Non-uniform Velocity Research Articles

Investigating the Flow Characteristics of Superhydrophobic U-Shaped Microchannels

Hydrophobicity has been widely reported for its superior behavior in drag reduction, self-cleaning, and anti-corrosion in many areas. Especially in engineering design, the research of the unique property of the slip flow with complex flow patterns is essential for practical applications. In this paper, the flow characteristics of a superhydrophobic U-shaped microchannel are systematically investigated, as the curved part is a fundamental component in microfluids. A slip model is established based on theoretical and experimental solutions. Various types of U-shaped microchannels, radii of curvature, and contact angles are studied with a wide range of Reynolds numbers from 0 to 300. We propose a velocity distribution to examine the non-uniformity of slip velocity on the cross-section. This imbalance is improved with an increase in the apparent contact angle and flow rate, and a decrease in the radius of curvature. The secondary flow and vortices generated by the centrifugal force are enhanced, and their positions are changed due to the slippery boundary. The results show a considerable drag reduction from 10% to 40% with different contact angles. The variation of curvature does not have a decisive impact on the final performance when the surface wettability maintains a steady state. Our research elucidates the physical principle of the slip model in curved channels, showing extensive applications of hydrophobicity in the design of complex microchips and the optimization strategy of heat transfer systems.

A flame technique to isolate the detonation/product interface relevant to rotating detonation engines

A novel experimental technique is proposed to study the detonation propagation in a layer of non-reacted gas weakly confined by combustion products. This problem is relevant to rotating detonation engines, where transverse detonations are confined by products of a previous rotation cycle, and other applications such as industrial safety. The experimental technique utilizes a flame ignited along the top wall in a long channel. The preferential growth of the flame along the long direction of the channel creates a finger flame and permits to create a narrow layer of unburned gas. A detonation ignited outside of this layer then propagates through the layer. This permits to conduct accurate observations of the detonation interaction with the inert gas and determine the boundary condition of the interaction. The present paper provides a proof-of-concept demonstration of the technique in a 3.4 m by 0.2 m channel, in which long finger flames were observed in ethylene-oxygen mixtures. The flame is visualized by high-speed direct luminosity over its entire travel, coupled with pressure measurements. A direct simulation of the flame growth served to supplement the experiments and evaluate the role of the induced flow by the flame growth, which gives rise to a non-uniform velocity distribution along the channel length. Detonation experiments were also performed at various layer heights in order to establish the details of the interaction. The structure was visualized using high speed Schlieren video. It was found that an inert shock always runs ahead of the detonation wave, which gives rise to a unique double shock reflection interaction.

Energy, exergy, economic, environmental (4E) and frequency distribution analysis of train wind gust with real-time data for energy harvesting

The wind gust velocity of trains are above the cut in speed of wind turbines. Multiple cases studies estimate the available wind energy and potential electrical output with numerical and computational models. These gust velocities are dynamic nature. This work collects real time data of wind gust using data acquisition, conducted 4E and Weibull frequency distribution analysis. The acquired data is further used as a velocity signal to Simulink and wind emulator wind energy harvesting systems. This distinguishes in producing benchmarking results when compared with numerical and computational models. From data interpretation and analysis, the wind gust are non-uniform and gust velocity ranges from 2.3 to 7.1 m s−1 is recorded with a Weibull scale parameter value(A) of 5.54 m s−1. The maximum power available for harvesting is after considering Betz limit is 159.6 W, whilst Simulink and emulator energy harvesting systems produces 126.4 W and 123.08 W with a maximum exergy efficiency of 49.38 and 49.14%. The estimated wind energy available for 1KM range with wind energy systems on both side of traction poles is about 3.3 KW/KM. The compared environmental and economic analysis reconfirms the feasibility of the proposed model with capacity factor 5.74%. Other findings are the corresponding variation in output with respect to dynamic-wind velocities is limited due to inertia and stored kinetic energy of system, the role of location, weather statistics and influence of tail winds in shaping wind gust velocity is also adjudged as crucial factors.

Thermo diffusion impacts on the flow of second grade fluid with application of (ABC), (CF) and (CPC) subject to exponential heating

The aim of this article is to investigate the exact solution by using a new approach for the thermal transport phenomena of second grade fluid flow under the impact of MHD along with exponential heating as well as Darcy’s law. The phenomenon has been expressed in terms of partial differential equations, then transformed the governing equations in non-dimentional form. For the sake of better rheology of second grade fluid, developed a fractional model by applying the new definition of Constant Proportional-Caputo hybrid derivative (CPC), Atangana Baleanu in Caputo sense (ABC) and Caputo Fabrizio (CF) fractional derivative operators that describe the generalized memory effects. For seeking exact solutions in terms of Mittag-Leffler and G-functions for velocity, temperature and concentration equations, Laplace integral transformation technique is applied. For physical significance of various system parameters on fluid velocity, concentration and temperature distributions are demonstrated through various graphs by using graphical software. Furthermore, for being validated the acquired solutions, accomplished a comparative analysis with some published work. It is also analyzed that for exponential heating and non-uniform velocity conditions, the CPC fractional operator is the finest fractional model to describe the memory effect of velocity, energy and concentration profile. Moreover, the graphical representations of the analytical solutions illustrated the main results of the present work. Also, in the literature, it is observed that to derived analytical results from fractional fluid models developed by the various fractional operators, is difficult and this article contributing to answer the open problem of obtaining analytical solutions the fractionalized fluid models.

Numerical modeling of sequential segmentation for enhancement of mixing inside microchannels

This study investigates sequential segmentation as an active mixing technique inside microchannels. In sequential segmentation, the solvent and anti-solvent streams are divided into segments in the axial direction. Due to the non-uniform velocity profile exhibited in laminar flow, Taylor-Aris dispersion can improve mixing by several orders of magnitude as compared with pure molecular diffusion. We built a 3D numerical model using COMSOL Multiphysics® to optimize this technique. We studied the effect of segmentation frequency, duty cycle (DC), flow velocity, microchannel aspect ratio and configuration of the inlet branches on the concentration distribution. We found that channel aspect ratio (H:W) and segmentation frequency had the most significant effect on mixing efficiency. Increasing segmentation frequency was found to increase the mixing efficiency up to a certain limit beyond which mixing efficiency decreased. This decrease was due to the non-complete segmentation of both streams at high frequencies and the formation of continuous trailing stream of both the solvent and anti-solvent. These trailing streams, which mitigate the segmentation effect, also appear in low aspect ratio (i.e. wider) channels and, therefore, reduces the mixing efficiency. Unequal mixing ratios were achievable by changing the segmentation duty cycle without changing the mixing efficiency considerably. Changing the flow velocity did not cause considerable changes in the mixing efficiency. Our 3D model confirms that sequential segmentation can considerably improve mixing inside microchannels and introduces the channel aspect ratio, for the first time, as an important parameter affecting the segmentation mixing efficiency.

Effect of Airflow Non-Uniformities on the Thermal Performance of Water–Air Heat Exchangers—Experimental Study and Analysis

The thermal performance of fin-and-tube heat exchangers (HX) is a crucial aspect in a multitude of applications and fields; several design and operational parameters influence this performance. This study focuses on the issue of flow maldistribution and its effect on the HX thermal performance. For this purpose, an experimental setup is designed and implemented to emulate the conditions under which an automotive heat exchanger operates in regard to the non-uniform upstream airflow velocity distribution over the HX surface. The setup allows obtaining various configurations of airflow velocity non-uniformity of some desired mean velocity and standard deviation. The experimental results reveal that a higher degree of non-uniformity (higher standard deviation of the velocity distribution) causes an increased deterioration of the HX thermal performance. For example, at a water flowrate of 200 L/h and a mean airflow velocity of 2 m/s, increasing the standard deviation from 0 to 2 m/s (i.e., moving from the lowest to highest degrees of non-uniformity) causes a total deterioration of 27% in the performance (3.78 to 2.75 kW, respectively), which can also be observed in the increased level of outlet water temperature (53.8 to 58.2 °C, respectively). The obtained results confirm the numerical results reported in the literature.

Eco-Efficient Vehicle Cooling Modules with Integrated Diffusers—Thermal, Energy, and Environmental Analyses

The automotive domain is a very good candidate for energy management, particularly due to the huge amounts of energy lost by heat through exhaust and water-cooling systems. This makes the optimization of vehicle cooling modules directly related to energy consumption and carbon dioxide emission. In this context, contemporary designs that employ diffusers between the forward-facing of an automobile and its heat exchanger subjected to airflow are presented in this work. The ultimate aim is to reduce the fuel consumption and carbon dioxide emissions of vehicles. Based on the aforementioned design, the intensity of the air velocity would be decreased but distributed over a larger exchanger surface. Consequently, the velocity non-uniformity of the airflow upstream would decrease and therefore, the thermal performance would increase. The above-mentioned conclusion is a result of parametric numerical analysis and its associated numerical results that disclosed the enhancement of water-air heat exchanger thermal performance. To perform a parametric numerical analysis of the heat exchanger thermal performance for the new suggested configurations, a computational code was developed and validated to estimate the thermal performance for a known set of parameters. It was shown that for an automobile, with engine power ranging from 100–200 kW that is utilized for three hours a day and equipped with a diffuser, reduction of up to 2.91 kg (3.89 L) of gasoline consumption and 9.51 kg of CO2 emission can be achieved per day. The originality of the present work resides in the use of diffusers in cooling modules of vehicles, the thermal modeling of heat exchanger’s thermal performance along with its associated code, and the parametric analysis performed to prove the potential enhancement related to the use of diffusers in the cooling modules.

Dispersion relation and instability for an anisotropic nonuniform flowing plasma

A generalized formula for wave instability is developed for an anisotropic nonuniform plasma with finite flows and temperatures. Six-moment fluid equations are solved to give the analytic expression for wave instability in arbitrarily nonuniform plasmas. The analytic formula explicitly states the dependence of wave instability on the nonuniformities of number density, flow velocity, and anisotropic or isotropic pressure. The accuracy of the formalism is verified by a numerical calculation of implicit dispersion relations in complex Fourier space. The analysis shows that nonuniformity plays a critical role in plasma instability, while the flow velocity and anisotropic pressures determine the growth rate of the instability. The instability diagram and associated instability criterion for anisotropy-driven instability are introduced as applications of the formalism.

Numerical investigation of the spark discharge process in a crossflow

The present study numerically investigates the spark discharge process under crossflow conditions using a thermal equilibrium plasma solver that fully couples the electromagnetic physics and fluid dynamics in a computational framework. Numerical results are validated by the comparison with experimental data. The spark discharge experiment is performed in a constant volume vessel using an inductive coil ignition system for automotive applications, and the evolution of the spark channel is measured using high-speed imaging. The crossflow in the gap between the spark-plug electrodes is generated by a rotating fan with two different fan speeds, and the flow velocity across the gap is characterized by particle image velocimetry (PIV) measurement. A computational fluid dynamics (CFD) solver is employed to simulate the crossflow and provide the flow field variables (velocity, pressure, temperature) to the plasma solver. The crossflow velocity predicted in the flow simulation agrees well with the PIV data in that the non-uniform velocity profiles at monitoring points are reproduced by the CFD code. With the crossflow initialization in the plasma solver, the simulated spark discharge process from the breakdown to spark discharge matches the experimental data, including the voltage and circuit waveforms and the high-speed images of the spark channel evolution. The stretch of spark channel captured by plasma simulations agrees with the measured data. The plasma simulation reveals that the mean temperature of the spark channel is maintained at 5000 K during the discharge phase, and the temperature varies along the spark channel so that the highest value is obtained at the spark root on the center electrode. Overall, the results presented in this paper are meant to provide valuable information about the properties of the plasma generated by the spark discharge.

Effect of Bottom Tuyere Arrangement Based on Impact Cavity Morphology on Kinetic Behavior of Molten Bath in Converter

The hydrodynamic interaction between the top blowing supersonic jet and the bottom plume significantly affects the kinetic behavior of the molten bath in the steelmaking converter. Herein, considering the influence of cavity morphology, four bottom tuyere arrangement cases are designed, and their effects on the gas–slag–metal interface characteristics, dynamic and mixing characteristics of molten bath, and abrasion characteristics of refractory lining are numerically studied. The results show that the gas–slag–metal interaction is more violent in the corner region of scattered cavities, with higher velocity, turbulent kinetic energy, and splashing. Arranging the tuyeres in the corner region of scattered cavities would exacerbate the nonuniformity of velocity and splashing, resulting in a large dead zone and high kinetic energy dissipation, which is not favorable to the efficient stirring and mixing of the molten bath. In contrast, arranging the tuyeres in the region of the extension line between the center of the molten bath and the center of scattered cavities may effectively enhance the relevant flow parameters. Furthermore, when the tuyeres are arranged in the corner region of scattered cavities, the bottom abrasion is more serious than that in other regions.

Influence of non-uniform flow on hysteresis effect of pitching hydrofoil

The hysteresis effect of pitching hydrofoil in uniform and non-uniform incoming flow are investigated theoretically and numerically. The numerical method is validated through the experimental results under steady uniform conditions. It has been shown that the simulation can capture the hysteresis effect of pitching hydrofoil. The added force caused by the pitching motion is an important factor of hysteresis phenomenon. When the sinusoidal periodical perturbations of incoming flow are carried, the hysteresis loop of lift coefficient curve are enlarged. According to the variation of the incoming flow, the lift coefficient curve are divided into four stages: S1(α+=5.0°∼α+=10.0°), S2(α+=10.0°∼α+=15.0°), S3(α-=15.0°∼α-=10.0°) and S4(α-=10.0°∼α-=5.0°). The flow structure and corresponding hydrodynamics of four stages under uniform and non-uniform conditions are compared and analyzed. Due to the non-uniform incoming velocity, the positive added lift increases in S1 stage and declines in S4 stage, thus leading to the increase of hysteresis loop. For the S2 stage, the “+” TEV is advanced because of the variational incoming flow. In S3 stage, the formation and development mode of “-” TEV are changed.

Effect of Compressor Unsteady Wakes on a Gas Turbine Combustor Flow

Abstract In gas turbines, combustor inlets are characterized by significant levels of unsteady circumferential distortion due to compressor wakes and secondary flows, together with additional radial nonuniformity induced by the adverse pressure gradients in the prediffuser. This can cause nonuniform velocity distributions across the fuel injector, although the exact interaction mechanism, and the effects it has on the downstream air–fuel mixing, is not fully understood. This paper investigates the flow in an a single sector of a fully featured isothermal rig comprising of compression and combustion systems, exploiting the synchronous coupling of a compressible unsteady RANS simulation with a low-Mach large eddy simulation (LES). Validation against five-hole probe measurements shows that the coupled approach can correctly predict distortion onset and development, with no solution discontinuity at the coupling interface, and is able to preserve unsteady information. The coupled prediction is then compared against a standalone combustor simulation carried out using a circumferentially uniform inlet profile, showing that the additional turbulence from the wakes interacts with the injector, reducing the coherence of the precessing vortex core (PVC) and potentially affecting the air–fuel mixing characteristics.

New Stage–Discharge Relationship for Triangular Broad-Crested Weirs

Simple hydraulic structures, such as weirs, allow measuring flow discharge by using the upstream flow depth and a stage–discharge relationship. In this relationship, a discharge coefficient is introduced to correct all the effects neglected in the derivation (viscosity, surface tension, velocity head in the approach channel, flow turbulence, non-uniform velocity profile, and streamline curvature due to weir contraction). In this paper, the dimensional analysis and the incomplete self-similarity theory are used to investigate the outflow process of triangular broad-crested weirs, characterized by different values of the ratio between crest height p and channel width B, and to theoretically deduce a new stage–discharge relationship. A new theoretical stage–discharge relationship is suggested for the free-flow condition, and it is tested using experimental data available in the literature for the hydraulic condition p/B > 0. The obtained stage–discharge equation, characterized by low errors in discharge estimate, is useful for laboratory and field investigations. Finally, specific analysis for the triangular broad-crested weirs with p/B > 0 was developed to modify the stage–discharge relationship obtained for the case p/B = 0. This specific stage–discharge relationship allows to reduce the errors, which are generally less than ±5%, in the estimate of discharge for triangular broad-crested weirs with p/B > 0 and is also applicable for the case p/B = 0.

The research of a plate fin cold plate characters based on numerical simulation

The cold plate which constructs by plate fins through 36% aqueous solutions of ethylene glycol to control the heated bottom face to below 28°C, was simulated by Fluent software. The properties such as density and specific heat and thermal conductivity and viscosity of 36% aqueous solutions of ethylene glycol were derived from the doctor thesis of R Wang1 by neural network method. The low Re k-ε model was used with shell wall type. The inlet boundaries from 1000Pa to 2500Pa were analysed, the two passes of cold plate flow detail patterns were given, non-uniform velocity field has shown, all parts of the cold plate are coupling. The distributions have shown that pressure and velocity fields nearly with 2D characters, while the temperature field own 3D characters, Performance of cold plate needs to improve the heat sink utilization. The full model shall be considered for more exact results.

Numerical study on the asymmetrical jets formation from active pre-chamber under super-lean combustion conditions

This paper aims to investigate the mechanism of the asymmetric jets of the TJI system. We analyzed the effects of fuel injection and air flows in pre-chamber and main chamber, initial flame kernel, and evolution of turbulent hot jets. The evaluation indexes of asymmetric jet were proposed. The results show when adding fuel injection into the homogeneous PC with off-central spark plug, the average relative velocity range and average degree of velocity non-uniformity increased by 50.8% and 32.8%, which increased by 75.1% and 46.7% for the real engine condition with intake airflow and fuel injection. A similar phenomenon was observed for central spark plug settings but with a much stronger impact on the asymmetry of jets velocity as proved by the increment of 138.5% and 93.2% under real engine condition. The asymmetric jets can be divided into the asymmetry of jets size and the asymmetry moment of flame jets. The former of different jets’ velocity is attributed to the tumble flow in main chamber. The latter is attributed to the irregular flame propagation caused by the off-central ignition and the uneven mixture distribution in pre-chamber. The optimization strategies to efficiency utilize the flow exchange between PC and MC were proposed.

Influence of Plasma Sheath’s Velocity Field on ISAR Imaging of Hypersonic Target

Plasma sheath poses a serious challenge to inverse synthetic aperture radar (ISAR) imaging of hypersonic targets. This paper investigated the distribution characteristics of the electron density and velocity field in the plasma sheath surrounding the hypersonic target in various flight scenes. The incident depth and reflective surface of electromagnetic (EM) waves with X-band, Ku-band, and Ka-band can be determined based on the plasma frequency. We established the echo model coupled with the velocity field of the plasma sheath on the reflective surface and obtained one-dimensional range profiles and ISAR images of the hypersonic target in various flight scenes. The simulation results indicated that the non-uniform velocity field on the reflective surface induced displacement and diffusion in the one-dimensional range profile, resulting in ISAR image distortion. A changing flight scene and radar frequency can have an impact on imaging results. Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity (SSIM) were utilized to assess the impact of plasma sheath on ISAR images. This study revealed the defocus mechanism of the ISAR image caused by the velocity field of the plasma sheath and provided a theoretical reference for the selection of radar frequency for hypersonic targets in various flight scenes.

Effects of obstacles on shock-induced perturbation growth

Effects of obstacles on interface evolution and mixing width induced by Richtmyer–Meshkov instability are investigated experimentally and numerically. In the experiment, the soap film technique is adopted to create an initial interface whose shape is governed by constraint strips protruding into the flow field. By varying the heights of constraint strips protruding into the flow field, effects of obstacles on post-shock flow features are highlighted. First, the interaction of a planar shock with an unperturbed interface is investigated numerically and experimentally. The results show that the obstacles have negligible effects on the transmitted shock velocity, but they greatly increase the reflected shock velocity. The obstacles induce the non-uniform pressure and velocity fields behind the shock, which change the interface evolution and mixing width. Then, experiments of planar shock wave interacting with single-mode interfaces with different initial amplitudes are performed. Induced by the non-uniform post-shock flow, the experimental schlieren images indicate that the spike tip becomes flat, but its size increases in the spanwise direction and the volume of the bubble is reduced. The effects of obstacles are magnified as their heights increase and are more pronounced when the initial interface amplitudes are small. The linear and nonlinear growth rates obtained from experiments show that the obstacles inhibit the perturbation growth, which is partially caused by less kinetic energy the interface obtains from the shock due to the block by the obstacles.

Investigation of the Dynamic Characteristics of Muck during EPB Shield Tunnelling in a Full Chamber Model Using a CFD Method

For the safety and efficiency of earth pressure balance (EPB) shield tunnelling, the dynamic characteristics of the muck in the excavation chamber and the screw conveyor should be considered. However, the velocity and pressure distribution of the muck in the EPB shield, reflecting muck flowability and face stability, are rarely observed at the jobsite. Therefore, a computational fluid dynamics (CFD) model with a modified inlet boundary was established to investigate the muck characteristics in the chamber and the screw conveyor during tunnelling in a full chamber mode. A compressible Bingham model was adapted to describe the muck compressibility and rheology. The time-dependent tunnelling parameters obtained from the simulation excellently matched the field-measured data. The simulation results show the nonuniform velocity distribution of the muck at the excavation face and the low-velocity zones where clogging potentially occurs. The muck pressure reaches a peak at the excavation face and then decreases along the cutterhead openings, chamber and screw conveyor for inflow. The simulation also captures the pressure imbalance at the cutterhead and bulkhead, which may destabilize the excavation face. The proposed discharge efficiency is 56.7% in the simulated condition, indicating that tunnelling with these muck properties has no risk of muck spewing.

Lie symmetry reductions and generalized exact solutions of Date–Jimbo–Kashiwara–Miwa equation

The propagation of nonlinear waves with nonuniform velocities is described by nonlinear evolution equations and their solutions involving arbitrary functions. When a nonlinear evolution equation is integrated, it reveals the several existing features of natural phenomena with continuous and fluctuating background. The Date–Jimbo–Kashiwara–Miwa equation is long water wave equation, which describes the propagation of nonlinear and weakly dispersive waves in inhomogeneous media. This work aims to extend the previous results and derive symmetry reductions of Date–Jimbo–Kashiwara–Miwa equation via Lie symmetry method. The infinitesimals involving four arbitrary functions are constructed by preserving invariance property of Lie groups under one parameter transformations. Then, the first symmetry reduction of test equation is determined using symmetry variables. The commutative and adjoint relations of four dimensional subalgebra are presented for reduced equation. Thereafter, the repeated utilization of Lie symmetry method results into the ordinary differential equations. These determining ODEs are solved under numeric constraints and provide exact solutions. The derived solutions retain all the four arbitrary functions appeared in infinitesimals and several arbitrary constants. Due to existing arbitrary functions, these solutions are generalized than previous established results. The deductions of previous results (Wang et al., 2014; Ali et al., 2021; Chauhan et al., 2020; Kumar and Kumar, 2020; Tanwar and Kumar, 2021; Kumar and Manju, 2022) show the novelty and significance of these solutions. Moreover, the derived results are expanded systematically with numerical simulation to analyze their physical significance and thus doubly soliton, multisoliton, line soliton, bell shape, parabolic nature are discussed.

New semi-analytical solutions to the radial advection-dispersion equation for solute transport in a transient divergent radial flow

Solute transport through radial advection–dispersion is a common phenomenon in many applications, such as aquifer decontamination, heat exchange for geothermal exploration, and tracer testing. Numerous analytical solutions to the problem of tracer testing in a radially divergent flow field are available for fitting breakthrough curves. However, all of these analytical solutions assume non-uniform flow velocity based on Thiem's solution, which varies only spatially, not temporally. Although an injection well with a constant injection rate in a tracer test causes both spatial and temporal variability of the radial flow field, previous analytical studies have employed a steady-state groundwater flow solution, i.e. Thiem's solution, to derive analytical solutions for breakthrough curves or spatial concentration distribution curves. To the best of our knowledge, no analytical solution to the radial advection–dispersion equation under spatially and temporally variable flow conditions driven by injection at a constant rate is currently available.Here, we propose a novel semi-analytical solution for describing solute transport in spatially and temporally variable flow induced by an injection well with a constant injection rate. We find that, above certain threshold values of dimensionless parameters related to aquifer flow properties, solute transport parameters, and the injection rate, the differences between previous analytical strategies our proposed semi-analytical solution are negligible. These threshold values tend to increase more sharply with an instantaneous injection source than a continuous injection source. In other words, it is more difficult for an instantaneous injection source to reach the condition where the difference between our approach and previous ones is negligible than for a continuous injection source type. Our findings clarify when existing analytical solutions can be reasonably used for parameter estimation through fitting of the breakthrough curve to field data and when the proposed semi-analytical solution is a better alternative.