speed (-500 ≤ Ω ≤ 500), Richardson number (0.01 ≤ Ri ≤ 100), volumetric nanoparticle fraction (0.02), and Grashof number (Gr = 104) on heat transfer rate or Nusselt number. The results were compared with previous literature, and the influence of the cylindrical obstacle rotational speed on convection flow was evaluated. An increase in the counterclockwise angular rotating speed resulted in higher nanofluid flux. The heat transmission coefficient increased as the Richardson number decreased. The use of nanofluid in the enclosure increased the coefficient of heat flow through mixed convection. Finally, the study showed that the convection heat exchange is enhanced with the increase in the radius. Moreover, an enhancement of the Nusselt number around 46% was reported for the cylinder, under Gr=10000, Φ=0.02, γ=45° and Ri= 10. [ABSTRACT FROM AUTHOR]

In view of the close relationship between gas-liquid mass transfer performance and flow regime, the coaxial mixer was selected as the research object, and the effects of rotation mode, inner impeller diameter, gas flow rate, and viscosity on flow regimes were experimentally investigated. The results showed that the main growth interval of the mass transfer coefficient was the loading regime, and the critical complete dispersion regime was an efficient and economic operation condition. In this state, the co-rotation mode had a low-power advantage compared to the counter-rotation mode. The anchor speed and gas flow rate could improve mass transfer performance, but at the expense of higher power consumption. Although it was easier to achieve the complete dispersion regime with a larger diameter impeller, the mass transfer performance was reduced at this time. The viscosity had a significant negative influence on gas-liquid dispersion and mass transfer performance. In terms of impeller combinations, PBTD+Anchor and RT+Anchor performed well in low and high viscous systems, respectively. In addition, applying the artificial neural network model to estimate the N cd for the coaxial mixer correlated with the studied parameters to recognize the complete dispersion regime. [Display omitted] • The critical complete dispersion regime was the most efficient operating condition. • Effects of rotation mode, inner diameter impeller, gas flow rate, and liquid viscosity were investigated. • PBTD+Anchor and RT+Anchor performed well in low- and high-viscous systems, respectively. • Mathematical model for accurate estimation of the N cd of the coaxial mixer were established. [ABSTRACT FROM AUTHOR]

The linear stability analysis of fluid flow, driven by an axial pressure gradient, inside the annular region partially filled with porous medium is investigated. The porous layer is attached to the inner cylinder. The flow is governed by the unsteady Darcy model in the porous region and the Navier–Stokes equation in the viscous region. The effect of the curvature parameter C (ratio of the inner cylinder radius to the gap between cylinders), the ratio of the fluid to the porous layer thickness (t ̂) , and the Darcy number (Da) on the stability characteristics are explored. In addition, the help of the radial velocity contours and the kinetic energy balance is taken to get an insight into the mode and the cause of instability, respectively. The results show that depending upon the value of t ̂ , a decrease in the value of C causes a shift in the neutral stability curve from bimodal to trimodal. For low values of t ̂ , when the onset of instability is dominated by a porous mode, C destabilizes the flow, whereas it has a stabilizing impact on the flow stability for the odd-fluid mode and the even-fluid mode. At high values of t ̂ , C has again destabilizing characteristics and instability is dominated by even-fluid mode. When axisymmetric disturbances are dominant, it is observed that the value of t ̂ for which similar instability characteristics are found varies directly as the square root of Da. However, the correlation between t ̂ and Da does not hold when the non-axisymmetric disturbances are least stable. Contrary to the unconditional stability of the annular Poiseuille flow under non-axisymmetric disturbances for C < 0.1325, the present system is unstable even for C = 0.005 and t ̂ ≤ 1. This shows the significant impact of the curved fluid–porous interface on the stability characteristics. [ABSTRACT FROM AUTHOR]

This paper investigates dynamics of Taylor bubble in counter-current flows, leveraging large eddy simulations combined with the volume of fluid method. Utilizing the OpenFOAM framework, we have implemented a high-order Runge–Kutta time-integration scheme, along with a piecewise linear interface calculation method for precise geometric reconstruction of the bubble interface. We examine the performance of algebraic vs geometric capturing techniques in the context of Taylor bubble breakup, focusing specifically on the transitional flow regime with a liquid Reynolds number of 1400. Our results reveal that the geometric capturing technique offers superior accuracy, improving our understanding of the breakup process and providing valuable insight for multiphase flow simulations in various engineering fields. Our study also reveals the emergence of a secondary vortex in the turbulent wake region behind the Taylor bubble, a phenomenon most prominent at finer mesh resolutions. This vortex represents a novel discovery in counter-current Taylor bubble flows. [ABSTRACT FROM AUTHOR]

This scientific paper investigates the flow and forced heat transfer characteristics of hybrid nanofluid in a channel with rotating circular cylinders. Two-dimensional simulations are used to analyze the effects of obstacle rotation, number, nanoparticle volume fraction, and Reynolds number on the Nusselt number and friction coefficient. The findings demonstrate that incorporating obstacles enhances heat transfer, while increasing the inlet flow frequency improves the system's performance coefficient. Interestingly, substituting hybrid nanoparticles with inlet frequency reduces the friction coefficient and further enhances the performance coefficient. The study underscores the potential of microscale heat exchangers with oscillating inlet velocity and rotating cylinders in optimizing heat transfer across various industries. Furthermore, it explores pulsating flow as a promising solution for high heat flux density challenges in single-phase cooling systems. Additionally, the investigation investigates the impact of inlet velocity frequency on heat transfer rate and friction coefficient, revealing that adjusting the cycle period and frequency improves the heat transfer rate. By modulating the flow frequency, the performance coefficient improves by mitigating the growth of the friction coefficient compared to a constant inlet velocity. By leveraging the advantages of nanofluids and manipulating the flow mode, we can optimize heat transfer performance without increasing the pump power. The investigation of different configurations of cylinders and oscillating flow conditions can provide valuable insights for engineering applications requiring efficient heat transfer and temperature regulation. The study concludes by highlighting the significance of investigating wake interactions in a three-cylinder system arranged in an intermediate triangular configuration under static and rotating conditions. These complex flow phenomena have substantial relevance to both fundamental fluid flow research and engineering applications. [ABSTRACT FROM AUTHOR]

The divergence free velocity field for a flow in the annulus between concentric circular pipes with a moving lateral boundary is studied. A dependence of velocity components on the domain deformation is determined. [ABSTRACT FROM AUTHOR]

Can the similarity between rotation and stratification provide quantitative predictions for complex turbulence? In this work, we focus on plane Couette turbulence as the background flow, which allows us to eliminate the key differences between Rayleigh–Bénard and Taylor–Couette turbulence, and facilitates a quantitative mapping across many complex turbulent systems involving rotation, stratification and curvature effects. To characterize the separated or coupled effects of rotation and stratification, we introduce an overall Richardson number Riχ which is the sum of the Coriolis Richardson number Riθ and the buoyancy Richardson number RiT. When the Prandtl number Pr = 1, the heat and inertial-frame momentum transport almost coincide and mainly depend on Riχ and the Reynolds number, regardless of the specific ratio between Riθ and RiT. When Pr varies, the weighted average Nuχ of the transport coefficients can still remain approximately invariant in most cases. Furthermore, the large-scale structures in purely rotating and purely stratified cases exhibit strikingly similar features. These findings not only indicate a more reasonable analogy for the ultimate Taylor–Couette turbulence but also pave the way for developing new predictive models for natural and industrial processes. [ABSTRACT FROM AUTHOR]

Flow between axially rotating concentric cylinders is well known to exhibit rich dynamics. Hence, Taylor instabilities have been studied, both experimentally and theoretically, for many years. Although usually studied in the abstract, such geometries arise in a range of practical situations including drilling, when a drilling fluid flow enters a well via a pipe that is the centre body and returns via the annulus between the pipe and the borehole wall. In drilling, the centre body rotates and the annular flow contains rock cuttings. Here, we report the development of an Eulerian-Eulerian solver, based on OpenFOAM, that solves for this cuttings transport problem in the presence of both gravity and Taylor vortices. To check the reliability of the solver, we conduct a set of experiments spanning a wide range of complex flow regimes. We show that the model successfully predicts, in all regimes, the observed complex redistribution of particulates. However, for suspension flows under viscously dominated conditions, high particle concentrations and in rectilinear flow, particle pressure and normal stress differences are sufficient to capture particle migration. Results show that in more complex flows exemplified by the Taylor–Couette flow studied here, more realistic predictions of non-Brownian particle migration require inclusion of forces arising through the relative velocity of the two phases including lift forces originating both from inertia and particle rotation [ABSTRACT FROM AUTHOR]