Evolution Of Vortex Structures Research Articles

Численное моделирование смешанной конвекции в жидком металле при подъемном течении по круглой обогреваемой трубе в условиях поперечного магнитного поля

Hydrodynamics and heat transfer in mixed convection of liquid metal (Pr=0.025) in a vertical pipe with an upward flow in a transverse magnetic field are considered. The study was carried out using a direct numerical simulation method. Simulations were performed at Reynolds numbers up to 12,000, Richardson numbers from 0 to 2.4 and Hartmann numbers up to 550 for two limiting cases of pipe wall conductivity. The simulation results for an isolated pipe were compared with the measurement data from the mercury experiments. The effect of the transverse magnetic field is manifested in the suppression of turbulent transfer and flow laminarization with the formation of strong inhomogeneity of the longitudinal velocity profile by angle and gradients of the streamwise velocity in the transverse direction. Depending on the pipe wall conductivity and the ratios of such parameters as Richardson and Hartmann numbers, the resulting flows with different topology of the longitudinal velocity profile determine the wall temperature distribution in the wall and the values of the heat transfer coefficients. It is shown that in a magnetic field in the case of isolated walls with an increase in the Richardson number the flow has a tendency to instability, which develops in the jets formed in the region of Roberts layers. The periodic process of development and breakage of jets leads to the emergence and development of vortex structures that persist in the magnetic field and cause fluctuations in the velocity and temperature components in the flow. The dependence of the friction factor and heat transfer (Nusselt number) on the Richardson number is obtained.

Inner–outer interaction of unsteady turbulent flow inside duct with wall corrugations: Augmentation and saturation

This study investigates the inner–outer interaction of the unsteady turbulent flow inside a duct with wall corrugations, where “inner” refers to the trapped flow in a corrugation-induced cavity array and “outer” relates to the mainstream flow transporting through a circular duct. Configurations with different pitch–diameter ratios (P/D) are used to demonstrate the effect of the cavity flow pattern on the mainstream flow variations. An improved delayed detached-eddy simulation with dynamic blending function is performed to acquire high-fidelity turbulent flow data, in which dynamic evolution of multi-scale vortex structures containing hairpin vortices and vortex fragments is clearly resolved. The subsequent statistical analysis reveals a nonlinear variation tendency of the inner–outer interaction intensity, experiencing sudden augmentation first and then a gradual attenuation toward the final saturation. Comparatively, a larger pitch ratio results in a stronger interaction under the effect of bicentric recirculation zones within the cavity array. Moreover, a proper orthogonal decomposition analysis allows for the visualization of energetic flow structures, as consistent velocity variations throughout the duct volume are identified for a larger pitch ratio, while discontinuity at the duct termination is found for a smaller pitch ratio. Finally, an advanced elliptical model based on the spatiotemporal cross correlation analysis is proposed to examine the convection velocity and sweeping velocity of the interactive mainstream flow and cavity flow. The results highlighted the presence of first-augmented and then saturated dynamics and kinematics inside the duct with the cavity array.

The vortex structure and enstrophy of the mixing transition induced by Rayleigh–Taylor instability

The mixing induced by the Rayleigh–Taylor (RT) instability occurs widely in various natural phenomena and engineering applications, such as inertial confinement fusion. The mixing transition in the RT mixing process is the key process affecting the mixing evolution. At present, research in RT mixing transition mainly involves mixing transition criteria based on global quantities, statistical analysis of mixedness parameters and kinetic energy, and so on. A few studies have paid attention to the evolution of vorticity and its intensity, enstrophy, during mixing transition process. However, previous studies have inferred that vorticity and enstrophy play important roles in mixing transition. In this paper, implicit large-eddy simulation for RT mixing is carried out to analyze the evolution of vorticity and enstrophy in mixing transition. First, the vortical motions throughout the whole mixing process are investigated by comparing the contours of mass fraction and vorticity. Then, for revealing the mechanism of vortical motions in transition stage, the vortex structures are extracted and the relationship between vortex structures and enstrophy in mixing transition is investigated. Finally, in order to quantify the vortical motions in the mixing transition, the probability density function (PDF) of enstrophy is introduced and analyzed. The main conclusions are as follows: (1) The evolution of vortical motions is closely related to the RT mixing transition process. Enstrophy can reflect the vortical motions in the mixing transition process. When the growth rate of averaged enstrophy reaches its maximum value, the transition occurs; (2) the PDFs of enstrophy can quantify the evolution of vortex structures during mixing transition and characterize the mixing transition process. The mixing transition begins when the PDF of enstrophy appears double peaks. The process of PDF right peak movement corresponds to the transition process, and the transition ends when the position of the right peak is no longer moving. Since the enstrophy studied in this paper is a local field quantity, the above results are expected to be used to construct local mixing transition criterion.

Identification of the vortex in the main flow passage of a multiphase pump and the relationship with pressure fluctuation

AbstractThis article investigates the evolution of vortex structures in the impeller channel of a multiphase pump. By capturing the vortices in the impeller channel using the vorticity and Q‐criterion, the generation location of the vortex structures is analyzed, and the pressure fluctuations induced by vortices in the main flow passage of the impeller are studied in terms of their time‐ and frequency‐domain characteristics. The relationship between the vorticity and the amplitude of pressure fluctuations at the main frequency of the impeller is further investigated. This study develops a method of identifying vortices in the impeller channel of the multiphase pump, and reveals the intrinsic connection between the vortices and pressure fluctuations in the main flow passage. These findings offer some suggestions for eliminating the influence of vortices and enhancing the pressurizing capabilities of multiphase pumps.

Spatial–temporal prediction model for unsteady near-wall flow around cylinder based on hybrid neural network

A hybrid neural network based on Densely Connected Convolutional Networks (DenseNet), Convolutional Long Short-Term Memory Neural Network (ConvLSTM), and Deconvolutional Neural Network (DeCNN) is employed to predict unsteady flow fields. The utilization of DenseNet makes the model more compact and makes the prediction of three-dimensional flow affordable. The ConvLSTM is implemented to predict multiple future time steps which improves prediction efficiency. The proposed model transforms the time sequences of velocity and pressure fields into uniform spatial–temporal topology as input and captures nonlinear feature information in the spatial–temporal domain. Numerical simulations are conducted for the flow around cylinder at different Reynolds numbers and the near-wall flow around cylinder with different gap ratios, and training samples for the neural network inputs are established. The predicted results are compared with the numerical simulation results, showing good agreement. From the prediction cycle, it can be seen that good prediction results can be maintained in the first three prediction cycles. The prediction results of the three-dimensional unsteady flow around a cylinder near a plane wall, exhibit remarkable accuracy, successfully capturing the evolution of turbulent vortex structures. This signifies that the prediction model is highly effective in capturing the spatial–temporal variations of complex unsteady flows.

Study on Aerodynamic Performance and Wake Characteristics of a Floating Offshore Wind Turbine in Wind–Wave Coupling Field

Floating offshore wind turbines (FOWTs) exhibit complex motion with multiple degrees of freedom due to the interaction of wind and waves. The aerodynamic performance and wake characteristics of these turbines are highly intricate and challenging to accurately capture. In this study, dynamic fluid body interaction (DFBI) and overset grid technology are employed to investigate the dynamic motion of a 5 MW FOWT. We use the volume of fluid (VOF) method and improved delayed detached eddy simulation (IDDES) model to investigate the aerodynamic performance and wake evolution mechanism for various wave periods and heights. According to the findings, the magnitude of the pitch motion increases with the period and height of the waves, leading to a decrease in both the power output and thrust; the maximum power was reduced by nearly 6.8% compared to a wind turbine without motion. The value of power and thrust reduction varies for different wave periods and heights, and is influenced by the relative speed and pitch angle, which play a crucial role. Wind–wave coupling has a significant impact on the evolution of both wake and vortex structures for FOWT. The wake shape downstream is also dynamically influenced by the waves. In the presence of wind and wave coupling, the interaction between the wind turbine and the wake is heightened, leading to the merger of two unstable vortex rings into a single, larger vortex ring. The research unveils a comprehensive picture of the offshore wind energy dynamics and wake field, which holds immense significance for the design of floating wind turbines and the optimization of wind farm layout.

Investigating three-dimensional vortex evolution in centrifugal pump under rotating stall conditions using tomographic particle image velocimetry

A rotating stall in centrifugal pumps commonly occurs under off-design operations, which is a detrimental phenomenon leading to flow instabilities, pressure fluctuations, and reduced performance. A time-resolved non-intrusive three-dimensional (3D) flow visualization method is developed for investigating complex vortex structures in centrifugal pumps based on Omega vortex identification and tomographic particle image velocimetry (tomo-PIV). A special-made centrifugal pump prototype was developed with acrylic glass allowing for optical access. This method enables both qualitative and quantitative analysis of high spatiotemporal resolution on flow behaviors and dynamics under various stall conditions. The ultra-high sampling frequency realized over 40 time-consecutive observations per revolution under 0.2 Qd, 0.4 Qd, 0.6 Qd, and 0.8 Qd. It captures the instantaneous evolution of vortex structures that undergoes a growth–breakup transition within 7–9 ms. The rotating stall mechanism is revealed experimentally from the evolution of the vortex structure. Our analysis shows the tomo-PIV's additional velocity component aids in understanding the 3D characteristics of the stall. A substantial region of reverse flow in the z-axis direction is observed under 0.2 Qd. Vortex structures are more prone to blockage at the impeller inlet, exacerbating the stall phenomenon. As the flow rate increases, the velocity distributions across different layers exhibit a laminar characteristic with a more uniform profile. The vortex structures extend radially and migrate toward the outlet. The evolutions of the stall vortex, wake vortex, and inlet vortex share the same dominant frequency components (4.75fn and 5.25fn), but the flow rate affects the proportion of different frequency components.

Wake characteristics and vortex structure evolution of floating offshore wind turbine under surge motion

Offshore wind energy is booming and floating offshore wind has been proved to be a promising clean energy. Wake effect is one of the biggest challenges in large-scale floating wind farm which results in power loss and increased fatigue load. In this paper, actuator line model (ALM) coupled with unsteady Reynolds-averaged Navier-Stokes (URANS) is adopted to investigate the wake characteristics and evolution of floating wind turbine under motion. Besides, modal analysis is adopted to extract typical features of turbine wake. Velocity fluctuation is the prominent characteristic of floating turbine wake. The increase of surge frequency results in the decrease of velocity fluctuating region and large surge amplitude leads to bigger velocity deficit fluctuations. In addition, the surge motion brings a faster recovery rate than fixed wind turbine. The induced velocity field generated by vortex ring structure is the key to understanding complex evolution of turbine wake and the vortex ring structure is analyzed in this paper. With the deep understanding of FOWT wake, an innovative vortex ring structure model is proposed in this paper. This work is the basic work of developing the engineering wake model of floating wind turbine, to cope with the wake effect challenge in the large-scale floating wind farm.

Study on flow regimes and mixing of vortex-inducing T-jet reactors with staggered inlet channel

Improving the jet reactor's mixing performance is critical and challenging. In this paper, a new type of vortex-inducing T-jet reactor proposed by researchers is studied. By designing the staggered inlet structure of the T-jet reactor, the vortex is induced in the flow field in the mixing channel to improve the mixing effect. Flow regimes and mixing characteristics in the vortex-inducing T-jet reactor were investigated through numerical simulations (CFD) at 30 ≤ Re ≤ 600. Results show that the staggered inlet channel induces vortices successfully, and the engulfment flow regime occurs at lower Reynolds numbers, thus promoting the mixing. The generation and evolution of vortex structures in the reactor were focused on. It was found that in the unsteady engulfment flow, the two fluids cross each other, which makes the vortex structure constantly twist and merge, which greatly improves the mixing effect. At larger Reynolds numbers, many attached small vortices are generated around the central vortex structure, and these vortices are wound together with the high-speed rotation of the central vortex, avoiding the unsteady symmetric flow pattern with a poor mixing effect. Finally, it is found that the mixing performance of the vortex-inducing T-jet reactor is greatly improved under all regimes.

Detecting the high/low-speed large-scale structures by a machine learning perspective in turbulent boundary

The current developments in machine learning have facilitated the application of data-driven strategies for predicting the evolution of vortex structures in turbulent boundary layers. This study combines the attached eddy hypothesis with the extreme gradient boosting model to forecast large-scale high/low-speed motion based on a series of input signals. The performance of the trained model, as quantified by the percentage of accurately predicted high/low-speed regions, exhibits variability concerning the deviation between the input velocity fluctuation and the predicted output value at z/δ=0.016, where ‘z’ represents the wall-normal height and ‘δ’ indicates the boundary layer thickness. Our findings underscore the significant potential of machine learning in predicting high/low-speed regions within large-scale motion.

Mechanistic study of noise source and propagation characteristics of flow noise of a submarine

Flow noise, part of hydrodynamic noise, significantly impacts the stealth capabilities of submarines. Investigating the noise source location and formation mechanism of flow noise is crucial for enhancing submarine stealth performance. This study uses large eddy simulation to examine the evolution of vortex structure in the SUBOFF submarine. The propagation of far-field noise from dipole and quadrupole noise sources of the SUBOFF is calculated, by rationally constructing a permeable integral surface in conjunction with the Ffowcs-Williams and Hawkings and “Collapsing-Sphere” formulations. The numerical results for both flow and acoustic fields are consistent with experimental data and previous numerical findings. Sail and rudder are the predominant dipole noise sources, due to turbulence effects from vortex shedding. The permeable integral surface effectively captures the quadrupole noise source. The quadrupole noise in the 2 kHz frequency range contributes 17 % to 26 % of the total noise energy. The location of the quadrupole noise source and its generation mechanism is explored, through the evolution of the vortex structure and the Lamb vector amplitude. Approximate scaling laws for far-field dipole and quadrupole noise spectra are given, thus illustrating the general laws in the frequency domain of the far-field noise.

Numerical simulation of fluid-particle flow of jet in supercritical water environment

Supercritical water gasification provides a new approach to the green transformation of coal. Nevertheless, the flow characteristics of fluids and particles injected into the reactor through the nozzle still need to be further investigated. In this paper, a three-dimensional numerical simulation of the particle jet in supercritical water environment is carried out coupling Large Eddy Simulation (LES) with Discrete Phase Model (DPM). The simulation focuses on the effects of particle size, Stokes number, mass flux ratio and initial fluid velocity on the flow characteristic of the fluid-particle flow, especially on the vortex structure evolution, particle distribution and velocity profile. The findings are that as the particle size decreases and the initial fluid velocity increases, the evolution of vortex structures become increasingly fierce, such as the destruction of large-scale coherent structures, the disappearance of vortex ribs, and the lateral expansion of vortices in the development section. Particle distribution exhibits characteristics similar to the vortex structure, as particles adhere closely to the carrier fluid at low Stokes number (St < 1). On the contrary, with increasing of Stokes number (St > 1), the particles are freer from fluid interference and keep their original motion state moving forward. The fluid velocity at the center of the transverse interface jet decrease with increased mass flow ratio. For higher initial fluid velocities, particle distribution is more uniform, and more particles are carried to areas with larger radial distances.

Evolution of Vortex Structures Generated by a Rigid Flapping Wing with a Winglet in Quiescent Water

This study aims to the utilization of vortex structures generated through wing flapping for achieving sustainable flight, and the motivation is elicited by the phenomenon observed in natural flyers. The vortex structures in the flow field generated by a flapping rigid wing are captured using vorticity and the LAMDA2 criterion. The study investigates a comparative analysis between a wing both with and without a winglet. Moreover, the influence of flapping frequency is examined as well. For the experiments, particle image velocimetry (PIV) measurements are employed for the flow field around mechanical flapping motion in a quiescent water condition. The flapping mechanism has one-degree freedom, showing a 1:3 ratio in motion, and tested wings at 1.5 and 2.0 Hz. A “modified” vortex filamentation and fragmentation phenomenon is proposed as a significant finding in the present study, based on a comprehensive analysis of the flow field around the wing with a winglet.

Study on mechanism of unsteady gas-liquid two-phase flow in liquid-ring vacuum pump

To investigate the gas-liquid two-phase flow mechanism in liquid-ring vacuum pump, the large-eddy simulation coupled with volume-of-fluid method (LES-VOF) was used to numerically simulate the unsteady gas-liquid flow characteristics in pump. The structure of gas-liquid flow within pump and its dynamic evolution characteristics were studied, and the hydraulic excitation characteristics induced by gas-liquid flow were analyzed. The result shows that the channel vortex inside the impeller on the pump suction side gradually flows out of the flow passage with the rotation of the impeller, and continuously mixes with the wake vortex at the outlet of the impeller and the separation vortex at the inner wall of the casing. The evolution and development of vortex structures with different intensities and scales near the inner wall of the suction side casing have a relatively large contribution to pressure fluctuation. The wake vortex at the impeller outlet of the exhaust side gradually enters the flow passage and dissipates gradually with time. There is an obvious high-intensity backflow vortex in the exhaust section due to the pressure difference between the flow channels near the leading edge of the exhaust port, which leads to obvious rotor-stator interaction in the exhaust section.

Effects of streamwise rotation on helicity and vortex in channel turbulence

Helicity plays a key role in the evolution of vortex structures and turbulent dynamics. The helicity dynamics and vortex structures in streamwise-rotating channel turbulence are discussed in this paper using the helicity budget equation and the differentiated second-order structure function equation of helicity. Generally, rotation and Reynolds numbers exhibit opposing effects on the interscale helicity dynamics and the vortices. Under the buffer layer, the positions of the helicity peaks are proportional to the ratio between the Reynolds and rotation numbers. The mechanism is related to the opposing effects of convection and rotation. Rotation directly affects the helicity balance through the Coriolis term and corresponding pressure term. In the buffer layer, the scale helicity is negative at small scales but positive at large scales, which is mainly induced by the spatial effects (the production and the spatial turbulent convection) but reduced by interscale cascades. Examination of structures reveals the close association between scale helicity and streaks, with streak lift angles exhibiting an increase with rotation and a decrease with Reynolds numbers. In the log-law layer, the Coriolis terms and corresponding pressure terms are proportional to the rotation numbers but remain independent of the Reynolds numbers. The negative scale helicity is forward cascaded towards small scales. Generally, spanwise vortices in the log-law layer are related to sweep events and forward cascades. Our findings indicate that these spanwise vortices are suppressed by rotation but recover with increasing Reynolds numbers, aligning with the effects observed in the scale helicity balance.

Numerical investigation of vortex structure and unsteady evolution in multi-stage double-suction centrifugal pump

Multi-stage double-suction centrifugal pump is constructed to handle situations with large flow rates and high head. However, due to the complex internal flow structure, the pump can experience hydraulic excitation caused by the presence of numerous vortical structures. Such excitation can lead to unstable pump operation and increased energy losses. In this study, we aim to analyze the multi-stage double-suction centrifugal pump by combining numerical simulation using detached eddy simulation (DES) and experiments to accurately capture the vortical structure and elucidate the mechanism of the rotor-stator interaction (RSI) formation. The results indicate that the omega vortex identification method can accurately capture the vortex structure in the pump, irrespective of the threshold value and without the influence of wall shear layers. Additionally, based on this identification method, we have analyzed the unsteady evolution characteristics of the vortex structure in the pump. Specifically, we have focused on the shedding of wake vortices and their collision with the tongue. The findings suggest that the rotor-stator interaction primarily arises from the periodic shedding of wake vortices near the impeller outlet. In summary, this study provides valuable insights into the flow dynamics of multi-stage turbomachines.

Investigation on unstable flow characteristics and energy dissipation in Pelton turbine

The internal flow scale of a Pelton turbine is variable, and the interaction between the jet and the bucket has strong transient characteristics, resulting in an incomplete understanding of its internal vortex structure evolution and energy dissipation mechanisms. In order to reveal the influence of vortices on the flow regime and energy dissipation mechanisms in the turbine, this paper establishes the correlation between the unsteady flow characteristics and energy dissipation inside the Pelton turbine based on the energy balance equation and quantifies the energy losses inside the turbine. The results indicate that vortex structures present a non-uniform distribution inside the jet, which disrupts the uniformity of the jet velocity distribution, resulting in an uneven distribution of high-vorticity zones on the bucket surfaces and intensifying flow interference among the buckets. The strong shear flow caused by the downstream flow detachment of the needle guide, the turbulent boundary layer on the jet surface, and the wake effect downstream of the needle tip are the main reasons for the dissipation of fluid kinetic energy. The distribution range of energy loss on the rotating bucket closely corresponds to the position of the high-vorticity zone. Energy dissipation inside the turbine is primarily in the form of turbulent kinetic energy, and the injectors and runner are the primary energy dissipation components. Moreover, inside the injectors, each form of energy loss remains relatively constant, whereas inside the runner, its rotation induces fluctuating energy losses attributed to Reynolds stress work. The results contribute to an enhanced understanding of the energy dissipation characteristics and complex flow mechanisms within the turbine, providing reference for the optimisation and efficient operation of the multi-nozzle Pelton turbine.

The Influence of Shear-Thinning Characteristics on Multiphase Pump Vortex Structure Evolution, Pressure Fluctuation, and Gas-Solid Distribution

The Influence of Shear-Thinning Characteristics on Multiphase Pump Vortex Structure Evolution, Pressure Fluctuation, and Gas-Solid Distribution

Numerical investigation on the cavitating wake flow around a cylinder based on proper orthogonal decomposition

The non-cavitating and cavitating wake flow of a circular cylinder, which contains multiscale vortices, is numerically investigated by Large Eddy Simulation combined with the Schnerr–Sauer cavitation model in this paper. In order to investigate the spatiotemporal evolution of cavitation vortex structures, the Proper Orthogonal Decomposition (POD) method is employed to perform spatiotemporal decomposition on the cylinder wake flow field obtained by numerical simulation. The results reveal that the low-order Proper Orthogonal Decomposition modes correspond to large-scale flow structures with relatively high energy and predominantly single frequencies in both non-cavitating and cavitating conditions. The presence of cavitation bubbles in the flow field leads to a more pronounced deformation of the vortex structures in the low-order modes compared to the non-cavitating case. The dissipation of pressure energy in the cylinder non-cavitating wake occurs faster than the kinetic energy. While in the cavitating wake, the kinetic energy dissipates more rapidly than the pressure energy.

Transformation of film cooling performance with evolution of vortex structure for coolant jet with various swirl intensities

Advanced H/J series heavy-duty gas turbines are widely equipped with film cooling to protect turbine blades. Previous studies have shown that coolant swirl has an important influence on film cooling performance. However, there are few studies on the quantitative analysis of the coolant swirl intensity because of the swirl generated by the specific structure, and most of the research was carried out at relatively low temperatures. In this paper, a new method of defining the velocity field in the coolant inlet is used to quantitatively investigate the coolant swirl intensity. A numerical study of the film cooling performance for the coolant jet with various swirl intensities is conducted in the operating conditions of H series turbine vanes. The effects of the blowing ratio and coolant vorticity on the vortex structure and mixing process of the flow field are investigated emphatically, and the mechanisms for the variation of the film cooling effectiveness are analyzed. The results show that the film cooling effectiveness is greatly improved with the swirling coolant jet at high blowing ratios. When Ωn=40,000 /s, the area-averaged (L = 5D) film cooling effectiveness is 54 % higher than the non-swirling case. Ωn=20,000 /s is a critical value of the coolant vorticity. When Ωn>20,000 /s, the film coverage transforms significantly. Furthermore, the main vortical structures in a typical SJICF flow field with high blowing ratios and large vorticities are proposed. Compared with the non-swirling JICF, the additional coolant vortex and induced vortex appear in the SJICF. Under the influence of the two vortices, the coolant momentum is sacrificed for enhanced attachment to the wall resulting in the momentum advantage gradually transforming into the effectiveness advantage with the increase of the vorticity.