فهرست مطالب

Applied Fluid Mechanics - Volume:18 Issue: 2, Feb 2025

Journal Of Applied Fluid Mechanics
Volume:18 Issue: 2, Feb 2025

  • تاریخ انتشار: 1403/09/14
  • تعداد عناوین: 15
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  • P. K. Shukla *, R. Mishra, R. P. Tewari Pages 304-316
    Ventricular assist devices (VADs) have emerged as an effective clinical tool for offering crucial aid to patients suffering with heart failure. To achieve optimal performance that matches a healthy ventricle, precise design and a thorough understanding of hydraulic and clinical factors are crucial. This research paper presents a comprehensive analysis using computational fluid dynamics (CFD) software ANSYS Fluent at different range of rotational speed and flow rate to examine the performance of an axial blood pump with three different straightener designs: conical, cylindrical, and paraboloid. The primary objective is to assess the impact of these straightener designs on the overall performance of the axial blood pump. Initially, the base axial pump employed conical straightener designs, which were subsequently modified to paraboloid and cylindrical shapes to evaluate their performance. Consistently, the results demonstrated that the paraboloid design outperformed the other designs. Specifically, the axial blood pump equipped with a paraboloid straightener exhibited an increased pressure head and lower intensity of turbulent kinetic energy compared to the other two designs. Additionally, the wall shear stress in the impeller region was lower in the paraboloid design. By employing CFD tool, this study provides valuable insights into the performance of different straightener designs for axial blood pumps. The findings highlight the superiority of the paraboloid design in terms of pressure head and wall shear stress reduction. These results contribute to enhancing the effectiveness and efficiency of left ventricular assist devices (LVADs), ultimately benefiting patients with heart failure.
    Keywords: Vads, Cvds, Heart Failure, Wall Shear Stress, Hemodynamic Complications
  • L. D. Boset *, Z. A. Debele, A. W. Koroso Pages 317-331
    Cyclone separators are commonly used in pneumatic conveyor systems due to their low cost and ability to separate solid particles from gas streams. Understanding pressure drop in cyclone separators is crucial for designing, developing, and optimizing efficient cyclone separators for pneumatic conveyors. The swirling motion within the cyclone during particle-gas separation can cause a pressure drop in the pneumatic conveyor. This study investigates the pressure drop across cyclone separators in pneumatic conveyor systems for Teff grain, both experimentally and computational fluid dynamics (CFD) with discrete particle modeling (DPM) simulation. The study utilized the Lapple cyclone separator model and examined the effects of varying cyclone size (0.75D, 0.9D, and 1D for D=200mm), inlet air velocity (10m/s, 14m/s, 18m/s, 22m/s), and material mass flow rate (0.009kg/s, 0.03kg/s, 0.044kg/s, 0.067kg/s) on the pressure drop across the cyclone separator. The results show that there is strong agreement between experimental and CFD-DPM simulation results. The simulation results accurately represent experimental results, with R-squared value of 0.99 and a residual sum of squares of 38.018. Furthermore, the best curve fit was obtained between the power losses due to pressure drop across the cyclone separator and air mass flow rate. These findings demonstrate that the pressure drop and associated power losses across cyclone separators in pneumatic Teff grain conveyors can be effectively determined using both experimental and simulation methods. This finding can inform the design and optimization of efficient cyclone separators for pneumatic Teff grain conveyor systems.
    Keywords: Cyclone Separator, Pressure Drop, Pneumatic Conveyor, Modeling, Teff Grain
  • G. Zhang, X. Wu, Z. Y. Wu, H. T. Zhang, H. D. Kim, Z. Lin * Pages 332-347
    Butterfly valves are critical control equipment widely used in transmission systems across various fields, including energy, water conservancy, materials and chemical industries, metallurgy, and aerospace engineering. Cavitation, induced when the local pressure is decreased to saturated vaporization pressure, is a common phenomenon in butterfly valves and causes severe damage to valve components. Numerical studies were conducted to explore the progression of dynamic cavitation in a butterfly valve under different actual conditions by using Large Eddy Simulation (LES) coupled with Schnerr-Sauer cavitation model.  The detailed evolution process of generation, development, and collapse was discussed by analyzing the corresponding vapor volume fraction. With the increase of valve opening, there is a corresponding increase in cavitation volume, leading to the rise of disturbance coefficient in full flow field as well as the decrease of shedding frequency of cavitation. The decline of shedding frequency of cavitation exhibits a sudden and pronounced drop at valve opening degree of 60%, which can be attributed to a shift in cavitation shedding behavior from unilateral to bilateral shedding. Periodic changes in cavitation evolution and the presence of attached cavitation on the upper surface of valve plate are obtained and discussed in detail. A comparative analysis of vortex distribution and structure within the flow field reveals insights into the spatial and temporal correlation between cavitation and vortices. The present study of the cavitation mechanism and the in-depth exploration of the evolution law of cavitation provide a clearer understanding of cavitation phenomenon, offering a reference for the structural optimization of butterfly valve in cavitation inhibition.
    Keywords: Butterfly Valve, Cavitation, Periodic Evolution, Attached Cavitation, Vortex Structure
  • N. Li, H. Meng, T. Li *, J. Zhang Pages 348-362
    In this study, a numerical simulation of the static leakage of a subway vehicle was conducted, based on the turbulence model of k-ω Shear Stress Transport (SST). The impact of the leak hole thickness and of the slenderness ratio, on the airtightness of the vehicle is analyzed with a single leak hole, as is the influence of the number, location, slenderness ratio, and area ratio of leak holes, on the airtightness of a train with multiple leak holes. The relative errors of the numerical simulation results are smallest when the leak hole slenderness ratio is 1:16. The relative errors in cases of a single leak hole, and of multiple leak holes are 4.93% and 3.68%, respectively. The pressure relief time first decreases, and then increases as the thickness of the leak hole increases, and is the smallest when the leak is 200 mm in thickness. Keeping the total area of leak holes unchanged, the location and number of leak holes have little impact on the pressure relief time. When door and window leak holes have different thicknesses, changing the area ratio of the door and window leak holes increases the pressure relief time, by a maximum of 1.23 seconds.
    Keywords: Subway Vehicle, Static Airtightness, Leakage Hole, Leak Characteristics, Numerical Simulation
  • S. Sadi, M. R. Asayesh, S. A. Moussavi * Pages 363-373
    Given the vast global capacity of wind turbines, even minor enhancements in their overall performance can substantially increase energy production. To achieve this, several techniques have been developed and implemented commercially to create advanced blades with improved efficiency. However, the fixed aerodynamic shape of these blades imposes certain constraints. This study conducts a numerical analysis of a 660 kW wind turbine, revealing that under specific operating conditions, the blades experience off-design conditions, leading to performance degradation. Simulations indicate that because the blades are designed for a single operating point, flow separation occurs on some sections of the blade surface in other situations. Further investigation demonstrates that the fixed geometry of the blades hinders the flow’s ability to adapt to their shape. To address this challenge, the method of boundary layer suction is proposed. Results indicate that by applying an appropriate level of suction intensity, the aerodynamic performance of the rotor can be enhanced by up to 8% under the specified working conditions by facilitating flow reattachment at the inboard section.
    Keywords: Wind Turbine, Boundary Layer Suction, Flow Separation, Aerodynamics, Performance
  • Y. Yu, K. Chen, D. Wang, Y. Qin, J. Liu * Pages 374-388
    In this paper, Large Eddy Simulation (LES) is employed to investigate the evolution characteristics of a spiral jet mill. This study aims to provide a theoretical reference for the numerical simulation and design optimization of spiral jet mills. To evaluate the influence of grids on simulation accuracy, grid convergence index (GCI) analysis was carried out on three sets of non-structural grids with equal proportion refinement. The visualization results demonstrate that the feeding gas traction and jet impact attenuation contribute to momentum conversion from the edge to the central domain, facilitating the development of the central swirl. The cross-scale chamber structure makes the turbulent coherent structure in the swirl evolution tend to be complex and disordered. A large-scale annular swirl is formed by stacking and winding multiple strip vortices. By comparing with the steady-state solution calculated using the k-epsilon model, it is confirmed that the aerodynamic characteristics in the micronization chamber stabilize at 400 ms. At this time, the combined action of the radial and tangential velocity forms a spiral airflow trajectory.
    Keywords: Swirl Topology, Flow Characteristic, Heat Transfer, Coherence Vortex, Large Eddy Simulation
  • D. Sahoo *, S. T. Kansara, P. Kumar Pages 389-398
    Understanding how protrusions, such as fins attached to flat or streamlined bodies, affect aerodynamics, especially in high-speed contexts, is vital for aerospace applications. These protrusions significantly influence overall aerodynamics and require a comprehensive understanding for accurate analysis and prediction of aerodynamic performance. This understanding is particularly critical in supersonic flight, where even minor aerodynamic disturbances can impact vehicle stability and efficiency. Therefore, a thorough understanding of protrusion-induced flow phenomena is essential for advancing aerospace engineering and improving supersonic vehicle performance and safety. The present paper focuses on the complex supersonic flow over a vertical fin, using a combination of experimental and computational methods. The study aims to understand how variations in fin height influence the behavior of the Lambda shock and any resulting changes in shock length. Specifically, the paper investigates different fin height-to-diameter (H/D) ratios ranging from 0.5 to 1.5 in steps of 0.25. To achieve this, both experimental testing in a supersonic wind tunnel and numerical simulations using the commercial CFD tool ANSYS-FLUENT are employed. Through this dual approach, the paper seeks insights into the characteristics of the Lambda shock and its effects on key aerodynamic parameters, such as shock strength and drag coefficient. By thoroughly investigating these aspects, the paper contributes to a deeper understanding of the complex flow phenomena associated with supersonic flow over vertical fins, potentially guiding the design and optimization of aerospace vehicles. The outcomes indicate that a fin height of 12 mm (H/D=1.0) provides the best balance in terms of pressure distribution, Lambda shock length, and drag coefficient, making it the optimal choice for enhancing aerodynamic stability and performance in supersonic conditions.
    Keywords: Protrusions, Vertical Fin, Aerodynamics, Experimental Testing, Supersonic Flow, Aerospace Applications, Flow-Phenomena
  • S. Kouah *, F. Fadla, M. Roudane Pages 399-418
    Understanding separated flow dynamics is crucial for implementing effective flow control techniques. These techniques help mitigate adverse effects on vehicle performance and environmental pollution. This research aims to improve flow control strategies by predicting separated flow dynamics solely through wall pressure measurements using artificial intelligence and numerical data. Initially, we identify numerical models that accurately replicate separated flow dynamics. Notably, the Detached Eddy Simulation (DES) model strongly agrees with experimental data, particularly in the turbulent regime at Reh= 89100, downstream of backward facing steps (BFS). Subsequently we conducted a correlational analysis that revealed a significant relationship between various wall pressure points and the velocity field, leading to the adoption of deep learning techniques such as Recurrent Neural Networks with Long Short-Term Memory (LSTM). These neural networks, tailored for time-dependent data, demonstrate high accuracy of low MSE of 13.48% using ten wall pressure points in predicting velocity magnitude contour over (BFS). To enhance predictions, Proper Orthogonal Decomposition (POD) is utilized to reduce system complexity while retaining essential dynamics, resulting in a lower MSE of 5.07%. Additionally, we identify the ideal wall pressure measurement region that accurately captures the entire dynamic behavior, achieving an acceptable MSE of 23.48% for predicting low order vorticity, with only three wall pressure points. This research aids in developing efficient flow control strategies with limited pressure data and offers valuable insights for closed-loop flow control applications.
    Keywords: Flow Separation, Backward Facing Step, Instabilities, LSTM Recurrent Neural Network, Dynamics Prediction, Machine Learning
  • P. Zhang, J. Yuan, Y. Fu *, X. Hou, J. Hao Pages 419-437
    The noise hazard posed by cavitation in pump-jet propellers is a significant concern during oceanic operations. This study evaluates the cavitation performance and associated noise characteristics of pump-jet propellers in underwater conditions, further examining the interplay between cavitation phenomena and noise radiation. Cavitation performance across varying advance coefficients was scrutinized using the k-ω SST turbulence model alongside the Zwart cavitation model. Employing Lighthill’s analogy method and bubble radiation theory, analyses of flow-induced noise and noise due to cavitation were conducted. The findings indicate an intensification of cavitation within the pump-jet with increased rotational speed and a reduction in cavitation number, aligning pressure and velocity distributions with observed cavitation patterns. Cavitation markedly elevates flow-induced noise levels, with noise under cavitation conditions found to be around 50 dB higher compared to non-cavitation conditions. Considering cavitation bubble radiation noise, the volumetric pulsations and their amplitudes in the pump-jet enlarge as the bubbles progress through initial growth to maturity. Predominantly, the noise levels from bubble volume pulsations occur within low to medium frequency ranges.
    Keywords: Pump-Jet Propeller, Numerical Simulation, Acoustic Computing, Cavitation, Noise
  • A. Bouhelal *, M. N. Hamlaoui, A. Smaili Pages 438-449
    The aerodynamic performance of wind turbines is significantly influenced by the design of their blades, which are engineered with advanced aerodynamic airfoils. However, the effectiveness of these designs is compromised by environmental factors such as dust, corrosion, sand, and insects, leading to alterations in blade shape and surface integrity over the turbine's operational period. These changes reduce the aerodynamic efficiency of the turbines. To assess these detrimental effects, this study utilizes a 3D Computational Fluid Dynamics (CFD) model based on the exact blade geometry. A modified version of the universal logarithmic wall function was implemented to quantify the influence of surface roughness. Comparative analyses between clean and rough blade surfaces under varying wind conditions showed that surface degradation significantly impacts the efficiency of wind turbines. Specifically, the findings indicate that surface roughness can lead to a substantial decrease in power output, with losses potentially reaching up to 35% under tested conditions. Notably, this roughness effect exhibits a critical value of  , beyond which the impact of roughness becomes negligible. Based on these results, an exponential correlation has been proposed. This study suggests that maintaining smooth blade surfaces or minimizing roughness is crucial for optimal turbine performance, especially under high wind conditions.
    Keywords: Wind Turbine Aerodynamics, Surface Roughness Effects, Logarithmic Wall Function, Computational Fluid Dynamics (CFD), Horizontal Axis Wind Turbine (HAWT), CFD Correlation
  • R. P. Jiang, P. Q. Liu, J. Zhang *, H. Guo Pages 450-467
    For square and circular finite wall-mounted cylinders (FWMCs) with an aspect ratio exceeding 10, the vortex shedding near the tip area leads to the generation of multiple tonal noises. The quantitative analysis of the spanwise distributions of the vortex modal energy with different frequencies was quite limited. This study employs dynamic mode decomposition to decompose the wake of FWMC into distinct frequencies to evaluate the modal energy distribution of pressure fluctuations at each frequency along the spanwise direction. Large eddy simulation combined with the Ffowcs Williams–Hawkings (FW–H) acoustics analogy is applied to a square and a circular FWMC with aspect ratio of 13.6 at a Reynolds number of 2.3 × 104. Two indicators to describe the spanwise energy contribution are proposed. The results reveal that, for square FWMC, the primary modal energy corresponding to Strouhal number ( St ) equal to 0.14 is concentrated below 30% of the cylinder height  owing to the 3D effect. A transition mode of  St ≈ 0.12 is identified in the midspan (0.3 L–0.7L ) without significant contribution to far-field noise spectrum. For circular FWMC, the modal energy is distributed over several frequencies, vortices cells corresponding to the main noise band (0.2 <St< 0.23) are distributed below 0.7 , and the vortices cells in the noise band of 0.15 <St < 0.19 distributed from the midspan to the upper part in a dispersed manner. The noise band with   St≈ 0.08 corresponds to tip-associated vortices gathering above 0.8 .
    Keywords: Aeroacoustics, Finite Wall-Mounted Cylinder, Dynamic Mode Decomposition, Spanwise Energy Distribution, Modal Energy Contribution
  • P. Dhiman, A. Bhat, A. Karn * Pages 468-484
    Hydropower is increasingly recognized as a sustainable energy source due to its minimal environmental impact, a crucial factor in meeting global energy demands. However, the efficiency of hydropower plants (particularly in the Himalayan region) can be hampered by wear and tear of essential components like hydroturbine blades, runners, guide vanes, and nozzles, caused by silt particles in water streams. This study proposes an innovative solution to mitigate silt erosion by implementing a partial air shield on the pressure surface of hydrofoils. Through numerical simulations, the study investigates the interaction between quartz particle-water suspension and injected air on a NACA 4412 hydrofoil. The Euler-Euler-Lagrange model combined with the K-omega SST turbulence scheme is observed to accurately predict erosion wear behavior with and without air injection. The investigation reveals two significant phases. Initially, a comparison between scenarios with and without air injection shows a noticeable reduction in erosion rate when air is introduced over the surface. To further illustrate this reduction, the study increases the silt suspension levels from 2500 ppm to 5000 ppm and the air injection speed from 7.5 m/s to 17.5 m/s, while maintaining a constant hydrofoil angle of attack at 10° and an air-injection angle of 30°. In the subsequent phase, detailed exploration of various air injection parameters reveals an inverse relationship between air injection speed and erosion rate. This study provides comprehensive data sheets illustrating results for different parameter ranges, suggesting that air entrainment on hydroturbine runners can effectively reduce wear due to silt.
    Keywords: Silt Erosion, Hydro Turbines, Air Injection, Erosion Mitigation, Cavitation
  • D. Wang, W. G. Zhao *, X. D. Han Pages 485-503
    This study investigated the effects of solid particles at varying concentrations on hydrodynamic cavitation within a nozzle in a solid particle-pure water-hydrodynamic cavitation flow system. Concentrations ranged from 5% to 10%, and mean diameters varied from 0.0015 mm to 0.040 mm. The Zwart-Gerber-Belamri cavitation model, originally developed for pure water-hydrodynamic cavitation flow, was adapted for the solid particle-pure water-hydrodynamic cavitation flow scenario. A novel algorithm integrating solid, liquid, and vapor phases was developed to facilitate numerical simulations of this flow. Comparisons were made between the vapor contents in solid particle-pure water-hydrodynamic cavitation flow under different concentrations and those in pure water-hydrodynamic cavitation flow to establish variation patterns. Solid particles consistently promoted cavitation evolution across all concentration conditions. However, the range of mean diameter promoting cavitation decreased with increasing concentration. The study analyzed variations in solid particle properties, flow fields, and the forces acting on solid particles to elucidate the underlying mechanisms. Solid particles induced a greater number of cavitation nuclei. In the solid particle-pure water-hydrodynamic cavitation flow, the maximum and minimum slip velocities, as well as the maximum and minimum turbulent kinetic energies, were higher than those in pure water-hydrodynamic cavitation flow, establishing these factors as primary influencers. Conversely, the Saffman lift force was relatively small, rendering its effects as secondary. The combined effects of these factors contributed to the distinctive evolution of hydrodynamic cavitation within the nozzle.
    Keywords: Solid Particle-Pure Water-Hydrodynamic Cavitation Flow, Solid Particle Concentration, Solid Particle Mean Diameter, Nozzle, Numerical Simulation
  • R. Bekkai, R. Mdouki *, R. Laouar Pages 504-517
    The goal of this research is to redesign the three-dimensional geometry of a micro horizontal-axis wind turbine blade using response surface methodology. The variation of the two influential design parameters, chord length and twist angle, along the blade is geometrically modelled using a fourth- and second- degree polynomial, respectively. Therefore, the optimization process is performed basing on eight input parameters that describe the initial blade design. The performance of the initial and the new optimized wind turbine are compared using CFD and BEM approaches. To well study fluid flow through the wind turbine and assess its performance, the CFD analysis step is carried out using the RANS equations with the k-ω SST turbulence model. Concerning the optimization step, The MOGA (Multi-Objective-Genetic Algorithm) method is employed in an automated manner based on a metamodel with non-parametric regression NPR to identify the best candidate with high efficiency. The performance of turbine rotor types is analyzed using the open source Qblade software and compared with CFD methodology for different TSR (Tip Speed Ratio) values. An increase of 14.65% and 17.17% in power coefficient is marked for CFD and Qblade, respectively, at the design TSR of 3. Compared to the initial blade, the optimal one produces more lift, has a lower separation area, and performs significantly better performance at all TSR values. The detailed representation of 3D flow via pressure distribution and limiting streamlines on both blade surfaces confirm the optimization target which leads to reduce separation zones and improve rotor torque. Additionally, a 37% improvement in starting operability at the lowest wind speed is achieved compared to the initial rotor.
    Keywords: CFD, MOGA, RANS, Response Surface Method, Qblade, Optimization
  • H. Li, Y. Chen, L. Bai, W. Shi, L. Zhou * Pages 518-534

    The centrifugal pump holds significant prominence as a widely adopted power machinery in mechanical industries. This study aims to uncover the influence of blade trailing edges on the energy performance of centrifugal pumps. Sixteen types of blade trailing edge models, including Bezier trailing edges, rounded pressure side, cut suction side, and original blade trailing edges, are examined both numerically and experimentally. Entropy production power and energy loss for each domain with different trailing edge models are computed using entropy production theory and the pressure drop method, respectively. The correlation between them and the interaction of energy loss in various domains are determined through Spearman correlation analysis. Furthermore, the energy loss and efficiency of the centrifugal pump are decomposed and explored. Finally, the impact of different trailing edges on each component of shaft power is analyzed. The study findings indicate that increasing the radius of the trailing edge leads to higher head, while a thinner trailing edge enhances efficiency. Consistent trends are observed in entropy production and energy loss across different blade trailing edges. Modifying the impeller trailing edge significantly affects not only the impeller but also the cavity, diffuser, and outlet chamber, with minimal impact on the inlet chamber. Thinning the blade trailing edge can decrease energy loss and entropy production. Proper design of the blade trailing edge can effectively reduce the pressure pulsation near the impeller outlet in the stator. This study serves as a valuable reference for the design and research of centrifugal pump blade trailing edges.

    Keywords: Bezier Curve, Blade Trailing Edge, Centrifugal Pump, Energy Loss, Entropy Production