Journal Of Applied Fluid Mechanics
Volume:16 Issue: 1, Jan 2023

The transonic turbulent two-dimensional airflow over a symmetric flat-sided double wedge is studied numerically. Solutions of the Reynolds-averaged Navier-Stokes equations are obtained with ANSYS-18.2 CFX finite-volume solver of second order accuracy on a fine mesh. The solutions demonstrate an extreme sensitivity of the flow field and lift coefficient to variation of the angle of attack α or free-stream Mach number M∞. Non-unique flow regimes and hysteresis in certain bands of  α  and  M∞ are identified. Interaction of shock waves and local supersonic regions is discussed. The study confirms a concept of shock wave instability due to a coalescence/rupture of supersonic regions. In addition to the instability of shock wave locations, the numerical simulation shows a buffet onset, i.e., self-exciting oscillations due to instability of a boundary layer separation at the rear of wedge. Curious flow regimes with positive lift at negative angles α and, vice versa, with negative lift at positive angles α, are pointed out. A piecewise continuous dependence of the lift coefficient on two free-stream parameters, α and M∞, is discussed.

Urban electric multiple units (EMUs) is based on high-speed trains and metro vehicle technology. Their design speeds are generally from 160km/h to 200km/h, which mitigates the low operating speeds of metro vehicles. Traditional crosswind calculations for the aerodynamic characteristics of trains often assume a 3-marshalling train. Urban trains are generally 4-marshalling and 6-marshalling. Evaluating the aerodynamic characteristics of urban EMUs of different marshalling lengths is instructive for system design. Based on CFD, aerodynamic models of urban trains are established. The train models include 3-marshalling, 4-marshalling and 6-marshalling. The aerodynamic characteristics of 200km/h urban trains subject to different crosswind velocities are numerically simulated. The research display that the aerodynamic performance of the head-car and the first middle-car, under the same crosswind velocity, of different marshalling lengths, are almost the same, whereas the aerodynamic characteristics of the tail-cars for different marshalling lengths are significantly different. The side forces of the 4 middle-cars of the 6-marshalling train decrease, sequentially. At a crosswind velocity of 35m/s, 34% difference in Fs of the tail-car of a 6-marshalling train compared to a 3-marshalling, and the overturning moment differs by 22.8%. Because of the significant difference in side force and overturning moment, the three-marshalling train model cannot represent the real train. Therefore, the real marshalling length should be used, as far as possible, when studying crosswind effects on the train.

Accurate predictions of the near wake of horizontal-axis wind turbines are critical in estimating and optimizing the energy production of wind farms. Consequently, accurate aerodynamic models of an isolated wind turbine are required. In this paper, the steady-state flow around an experimental horizontal-axis wind turbine (known as the MEXICO model) is investigated using full-geometry computational fluid dynamics (CFD) simulations. The simulations are performed using Reynolds-Averaged Navier-Stokes (RANS) equations in combination with the transitional k-kl-w turbulence model. The multiple reference frame (MRF) approach is used to allow the rotation of the blades. The impacts of the nacelle and blade rotation on the induction region and near wake are highlighted. Simulation cases under attached and detached flow conditions with and without the nacelle were compared to the detailed particle image velocimetry (PIV) measurements. The axial and radial flow behaviors at the induction region have been analyzed in detail. This study attempts to highlight the nacelle effects on the near wake flow and on numerical prediction accuracy under various conditions, as well as the possible reasons for these effects. According to simulation results, the rotation of blades dominates the near wake region, and including the nacelle geometry can improve both axial and radial flow prediction accuracy by up to 15% at high wind speeds. At low wind speeds, the nacelle effects can be ignored. The presence of the nacelle has also been shown to increase flow separation at the trailing edges of the blade airfoils, increasing both root and tip vorticities. Finally, small nacelle diameters are recommended to reduce flow separation on the blades and increase the average velocity downstream of the rotor, thereby optimizing wind farm output power.

Particle imaging velocimetry (PIV) was used to study the near-field variation of a pyramid rough element in clear water and a liquid–solid boundary layer (thickness: 60 mm). Particles with an average diameter of 355 µm and Stokes number of 4.3 were injected into a 1:1000 mass ratio (solid particles: water) liquid–solid two-phase solution. Experiments were conducted to collect instantaneous velocity field information in the streamwise–normal direction and streamwise–spanwise direction at a Reynolds number of 8350. Then, the average velocity field and turbulence intensity of the rough element wake under single-phase and two-phase conditions were compared, and the morphology and periodicity of the shedding structure were analyzed by using proper orthogonal decomposition (POD) combined with the power spectral density function (PSD). Particles were shown to have no significant impact on the recirculation area in the streamwise–spanwise plane but did result in a reduction of the recirculation zone in the streamwise–normal plane and a 0.2h closer location of the streamline's origin to the obstacle. Along with the weakening of the upwash structure, the particle phase diminishes the velocity gradient along the span direction and turbulence intensity. Structural shedding at the top of the pyramid and near the wall occurred simultaneously, and the same shedding period was maintained. Particularly, in the first two POD modes, the energy of the shedding structure near the wall was higher than that at the obstacle tip, with a maximum energy differential of approximately 6%. The Strouhal number of the shedding structure decreased by particles from 0.217 to 0.209. The concentration distribution and degree of dispersion in the particle-laden flow illustrate different results, with lower statistics in the wake flow field.

An experimental investigation was conducted to investigate the effects of different cold orifice diameters and operating pressures of the vortex tube. A vortex tube test rig was employed to conduct the experiments for various cold orifice diameters and operating pressures. Cold orifice diameters range from 1 mm to 6 mm, whereas the pressure condition ranges from 2 to 5 bar. The vortex generators were made up of brass material having six inlet nozzles. It was found that the temperature separation of the vortex tube significantly depends on the cold orifice diameter of the vortex tube and operating pressure. The study demonstrates the deviation of cold temperature separation with respect to the cold orifice diameters and inlet pressure for different cold mass fractions. In addition, present experimental results are used to determine the optimum cold orifice diameter, which is 5 mm at 5 bar inlet pressure. The percentage improvement in average cold temperature separation for 5 mm cold orifice diameter is 66.18% compared to rest of the cold orifice diameters at an inlet pressure of 5 bar. The maximum cooling power separation is 0.08 kW at 0.3 cold mass fraction and inlet pressure of 5 bar. The CFD technique was approached to discuss the complex fluid flow inside the tube at various radial distances. A three-dimensional numerical study was done and validated with the present experimental work. It was found that the numerical results are in good agreement with the present experimental data.

The separator with inner channels is designed to solve the inefficiency caused by particle collisions with the wall. Then the response surface methodology is used to optimize this novel separator with the aim of maximizing the separation efficiency and exhaust rate while minimizing the pressure drop simultaneously. Firstly, the Reynolds Stress model and the Euler-Euler model are taken to compare the novel and conventional structures. Secondly, five factors, including inlet velocity (v) of air-particle flow, number of inner channels, axial angle of inner channels, height of inner channels, and number of guide vanes, are selected in the Box-Behnken Design. Thirdly, the quadratic regression equations are established in multi-objective optimization. The research results demonstrate that separation efficiency is improved but the pressure drop is increased in the novel design. Additionally, too large inner channels can lead to a decrease in separation efficiency. The increased height of the inner channels has the most positive impact on the exhaust rate. And the optimization amplitude of the pressure drop is the most remarkable, which is presented as 13.87% at v = 2.5 m/s, 34.49% at v = 4.5 m/s, and 75.49% at v = 6.5 m/s, respectively. Furthermore, the separation efficiency of optimized designs is higher than that of conventional ones at each velocity. The relevant research results can provide an effective guide for improving the efficiency of separators.

Wake models based on Actuator Disk theory are usually applied to optimize the wind farm layouts and improve their overall efficiency and expected AEP. Despite the effectiveness of the existing models, most Actuator Disk approaches are based on the flow axisymmetric assumption, without considering the ground effect on the wake behavior. However, it has been shown that the mast’s height, or distance from the wind turbine to the ground, has an influence on the wake expansion on both hub’s side and at downstream of the wind turbine. Therefore, in this study, a hybrid CFD-BEM-Actuator Disk approach is developed to address the lack of the existing models. In the proposed model, the 3D wind rotor is modeled by a set of blade elements. Then, the local lift and drag forces acting on each blade element are calculated using BEM theory and incorporated into the momentum equation. This BEM-AD model is implemented in a User Defined Function (UDF) that is loaded into the CFD software. Thereby, ground effects are considered to be a wall boundary and defining a wind boundary layer profile at the inlet boundary, which describes the Atmospheric Boundary Layer (ABL). For the validation of this new Actuator Disk model, an enhanced experimental study is conducted at the Dynfluid Laboratory wind tunnel (ENSAM School Paris Tech). The Particle Image Velocimetry (PIV) measurements are used for the experimental wake explorations applied to a miniature two-bladed wind turbine. The wake developments are analyzed at two different hub heights ratio, h/D = 0.7 and 1.0 (where h is the hub height, and D is the wind rotor diameter). The analysis of the outcomes showed that the numerical simulations are in good correlation with the experimental measurements of the ENSAM wind tunnel. The numerical results show that for h/D=0.7, the upper half of the rotor operates within the boundary layer whereas the lower tip vortices are mainly developed in the horizontal direction with lower intensity compared to the upper tip vortices. This effect was not observed for the case h/D=1.0 where the rotor operates outside of the boundary layer; however, the wake centerline is upward deflected at about 0.3D. The main conclusion is that a distance above 7D must be observed between wind turbines to optimize the wind farm performance and over 1D hub height be required to limit the influence of the ground boundary layer effect.

This study proposes a new intermediate warehouse to prevent the problem that the ore in the vertical pipeline hydraulic lifting system blocks the nozzle during the process of being sucked into the lifting hard tube. A mathematical model and a finite element model of ore transport are established. Numerical simulation and experimental research are performed on the process of ore injection into the intermediate warehouse. Results show that the proposed intermediate warehouse has higher ore transmission efficiency, smaller pressure pulsation, and more stable transmission process compared with the traditional structure scheme. The larger the slurry conveying flow, the shorter the time required to convey the ore. In the process of slurry injection, the closer to the inlet, the more uneven the velocity distribution on the Z section, and the more concentrated the dynamic pressure. The closer to the lower end of the intermediate warehouse, the more dispersed the dynamic pressure, and the fluctuation of dynamic pressure in the intermediate warehouse is the main cause of unstable flow. The feed flow has a great influence on the stress state of the intermediate warehouse structure, the ore transfer time, and the stress decay period. The feed flow rate should be reasonably selected to meet the working requirements.

This paper investigates the performance of a non-symmetric airfoil in a perturbed flow for a low Reynolds number by creating small vortical structures. A newly designed two-dimensional numerical tool is used to examine the interaction between the NACA 23015 airfoil and the vortex shedding from a square cylinder. Different airfoil position ratios are numerically simulated concerning the square cylinder G/D (D: square cylinder diameter), the channel centerline T/d (d=D/2), and the vortices scale size D/c (c: airfoil chord length). Results show that the maximum values of the lift and drag aerodynamic coefficients are influenced by the airfoil’s lateral and longitudinal positions. The Proper Orthogonal Decomposition (POD) method is used to identify the most energetic flow structures. For all simulated scenarios, it was found that the first two modes reflect the dominating coherent structures in the flow field. The results also show that a leading-edge vortex is formed over the airfoil. The observed phenomena of symmetric and antisymmetric shedding vortex mechanisms essentially depend on the lateral distance of the airfoil T/d and the vortex scale size D/c. However, the spectral analysis demonstrates that the shedding frequency mainly depends on the gap distance G/D.

Loading a dielectric barrier discharge plasma excitation on a Savonius wind turbine can improve its performance. To study the mechanism of performance improvement, the effects of three pairs of plasma excitations at different positions were comparatively studied by means of numerical simulation, and it was found that the middle position was the best for plasma excitation loading. Then, two plasma excitations in opposite directions were set at this position, and the effects were compared. The results show that plasma excitation can generate a high-velocity airflow, which can affect the velocity distribution around the blade, thereby changing the pressure distribution, and therefore the performance of the Savonius wind turbine. The effect of two opposite directions of plasma excitation loaded at the optimum position on the performance of the wind turbine varies with the change of the TSRs. When the TSR is relatively small, the influence of the two directions is very different. While the TSR is relatively large, the influence of the two directions is close.

Numerical simulations of (water-Fe3O4) ferrohydrodynamics (FHD) mixed convection inside a vertical channel are performed. The magnetic field is produced by three sources positioned outside the channel’s right wall. The latter is provided with localized heat sources surmounted by variously shaped porous blocks: rectangular, trapezoidal, and triangular. The general model of Darcy-Brinkman-Forchheimer is employed to describe the fluid flow in the porous regions, and the resulting equations are numerically solved by the finite volume approach. The influence of significant parameters, including the magnetic number (Mn), the Richardson number (Ri), and the shape of blocks, is examined. The results essentially reveal that the enhanced heat transfer brought by the magnetic field and its intensity increase is suppressed by the augmentation of Ri until a critical value, rising with Mn, beyond which the global Nusselt number increases again. The mean friction coefficient increases with increased Mn and reduced Ri. Compared to the case with no magnetic field, the maximum enhancement in heat transfer rate is around 132% for the rectangular blocks, 146% for the trapezoidal blocks, and 160% for the triangular blocks, while the maximum increase in pressure drop is approximately 45% for all the shapes. The triangular shape seems the most efficient because it leads to high heat transfer rates and low mean friction coefficients; its performance factor is 2.32 for a dominant magnetic field and 2.62 for a dominant buoyancy force. The current research's conclusions will help optimize the operation of various thermal engineering systems, including electronic devices, where the improved heat removal rate will keep the electronic components at a safe operating temperature.

To reduce the noise in a pump piping system and increase the usage time of equipment, a new type of porous muffler is proposed in this paper. A water guide cone is incorporated into the muffler structure, which may help to redirect the fluid media in the piping system. The porous structure is adapted from a muffler shell, water cone wall and round bottom plate. According to this structure, the muffler sample is made, and a pump pipeline system test bench is set up. The outlet noise of the pump pipeline system is measured after installing the muffler. At the same time, the muffler is numerically simulated by combining computational fluid mechanics and Lighthill acoustic theory. The characteristics of the flow field and external sound field under three different flow conditions of 200 m3/h, 400 m3/h and 600 m3/h are assessed. The numerical simulation results show the same dominant frequency and trend as the experimental results. The rationality of the numerical simulation is verified. Research shows that: the level of sound pressure at the muffler's outlet is lower than at the inlet, causing muffling, and the characteristics of a quadrupole sound source appear at the outlet. The proposed muffler has a certain effect in reducing noise in the pump pipeline system.

This study investigated the effect of leading-edge protuberances on the aerodynamic performance of two distinct airfoils with low Reynold’s number (Re): E216 and SG6043. Three protuberance shapes, namely sinusoidal, slot, and triangular, were considered. The amplitudes (A) of protuberances considered were 0.03c, 0.06c, and 0.11c, and the wavelengths (W) were 0.11c, 0.21c, and 0.43c, where c is the chord of the airfoil. The numerical and experimental analyses were performed in the angle of attack (AoA) range of 0° to +20° at and Re of 105. The numerical investigation was performed using the commercial computational fluid dynamics package ANSYS FLUENT. The SST k-ɷ model was used to simulate turbulent flow. The experimental force measurements were conducted using a highly sensitive three-component force balance in a subsonic wind tunnel facility. The flow physics was analyzed using vorticity contours in streamwise and spanwise slices and static pressure distribution contours. The smoke flow visualization technique was used to observe flow streamlines, boundary layer separation, and reattachment over the airfoil surface. The result indicated that the triangular and slot protuberances were the most beneficial for improving poststall lift and reducing skin friction drag. The operating mechanism involved a shift in pressure distribution due to leading-edge alterations and flow energization by secondary flow emanating from the protuberances.

To discuss the lubrication characteristics of roller-sliding bearing under oil-injected lubrication, the CFD method was introduced into the fluid-structure interaction model to realize the transient simulation of oil-air two-phase flow (OATPF). The volume of fluid (VOF) method is applied to capture the oil-air interface. A sliding mesh is established between the inner flow field and outer flow field. Moreover, the two design schemes of roller-sliding bearing are compared. A reasonable design scheme is obtained. The bearing rotation speed, oil velocity, oil viscosity, oil ratio, and oil temperature have a significant impact on the lubrication performance of the roller-sliding bearing. The results show that the oil distribution inside the bearing is uneven. The lowest oil volume fraction exists in the basin, which is near the upstream of the nozzle, and gradually increases from the inner raceway to outer raceway. The oil storing rate increases with the increase of oil viscosity. A novel method of oil volume prediction is proposed in a deep manner. It also provides some reference for the design of roller-sliding bearing and other bearings.

The aerodynamic performance of high-speed trains is closely related to their head shape. As each train set has its own modelling system, the aerodynamic shape design of head type of a high-speed train must follow a pedigree modelling approach. The train pedigree characteristics directly reflect its aerodynamic performance. The focus of this study is the extraction and application of the Shinkansen train pedigree features. First, large-scale differences in train head shapes are eliminated by dimensionless elimination. Second, the Shinkansen model pedigree features are extracted using similarity calculations. Finally, based on the constraint range of pedigree features on the new model shape, the concept head shape is designed with reference to the shape and surface parameters of high-speed trains, and its aerodynamic performance is verified. The results of the aerodynamic verification indicate that the concept design reduces the drag by more than 10% compared to the E2 Series base model. Additionally, the results demonstrate that the extraction and design application of train pedigree features can help a new train to develop its aerodynamic shape within its own pedigree modelling system, thus enabling the synergistic design of train shape and aerodynamic performance.