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An airlift pump is a vertical tube that utilizes the buoyant effects of a gas to lift a liquid. Unlike a standard mechanical pump, the liquid flow rate through the airlift pump is not directly controlled; rather, it depends on the supplied gas flow rate, the tube length and diameter, and the relative height of the liquid supply free surface (submergence ratio). The present study uses the commercial CFD code ANSYS CFX to model the isothermal, 3D, transient flow in an airlift pump using water and air. The model applies pressure boundary conditions at both ends of the tube and specifies the mass flow rate of air through multiple openings in the side of the tube. The bottom of the tube is an inlet of water only and the outlet is a two-phase flow opening. A time-dependent, homogeneous, VOF two-phase RANS CFD modelling approach is used with the air treated as an ideal gas. This work found that a complete 3D domain was necessary for consistent prediction of the airlift performance and physically realistic two-phase flow structures. Statistical analysis of the two-phase flow structures was applied to characterize airlift pump instability and better understand the physics of the airlift pump. [ABSTRACT FROM AUTHOR]

Airfoil leading-edge fluid-blowing control is computationally studied to improve aerodynamic efficiency. The fluid injection momentum coefficient Cμ (the ratio of injection to incoming square velocities times the slot's width to airfoil's half chord length) varies from 0.5% to 5.4%. Both static and dynamic conditions are investigated for a NACA0018 airfoil at low speed and Reynolds number of 250 k based on the airfoil's chord length. The airfoil is dynamically pitched at a reduced frequency (the pitching tangential speed to the freestream speed ratio), varying between 0.0078 and 0.2. Reynolds-averaged Navier–Stokes (RANS) and unsteady RANS (URANS) is used in the simulations as based on the Transition SST and Spalart–Allmaras models, generally achieving good agreement with experimental results in lift and drag coefficients and in the pressure coefficient distributions along the airfoil. It is found that oscillating the airfoil can delay stall, as expected, in dynamic stall (DS). Leading-edge blowing control can also significantly delay stall both in static and dynamic conditions as long as sufficient momentum is applied to the control. On the other hand, for a small Cμ such as 0.5%, the leading-edge control worsens the performance and hastens the appearance of stall in both static and dynamic conditions. The airfoil's oscillation reduces the differences between pitch-up and pitch-down aerodynamic performances. Detailed analysis of vorticity, pressure, velocity, and streamline contours is given to provide plausible explanations and insight to the flow. [ABSTRACT FROM AUTHOR]

Thick airfoils are often used in the inboard sections of blades in commercial wind turbines. The main reason for this is to give the blade greater structural strength, but it is well known that thick airfoils degrade aerodynamic performance by stalling at relatively small angles of attack. The adoption of flatback airfoils instead of sharp trailing edges allows high lift coefficient to be maintained in thick airfoils. In this paper, we propose a novel airfoil design based on a passive flap to further improve the lift coefficient. This new design was tested by numerical simulation on several airfoils with different maximum thickness and different TE thickness. The improved design for flatback airfoils yields a higher lift coefficient, while the drag behaviour is strictly related to the baseline airfoil shape: some airfoils show a decrease in drag at certain angles of attack, while others exhibit a drag increase. In conclusion, the practical implications of the flap's utilisation on a state-of-the-art blade designed for a 5 MW wind turbine are analysed. The findings demonstrate that, due to the enhanced lift coefficient, it is feasible to shorten the chord while maintaining the power output, thereby reducing material costs. [ABSTRACT FROM AUTHOR]

This article details the development and evaluation of a novel wave energy converter (WEC) aimed at efficiently capturing wave energy for electricity production. The study employs Computational Fluid Dynamics (CFD) techniques, specifically the URANS method and the k-ω SST turbulence model, to solve the Navier-Stokes equations and capture the free surface using the Volume of Fluid (VOF) model. The CFD results are validated against experimental data to ensure accuracy. Various design parameters of the proposed device were tested, revealing that the arms and bottom angle significantly affect its performance. Unlike the floating Wave Dragon (WD) device, which utilises potential energy and is set in deep water, the new fixed-seabed device is positioned in the transitional wave region near the shore, where waves retain 80% of their energy. It can be constructed from environmentally friendly cement, making it resistant to hurricanes and suitable for any wave turbine in the open sea. The MP687 turbine was used to capture the wave energy in the proposed device, testing its performance in three positions: in the open sea, in the middle of the device, and at the device's outlet. The results show that the device significantly enhances wave energy concentration, especially when the turbine is placed at the outlet. The proposed device offers numerous advantages, including its fixed position in a high-energy wave zone, the efficient use of turbulent kinetic energy, and robust construction that can withstand storms. [ABSTRACT FROM AUTHOR]

Vortex distribution affects the flow field variation of offshore structures at different aspect ratios (ARs; length-to-draft). This paper investigates the relationship between the vortex distribution and flow field around a semi-submerged rectangular cylinder (SRC) at a low Reynolds number (Re = 36300) using the OpenFOAM platform. First, vortex generation is explored, and then the influence of vortex distributions on the flow field around the SRC is analyzed. The numerical results show that when the SRC pierces the free surface, the wake region is different from the Kármán vortex street due to the free surface. AR mainly affects the range of the wake region and the SRC bottom vortex. In terms of vortex generation and differences in vortex structure, the variations of the flow field around the SRC are significantly affected by the vortex at the bottom of the SRC. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the air wake around a generic surface combatant ship called NATO-Generic Destroyer (NATO-GD). Naval surface combatants with flight decks must be designed taking aerodynamic concerns into account. Most of the previous studies have employed the Simple Frigate Shape (SFS) and its modified version (SFS2) to investigate the airwake. However, these generic geometries do not accurately represent modern warship designs. To address this, a modern geometry called NATO-GD proposed by the NATO Research Task Group, which represents the features of a modern destroyer, was utilized in the present work. The objective is to examine the air wake on the helicopter deck to ensure the safe operation of air vehicles such as helicopters and drones. The three-dimensional, transient airflow around the ship was solved using the unsteady Reynolds-Averaged Navier-Stokes (URANS) and Detached Eddy Simulation (DES) turbulence models. Besides, the effect of inflow was investigated by comparing uniform velocity inlet and atmospheric boundary layer (ABL) for various wind-over-deck (WOD) angles. The numerical approach was verified for the URANS turbulence model using the Grid Convergence Index (GCI) method. The numerical uncertainty was calculated with four different methods and the uncertainty was found between 1.6% and 2.1%. A detailed discussion of the flow field above the flight deck was conducted to compare the URANS and DES models fairly. It was concluded that employing the ABL profile as a boundary condition is more suitable for achieving accurate ship aerodynamics calculations. The ABL velocity profile makes a significant difference in the velocity components. According to the URANS results, these deviations are found as 8.23% in the x-component, 1.25% in the y-component and 4.89% in the z-component. The deviations were calculated using the root mean square error (RMSE) method. Furthermore, although the numerical results of the URANS and DES models were similar at some points, detailed flow field analysis is only possible with the DES results to determine safe approach patterns for air vehicles. Various wind speeds, directions, and the resulting wake structures were tested to analyze the wake behavior over the helicopter deck under different air conditions. When the wind comes from the port side with 15 degrees (R15) it wind changes the intense turbulence region and creates a low turbulence area on the starboard side while R30 wind causes small scale vortices breaking this region.

Thick airfoils are often used in the inboard sections of blades in commercial wind turbines. The main reason for this is to give the blade greater structural strength, but it is well known that thick airfoils degrade aerodynamic performance by stalling at relatively small angles of attack. The adoption of flatback airfoils instead of sharp trailing edges allows high lift coefficient to be maintained in thick airfoils. In this paper, we propose a novel airfoil design based on a passive flap to further improve the lift coefficient. This new design was tested by numerical simulation on several airfoils with different maximum thickness and different TE thickness. The improved design for flatback airfoils yields a higher lift coefficient, while the drag behaviour is strictly related to the baseline airfoil shape: some airfoils show a decrease in drag at certain angles of attack, while others exhibit a drag increase. In conclusion, the practical implications of the flap’s utilisation on a state-of-the-art blade designed for a 5 MW wind turbine are analysed. The findings demonstrate that, due to the enhanced lift coefficient, it is feasible to shorten the chord while maintaining the power output, thereby reducing material costs.

The effect of helicopter rotor blade tip shape on overall aircraft noise level is analyzed, in forward flight conditions. The infamous HART-II experimental data is used as a base for validations. Commercially available CFD software Ansys/Fluent is used to model the flow field over rotor-body interaction with movingdeforming meshes. The blades are modelled individually to match the actual physical blade geometry and blade movement. The acoustics calculations are performed via FW-H modelling of the software. HART-II blade has a rectangular blade tip planform. Four different tip shapes are modeled and analyzed in addition to the original blade planform. Baseline case of HART-II experiment is used for the predictive calculations. The predictions show that the least sound pressure level obtained by %10 c notched tip shape having 10-degree anhedral angle with 2,23 dB(A) difference amongst other four tip shapes.

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The flow structure around limited-size vegetation patches is crucial for understanding sediment transport and vegetation succession trends. While the influence of vegetation density has been extensively explored, the impact of the relative diameter of vegetation stems remains relatively unclear. After validating the reliability of the numerical model with experimental data, this study conducted 2D-URANS simulations (SST k-ω) to investigate the impact of varying relative diameters d/D under different vegetation densities λ on the hydrodynamic characteristics and drag force of vegetation patches. The results show that increasing d/D and decreasing λ are equivalent, both contributing to increased spacing between cylinder elements, allowing for the formation of element-scale Kármán vortices. Compared to vegetation density λ, the non-dimensional frontal area aD is a better predictor for the presence of array-scale Kármán vortex streets. Within the parameter range covered in this study, array-scale Kármán vortex streets appear when aD ≥ 1.4, which will significantly alter sediment transport patterns. For the same vegetation density, increasing the relative diameter d/D leads to a decrease in the array drag coefficient C ¯ D and an increase in the average element drag coefficient C ¯ d . When parameterizing vegetation resistance using aD, all data points collapse onto a single curve, following the relationships C ¯ D = 0.34 ln a D + 0.78 and C ¯ d = − 0.42 ln a D + 0.82 . [ABSTRACT FROM AUTHOR]

The flow rate is a significant factor in the operation of centrifugal pumps. The characteristic curve of the pump head is frequently employed in the calculation of the flow rate. Nevertheless, this may be subject to alteration because of prewhirl on the suction side of the pump. Calculating the changes in the head's characteristic curve reveals a change in hydraulic losses. The impact of prewhirl on hydraulic losses is investigated by experimental and numerical analysis of two radial centrifugal pumps. It is demonstrated that the primary changes occur in the pump impeller losses. Relative velocity is a significant factor in this context. Alterations in the pumps' configurations result in a range of secondary flows and shock losses at the leading edge of the blades. A physical model, derived on the basis of the relative velocity, is used to predict the characteristic curves of radial centrifugal pumps with prewhirl with a high degree of accuracy. The results demonstrate a notable enhancement in comparison to modelling techniques that do not incorporate the fluctuating hydraulic losses. [ABSTRACT FROM AUTHOR]

A computational investigation of New MEXICO test cases operating under axial flow conditions is reported. Three wind speed cases (10, 15, 24 m/s) corresponding to three different tip speed ratios (10, 6.67, 4.17) when the turbine operates at 425.1 rpm were considered. ANSYS CFX 2021R1 was employed to perform simulations using Single Reference Frame (SRF) and Multiple Reference Frame (MRF) approaches. The flow field is computed by solving unsteady Reynolds Averaged Navier-Stokes (uRANS) equations coupled with SST k - ω turbulence model and Gamma-Theta transition model. Validation involved comparing CFD-predicted integral quantities, static pressure distributions, and loads with corresponding experimental values demonstrating reasonably good agreement at all three wind speeds. Overall, SRF exhibited slightly better wake predictions (hypothetical), while MRF predictions were closer to measurements for integral quantities, static pressure and loads. This study demonstrates the utility of uRANS-based 3D CFD computations in wind turbine aerodynamics studies. [ABSTRACT FROM AUTHOR]

Crosswind can reduce the operability of an aeroengine significantly at static or near static operating conditions. Computational fluid dynamics predictions of the flow at crosswind conditions will play an important part in future designs, however, accurate numerical predictions of the flow within the intake remain challenging even for simulations of cases with intake only. The main objective of this paper is to demonstrate the importance of numerical setup and to determine an optimal computational model for crosswind investigations that can be used by other researchers. By considering the flow to be inherently unsteady, the influence of inlet and exit boundary conditions, and grid sensitivity is studied by using unsteady Reynolds-average Navier-Stokes (URANS) simulations. Numerical predictions of the time-averaged intake pressure recovery (IPR) and the motion of the vortex on the ground are compared against the existing experimental data. The results show that for an intake under crosswind, the ground vortex that forms under the intake and the in-duct separation, when present, exhibit unsteady behavior that become stronger as the crosswind velocity is increased. The steady-state simulation is only representative at lower crosswinds. The intake flow separation and ground vortex predictions are influenced by the inlet boundary layer profiles. Moreover, acoustic reflection was observed at the intake exit boundary which propagates upstream, creating artificial unsteady frequencies in IPR. The reflection is generated from the use of uniform boundary conditions at the intake exit and is not dissipated in the grid due to the long wavelength; this can be mitigated by using a choked nozzle at the intake exit. Acoustic reflection was also observed at the far-field exit boundary. These reflections are caused from interaction of trailing vortex and far-field boundary. [ABSTRACT FROM AUTHOR]

The effect of helicopter rotor blade tip shape on overall aircraft noise level is analyzed, in forward flight conditions. The infamous HART-II experimental data is used as a base for validations. Commercially available CFD software Ansys/Fluent is used to model the flow field over rotor-body interaction with moving-deforming meshes. The blades are modelled individually to match the actual physical blade geometry and blade movement. The acoustics calculations are performed via FW-H modelling of the software. HART-II blade has a rectangular blade tip planform. Four different tip shapes are modeled and analyzed in addition to the original blade planform. Baseline case of HART-II experiment is used for the predictive calculations. The predictions show that the least sound pressure level obtained by %10 c notched tip shape having 10-degree anhedral angle with 2,23 dB(A) difference amongst other four tip shapes. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) computational fluid dynamics (CFD) simulations have been carried out to investigate the complex interaction of a planar shock wave (Ma = 1.22) with a cylindrical bubble. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach with a level set coupled with volume of fluid (LSVOF) method has been applied in the present study. The predicted velocities of refracted wave, transmitted wave, upstream interface, downstream interface, jet, and vortex filaments are in very good agreement with the experimental data. The predicted non-dimensional bubble and vortex velocities also have great concordance with the experimental data compared with a simple model of shock-induced Rayleigh–Taylor instability (i.e., Richtmyer–Meshkov instability) and other theoretical models. The simulated changes in the bubble shape and size (length and width) against time agree very well with the experimental results. Comprehensive flow analysis has shown the shock–bubble interaction (SBI) process clearly from the onset of bubble compression up to the formation of vortex filaments, especially elucidating the mechanism on the air–jet formation and its development. It is demonstrated for the first time that turbulence is generated at the early phase of the shock cylindrical bubble interaction process, with the maximum turbulence intensity reaching about 20% around the vortex filament regions at the later phase of the interaction process. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) computational fluid dynamics (CFD) simulations have been carried out to investigate the complex interaction of a planar shock wave (Ma = 1.22) with a cylindrical bubble. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach with a level set coupled with volume of fluid (LSVOF) method has been applied in the present study. The predicted velocities of refracted wave, transmitted wave, upstream interface, downstream interface, jet, and vortex filaments are in very good agreement with the experimental data. The predicted non-dimensional bubble and vortex velocities also have great concordance with the experimental data compared with a simple model of shock-induced Rayleigh–Taylor instability (i.e., Richtmyer–Meshkov instability) and other theoretical models. The simulated changes in the bubble shape and size (length and width) against time agree very well with the experimental results. Comprehensive flow analysis has shown the shock–bubble interaction (SBI) process clearly from the onset of bubble compression up to the formation of vortex filaments, especially elucidating the mechanism on the air–jet formation and its development. It is demonstrated for the first time that turbulence is generated at the early phase of the shock cylindrical bubble interaction process, with the maximum turbulence intensity reaching about 20% around the vortex filament regions at the later phase of the interaction process.

Ducted rotor-only low-pressure axial fans play an integral role in automotive thermal management. The tightly packed under-hood region and down-stream heat-exchanger shape limit the fan diameter. In order to circumvent this limitation, multiple cooling fans of small diameters are tightly packaged and placed in parallel. Currently, there is limited scientific work, that study the aerodynamics of low-pressure axial fans when installed in parallel. This work aims to quantify the aerodynamic performance and the flow-field as a result of installing low-pressure axial fans in parallel through computational fluid dynamics (CFD). Publicly available experimental data from Friedrich-Alexander University is used for the validation of the numerical setup. Three-dimensional, full-annulus, unsteady Reynolds-averaged Navier-Stokes (URANS) analysis has been performed for both a single-fan and two-fans installed in parallel and their respective aerodynamic performance has been compared for the operation condition identified as the best efficiency point in experiments. Only small differences are observed in the overall aerodynamic performance of the two-fans in parallel compared to a single-fan. A circumferential nonuniformity in the form of a local high-pressure zone at the inlet of the fan is observed when the two-fans are placed in parallel. The resulting circumferential nonuniformity is quantified, both in space and time. A strong correlation is found between the pressure fields of the two-fans installed in parallel. [ABSTRACT FROM AUTHOR]

In this study, the unsteady Reynolds-averaged Navier–Stokes (URANS) equations are employed in conjunction with the Menter Shear Stress Transport (SST)-Scale-Adaptive Simulation (SAS) turbulence model in compressible flow, with an unstructured mesh and complex geometry. While other scale-resolving approaches in space and time, such as direct numerical simulation (DNS) and large-eddy simulation (LES), supply more comprehensive information about the turbulent energy spectrum of the fluctuating component of the flow, they imply computationally intensive situations, usually performed over structured meshes and relatively simple geometries. In contrast, the SAS approach is designed according to "physically" prescribed length scales of the flow. More precisely, it operates by locally comparing the length scale of the modeled turbulence to the von Karman length scale (which depends on the local first- and second fluid velocity derivatives). This length-scale ratio allows the flow to dynamically adjust the local eddy viscosity in order to better capture the large-scale motions (LSMs) in unsteady regions of URANS simulations. While SAS may be constrained to model only low flow frequencies or wavenumbers (i.e., LSM), its versatility and low computational cost make it attractive for obtaining a quick first insight of the flow physics, particularly in those situations dominated by strong flow unsteadiness. The selected numerical application is the well-known M219 three-dimensional rectangular acoustic cavity from the literature at two different free-stream Mach numbers,  M ∞  (0.85 and 1.35) and a length-to-depth ratio of 5:1. Thus, we consider the "deep configuration" in experiments by Henshaw. The SST-SAS model demonstrates a satisfactory compromise between simplicity, accuracy, and flow physics description. [ABSTRACT FROM AUTHOR]

This paper presents the results of a study of self-sustained processes excited in a Helmholtz resonator after a flow over its orifice. A comparative analysis of various approaches to the numerical modeling of this problem was carried out, taking into account both the requirements for achieving the required accuracy and taking into account the resource greediness of calculations, the results of which were verified by comparison with data obtained during a special experiment. The configuration with a spherical resonator with a natural frequency of 260 Hz and an orifice diameter (about 5 cm) in an air flow with a speed of 6 to 14 m/s was considered. A comparison of the calculation results with data obtained in experiments carried out in the wind tunnel demonstrated that the accuracy of calculations of the characteristics of the self-sustained mode using the simplest URANS class model tends to the accuracy of calculations within the large eddy simulation approach formulated in the WMLES model. At the same time, when using WMLES, it is possible to better reproduce the background level of pulsations. From the point of view of resource greediness, expressed in the number of core hours spent obtaining a solution, both models of the turbulence turned out to be almost equivalent when using the same grid models. [ABSTRACT FROM AUTHOR]

Airflow over a square cross-section at high Reynolds numbers and different angles of incidence is investigated with the aim of providing deeper insight into wind actions on elongated structures and, in particular, tall buildings. The flow around bluff bodies is characterized by separation at sharp corners, as well as possible flow reattachment at side surfaces. The alternate shedding of vortices is also generated in the wake of bluff bodies due to the unsteady nature of flow separation. Two-dimensional (2D) URANS numerical simulations were conducted in order to model transient flow and examine wind actions on a square used as a model of a typical cross-section of a tall building far from its roof and the ground. For validation purposes, the study's numerical results on drag and lift coefficients, Strouhal numbers, as well as pressure coefficient distribution were found to be in good agreement with available experimental and numerical results in the literature for relatively low Reynolds numbers. The numerical study was then extended to higher Reynolds numbers, approaching values that are pertinent for wind flow around buildings, thus addressing the lack of such results in the literature. On the basis of these results, the impact of Reynolds numbers and angles of incidence on drag and lift coefficients, as well as the pressure coefficient distribution along the walls of the cross-section, is highlighted. [ABSTRACT FROM AUTHOR]

In the present work, a numerical methodology is developed for affordable and accurate analysis of helicopter rotor noise in forward flight. The flow field is obtained using unsteady compressible flow analyses using commercially available computational fluid dynamics (CFD)solver FLUENT. Azimuthal variations of the flap and pitch motions of the blades are prescribed a priori as high order polynomial equations through a user-defined function. The rotor noise is predicted using Fluent Acoustic module based on the Ffowcs-Williams and Hawking's equations. Methodology is verified using HART II experimental setup and data which includes flapping, pitching, lead-lag motion and elastic torsion for each rotor blade. Individually defined blade motion is found critical for an accurate acoustic prediction based on the obtained CFD calculation. The present predictions compare very well and performs better than other previous CFD analysis both for minimum noise and baseline case at maximumsound pressure level(SPL)of HART II data, even with meshes of 7-8 million cells rather than tens of millions usually needed.

This paper presents the results of a study of self-sustained processes excited in a Helmholtz resonator after a flow over its orifice. A comparative analysis of various approaches to the numerical modeling of this problem was carried out, taking into account both the requirements for achieving the required accuracy and taking into account the resource greediness of calculations, the results of which were verified by comparison with data obtained during a special experiment. The configuration with a spherical resonator with a natural frequency of 260 Hz and an orifice diameter (about 5 cm) in an air flow with a speed of 6 to 14 m/s was considered. A comparison of the calculation results with data obtained in experiments carried out in the wind tunnel demonstrated that the accuracy of calculations of the characteristics of the self-sustained mode using the simplest URANS class model tends to the accuracy of calculations within the large eddy simulation approach formulated in the WMLES model. At the same time, when using WMLES, it is possible to better reproduce the background level of pulsations. From the point of view of resource greediness, expressed in the number of core hours spent obtaining a solution, both models of the turbulence turned out to be almost equivalent when using the same grid models.

The aim of this study is to evaluate the aerodynamic efficiency of a Savonius vertical-axis wind turbine. The approach used relies on resolving the Unsteady Reynolds Averaged Navier-Stokes equations (URANS), the turbulence being modeled by the k-ω SST model. The flow around the wind turbine is simulated using the arbitrary sliding interfaces technique. First, the study investigates the impact of blade shape on wind turbine efficiency by examining seven Savonius rotors constructed with distinct blade configurations. The results indicate that the highest aerodynamic performance is provided by the rotor with the elliptical blades, with a notable increase in the power coefficient of about 80% in comparison to the classic semi-circular profile. To further enhance the efficiency of the Savonius wind turbine, a twin-rotor configuration using the elliptical blades was studied. The results indicate a further enhancement in the power coefficient, reaching 110% compared to a single rotor with semicircular blades.

The aim of this study is to evaluate the aerodynamic efficiency of a Savonius vertical-axis wind turbine. The approach used relies on resolving the Unsteady Reynolds Averaged Navier-Stokes equations (URANS), the turbulence being modeled by the k-ω SST model. The flow around the wind turbine is simulated using the arbitrary sliding interfaces technique. First, the study investigates the impact of blade shape on wind turbine efficiency by examining seven Savonius rotors constructed with distinct blade configurations. The results indicate that the highest aerodynamic performance is provided by the rotor with the elliptical blades, with a notable increase in the power coefficient of about 80% in comparison to the classic semi-circular profile. To further enhance the efficiency of the Savonius wind turbine, a twin-rotor configuration using the elliptical blades was studied. The results indicate a further enhancement in the power coefficient, reaching 110% compared to a single rotor with semicircular blades. [ABSTRACT FROM AUTHOR]

In the present work, a numerical methodology is developed for affordable and accurate analysis of helicopter rotor noise in forward flight. The flow field is obtained using unsteady compressible flow analyses using commercially available computational fluid dynamics (CFD) solver FLUENT. Azimuthal variations of the flap and pitch motions of the blades are prescribed a priori as high order polynomial equations through a user-defined function. The rotor noise is predicted using Fluent Acoustic module based on the Ffowcs-Williams and Hawking's equations. Methodology is verified using HART II experimental setup and data which includes flapping, pitching, lead-lag motion and elastic torsion for each rotor blade. Individually defined blade motion is found critical for an accurate acoustic prediction based on the obtained CFD calculation. The present predictions compare very well and performs better than other previous CFD analysis both for minimum noise and baseline case at maximum sound pressure level (SPL) of HART II data, even with meshes of 7-8 million cells rather than tens of millions usually needed. [ABSTRACT FROM AUTHOR]

The present study treats numerically the performance of a straight-bladed, vertical axis, Darieus wind turbine. A two-dimensional (2D), Unsteady Reynolds-averaged Navier–Stokes (URANS) simulations were performed out by the solver ANSYS/FLUENT using the sliding mesh method. Four turbulence models, namely the one-equation Spalart– Almaras (SA) model, the two-equation Shear Stress Transport (SST) k-ω, the Transitional Shear Stress Transport (TSST), and the realizable k-ϵ models, with low Reynolds number capabilities, were tested.The dependency of the power curve upon the torque coefficient and the Tip Speed Ratio (TSR) was evaluated under identical conditions to previously published experimental studies. The results suggest that the realizable k-ϵ model outperformed other turbulence models and matched better with the experimental data. Further numerical investigations were performed to determine the conditions for an optimal performance of the VAWT in question.

The flow rate is a significant factor in the operation of centrifugal pumps. The characteristic curve of the pump head is frequently employed in the calculation of the flow rate. Nevertheless, this may be subject to alteration because of prewhirl on the suction side of the pump. Calculating the changes in the head’s characteristic curve reveals a change in hydraulic losses. The impact of prewhirl on hydraulic losses is investigated by experimental and numerical analysis of two radial centrifugal pumps. It is demonstrated that the primary changes occur in the pump impeller losses. Relative velocity is a significant factor in this context. Alterations in the pumps’ configurations result in a range of secondary flows and shock losses at the leading edge of the blades. A physical model, derived on the basis of the relative velocity, is used to predict the characteristic curves of radial centrifugal pumps with prewhirl with a high degree of accuracy. The results demonstrate a notable enhancement in comparison to modelling techniques that do not incorporate the fluctuating hydraulic losses.

The flow structure around limited-size vegetation patches is crucial for understanding sediment transport and vegetation succession trends. While the influence of vegetation density has been extensively explored, the impact of the relative diameter of vegetation stems remains relatively unclear. After validating the reliability of the numerical model with experimental data, this study conducted 2D-URANS simulations (SST k-ω) to investigate the impact of varying relative diameters d/D under different vegetation densities λ on the hydrodynamic characteristics and drag force of vegetation patches. The results show that increasing d/D and decreasing λ are equivalent, both contributing to increased spacing between cylinder elements, allowing for the formation of element-scale Kármán vortices. Compared to vegetation density λ, the non-dimensional frontal area aD is a better predictor for the presence of array-scale Kármán vortex streets. Within the parameter range covered in this study, array-scale Kármán vortex streets appear when aD ≥ 1.4, which will significantly alter sediment transport patterns. For the same vegetation density, increasing the relative diameter d/D leads to a decrease in the array drag coefficient C¯D and an increase in the average element drag coefficient C¯d. When parameterizing vegetation resistance using aD, all data points collapse onto a single curve, following the relationships C¯D=0.34lnaD+0.78 and C¯d=−0.42lnaD+0.82.

Measuring the flow rate in High-Temperature Gas-cooled Reactors is a challenge for traditional flowmeters due to the high flow rate (10 to 15 m/s at nominal operating conditions), high operating temperatures (>700°C), and high neutron flux and gamma fields in the reactor core. This paper discusses developing a novel flowmeter that can work under these extreme conditions. Oak Ridge National Laboratory first proposed using acoustics to measure the flow in the reactor, more specifically, using a Kelvin-Helmholtz resonator to correlate the gas flow rate with vibration frequency. With the primary goal of developing an acoustic measurement technique, we propose an acoustic corrugated pipe as a candidate for the development of a novel gas flowmeter. Experimental investigations on corrugated pipes have confirmed the dependence of the whistling frequency on the gas flow rate. Also, a tube-in-tube configuration is proposed for the flowmeter prototype, which can help mitigate resonance between the system and the flowmeter. Experimental investigation using the prototype has shown good independence from the piping system. Furthermore, Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations have been performed and validated with a satisfactory agreement, providing confidence that URANS models can adequately predict the characteristic curve (flow rate versus frequency) of the corrugated pipe and can therefore be used to optimize the flowmeter designs cost-effectively. [ABSTRACT FROM AUTHOR]

A fully developed turbulent flow in a concentric annulus, Re = 10,000 , r i / r o = 0.5 , with an inner rotating cylinder in the velocity range N = U ω / U b = 0 ÷ 4 , is studied via a large-eddy simulation. Also, for comparison, simulations by steady-state, unstatiounary RANS k- ω SST (URANS), and Elliptic Blending Model (EBM) were made. The main focus of this study is on the effect of high rotation on the mean flow, turbulence statistics, and vortex structure. Distribution of the tangential velocity and the Reynolds stress tensor change their behaviour at N > 0.5 ∼ 1 . With rotation increases, the production of tangential fluctuation becomes dominant over axial ones and the position of turbulent kinetic energy maximum shifts towards the wall into the buffer zone. URANS and EBM approaches show good agreement with LES in mean flow, turbulent statistics, and integral parameters. The difference in pressure loss prediction between LES and URANS does not exceed 20%, but the average difference is about 11%. The EBM approach underestimates pressure losses up to 9% and on average not more than 5%. Vortex structures are described well by URANS. [ABSTRACT FROM AUTHOR]

In this study, the unsteady Reynolds-averaged Navier–Stokes (URANS) equations are employed in conjunction with the Menter Shear Stress Transport (SST)-Scale-Adaptive Simulation (SAS) turbulence model in compressible flow, with an unstructured mesh and complex geometry. While other scale-resolving approaches in space and time, such as direct numerical simulation (DNS) and large-eddy simulation (LES), supply more comprehensive information about the turbulent energy spectrum of the fluctuating component of the flow, they imply computationally intensive situations, usually performed over structured meshes and relatively simple geometries. In contrast, the SAS approach is designed according to “physically” prescribed length scales of the flow. More precisely, it operates by locally comparing the length scale of the modeled turbulence to the von Karman length scale (which depends on the local first- and second fluid velocity derivatives). This length-scale ratio allows the flow to dynamically adjust the local eddy viscosity in order to better capture the large-scale motions (LSMs) in unsteady regions of URANS simulations. While SAS may be constrained to model only low flow frequencies or wavenumbers (i.e., LSM), its versatility and low computational cost make it attractive for obtaining a quick first insight of the flow physics, particularly in those situations dominated by strong flow unsteadiness. The selected numerical application is the well-known M219 three-dimensional rectangular acoustic cavity from the literature at two different free-stream Mach numbers, M∞ (0.85 and 1.35) and a length-to-depth ratio of 5:1. Thus, we consider the “deep configuration” in experiments by Henshaw. The SST-SAS model demonstrates a satisfactory compromise between simplicity, accuracy, and flow physics description.

This paper obtains the rotational speed effects of an immersed rotating vertical cylinder at constant temperatures on convection (heat transfer) within stationary fluids. The finite volume method in accordance with the commercial CFD solver FLUENT is used to solve the main equations. The wall domain effect is also examined by changing the aspect ratio. The results show that the effect of the walls disappears in the aspect ratios of 4 and above. Various Reynolds numbers (Re) (200–3000) are taken into account to perform simulations. The flow and thermal field details are described by exploring variations of flow vector, isothermal contour, iso-surface temperature, local and average Nusselt number (Nu), and velocity components. The results indicate convincing evidence of the strong impact of the increase in rotation speed or Re on the flow patterns and heat transfer. An increase in Re from 200 to 3000 enhances the average Nu by 68%. The laminar-to-turbulent flow changes are reported when Re values are close to 1000. Performance evaluations are carried out for LES and UARNS turbulence methods at Re = 3000 to accurately obtain the simulating vortices and their effect on heat transfer. Based on the findings, only LES could provide an accurate structure for the secondary flows with a significant effect on heat transfer. [ABSTRACT FROM AUTHOR]

This study aims to investigate the critical state of the saddle-shaped unstable region of the axial-flow pump and propose a suitable criterion for identifying this state. The bifurcation SST k – ω model considered the rotation effect is used in the present work and verified in the numerical calculation of a water jet pump. Then, it is used to simulate the critical state of the axial-flow pump. Results show that the leading-edge separation vortex generates at 0.6Qd, while the head declines only at 0.55Qd. Therefore, using the inflection point of the head-flow curve as the critical state criterion is unsuitable. In addition, the fixed monitoring point is unsuitable for identifying the critical state due to the insensitivity to the amplitude, main frequency, and periodicity changes at the critical state. Finally, to identify the critical state, it is essential to arrange a monitoring point at the leading edge of the blade suction near the shroud, which should rotate with the impeller. The critical state criterion is that the main frequency position of the pressure fluctuation signal is offset at the monitoring point, and the amplitude is increased by 10 times. [ABSTRACT FROM AUTHOR]

Nanoparticles released from tailpipes of ground vehicles are a major contributor to air quality worsening. As it is question of small sized solid particles, they can cause important damages, particularly to human health. The dynamics of these ultrafine particles (UFP) is strongly related to the characteristics of the airflow. The present work focused on a numerical study of a two-phase flow (air and UFP) developing in the wake of a simplified vehicle (square back Ahmed body). The goal is to get a better understanding of the dynamics of these UFP in order to prevent their infiltration in a following vehicle. Two cases are analyzed corresponding to two different upstream velocities U ∞ = 10 and 15 m/s . A URANS model coupled with a tracking Lagrangian particle approach was used to predict the two-phase flow. Results show a decrease of the UFP concentration with the distance to the emission point. In addition, the concentration profiles show a highly dissymmetrical shape in the near wake recirculation region, which can be explained by the off-centered position of the exhaust pipe and the impact of the large coherent structures. In the far wake, the particles concentration profiles tend to homogenize. Furthermore, dispersion increases with the distance to the vehicle and its speed due to the dispersion process. Article Highlights: The concentration profiles show a highly dissymmetrical shape in the near wake recirculation region [ABSTRACT FROM AUTHOR]

The volute-free centrifugal fans are widely used in air conditioning and ventilation applications due to their simple configuration and easy maintenance. The motor normally intrudes towards the fan interior from the rear disc to reduce the size and achieve better compatibility. The size and geometry of the motor affects the local flow field, which may have a serious impact on the aerodynamic performances of the fan. In this paper, we numerically investigated the effect of motor intrusion on the performances and internal flow characteristics of the volute-free centrifugal fan at different flow rates. The numerical results reveal that the internal flow field varies with the increasing of motor height at all flow rates. Under large flow rate, the height of motor has more obvious influence on the static pressure and static pressure efficiency of the fan; as the height of motor increases from 0 to 4 H /8, the two values decrease by 82.8% and 49.7%, respectively. The flow is more unstable which are manifested by the intensified velocity and pressure fluctuations. Therefore, the blocking effect of motor cannot be ignored and must be taken into account for the design the volute-free centrifugal fans. [ABSTRACT FROM AUTHOR]

Turbulent mixing in buoyant flows is an essential mechanism involved in many scenarios related to nuclear safety in nuclear power plants. Comprehensive understanding and accurate predictions of turbulent buoyant flows in the reactor are of crucial importance, due to the function of mitigating the potential detrimental consequences during postulated accidents. The present study uses URANS methodology to investigate the buoyancy-influenced flows in the reactor pressure vessel under the main steam line break accident scenarios. With a particular focus on the influence of turbulent heat flux closure models, various combinations of two turbulence models and three turbulent heat flux models are utilized for the numerical simulations of three ROCOM tests which have different characteristic features in terms of the flow rate and fluid density difference between loops. The simulation results are compared with experimental measurements of the so-called mixing scalar in the downcomer and at the core inlet. The study shows that the anisotropic turbulent heat flux models are able to improve the accuracy of the predictions under conditions of strong buoyancy whilst in the weak buoyancy case, a major role is played by the selected turbulence models with essentially a negligible influence of the turbulent heat flux closure models.

Wind energy conversion is contributing significantly to the new clean energy transition but, up to now, such contribution is mainly driven by large multi-MW windfarms. On the other hand, the recent needs in terms of distributed energy production are revealing a new interest in small-scale wind energy conversion technologies for residential and urban applications. In this context, vertical axis wind turbines (VAWT) are often the most valid choice from the point of view of building integrating possibilities, but it is well known that they suffer a considerable gap in performance when compared to horizontal axis technologies. Based on these premises, in the present work some innovative solutions for improving the performance of a Darrieus vertical axis wind turbine are investigated through numerical and engineering approaches. The investigated interventions include rotor hybridization by the use of an inner Savonius section for improving the machine's start-up and the possible application of dimples for improving the NACA 0021 airfoil performances on the outer rotor section. Results demonstrate the effectiveness of the interventions in obtaining a rotor with stable performances in a very wide range of wind regimes. • Vertical axis wind turbines (VAWT) are the most valid choice for building integration • It can be important to fill the performance gap compared to horizontal axis machines • The improvement of the Darrieus VAWT is studied by the use of a hybrid rotor • The maximum performance can be also achieved with the possible application of dimples [ABSTRACT FROM AUTHOR]