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Motivated by a bioinspired optimal aerodynamic design of a multi-propeller configuration, here we propose a ducted multi-propeller design to explore the improvement of lift force production and FM efficiency in quadrotor drones through optimizing the ducted multi-propeller configuration. We first conducted a CFD-based study to explore a high-performance duct morphology in a ducted single-propeller model in terms of aerodynamic performance and duct volume. The effect of a ducted multi-propeller configuration on aerodynamic performance is then investigated in terms of the tip distance and the height difference of propellers under a hovering state. Our results indicate that the tip distance-induced interactions have a noticeable effect in impairing the lift force production and FM efficiency but are limited to small tip distances, whereas the height difference-induced interactions have an impact on enhancing the aerodynamic performance over a certain range. An optimal ducted multi-propeller configuration with a minimal tip distance and an appropriate height difference was further examined through a combination of CFD simulations and a surrogate model in a broad-parameter space, which enables a significant improvement in both lift force production and FM efficiency for the multirotor, and thus provides a potential optimal design for ducted multirotor UAVs.

The S1 stream surface inverse method for a single-stage axial transonic compressor was developed and studied under the guidance of the quasi-three-dimensional viscous design theory, based on computational fluid dynamics (CFD), in order to establish and solve governing equations. The re--adjusted load distribution of the S1 stream surface was imposed to obtain a special profile for the rotor, which is known as S-shaping. This changed the structure and weaken the strength of the shock due to the pre-compression effect. In order to match the inlet flow angle of the downstream stator, inverse modification for multiple S1 stream surfaces of the stator blade was conducted in the present study at the same time. As it is known, the S-shaped profile is commonly applied to supersonic flows. Therefore, NASA stage 37 was selected as the design case to verify whether the present inverse method is effective and reliable. Stage 37 was re-designed by stacking five S1 stream surfaces. The profiles of these surfaces were renewed through the inverse design. The results revealed that, compared to the prototype, the aerodynamic performance of the re-designed one was apparently promoted within the stable working range. The adiabatic efficiency at the design point increased by approximately 0.4%, and the pressure ratio improved by approximately 4.5%. In addition, analysis result for the characteristic line revealed that the performance of the redesigned blades at off-design points significantly improved.

The development of high-performance aero engines put forward higher performance requirements for compressor components, and the improvement of aerodynamic performance of compressors has important engineering application value. The blade tip recess has great potential and advantages in improving the aerodynamic performance of compressors. In order to better understand the effect of the blade tip recess on the compressor aerodynamic performance, in this paper, the influence mechanism of the blade tip recess on the aerodynamic performance of the isolated rotor of a transonic axial compressor stage is discussed. Under the premise that the numerical method's results are almost consistent with the experimental test results, the full three-dimensional unsteady numerical results show that the main reason for the original blade rotor stall is the leading edge of the blade tip blockage, which is caused by blade tip clearance leakage vortex breakage. After adopting the measures of the blade tip recess, the study shows that the blade tip recess can increase the rotor stall margin by 2.10% without reducing the rotor efficiency and the total pressure ratio. A detailed analysis of the blade tip flow field shows that the blade tip recess can reduce the intensity of the tip clearance leakage flow by increasing the turbulence intensity of the blade tip near the casing wall, and reduces the leading edge of blade tip blockage, improves the rotor blade tip flow field, thereby achieving the purpose of enhancing rotor stability.

In order to improve the performance of a high-load transonic axial compressor, this paper proposes a method of applying endwall synthetic jet to the casing for active flow control. Taking NASA Rotor35 as the research object, the aerodynamic performance of the compressor is numerically calculated by applying three sets of synthetic jets with different excitation parameters at five different axial positions of 0%Ca, 25%Ca, 50%Ca, 75%Ca and 96.15%Ca. The results show that the three parameters of excitation position, jet peak velocity and jet frequency all have an effect on the performance of the compressor. The excitation position has the greatest influence on the flow margin of the compressor, and the best position is 25%Ca. After the jet peak velocity is increased from 100m/s to 150m/s, the flow margin, total pressure ratio and efficiency of the compressor are not greatly improved, which shows that the impact of the jet peak velocity is not as good as the excitation position. After continuing to increase the excitation frequency of the synthetic jet from 600Hz to 1200Hz, although the flow margin of the compressor is slightly reduced, the total pressure ratio and efficiency are further improved. This shows that there may be a threshold for the jet frequency, and only when the jet frequency is greater than the threshold can the overall aerodynamic performance of the compressor be improved.

The condensation of wet steam has important effects on the behavior of the flow field. To evaluate the aerodynamic performance of exhaust passage influenced by wet steam phase change condensation, a numerical investigation was conducted. Taking a 600 MW steam turbine as an example with consideration of the wet steam from the last stage blade and the steam exhaust of the BFPT (boiler feed water pump turbine), the governing equations of wet steam two-phase flow were adopted by the Eulerian-Eulerian approach. Results show that the wetness in the stator domain increases gradually while the wetness in the rotor domain varies little on the pressure surface and is in small increment on the suction surface. The velocity uniformity can be improved at condenser throat outlet as the mass flow or wetness increases. Moreover, the trend to improve the aerodynamic performance of exhaust passage benefits from the improvement of wetness at the last stage blade inlet. Conversely, with the increment of wetness at the BFPT inlet, the static pressure recovery coefficient reduces by 5.8% and the total pressure loss coefficient increases by 2.4%, resulting in a reduction of aerodynamic performance of exhaust passage.

The pantograph monitoring device on high-speed trains bears not only its own strength but also the aerodynamic load applied by the air flow when the train is running at high speed. A well designed shape of the pantograph monitoring device on high-speed trains reduces the loads and pressure fluctuations acting on it, and therefore, increases its function stability and life cycle. In this paper, we present an aerodynamic shape design method for such device. Firstly, an efficient and reliable numerical simulation approach is established for the evaluation of the aerodynamic loads acting on the device. According to the numerical computations, a basic shape for the monitoring device is formed, with which the minimum functional space of the device is reserved. Then, the corners of the basic shape are smoothed out with three types of continuous transitions. By comparing the numerical results of the three smoothed shapes, we obtain an optimal aerodynamic shape for the pantograph monitoring device. The design method is not limited to the monitoring device studied in this manuscript. The aerodynamic shape of other small functional devices on high-speed trains can also be generated or optimized with the method presented herein.

In this paper, the aerodynamic performance of the head car of a CRH2 train running in sandstorms was investigated. A numerical simulation method based on Realizable k-ε turbulence model was used to explore the flow features around the high-speed train. The accuracy of mesh resolution and methodology of CFD was validated by wind tunnel tests. A discrete phase model (DPM) was adopted to investigate the effects of sand particle properties (diameter and restitution coefficient) on the aerodynamic performance of the head car. Yaw angle effects with the sand-laden flow on the aerodynamic coefficient were also discussed. The results show that the drag force, lift force, lateral force, and overturning moment of the head car increase significantly due to the sand, and the sand particle properties have dominant effects on the aerodynamic performance of the head car. The impact probability of sand particles on the vehicle increases with the sand particle diameter and the yaw angle increasing. Larger restitution coefficients lead to lager forces of the head car, resulting in more contribution to the aerodynamic coefficients. Owing to the sand collision, a larger yaw angle causes more contribution to the aerodynamic performance of the head car, and the influence of sand properties on the drag force, lateral force and overturning moment are enhanced with the increase of the yaw angle. Using appropriate coatings around the high-speed train can not only reduce the energy consumption, but also improve the lateral stability and the critical operational speed of the high-speed train in the sandstorms.

All human actions necessitate energy resources. Currently, a major part of our energy requirements is supplied by fossil fuels, which are faced with uncertainties concerning their availability in the forthcoming decades. However, the combustion of fuels results in adverse environmental aftermaths. Energy of wind falls into one of the clean and renewable energy resources. Wind power is generated using horizontal and vertical axis wind turbines (HAWTs & VAWTs). VAWTs operate appropriately under low wind velocity conditions to generate power in small scales. On the other hand, numerous VAWT designs have been presented to enhance their performance in such circumstances. This study is aimed at developing cost-effective aerodynamic calculations models for both Gorlov and Darrieus straight-bladed VAWT types. Thus, DMST (double multiple stream tube) models, which act on the basis of BEM (blade element momentum) theory, have been designed for Gorlov and Darrieus VAWTs. By comparison of the results obtained with those available in the literature, the models developed are validated. Additionally, the performance of the Darrieus-type straight-bladed VAWTs was compared to that of the Gorlov VAWTs. According to the findings, although the peak power coefficient (C_P) decreased slightly for Gorlov VAWTs in comparison with the Darrieus straight-bladed type, the Gorlov rotor showed improved performance in terms of fluctuation and effectiveness based on the torque coefficient curve of helical blades.

The Actuator Disk Method (ADM), in its analytical formulation or combined with Navier-Stokes equations, is widely used for design and/or for aerodynamic analysis of Horizontal Axis Wind Turbines (HAWT). This method has demonstrated its capabilities for performance predictions of HAWT rotors for limited range of wind speeds with lower angles of attack values, i.e. attached flow conditions. However, for typical wind speeds that rotor encounters, under higher angles of attack i.e. stall conditions, such a method cannot describe accurately the flow characteristics around rotor-blades due to severe flow separations coupled with the effects of blades rotation as well as the radial flow over the blades. In this paper, original correction approaches have been proposed for the existing stall delay models to take into account both the blade rotation and the radial flow effects over the rotor blades. For this purpose, the ADM combined with 3D- NavierStokes equations formulation using Large Eddy Simulations (LES) model has been considered to describe the incompressible turbulent flow field around HAWT rotor blades. The resulting mathematical model has been solved using a 3D in-house subroutine developed with OpenFOAM code. The proposed numerical method has been validated against the well recognized reference measurements obtained using the New MEXICO and the NREL Phase VI experimental wind turbines. In comparison with existing stall delay models, the proposed correction approaches, especially the radial flow approach, have shown noticeable enhancements on performance predictions of HAWT rotors compared to the experimental measurements. It has been found very low discrepancies to the experimental torque and thrust values, up to 1% and 10% have been recorded respectively.

The braking system of a train, which is one of its key technologies, plays a vital role in ensuring the safe operation of the train. Plate braking can be used as a method for braking of a high-speed train during an emergency; the higher the train speed, the better the braking effect. In this study, dynamic grid technology is used to simulate plate movement, and the IDDES with the SST k-w turbulence model is used to simulate the unsteady aerodynamic characteristics of the train during the plate braking process. The aerodynamic force and torque obtained from the fluid simulation are then fed as an external load into the simplified center of the multibody system dynamics model of the high-speed train. Finally, the safety indices of the plate-braking train under crosswinds of different speeds are obtained. The results show that the rapid opening of the plate provides a large braking force to the train but destabilizes the flow field around and especially above it; this phenomenon is further aggravated by crosswinds. The aerodynamic force distribution of each car in the train is considerably changed by the crosswind, and the proportion of lateral force acting on the head car significantly increases. The aerodynamic forces acting on each car further increase due to the opening of the plate. From the perspective of vehicle dynamics performance, an increase in the crosswind speed leads to noticeable increases in the derailment coefficient and wheel load reduction rate of each car; derailment and overturn may also occur. Under crosswind, the head car presents the worst dynamic performance, which is further deteriorated by the opening of the plate. The impact of the tail car is relatively small. The dynamic performance of the head car can thus be used to evaluate train safety. As a precaution, the plate of the head car should never be opened during strong crosswinds.