Journal Of Applied Fluid Mechanics
Volume:17 Issue: 7, Jul 2024

In order to determine the most suitable turbulence model for studying the aerodynamic performance of bus, the effects of different turbulence models on the aerodynamic characteristics of bus were investigated. A comparative analysis was conducted on five turbulence models (IDDES, DDES, DES, LES, URANS). The pressure distribution on the cross section at x=0 and y=0 is also analyzed for each model. The results reveal that IDDES accurately captures the negative pressure at the rear of the bus and predicts the pressure gradients more effectively than other models. IDDES also captures more vortices at the head of the bus and predicts the wake flow more widely than other models. DDES has obvious shedding phenomenon in the wake flow, while IDDES provides a relatively smooth airflow trajectory, but its prediction of airflow trajectory at a distance is less clear. Through quantitative and qualitative analyses of the aerodynamic characteristics of bus under different turbulence models, it can be concluded that IDDES is the most suitable turbulence model to study the aerodynamic characteristics of bus.

The article presents a methodology for determining the hydraulic resistance multiplier, used for a rapid estimation of linear losses in pipes with non-circular cross-sections. The numerical approach was applied using the Finite Volume Method and the ANSYS Fluent software. The research was conducted under turbulent flow conditions, covering two Reynolds number ranges: 10,000 to 100,000 (10 cases) and 100,000 to 1,000,000 (5 cases). The first section of the article presents calculations of losses for a circular pipe, accompanied by a mesh test and error estimation. The second section includes calculations conducted for a series of pipes with various selected cross-sectional shapes: half-circle, quarter-circle, square, rectangles with aspect ratios of 2:1 and 3:1, isosceles triangle, and equilateral triangle. The last section of the article discusses the calculation of linear losses and the hydraulic resistance multiplier for each tested shape. It was found that this coefficient ranged from 1.33 to 2.2, depending on the shape, with the influence of the Reynolds number being relatively insignificant.

Accurate predictions of aerodynamic performance and near wake expansion around Horizontal Axis Wind Turbine (HAWT) rotors is pivotal for studying wind turbine wake interactions and optimizing wind farm layouts. This study introduces a novel engineering model centered on stall delay correction to enhance the precision of the Actuator Disk Method (ADM) predictions in both aerodynamic performance and near wake expansion around HAWT rotors. The model is developed based on a comprehensive study of the 3D lift coefficient evolution over the rotor blade, incorporating a shift parameter that considers both stall angle detection and radial decrement. The proposed approach demonstrates remarkable agreements, showcasing discrepancies as low as 7% for both loads and axial wake predictions. These quantifiable results underscore the effectiveness of the model in capturing intricate aerodynamic phenomena. Looking forward, the success of this approach opens avenues for broader applications, guiding future research in wind energy towards improved simulation accuracy and optimized wind farm designs.

The interior noise caused by the pantograph area is greater than that caused by other areas, and the impact of this pantograph area becomes more significant as the speed of high-speed trains increases, especially above 350 km/h. This study proposes an active jet method for pantograph cavities to control noise at the source. First, a predictive model for the interior noise of pantograph carriages was established by jointly adopting large eddy simulation–statistical energy analysis methods. Then, numerical simulations were conducted to determine the external noise sources and interior sound pressure level at different speeds (300, 350, 400, and 450 km/h). Finally, active jets at different speeds (97.2, 111.1, 125, and 140 m/s) were used to analyze the reduction in interior noise. Results showed that the active jet method decreased the average overall sound pressure level of the acoustic cavity in the horizontal plane. When the train speed reached 450 km/h, the optimal reduction in interior noise was approximately 7.5 dB in the horizontal plane for both the standing and sitting postures. The proposed method can efficiently reduce interior noise in the pantograph area.

The vertical drop is one of the most widely used hydraulic structures for dissipating the destructive energy of water. The purpose of this research is to investigate the effect of the two difference height, and five vertex angles of a triangular plan form vertical drop on energy dissipation and average velocity using the volume of fluid (VOF) method. The findings revealed that by decreasing vertex angle of the triangular plan form vertical drop, energy dissipation increases. The lowest relative depth of the pool occurs with this drop. In contrast, as the vertex angle of the triangular plan form vertical drop decreases, the average velocity at the foot of the drop increases and the maximum average velocity in the triangular plan form vertical drop with an angle of 60 degrees and a height of 0.2 m is higher than other models. The average downstream velocity also decreases by decreasing the angle  and this decrease is more intense in the center of the channel than on the sides.

Noise is one of the key indicators to evaluate axial flow fans, and in many cases, it is also the only indicator for determining their suitability for use. In this study, a new method to reduce axial fan’s noise was proposed for changing the section chord length to transform the blades of two axial fans with the same design parameters but distinct chord lengths to wavy blades. The aerodynamic calculations and noise reduction mechanism of the wavy configuration of the two fans were studied by combining CFD of large eddy simulation with the Lighthill acoustic analogy method. The results showed that the main mechanism contributing to noise reduction through wavy configuration was the promotion of transformation of the blade surface's layered vortex structure into an uncorrelated comb vortex structure. For fan blades with smaller chord lengths, the comb structure with low spanwise correlation was still maintained after the trailing edge, while for fan blades with larger chord lengths, the comb structure of the shedding vortex rapidly dissipated downstream of the trailing edge. Under the rated design conditions, the implementation of wavy leading edge blades resulted in noise reductions of 1.9 dB and 1.5 dB for the two fans, respectively, while wavy trailing edge blades yielded reductions of 2.6 dB and 2.1 dB, respectively. Furthermore, the adoption of wavy configuration induced a phenomenon of pressure increase and efficiency decrease in both axial fans at medium and low flow rates, with minimal impact at high flow rates. These outcomes underscored the superior noise reduction efficacy of the wavy trailing edge blades, offering a promising way for the noise reduction design of axial flow fans.

Today, due to advances in computing power, Reynolds Averaged Navier-Stokes (RANS) solvers are widely preferred for quasi-three-dimensional (Q3D) blade-to-blade analysis. This study investigates the performance of different flux calculation methods and turbulence models with a density-based RANS solver (Numeca®) in blade-to-blade analysis. A block-structured mesh topology is used to create a solution grid around the airfoil. Spatial discretization is performed in the pitchwise direction to represent the quasi three-dimensional flow, while only one computational cell is used in the radial direction to simulate the flow through the Q3D cascade. The computational grid around the airfoil is created with the Autogrid® tool using the block mesh topology. For the convective flow calculations, both the central and upwind methods available in Numeca® are applied separately. The Baldwin Lomax (BL), Spalart Allmaras (SA), Shear Stress Transport (SST), Explicit Algebraic Reynolds Stress Model (EARSM) and k-ε (KEPS) turbulence models are used for the turbulent shear stress calculations. In order to evaluate the aerodynamic performance of the spatial discretization methods and turbulence models, the isentropic Mach distribution on the airfoil surface, the total pressure loss and the exit flow angle behind the blade are compared with the experimental data of six test cases. In the compressor cases, the Spalart-Allmaras turbulence model with the Central scheme gives the best results in terms of average loss prediction, while no turbulence model is superior to the other in terms of exit angle prediction. On the turbine side, EARSM and KEPS give better performance in terms of loss prediction for the low Reynolds case compared to others, while the Spalart-Allmaras turbulence model is better for the high Reynolds cases.

In this work, to comprehensively analyze the flow field characteristics of a normal slot plasma synthetic jet actuator, three-dimensional simulation models are established for both normal slot and normal orifice actuators. A detailed comparative analysis of the three-dimensional flow field characteristics of these two actuators is performed. The results indicate that the motion shockwaves and jets generated by the normal slot actuator cover a larger and more uniform region, showing planar characteristics and excellent flow control uniformity. The total pressure ratio for the normal slot actuator is 3.59, significantly higher than the value of 3.50 for the normal orifice actuator, indicating lower pressure loss in the former. Additionally, the normal slot has a larger average exit Mach number (Ma), indicating a stronger flow control capability. It also achieves the peak Ma in a shorter time, indicating a faster momentum output response. Therefore, compared with the normal orifice actuator, the normal slot actuator has better potential for flow control.

Stall, a complex flow phenomenon in centrifugal pump, plays a crucial role in pump safety and stability under part-load conditions. In this paper, a verified numerical simulation method is employed to analyze the three-dimensional flow field under the stall inception conditions. The results reveal the initial stall vortex occurs near the Q=0.7Qd condition in the prototype impeller. Based on stall formation mechanism, the high-velocity fluid near the blade pressure side is sucked into suction side of next impeller channel by setting a groove near the blade leading edge. This jet flow can prevent the narrow vortices near the impeller shroud from moving towards the blade suction side, thereby suppressing the formation of stall vortex. By comparing the effects of different groove locations, directions, and sizes on stall vortex control, the optimal groove width is determined to be approximately 1mm. Compared with the prototype impeller, the grooved impeller can completely eliminate the stall vortex and significantly reduce pressure pulsation under part-load conditions. Moreover, the head of grooved impeller is increased by nearly 15% under Q=0.6Qd condition, and the potential suppression mechanism is also explained. Based on the stall formation mechanism, this paper puts forward an effective stall control method, which delays the stall inception significantly.

This paper presents a comprehensive investigation of flow-induced noise characteristics in ethylene cracking furnace tubes, covering both pre- and post-coking conditions. Large-eddy simulation (LES) was employed in conjunction with a generalized Lighthill’s acoustic analogy model. The results indicate that noise sources can be classified as dipole acoustic sources, with energy primarily concentrated ranged from 300 to 1500 Hz, in comparison to standard conditions. The primary location of the acoustic source was identified in the region commonly referred to as the “necking” of the furnace tube, demonstrating a strong correlation with turbulence intensity near the tube wall. As the coke layer thickness in the furnace tube increased from 5 mm to 15 mm, both the sound power level and turbulence intensity exhibited significant growth. Specifically, the sound power level increased by 60.5% while the turbulence intensity increased by 58.5%. Variations in the overall sound pressure level (OASPL) curve measured within the tube could be utilized to assess coking levels. Significant peaks in the OASPL curve were observed as the furnace tube underwent substantial coking, with coke layer thicknesses of 10 mm and 15 mm. The corresponding OASPL values recorded were 79.25 dB and 119.08 dB, respectively. The findings of this work offer significant insights that may contribute to enhanced safety measures in the operation of ethylene cracking furnace tubes.

This study investigates pressure gradient dynamics within a porous medium in the context of two-phase fluid flow, specifically water and sand particle interactions. Using experimental data, we refine pressure correction coefficients within a numerical solution framework, employing the Semi-Implicit Method for the Pressure-linked Equations algorithm. Our findings highlight the relative nature of pressure gradient phenomena, with particle size and volume fraction emerging as crucial determinants. Graphical representations reveal a clear trend: an increase in volume fraction, up to 40%, across varying Reynolds Numbers, leads to a transition towards non-Newtonian behavior in the two-phase fluid system. Unlike the linear pressure gradient seen in single-phase fluid flow, the interplay between liquid and solid phases, along with drag forces, imparts a distinctly nonlinear trajectory to the pressure gradient in two-phase fluid flow scenarios. As the two-phase flow enters a porous medium, numerous factors come into play, resulting in a pressure drop. These factors include changes in cross-sectional geometry, alterations in boundary layer dynamics, and ensuing momentum fluctuations. Interestingly, an increase in porosity percentage inversely correlates with pressure gradient, resulting in reduced pressure gradient with higher porosity levels.

The boundary layer's separation loss in compressor cascades constitutes a significant portion of profile loss, critically influencing aerodynamic performance optimization and control. This study employs Large Eddy Simulation (LES) to examine separation losses at varying attack angles, focusing on a rectangular compressor cascade. Specifically, it explores the long separation bubble at a 45% blade height cross-section under designed incidence. Analysis of the separation bubble's transition process revealed a notable surge in total pressure loss rate prior to transition, which stabilized following reattachment. The study thoroughly investigates the evolution of long bubbles, employing quadrant analysis of Reynolds stress, critical point theory, and an in-depth examination of individual vortex dynamics. The findings indicate that the peak of cross-flow within the separation bubble acts as the primary mechanism initiating the transition. This insight is corroborated by DNS calculations of natural transitions on flat plates. Building upon these findings, the study discusses the effects of varying attack angles on transition processes. Notably, increased incidence prompted the upstream migration of the long separation bubble, transforming it into a short bubble at the leading edge. This shift led to a fivefold increase in separation loss and doubled the frequency of transverse flow fluctuations.

Air can have an adverse effect on the performance of an aero-engine lubrication system. A numerical analysis was conducted to explore the influence of inlet void fraction and pipe layout on the characteristics of oil-gas two-phase flow in a 90° elbow. The pipes were arranged horizontally and vertically with inlet void fractions of 0.05-0.15. The laws governing flow velocity, void fraction, and pressure along the pipe were determined separately. The results revealed the formation of large-scale vortices with high gas volume fractions inside both types of elbows, which exacerbate oil-gas separation and cause additional head loss. The maximum pressure drop was observed at approximately one pipe diameter downstream of the elbow outlet, which initially increases with the inlet void fraction and then gradually stabilizes. Asymmetric secondary flow vortices in the horizontal elbow were found to enhance oil-gas separation and accelerate lubricating oil to greater extent than in a vertical elbow under the same conditions. Consequently, the maximum pressure drop caused by flowing through the horizontal elbow is higher than that in the vertical elbow.

The pantograph is a critical instrument that significantly affects the aerodynamics of high-speed trains, posing a considerable challenge to the energy conservation and environmental protection of trains. This study explores the feasibility and efficiency of a jet-flow control technique in optimising the aerodynamic characteristics of the pantograph. A numerical method was adopted to investigate the effects of various jet-flow parameters, such as the jet positions, velocities and jet-slot widths, on the flow changes around the pantograph and subsequent reduction in aerodynamic drag of the pantograph. The results show that the impact of the jet position is negligible when the jet velocity is lower than the train speed. The aerodynamic drag reduction rate decreased with increasing distance from the pantograph as the jet velocity increased. When the distance between the jet slot and pantograph is less than 0.6 times the height of the pantograph, the aerodynamic drag reduction rate continuously increased with the jet velocity. As the jet slot moved away from the pantograph, the aerodynamic drag reduction rate initially increased rapidly with the jet velocity and then gradually decreased when the velocity surpassed 1.2 times the train speed. In addition, the aerodynamic drag of the pantograph decreased as the width of the jet slot decreased. However, the energy of the whole train can be only saved when the jet velocity is below 0.6 times the train speed. Findings in this study verified the effectiveness of the jet-flow method in reducing the aerodynamic drag of pantographs and provide important engineering guidance for the energy-saving of high-speed trains.

This study presents an efficient surrogate-based optimization (SBO) method of the aerodynamic performance of a contra-rotating open rotor (CROR). The objective was to maximize propulsion efficiency while reaching the target thrust coefficient at the cruise condition. To reduce the sample size and improve the optimization convergence speed, an infilling criterion was proposed based on the features of the interaction between the CROR front and rear rotors. The efficient front and rear rotors of the initial samples were selected and then combined to form the infilled samples. The results show that the infilled samples were closer to the Pareto front than the initial samples. For the six optimization parameters, 20 initial sample points were used, 11 samples were infilled, and the surrogate-based optimization was completed in five iterations. In total 43 samples were calculated during the optimization. The number of overall samples is approximately seven times the number of optimization parameters. The optimization results in parameter changes compared to the baseline and improved propulsion efficiency while meeting the thrust target. The optimization process increases the torque share of the rear rotor and changes the flow state at different radial positions, leading to a more uniform total pressure distribution at the outlet position, both circumferentially and radially.