Vortex Velocity Field Research Articles

Investigation of wake dynamics near the cylinder with the variation of length-to-diameter ratio

The study investigates the impact of drag forces on underwater pipeline structures at junctions where diameter changes occur, utilizing three-dimensional numerical simulations to analyze flow patterns around dual-step cylinders at a Reynolds number of 5000. The numerical model is validated against benchmark studies for a plain circular cylinder at heating condition, demonstrating strong agreement. Subsequently, cylinder temperatures were varied to assess their influence on drag force. The study further explores the effect of varying the aspect ratio L/D from 1 to 3 on drag reduction. Analysis of the flow structures is conducted, focusing on vortex patterns, velocity fields, and force coefficients surrounding the cylinders. Additionally, streamwise and crosswise velocities are analyzed at different sectional planes of the step cylinder. Results indicate that the drag force diminishes with increasing aspect ratio, correlating with formation length (Lf). At lower aspect ratios, step-induced vortices migrate toward the wall, while at higher aspect ratios, they drift midway. This research provides insights for designing stepped diameter cylinder systems in the subcritical regime, contributing to optimized underwater pipeline structures.

Numerical analysis of the flow over four side-by-side square cylinders with different gaps

This study conducts two-dimensional numerical simulations of the flow over four square cylinders arranged side by side at a low Reynolds number (Re) of 100. The investigation primarily centers on the influence of the gap to a square cylinder width ratio (g*) on the flow. The range of g* spans from 0.1 to 7.0. Within this parameter range, three distinct flow regimes emerge based on the inherent flow characteristics. These regimes are defined as follows: (1) single bluff body flow (g* ≤ 0.3), (2) flip-flopping flow (0.3 < g* < 2.0), and (3) modulated periodic flow (g* ≥ 2.0). Additionally, the modulated periodic flow is further categorized into three distinct flow patterns. Various aspects of these different flow regimes are examined, including vortex contours, velocity fields, and liquid force coefficients around the cylinders. Moreover, detailed illustrations are provided for the modulation behaviors in vortex structures and liquid force coefficients. Finally, the proper orthogonal decomposition technique is employed to identify and analyze the underlying spatial coherent structures in the flow field, offering further insights into the dynamic features of wakes.

Hydrodynamics of quantum vortices on a closed surface

We develop a neutral vortex fluid theory on closed surfaces with zero genus. The theory describes collective dynamics of many well-separated quantum vortices in a superfluid confined on a closed surface. Comparing to the case on a plane, the covariant vortex fluid equation on a curved surface contains an additional term proportional to Gaussian curvature multiplying the circulation quantum. This term describes the coupling between topological defects and curvature in the macroscopic level. For a sphere, the simplest nontrivial stationary vortex flow is obtained analytically and this flow is analogous to the celebrated zonal Rossby-Haurwitz wave in classical fluids on a nonrotating sphere. For this flow the difference between the coarse-grained vortex velocity field and the fluid velocity field generated by vortices is solely driven by curvature and vanishes in the corresponding vortex flow on a plane when the radius of the sphere goes to infinity. Published by the American Physical Society 2024

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.

Investigation on the flow around a submarine under the rudder deflection condition by using URANS and DDES methods

Considering the complex flow features of a submarine under the maneuvering condition and their close relationship with the maneuverability, a numerical study is performed for the flow around the submarine under the rudder deflection condition. Taking the submarine model DARPA SUBOFF as the study object, the rudder force test with a wide range of rudder angles is simulated based on the open source CFD platform OpenFOAM. Two numerical methods of Unsteady Reynolds-Averaged Navier-Stokes (URANS) method and Delayed Detached Eddy Simulation (DDES) method are employed. A systematic convergence study is performed to clarify the effect of grid spacing and time step on the numerical solution. The computed hydrodynamic forces are compared with the available experimental data, and the effectiveness of the adopted numerical methods is validated. Besides, the obtained rudder forces and rudder parameters are compared and discussed. Based on the obtained flow field details of vortex structures, velocity fields, streamline distribution, turbulence levels, and pressure distribution, the flow mechanisms of the deflected rudders are analyzed. Compared with URANS method, DDES method presents a better capability of capturing the flow field details around the submarine under the tight maneuvering condition, where remarkable flow separation is involved.

Detached eddy simulation on the structure of swirling jet flow field

The improved delayed detached eddy simulation method with shear stress transport model was used to analyze the evolution of vortex structure, velocity and pressure fields of swirling jet. The influence of nozzle pressure drop on vortex structure development and turbulence pulsation was investigated. The development of vortex structure could be divided into three stages: Kelvin-Helmholtz (K-H) instability, transition stage and swirling flow instability. Swirling flow could significantly enhance radial turbulence pulsation and increase diffusion angle. At the downstream of the jet flow, turbulence pulsation dissipation was the main reason for jet velocity attenuation. With the increase of pressure drop, the jet velocity, pulsation amplitude and the symmetry of velocity distribution increased correspondingly. Meanwhile the pressure pulsation along with the axis and vortex transport intensity also increased significantly. When the jet distance exceeded about 9 times the dimensionless jet distance, the impact distance of swirling jet could not be improved effectively by increasing the pressure drop. However, it could effectively increase the swirl intensity and jet diffusion angle. The swirling jet is more suitable for radial horizontal drilling with large hole size, coalbed methane horizontal well cavity completion and roadway drilling and pressure relief, etc.

Numerical investigation of full scale coaxial main rotor aerodynamics in hover and vertical descent

The original free vortex wake model was used for numerical investigation. Calculation of the aerodynamic characteristics in hover and vertical descent modes in the range of vertical descent speed of 0–30 m/s including the Vortex Ring State (VRS) area was performed. The calculations were carried out under the condition of variable blade pitch angle values providing a fixed time-average thrust value. Visualization data of free vortex wake shapes, flow structures, and velocity fields were obtained and analyzed. The time-dependences of the rotor’s thrust and torque coefficients were obtained and analyzed. The obtained data allows determining the boundaries of the VRS area by various criteria such as rotor thrust and torque pulsations, growth of rotor power consumption relative to the hover, growth of rotor induced velocities relative to the hover, and growth of the required rotor blade pitch angles values. The results of the study are compared with experimental and calculated data of other authors and can significantly supplement the available results of experimental and computational studies in this area.

Wandering corrections from PIV measurements of tornado-like vortices

Wandering of tornado-like vortices consists in random oscillations of the vortex core from its time-averaged position, which complicates efforts to characterize vortex characteristics. A procedure is then necessary to retrieve characteristics of tornado-like vortices not affected by wandering smoothing effects. This study explores two procedures to correct wandering effects on Particle Image Velocimetry data obtained from a down-scaled model of the WindEEE Dome simulator. The first procedure re-centers the velocity data as a function of the instantaneous location of the vortex center. The second procedure treats the time-averaged vortex velocity field as the convolution of a bi-variate normal probability density function, which represents the distribution of vortex center locations over horizontal planes orthogonal to the vortex axis and the instantaneous tornado velocity field. Depending on swirl ratio and vortex height, wandering amplitude was generally from 5% to 9% of the updraft radius. The re-centering procedure was found to be more accurate than the deconvolution procedure. When applied to the turbulence statistics of the velocity field, the correction revealed that a higher level of turbulence at the center of low swirl tornadoes is a result of wandering. Also, the corrected shear stresses revealed a spiral pattern for cases of higher swirl. Wandering effects increases with reducing swirl ratio. For this experiment, swirl ratio is reduced from 0.96 down to 0.22 and errors on the vortex core radius as high as 50% and reduction of the maximum tangential velocity of 13% were observed.

Evaluation of curvature correction methods for tip vortex prediction in SST [formula omitted] turbulence model framework

This paper presents and studies effects of curvature correction (CC) methods to improve two equation RANS simulations of tip vortex flows, exemplified using the SST k−ω turbulence model. Performance of the CC models is first evaluated in the classical Rankine vortex flow field, and then extended into the study of tip vortex flows over an elliptical foil. The results have been compared with experimental measurements in terms of the vortex strength and velocity field, and the importance of the turbulence closure in tip vortex simulations is highlighted.Contribution of the CC models in different terms of the turbulent kinetic energy and specific dissipation transport equations are described, and it is discussed why a CC model may have mesh resolution dependent results. By considering the distribution of the CC function, it is shown that although some of the models can predict the location of the tip vortex core accurately, they still do not significantly improve the vortex prediction as the impact on the turbulent viscosity is wrong or not enough. It is further noted that as some of these models have been calibrated on specific vortex flows, they may not be completely applicable for other cases without recalibration. It is shown that some CC models provide accurate tip vortex predictions, primarily the ones based on the sensitization of the turbulent viscosity. Further, it is noteworthy that the successful models are active not only around the vortex, but also change the boundary layer characteristics on the foil, and the boundary layer separation lines, which consequently can provide the required momentum for the vortex core accelerated axial velocity.

Submesoscale Tidal-Inlet Dipoles Resolved Using Stereo WorldView Imagery

A pair of high-resolution visible-band satellite images, acquired 65 s apart and analyzed using an optical-flow algorithm, is shown to provide a realistic snapshot of the velocity field of dipolar vortices (dipoles) emitted from the Gulf of San Jose, Argentina. The results reveal the expected counter-rotating vortices within three dipoles, as well as one monopole; the magnitude of vorticity ranges from 8 to 29 times the local Coriolis parameter. Analysis of the derived velocity and vorticity fields yields an estimate of 0.27 ms $^{-1}$ for the dipole self-propagation velocity, which is the part of the physics that allows transport of gulf-derived material into open waters. Dipole size, measured as the distance between vortex centers, increases with distance from the source area in a manner consistent with the effects of entrainment of ambient sea water. Given favorable imaging conditions, the general approach used here can provide a detailed portrayal of local circulation patterns—one suitable for use in validating high-resolution numerical models, especially in coastal areas having complex bathymetry.

Numerical simulation study on structural change of high-lift working condition of vortex pump

To understand the internal flow characteristics of the high-lift working condition of vortex pump, obtain the import and export structure parameters influence on the performance of vortex pump, vortex pump has established three-dimensional flow model of fixed channel, simulation of vortex pump numerical study of structural parameters of the import and export of the dimensions and angles by means of ANSYS FLUENT, comparative analysis of the structural parameters on the vortex pump field effect process, work efficiency, flow structure etc. The inlet flow rate of the vortex pump is 10 L/min, and the impeller speed is 3000 r/min. The research reveals the internal pressure of the vortex pump field and velocity field distribution, presented with fixed channel structure under the import and export of parameters to the design ideas to improve the efficiency of vortex pump, has certain reference value for the optimization of rotary pump interface design.

Aerodynamic forces and flows of the full and partial clap-fling motions in insects.

Most of the previous studies on Weis-Fogh clap-fling mechanism have focused on the vortex structures and velocity fields. Detailed pressure distribution results are provided for the first time in this study to reveal the differences between the full and the partial clap-fling motions. The two motions are studied by numerically solving the Navier–Stokes equations in moving overset grids. The Reynolds number is set to 20, relevant to the tiny flying insects. The following has been shown: (1) During the clap phase, the wings clap together and create a high pressure region in the closing gap between wings, greatly increasing the positive pressure on the lower surface of wing, while pressure on the upper surface is almost unchanged by the interaction; during the fling phase, the wings fling apart and create a low pressure region in the opening gap between wings, greatly increasing the suction pressure on the upper surface of wing, while pressure on the lower surface is almost unchanged by the interaction; (2) The interference effect between wings is most severe at the end of clap phase and the start of the fling phase: two sharp force peaks (8–9 times larger than that of the one-winged case) are generated. But the total force peaks are manifested mostly as drag and barely as lift of the wing, owing to the vertical orientation of the wing section; (3) The wing–wing interaction effect in the partial clap-fling case is much weaker than that in the full clap-fling case, avoiding the generation of huge drag. Compared with a single wing flapping with the same motion, mean lift in the partial case is enhanced by 12% without suffering any efficiency degradation, indicating that partial clap-fling is a more practical choice for tiny insects to employ.

Simulation Methods for Aircraft Encounters with Deformed Wake Vortices

Simulation of aircraft wake-vortex encounters is regularly applied in the research toward revised aircraft separation minima. Most encounter flight simulation studies have used a pair of straight counter-rotating vortices. However, after being shed by the generating aircraft, the wake vortices begin to develop a growing deformation due to the long-wave Crow instability, especially under low-turbulent atmospheric conditions when vortices decay slowly. In this study, two methods for simulation of aircraft encounters with such perturbed wake vortices are described and compared. The first method uses a vortex simulation model, which is based on analytical models of the deformation that consider the large-scale vortex shapes. The second method applies vortex-velocity fields from high-fidelity large-eddy simulations. It offers the highest possible level of realism and is used as the reference. The resulting aircraft responses induced by wavy vortices and vortex rings are reproduced with good quality by the vortex simulation model. The comparison of both methods shows that the vortex simulation model captures the overall impact of vortex deformation on the aircraft upsets. It can be used for future safety assessments that require a large number of encounter simulations.

Experimental characterization of wingtip vortices in the near field using smoke flow visualizations

In order to predict the axial development of the wingtip vortices strength, an accurate theoretical model is required. Several experimental techniques have been used to that end, e.g. PIV or hot-wire anemometry, but they imply a significant cost and effort. For this reason, we have performed experiments using the smoke-wire technique to visualize smoke streaks in six planes perpendicular to the main stream flow direction. Using this visualization technique, we obtained quantitative information regarding the vortex velocity field by means of Batchelor’s model for two chord-based Reynolds numbers, $$Re_c=3.33\times 10^4$$ and $$10^5$$ . Therefore, this theoretical vortex model has been introduced in the integration of ordinary differential equations which describe the temporal evolution of streak lines as function of two parameters: the swirl number, S, and the virtual axial origin, $$\overline{z_0}$$ . We have applied two different procedures to minimize the distance between experimental and theoretical flow patterns: individual curve fitting at six different control planes in the streamwise direction and the global curve fitting which corresponds to all the control planes simultaneously. Both sets of results have been compared with those provided by del Pino et al. (Phys Fluids 23(013):602, 2011b. doi: 10.1063/1.3537791 ), finding good agreement. Finally, we have observed a weak influence of the Reynolds number on the values S and $$\overline{z_0}$$ at low-to-moderate $$Re_c$$ . This experimental technique is proposed as a low cost alternative to characterize wingtip vortices based on flow visualizations.

A framework for synthetic validation of 3D echocardiographic particle image velocimetry

Particle image velocimetry (PIV) has been significantly advanced since its conception in early 1990s. With the advancement of imaging modalities, applications of 2D PIV have far expanded into biology and medicine. One example is echocardiographic particle image velocimetry that is used for in vivo mapping of the flow inside the heart chambers with opaque boundaries. Velocimetry methods can help better understanding the biomechanical problems. The current trend is to develop three-dimensional velocimetry techniques that take advantage of modern medical imaging tools. This study provides a novel framework for validation of velocimetry methods that are inherently three dimensional such as but not limited to those acquired by 3D echocardiography machines. This framework creates 3D synthetic fields based on a known 3D velocity field $${\mathbf{V}}$$ and a given 3D brightness field $${\mathbf{B}}$$ . The method begins with computing the inverse flow $${\mathbf{V}}^{\varvec{*}} $$ based on the velocity field $${\mathbf{V}}$$ . Then the transformation of $${\mathbf{B}}$$ , imposed by $${\mathbf{V}}$$ , is calculated using the computed inverse flow according to $${\mathbf{B}}^{\varvec{*}} \left( {\mathbf{x}} \right) = {\mathbf{B}}\left( {{\mathbf{x}} + {\mathbf{V}}^{\varvec{*}} \left( {\mathbf{x}} \right)} \right)$$ , where x is the coordinates of voxels in $${\mathbf{B}}^{\varvec{*}} $$ , with a 3D weighted average interpolation, which provides high accuracy, low memory requirement, and low computational time. To check the validity of the framework, we generate pairs of 3D brightness fields by employing Hill’s spherical vortex velocity field. $${\mathbf{B}}$$ and the generated $${\mathbf{B}}^{\varvec{*}} $$ are then processed by our in-house 3D particle image velocimetry software to obtain the interrelated velocity field. The results indicates that the computed and imposed velocity fields are in agreement.

Simulation of Airborne Radiometric Detection of Wake Vortices

This paper describes an analysis of the potential of using an airborne Fourier transform spectrometer (FTS) or radiometer to detect wake vortices. The goal was to determine the requirements for an infrared (IR) FTS to effectively detect wake vortices. Initially, a theoretical analysis of wake vortex detection by thermal radiation was realized in a series of simulations. The first stage used the Terminal Area Simulation System (TASS) dynamic model to simulate wake vortex temperature, moisture, and velocity fields. The second stage used these fields as input to the line-by-line radiative transfer model (LBLRTM) to simulate responses from both an imaging IR hyperspectral FTS and an IR imaging radiometer. These numerical simulations generated FTS and radiometer imagery that was compared with the original temperature data. This research supported an effort, using ground-based imaging FTS instruments, to make measurements of wake vortices of various landing aircraft. Results from two different field campaigns have been previously reported. Instrument specifications for wake vortex thermal detection are recommended for an imaging radiometer sensitive within the following two narrow spectral bands: 670–750 cm−1 and 2200–2350 cm−1 . The instrument must have at the very minimum a noise equivalent differential temperature < 2 mK and a spectral resolution of at least 32 cm−1.

Broadband acoustic trapping of a particle by a soft plate with a periodic deep grating

We investigated the acoustic radiation force (ARF) acting on a cylindrical brass particle near an acoustically soft plate patterned with a periodic deep grating. The existence of a negative ARF by which the particle can be pulled towards the sound source is confirmed. In addition, the bandwidth for negative ARF in this soft-plate system is found to be considerably broader than in the stiff-plate systems typically used in previous studies. It is further demonstrated by field distribution analysis that the negative ARF is caused by the gradient force induced by the gradient vortex velocity field near the surface, which stems from the collective resonance excitation of the antisymmetric coupling of Scholte surface waves in the thin plate. The effects of particle location and size on the ARF were also investigated in detail. The negative ARF has potential use in applications requiring particle manipulation using acoustic waves.

Characteristics of Flow around a Modified Square Prism

The present investigations focus attention on PIV measurements and three-dimensional numerical investigation on flow past a modified square prism with a sinusoidal surface. The detailed near wake vortex structures and velocity fields of such modified square prism are captured and compared with a straight square prism in the same flow conditions. The three-dimensional free shear layers behind the modified square prism extend further downstream and become more stable than those of the straight one. For a modified square prism, significant mean drag coefficient reduction and fluctuating lift suppression are obtained at the Reynolds numbers of 600 and 5900. It indicates that such modified square prism is capable of minimization of flow induced vibration and force reduction.

Application of Image Preprocessing on PIV Experimental Study of Wake Vortex Interactive Instability

A basic experiment study with a simplified wing model was carried out to introduce a opposite wake vortex by different sizes of spoilers. A PIV measurement was employed to obtain a two dimension wing wake vortex velocity field data of different spoilers in a water towing tank and confirm the existence of Reyleigh-Ludwieg instability in aircraft wake vortex. The image preprocessing is useful in PIV post-processing by comparing the three kinds of image preprocessing results with normal PIV image processing results.

PIV measurements of air-core intake vortices

Particle Image Velocimetry (PIV) measurements were conducted in a large-scale hydraulic model to obtain the horizontal velocity field of intake vortices with considerable air entrainment. Vortex wandering and air-core diameter were determined by object detection using PIV images and velocity vector maps. This allowed for the calculation of ensemble- and azimuthally-averaged velocity fields. The results of the horizontal velocity field around the vortex, whose circulation affects a wide area, confirm the applicability of the analytic solution of the 2D Navier–Stokes equation for a potential (free) vortex. The setup and the improvement of the PIV measurements and their limitations are presented. For example, a completely resolved velocity field close to the air-core did not exist due to reflections at the interface between air and water. Vortex characteristics like vorticity and circulation are derived from the velocity fields and turbulence is shown by representing in-plane Turbulent Kinetic Energy (TKE).