Low-pressure Vortex Research Articles

Analysis of Flight Mechanism and Fluid Dynamics Based on Dandelion Model

The elevation angle of dandelion crown hairs is investigated by controlling the angle of the fan to investigate the stability factors of dandelion flight, and the theoretical knowledge is deduced and analyzed to get the conclusion. Taking the structure of badminton ball as the basic reference, three motion states of dandelion are set, and the exit wind speed of the wind tunnel is measured respectively. Through Fluent flow field analysis, a low-pressure vortex detached from the body exists above the center of the dandelion model during flight, and a high-pressure airflow field exists below, the size of which decreases with the increase of elevation angle. The velocity field is unfolded by the semicircular arcs on both sides of the vortex and decreases gradually, and the velocity maxima appear at the left and right vertices of the elliptical vortex, and the range of the semicircular arcs of the velocity field increases gradually with the increase of elevation angle. In the elliptical vortex, there are two vortices with different velocity directions, the left velocity direction is clockwise and the right is counterclockwise.

Validation of numerical simulations and experiments on impulse characteristics induced by self-excited oscillation

The high-frequency pulse flow, equivalent to the natural frequency of rocks, is generated by a self-excited oscillating cavity to achieve resonance rock-breaking. The flow field and oscillating mechanism of the self-excited oscillating cavity were simulated using the large eddy simulation method of Computational Fluid Dynamics (CFD). A field-scale testing apparatus was developed to investigate the impulse characteristics and verify the simulation results. The results show that the fluid at the outlet at the tool is deflected due to the pulse oscillation of the fluid. The size and shape of low-pressure vortices constantly change, leading to periodic changes in fluid impedance within the oscillating cavity. The impulse frequency reaches its highest point when the length–diameter ratio is 0.67. As the length–diameter ratio increases, the tool pressure loss also increases. Regarding the cavity thickness, the impulse frequency of the oscillating cavity initially decreases, then increases, and finally decreases again. Moreover, both the impulse frequency and pressure loss increase with an increase in displacement. The numerical simulation findings align with the experimental results, thus confirming the validity of the theoretical model. This research provides theoretical guidance for the practical application of resonance rock-breaking technology.

Energy Performance Prediction Model for Mixed-Flow Pumps by Considering the Effect of Incoming Prerotation

Abstract Mixed-flow pump is one of the most broadly applied sorts of power equipment in the field of petrochemical and water conservancy. The effect of inlet prerotation on the energy characteristics and operational stability of a pump is a significant consideration. The aim of this study is to analyze the relationship between inlet prerotation and the total energy consumption of a mixed flow pump by developing a predictive model. The impact of prerotation on the pumping performance and energy conversion for a typical mixed-flow pump has been investigated by a combined approach of theoretical derivation, numerical simulation, and experimental verification. Validation of the numerical methods was achieved by comparing the results to the experimentally obtained data. A prediction model was developed for head and power, which incorporated inlet prerotation. The study utilized a mathematical model and numerical simulation to compute the head and power output of a mixed-flow pump for a wide range of inlet prerotation angles. The results of the two methods were highly consistent. Moreover, the effects of prerotation on the flow structure of the mixed-flow pump were analyzed. It was found that prerotation led to an increase in the incoming flow angle, resulting in unstable flow patterns causing secondary flows and low-pressure vortex in the impeller flow path. This induced a rise in energy consumption of the impeller. The prediction model and analysis of the internal flow structure provide a theoretical foundation for predicting the hydraulic performances of mixed-flow pumps under prerotation conditions and improving their stability of operation.

Research on Aerodynamic Test Validation and the Vector Force Control Method for an E-STOL Fan Wing Unmanned Aerial Vehicle

The concept of the Fan Wing, a novel aircraft vector-force-integrated device that combines a power unit with a fixed wing to generate distributed lift and thrust by creating a low-pressure vortex on the wing’s surface, was studied. To investigate the unique propulsion mechanism of the Fan Wing, a Fan Wing test platform was developed, and experiments were conducted in a wind tunnel. At the same time, numerical simulations were established. In order to further improve the aerodynamic efficiency of the Fan Wing and decouple the control of lift and thrust, an improved scheme for the leading-edge structure of the Fan Wing was proposed, and a numerical analysis was conducted. A Fan Wing unmanned aerial vehicle (UAV) was designed and manufactured using the Fan Wing as the source of lift and thrust for the aircraft, and flight verification was conducted. The wind tunnel tests have proven that the main factors influencing the lift and thrust of the Fan Wing are rotation speed of cross flow fan, angle of attack, and incoming flow. The numerical analysis results of slotting on the leading edge show that the lift and thrust of the Fan Wing can be improved, but also the strength and position of the low-pressure vortices can be controlled. The results of flight tests show that the distributed lift and thrust of the Fan Wing can be directly applied to aircraft without the need for additional propulsion devices. In summary, the aerodynamic characteristics of the Fan Wing can be applied to electric short takeoff and landing (E-STOL) scenarios in urban air traffic.

Vortex dynamics characteristics in the tip region based on Wray–Agarwal model

In order to solve the blockage effect and energy dissipation phenomenon caused by cavitation in the low-pressure vortex core region, this paper analyzes the spatial evolution of vorticity intensity and turbulent kinetic energy intensity under different cavitation conditions based on the Wray–Agarwal (WA) model. First, the tip leakage flow characteristics are studied, the evolution of vorticity and vorticity intensity is analyzed, then the distribution of turbulent kinetic energy distribution in the blade tip region is studied, and finally, the vorticity transport characteristics of the tip region are analyzed. It is found that the tip leakage rate is less affected by the vortex cavitation of the tip leakage, and there is a strong interaction between the leakage flow at the tip leading edge and the trailing edge, and the separation vortices and low-speed regions formed in the end-wall region cause blockage of the flow passage. Low pressure causes cavitation to cover most regions of the suction surface, inhibiting the formation and development of the tip leakage vortices. The distribution range of high turbulent kinetic energy region is almost the same as that of high-vorticity region, and there is a positive correlation between the two intensities. Severe cavitation causes the high turbulent kinetic energy region at the outlet of the flow passage to develop in the radial and axial directions of the impeller, which increases the turbulent dissipation and energy loss. The change of vorticity transport intensity caused by cavitation is mainly reflected in the expansion contraction term, and the Coriolis force term plays a dominant role in the vorticity transport process. This paper provides a reference for further improving the performance of mixed-flow pumps.

Numerical investigation of the effect of dielectric barrier discharge plasma actuator-induced momentum jet parameters on flow control of an oscillating wind turbine airfoil

Wind turbine blades that undergo pitch-oscillating motion exhibit dynamic stall behavior, deteriorating their aerodynamic performance. In this regard, dielectric barrier discharge (DBD) plasma actuators are promising tools for controlling flow by inducing momentum jet flow. Four key parameters typically determine the momentum jet: length, power, angle, and injection location. This paper presents a numerical study that investigates the effect of these parameters on flow control around an SG6042 wind turbine airfoil at a Reynolds number of Rec=1.35×105. For this sake, the study considers various numbers of actuators, force directions, and installation locations. This study utilizes two-dimensional, unsteady Reynolds-averaged Navier–Stokes equations with the γ–Reθ transition model. The results demonstrate the significant effects of momentum jet parameters on flow control. As the location of single-DBD moves closer to the leading edge, its effectiveness on the low-pressure vortex growth increases, resulting in a smaller vortex and a lower drag coefficient. Furthermore, an increase in the power and length of the jet leads to effective flow control. Vortices on the airfoil's suction surface are recognized as influential factors in the aerodynamic performance. As a result, the co-flow actuator significantly improves the performance of the airfoil by inducing the momentum jet in this region. Flow control is augmented when the actuator is installed at a location with a near-surface jet angle. The leading edge case with a co-flow single-DBD achieves the best control performance. In this instance, the dynamic stall occurs approximately 5% later than in the case of the clean configuration.

Characteristics of water vapor isotopes and moisture sources for short-duration heavy rainfall events in Nanjing, eastern China

Short-duration heavy rainfall causes severe urban flooding, threatening urban security and socio-economic development. The lower reaches of the Yangtze River are one of the regions with the highest frequency of short-duration heavy rainfalls in China. In this study, hourly stable isotope compositions in water vapor (δ18Ov and d-excessv) are analyzed for nine short-duration heavy rainfall events during the summer monsoon season (June to September) from 2013 to 2021 in Nanjing, eastern China. The circulation patterns that lead to these events can be divided into four types: tropical cyclone, low-pressure vortex, cold front, and western North Pacific subtropical high. During these events, δ18Ov is enriched, ranging from −18.8 ‰ to −13.7 ‰ with a pre-storm increasing trend. This is largely caused by strongly isolated meso- and small-scale convections and the close proximity of oceanic moisture sources. The d-excessv shows three different variation patterns during rainfall events: increasing, decreasing, and irregular fluctuations. They each correspond to the increasing contribution of terrestrial moisture from eastern China, proximal oceanic moisture from China’s offshore waters (including the South China Sea, East China Sea, and Yellow Sea), and the mixing influence of these two moisture sources. Multiple patterns of the d-excessv variations reflect both importance of terrestrial moisture from eastern China and oceanic moisture from China’s offshore waters for summer short-duration heavy rainfall events in eastern China.

Flow Characteristics and Parameter Influence of the Under-Expansion Jet on Circulation Control Airfoil

The enhancement in the jet pressure ratio and jet velocity contributes to expanding the control efficiency and control boundary of circulation control airfoil under high subsonic incoming flow. However, because of an excessive jet pressure ratio, the jet separates prematurely on the Coanda surface, resulting in control failure. In a bid to improve the adhesion capability of the jet under a high pressure ratio, a circulation control airfoil with a converging nozzle and back-facing step structure at the trailing edge was numerically simulated based on the Reynolds averaged Navier−Stokes equation (RANS), and a study was conducted on the complex flow structure of the under-expansion jet on the Coanda surface and the impact of design parameters such as jet pressure ratio, ellipticity, and nozzle height on the jet separation. The results show that the back-facing step provides an expansion space for the under-expansion jet and changes the shock-boundary layer interaction form. As the jet pressure ratio and nozzle height increase, the size of the shock cell increases, the strength of the intercepting shocks on both sides increases, and Mach reflection occurs, resulting in jet stratification and in a decline in the adhesion capability of the jet. The combination design of proper ellipticity and the back-facing step contributes to forming a closed low-pressure vortex area behind the step and promote jet attachment. Reducing the nozzle height can improve the adhesion capability of the jet under a high pressure ratio.

The Influence of Tip Clearance on the Performance of a High-Speed Inducer Centrifugal Pump under Different Flow Rates Conditions

The influence mechanism of the blade tip clearance (TC) of an inducer on the performance of a centrifugal pump at high speed was researched under different flow rate conditions in this work. An experiment on the pump’s external performance was carried out, and numerical calculation was also performed under four different TCs. The full characteristic performance curves, static pressure and pressure pulsation distributions of the pump were obtained. Through the research and analysis, it was found that the influence of the TC on the efficiency and the head of the centrifugal pump are related to the flow rate. Under the influence of a large flow rate, the increase in the TC is helpful to improve the efficiency and the head of the pump. The increase in the TC helps to weaken the gap jet effect on the inducer. The inlet jet of the inducer, caused by TC leakage, will form a low-pressure vortex zone at the inlet of the inducer. The splitter-bladed inducer’s pressure pulsation is affected by the TC. The peak pressure pulsation at the monitoring point at the short blades is larger than that at the long blades. With the increase in TC, the cavitation degree at the inlet of the long blade of the inducer is decreased, while the cavitation degree at the short blade is deepened. It is also found that the TC has little effect on the radial force of the inducer and the impeller. These results will provide the design basis for the tip clearance of an inducer.

Two-dimensional characterization of particle entrainment over a downstream obstacle

This study conducts a two-dimensional numerical investigation of the particle entrainment in the presence of a large downstream circular obstacle, and aims to deepen the understanding of the entrainment mechanism and propose a dimensionless number to describe the entrainment threshold. With the coupled lattice Boltzmann and discrete element method, direct numerical simulations are conducted for a wide range of obstacle-to-particle radius ratios ( 2 < R o / R p < 8 ) and Reynolds numbers ( 38 ≤ R e ≤ 160 ). The results show that the low-pressure separated vortex due to the obstacle is the fluid structure to entrain the particle by vortex stretching at low R o / R p ( 2 < R o / R p < 3 . 8 ). Based on this mechanism, a dimensionless number defined as the ratio of the vertical drag force to the particle gravity is proposed, which can be understood as a generalized Shields number including the additional effect of the obstacle radius, and it approaches the constant value of 2.5198 at the critical conditions of entrainment. In addition, we find that at high R o / R p ( 5 . 6 ≤ R o / R p ≤ 8 . 0 ), the particle is too far from the separated vortex to undergo sufficient vertical drag force, and thus its inertia should be small enough to ensure the entrainment. In this situation, the entrainment threshold is well described by the Stokes number, which approaches the constant value of 0.2134. At the moderate R o / R p ( 3 . 8 ≤ R o / R p ≤ 5 . 6 ) the drag force of separated vortex and the low particle inertia are both essential to realize the particle entrainment. • Particle entrainment over a downstream obstacle is investigated by LB-DEM. • Two kind of entrainment mechanisms are identified. • Two dimensionless numbers are proposed to describe critical entrainment condition.

Interaction of a rigid buoyant sphere and a deforming bubble with a vortex ring: The role of deformability

Bubbly turbulent flows involve complex interactions between the water and the bubbles in which bubble deformation is known to play an important role. We experimentally investigate an idealization, namely, the interaction of a deforming air bubble and a rigid buoyant particle (non-deforming bubble) with a single vortex ring. On the bubble/particle dynamics side, after capture by the low-pressure vortex core, the bubble undergoes large azimuthal expansion within the ring, unlike the particle. This difference in the shape of the bubble compared to the particle, within the ring, leads to distinct differences in the ring time evolution, which brings us insight into the role of deformability in these interactions.

Optimization of wind resistance in commercial vehicles with consideration of sunroof buffeting noise

The aim of this study was to reduce the aerodynamic resistance coefficient of heavy commercial vehicles and buffeting noise that occurs when a sunroof is opened. Computational fluid dynamics was used to establish a numerical simulation model of wind resistance and buffeting noise. Analysis shows that the main influences on aerodynamic resistance are the airflow impact on the roof dome and the wake vortex behind the cab. Sunroof buffeting noise is caused by a low-pressure turbulent vortex generated at the forehead of the cab and falls off at the sunroof. A strategy for reducing the wind resistance coefficient and sunroof buffeting noise is proposed, which involves optimizing the sun visor installation angle and roof dome shape. The optimal strategy is to increase the sun visor installation angle by 4° and use the Scheme 2 roof dome. After optimization, the aerodynamic resistance coefficient of the commercial vehicle was reduced by 2.07% and the sound pressure level of sunroof buffeting noise was reduced by 18.3 dB. Hence, the effect of optimization was obvious. This work provides guidance for reducing drag and buffeting noise in commercial vehicles.

Low- and mid-frequency wall-pressure sources in a turbulent boundary layer

Abstract

Retrieval of rainstorm similarity system based on deep learning

Abstract Weather map contains abundant meteorological information.The analysis of high-altitude low-pressure vortex plays a crucial role in rainstorm weather forecast. Using the graphical and numerical information of the low-pressure vortex, the similar weather system in the historical storm database can be searched for the accurate prediction of weather conditions.A deep learning convolutional neural network and YOLO V3 algorithm are studied. A similarity search algorithm for weather systems based on deep learning and high-altitude low-pressure vortex is designed.The method first pre-processes the height field meteorological data and generate a height field contour image;Then use YOLO V3 algorithm to locate and identify the vortex in the image and superimpose the wind field to remove the high-voltage vortex.Finally, the center position, center strength, system intensity, and influence region information of each low-pressure vortex are calculated and used for similar retrieval of weather systems.The experimental results show that the method works well and it provides a great help for the accurate prediction of heavy rain.

Study on velocity and pressure characteristics of self-excited oscillating nozzle

The mechanism of the self-excited oscillation pulsed jet is still in a period of continuous improvement, and the application breadth and depth of the self-excited oscillation nozzle are significantly limited since the turbulent flow field inside the self-excited oscillation nozzle is exceptionally complex. In this study, computational fluid dynamics numerical simulation and experimental research methods are combined. By changing the structural parameters of self-excited oscillating nozzles, the velocity and pressure characteristics of the self-excited oscillating nozzles were compared and analyzed to determine the important factors that influenced the effect of pulsed jet. These findings demonstrated that the periodic expansion and contraction of the low-pressure vortex ring in the self-excited oscillation nozzle chamber will cause the pressure of the nozzle outlet section to change periodically. The periodic high-voltage blocking and low-pressure speedup of the outlet directly correlate with the pulsed jet. This study shows the reasonable size design range of the self-excited oscillation nozzle, which can guide the design and practical application of the nozzle, as well as provide the theoretical basis for perfecting the self-excited oscillation pulsed jet theory.

Transient characteristics during the co-closing guide vanes and runner blades of a bulb turbine in load rejection process

The transient characteristics associated with the machine load rejection process are investigated due to its close relationship with hydraulic facilities’ safety in low-head tubular turbines, especially in double-regulated bulb turbine. Load-rejection transient characteristics of a double regulated bulb turbine during the co-closing guide vanes (GVs) and runner blades (RBs) procedure were numerically and experimentally studied in this paper. The co-adjusting of GVs and RBs is realized through the usage of dynamic mesh technology. During the process, the speed is augmented by 33.9% by the time it reaches its peak value, where the corresponding maximum negative axial force is 43% of the initial value. The three-dimensional (3D) unsteady numerical simulation method reveals that severe impact of backflow from RBs causes significant fluctuation of torque and axial force during the co-closing procedure. A low-pressure vortex rope is also cut off by gravity-dominant pressure distribution, which leads to hydraulic instability and complex flow structure. The system performance characteristics have been obtained through experimental testing, where the dynamic instability during the co-closing procedure has been showcased. This article presents a novel prediction model and a foundations for further transient characteristics studies and prediction for turbine system with adjustable RBs and GVs.

Identification of the vortex around a vehicle by considering the pressure minimum

This paper proposes a method to identify low-pressure vortices with swirling motions around a vehicle by considering the two-dimensional pressure minimum. The existing sectional-pressure-minimum-and-swirl method combined with the finite difference method can be used to identify low-pressure vortices with swirling motions in homogeneous isotropic turbulence. To apply this method to the flow field around a vehicle, a method that extends the existing method to the finite volume method on unstructured grids and prevents the fragmentation of the vortex core lines was developed. To verify the proposed method, it was applied to the von Karman vortices of the square cylinder on the unstructured grids. The results indicate that the von Karman vortices, which involve low-pressure vortices with swirling motions, could be effectively captured using the proposed method. Finally, the proposed method was applied to the flow field around a vehicle. Compared with the existing method, the proposed method could better prevent the fragmentation of the vortex core lines. In addition, four known vortex structures around the vehicle could be identified by using the proposed method in combination with the isosurface method. Compared to other generally used methods in the field of vehicle aerodynamics, the proposed method could better identify the vortex core lines within a few minutes. These results demonstrate that the proposed method is effective for identifying the vortices around a vehicle.

Numerical Simulation of the Effect of Different Number Leading Edge Winglets on the Fan-Wing Aerodynamic Characteristics

The generation of lift and thrust mainly depends on the formation of low-pressure vortices above the arc groove on the leading edge of the Fan-wing, which makes the lift and thrust have a strong coupling relationship. How to decouple and control the lift and thrust is the key to further engineering application of the Fan-wing. Normally, the geometric parameters of the Fan-wing airfoil were determined; the leading edge opening angle has the greatest influence on the aerodynamic performance. Therefore, the method of installing leading edge winglets on the leading edge of a base Fan-wing airfoil was considered to change the opening angle of the leading edge of the Fan-wing. Through numerical simulation, the effects of single, double, and triple leading edge winglets on lift and thrust of the Fan-wing at different installation angles, inflow velocities, and angles of attack were compared and analyzed. The results show that by controlling the angle of the leading edge winglet, not only the lift and thrust of the fan can be improved but also the strength and position of the low-pressure vortices can be controlled, so as to meet the active control requirements of the aerodynamic moment of the Fan-wing, and then the attitude of the Fan-wing aircraft can be controlled.

Effect of fuel jet arrangement on the mixing rate inside trapezoidal cavity flame holder at supersonic flow

Fuel mixing inside the supersonic combustion chamber is a significant process for development of modern scramjets. In this article, computational fluid dynamic (CFD) approach is applied to investigate the effect of various fuel injections on the mixing rate inside the supersonic combustion chamber. The mixing of hydrogen jets with four different arrangements inside the cavity flame holder is comprehensively studied. In order to examine the effect of multi jets within a cavity flameholder, a three-dimensional model is established and Navier-stocks equations are solved to simulate the flow and mixing zone inside a cavity region. Obtained results show that the injection of hydrogen jet from the bottom of cavity flame holder considerable enhances the ignition zone within the cavity. Moreover, the backward fuel injection is more superior to forward fuel injection since low-pressure vortex could significantly distribute the fuel and enlarge the mixing zone inside the cavity flame holder.

Enhancing the penetration stability of plasma jet in liquid by improving the chamber structure

Two kinds of chamber structures were designed to improve the penetration stability of plasma jet in the working fluid. They are the cone-truncated chamber with an inclined wall structure and the stepped-wall chamber with an abrupt expanding structure. Based on the experiment, the penetration characteristics of plasma jet in liquid were investigated by the numerical calculation, especially the multiphase flow field structure and the jet penetration stability. In the cone-truncated chamber, the arc-like pressure wave does not evolve into the plane wave like the wave evolution rule in the traditional cylindrical chamber, and an alternating structure of arc-like wave and the plane wave is formed in the stepped-wall chamber. The stability control of stepped wall is more significant than that of inclined wall because the low-pressure vortex formed at the corner of step enhances the radial induction effect. Besides, the stepped-wall structure can restrain the moving randomness of the big vortex's core. The axial expansion speed of plasma jet in the ambient fluid is decreased exponentially and the speed estimation formula was obtained in this work.