Journal of Applied Fluid Mechanics
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Journal of Applied Fluid Mechanicsendaily1Sat, 24 Feb 2024 00:00:00 +0330Sat, 24 Feb 2024 00:00:00 +0330Numerical Simulations of Turbulent Gas-solid Flow in a Gradual Expansion
https://www.jafmonline.net/article_2397.html
Modeling efforts on turbulent gas-solid flows have mainly focused on studying particle-laden flows in channels and pipes. Despite its significance for industrial applications, the study of gas-solid flows in sudden or gradual expansions is less common in the literature. This paper challenges current two-phase flow models to compute the dilute turbulent gas-solid flow in a vertically oriented 12&deg; conical diffuser. The solids phase is modeled in two ways: the Two-Fluid Model approach that incorporates closure relations derived from the kinetic theory of granular flow, and the Euler-Lagrange particle tracking model with two-way coupling. In both cases, turbulence in the gas phase is estimated by the Reynolds stress model with additional modulation terms that account for the effect of the particles on the gas-phase turbulence. Simulation results are validated versus experimental benchmark data not only for gas axial velocity but also for streamwise and radial turbulence intensity, as comparison with such turbulent variables has not been detailed in previous studies. Nevertheless, due to the lack of experimental data for validation, profiles of solids axial velocity are only compared numerically. Contours of turbulence kinetic energy and granular temperature in the diffuser region reveal a high shear area responsible for the production of turbulence in both phases. Moreover, results obtained from the Euler-Lagrange model show an intense particle fluctuating velocity in the streamwise direction downstream of the diffuser.Study of the Effect of a Cooling Load on a Fluid Surface (Water) in an Open Channel
https://www.jafmonline.net/article_2398.html
In this study the effect of cooling load on the surface water of an open channel with different flow depths is investigated. The method, which was used, involves an experimental laboratory set-up that contains a well-insulated cooling load over a finite area of the water surface, without direct contact with the free water surface so that losses of load to the environment should be avoided. The different cooling loads for each experiment were achieved with the use of insulating films. The insulating film is placed at the bottom of the experimental set-up where there was an empty surface (gap - D), through which the cooling load is allowed to pass. The measurement of velocities was carried out at a two-dimensional (XZ) field, with the help of a digital camera. The recording of motion of the dye (rhodamine) along the channel per unit of time, allows the calculation of the values of the velocity fields. Measurements were conducted when the phenomenon becomes steady. The results for the determination of the cold mass length as a function of the flow depth, and the temperature difference &Delta;T, in a state of thermal equilibrium, led to the formation of a new mathematical relationship. Further study of the phenomenon is essential for the improvement of this study, in combination with other parameters that affect the aquatic ecosystem.Examining the Influence of the Water Entry Velocity of Projectiles on Supercavity Flow and Ballistic Characteristics under Wave Conditions
https://www.jafmonline.net/article_2399.html
In this study, we aim to examine the influence of water entry velocity of a single and two tandem projectile(s) on the supercavitation flow and projectile loading under wave conditions using numerical simulation. The volume of fluid model, renormalization group (RNG) &kappa;-&epsilon; turbulence model, and Schnerr&ndash;Sauer cavitation model are adopted to simulate the multiphase, turbulent, and cavitation flow, respectively. The projectile movement is considered using overlapping grids and a six-degree-of-freedom model. The results show that as the projectile velocity increases, both the dimensionless maximum radius and length of the cavity, as well as the yaw angle, also increase with the rising water entry velocity. For the two tandem projectiles, the cavity pattern on the second projectile varies as the projectile velocity changes. With a lower projectile velocity, the second projectile cannot directly access the front cavity, and there may be situations wherein the part of the second projectile is not enveloped by cavity. As the projectile velocity increases, the second one can directly enter the cavity of the first projectile without forming a separate cavity around itself. In all of the examined cases, the peak pressure on the first projectile is approximately an order of magnitude higher than that on the second one. Furthermore, with increasing projectile velocity, the pressure peak ratio between the first and second projectiles increases.&nbsp;Acoustic Noise Measurement Downstream of an Oscillating Wind Turbine Blade Section
https://www.jafmonline.net/article_2400.html
Acoustic measurements were performed using microphone downstream of a 2-D wind turbine blade section in wind tunnel. The experiments have been carried out in both static and oscillatory pitching cases. The latter is usually experienced by the blades in actual circumstances. The microphone was 1.5 chords downstream of the airfoil and the measurements were conducted at three transverse positions, i.e. behind the trailing edge, midway between the trailing edge and the ground and very close to the ground. A CFD simulation of the flowfield has also been conducted using Fluent to correlate the acoustic behavior to the phenomena observed in the flowfield around the blade. The results show that the acoustic noise heard by a listener located on the ground is higher and stronger than that positioned downstream of the trailing edge, showing the ground effect on acoustic noise reverberation. The aerodynamic noise heard by the listener, changes from a treble to bass sound as the angle of attack increases. Beyond stall, the flow is dominated by the vortices shed into wake and the acoustic noises would be at very low frequencies which would result in a bass sound accompanied by structural vibration. In high angle of attack range, such noises can hardly be heard by a normal person but have a very destructive role on blade structure.Examining the Uniformity of Flow Distribution in Manifolds
https://www.jafmonline.net/article_2401.html
Flow distribution uniformity in manifolds is important in various engineering applications. In this study, the effect of manifold design on flow distribution is examined using both experimental and numerical methods. A comparison was made between a straight manifold and a gradually decreasing cross-sectional design considering two different inlet diameters. In addition, the staggered manifold case with the most homogeneous outlet was compared with the conical manifold under the same conditions. The results demonstrate that the gradually decreasing manifold design significantly improves the flow rate uniformity compared with the straight manifold. This improvement is achieved by reducing the flow rate differences between the distribution branches, leading to a more balanced fluid distribution. The gradual reduction in the cross-sectional area allows the fluid to traverse at lower velocities in regions with higher resistance, effectively minimizing flow rate discrepancies and pressure drops. In addition, the effect of varying the inlet diameter on flow rate uniformity was investigated, revealing that larger inlet diameters contribute to improved flow distribution. The outlet uniformity of the staggered manifold matches the effective performance of the conical manifold, demonstrating similar performance at a lower cost. The results highlight the importance of designing an appropriate manifold, considering factors such as inlet diameter, channel geometry, and staggered ratio, to achieve efficient and uniform fluid distribution.&nbsp;Numerical Simulation of Inverted Bucket Steam Valve Noise based on Multiband Analysis
https://www.jafmonline.net/article_2402.html
As an important control element in steam heating piping systems, the safety and stability of inverted bucket steam valves determine the reliable operation of the system. Therefore, it is necessary to investigate the acoustic mechanism of inverted bucket steam valves. Aiming at the difficulty of numerical simulation in accurately predicting the aerodynamic noise of inverted bucket steam valves, this paper proposes a new method for simulating the aerodynamic noise of inverted bucket steam valves based on multiband analysis (LES). The flow field of the inverted bucket steam valve is numerically simulated using the LES method to obtain wall pressure pulsation information and fluid velocity pulsation information, which are used as excitation sources for acoustic simulation. The characteristics of dipole and quadrupole sound sources were obtained by applying the FW-H method and experimentally verified. The results show that a new multifrequency band analysis method for inverted bucket steam valves is effective by comparing the numerical simulation results, in which the dipole source dominates in the low-frequency band, in the medium frequency range, the quadrupole source outperforms the dipole source, but in the high frequency range, the quadrupole source is dominant. The experimental results are in good agreement with the simulation results, and the correctness of the numerical simulation is confirmed by the fact that there is less than a 3% difference between the findings of the numerical simulation and the experimental data.Aerodynamic Characterization of Bullet Heads with Different Arcuate Curves
https://www.jafmonline.net/article_2403.html
The bullet shape is critical in efficient bullet design because it affects the lift and drag forces. This paper proposes a new bullet shape with a logarithmic curve and analyzes the lift and drag coefficients of bullets with different curves under different angles of attack. The results are compared with a bullet whose shape is described by the power law curve. Fluent simulations demonstrate that the optimal power exponent values are 0.65, 0.6, and 0.65 for the bullet with the power law curve and 1.3, 1, and 1 for the bullet with the logarithmic curve at 0&deg;, 30&deg;, and 40&deg; angles of attack, respectively. At a 0&deg; angle of attack, the lift coefficient of the logarithmic curve is the largest. The lift force of the bullet with the logarithmic curve is 129.4% higher than that with the von Karman curve. The drag coefficient is the largest for the bullet with the rectilinear curve; it is 1.30% larger than that of the bullet with the logarithmic curve. At 30&deg; and 40&deg; angles of attack, the lift coefficient of the bullet with the power law curve is larger. The difference in the lift coefficients between the two angles of attack is 18.47%. The bullet&rsquo;s drag coefficient is the largest for the logarithmic curve, and the difference in the drag coefficients between the two angles of attack is 18.59%.Manipulation of Supersonic Jet using Grooved Tabs
https://www.jafmonline.net/article_2404.html
This study experimentally evaluated the mixing augmentation of twin tabs mounted along a diameter at the outlet of a convergent-divergent Mach 1.62 circular nozzle. The usefulness of the plain and grooved tabs is examined at various expansion levels prevailing at nozzle outlet. The tab's performance is assessed through pitot pressure distribution measured along and perpendicular to the jet centerline at different nozzle pressure ratios (NPRs). The shadowgraph technique visualized the shocks and expansion fans in uncontrolled and controlled jets. With the introduction of uncorrugated or plain tabs at the nozzle outlet operating under overexpanded conditions corresponding to NPR 4, the supersonic length (SL) was decreased only by 35.4%. On the other hand, the corrugated or grooved tabs under similar conditions decreased the SL substantially. Interestingly, the performance of grooved tabs was best at underexpanded conditions associated with NPR 6, where the SL was reduced by about 88%. The pressure profiles also established the superiority of tabs with grooved edges in mixing augmentation without introducing any significant asymmetry to the flow field. In addition, the Shadowgraph images also confirmed the weakening of shock strength and reduction of shock-cell length in the case of grooved tabs at the nozzle exit compared to the plain nozzle.Investigation of Distribution Characteristics of Cylindrical Particles after the Rupture of Modular Cartridges in a Simulator Chamber
https://www.jafmonline.net/article_2405.html
Modular charging is an advanced technique designed to meet the requirements of auto-loading artillery, whereby granular propellants are stored within modular cartridges that are loaded into the gun chamber. This study employed an extended coupled computational fluid dynamics-discrete element method (CFD-DEM) approach to investigate the gas-particle flow within modular charges. After model validation, we analyzed the distribution characteristics, velocity, coordination number, and orientation of cylindrical pellets in a simulator chamber. Four different loading positions for modular cartridges were examined to assess their impact on particle distribution. Numerical simulations revealed a combination of gentle, horizontal, and steep slopes in the particle distribution. The maximum particle velocity experienced a rapid increase during the initial phase, followed by a zigzag decline after reaching its peak. High-coordination number particles tended to accumulate primarily in the middle layer of steep accumulation. Additionally, the particles exhibited an inverted V-shape orientation range from 0&deg; to 180&deg;, suggesting their tendency to assume upright positions. This established model significantly enhanced our understanding of particle distribution following module cartridge rupture and provided valuable guidance for optimizing the design of large-caliber artillery charges.Effect of Synthetic Jet on NACA0012 Airfoil Vortex Structure and Aerodynamic Characteristics
https://www.jafmonline.net/article_2406.html
In this manuscript, the vortex generated by the main frequency excitation of the shedding vortex at various attack angles is investigated by employing the synthetic jet control technique. We also analyzed the impact of the vortex structure on the fled flow around the wing and the spectral characteristics corresponding to the vortex. The dominant frequency and harmonic frequency corresponding to the wave rule of the shedding vortex at various attack angles without the absence of a synthetic jet are selected as the synthetic jet excitation frequency. The results indicate that under the excitation of fixed frequency synthetic jet, the shape of the shedding vortex in the flow field turns correspondingly. Compared with the flow field without jet excitation, it is found that the field with the jet at most attack angles is stable in 2S (Single) mode, and the flow field at a small attack angle is stable in a chaotic state. The angle of attack with a chaotic state is delayed by adding a jet, which makes the curves and corresponding spectral characteristics more orderly. At a defined attack angle, the combined frequency synthetic jet will cause the lift coefficient to fluctuate regularly. At this time, the multiple small-scale vortex structures lead to lift reduction.URANS Simulation of Self-Recirculation Casing Treatment in a Transonic Compressor
https://www.jafmonline.net/article_2407.html
Time-accurate numerical calculations are performed to investigate the effect of air recirculation on NASA Rotor 37. An annular casing-mounted recirculation passageway is designed and located over the blades. Because the investigated rotor does not have any stator, the bleed air has a high circumferential velocity component (in the same direction of the rotor). Therefore, the injected air would have a high swirl component, reducing the injection's effectiveness. As a result, anti-swirl blades have been installed within the recirculation duct, to reduce flow swirl and improve injector effectiveness. Different anti-swirl vanes have been simulated in order to determine the best vanes in terms of minimum pressure loss and zero injection yaw angle (axial injection). Results show that these vanes can effectively turn the circulated fluid to the axial direction and provide a high velocity axial injection upstream of the rotor blades. As a result of the effective injection, the leakage flow moves downstream, improving stability by shifting the stalling point to lower mass flow rates. Because the injection port is close to the blade, the interaction of the passage shock and the injection port causes unsteadiness in the injection mass flow, which is discussed in the paper.Sessile Droplet Evaporation on Wall with Radial Temperature Gradient
https://www.jafmonline.net/article_2408.html
Droplet evaporation coupled with gravity and surface tension on a wall with the radial temperature gradients is numerically studied with the arbitrary Lagrangianâ€’Eulerian method. The influence of the wall temperature distribution on the droplet evaporation process, which is less considered in the existing literature, is mainly discussed. The droplet temperature coefficient of the surface tension and the viscosity on the droplet profile evolution, flow, heat and mass transfer characteristic are also discussed. The results indicate that the droplets become flat first and then retract under the gravity and Marangoni convection during droplet evaporation. There are two high-velocity regions inside the evaporating droplet. One region is at the droplet axis, in which fluid flows to the wall from the droplet top. The other region is near the droplet surface, where fluid flows to the droplet top. There are turning points on the two sides of which the influence of wall temperature distribution on the ratio between the droplet height and the radius of the three-phase contact line (h/Rc), the velocity in the droplet and the surface temperature converts. All of them are larger before the turning point when the wall temperature slope is positive. After the turning point, these are reversed. For both h/Rc and average surface temperature, there is one turning point, which are t*=1.63&times;10-4 and t*=1.05&times;10-4, respectively. For maximum velocity and average velocity in droplet, there are two turning points, which are both t*=1.63&times;10-4 and t*=1.7&times;10-5. The droplet morphology changes more obviously when it is with a greater temperature coefficient of surface tension. Moreover, the turning point is delayed from t*=6.41&times;10-5 while &alpha; is 8 K/m to t*=7.91&times;10-5 while &alpha; is -8 K/m, which indicates that the negative wall temperature slope is beneficial to inhibit the Marangoni effect on droplet evaporation.Enhancement of Jet Mixing using Stepped Tandem Tabs
https://www.jafmonline.net/article_2409.html
Mixing characteristics of jet emerging from a subsonic nozzle exit has been experimented and the results are compared with uncontrolled jet and controlled jet configurations. The mixing enhancement was achieved using a passive method of jet control in which tandem tabs arrangement with rectangular cross section are fixed at the nozzle exit. Two Tab configurations, the Tandem tab (TT) and Stepped Tandem Tab (STT) are used to enhance the mixing characteristics of the jet, the aspect ratio (length /width) of the tabs was 1.67 offering a blockage ratio of 9.55% to the nozzle exit. The blockage ratio of TT and STT configurations are maintained to be equal so that the mixing characteristics can be compared. The axial and radial jet spread are compared for nozzle exit Mach numbers of 0.6, 0.8 and 1.0. The TT controlled jet offered a potential core reduction of 63%, 78% and 82% for Mach numbers 0.6, 0.8 and 1.0 respectively. The STT controlled jet offered a potential core reduction of 89%, 90% and 85% for Mach numbers 0.6, 0.8 and 1.0 respectively. The radial spread of uncontrolled jet, controlled jet with TT and STT are plotted at several X/D locations and found that the controlled jets have more jet spread in both radial directions. A simulation is conducted for jets with exit Mach number 0.8 and the results are validated with the experimental findings. Based on the preliminary experimentation and computation, the STT controlled jet achieved better jet mixing through more potential core reduction and radial spread characteristics as compared to the TT configuration and base nozzle.&nbsp;Integral Airfoil Generation of Multi-rotor Aircraft Based on Optimization of Upper Wing Contour and CFD Simulation
https://www.jafmonline.net/article_2410.html
Regarding the airfoil optimization design of multi-rotor unmanned aerial vehicles, this paper proposes an integral airfoil design method based on upper airfoil contour optimization. Firstly, by designing concave descent input curves with 0-1 distribution, the upper arc of different optimized airfoils is obtained using the Tangent circles method. Secondly, an integral airfoil generation method is developed after establishing the middle arc. As the upper and lower arcs of different shapes are randomly combined, various airfoil profiles are obtained by random assortment. Finally, the effectiveness and accuracy of the designed airfoil are validated through Python programming. The airfoil is generated by the XFOIL program, and the optimal airfoil is output with a lift-to-drag ratio as the target. Meanwhile, an accurate Fluent analysis model is established, and a comparison verification is conducted on the data with the attack angle falling within [-8.02, 12.04] and lift-to-drag ratio falling within [-50, 100]. After Fluent modeling of the designed airfoil, the Euclidean distance between the calculated angle-lift-drag ratio data curve and the data curve tested by the wind tunnel is 0.0331, while the Euclidean distance between the simulated data in the literature and the wind tunnel data is 0.0408. It indicates that our precise model achieves 18.9% higher accuracy than the literature model. Testing and verification results indicate that our designed airfoil based on upper arc optimization and its corresponding airfoil library can meet the design requirements for the aerodynamic performance of airfoils in practical applications. It provides a valuable reference for the development of airfoil design, optimization, and generation methods.Numerical Study on Drag Reduction of Superhydrophobic Surfaces with Conical Microstructures in Laminar Flow
https://www.jafmonline.net/article_2411.html
Superhydrophobic surfaces have garnered attention for their ability to decrease fluid resistance, which can significantly reduce energy consumption. This study aims to accurately capture critical flow phenomena in a microchannel and explore the internal drag-reduction mechanism of the flow field. To achieve this, the three-dimensional (3D) superhydrophobic surface flow field with conical microstructure is numerically simulated using the gas&ndash;liquid two-phase flow theory and Volume of Fluid (VOF) model, combined with a Semi-implicit method for the pressure-linked equation (SIMPLE) algorithm. The surface drag-reduction effect of the conical microstructure is investigated and compared it to that of the V-longitudinal groove and V-transverse groove surfaces. Additionally, the changes in the drag-reduction effect during the wear of the conical microstructure were explored. The numerical results reveal that the drag-reduction effect improves with a larger period spacing of the conical microstructure, the drag reduction rate can reach 25.23%. As the height of the conical microstructure increases, the aspect ratio (ratio of width to height) decreases, and the dimensionless pressure drop ratio and the drag-reduction ratio increase. When the aspect ratio approaches 1, the drag reduction rate can reach over 28%. indicating a more effective drag-reduction. The microstructure is most effective in reducing drag at the beginning of the wear period but becomes less effective as the wear level increases, when the high wear reaches 10, the drag reduction rate decreases to 3%. Compared to the V-shaped longitudinal groove and V-shaped transverse grooves, the conical microstructure is the most effective in reducing drag.&nbsp;