Vortex Trajectories Research Articles

Near wake study of counter-rotating horizontal axis tidal turbines based on PIV measurements in a wind tunnel

The reveal of the near wake structure and development is of vital importance to the understanding of the working principle and power coefficient of tidal turbines. In this paper, the detailed near wake flow field was visualized by high resolution particle image velocimetry (PIV) technique, and the time-averaged mean velocity (both axial and tangential) and phase-locked transient velocity were measured, respectively, for a counter-rotating horizontal axis tidal turbine in the wind tunnel. The vortex was captured by vorticity and swirling strength calculation of the vector field for each phase, and the vortex formation and development were analyzed based on the summation of the data from all the different phases. To achieve more information about vortex, the experiment was conducted for two different axis distances and three different tip speed ratios. The existence of the rear rotor affects the trajectory of vortices shed from the front rotor and enhances the velocity deficit of the free stream. The experimental result can serve as a reference for numerical simulations.

Parametric instability of a many point-vortex system in a multi-layer flow under linear deformation

The paper deals with a dynamical system governing the motion of many point vortices located in different layers of a multi-layer flow under external deformation. The deformation consists of generally independent shear and rotational components. First, we examine the dynamics of the system’s vorticity center. We demonstrate that the vorticity center of such a multi-vortex multi-layer system behaves just like the one of two point vortices interacting in a homogeneous deformation flow. Given nonstationary shear and rotational components oscillating with different magnitudes, the vorticity center may experience parametric instability leading to its unbounded growth. However, we then show that one can shift to a moving reference frame with the origin coinciding with the position of the vorticity center. In this new reference frame, the new vorticity center always stays at the origin of coordinates, and the equations governing the vortex trajectories look exactly the same as if the vorticity center had never moved in the original reference frame. Second, we studied the relative motion of two point vortices located in different layers of a two-layer flow under linear deformation. We analyze their regular and chaotic dynamics identifying parameters resulting in effective and extensive destabilization of the vortex trajectories.

Numerical simulation of wake vortices of crop spraying aircraft close to the ground

A study of the velocity field of the wake of a Thrush 510G carrying out spray application close to the ground was investigated. The aircraft normally operates in extreme ground effect and wake vortices play a major role in the dispersal and deposition of pesticide spray released behind the aircraft. The evolution of wake vortices over time, and their interaction with the ground were simulated using computational fluid dynamics (CFD) techniques. The simulation was initialised with meteorological data from a field experiment. Information given by CFD simulation included the trajectory of the wake vortices as well as circulation, which indicates the strength of vortex. This information provided insights into the velocity field in the wake, including the upwash near the wing tips and the downwash in between. The effect of turbulence intensity as well as crosswind on the wake vortices and the resulting velocity field were investigated.

Effect of the wind direction on the near wake structures of an Archimedes spiral wind turbine blade

For urban usage of an Archimedes spiral horizontal axis wind turbine, the effect of wind direction was examined for the confront measurement planes from 0° to 15°. Aerodynamic characteristics in the near wake of wind turbine blade were investigated using the particle image velocimetry method. Ensemble and phase-averaged velocity fields were obtained in both front and lee sides of the blade with various angles of attack. The streamlines are parallel with the incoming flow for the range of attack angles; however, wake velocity is decreasing with increasing angle of attack. It has been found that the trajectory of tip vortices varies significantly in the front side plane compared to that of lee side plane when the angle of attack is higher than 10°. Distance between two consecutive vortices in the front plane decreases monotonically with increasing attack angle, while that in the lee side shows an opporsite manner.

Analytical model of the optical vortex microscope.

This paper presents an analytical model of the optical vortex scanning microscope. In this microscope the Gaussian beam with an embedded optical vortex is focused into the sample plane. Additionally, the optical vortex can be moved inside the beam, which allows fine scanning of the sample. We provide an analytical solution of the whole path of the beam in the system (within paraxial approximation)-from the vortex lens to the observation plane situated on the CCD camera. The calculations are performed step by step from one optical element to the next. We show that at each step, the expression for light complex amplitude has the same form with only four coefficients modified. We also derive a simple expression for the vortex trajectory of small vortex displacements.

Steady and perturbed motion of a point vortex along a boundary with a circular cavity

The dynamics of a point vortex moving along a straight boundary with a circular cavity subjected to a background flow is investigated. Given the constant background flow, this configuration produces regular phase portraits of the vortex motion. These phase portraits are discriminated depending on the cavity's circular shape, and then the transition to chaos of the vortex motion is investigated given an oscillating perturbation superimposed on the background flow. Based on the steady-state vortex rotation, the forcing parameters that lead to effective destabilization of vortex trajectories are distinguished. We show that, provided the cavity aperture is relatively narrow, the periodic forcing superimposed on the background flow destabilizes the vortex trajectories very slightly. On the other hand, if the cavity aperture is relatively wide, the forcing can significantly destabilize vortex trajectories causing the majority of the trajectories, which would be closed without the forcing, to move towards infinity.

Effects of non-axisymmetric endwall contouring and film cooling on the passage flowfield in a linear turbine cascade

The time-resolved flowfield is measured in the passage of a linear turbine cascade to show the effects of endwall film cooling and non-axisymmetric endwall contouring on the passage secondary flows. A particle image velocimetry system is used in three measurement planes: the plane at the exit of the passage and two streamwise planes along the blade suction side. In the downstream half of the passage, the passage vortex moves away from the endwall toward the midspan, but closely follows the profile of the blade suction side. The secondary velocity vectors and vorticity fields in the passage exit plane indicate the large size of the passage vortex. The measured velocities in the streamwise measurement planes reveal the trajectory of the passage vortex as well as steep gradients in the direction normal to the blade surface. The passage vortex can also be identified by elevated flow unsteadiness as reported by turbulent kinetic energy levels. When passage film cooling is added, the size of the passage vortex, secondary velocities, and exit plane turbulent kinetic energy are all increased. Endwall contouring has the opposite effect, reducing the passage vortex size, the secondary velocities, and exit plane turbulent kinetic energy.

Unsteady aerodynamic characteristics of a translating rigid wing at low Reynolds number

Rectilinearly surging wings are investigated under several different velocity profiles and incidence angles. The primary wing studied here was an aspect ratio 4 rectangular flat plate. Studies on acceleration distance, ranging from 0.125c to 6c, and incidence angles 5°–45° were performed to obtain a better understanding of the force and moment histories during an extended surge motion over several chord-lengths of travel. Flow visualization and particle image velocimetry were performed to show the flow structures responsible for variations in force and moment coefficients. It was determined that the formation and subsequent shedding of a leading edge vortex correspond to oscillations in force coefficients for wings at high angle of attack. Comparing unsteady lift results to static force measurements, it was determined that for cases with large flow separation, even after 14 chords traveled at a constant velocity, the unsteady forces do not converge to the fully developed values. Forces were then broken up into circulatory and non-circulatory components to identify individual contributors to lift. Although it was observed that the “fast” and “slow” cases produced nearly identical vortex trajectories, circulation measurements confirmed that the faster acceleration case generates more vorticity in the form of a tighter, more coherent vortex and produces significantly more circulation than the slower acceleration case, which is consistent with the difference in force production.

Study of the vortex-induced pressure excitation source in a Francis turbine draft tube by particle image velocimetry

Francis turbines operating at part-load experience the development of a precessing cavitation vortex rope at the runner outlet, which acts as an excitation source for the hydraulic system. In case of resonance, the resulting pressure pulsations seriously compromise the stability of the machine and of the electrical grid to which it is connected. As such off-design conditions are increasingly required for the integration of unsteady renewable energy sources into the existing power system, an accurate assessment of the hydropower plant stability is crucial. However, the physical mechanisms driving this excitation source remain largely unclear. It is for instance essential to establish the link between the draft tube flow characteristics and the intensity of the excitation source. In this study, a two-component particle image velocimetry system is used to investigate the flow field at the runner outlet of a reduced-scale physical model of a Francis turbine. The discharge value is varied from 55 to 81 % of the value at the best efficiency point. A particular set-up is designed to guarantee a proper optical access across the complex geometry of the draft tube elbow. Based on phase-averaged velocity fields, the evolution of the vortex parameters with the discharge, such as the trajectory and the circulation, is determined for the first time. It is shown that the rise in the excitation source intensity is induced by an enlargement of the vortex trajectory and a simultaneous increase in the precession frequency, as well as the vortex circulation. Below a certain value of discharge, the structure of the vortex abruptly changes and loses its coherence, leading to a drastic reduction in the intensity of the induced excitation source.

Comparison of Rotating and Translating Wings: Force Production and Vortex Characteristics

Experiments were conducted on a rectangular flat plate wing at high incidence to compare the force production and leading-edge vortex location and strength as a function of time for eight sets of wing kinematics: rectilinear and rotational pitch and surge, with fast and slow accelerations. The primary differences observed in the force histories were between the pitch and surge cases, and they occurred during the acceleration or pitching portions of the motions. The force histories were found to be similar between the rectilinear and rotational cases. The leading-edge vortex trajectories in the rectilinear cases were found to be similar when compared on a new time scale , related to the integrated leading-edge-normal velocity. The vortex trajectory of the rotational cases matched those of the rectilinear cases until the leading-edge vortex in the rotational case settled into a position attached to the wing. Circulation values of the leading-edge vortex for all cases were found to collapse when plotted versus and scaled by the instantaneous leading-edge-normal velocity. The results demonstrate a successful collapse of both vortex quantities and force production using only leading-edge kinematics.

Dynamics of vortex evolution in a 2D baffled tank

A finite difference method with coordinate transformation and fictitious cell approach were used to analyze the vortex generation and shedding phenomenon for sloshing liquid in 2D tanks with baffles. The detailed description of the dynamics of vortex evolution for sloshing fluid in a tank with baffles is seldom seen in the literatures and is firstly reported in this study. The exploration of liquid sloshing in a 2D tank with vertically bottom-mounted baffles is demonstrated. The benchmark tests of a tuned liquid damper (TLD) solved by the present numerical scheme show good agreements with the reported results. The experimental measurement is also carried out in this study and the present numerical simulation has excellent accuracy according to the comparison between the computational results and experimental measurement. The evolution of vortices inside a baffled tank in terms of vortex generation, vortex shedding and the trajectories of vortices are analyzed. Four phases of interaction processes of vortices are categorized in this work. The comprehensive discussions include the evolution of vortices and vortex shedding around the baffle tip, the vortex size generated in the vicinity of the baffle tip, the shedding frequency of the vortices, and the interaction of vortices inside the tank with various heights of the baffles and liquid depths. The vortex shedding phenomenon in the vicinity of the baffle tip is tightly correlated with the strength of the vertical jet along the baffle walls and the excitation frequency of the tank. Vortex size is closely correlated with the baffle height. When the baffle height is small (db≤0.2d0), the vortex size mainly grows in the horizontal direction. Instead, the vortex size dominantly develops in the vertical direction as db≥0.3d0. The period of the generation and shedding of vortices near the baffle tip is nearly about one half of the excitation period of the tank. The dynamics of vortex evolution is closely related to the growth and the hydrodynamic interaction of the vortices and sensitively depends on the baffle height, liquid depth, excitation frequency, and excitation displacement of the tank.

RANS computations of tip vortex cavitation

The present study is related to the development of the tip vortex cavitation in Kaplan turbines. The investigation is carried out on a simplified test case consisting of a NACA0009 blade with a gap between the blade tip and the side wall. Computations with and without cavitation are performed using a R ANS modelling and a transport equation for the liquid volume fraction. Compared with experimental data, the R ANS computations turn out to be able to capture accurately the development of the tip vortex. The simulations have also highlighted the influence of cavitation on the tip vortex trajectory.

Benchmark CFD validation data for surface combatant 5415 in PMM maneuvers – Part II: Phase-averaged stereoscopic PIV flow field measurements

Part II of this two-part paper presents computational fluid dynamics (CFD) benchmark local flow field measurements for a surface combatant in dynamic planar-motion-mechanism (PMM) maneuvers. Phase-averaged stereoscopic particle-image-velocimetry (PIV) measurements are performed for pure sway and pure yaw maneuvers. The geometry is David Taylor Model Basin (DTMB) model 5512, a 3.048m geosim of DTMB model 5415. The experiments are done in a 100m×3m×3m towing tank. The measurement system is a custom-designed towing tank maneuvering test flow-mapping system, which features a PMM for captive model testing with an integrated stereoscopic PIV. Statistical convergence error is defined and monitored by using the confidence intervals of the mean and variance. The quality of data is also assessed by following standard uncertainty assessment procedures. The present experimental data are cross-referenced with computational simulations for identification of the major vortices in the flow. The core locations and trajectories of the major vortices are traced and the flow variables at the core locations are analyzed. The tests of both Parts I and II are sufficiently documented and detailed to be useful as experimental benchmark data for validation of CFD codes. Part I presents the force/moment/motion measurements.

Effect of Sideslip on High-Angle-of-Attack Vortex Flow over Close-Coupled Canard Configuration

The aerodynamic response to sideslip is crucial for an aircraft’s lateral stability, especially for configurations incorporating vortex flow, on which sideslip causes asymmetric vortex behavior and brings extra complexity. In this work, vortex flow over a close-coupled canard configuration under sideslip is simulated using delayed detached-eddy simulation, with the angle of attack varying up to the poststall regime. Sideslip-induced distinctions in both vortex dynamics and canard/wing-vortex interaction are analyzed to explain the nonlinear response of the configuration. The results reveal that the effect of sideslip differs significantly upon different angle-of-attack regimes. At a medium angle of attack, the spanwise extension of the low-pressure area due to windward vortex core expansion results in superior windward lift against the leeward side, but the relatively low pressure retained due to delay of the leeward vortex breakdown is the main motivation that leeward lift surpasses windward at a high angle of attack. The canard vortex trajectory deflected under sideslip induces asymmetric vortex interaction, enhancing the leeward side, which not only leads to an unusual rolling moment toward the higher lift side at a medium angle of attack but also causes an abrupt loss of lateral stability at the poststall angle of attack. A featured demarcation line exists on both the canard and the wing to separate the lift imbalance area, which is found useful in interpreting the nonlinear behavior.

Experimental and numerical investigation of tip vortex generation and evolution on horizontal axis wind turbines

AbstractThe tip vortex of a wind turbine rotor blade is the result of a distribution of aerodynamic loads and circulation over the blade tip. The current knowledge on the generation of the tip vorticity in a 3D rotating environment still lacks detailed experimental evidence, particularly for yawed flow. The aim of this paper is to investigate how circulation at the blade tip behaves and how vorticity is eventually released in the wake, for both axial and 30° yawed flow conditions through the combination of experimental and numerical simulations. Stereo particle image velocimetry is used to measure the flow field at the tip of a 2m diameter, two‐bladed rotor at the TU Delft Open Jet Facility, for both axial and yawed flow; numerical simulations of the experiments are performed using a 3D, unsteady potential flow free‐wake vortex model. The generation mechanisms of the tip vorticity are established. The spanwise circulation along the blade exhibits a similar variation in both axial and yaw cases. A comparison of the chordwise directed circulation variation along the chord between axial and yawed flow is also presented and shown to be different. The analysis is based on contour integration of the velocity field. The tip vortex trajectory for axial flow confirms previous observations on the MEXICO rotor. The experimental results for yawed conditions have clearly shown how vorticity is swept radially away from the blade under the influence of the in‐plane radial component of flow. Such phenomena were only partially captured by the numerical model. The results of this work have important implications on the modelling of blade tip corrections. Copyright © 2015 John Wiley & Sons, Ltd.

Visualisation of underexpanded jet flowfields in the presence of an intrusive tab

This paper investigates the influence of a single intrusive tab has on the flowfield of a supersonic jet. When a tab is added to the jet, an increase in the fundamental screech frequency is observed in acoustic spectra. High resolution schlieren imaging reveals alteration of both the shock structure and preferred instability mode in the jet. Two-point spatial correlation of schlieren images shows the transition from a lateral flapping mode to a punctuated toroid screech mode. This change in dominant instability mode produces a change in coherent structure wavelength that correlates well to the observed screech frequency shift in the acoustic data. The high quality of the schlieren images also allows the counter-rotating vortex pair produced by an individual tab to be identified. Images of the radial density gradient are used to estimate vortex pair location, and a statistical estimate for the trajectory of the vortex pair is produced. The vortex pair path identification technique shows that the path of the vortex pair is constrained close to the exit of the nozzle and increasingly variant in space with greater downstream distance. The vortex trajectory is shown to alter the sonic line and thus the spatial extent of the embedded shock structures.

Method of radial velocities for the estimation of aircraft wake vortex parameters from data measured by coherent Doppler lidar.

The method of radial velocities (RV) is applied to estimate aircraft wake vortex parameters from measurements conducted with pulsed coherent Doppler lidar (PCDL). Operations of the Stream Line lidar and the 2-µm PCDL are simulated numerically to analyze the accuracy of the estimated wake vortex parameters with the RV method. The RV method is also used to estimate wake vortex trajectories and circulation from lidar measurements at Tomsk and Munich airports. The method of velocity envelopes and the RV method are compared employing data gathered with the 2-µm PCDL. The domain of applicability of the RV method is determined.

Three-dimensional streaming flow in confined geometries

Steady streaming vortex flow from microbubbles has been developed into a versatile tool for microfluidic sample manipulation. For ease of manufacture and quantitative control, set-ups have focused on approximately two-dimensional flow geometries based on semi-cylindrical bubbles. The present work demonstrates how the necessary flow confinement perpendicular to the cylinder axis gives rise to non-trivial three-dimensional flow components. This is an important effect in applications such as sorting and micromixing. Using asymptotic theory and numerical integration of fluid trajectories, it is shown that the two-dimensional flow dynamics is modified in two ways: (i) the vortex motion is punctuated by bursts of strong axial displacement near the bubble, on time scales smaller than the vortex period; and (ii) the vortex trajectories drift over time scales much longer than the vortex period, forcing fluid particles onto three-dimensional paths of toroidal topology. Both effects are verified experimentally by quantitative comparison with astigmatism particle tracking velocimetry (APTV) measurements of streaming flows. It is further shown that the long-time flow patterns obey a Hamiltonian description that is applicable to general confined Stokes flows beyond microstreaming.

Scale-model tests of coaxial rotors in water tunnel via particle image velocimetry technique

In this work, the flow features of coaxial rotors measured in water tunnel are investigated via particle image velocimetry technique. The experiments are conducted using a two-bladed model-scale coaxial rotor and a single rotor with the same geometry. Some noticeable flow features are found. For the coaxial rotor in hover, there is an annular region radially inboard of the lower rotor disk, where there are almost no intrusions from the upper rotor wake. At the same time, upwash occurs at the outboard edge of the lower rotor, while no such phenomenon occurs near the upper rotor blade tip. Sometimes, two upwash phenomena are found both at the blade tip and root in the single-rotor case. For both the single and coaxial rotors in forward flight, there is a sudden velocity change at about 0.7 R on the retreating side. From comparisons with the prior performance experiments, it can also be found that mutual interferences are playing a positive role in the hovering coaxial rotor configuration. The previously prescribed wake models can well describe the tip vortex trajectories of the single rotor and coaxial rotor in hover and forward flight, especially at high advance ratios. However, in this experiment, some special features are also found slightly different with that of the experiment conducted in air. Some valuable results obtained from this experiment would be helpful to reasonable designs of coaxial helicopters in the future.

Vortex dynamics during blade-vortex interactions

Vortex dynamics during parallel blade-vortex interactions (BVIs) were investigated in a subsonic wind tunnel using particle image velocimetry (PIV). Vortices were generated by applying a rapid pitch-up motion to an airfoil through a pneumatic system, and the subsequent interactions with a downstream, unloaded target airfoil were studied. The blade-vortex interactions may be classified into three categories in terms of vortex behavior: close interaction, very close interaction, and collision. For each type of interaction, the vortex trajectory and strength variation were obtained from phase-averaged PIV data. The PIV results revealed the mechanisms of vortex decay and the effects of several key parameters on vortex dynamics, including separation distance (h/c), Reynolds number, and vortex sense. Generally, BVI has two main stages: interaction between vortex and leading edge (vortex-LE interaction) and interaction between vortex and boundary layer (vortex-BL interaction). Vortex-LE interaction, with its small separation distance, is dominated by inviscid decay of vortex strength due to pressure gradients near the leading edge. Therefore, the decay rate is determined by separation distance and vortex strength, but it is relatively insensitive to Reynolds number. Vortex-LE interaction will become a viscous-type interaction if there is enough separation distance. Vortex-BL interaction is inherently dominated by viscous effects, so the decay rate is dependent on Reynolds number. Vortex sense also has great impact on vortex-BL interaction because it changes the velocity field and shear stress near the surface.