European Journal of Mechanics B-fluids最新文献

Flow control techniques used on wind turbines have been shown to significantly increase energy generation when compared to traditional wind turbines. Although various flow control methods have been introduced in the last two decades, the comparison between these methods is still the least conducted by researchers. Therefore, the present study aims to evaluate the performance of an airfoil utilizing both single and dual passive flow control methods, such as droop, flap, microcylinder, slot, and spoiler with optimal parameters. In this study, a numerical model was developed and applied with the same boundary conditions as those in the experiment. The results of the developed numerical simulation were then validated with experimental and other numerical studies. Mosaic mesh was utilized and the results were compared with conventional mesh types. Even though the mosaic mesh requires a lower number of computational elements, it demonstrated higher computational accuracy when compared to hexcore, polyhedra and tetrahedral type meshes. After obtaining an accurate numerical model, parametric studies were then conducted. The findings mainly highlighted that the airfoil with a microcylinder consistently generated higher performance than droop, flap, spoiler, slot and conventional airfoil. The mean relative improvement was about 2.6%. In an extensive study, eight combinations of flow controls were proposed and evaluated. The highest performances were achieved with the combination of microcylinder and flap, up to 27.9% and the combination of microcylinder and slot, reaching up to 50.2%, for low and high AOAs, respectively.

This review delves into the dynamics of fibre-laden turbulent flows, a field that has garnered substantial attention due to its relevance in both natural and engineering contexts. The focus here is mainly on finite-size fibres, those exceeding the Kolmogorov scale, diverging from the commonly studied smaller ones. The study synthesises current understanding of the behaviour and organisation of both rigid and flexible finite-size fibres within turbulent flows, underscoring the added complexity these anisotropic particles introduce compared to their spherical counterparts. The influence of the length, the curvature and the inertia on the dynamics of rigid and flexible fibres is addressed. Fibre-based novel experimental methods, such as Fibre Tracking Velocimetry, are highlighted. Ultimately, this paper seeks to provide a clearer picture of the intricate dynamics at play in fibre-laden turbulent flows and their practical implications in various fields.

This paper explores the combined influence of an axial electric field and a perpendicular magnetic field imposed on rotating micro-parallel plates immersed in an electrolyte solution. A specialized computer program was developed to solve the velocity as well as the EDL potential fields using the finite difference method, employing the Debye-Hückel (DH) approximation to linearization the EDL potential. The study examines the influence of various non-dimensional parameters, including rotational speed ($\omega$), Hartmann number (Ha), Debye-Hückel parameter ($\kappa$), and the non-dimensional parameter ‘S’, on axial, and transverse velocities, wall shear stress, and net flow rate. Results demonstrate that, both velocity components decrease with increased rotational speed and Hartmann number, while the net flow rate increases with the Debye-Hückel parameter for both rotating and non-rotating systems. The impact of these parameters on shear stress was also analyzed. Analysis of Ekmann spirals in the velocity plane revealed closed spirals at a higher rotational speed and open spirals at lower speeds, with spiral size reducing as rotational speed increases.

This paper provides a detailed historical review of the cavity flow phenomena in fluid mechanics, from recorded studies in the late 19th century to more recent work. Research has been reviewed, independently and in culmination with other studies, to summarise the major and minor governing parameters of the flow. Outlined are influences of technology, regarding numerical models, experimental methods, analysis, and control techniques. All Mach regimes are assessed; low incompressible, sub-, trans-, super- and hypersonic where substantial research was available. A large variety of cavity geometry was presented, mostly rectangular, with more complex features akin to industry application, and where available, assessment of the boundary layer structure is also included. Conclusions on present understanding, and requirements for future work are given, with an aligned set of available data.

Cavity flow-field initialisation and development is dependent on; upstream (U/S) flow conditions of; airspeed $M_{\infty }$, boundary layer (BL) disturbance ($\delta$), displacement (${\delta }^{\ast }$) and momentum ($\theta$) thickness, either laminar or turbulent, and cavity geometry; length ($L$), depth ($D$) and width ($W$), with ratios $L/D,L/W,\delta /D$ and $L/\theta$ defining cavity response. I.e., a narrow cavity with a thin BL U/S tends toward a periodic 3D flow-field, with 3D effects and periodicity decreasing as $W$ and $\delta$ increase. Control is achievable through SL stabilisation via spanwise disturbance from the leading edge (LE), or thickening the BL, thus shear layer (SL). Experiments are preferred over numerical models, due to the inefficiency and high cost of required models (Colonius, 2001; Rowley and Williams, 2006; Lawson and Barakos, 2011). We understand effects of $L$, $D$, $L/D$, and $M_{\infty }$, thus future work should focus on $W$, BL and how they impact mode switching and stream/spanwise flow propagation. Also introducing more complex geometry, realistic to application, to observe additional 3D effects and U/S momentum change, in contribution to a scaling parameter and determination of criteria for activation of material displacement.

Developing the ideas from the author’s previous publication in the European Journal of Mechanics / B Fluids (https://doi.org/10.1016/j.euromechflu.2024.02.008), an interpolation formula is proposed for the angular velocity of mean retrograde flow in a fluid-filled oblate spheroid with arbitrary eccentricity, in the limit of very low longitudinal libration frequencies.

In the design of high-performance heat and mass transfer devices such as liquid-cooled heat sinks, catalytic reactors, and catalytic convertors, parallel mini/microchannels are favored owing to their special potentials. Offering low pressure drop, providing high transfer surface area to volume ratio, and being easy to manufacture and optimize have been drawing thermal and chemical engineers attention to parallel channels for past decades. When working with parallel channels, the challenge of flow maldistribution is commonly faced which decreases their efficiency significantly. System total pressure drop and flow uniformity are two parameters that determine the system performance. In the present study, a variety of practical ideas, aiming to enhance parallel channels performance, are studied numerically. Inventive manifold designs with high hydraulic performance are created through the course of this study. The results of these designs are compared with basic conventional designs which show substantial enhancement. Analyzing less successful designs lead us to deep understanding of fluid dynamics in parallel channel heat and mass transfer devices.

The purpose of this research is to develop a combustion model that can be applied uniformly to laminar and turbulent premixed flames while considering the effect of the Lewis number (Le). The model considers the effect of Le on the transport equations of the reaction progress, which varies with the chemical species and temperature. The distribution of the reaction progress variable is approximated by a hyperbolic tangent function, while the other distribution of the reaction progress variable is estimated using the approximated distribution and transport equation of the reaction progress variable considering the Le. The validity of the model was evaluated under the conditions of propane and iso-octane with Le ≠ 1 and methane with Le = 1 (equivalence ratios of 0.5 and 1). The estimated results were found to be in good agreement with those of previous studies under all conditions. A method of introducing a turbulence model into this model is also described. the validity of the model is confirmed by a comparison with the experimental results of a turbulent methane flame. It was confirmed that the model is in good agreement with experimental results and other turbulence models, and represents approximately a conventional turbulence model.

The unsteady pressure field over an axisymmetric backward-facing step was investigated experimentally at transonic freestream Mach numbers of 1.05, 1.2, and 1.4. The study was aimed at examining the influence of transonic freestream Mach numbers on the spatio-temporal character of the unsteady pressure field and on the dominant modes/mechanisms driving it. Surface flow visualization, schlieren, and unsteady pressure measurements were carried out as part of the experimental investigation. From oil flow visualization and schlieren, the reattachment region was identified, and consequently, the mean reattachment length was estimated. The mean reattachment length shows an increase with the increase in freestream Mach number. The coefficient of mean pressure along the rearbody imitates a classical backward-facing step flow profile and can be divided into three distinct regions. The peak values of the coefficient of mean pressure and the coefficient of root mean square of the fluctuation are seen to decrease with an increase in the freestream Mach number. Conventional spectral analysis reveals that as the freestream Mach number increases, the dominant peak in the spectra shifts to lower frequencies. From the spectra, three dominant fluid dynamic mechanisms depending on the freestream Mach number have been identified. Proper Orthogonal Decomposition (POD) analysis shows that 79–84 % of the total energy contribution comes from the first six modes. The temporal dynamics of the POD modes indicate three prominent mechanisms are responsible for the unsteady pressure field. Spectral analysis of POD modes indicates that the spectra are primarily driven by the first three POD modes for freestream Mach number of 1.05 and the first two modes for freestream Mach numbers of 1.2 and 1.4. Moreover, it reveals the presence of three dominant modes, and the freestream Mach number strongly dictates the dominant mode that is driving the pressure field.

The cochlea, situated within the inner ear, is a spiral-shaped, liquid-filled organ responsible for hearing. The physiological significance of its shape remains uncertain. Previous research has scarcely addressed the occurrence of transverse flow within the cochlea, particularly in relation to its unique shape. This study aims to investigate the impact of the geometric features of the cochlea on fluid dynamics by characterizing transverse flow induced by harmonically oscillating axial flow in square ducts with curvature and torsion resembling human cochlear anatomy. We examined four geometries to investigate curvature and torsion effects on axial and transverse flow components. Twelve frequencies from 0.125 Hz to 256 Hz were studied, covering infrasound and low-frequency hearing, with mean inlet velocity amplitudes representing levels expected for normal conversation or louder situations. Our simulations show that torsion contributes significantly to transverse flow in unsteady conditions, and that its contribution increases with increasing oscillation frequency. Curvature alone has a small effect on transverse flow strength, which decreases rapidly with increasing frequency. Strikingly, the combined effect of curvature and torsion on transverse flow is greater than expected from a simple superposition of the two effects, especially when the relative contribution of curvature alone becomes negligible. These findings may be relevant to understanding physiological processes in the cochlea, including metabolite transport and wall shear stress. Further studies are needed to investigate possible implications for cochlear mechanics.

We study the capability of Large Eddy Simulation (LES) to predict fluid residence time in a ventilated room. Validation is performed against an experiment where the inlet vent slot width matches that of the room. On a coarse grid, the Smagorinsky subgrid-scale model has a detrimental effect on flow statistics, whilst the WALE and Germano–Lilly models perform well. A refined grid produces close agreement with the reference data. A simulation with a narrow inlet slot demonstrates that the flow becomes three-dimensional, with pairs of spiral vortices forming in the room and altering the recirculation pattern when compared to the wide inlet slot configuration. The obtained LES statistics show improvements in the prediction of velocity field over conventional RANS modelling techniques. Fluid age probability density functions show that a wide range of residence time values around the mean value can be observed within the room. LES is capable of providing accurate predictions in a simplified ventilated room, and residence time probability density function distributions can be useful for the improvement of ventilation strategies.