Showing papers on "Starting vortex published in 2014"

Mind the gap: a new insight into the tip leakage vortex using stereo-PIV

Abstract: The tip leakage vortex (TLV), which develops in the clearance between the rotor and the stator of axial hydro turbines, has been studied for decades. Yet, many associated phenomena are still not understood. For instance, it remains unclear how the clearance size is related to the occurrence of cavitation in the vortex, which can lead to severe erosion. Experiments are here carried out on the influence of the clearance size on the tip vortex structure in a simplified case study. A NACA0009 hydrofoil is used as a generic blade in a water tunnel while the clearance between the blade tip and the wall is varied. The 3D velocity fields are measured using Stereo Particle Image Velocimetry (SPIV) in three planes located downstream of the hydrofoil for different values of the upstream velocity, the incidence angle and a large number of tip clearances. The influence of the flow conditions on the structure of the TLV is described through changes in the vortex intensity, core axial flow, vortex center position and wandering motion amplitude. Moreover, high-speed visualizations are used to highlight the vortex core trajectory and clearance flow alteration, turning into a wall jet as the tip clearance is reduced. The measurements clearly reveal the existence of a specific tip clearance for which the vortex strength is maximum and most prone to generating cavitation.

Vorticity transport in the leading-edge vortex on a rotating blade

Abstract: Vorticity transport is analysed within the leading-edge vortex generated on a rectangular flat plate of aspect ratio 4 undergoing a starting rotation motion in a quiescent fluid. Two analyses are conducted on the inboard half of the blade to better understand the vorticity transport mechanisms responsible for maintaining the quasi-equilibrium state of the leading-edge vortex. An initial global analysis between the and spanwise positions suggests that, although spanwise velocity is significant, spanwise convection of vorticity is insufficient to balance the flux of vorticity from the leading-edge shear layer. Subsequent detailed analyses of vorticity transport in planar control volumes at the and spanwise positions verify this conclusion and demonstrate that vorticity annihilation due to interaction between the leading-edge vortex and the opposite-sign layer on the plate surface is an important, often dominant, mechanism for regulation of leading-edge-vortex circulation. Thus, it provides an important condition for maintenance of an attached leading-edge vortex on the inboard portion of the blade.

Dynamics of revolving wings for various aspect ratios

Abstract: High-fidelity, direct numerical simulations (DNSs) are conducted to examine the vortex structure and aerodynamic loading of unidirectionally revolving wings in quiescent fluid. Wings with aspect ratios , 2 and 4 are considered at a fixed root-based Reynolds number of 1000. Each wing is shown to generate a coherent leading-edge vortex (LEV) that remains in close proximity to the surface and provides persistent suction throughout the motion. Towards the tip, the LEV lifts off as an arch-like structure and reorients itself along the chord through its connection with the tip vortex. The substantial and sustained aerodynamic loads achieved during the motion saturate with aspect ratio resulting from the chordwise growth of the LEV along the span eventually becoming geometrically constrained by the trailing edge. Further, for , substructures develop in the feeding sheet of the LEV, which appear to directly correlate with the local, span-based Reynolds number achieved during rotation. The lower-aspect-ratio wings do not have sufficient spans for these transitional elements to manifest. In contrast, vortex breakdown, which occurs around midspan for each aspect ratio, shows a strong dependence on the spanwise pressure gradient established between the root and tip of the wing and not local Reynolds number. This independent development of shear-layer substructures and vortex breakdown parallels very closely with what has been observed in delta wing flow. Next, the centrifugal, Coriolis and pressure gradient forces are also analysed at several spanwise locations across each wing, and the centrifugal and pressure gradient forces are shown to be responsible for the spanwise flow above the wing. The Coriolis force is directed away from the surface at the base of the LEV, indicating that it is not a contributor to LEV attachment, which is contrary to previous hypotheses. Finally, as a means of emphasizing the importance of the centrifugal force on LEV attachment, the wing is simulated with the addition of a source term in the governing equations to oppose and eliminate the centrifugal force near the surface. The initial formation and development of the LEV is unhindered by the absence of this force; however, later in the motion, the outboard lift-off of the LEV moves inboard. Without the opposing outboard-directed centrifugal force to keep the separation past midspan, the entire vortex eventually separates and moves away from the surface.

Experimental study of flow separation control on a low-Re airfoil using leading-edge protuberance method

Abstract: An experimental study of flow separation control on a low-Re c airfoil was presently investigated using a newly developed leading-edge protuberance method, motivated by the improvement in the hydrodynamics of the giant humpback whale through its pectoral flippers. Deploying this method, the control effectiveness of the airfoil aerodynamics was fully evaluated using a three-component force balance, leading to an effectively impaired stall phenomenon and great improvement in the performances within the wide post-stall angle range (22°–80°). To understand the flow physics behind, the vorticity field, velocity field and boundary layer flow field over the airfoil suction side were examined using a particle image velocimetry and an oil-flow surface visualization system. It was found that the leading-edge protuberance method, more like low-profile vortex generator, effectively modified the flow pattern of the airfoil boundary layer through the chordwise and spanwise evolutions of the interacting streamwise vortices generated by protuberances, where the separation of the turbulent boundary layer dominated within the stall region and the rather strong attachment of the laminar boundary layer still existed within the post-stall region. The characteristics to manipulate the flow separation mode of the original airfoil indicated the possibility to further optimize the control performance by reasonably designing the layout of the protuberances.

On the wake dynamics of a propeller operating in drift

Abstract: The onset and the nature of dynamic instabilities experienced by the wake of a marine propeller set in oblique flow are investigated by means of detached eddy simulations. In particular, the destabilization process is inspected by a systematic comparison of the wake morphology of a propeller operating in pure axisymmetric flow and in drift with angle of 20°, under different loading conditions. The wake behaviour in oblique flow shows a markedly different character with respect to the axisymmetric condition: in the latter, the destabilization is triggered by an increasing interaction of the main vorticity confined in the tip vortex; whereas, in the former, the role of the secondary vorticity (oriented in the streamwise direction) as well as the hub vortex seems to be crucial. The features of the wake have been investigated by the criterion (Jeong & Hussain, J. Fluid Mech., vol. 285, 1995, pp. 69–94) and typical flow variables (pressure, velocity and vorticity), for both the averaged and instantaneous flow fields. Moreover, in order to further inspect the evolution of the vortical structures, as well as their interaction and destabilization, the spectra of the kinetic energy have been considered. This investigation aims to broaden the knowledge from previous works on the subject of rotor wake instabilities, focusing on the differences between an ideal (axisymmetric) and actual operating conditions occurring in typical engineering applications.

A regular Strouhal number for large-scale instability in the far wake of a rotor

Abstract: The flow behind a model of a wind turbine rotor is investigated experimentally in a water flume using particle image velocimetry (PIV) and laser Doppler anemometry (LDA). The study performed involves a three-bladed wind turbine rotor designed using the optimization technique of Glauert (Aerodynamic Theory, vol. IV, 1935, pp. 169–360). The wake properties are studied for different tip speed ratios and free stream speeds. The data for the various rotor regimes show the existence of a regular Strouhal number associated with the development of an instability in the far wake of the rotor. From visualizations and a reconstruction of the flow field using LDA and PIV measurements it is found that the wake dynamics is associated with a precession (rotation) of the helical vortex core.

Dynamics of vortex structures in a stratified rotating fluid

Abstract: 1. The Introductory Chapter 1.1 Introduction 1.2 The mathematical introduction 1.2.1 The derivation of potential vortex conservation equations 1.2.2 Formal solution. Integral invariants 1.2.3 Contour dynamics method 1.2.4 Stationary axisymmetric solution 1.2.5 An approach to studying the stability of a axisymmetric two-layer vortex 1.2.6 The structure of simplest types of external field 1.2.7 A limiting case of discrete vortices 1.2.8 Phase portraits. Choreographies 1.2.9 Three-layer model equations 2. Dynamics of Discrete Vortices 2.1 Two vortices in a two-layer fluid 2.2 2A vortices in a two-layer fluid 2.2.1 The case of arbitrary A 2.2.2 Case A = 2 2.2.2.1 Two hetons with zero total linear momentum and nonzero angular momentum 2.2.2.2 Two hetons with nonzero total linear momentum and zero angular momentum 2.2.2.3 Two hetons with zero total linear and angular momenta 2.2.2.4 Vortex structures: warm heton-cold heton, two antihetons, two "horizontal" pairs 2.3 A + 1 vortices in a two-layer fluid 2.3.1 Vortex structures with zero total momentum at A => 2 (free motion) 2.3.2 Vortex structures with zero total momentum at A => 2 (motion in an external field) 2.3.2.1 Analysis of static states 2.3.2.2 Stationary solutions at A = 2 2.3.2.3 Analysis of optimal perturbation frequencies at A = 2 2.3.2.4 Origination of chaos at A = 2 2.3.3 The case of nonzero total momentum at A = 2 2.3.3.1 Phase portraits in trilinear coordinates 2.3.3.2 Analysis of steady states 2.3.3.3 Classification of motions of triangular vortex structures: trajectories of absolute motion, choreographies 2.3.3.4 Analysis of weakly perturbed tripolar collinear states 2.3.3.5 Regular advection near stationary configurations 2.3.3.6 Chaotic advection near stationary configurations 2.4 Heton structures in a three-layer fluid 3 Dynamics of Finite-core Vortices 3.1 Studying the linear stability of a two-layer vortex 3.1.1 A vortex with a vertical axis: two circular vortex patches 3.1.1.1 Heton with vertical axis 3.1.1.2 "Ballistic" propagation law of vortex domain boundary: application to deep convection in the ocean 3.1.1.3 Analogy with A-symmetric structure of discrete hetons 3.1.1.4 Noncompensated two-layer vortex 3.1.2 Annular two-layer vortex: four circular vortex patches 3.1.2.1 Studying the stability of rings 3.1.2.2 Modeling the transformations of an oceanic ring into smaller vortex structures 3.2 The impact of finite perturbations 3.2.1 Heton with a tilted axis: two initially circular patches 3.2.2 Stationary translation hetonic V-states 3.2.3 Heton with a vertical axis: two initially elliptic vortex patches 3.3 Interaction between two hetons 3.3.1 Two hetons with vertical axes 3.3.2 Heton with a vertical axis and heton with a tilted axis 3.3.3 Two hetons with tilted axes, the case of zero total momentum 3.3.4 Two hetons with tilted axes, the case of nonzero total momentum 3.3.5 Interaction between a warm and a cold hetons 3.4 The effect of external field on heton motion 3.5 Vortex patch dynamics in a three-layer model 3.5.1 Stability study of a three-layer vortex 3.5.2 Modeling the motion of meddies 3.5.2.1 Merging of two initially circular vortex patches 3.5.2.2 Evolution of elliptic vortex patch 3.5.2.3 On detecting lenses on oceanic surface 3.5.2.4 On the effect of bottom topography on the motion of lenses 3.5.2.5 Dynamics of medies in the flow over submerged hills 3.5.3 Examples of interaction between three-layer vortices 4 The Concluding Chapter 4.1 Concluding remarks 4.2 Outlook to heton problems 4.3 Discussion Appendix A. E.J. Hopfinger. Experimental study of hetons Appendix B. M.A. Sokolovskiy. In memory of my Teacher Index

Computational and Experimental Analysis of a High-Performance Airfoil Under Low-Reynolds-Number Flow Condition

Abstract: A high-performance Ishii airfoil was analyzed using both a wind-tunnel and large-eddy simulations at a low-Reynolds-number condition (Re=23,000). The design guidelines for an airfoil shape with a high lift-to-drag ratio under the aforementioned condition are described by analyses of flowfields and aerodynamic characteristics of the Ishii airfoil. Compared with conventional airfoils, such as the NACA 0012 and NACA 0002, the shape characteristic effects of the Ishii airfoil on its flowfield and aerodynamic characteristics are discussed. The shape on the suction side of the Ishii airfoil can cause delays in the flow separation at low angle of attacks. The separated flow reattaches, and a separation bubble forms even when trailing-edge separation changes to leading-edge separation. The separation bubble contributes to an increase in lift coefficient. In addition, the Ishii airfoil can gain a high positive pressure on the pressure side as compared with the other two symmetric airfoils due to the camber near th...

Pinch-off of non-axisymmetric vortex rings

Abstract: The formation and pinch-off of non-axisymmetric vortex rings is considered experimentally. Vortex rings are generated using a non-circular piston–cylinder arrangement, and the resulting velocity fields are measured using digital particle image velocimetry. Three different nozzle geometries are considered: an elliptical nozzle with an aspect ratio of two, an elliptical nozzle with an aspect ratio of four and an oval nozzle constructed from tangent circular arcs. The formation of vortices from the three nozzles is analysed by means of the vorticity and circulation, as well as by investigation of the Lagrangian coherent structures in the flow. The results indicate that, in all three nozzles, the maximum circulation the vortex can attain is determined by the equivalent diameter of the nozzle: the diameter of a circular nozzle of identical cross-sectional area. In addition, the time at which the vortex rings pinch off is found to be constant along the nozzle contours, and independent of relative variations in the local curvature. A formation number for this class of vortex rings is defined based on the equivalent diameter of the nozzle, and the formation number for vortex rings of the three geometries considered is found to lie in the range of 3–4. The implications of the relative shape and local curvature independence of the formation number on the study and modelling of naturally occurring vortex rings such as those that appear in biological flows is discussed.

Vortex stretching and compression near the turbulent/non-turbulent interface in a planar jet

Abstract: Vortex stretching and compression, which cause enstrophy production by inviscid processes, are investigated near the turbulent/non-turbulent (T/NT) interface in a planar jet by using a direct numerical simulation (DNS). The enstrophy production is investigated by analysing the relationship among a vorticity vector, strain-rate eigenvectors and strain-rate eigenvalues. The statistics are calculated individually for three different interface orientations. The vorticity near the T/NT interface is oriented in the tangential direction to the interface. The enstrophy production is affected by the interface orientation because the intensity of vortex stretching depends on the interface orientation, and the alignment between the vorticity vector and the strain-rate eigenvectors is confined by the interface. The enstrophy production near the T/NT interface is analysed by considering the motion of turbulent fluid relative to that of the interface. The results show that the alignment between the interface and the strain-rate eigenvectors changes depending on the velocity field near the T/NT interface. When the turbulent fluid moves toward the T/NT interface, the enstrophy is generated by vortex stretching without being greatly affected by vortex compression. In contrast, when the turbulent fluid relatively moves away from the T/NT interface, large enstrophy reduction frequently occurs by vortex compression. Thus, it is shown that the velocity field near the T/NT interface affects the enstrophy production near the interface through the alignment between the vorticity and the strain-rate eigenvectors.

On the absence of asymmetric wakes for periodically plunging finite wings

Abstract: It has previously been shown that, at high Strouhal numbers, oscillating airfoils can produce deflected jets that can create very high lift-coefficients for otherwise symmetric scenarios. These deflected jets form through pairing of the trailing-edge vortices to create asymmetric vortex couples that self-propel at an angle to the freestream, resulting in an asymmetric flow field and non-zero lift. In this paper results are presented that indicate these high-lift deflected jets cannot form for finite wings. Instead of the straight vortex tubes that pair and convect at an angle to the freestream observed for effectively infinite wings, finite wings exhibit vortex tubes that break into two branches near the tip forming double helix structures. One branch connects with the last vortex; one branch connects with the next vortex. This creates a long “daisy chain” of interconnected trailing edge vortices forming a long series of vortex loops. These symmetric flow fields are shown to persist for finite wings even to Strouhal numbers more than twice those required to produce asymmetric wakes on plunging airfoils. Two contributing reasons are discussed for why deflected jets are not observed. First the tip vortex creates three-dimensionality that discourages vortex coupling. Second, the symmetry of the circulation of the interconnected vortex loops, which has been confirmed by the experiments, is a natural consequence of the vortex topology. Therefore, the asymmetry in trailing edge vortex strength previously observed as characteristic of deflected jets cannot be supported for finite wings.

Reconnection of skewed vortices

Abstract: Based on experimental evidence that vortex reconnection commences with the approach of nearly antiparallel segments of vorticity, a linearised model is developed in which two Burgers-type vortices are driven together and stretched by an ambient irrotational strain field induced by more remote vorticity. When these Burgers vortices are exactly antiparallel, they are annihilated on the strain time-scale, independent of kinematic viscosity in the limit . When the vortices are skew to each other, they are annihilated under this action over a local extent that increases exponentially in the stretching direction, with clear evidence of reconnection on the same strain time-scale. The initial helicity associated with the skewed geometry is eliminated during the process of reconnection. The model applies equally to the reconnection of weak magnetic flux tubes under the action of a strain field, when Lorentz forces are negligible.

Turbulent wake behind a curved circular cylinder

Abstract: This paper reports results from a direct numerical simulation of the flow past a circular cylinder with axial curvature. The main objective is to explore the effects of spanwise curvature on the stability of the shear layers and the turbulent wake at the subcritical Reynolds number of 3900. The bluff-body geometry is adapted from a previous study conducted at lower Reynolds numbers, in which a quarter segment of a ring represented the deformed cylinder. A convex configuration in which the free-stream direction is towards the outer face of the ring is adopted here. The present results show a striking distinction between the upper and lower wake regions. Despite the turbulent character of the wake, the upper wake region is more coherent due to the periodic vortex shedding of primary vortical structures, which are in close alignment with the axial curvature. A mild axial flow develops upwards along the lee face of the curved cylinder, displacing the vortex formation region further downstream from the location expected for a straight cylinder at the same Reynolds number. In the lower wake region the vortex shedding strength is drastically reduced due to larger local inclination, resulting in higher three-dimensionality and loss of coherence. A strong downdraft with a swirling pattern is the dominating feature in the lower base region. This is associated with a substantial decrease of the base suction, and the suppression of the characteristic recirculating backflow.

Role of slipstream instability in formation of counter-rotating vortex rings ahead of a compressible vortex ring

Abstract: Counter-rotating vortex rings (CRVRs) are observed to form ahead of a primary compressible vortex ring that is generated at the open end of a shock tube at sufficiently high Mach numbers. In most of the earlier studies, the embedded shock strength has been asserted as the cause for the formation of CRVRs. In the present study, particle image velocimetry (PIV) measurements and high-order numerical simulations show that CRVRs do not form in the absence of a Mach disk in the sonic under-expanded jet behind the primary vortex ring. Kelvin–Helmholtz-type shear flow instability of the slipstream originating from the triple point of the Mach disk and subsequent eddy pairing, as observed by Rikanati et al. (Phys. Rev. Lett., vol. 96, 2006, art. 174503) in shock-wave Mach reflection, is found to be responsible for CRVR formation. The growth rate of the slipstream in the present problem follows the model proposed by them. The parameters influencing the formation of CRVRs as well as their dynamics is investigated. It is found that the strength of the Mach disk and its duration of persistence results in an exit impulse that determines the number of CRVRs formed.

Stationary cavitation bubbles forming on a delta wing vortex

Abstract: Vortex cavitation forming in the leading-edge vortices of a delta wing was examined to determine how the individual cavitation bubbles incepted, grew, interacted with the underlying vortical flow and produced acoustic tones. The non-cavitating vortical flow over the delta wing was chosen to be similar to those previously reported in the literature. It was found that vortex breakdown was unaffected by the presence of incipient and developed vortex cavitation bubbles in the vortex core. While some cavitation bubbles incepted, grew, and collapsed relatively quickly, others reached an equilibrium position wherein the bubble tip was stationary in the laboratory frame at a particular location along the vortex axis. For a given attack angle, the equilibrium location moved upstream with a reduction in free stream cavitation number. It is shown that the existence of these stationary vortex bubbles is possible when there is a balance between the axial growth of the bubble along the vortex axis and the opposite motion of the axial jetting flow in the vortex core, and only a single equilibrium position is possible along the axially evolving vortex for a given free stream cavitation number. These transient and stationary vortex bubbles emit significant cavitation noise upon inception, growth, and collapse. The spectral content of the noise produced was expected to be related to the interaction of the bubble with the surrounding vortical flow in a manner similar to that reported in previous studies, where sustained tones were similar to the underlying vortex frequency. However, in the present study, the dominant frequency and higher harmonics of the tones occur at a higher frequency than that of the underlying vortex. Hence, it is likely that the highly elongated stationary bubbles have higher-order volume oscillations compared to the two-dimensional radial mode of the vortex cores of vortex cavitation bubbles with much smaller diameter-to-length ratios.

Three-dimensional vortex wake structure of flapping wings in hovering flight

Abstract: Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.

Modification of a circular cylinder wake with synthetic jet: Vortex shedding modes and mechanism

Abstract: The wake behind a circular cylinder is modified by a synthetic jet positioned at the front stagnation point. The flow field is measured with a time-resolved particle image velocimetry (PIV) system, and the proper orthogonal decomposition (POD) and λ c i methods are used to analyze the vortex dynamics. The synthetic jet vortex pair is induced near the exit orifice periodically and then moves upstream. The interaction between the synthetic jet and the oncoming flow gives rise to an envelope formed upstream of the circular cylinder, which acts as a virtual aerodynamic shape. It is found that the envelope can be categorized into the periodic closed envelope and the quasi-steady open envelope, leading to different shedding modes for the wake around the circular cylinder. In the present investigation, six kinds of vortex shedding modes under synthetic jet control have been classified as natural Karman vortex mode, bistable state mode I, symmetric mode, bistable state mode II, antisymmetric mode with shortened vortex formation length, vortex generation close to the rear stagnation point. The vortex dynamics analysis indicates that the wake vortex trajectory, vortex circulation, and convection velocity at the vortex core all exhibit regular variations for these typical shedding modes. The formation mechanisms for these shedding modes have been further revealed, which present some novel formation processes in comparison with the natural Karman vortex street. Moreover, the effects of the synthetic jet momentum coefficient and excitation frequency on the control are also compared, which suggests that the type of the front envelope is most important for the vortex shedding modes.

DNS study on Λ-vortex and vortex ring formation in flow transition at Mach number 0.5

Abstract: Large vortex structure in late boundary layer transition with an inflow Mach number of 0.5 is studied by DNS (Direct Numerical Simulation) in this paper. First, we found that there are no Λ-vortex tubes, contradicting to what the existing literatures and textbooks addressed. The so-called Λ-vortex is always open on head, which has a different shape from Λ. Λ-vortex is really a pair of open rotation cores with a lower half of the Λ shape. It is also found that the Λ-vortex and ring-like vortex are formed separately and independently. There is no such a process that the Λ-vortex self-deforms to a hairpin vortex at the tip as many literatures indicated. Λ-vortex and ring-like vortex can be visualised by the iso-surface of λ2. However, the iso-surfaces of λ2 only represent rotation cores but not necessarily vortex tubes. In fact, many spanwise vortex filaments can easily penetrate the so-called Λ-vortex (iso-surface of λ2), change the direction toward the streamwise direction, and then leave the iso-surface o...

Enhancement of aircraft wake vortex decay in ground proximity: Experiment versus Simulation

Abstract: Aircraft wake vortex evolution in ground proximity is investigated experimentally in a water towing tank, as well as numerically with wall-resolved large eddy simulation (LES). With these complementary instruments the enhancement of wake vortex decay by obstacles, introduced at the ground surface, is analyzed. The experimental methods include time-resolved stereo particle image velocimetry and vortex core visualization. For comparison with the experiment, the LES considers the turbulent wake of the strut, holding the towed aircraft model. Wake vortex trajectories and circulation decay are compared at different distances from the obstacle. Tracers are employed to visualize the obstacle’s effects on the vortex core, in LES and experiment. The experimentally obtained trajectories and decay characteristics are reproduced qualitatively by simulations, whereas the agreement is degraded at later times. Beyond that, the vortex dynamics, deduced from the LES results, help to understand the experimental observations. The obstacles trigger helical secondary vortex structures, propagating along the primary vortices. The observed propagation speed of the helical disturbance is fairly well predicted by the suggested simple model. It is shown that the obstacles significantly modify the vortex interaction with the ground and substantially accelerate vortex decay. Two neighboring obstacles lead to colliding disturbances that further enhance vortex decay rates.

Parameter Variation and the Leading-Edge Vortex of a Rotating Flat Plate

Abstract: Particle image velocimetry was used to characterize the flowfield on flat plates of aspect ratios 2 and 4 undergoing a starting rotation motion at Reynolds numbers, based on tip velocity, of 4,000, 8,000, and 16,000. The starting motion was performed in a tank of quiescent water. For both aspect ratios, a leading-edge vortex was observed on the suction surface of the plate, and its evolution and circulation were characterized with variations in angle of attack, Reynolds number, and azimuthal and spanwise positions. A region of strong counter-rotating vorticity was also observed between the leading-edge vortex and the plate surface, which becomes entrained by the leading-edge vortex. The circulation of the leading-edge vortex, when rendered dimensionless by the plate chord length and tip speed, was found to be relatively insensitive to Reynolds number and the azimuthal position of the plate within the ranges of the measurements; however, a strong (approximately linear) dependence on angle of attack was obs...

Aerodynamic behavior of an airfoil with morphing trailing edge for wind turbine applications

Abstract: The length of wind turbine rotor blades has been increased during the last decades. Higher stresses arise especially at the blade root because of the longer lever arm. One way to reduce unsteady blade-root stresses caused by turbulence, gusts, or wind shear is to actively control the lift in the blade tip region. One promising method involves airfoils with morphing trailing edges to control the lift and consequently the loads acting on the blade. In the present study, the steady and unsteady behavior of an airfoil with a morphing trailing edge is investigated. Two-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulations are performed for a typical thin wind turbine airfoil with a morphing trailing edge. Steady-state simulations are used to design optimal geometry, size, and deflection angles of the morphing trailing edge. The resulting steady aerodynamic coefficients are then analyzed at different angles of attack in order to determine the effectiveness of the morphing trailing edge. In order to investigate the unsteady aerodynamic behavior of the optimal morphing trailing edge, time- resolved RANS-simulations are performed using a deformable grid. In order to analyze the phase shift between the variable trailing edge deflection and the dynamic lift coefficient, the trailing edge is deflected at four different reduced frequencies for each different angle of attack. As expected, a phase shift between the deflection and the lift occurs. While deflecting the trailing edge at angles of attack near stall, additionally an overshoot above and beyond the steady lift coefficient is observed and evaluated.

Predicting Free-Surface Vortices with Single-Phase Simulations

Abstract: In this article, single-phase, computational-fluid-dynamics simulations of free-surface vortices are presented. The purpose of the simulations is to determine the appropriate turbulence model for free-surface vortices, which could later be applied to simulations of flow in various engineering systems. The water flow in the laboratory model of a free-surface vortex was numerically simulated by unsteady single-phase computations. The vortex circumferential velocity, the downward velocity inside the vortex core and the predicted length of the free-surface vortex gas core were compared with available measurements. For the two-equation turbulence models, the results indicated the importance of the curvature correction (CC). The effect of the time-step size and the choice of the advection scheme were analyzed. For the tested case, it was determined that the unsteadiness of the flow was insufficient for the correct behavior of the scale-adaptive simulation (SAS) turbulence model. With the CC option, the ...

Parameter dependence of vortex interactions on a two-dimensional plunging plate

Abstract: The structure and dynamics of the flow field created by a plunging flat-plate airfoil are investigated at a chord Reynolds number of 10,000 while varying plunge amplitude and Strouhal number. Digital particle image velocimetry measurements are used to characterize the shedding patterns and the interactions between the leading- and trailing-edge vortex structures (LEV and TEV), resulting in the development of a wake classification system based on the nature and timing of interactions between the leading- and trailing-edge vortices. The streamwise advancement of the LEV during a plunge cycle and its resulting interaction with the TEV is primarily dependent on reduced frequency; however, for Strouhal numbers above approximately 0.4, significant changes are observed in the formation of vortices shed from the leading and trailing edges, as well as the circulation of the leading-edge vortex. The functional form of the relationship between leading-edge vortex circulation and Strouhal number suggests that the Strouhal number dependence is more specifically a manifestation of the effective angle of attack. Comparison with low-Reynolds-number studies of plunging airfoil aerodynamics reveals a high degree of consistency and suggests applicability of the classification system beyond the range examined in the present work.

Introduction of Annular Vortex Tube and experimental comparison with Ranque–Hilsch Vortex Tube

Abstract: In order to enhance the Ranque–Hilsch Vortex Tube (RHVT) performance, an improved design of a Vortex Tube (VT) is introduced here. The new design of the VT which called Annular Vortex Tube (AVT) is experimentally tested and compared with a similar design RHVT. The hot stream, after passing the hot conical valve, redirected along the outer wall of hot tube in the AVT. This increases heat lost from outer wall of the hot tube. Results show that AVT has higher thermal separation performance comparing to RHVT. For optimum cold mass fraction, AVT improves cooling efficiency about 24% comparing to RHVT. The conical valve angle has also no effect on thermal performance of AVT. For AVT, three vortex generators are tested with different nozzle areas. All vortex generators have the same coefficient of performance in heating while vortex generator with higher nozzle area ratio represents better cooling efficiency among the others.

Stereo particle image velocimetry measurements of perpendicular blade–vortex interaction over an oscillating airfoil

Abstract: The aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 oscillating airfoil was investigated using stereo particle image velocimetry. The experimental rig enabled the study of the aerodynamic effects due to the blade pitching motion in the interaction with the vortex. The experimental study focused on the light dynamic stall regime, which represents a typical condition of the retreating blade of a helicopter in forward flight. Particle image velocimetry was applied to a measurement volume close to the airfoil upper surface in order to obtain the three-dimensional interacting flow field. In particular, the experimental results show that during the airfoil downstroke motion, the vortex impact triggers the stall of the local blade section, indicating that detrimental effects on the blade performance can be introduced by perpendicular vortex interactions.

Numerical Investigation into Vortex Generators on Heavily Cambered Wings

Abstract: A computational model has been implemented that is capable of accurately predicting the flow patterns around a vortex generator. The aim in development was the resolution of small-scale flow features. By observing the progression of the vortex structure along the wing, the causes of flow separation could be ascertained. It was found that flow separation occurred when the vortex structure produced by the generators broke down. This caused a localized flow separation that, at high angles, extended to cause globalized separation and generator engulfment. The computational fluid dynamics model was tested with a wind tunnel model and found to accurately describe the aerodynamic characteristics of the wing in the unstalled and lightly stalled condition. It was found that the severity of stall, experienced by wings fitted with ramp-type vortex generators, is largely due to a feedback loop mechanism that is created by the flow being drawn through the generators by attached trailing flows. An additional consequenc...

Investigation of three-dimensional dynamic stall on an airfoil using fast-response pressure-sensitive paint

Abstract: Dynamic stall on a pitching OA209 airfoil in a wind tunnel is investigated at Mach 0.3 and 0.5 using high-speed pressure-sensitive paint (PSP) and pressure measurements. At Mach 0.3, the dynamic stall vortex was observed to propagate faster at the airfoil midline than at the wind-tunnel wall, resulting in a “bowed” vortex shape. At Mach 0.5, shock-induced stall was observed, with initial separation under the shock foot and subsequent expansion of the separated region upstream, downstream and along the breadth of the airfoil. No dynamic stall vortex could be observed at Mach 0.5. The investigation of flow control by blowing showed the potential advantages of PSP over pressure transducers for a complex three-dimensional flow.