Tail Vortex Research Articles

Flow Characteristics and Pressure Pulsation Analysis of Cavitation Induced in a Double-Volute Centrifugal Pump

Cavitation is a complex multiphase flow phenomenon, and the generation of transient phase transitions between liquid and vapor during cavitation development leads to multi-scale vortex motion. The transient cavitation dynamics and centrifugal pump’s rotor–stator interaction will induce pressure fluctuations in the impeller and the volute fluid of the centrifugal pump, resulting in a complex flow field structure. Based on the Schnerr–Sauer cavitation model and SST k-ω turbulence model, this paper studies the transient characteristics of the cavitation-induced unsteady flow in the centrifugal pump and the excitation response to the pressure pulsation in the volute under different flow conditions, taking the large vertical double-volute centrifugal pump as the research object. The results indicate the following: As the impeller rotates, in the external excitation response, the jet-wake flow structure at the centrifugal pump blade outlet shows an increase in the blade frequency signal. This is evident near the measurement points of the volute tongue and separator. When severe cavitation occurs, the maximum amplitude at the blade frequency in the volute shifts from the pump tongue (30°) to the downstream of the tongue (45°). The value of fpmax is 3.1 times that when NPSHa = 8.88 m. By applying the Omega vortex identification method, it can be seen that the interaction between the vortices at the blade trailing edge and the stable vortex in the volute tongue undergoes a process of elongation, fusion, separation, and recovery. This represents the downstream influence of the impeller on the volute. When Q = 0.9Qd, the process of the blade passage vortex tail detaching and dissipating in the impeller flow path can be observed, demonstrating the upstream influence of the volute on the impeller.

Experimental research on pulsation shedding characteristics of tail cavities attached to underwater vehicles based on the U-net cavity boundary identification method

Tail cavities are separate bubbles of initially non-condensable gas attached to the bottom of the vehicle, affecting the dynamics characteristics of the vehicle. The evolution of the tail cavity under various launch pressures is experimentally simulated, and the pulsation shedding characteristics of the tail cavity are investigated with the U-net method. Additionally, the influence of the tail shape on the bubble-carrying capability of tail cavity is discussed. Research shows that gas break-off and environmental pressure reduction induce a quasi-periodic pulsation process in the tail cavity, maintaining an approximately fixed length. Due to the asymmetric flow disturbances and the formation of an end closure by bubble cluster, the tail cavity shedding is transformed from regular vortex ring to hairpin vortex. The cavity volume pulsation frequency is consistent with cavity length. The continuous expansion of the cavity induced by environmental depressurization compensates for the loss of cavity volume due to hairpin vortex shedding. Increasing the launch pressure leads to a change in the tail vortex morphology, but does not significantly increase the size of tail cavity. The bottom extension can improve the bubble-carrying capacity and effectively solve the exit angle and air column caused by the increase of launch pressure.

Numerical study of vortex–bubble interaction mechanism in bubble breaker using the volume of fluid and large eddy simulation method

Effectively removing or reducing the number of bubbles doped in a material is of great significance to stabilize product characteristics and improve production safety. In this paper, the volume of fluid method and large eddy simulation method are used to numerically simulate the bubble dynamics in unsteady turbulence near a four-blade propeller in a bubble breaker at different speeds. The results show that the final rupture position of the bubble occurs in the main vortex region. The interaction mechanism between the vortex and the bubble involves not only the impact of the small vortex on the bubble but also the influence of the tail vortex pair, which exerts two strong torsional stresses in opposing directions. This interaction ultimately results in the breakup of the bubble. In addition, three different fracture modes were observed: Ring Broken Mode (RBM), Dented pull out Broken Mode (DBM), and Tear Broken Mode (TBM). The bubble breaking process is quantitatively described in terms of vorticity change rate and critical Weber number (We). It is found that DBM does not occur at high rotational speed, and TBM occurs faster and more violently with increase in vorticity change rate. After multiple cycles, the vorticity change in RBM is similar to that at medium speed, and when the change rate reaches its maximum, the size of the fragmentation bubble becomes smaller.

Investigation of the characteristics of the heaving lock-in region of the 10:1 π-shaped section

To gain a comprehensive understanding of the intrinsic characteristics of the lock-in region of the π-shaped section, a study was conducted to investigate the influence of the Angle of Attack (AOA), damping ratio, and mass ratio on the characteristics of the two lock-in regions using a 1:50 segment model test. The Proper Orthogonal Decomposition (POD) method was employed to analyze the characteristics of the two lock-in regions, and the flow field difference between the two lock-in regions was analyzed using Computational Fluid Dynamics (CFD). The findings indicate that both the Angle of Attack (AOA) and mass ratio have comparable effects on the lock-in region. However, it is noteworthy that the AOA appears to exert a more pronounced influence on the second lock-in region, whereas the damping ratio predominantly impacts the amplitude of the lock-in region. The analysis of the distribution and jumping degree of the surface time-averaged pressure enables us to infer the key-action zone number and action position of vortices present around the given section. In the two-lock-in region, there exist two different distribution forms for the energy ratio of POD modes. The first lock-in region is primarily affected by the Karman vortex street phenomenon, whereas the second lock-in region is mainly influenced by the long tail vortex shedding.

Numerical study on noise reduction of ground-effect wing with serrated trailing edge

This study proposes a passive bionic noise reduction strategy using serrated trailing edge (TE) designs to be applied to the aero-train's ground-effect wing. The aim is to mitigate TE noise by suppressing the tail vortex under high-speed and near-wall conditions. Three different serrated TE designs were numerically investigated via large eddy simulation, combined with Möhring acoustic analogy theory under Ma = 0.3 to reveal their noise reduction effects. The noise reduction mechanism was analyzed from the perspective of the flow characteristics in the TE boundary layer. The results indicate that all serrated TE designs achieve noise reduction, with the TE2 exhibiting superior performance. This outcome is linked to the ability of serrated TE designs to moderate boundary layer airflow separation and backpressure gradients, facilitating smoother transitions and lessening wake vortex intensity, thereby reducing turbulence-induced noise. Designs featuring wider serration gaps and sharper edges further enhance noise attenuation.

Research on the Flow-Induced Vibration of Cylindrical Structures Using Lagrangian-Based Dynamic Mode Decomposition

An oscillating flow past a structure represents a complex, high-dimensional, and nonlinear flow phenomenon, which can lead to the failure of structures due to material fatigue or constraint relaxation. In order to better understand flow-induced vibration (FIV) and coupled flow fields, a numerical simulation of a two-degrees-of-freedom FIV in a cylinder was conducted. Based on the Lagrangian-based dynamic mode decomposition (L-DMD) method, the vorticity field and motion characteristics of a cylinder were decomposed, reconstructed, and predicted. A comparison was made to the traditional Eulerian-based dynamic mode decomposition (E-DMD) method. The research results show that the first-order mode in the stable phase represents the mean flow field, showcasing the slander tail vortex structure during the vortex-shedding period and the average displacement in the in-line direction. The second mode predominantly captures the crossflow displacement, with a frequency of approximately 0.43 Hz, closely matching the corresponding frequency observed in the CFD results. The higher dominant modes mainly capture outward-spreading, smaller-scale vortex structures with detail displacement characteristics. The motion of the cylinder in the in-line direction was accompanied by symmetric vortex structures, while the motion of the cylinder in the crossflow direction was associated with anti-symmetric vortex structures. Additionally, crossflow displacement will cause a symmetrical vortex structure that spreads laterally along the axis behind the cylinder. Finally, when compared with E-DMD, the L-DMD method demonstrates a notable advantage in analyzing the nonlinear characteristics of FIV.

Effect of Obstacle Gradient on the Deflagration Characteristics of Hydrogen/Air Premixed Flame in a Closed Chamber

In this paper, computational fluid dynamics (CFD) numerical simulation is employed to analyze and discuss the effect of obstacle gradient on the flame propagation characteristics of premixed hydrogen/air in a closed chamber. With a constant overall volume of obstacles, the obstacle blocking rate gradient is set at +0.125, 0, and −0.125, respectively. The study focuses on the evolution of the flame structure, propagation speed, the dynamic process of overpressure, and the coupled flame–flow field. The results demonstrate that the flame front consistently maintains a jet flame as the obstacle gradient increases, with the wrinkles on the flame front becoming increasingly pronounced. When the blocking rate gradients are +0.125, 0, and −0.125, the corresponding maximum flame propagation speeds are measured at 412 m/s, 344 m/s, and 372 m/s, respectively, indicating that the obstacle gradient indeed increases the flame propagation speed. Moreover, the distribution of pressure is closely related to changes in the flame structure, with the overpressure decreasing in the obstacle channel as the obstacle gradient increases. Furthermore, the velocity vector and vortex distribution in the flow field are revealed and compared. It is found that the obstacle tail vortex is the main factor inducing flame evolution and flow field changes in a closed chamber. The effect of the blocking rate gradient on flow velocity is also quantified, with instances of deceleration occurring when the blocking rate gradient is −0.125.

Extended State Observer-Based Sliding-Mode Control for Aircraft in Tight Formation Considering Wake Vortices and Uncertainty

The tight formation of unmanned aerial vehicles (UAVs) provides numerous advantages in practical applications, increasing not only their range but also their efficiency during missions. However, the wingman aerodynamics are affected by the tail vortices generated by the leading aircraft in a tight formation, resulting in unpredictable interference. In this study, a mathematical model of wake vortex was developed, and the aerodynamic characteristics of a tight formation were simulated using Xflow software. A robust control method for tight formations was constructed, in which the disturbance is first estimated with an extended state observer, and then a sliding mode controller (SMC) was designed, enabling the wingman to accurately track the position under conditions of wake vortex from the leading aircraft. The stability of the designed controller was confirmed. Finally, the controller was simulated and verified in mathematical simulation and semi-physical simulation platforms, and the experimental results showed that the controller has high tight formation accuracy and is robust.

Numerical Optimization on Aircraft Wake Vortex Decay Enhancement

Blowing air at the end of the airport runway can accelerate the decay of the near-ground aircraft wake vortex, thereby reducing the negative impact of the vortex on the following aircraft. However, the benefits of accelerating wake dissipation vary for different blowing parameters, so it is necessary to set appropriate parameters in order to obtain better acceleration results. Because of the high cost of traditional optimization methods, this research uses a Kriging surrogate model to optimize the blowing parameters. The results show that the current optimization algorithm can deal with the global optimization problem well. After obtaining 205 sample points, the response surface model of the blowing parameters and blowing yield was accurately established. A relatively optimal parameter setting range was given, and numerical simulation shows that the current parameter setting can obtain improved benefits from accelerated vortex dissipation. In addition, since the optimization process is partially dimensionless, the above optimization results can be used to achieve multi-objective design, that is, the same set of blowing devices can efficiently accelerate the dissipative process of the tail vortices of different aircraft types, thus improving the engineering feasibility of the current blowing method.

Numerical investigation of thermal-hydraulic characteristics in cross-flow heat exchangers with different twisted oval tubes

This study presents a new strategy to strengthen the flow and thermal performance of gas-liquid tube bundle heat exchangers by combining oval tubes with variable-direction twist method. The effects of twist direction, twist angle and flow velocity on the overall performance were investigated. The variable-direction twisted oval tube enhances the mixing degree of the main stream and the tail vortex, thereby improving the thermal performance and reducing the pressure drop. Both heat transfer coefficient and pressure drop increase with the increase of twist angle, and the overall performance is negatively correlated. Twist period has little effect on overall performance. Convective heat transfer coefficient of the variable-direction twisted oval tube with a twist angle of 120° is increased by 5.8%–10.2%, pressure drop is reduced by 5.0%–7.8% and overall performance is increased by 7.4%–10.7%, as compared circular tube bundle. The proposed variable-direction twisted oval tube realizes the synergistic enhancement of thermal-hydraulic performance, and can directly replace the traditional circular tube. The work of this paper provides technical guidance for improving the processing technology of twisted oval tube and developing new heat exchangers.

Hydrodynamic and heat transfer characteristics of two circular cylinders in series under two-degrees-of-freedom vortex-induced vibration

The heat transfer and hydrodynamic properties of two rigid cylinders in series with fluid disturbance at the laminar flow under vortex-induced vibration were studied numerically. The six DOF model was chosen and combined with user-defined functions (UDF) to seek a solution for the cylindrical equations of motion. The coupled calculation between fluid flow and rigid body motion was realized by combining the dynamic mesh technique. The effects of two types of motion conditions of the upstream cylinder, i.e. fixed (referred to as model A) and with two degrees-of-freedom (referred to as model B), on the flow field and heat transfer of the downstream cylinder with two degrees-of-freedom were investigated. The numerical results showed that in the reduced velocity (Ur ) range considered in this work, the heat transfer and motion conditions of the downstream cylinder were subjected to the self-shedding vortices and the shedding vortices of the upstream cylinder. The comparison of models A and B at Ur = 4 and Ur = 5, respectively, displayed that the force frequency of the downstream cylinder was close to the natural frequency when it was impacted by the tail vortices shedding of the upstream cylinder and the self-shedding vortices. This resulted in the maximum transverse amplitude and strongest heat transfer of the downstream cylinder.

Investigation of the intrinsic characteristics of two lock-in regions in a 10:1 π-shaped deck

The π-shaped deck, as a typical blunt body, is prone to vortex-induced vibration due to the surrounding flow. However, the intrinsic characteristics of sectional vibration are frequently disregarded, leading to a lack of a comprehensive understanding of the lock-in region characteristics. Therefore, a deep analysis of the spectrum characteristics was carried out using a combination of simultaneous vibration–pressure testing and computational fluid dynamics (CFD) to gain a deeper understanding of this phenomenon. In this study, the differences between the two lock-in regions were analyzed, and it was observed that the predominant frequency of vibration increased with an increase in reduced wind velocity in a certain range. Furthermore, it was found that, in the lock-in region, when the maximum amplitude occurred, the vibration predominant frequency was greater than the natural frequency. The Strouhal number (St number) of each lock-in region generally exhibited a step-like downward trend as the frequency ratio increased. Moreover, the first lock-in region was associated with the Karman vortex street in the wake region, as well as with the slow-moving vortex on the upper surface and the two vortices on the lower surface. The long tail vortex shedding in the wake region and the single vortex on the lower surface were the key reasons for the larger amplitude of the second lock-in region when compared to the first lock-in region.

Fluid dynamic analysis and lift enhancement of material transport and mixing around airfoil through Lagrangian coherent structures

Flow separation around an airfoil can be effectively controlled by appropriately positioning a synthetic jet (SJ) on the airfoil surface, subsequently enhancing the airfoil's lift. This study conducts a fluid dynamics analysis of material transport and mixing related to momentum transport and mixing around an airfoil. The detailed analysis of the corresponding effects on lift improvement aids in understanding fluid dynamics through Lagrange Coherent Structures (LCSs) and their nonlinear dynamics. An additional SJ is introduced in the airfoil's rear section, combined with a high-speed, low-frequency SJ in the front section to form a combined SJ. The flow around the airfoil with this combined SJ is simulated. LCSs surrounding the SJs are identified and utilized to analyze the complex material transport and mixing processes. The impact of material transport and mixing on lift enhancement is explored through the evolution of LCSs and pressure distribution on the airfoil surface. Results demonstrate that the combined SJs further improve the airfoil's lift coefficient compared to single SJs, with the enhanced material transport and mixing of SJs playing a vital role in lift enhancement. The rear jet accumulates downstream of the rear slot, forming a tail vortex with high momentum and low pressure. The vortex generated by the front jet carries the tail vortex, continuing to move downstream when it reaches the rear slot. A pressure drop forms where the two vortices flow, enhancing the airfoil's lift. The rear jet can absorb fluid downstream and upstream of the jet slot, reducing pressure on the suction surface near the jet slot and significantly contributing to lift improvement.

Simulation and experimental study on the aerodynamic characteristics of a high-speed elevator with three-sided arc fairings

To address the crucial issue of optimizing the design of fairings for drag and noise reduction, this study focuses on a high-speed elevator in operation as the research subject. The numerical and experimental analysis investigates the influence mechanism of a three-sided arc fairing on the surface pressure distribution, surrounding airflow velocity, and aerodynamic drag of elevator cars. The results indicate that the fairing is effective in reducing the high-pressure area on the top of the car. It also helps to prevent vibrations caused by early separation of the boundary layer and periodic vortex separation, and mitigates the impact of aerodynamic forces on the car when there is a sudden change in airflow direction. As the arc radius of the fairing increases, the airflow velocity between the car and the shaft decreases. This leads to a reduction in the tail vortex area at the bottom of the car, resulting in decreased aerodynamic drag. Among the four schemes, scheme D with the largest arc radius of the fairing demonstrates the best optimization performance. Compared to the case without fairing, the car surface pressure decreases by approximately 35%. The aerodynamic drag is reduced to 129.51 N, which corresponds to a 60.13% decrease. However, the fairing has little impact on the lateral lift. The airflow velocity variations in the numerical simulation and experimental tests exhibit the same trend. The average relative error is about 10%, verifying the effectiveness of the numerical calculation method.

Investigation of boiling heat transfer process in the barrel of a gravity heat pipe type extruder

Gravity heat pipe barrels produce gas films during the processing of thermoplastic materials, which affect the heat transfer, so there is an urgent need to study the nucleate boiling process and mechanism at different locations in the barrel. The results showed that the local heat flux at the vapor-liquid-solid three-phase contact was the largest, and different process conditions had different effects on the departure diameter and departure time of boiling bubbles. The tail vortex disturbance generated by bubble departure drives the merging and growth of bubbles to departure, which in turn improves the heat transfer. The rounded bottom of the barrel reduced the air film and increased the heat transfer efficiency compared to the horizontal bottom of the barrel. This study can deepen the understanding of the mechanism of bubble boiling heat exchange in gravity heat pipe-type barrels and provide theoretical support for more relevant application scenarios.

Body effect on inflow distortion in civil engine intake under crosswind condition

The effect of body on the inlet aerodynamic performance of distortion under crosswind condition for a high pass ratio turbofan engine was investigated. The work is carried out by using computational fluid dynamics on civil nacelle and body. The investigation suggests that: due to the shelter of the body, the inlet distortion on the windward side and leeward side is very different under crosswind conditions. The nacelle on the windward side mainly takes in air from the side while the engine on the leeward side takes in air from above and below. The existence of body makes it easier to produce fuselage vortex and tail vortex near the nacelle. The body has an adverse impact on the intake of the engine on the leeward side and improves the requirement of the crosswind resistance of the upper section of the nacelle.

Motion Behaviors of Two Ascending Bubbles in a Wide Range of Viscous Liquid

Foaming devolatilization is a common phenomenon in a process of viscous solution. Bubble growth and motion behaviors depends on the flow conditions and the properties of liquid phase and that is closely related to heat and mass transfer efficiency in devolatilization process. Herein, experimental study of two ascending bubbles in a wide range of viscous fluid were conducted to achieve an understanding of the formation, coalescence and escape. As a supplement and verification, the motion behaviors rising bubbles were also investigated numerically. Results showed that the first bubble generated a tail vortex to affect the next bubble entering the region. The escape time also increased from 0.3 s to 65 s as the fluid viscosity charged from 0.05.Pa · s to 447 Pa · s, and the bubble broke and it could not formed completely with a high viscous fluid in large rate. It was expected the experimental escape time of bubble in different flow rate was in good agreement with the numerical results.

Comparative Analysis of the Hydrodynamic Performance of Dual Flapping Foils with In-Phase and Out-of-Phase Oscillations

In the context of the plain river network, conventional water pumps suffer several drawbacks, including inadequate efficiency, poor security, and costly installation costs. In order to improve the hydrodynamic insufficiency problem and enhance the hydrodynamic performance and applicability of flapping hydrofoils, this paper proposes a bionic pumping device based on dual flapping foils. Based on the finite volume method and overlapping grid technology, the numerical simulation and experimental verification of the hydraulic performance of two typical motion modes of in-phase and out-of-phase oscillations are conducted, thereby providing a theoretical foundation for improving and optimizing the design of flapping hydrofoils. The results show that the out-of-phase oscillation has better hydraulic performance compared to the in-phase oscillation. The formation of the tail vortex structure plays a crucial role in determining the hydraulic efficiency of dual flapping foils, with in-phase oscillation forming a pair of vortex streets and out-of-phase oscillation forming two pairs of vortex streets. The pumping efficiency of the out-of-phase oscillation is significantly higher than that of the in-phase oscillation, reaching up to 38.4% at a fixed frequency of f = 1 Hz, which is an increase of 90.5% compared to the in-phase oscillation. The characteristic curve of the in-phase oscillation shows an “S” type unstable oscillation phenomenon, namely the hump phenomenon, while the out-of-phase oscillation does not show such a phenomenon, which can effectively expand its application range. In addition, the applicable head of the out-of-phase oscillation hydrofoil is lower, which can better meet the requirements of ultra-low head conditions.

Numerical study on unsteady cavitation flow and vortex dynamics characteristics around the Delft Twist 11 hydrofoil

The objective of this study is to explore the unsteady cavitation characteristics and the mechanism of cavitation-vortex interaction around a twist hydrofoil. The cavitation conditions were simulated using the Partially-Averaged Navier-Stokes (PANS) method in conjunction with a homogeneous flow cavitation model. The numerical method employed in this study successfully captures the intricate shedding process of cavitation on the hydrofoil surface. Furthermore, the cavity morphology obtained at various time instants exhibits a remarkable agreement with the experimental results. The simulation results of the unsteady characteristics of cavitation shedding on the hydrofoil surface revealed that the shedding process could be divided into primary and secondary shedding phases. The primary shedding happened as a result of the re-entrant jet flow generated by the adverse pressure gradient at the cavitation tail. Notably, the secondary shedding was predominantly caused by the combined effects of the re-entrant jet and the side re-entrant jet flow. During the primary shedding process, the cavitation experienced disturbances from the side re-entrant jets, causing it to roll and be drawn upwards into a U-shape cavity structure. Subsequently, as the secondary shedding happened, the side re-entrant jets contributed to the formation of a secondary U-shape cavity at the cavitation tail. The investigation on the cavitation-vortex interaction around the hydrofoil showed a strong consistency between the distribution of vortices and cavitation. The foot of the primary U-shape vortex was attached to the hydrofoil surface, while the shedding vortex tail on both sides exhibited secondary U-shape vortex structures. The primary U-shape vortex formation resulted from the combined effects of suction and rotation generated by the side re-entrant jets and re-entrant jets on both sides of the hydrofoil. The secondary U-shape vortex was induced by secondary shedding cavitation which produced by rotational effects at the stable cavitation tail.

Effects of surge and roll motion on a floating tidal turbine using the actuator-line method

This paper employs a dynamic and sliding mesh in the simulation of both uncoupled and coupled surge and roll motions of a tidal stream turbine, utilizing a modified actuator-line method. The modification involves the relocation of blade elements in relation to the grid. Detailed analyses are conducted on the Cp and Cz variations in surge, roll, and coupled motions at various frequencies and amplitudes. It is observed that changing the amplitude and frequency of surge and roll motions both impacts the amplitude of Cp and Cz. Interestingly, the Cp and Cz variations in surge motion are inversely proportional to velocity variations, while they are directly proportional in roll motion. The influence of the surge motion on Cp Cz plays a major role, while the addition of the roll motion increases the mean values of Cp and Cz. Due to the combination of the wake characteristics of both surge and roll, the coupled motion wake exhibits a contraction–expansion oscillation pattern. In a coupled motion with equal periods, the ring and strip tail vortex characteristics of both motions are apparent. A surge period increment diminishes the surge's tail vortex characteristic, whereas an increase in the roll period gradually erodes the roll's tail vortex characteristic. The coefficient variation of the tangential and normal forces (cn, ct) in combined motion mirror that of surge motion, presenting a convex table per surge cycle with depressions at the 1/2T and 1T points. The peak of cn and ct in surge motion are approximately 0.28 and 0.03, respectively, while in roll motion, they are around 0.261 and 0.025. The exploration of cyclic stress impacts on the turbine, and the potential instability on the platform could be valuable directions for future research.