Fluid Dynamics最新文献

The winglets located at aircraft wing ends are used to increase the aerodynamic performance and reduce fuel consumption by regulating negatively affecting the performance at the wings end. Therefore, aircraft wings have a critical importance on aerodynamic efficiency. The most efficient winglet model should be used in each of the flight positions. This study is aimed to investigate the effects of concave, convex, and plain winglet geometry on wing performance. The effects of plain, concave and convex winglet models with cant angles of 30°, 45°, and 60° were experimentally investigated at the angles of attack ranging from 0 to 20°. The aerodynamic lift and drag coefficients generated by each design were measured in wind tunnel tests conducted at a Reynolds number of 2.5 × 10⁵, and the effect of the winglet’s shape on wing performance was compared. The data obtained reveal that various wingtip designs have significant effects on the aerodynamic properties of the wing. At low, moderate and high angles of attack, the highest lift is achieved with the 30° cant angle winglet. The best results for all winglet models in terms of the aerodynamic quality were obtained in the plain winglet model. Furthermore, the aerodynamic quality generally increases with decrease in the cant angles. It is anticipated that these findings, obtained from winglet models with various geometric designs, could contribute to the development of more efficient winglet geometry.

This study presents a three-dimensional numerical investigation of droplet breakup in asymmetric Y-junction microchannels using a coupled volume of fluid-level set method. The motivation arises from the limited understanding of how geometric asymmetry and viscosity contrast jointly influence droplet splitting, which has been extensively explored only in symmetric T-junctions. The effects of the viscosity ratio ({{lambda }}) and the outlet width ratio ({{w}_{2}}{text{/}}{{w}_{1}}) on the critical capillary number ({text{Ca}}) and the droplet length ratio l/w governing the transition between the breakup and non-breakup regimes were systematically analyzed. The results reveal that the higher viscosity ratios promote breakup by enhancing the viscous stresses with respect to interfacial tension, while the larger outlet width ratios favor non-breakup as droplets tend to move into the wider branch. A modified predictive model, developed by extending the existing T-junction framework, successfully captures the regime transition behavior in asymmetric Y-junctions. Furthermore, the daughter droplet length ratio after breakup deviates increasingly from ideal geometric scaling with greater outlet asymmetry. These findings provide new insight into the coupled influence of the viscosity ratio and geometric asymmetry on droplet dynamics and offer a predictive approach for designing microfluidic systems with controlled droplet splitting.

The effect of internal square cross-section grooves in a circular convergent-divergent nozzle as vortex generators on the spreading characteristics of supersonic jets ejected from the nozzle has been studied experimentally. The area ratio of the nozzle was equal to 1.44 and the outlet Mach number for the optimum expansion level was equal to 1.8. The experiments were carried out at three nozzle pressure ratios equal to 3.6, 5.5, and 7.2 corresponding to the over expansion, near optimum, and under expansion conditions, respectively, using both a plain circular nozzle and a nozzle with two diametrically opposite grooves. The radial Pitot pressure distributions were obtained at various axial distances from the nozzle exit. At overexpansion and near optimum expansion, the spread was lowered by introduction of grooves. At underexpansion conditions, the grooves increased the jet spread. The shear layer width of the jets from both plain and grooved nozzles increased with the growth of the axial distance but the variations revealed that the effect of grooves was to reduce the width at all axial locations at the over expansion and near optimum expansion levels. At the under expansion conditions, the jet from the grooved nozzle exhibited the higher shear layer width. The variation in the jet full width of both the jets exhibited a similar behaviour as that of the shear layer width. From the experimental data, it can be inferred that the grooves increased the spread at the underexpansion conditions and decreased the spread at the overexpansion and near optimum expansion conditions.

The results of an experimental study of incompressible, isothermal turbulent free jets issuing from sharp-edged isosceles triangular orifices with the 10°, 70°, and 160° apex angles are presented. The aspect ratios, defined herein as the ratios of the base to the height of the triangular orifices, are equal to 0.18, 1.40, and 11.32 for the 10°, 70°, and 160° orifices, respectively. The results for a round jet, also issuing from a sharp-edged orifice, are presented for comparison. All the orifices had the same exit area of 1613 mm². The Reynolds number, based on the equivalent diameter of the triangular orifices (calculated as ({{D}_{e}} = sqrt {4A{text{/}}pi } ), where A is the orifice exit area) or the diameter of the round orifice, was equal to (~left( {1.67 pm 0.08} right) times {{10}^{5}}). A Pitot-static tube was used to measure the mean streamwise velocities, and a hot-wire probe was used for all other measurements. The mean streamwise centerline velocity decays at the fastest rate in the near field of the 160° triangular jet. The Strouhal number, based upon the preferred mode frequency and initial momentum thickness, in all the jets tested has its highest value in the 160° triangular jet. Also, the inertial subrange in the one-dimensional energy spectra occurs closest to the exit plane in this jet compared to the other jets. The autocorrelation coefficients of the fluctuating streamwise velocities exhibit long tails in the 10° and 160° triangular jets, consistent with the nearly uniform initial mean streamwise velocity profiles in these jets.

This study explores the impact dynamics and morphological evolution of ADN-based green liquid propellant droplets on flat, non-heated surfaces with systematically varied surface roughness Ra from 0.015 to 2.166 µm. Using high-speed imaging, the droplet interactions were captured across three Weber numbers (We = 46.29, 104.15, and 186.15), corresponding to the impact velocities of 1, 1.5, and 2 m/s for the 2 mm diameter droplets. The spreading behaviour was quantified through time-resolved measurements of the spreading ratio β, while morphological features, such as lamella expansion, rim formation, and contact line stability, were evaluated. Results reveal that surface roughness critically controls the maximum spreading, retraction rate, and energy dissipation. The maximum spreading ratio β_max was found to scale with the Weber number, with rapid retraction observed. A curve fitting analysis was performed for this scaling relationship, aligning well with classical inertial-capillary dynamics. Moderately rough surfaces (Ra = 0.2915 µm) enhanced spreading due to optimal capillary attachment, but beyond the roughness Ra = 0.318 µm, micro texture-induced damping suppressed further spreading, reduced β–We sensitivity, and halted retraction. Compared to conventional fluids, ADN droplets exhibited higher maximum spreading on smooth substrates and sharper saturation on rough ones.

In air reverse circulation drilling using double-wall drill pipes, the elbow swivel is prone to failure and leakage under the high-speed impact of rock cuttings. To address these challenges, in this study a novel spherical swivel was proposed. A coupled CFD-DPM method was employed within Euler–Lagrange framework and Huser–Kvernvold erosion model to investigate the erosion behavior and the mechanism of rock cuttings in the spherical swivel. Simulation results reveal that, in contrast to the severe erosion of the traditional elbow swivel caused by repeated collisions of rock cuttings in local areas, the erosion in the spherical swivel is dominated by the initial impact of rock cuttings. Within the spherical chamber, rock cuttings are effectively dispersed without significant superposition of secondary impacts. The maximum erosion rate of the spherical swivel is approximately 42% lower than that of the conventional elbow swivel. An increase in the chamber diameter of the spherical swivel can further reduce erosion by enhancing particle kinetic energy dissipation and minimizing impact superposition, while the angle between the inlet and outlet pipes shows a negligible influence on the erosion characteristics. Furthermore, a higher drilling rate substantially intensifies erosion due to the increased generation of rock cuttings per unit time. The greater cuttings velocities correspond to the higher kinetic energy, and, consequently, more severe erosion. Given that the size of rock cuttings is inherently small, variations in their size have a limited effect on the erosion behavior. The findings provide crucial insights for improving the drilling efficiency and operational safety in air reverse-circulation drilling systems.

Although existing unsupervised particle image velocimetry (PIV) methods avoid the reliance on large-scale labeled flow data, they often suffer from low reconstruction accuracy. To address this, we propose a new unsupervised deep learning framework – UnLiteFlowNet with Multi-Scale Inception and Subpixel Upsampling (UnLiteFlowNet-MSI-SU). Built upon the classic LiteFlowNet, our method incorporates a multi-scale inception depthwise convolution module to enhance feature extraction and replaces traditional bilinear interpolation with a subpixel upsampling layer for finer reconstruction. The network is trained with an unsupervised loss function, including structural similarity loss to preserve image details and improve estimation accuracy. Experiments on synthetic datasets show that our method reduces the average endpoint error (AEE) by 11–21.4% for five typical flow scenarios compared to the baseline UnLiteFlowNet. Tests on real images from the third international PIV challenge further confirm its superior performance in reconstructing fine-scale flow structures.

The bubble dynamics in ultrasonic fields have extensive application prospects in industrial and medical fields such as industrial cleaning, marine technologies, underwater explosions, targeted drug delivery, and non-intrusive measurement of blood pressure owing to the merit of non-pollution, non-invasiveness, and fine control of bubbles. Consequently, the comprehensive understanding of bubble dynamics in ultrasound fields is of great significance. The effect of the bubble deformation, the bubble equivalent diameter, and the bubble ascending trajectory in an ultrasonic travelling wave field are investigated. The impacts of the needle specification, the advance speed of the injection pump, and the ultrasonic transducer height on the bubble dynamics in an ultrasonic travelling wave field are analyzed. The present investigations indicate that a small bubble can levitate in liquid, and the ultrasonic travelling wave can delay the bubble generation, leading to the bubble dynamics becoming complex. Namely, the equivalent diameter of bubbles and their deformation increase under the impacts of the ultrasound.

To meet the increasingly stringent carbon emission requirements, the development of distributed electric propulsion aircraft has received growing attention. As the primary power component of electric aircraft, propeller performance profoundly influences the aircraft flight characteristics. This study aims to provide a reference for proposing new high aerodynamic efficiency design method for distributed electric propulsion heavy-loaded propeller by comparing the Aprop method with the widely used Adkins design approach, which are both suitable for heavy-loaded propeller. A thorough comparative analysis was conducted, focusing on the blade geometries, the thrust accuracy, the circulation distribution, the thrust distribution, and the induced velocity distribution. The results show that, under the same design conditions, the blades designed using the Aprop method have larger pitch angles and smaller chord lengths, except of a small region near the blade root. The Aprop method provides superior accuracy in the thrust, with actual thrust closely matching the required thrust. Although the Adkins method is applicable to the high propeller disc loading, it tends to underestimate the axial induced velocity, resulting in smaller pitch angles and discrepancies between actual and required thrust.

Numerical and experimental results for the problem of melting of a paraffin sample in a high-temperature gas flow are given. Two-dimensional numerical simulation was carried out using the volume-of-fluid (VOF) method within the framework of the OpenFOAM software package. A good agreement between the shape of samples obtained as a result of experiments and that found from numerical calculations is observed. Acceptable agreement for the sample regression rate in the cases corresponding to various temperatures and air flow rates is obtained. The observed formation of a liquid paraffin layer and its disturbances due to the development of Kelvin–Helmholtz instability are in qualitative agreement for both methods of study. Quantitative comparison of the wavelengths that characterize the melt surface disturbances makes it possible speak about the potential of the VOF method for problems requiring detailed resolution of the interphase boundary structure.