Vortex Flow Field Research Articles

Investigation into the effect of vortex distribution on the flow field around a semi-submerged rectangular cylinder with different aspect ratios

ABSTRACT Vortex distribution affects the flow field variation of offshore structures at different aspect ratios (ARs; length-to-draft). This paper investigates the relationship between the vortex distribution and flow field around a semi-submerged rectangular cylinder (SRC) at a low Reynolds number (Re = 36300) using the OpenFOAM platform. First, vortex generation is explored, and then the influence of vortex distributions on the flow field around the SRC is analyzed. The numerical results show that when the SRC pierces the free surface, the wake region is different from the Kármán vortex street due to the free surface. AR mainly affects the range of the wake region and the SRC bottom vortex. In terms of vortex generation and differences in vortex structure, the variations of the flow field around the SRC are significantly affected by the vortex at the bottom of the SRC.

Fast and accurate flow measurement through dual-camera light field particle image velocimetry and ordered-subset algorithm

Light field particle image velocimetry (LF-PIV) can measure the three-dimensional (3D) flow field via a single perspective and hence is very attractive for applications with limited optical access. However, the flow velocity measurement via single-camera LF-PIV shows poor accuracy in the depth direction due to the particle reconstruction elongation effect. This study proposes a solution based on a dual-camera LF-PIV system along with an ordered-subset simultaneous algebraic reconstruction technique (OS-SART). The proposed system improves the spatial resolution in the depth direction and reduces the reconstruction elongation. The OS-SART also reduces the computational time brought by the dual-camera LF-PIV. Numerical reconstructions of the particle fields and Gaussian ring vortex field are first performed to evaluate the reconstruction accuracy and efficiency of the proposed system. Experiments on a circular jet flow are conducted to further validate the velocity measurement accuracy. Results indicate that the particle reconstruction elongation is reduced more than 10 times compared to the single-camera LF-PIV and the reconstruction efficiency is improved at least twice compared to the conventional SART. The accuracy is improved significantly for the ring vortex and 3D jet flow fields compared to the single-camera system. It is therefore demonstrated that the proposed system is capable of measuring the 3D flow field fast and accurately.

Application of Vortex Identification Methods in Vertical Slit Fishways

The reproduction and survival of fish are often negatively affected by the construction of dams and other hydroelectric projects, which cut off their migratory routes. Building effective fish passage facilities that allow fish to pass through dams smoothly alleviates the negative impact of hydroelectric projects on the ecological environment, thus protecting the diversity of aquatic species and preventing the extinction of indigenous fish. Vertical slit fishways are highly effective, but turbulence inside the fishway pools directly affects fish passage. In this study, the large-eddy simulation framework is used to capture the vortex characteristics in the interior of vertical slit fishway pools, and the volume of fluid method is applied to simulate the free surface. The independence of the grid is assessed by the large-eddy simulation quality index, and the simulation results are compared with experimental acoustic Doppler velocimetry data. This work characterizes the vortex flow field inside the vertical slit fishway using the Q-criterion, Omega method, and Liutex vortex identification method. The results show that the vortex structure inside the fishway pool has obvious three-dimensional characteristics and vortex structure varies within the different fishway pool chambers. The analysis and comparison of the three different vortex identification methods show that the vortex structure captured by the Liutex method is more consistent with the actual motion pattern of the fishway water flow.

Experimental characterization of the flowfield and cavitation physics of a tip-loaded hydrofoil

This paper reports an experimental study of tip vortex flowfield and cavitation inception of a tip-loaded hydrofoil. Vortex strength, wandering, and turbulence statistics are characterized using stereo particle image velocimetry (SPIV) in a water tunnel facility, at a chord Reynolds number of 1.3×106. Cavitation physics are characterized using high-speed videography and dual-hydrophone acoustic cavitation measurements. The loading of the rectangular planform hydrofoil has a maximum at 65% span, 56% greater than that at the root, i.e., the hydrofoil loading is representative of non-elliptically loaded open propellers. Acoustic cavitation inception is quantified and is observed to precede visual cavitation onset using unaided and high-speed imaging. Measurements reported here show that vorticity fluctuations are nearly of the same magnitude as the ensemble vorticity. Instantaneous measurements of vorticity at the trailing edge, 12-chord downstream, and one-chord downstream positions are reported. Their peak magnitudes are located adjacent to the ensemble vortex center and are between four and five times the ensemble mean. The fluctuating vorticity measurements, taken in conjunction with high-speed video observations, provide insight into the hydrodynamic conditions responsible for intermittent cavitation events. The reported measurements elucidate instantaneous and mean turbulence physics associated with vortex cavitation and can provide a validation basis for numerical simulations.

Sparse sensor reconstruction of vortex-impinged airfoil wake with machine learning

Reconstruction of unsteady vortical flow fields from limited sensor measurements is challenging. We develop machine learning methods to reconstruct flow features from sparse sensor measurements during transient vortex–airfoil wake interaction using only a limited amount of training data. The present machine learning models accurately reconstruct the aerodynamic force coefficients, pressure distributions over airfoil surface, and two-dimensional vorticity field for a variety of untrained cases. Multi-layer perceptron is used for estimating aerodynamic forces and pressure profiles over the surface, establishing a nonlinear model between the pressure sensor measurements and the output variables. A combination of multi-layer perceptron with convolutional neural network is utilized to reconstruct the vortical wake. Furthermore, the use of transfer learning and long short-term memory algorithm combined in the training models greatly improves the reconstruction of transient wakes by embedding the dynamics. The present machine-learning methods are able to estimate the transient flow features while exhibiting robustness against noisy sensor measurements. Finally, appropriate sensor locations over different time periods are assessed for accurately estimating the wakes. The present study offers insights into the dynamics of vortex–airfoil interaction and the development of data-driven flow estimation.Graphic abstract

Acoustic reconstruction of the vortex field in the nonuniform temperature field of a simulated furnace

The high-temperature thermal vortex flows generated by the tangential combustion of large-scale coal-fired power plant boilers represent a complex medium that causes acoustic ray bending. In this paper, nonlinear acoustic tomography is combined with particle swarm optimization as a means of reconstructing the two-dimensional thermal vortex field. The nonuniform and large-gradient temperature distribution is the main cause of acoustic ray bending. We establish curved paths of acoustic propagation that are closely related to the temperature distribution. Based on these curved paths, a set of radial basis functions combined with Tikhonov regularization is derived to achieve more accurate reconstruction of the temperature field. The nonuniform temperature field affects the reconstruction of the flow field information. Our model considers a variation in the angle between the direction vector of the acoustic curved paths and the horizontal and vertical components of the flow velocity, and incorporates a priori information and a six-parameter hypersurface into the flow field reconstruction scheme. Particle swarm optimization is applied to minimize the objective function and obtain the vortex velocity field. Numerical experiments show that the proposed mathematical model of acoustic ray bending improves the reconstruction accuracy of the temperature distribution and vortex flow field. The acoustic time-of-flight is experimentally measured through the generalized cross-correlation with different window functions. The temperature and vortex fields reconstructed based on the actual time-of-flight are compared with thermocouple and anemometer measurements to demonstrate the validity and robustness of the proposed reconstruction model.

An integrated Lattice-Boltzmann model of immiscible two-phase flow and bulk mass transfer with Marangoni effect

The microscale reactive immiscible multiphase flow often happens for the miniaturized chemical reactor such as micro fixed-bed-reactor, but it is significantly challenging in solving concurrently hydrodynamic, mass transfer and reaction behaviors in the porous packing media. Based on the phase-field (PF) and the revised Fick's law mass transfer approaches, a new multicomponent-multiphase Lattice-Boltzmann (LB) model with chemical reaction and the Marangoni effect was built up. Both the bulk homogeneous and fluid-solid interface heterogeneous reactions were resolved for the reactive two-phase fluid. The curved boundary treatment in Cartesian grids was upgraded to describe the heterogeneous reaction in the LB framework. Those mathematical models were validated respectively and showed good agreement with the analytical solutions. Then we applied our new models in the vortex flow field in a micro-structured reactive substrate. The numerical results showed that the newly developed LB model is able to simulate reactive multiphase flow in micro-structured porous media, which can provide a strong basis for future industrial applications.

Bidirectional vortex stabilization of a supersonic inductively coupled plasma torch

Radio-frequency (RF) inductively coupled plasma (ICP) torches using a supersonic nozzle have many industrial materials processing applications and have also been proposed as novel electrothermal plasma thrusters for space propulsion. The gas injection method in plasma torches plays an important role in both gas heating dynamics and overall discharge stabilization. Here, we investigate reverse vortex gas injection into a supersonic ICP torch for RF powers up to 1 kW, argon mass flow rates between 15 and 180 mg s−1, and plasma torch pressures from ∼270 Pa to ∼50 kPa. In this configuration, gas is injected tangentially just upstream of the nozzle inlet. This produces a bidirectional vortex flow field where gas first spirals upwards along the outer edge of the plasma torch walls, before then reversing direction at the torch end and spiralling back down through the central plasma region towards the nozzle exit. Results are compared to a more conventional forward vortex configuration where gas is instead injected tangentially from the upstream end of the torch, and which forms a unidirectional vortex that spirals towards the downstream nozzle. While performance is similar for gas flows below 80 mg s−1, we show that at higher mass flow rates both the effective torch stagnation temperature and thermal efficiency can be increased by almost 50% with reverse vortex injection. Considering that the measured RF antenna-plasma power transfer efficiency is similar for both configurations, this enhancement occurs because of the unique bidirectional vortex flow field which leads to reduced gas-wall heat losses and consequently an increased enthalpy flow leaving the torch.

Numerical simulation of vortex flow field generated in a novel nested-bottled photobioreactor to improve Arthrospira platensis growth

In this study, computational fluid dynamics were employed to examined clockwise and anticlockwise vortexes in the rising and down coming sections of novel nested-bottle photobioreactor. The radial velocity was increased by four times which significantly reduced dead zones compared to traditional PBR. The (NB-PBR) comprised of integrated bottles connected by curved tubes (d = 4 cm) that generated dominant vortices as the microalgae solution flows through each section (h = 10 cm). The (NB-PBR) was independent of the inner and outer sections which increased the mixing time and mass-transfer coefficient by 13.33 % and 42.9 %, respectively. Furthermore, the results indicated that the (NB-PBR) showed higher photosynthesis efficiency preventing self-shading and photo-inhibition, resulting in an increase in biomass yield and carbon dioxide fixation by 35 % and 35.9 %, respectively.

2D Numerical investigation of surface wettability induced liquid water flow on the surface of the NACA0012 airfoil

The accumulation of ice on wind turbine blades is a serious threat to the safety of wind power generation. Anti-icing techniques with low energy consumption, especially when combined with superhydrophobic surfaces, have recently attracted increasing levels of interest. In this work, the anti-icing mechanism of superhydrophobic surfaces was investigated by conducting the two-dimensional numerical analysis of the flow patterns of liquid water on surfaces with different wettability, which was based on the phase field method (PFM) and a dynamic contact angle (DCA) model. It is found that the wettability of surfaces significantly influences the flow, break, and shedding behaviors of liquid water on the surfaces. An increase in contact angle (CA) leads to more frequent break and shedding behaviors, and the flow pattern of liquid water gradually transformed from continuous water film to fragmented droplets because of the vortical flow field. In addition, it is also investigated that when the CA of surfaces exceeded 145°, the broken liquid water completely shed away in 0–0.1 chord length. Moreover, the average chordwise location (x‾/c) and maximum chordwise location (xmax/c) of the liquid water that had traveled the farthest on the surface in 30 ms gradually decreased with increasing contact angle, which is beneficial to superhydrophobic-dry anti-icing. What's important, the wettability characteristics should be responsible for the unique water flow patterns coupled with shear stress, which has been validated again, and previously proposed by our work. The results of this study provide theoretical support for the incorporation of superhydrophobic surfaces in anti-icing technology applied to wind turbine blades, which is promising for solving the icing issue of wind turbine blades with low energy consumption.

Pix2Pix and Deep Neural Network-Based Deep Learning Technology for Predicting Vortical Flow Fields and Aerodynamic Performance of Airfoils

Traditional computational fluid dynamics (CFD) methods are usually used to obtain information about the flow field over an airfoil by solving the Navier–Stokes equations for the mesh with boundary conditions. These methods are usually costly and time-consuming. In this study, the pix2pix method, which utilizes conditional generative adversarial networks (cGANs) for image-to-image translation, and a deep neural network (DNN) method were used to predict the airfoil flow field and aerodynamic performance for a wind turbine blade with various shapes, Reynolds numbers, and angles of attack. Pix2pix is a universal solution to the image-to-image translation problem that utilizes cGANs. It was successfully implemented to predict the airfoil flow field using fully implicit high-resolution scheme-based compressible CFD codes with genetic algorithms. The results showed that the vortical flow fields of the thick airfoils could be predicted well using the pix2pix method as a result of deep learning.

Fluid–structure interaction simulation for studying hemodynamics and rupture risk of patient-specific intracranial aneurysms

Purpose The role of regional hemodynamics in intracranial aneurysms (IAs) hemodynamics and rupture risk has been widely discussed based on numerical models over the past decades. The aim of this paper is to investigate hemodynamics and rupture risk with a complicated IA model. Methods Fluid-structure interaction (FSI) simulations were performed to quantify the hemodynamic characteristics of the established IA models. Hemodynamic parameters, including wall shear stress (WSS), flow velocity, and flow pattern, were calculated and analyzed. Results In this paper, the risk assessment of intracranial aneurysms focuses on the mechanical properties of blood flow and blood vessel walls. Vortex flow and concentrated impact field during blood flow play a decisive role in the rupture and development of aneurysms. The uneven distribution of wall shear stress on the vessel wall has a great influence on growth and rupture. By observing the simulation results of rigid walls, risks can be predicted efficiently and accurately. Conclusions This paper focuses on the relationship between hemodynamics and ruptures risk with a double intracranial aneurysms disease mode and provides a new perspective on the treatment of intracranial aneurysms.

Optimization of Transducer Array for Cross-Sectional Velocity Field Reconstruction in Acoustic Tomography

As a part of industrial process tomography, acoustic tomography (AT) is a promising method for high-quality velocity field measurement, which is widely utilized in the monitoring of multiphase flow, atmospheric environment, and so on. However, subjected to the safety and reliability requirements of furnaces or industrial pipes, the number of installed transducers is restricted, which results in sparse valid data, and, thus, ill-posed and rank-deficient limited-data linear AT. Therefore, special attention should be given to the transducer arrangement in the framework construction module to maximize the reconstruction accuracy. Unlike methods that minimize relative reconstruction error including true value of interesting field, finding an alternative prior knowledge-based predictor that maps the reconstruction error is more practical and preferred. In this work, based on the prior information that the linear independence of vectors incorporates maximum measurement information, a linear independence degree (LID) metric is proposed to optimize the transducer array. It contains no prior information about flow velocity fields, which enables the procedure to be performed in an off-line mode. A covariance matrix adaptation evolution strategy (CMA-ES) algorithm is employed to solve this minimization problem. Proof-of-concept demonstrations, including representative synthetic vortical flow fields with a biased single vortex or with double vortices and experimental data-based velocity field, are performed by comparison with existing representative optimization strategies. Qualitative and quantitative results show the feasibility and robustness of the LID metric in improving imaging accuracy of arbitrary practical velocity fields based on AT. This prior metric is expected to provide guidance for better diagnosis of multiphase flow and optimization of industrial flow process.

CFD simulation and parameter optimization of the internal flow field of a disturbed air cyclone centrifugal classifier

In the present study, a new disturbance structure is designed to improve the performance of classification air cyclones. To this end, numerical simulations and experiments are carried out to analyze the flow field inside the disturbance structure. It is found that the perturbation structure has a great effect on the velocity and pressure distributions, properties of gas and particles, and the classification performance of the cyclonic air centrifugal classifier. The obtained results show that the pressure and velocity profile in different radial sections has W-shaped and Rankin distributions, respectively. The outer layer is quasi vortex-free, while the inner layer is quasi forced vortex, and the forced vortex is the dominant force field that affects the classification performance. The critical speed of the perturbation structure to reach a uniform stability of the graded flow field is 1500 rpm. When the particle motion path is completed and the trajectory of different particle sizes is regular, the helix angle of the gas flow trajectory is 28°, the gas flow has low speed and high density, the effective graded flow field occupies more space, and the vortex flow field is strong, uniform, and stable. It is found that any speed other than the critical speed (1500 rpm) affects the uniformity and stability of the airflow. The obtained results show that for a rotational speed of 1500 rpm, a feed speed of 0.3 kg/s, and an inclination angle of 4°, macroscopic and microscopic classification efficiencies exceed 91 %. The order of significant effects between parameters was determined as Rf > v > β, and the correlation model between classification index and parameter factors was established. This article is expected to provide a reference to study the influence of internal flow field and classification performance of large-scale industrial air classifiers.

Microfluidic vortex focusing for high throughput synthesis of size-tunable liposomes

Control over vesicle size during nanoscale liposome synthesis is critical for defining the pharmaceutical properties of liposomal nanomedicines. Microfluidic technologies capable of size-tunable liposome generation have been widely explored, but scaling these microfluidic platforms for high production throughput without sacrificing size control has proven challenging. Here we describe a microfluidic-enabled process in which highly vortical flow is established around an axisymmetric stream of solvated lipids, simultaneously focusing the lipids while inducing rapid convective and diffusive mixing through application of the vortical flow field. By adjusting the individual buffer and lipid flow rates within the system, the microfluidic vortex focusing technique is capable of generating liposomes with precisely controlled size and low size variance, and may be operated up to the laminar flow limit for high throughput vesicle production. The reliable formation of liposomes as small as 27 nm and mass production rates over 20 g/h is demonstrated, offering a path toward production-scale liposome synthesis using a single continuous-flow vortex focusing device.

Numerical Investigation of Secondary Vortex Structures in a Rotor Wake

In hovering rotor computational fluid dynamics simulations, a common phenomenon is the breakdown of the primary vortex system due to secondary vortices, which are characterized by S-shaped vortical structures appearing between the primary blade-tip vortices. Since the presence of the secondary vortices is strongly influenced by the numerical settings, the degree to which secondary vortices physically occur warrants investigation. Therefore, this study investigates how the numerical settings affect the development of secondary vortices in hover simulations using the rotating blade geometry from previous experiments, where the focus is on temporal convergence. The computed near-wake vortical flow field is qualitatively compared to the experimental results. The calculations are then used to elucidate the secondary vortex development process, where it is shown that the secondary vortices develop by the interaction of the vortex sheet with the primary vortex structure. A number of parametric studies are done on the rotating blade geometry to investigate how the numerical settings affect the secondary vortex development process. Although the focus is on understanding the impact of temporal convergence, the effect of the off-body grid structure, grid rotation, mesh resolution, numerical dissipation, and time-step scaling is also studied. In contrast to previous work, the resulting wake breakdown from using the different numerical settings is compared both qualitatively and quantitatively. For relatively high flow solver subiteration convergence, the off-body grid structure and grid rotation have little effect on the development of the secondary vortex structures. However, at low solver subiteration convergence, larger differences occur between the torus and no torus grid structures. A parametric sweep on subiterations shows that as the subiteration residual drop increases the vortex sheet is broken down less and, correspondingly, both the rotor performance data and peak number of secondary vortices converge. The analysis shows that when the subiteration convergence or grid resolution is not high enough, the vortex sheet and primary vortex are broken down, which disrupt the development process and leads to fewer secondary vortices. To better understand the rotating blade case, a separate vortex ring study is done to observe how vortex breakdown is influenced by off-body grid structure and different levels of subiteration convergence. In agreement with the rotating blade case, when all flow solvers are sufficiently converged, vortex breakdown is minimized and the solution to the rotating and stationary torus is identical.

Numerical study on the effect of buoyancy-driven pollution source on vortex ventilation performance

High-temperature buoyancy-driven pollutants that generally exist in industrial buildings and kitchens often be eliminated by local exhaust ventilation systems. The high-temperature pollutant plume driven by buoyancy significantly affects the local exhaust process, resulting in the change in the exhaust flow field and reduction of the pollutant capture efficiency. In this study, the effects of thermal buoyancy-driven pollution sources on the vortex flow field and pollutant removal efficiency of a vortex ventilation system are evaluated through experiments and numerical simulations. The results show that with an increase in the buoyancy flux of the pollution source, the minimum negative pressure and maximum tangential velocity in the vortex flow first increase and then decrease, that is, the buoyancy plume first enhances and then weakens the vortex flow. A critical buoyancy flux exists, and the value of the critical buoyancy flux increases with an increase in the vortex ventilation intensity. Furthermore, the capture of high-temperature pollutants by vortex flow can be categorized as “plume-dominated” and “vortex-dominated”. These two types exhibit significant differences in pollutant transport paths and pollutant capture times during pollutant removal. In addition, on the basis of the pollutant capture efficiency, the vortex airflow performance with respect to capturing high-temperature buoyancy-driven pollutants can be classified as low-efficiency, transition, high-efficiency, and invalid zones. In the process of capturing high-temperature pollutants, vortex ventilation systems should be avoided in the low-efficiency and invalid zones.

A Variational Method for Analyzing Vortex Flows in Radar-Scanned Tornadic Mesocyclones. Part III: Sensitivities to Vortex Center Location Errors

Abstract When the vortex center location is estimated from a radar-scanned tornadic mesocyclone, the estimated location is not error-free. This raises an important issue concerning the sensitivities of analyzed vortex flow (VF) fields by the VF-Var (formulated in Part I of this paper series and tested in Part II) to vortex center location errors, denoted by Δxc. Numerical experiments are performed to address this issue with the following findings: The increase of |Δxc| from zero to a half of vortex core radius causes large analysis error increases in the vortex core but the increased analysis errors decrease rapidly away from the vortex core especially for dual-Doppler analyses. The increased horizontal-velocity errors in the vortex core are mainly in the Δxc-normal component, because this component varies much more rapidly than the other component along the Δxc direction in the vortex core. The vertical variations of Δxc distort the vertical correlation structure of Δxc-dislocated VF-dependent background error covariance, which can increase the analysis errors in the vortex core. The dual-Doppler analyses have adequate accuracies outside the vortex core even when |Δxc| increases to a half of vortex core radius, while single-Doppler analyses can also have adequate accuracies outside the vortex core mainly for the single-Doppler-observed velocity component. The sensitivities to Δxc are largely unaffected by the vortex slanting. The above findings are important and useful for assessing the accuracies of analyzed VFs for real radar-observed tornadic mesocyclones. Significance Statement When the vortex center location is estimated from a radar-scanned tornadic mesocyclone, the estimated location is not error-free. This raises an issue concerning the sensitivity of analyzed vortex flow (VF) by the VF-Var (formulated in Part I of this paper series and tested with simulated radar observations in Part II) to vortex center location error. This issue and its required investigations are very important for the VF-Var to be applied to real radar-observed tornadic mesocyclones, especially in an operational setting with the WSR-88Ds. Numerical experiments are performed to address this issue. The findings from these experiments are important and useful for assessing the accuracies of VF-Var analyzed VF fields for real radar-observed tornadic mesocyclones.

Research on the characteristics of the leeward vortex flow field on a blunt-headed rotating body at high angles of attack

Detached eddy simulation was used to numerically compute the leeward vortex flow field on a blunt-headed rotating body. The effects of changes in the angle of attack and flight velocity on the evolutionary law of the counter-rotating vortexes and the fluctuation characteristics of the lateral force coefficient were analyzed. The results show that the counter-rotating vortexes on the leeward surface of the blunt-headed rotating body gradually separate from the wall along the direction from the tail to the head of the projectile as the angle of attack increases. In addition, it shows a periodic variation law of alternating expansion and detachment of the unilateral separated vortex. When the angle of attack is greater than 30°, the fluctuation amplitude of the lateral force coefficient initially rises and then drops with an increase in the angle of attack, while the fluctuation frequency keeps increasing. The maximum amplitude and corresponding frequency of the lateral force coefficient under the main fluctuation remain in the range of 100–230 Hz and have a linear positive feedback relationship with the flight Mach number. With the angle of attack and flight velocity as independent variables, the fittings were performed on the varying laws of maximum amplitude and the corresponding frequency of the lateral force coefficient under the main fluctuation. An unsteady calculation equation for the lateral force coefficient under the change in coupling with the angle of attack and flight velocity was established.

Producing and imaging quantum turbulence via pair-breaking in superfluid He3−B

Turbulence remains one of the last unsolved problems of classical physics. Here, the authors present insights from the nucleation and evolution of the simpler quantum turbulence. They generate turbulence mechanically, and image the dynamics nondestructively through illumination of thermal quasiparticles. At low temperatures, the ballistic quasiparticles are Andreev-reflected by the vortex flow fields, producing a shadow which shows that the tangle is asymmetrical. This suggests that the vortex rings - precursors of quantum turbulence - are unevenly created by the generator.