Vortex Transport Research Articles

Cavitating flow investigation in low specific speed axial flow waterjet pumps

Abstract In this paper, the cavitating flow in low specific speed axial flow waterjet pumps are treated at different cavitation conditions. In order to analyze the cavitating turbulent flow comprehensively, three axial flow waterjet pumps with the specific speed less than 650 min-1·m3s-1·m are chosen to compare the internal flow phenomenon. The investigations are conducted by numerical simulation, where RANS method is applied with k-ω SST turbulence model and homogeneous cavitation model. The results show that there are several types of cavitation such as tip leakage vortex cavitation, sheet cavitation attached on blade suction surface, and shedding cavity in the pumps. The vortex structure in the low specific speed waterjet pump contains tip leakage vortex, shedding vortex downstream of the sheet cavitation and perpendicular cavitation vortex. It is noted that the cavity firstly occurs at the leading edge near blade tip, and the sheet cavity covering the blade suction surface extends from leading edge to the middle part, while the tip leakage vortex cavitation develops with the decreasing cavitation number. Further analysis using relative vortex transport equation depicts that the vortex stretching term is the main contributor for the vorticity production in the pump, and the vortex dilation and baroclinic torque occur at the interface between cavity and water.

Experimental investigation and energy conversion modelling of radial diffuser for centrifugal compressor

Centrifugal compressor is seen as the predominant choice in turbocharging and natural gas transportation. The energy loss measurement in centrifugal compressor is of great essence, especially under instability conditions. Based on one-dimensional model and simulations, each energy loss mechanism in diffuser was evaluated by defining the weight coefficient representing the influence of energy loss on compressor performance. The thermodynamics in the diffuser was investigated under design and stall inception conditions. Vortex transportation and synergy angle analysis was realized to identify the correlation between energy losses and stall identification. The results indicate that the skin friction loss accounts for 75.06% of the total energy loss in diffuser, while 16.96% and 7.98% for diffusion and secondary loss near stall condition, respectively. The determination of stall in vanless diffuser may be confirmed according to the combination of secondary vorticity coefficient and synergy angle. Due to small inflow angle and large width ratio of the diffuser, stall inception is the results of secondary flow sweep and momentum loss induced by diffusion. The weight function was originally modelled to quantize the influence of energy loss on flow aerodynamics. The present work may shed lights on stall identification and energy loss reduction in industrial compressor.

Eulerian–Lagrangian modeling of spray G atomization with focus on vortex evolution and its interaction with cavitation

Internal flow is closely related to energy consumption and emissions because of its dominant role in the atomization process. The intricate three-phase flow patterns in an orifice were studied based on modified cavitation and turbulence models. Interacting vorticities and their interactions with cavitation were analyzed using vorticity identification methods and the vorticity transport equation. The paired vortex was formed before cavitation inception, and the cavitation inception occurred invariably inside the vortex core, indicating that cavitation was triggered by vortex evolution. Cavitation patterns affect the vorticity transport that further affects atomization. The internal flow data were then extracted and coupled to a hybrid break-up model with finer characteristic scales.The spray morphologies of different types of fuel at various operating conditions were obtained using diffused back illumination method to validate the proposed model. The predictions obtained from the coupled model were consistent with experimental results, exhibiting less dependence on the grid at the same time. The results of this study can supplement experimental measurements to explore the correlation between cavitation-modulated vortex transport and spray behavior.

Berezinskii-Kosterlitz-Thouless transition transport in spin-triplet superconductor

As the spin-triplet superconductivity arises from the condensation of spinful Cooper pairs, its full characterization requires not only charge ordering, but also spin ordering. For a two-dimensional (2D) easy-plane spin-triplet superconductor, this na\"{i}vely seems to suggest the possibility of two distinct Berezinskii-Kosterlitz-Thouless (BKT) phase transitions, one in the charge sector and the other in the spin sector. However, it has been recognized that there are actually three possible BKT transitions, involving the deconfinement of, respectively, the conventional vortices, the merons and the half-quantum vortices with vorticity in both the charge and the spin current. By considering equal-spin-pairing spin-triplet superconductors with bulk spin degeneracy, we show how all the transitions can be characterized by the relation between the voltage drop and the spin-polarized current bias. This study reveals that, due to the hitherto unexamined transport of half-quantum vortices, there is an upper bound on the spin supercurrent in a quasi-long range ordered spin-triplet superconductor, which provides a means for half-quantum vortex detection via transport measurements and deeper understanding of fluctuation effects in superconductor-based spintronic devices.

Stereoscopic PIV measurements in a two-stage axial turbine

In the present work, the three-dimensional flow field in the interstage region of a twostage axial turbine has been measured by a stereoscopic PIV system. The stereoscopic method is used to compensate for perspective as well as to observe the highly threedimensional flows. The digital images are recorded with a set of two cameras positioned perpendicularly to the measurement plane and inclined by an angle varying between 22° and 30° to allow stereoscopic measurements. The laser beam is delivered to a laser endoscope able to access the measurement areas. By traversing radially, several blade-to-blade planes can be illuminated with the laser endoscope from 66 to 96% blade span. To compensate for the perspective distortion of the field of view due to the tilt angle of camera B as well as the optical distortion through the double-curvature windows, a threedimensional calibration method is used. In the current investigation, a Monte Carlo simulation has been conducted to evaluate measurement errors of PIV. Results of these measurements are compared with velocities derived from time resolved pressure measurements using fast aerodynamic response probe (FRAP). A good agreement is found at the exit of the second rotor. The present work present a unique set of steady and unsteady data measured in a two-stage axial turbine. Measured data in a volume can be used for numerical tool validation as well as improve existing kinematic model of vortex transport and dissipation.

Flow-Induced Forces and Vortex Transportation Characteristics of Three Circular Cylinders at Subcritical Reynolds Number

Flow-Induced Forces and Vortex Transportation Characteristics of Three Circular Cylinders at Subcritical Reynolds Number

The study of the influence of protective screen on reducing the level of air pollution

The task of assessing the areas of chemical pollution near the highway, where the protective screen is located, is considered. The protective screen locally changes the aerodynamics of the air flow near the highway, which contributes to the minimization of air pollution in the working zones near the highway. Recently, interest has increased in studying the effectiveness of the use of protective screens of complex shape, which are located near highways. The purpose of the work is to develop a numerical model for calculating pollution zones formed near the protective screen, as well as conducting a laboratory experiment to analyze the patterns of formation of pollution zones near the T-shaped screen. For mathematical modeling of the process of formation of pollution zones near the protective screen, the equation of convective-diffusion transfer of impurities is used. This equation takes into account atmospheric diffusion, wind speed, emission intensity of a chemically hazardous substance, the location of the emission source, and the shape of the protective screen. Two models of aerodynamics are used to calculate the air flow velocity field when flowing around a protective screen. The first model is the Navier-Stokes equations. These equations are written in the variables "eddy current function". The second model is a two-dimensional equation for the velocity potential. For the numerical integration of the vortex transport equation, the splitting method is used, followed by the explicit finite-difference method. For the numerical integration of the Poisson equation for the stream function, the Liebman method is used. To numerically integrate the equation for the velocity potential and the equation for the convective-diffusion transfer of impurities, a locally one-dimensional difference scheme is used. The calculation of unknown parameters is implemented by an explicit formula. A package of application programs was created on the basis of the developed numerical model. The constructed numerical model and software package allow to study the process of the formation of pollution areas near the highway almost in real time. The results of the computational experiment are presented. The results of a laboratory experiment on the study of the structure of the contamination zone near the protective screen are presented.

Study on a horizontal axial flow pump during runaway process with bidirectional operating conditions

The ultra-low head pump stations often have bidirectional demand of water delivery, so there is a risk of runaway accident occurring in both conditions. To analyze the difference of the runaway process under forward runaway condition (FRC) and backward runaway condition (BRC), the whole flow system of a horizontal axial flow pump is considered. The Shear-Stress Transport (SST) k–ω model is adopted and the volume of fluid (VOF) model is applied to simulate the water surface in the reservoirs. Meanwhile, the torque balance equation is introduced to obtain the real time rotational speed, then the bidirectional runaway process of the pump with the same head is simulated. In addition, the vortex transport equation and swirl number are proposed to reveal the flow characteristics during the runaway process. The results show that the runaway process can be divided into five stages: the drop, braking, rising, convergence and runaway stages, according to the changing law of torque curve. In the rising stage, the pressure difference on the blade surface continues to increase, which contributes to the abnormal torque increase. In this stage, the flow hits the pressure surface (PS) at a faster speed enlarging the pressure on PS, and the flow separation takes place on the suction surface (SS) weakening the pressure on SS. During the convergence and runaway stage, the pulsation amplitude of torque and axial force under FRC is obviously larger than those under BRC. This is because the rotation frequency of the vortex rope is the same as main pressure fluctuation frequency in impeller under FRC, which enhances the pulsation amplitude. Whereas the vortices are broken due to the inhibitive effect from guide vanes under BRC.

Seasonal Modulation of Dissolved Oxygen in the Equatorial Pacific by Tropical Instability Vortices

AbstractTropical instability vortices (TIVs) have a major influence on the physics and biogeochemistry of the equatorial Pacific. Using an eddy‐resolving configuration of the Community Earth System Model (CESM‐HR) and Lagrangian particle tracking, we examine TIV impacts on the three‐dimensional structure and variability of dissolved oxygen (O2) in the upper equatorial Pacific water column. In CESM‐HR, the simulated generation and westward propagation of TIVs from boreal summer through winter lead to the seasonal oxygenation of the upper northern equatorial Pacific, exhibited as a deepening of hypoxic depth west of 120°W. TIV effects on the equatorial Pacific oxygen balance are dominated by eddy‐advection and mixing, while indirect TIV effects on O2 consumption play minor roles. These advective effects reflect the transient displacements of isopycnals by eddy pumping as well as vortex transport of oxygen by eddy trapping, stirring, and subduction. TIVs influence on the upper equatorial Pacific O2 distribution and variability has important implications for understanding and modeling marine ecosystem dynamics and habitats, and should be taken into consideration in designing observation networks in this region.

Vortex transport entropy in cuprate superconductors and Boltzmann constant

The vortex transport entropy in the mixed state of the cuprate superconductors YBa2Cu3O7−δand Bi2Sr2CaCu2O8+x at the maximum Nernst or Ettingshausen signal below Tc is discussed. The vortex transport entropy per Cu02 double layer in the cuprate epitaxial films is found to be close to the Boltzmann constant, in agreement with the recent results of C. W. Rischau et al. on two-dimensional superconductors.

Lattice Boltzmann method for computational aeroacoustics on non-uniform meshes: A direct grid coupling approach

The present study proposes an accurate lattice Boltzmann direct coupling algorithm, well suited for industrial purposes, making it highly valuable for aeroacoustic applications. It is indeed known that the convection of vortical structures across a grid refinement interface, where cell size is abruptly doubled, is likely to generate spurious noise that may corrupt the solution over the whole computational domain. This issue becomes critical in the case of aeroacoustic simulations, where accurate pressure estimations are of paramount importance. Consequently, any interfering noise that may pollute the acoustic predictions must be reduced.The proposed grid refinement algorithm differs from conventionally used ones, in which an overlapping mesh layer is considered. Instead, it provides a direct connection allowing a tighter link between fine and coarse grids, especially with the use of a coherent equilibrium function shared by both grids. Moreover, the direct coupling makes the algorithm more local and prevents the duplication of points, which might be detrimental for massive parallelization. This work follows our first study (Astoul et al. 2020 [1]) on the deleterious effect of non-hydrodynamic modes crossing mesh transitions, which can be addressed using an appropriate collision model: the hybrid recursive regularization. The grid coupling algorithm is assessed in the context of computational aeroacoustics and compared to a widely-used cell-vertex algorithm. The validation benchmark includes the simulation of (1) an acoustic pulse, (2) a vortex transport by a mean flow, and finally, (3) a turbulent circular cylinder wake flow at high Reynolds number. In the end, the proposed approach is proven to drastically reduced the spurious noise generated at grid interfaces, hence, paving the way for accurate and efficient aeroacoustic simulations based on lattice Boltzmann methods.

Circulation by Microwave-Induced Vortex Transport for Signal Isolation

A chip-scale circulator is proposed that operates via the rotation of persistent-current vortices, promising signal routing with moderate bandwidth, low insertion loss, and operation at microwave frequencies relevant for experiments with qubits.

Numerical Simulation of the Streamwise Transport of a Delta Wing Leading-Edge Vortex

Longitudinal vortices are a common flow phenomenon in the flow around aircraft. They emerge wherever sharp edges are encountered and affect the stability of the boundary layers with which they interact. Hence, for an optimal aircraft design, it is necessary to accurately predict not only the formation of these vortices, but also their downstream evolution. This paper presents a numerical simulation approach for the formation and downstream transport of longitudinal vortices. The approach consists in a hybrid Reynolds-averaged Navier–Stokes/large eddy simulation (RANS/LES) method with synthetic turbulence forcing for which different RANS/LES interface locations are investigated. To provide a quantitative analysis, an isolated sharp-edged delta wing configuration is adopted that allows to get rid of unwanted uncertainties. The presented RANS/LES approach is validated against particle image velocimetry data and compared with a wall-modeled LES and to a differential Reynolds stress model simulation. The results show that the RANS simulation is able to accurately predict the formation of the bulk vortical structure. However, its downstream transport can be accurately predicted only by a scale-resolving simulation. The hybrid RANS/LES approach allows to achieve this while saving computational resources by modeling the roll-up of the vortex. Nevertheless, its predictive accuracy highly depends on the appropriate placement of the RANS/LES interface.

Vortex dynamics and transport phenomena in stenotic aortic models using Echo-PIV

Atherosclerosis is the most fatal cardiovascular disease. As disease progresses, stenoses grow inside the arteries blocking their lumen and altering blood flow. Analysing flow dynamics can provide a deeper insight on the stenosis evolution. In this work we combined Eulerian and Lagrangian descriptors to analyze blood flow dynamics and fluid transport in stenotic aortic models with morphology, mechanical and optical properties close to those of real arteries. To this end, vorticity, particle residence time (PRT), particle's final position (FP) and finite time Lyapunov's exponents (FTLE) were computed from the experimental fluid velocity fields acquired using ultrasonic particle imaging velocimetry (Echo-PIV). For the experiments, CT-images were used to create morphological realistic models of the descending aorta with 0%, 35% and 50% occlusion degree with same mechanical properties as real arteries. Each model was connected to a circuit with a pulsatile programmable pump which mimics physiological flow and pressure conditions. The pulsatile frequency was set to ≈0.9 Hz (55 bpm) and the upstream peak Reynolds number (Re) was changed from 1100 to 2000. Flow in the post-stenotic region was composed of two main structures: a high velocity jet over the stenosis throat and a recirculation region behind the stenosis where vortex form and shed. We characterized vortex kinematics showing that vortex propagation velocity increases with Re. Moreover, from the FTLE field we identified Lagrangian coherent structures (i.e. material barriers) that dictate transport behind the stenosis. The size and strength of those barriers increased with Re and the occlusion degree. Finally, from the PRT and FP maps, we showed that independently of Re, the same amount of fluid remains on the stenosis over more than a pulsatile period.

Collapse of n Point Vortices, Formation of the Vortex Sheets and Transport of Passive Markers

In this paper, the motion of the n-vortex system as it collapses to a point in finite time is studied. The motion of vortices is described by the set of ordinary differential equations that we are able to solve analytically. The explicit formula for the solution demands the initial location of collapsing vortices. To find the collapsing locations of vortices, the algebraic, nonlinear system of equations was built. The solution of that algebraic system was obtained using Newton’s procedure. A good initial iterate needs to be provided to succeed in the application of Newton’s procedure. An unconstrained Leverber–Marquart optimization procedure was used to find such a good initial iterate. The numerical studies were conducted, and numerical evidence was presented that if in a collapsing system n=50 point vortices include a few vortices with much greater intensities than the others in the set, the vortices with weaker intensities organize themselves onto the vortex sheet. The collapsing locations depend on the value of the Hamiltonian. By changing the Hamiltonian values in a specific interval, the collapsing curves can be obtained. All points on the collapse curves with the same Hamiltonian value represent one collapsing system of vortices. To show the properties of vortex sheets created by vortices, the passive tracers were used. Advection of tracers by the velocity induced by vortices was calculated by solving the proper differential equations. The vortex sheets are an impermeable barrier to inward and outward fluxes of tracers. Arising vortex structures are able to transport the passive tracers. In this paper, several examples showing the diversity of collapsing structures with the vortex sheet are presented. The collapsing phenomenon of many vortices, their ability to self organize and the transportation of the passive tracers are novelties in the context of point vortex dynamics.

Phonon vortex dynamics in graphene ribbon by solving Boltzmann transport equation with ab initio scattering rates

In this work, we study thermal phonon vortex in graphene ribbon by a discrete-ordinate solution of phonon Boltzmann equation under Callaway's dual relaxation model. The phonon scattering rates of normal and resistive processes are acquired from ab initio calculation without need of any empirical input parameters. The temperature, size and isotope effects on transition from phonon vortex transport to conventional Fourier's heat conduction in both simple and complex geometries are investigated. The physical mechanism for the evolution of phonon vortex is declared by wide phonon mean free path distribution of resistive processes. A hierarchical vortex series is obtained with primary, secondary and terniary vortexes in a complicated geometry. The present work provides an accurate and efficient multi-scale numerical framework for modeling hydrodynamic phonon transport in high-thermal-conductivity materials and also sheds light on the heat dissipation applications.

Pressure Losses in Power-Law Fluid Flow through a Tube of Variable Cross-Section

Steady laminar flow of an incompressible power-law fluid is a tube with an obstacle of given shape is numerically simulated. The mathematical description of the process is based on the vortex transport equation and the Poisson equation for the stream function, while the rheological properties of the medium are described by the Ostwald—de Waele power law. The steady solution of the problem is obtained using a time-dependent method based on the finite-difference approximation of the governing equations. The pressure distribution is determined by numerically solving the Poisson equation. A parametric investigation of the kinematic and dynamic flow parameters, as functions of the control parameters of the problem, is performed for non-Newtonian media. The effect of the Reynolds number, the nonlinearity exponent, and the obstacle geometry on the coefficient of the local fluid resistance is demonstrated.

Comparison study of URANS and hybrid RANS-LES models on predicting vertical axis wind turbine performance at low, medium and high tip speed ratio ranges

Comparison study of unsteady Reynolds-averaged Navier-Stokes (URANS) and hybrid RANS-LES models is carried out for predicting the performance of three-straight-bladed vertical axis wind turbine operating within tip speed ratios (TSRs) ranges of (1.44–3.3). The evaluation is focused on power coefficient, moment coefficient and vortex structure generation, growth and transportation predictions. It was found that URANS turbulence modelling is sufficient for averaged power coefficient prediction and specific range of TSR evaluation, and it also benefits from short simulation run time. To further evaluate flow field details and to understand the underlying flow physics such as dynamic stall behaviour, hybrid RANS-LES turbulence modelling is necessary. Comparing between hybrid models adopted, stress-blended eddy simulation based on transition shear-stress transport (TSST) model has achieved an overall better performance, such as a reduction of simulation discrepancy by 50% in low TSRs range compared to TSST turbulence model. In both medium and high TSRs ranges, the modelling discrepancies are less than 3% compared to TSST turbulence models at about 25% extra computational time. Furthermore, in high TSRs range, hybrid RANS-LES models are able to predict the appearance of vortex shedding at high azimuthal angles (θ ≥ 180°) for which URANS models failed to capture.

Thermodynamics Investigation and Spike-stall Identification Based on Energy Loss of Centrifugal Compressor

Although quantities of investigations concerning the stall characteristics and thermal-dynamics of centrifugal compressor were carried out, rare discussions, however, were employed on the relationship between energy loss and flow unsteadiness, especially the losses originating from spike-type stall in the impeller. In the present work, a combined unsteady numerical and experimental analysis of energy loss features under stall condition in the impeller of centrifugal compressor was presented. Firstly, an energy loss prediction model based on one-dimensional equations considering multiple components was proposed and validated against experimental data. The dominating loss mechanisms in the impeller were quantitatively evaluated and the corresponding effect of each loss on various operating conditions was measured by proposing a weight coefficient. In order to prove the possibility of the potential application of the present approach, the weight coefficient method was also employed in another centrifugal compressor and the results were well validated. Then the incidence loss, which has the leading three weight coefficients during the occurrence of stall, were investigated with the distribution of inflow angle and entropy near stall condition. Then, applying blade loading variations and vortex transport analysis allowed the relevance of blade loading loss and clearance loss to be taken into account. Finally, with the synergy angles introduced, the criteria of spike stall through losses identification and qualification were established. The results imply that friction loss, clearance loss and incidence loss account for the major part of the total losses with the weight coefficient of 48%, 29% and 19% near stall condition. However, the weight coefficient of incidence loss increases remarkably with those of other losses decreasing from design condition to stall condition, indicating the significant contribution of incidence loss to the occurrence of stall. Correspondingly, the dramatic increase of inflow angle at limited circumferential region can be treated as a general criterion for spike-stall. The influence of tip clearance leakage on the leading edge of splitter blade results in the decrease of blade loading at reduced mass flow. Based on the synergy angle distribution, mutation region of field synergy angle accompanied with the increase of flow resistance at the impeller inlet may lead to a more efficient determination of the spike-stall for compressor impeller.

Construction of1D Vortex Chain Using a Chiral Nanostructure.

The construction and control of high‐order coupled vortices are a significant challenge for promoting the application of magnetic vortices. Thus far, only double‐coupled vortices have been produced and modulated in some ferromagnetic nanostructures. Here, an effective approach is provided to obtain a high‐order coupled vortex structure by using a chiral nanostructure. Double‐vortex, triple‐vortex, and n‐vortex chains can be successfully constructed using structured Fe4N nanostrips and bias nanomagnets. The designed chiral nanostructure cannot only control the transport and hybridization of vortices but also modulate the domain walls of the vortex chain for spin wave (SW) propagation. At the exciting frequency of 1.2 GHz, the SW propagates along the domain walls formed in the vortex chain. Upon increasing the frequency to 5.0 GHz, the SW gradually spreads from the domain walls into domains. This technique will present a new perspective for the design and application of magnetic vortex‐based devices.