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Showing papers on "Starting vortex published in 2012"


Journal ArticleDOI
TL;DR: In this article, the three-dimensional structure of the leading-edge vortex on a rotating wing is addressed using a technique of particle image velocimetry, where organized patterns of chordwise-oriented vorticity arise from the spanwise flow along the surface of the wing, which can attain a velocity the same order as the wing at its radius of gyration.
Abstract: The three-dimensional structure of the leading-edge vortex on a rotating wing is addressed using a technique of particle image velocimetry. Organized patterns of chordwise-oriented vorticity, which exist within the vortex, arise from the spanwise flow along the surface of the wing, which can attain a velocity the same order as the velocity of the wing at its radius of gyration. These patterns are related to the strength (circulation) and coherence of the tip and root vortices. The associated distributions of spanwise-oriented vorticity along the leading-edge vortex are characterized in relation to the vorticity flux and downwash along the wing.

90 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the use of dielectric-barrier-discharge plasma actuators as vortex generators for flow separation control applications, and they found that the vortex generators were successful in reducing the separation region, even at plasma-to-free-stream velocity ratios of less than 10%.
Abstract: We investigated the use of dielectric-barrier-discharge plasma actuators as vortex generators for flow separation control applications. Plasma actuators were placed at a yaw angle to the oncoming flow, so that they produced a spanwise wall jet. Through interaction with the oncoming boundary layer, this created a streamwise longitudinal vortex. In this experimental investigation, the effect of yaw angle, actuator length and plasma-induced velocity ratio was studied. Particular attention was given to the vortex formation mechanism and its development downstream. The DBD plasma actuators were then applied in the form of co-rotating and counter-rotating vortex arrays to control flow separation over a trailing-edge ramp. It was found that the vortex generators were successful in reducing the separation region, even at plasma-to-free-stream velocity ratios of less than 10%.

86 citations


Journal ArticleDOI
TL;DR: In this article, the flow field around an asymmetric dielectric-barrier discharge (DBD) plasma actuator in quiescent air is studied using particle image velocimetry (PIV) and smoke-flow visualization.
Abstract: The flow field around an asymmetric dielectric-barrier-discharge (DBD) plasma actuator in quiescent air is studied using particle image velocimetry (PIV) and smoke-flow visualization. On initiation of DBD plasma a starting vortex is created, which rolls up to form a coherent structure. The starting vortex becomes self-similar when the maximum velocity induced by the DBD plasma actuator reaches a steady state. Here, the plasma jet momentum increases linearly with time, suggesting that the DBD plasma actuator entrains and accelerates the surrounding fluid with a constant force. The wall-parallel and wall-normal distances of the vortex core are observed to scale with as it travels at to the wall. The velocity of the starting vortex is found to scale with , while the circulation induced by the plasma actuator scales with .

79 citations


Journal ArticleDOI
TL;DR: In this paper, the root-to-tip vortex formation of a trapezoidal flat-plate fin undergoing rotation from rest at a 90° angle of attack and Reynolds numbers of O(103) is investigated.
Abstract: We investigate experimentally the unsteady, three-dimensional vortex formation of low-aspect-ratio, trapezoidal flat-plate fins undergoing rotation from rest at a 90° angle of attack and Reynolds numbers of O(103). The objectives are to characterize the unsteady three-dimensional vortex structure, examine vortex saturation, and understand the effects of the root-to-tip flow for different velocity programs. The experiments are conducted in a water tank facility, and the diagnostic tools are dye flow visualization and digital particle image velocimetry. The dye visualizations show that the low-aspect-ratio plate produces symmetric ring-like vortices comprised mainly of tip-edge vorticity. They also indicate the presence of the root-to-tip velocity. For large rotational amplitudes, the primary ring-like vortex sheds and a secondary ring-like vortex is generated while the plate is still in motion, indicating saturation of the leading vortex. The time-varying vortex circulation in the flow symmetry plane provides quantitative evidence of vortex saturation. The phenomenon of saturation is observed for several plate velocity programs. The temporal development of the vortex circulation is often complex, which prevents an objective determination of an exact saturation time. This is the result of an interaction between the developing vortex and the root-to-tip flow, which breaks apart the vortex. However, it is possible to define a range of time during which the vortex reaches saturation. A formation-parameter definition is investigated and is found to reasonably predict the state corresponding to the pinch-off of the initial tip vortex across the velocity programs tested. This event is the lower bound on the saturation time range.

74 citations


Journal ArticleDOI
TL;DR: In this article, the influence of three-dimensional effects in hovering flapping flight was analyzed using numerical simulations at a Reynolds number of 1000 to compare two types of flapping kinematics whose plunging phase is characterized by either a rectilinear translation or a revolving motion.
Abstract: This paper aims at understanding the influence of three-dimensional effects in hovering flapping flight. Numerical simulations at a Reynolds number of 1000 are performed to compare two types of flapping kinematics whose plunging phase is characterized by either a rectilinear translation or a revolving motion. In this way, we are able to isolate the three-dimensional effects induced by the free end condition from that induced by the spanwise incident velocity gradient (and the associated implicit Coriolis and centrifugal effects). In the rectilinear translation case, the analysis of the wake and of the aerodynamic loads reveals that the wingspan can be compartmented into three distinct regions whether it is predominantly subjected to an unstable two-dimensional flow, a stable three-dimensional flow or both two-dimensional and three-dimensional effects. It is found that this partitioning exhibits common features for three different aspect ratios of the wing. In conjunction with the previous results of Ringuette, Milano & Gharib (J. Fluid Mech., vol. 581, 2007, pp. 453–468), this suggests that the influence of the tip vortex over the wingspan is driven by a characteristic length scale. In addition, this length scale matches the position of the connecting point between leading and tip vortices observed in the revolving case, providing insight into the connecting process. In both translating and revolving cases, leading edge vortex attachment and strong spanwise velocities are found to be strongly correlated phenomena. Spanwise velocities (that mostly confine at the periphery of the vortices), together with downward velocities, do not only affect the leading edge vortex but also act as an inhibitor for the trailing edge vortex growth. As a consequence, cross-wake interactions between leading and trailing edge vortices are locally limited, hence contributing to flow stabilization.

71 citations


Journal ArticleDOI
TL;DR: In this paper, a NACA0012 airfoil of 30 cm chord and aspect ratio of 1 placed at the exit of an open-circuit blower type wind tunnel was used in the investigation of the relationship between the vortical velocity field and its surface pressure signature.

68 citations


Journal ArticleDOI
TL;DR: In this article, large-eddy simulations of a coherent counter-rotating vortex pair in different environments are performed and it is revealed that the vortex bursting phenomenon, known from photos of aircraft contrails or smoke visualization, is caused by collisions of secondary vortical structures traveling along the vortex tube which expel material from the vortex but do not result in a sudden decay of circulation or an abrupt change of vortex core structure.
Abstract: Large-eddy simulations of a coherent counter-rotating vortex pair in different environments are performed. The environmental background is characterized by varying turbulence intensities and stable temperature stratifications. Turbulent exchange processes between the vortices, the vortex oval, and the environment, as well as the material redistribution processes along the vortex tubes are investigated employing passive tracers that are superimposed to the initial vortex flow field. It is revealed that the vortex bursting phenomenon, known from photos of aircraft contrails or smoke visualization, is caused by collisions of secondary vortical structures traveling along the vortex tube which expel material from the vortex but do not result in a sudden decay of circulation or an abrupt change of vortex core structure. In neutrally stratified and weakly turbulent conditions, vortex reconnection triggers traveling helical vorticity structures which is followed by their collision. A long-lived vortex ring links once again establishing stable double rings. Key phenomena observed in the simulations are supported by photographs of contrails. The vertical and lateral extents of the detrained passive tracer strongly depend on environmental conditions where the sensitivity of detrainment rates on initial tracer distributions appears to be low.

64 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the mechanism of temperature separation in a counter-flow vortex tube and provided enough evidence that the flow in the tube consists of a forced vortex formed near the inlet gradually transforming to a free vortex at the hot end.

60 citations


Journal ArticleDOI
TL;DR: In this article, the effect of hydrofoil trailing edge shape on the wake dynamic and flow induced vibration is investigated at high Reynolds number, Re ¼0.5 � 10 6 −2.9 x 10 6.

60 citations


Journal ArticleDOI
TL;DR: In this paper, the authors conducted a Detached Eddy Simulation on a 13m grid to identify and analyze the vortical structures, instabilities, and turbulent structures with analogy to the vortex breakdown and helical instability analysis for delta wing at large angles of attack.

59 citations


Journal ArticleDOI
TL;DR: In this paper, large-eddy simulations are conducted for a NACA0012 airfoil with rounded trailing edge for two flow configurations with different freestream Mach numbers (M1 � 0:115 and 0.4) for an angle of incidence of 5 deg.
Abstract: The present investigation of airfoil trailing-edge noise generation and propagation concerns the broadband noise that arises from the interaction of turbulent boundary layers with the airfoil trailing edge and the tonal noise that arises from vortex shedding generated by laminar boundary layers. Large-eddy simulations are conducted for a NACA0012 airfoil with rounded trailing edge for two flow configurations with different freestream Mach numbers (M1 � 0:115 and 0.4) for an angle of incidence of 5 deg. The Reynolds number based on the airfoil chord is fixed at Rec � 408;000. The acoustic predictions are performed by the Ffowcs Williams and Hawkings acoustic analogy formulation and incorporate convective effects. Surface and volume integrations of dipole and quadrupole source termsappearingintheFfowcsWilliamsandHawkingsequationareperformedusingathree-dimensionalwideband multileveladaptivefastmultipolemethodtoacceleratethecalculationsofaeroacousticintegrals.TheeffectsofMach numberandnonlinearquadrupolesources areassessed.Nonlinear quadrupolenoise sourcesplay animportant role infar-fieldsoundradiationforthehigherMachnumber flowconfiguration,M1 � 0:4.Ascalingstudyisperformed and, although surface pressure spectra scale with hydrodynamic scaling, far-field acoustics has a more complex behavior. The broad vortex-shedding tone generated by the laminar boundary layer vortex shedding noise causes a departure from the classical fifth power-law scaling, a behavior also seen in previous experiments.

Journal ArticleDOI
TL;DR: In this paper, a model for early-stage vortex growth based on the transport of circulation from the shear layer into the vortex was proposed and found to agree well with experimental data.
Abstract: The evolution of an isolated line vortex generated by a starting two-dimensional jet is studied experimentally using time-resolved particle image velocimetry. The vortex growth in this current configuration is not linked to any externally imposed length scales or interactions with other vortical structures or walls that could potentially influence vortex growth. A model for the early-stage vortex growth, based on the transport of circulation from the shear layer into the vortex, is proposed and found to agree well with experimental data. The model provides a scaling scheme for vortex growth using shear-layer characteristic velocity and shear-layer thickness. The vortex growth is limited through a gradual separation of the vortex from the feeding shear layer, arising from decreased shear-layer curvature. This phenomenon is linked to a competition between the shear-layer tendency to remain in the streamwise direction and the induced velocity from the vortex on the shear layer. Finally, a dimensionless number representing this competition is introduced, which in turn is able to describe the gradual separation of the vortex from the shear layer.

Journal ArticleDOI
TL;DR: In this paper, the authors quantify the vortex dynamics of the primary vortex reconnection that leads to the formation of elliptical rings with axial flow and report for the first time a subsequent collision of these rings.
Abstract: We present simulations of the long-time dynamics of two anti-parallel vortex tubes with and without initial axial flow, at Reynolds number Re = Γ/ν = 104. Simulations were performed in a periodic domain with a remeshed vortex method using 785 × 106 particles. We quantify the vortex dynamics of the primary vortex reconnection that leads to the formation of elliptical rings with axial flow and report for the first time a subsequent collision of these rings. In the absence of initial axial flow, a −5/3 slope of the energy spectrum is observed during the first reconnection of the tubes. The resulting elliptical vortex rings experience a coiling of their vortex lines imparting an axial flow inside their cores. These rings eventually collide, exhibiting a −7/3 slope of the energy spectrum. Studies of vortex reconnection with an initial axial flow exhibit also the −7/3 slope during the initial collision as well as in the subsequent collision of the ensuing elliptical vortex rings. We quantify the detailed vortex dynamics of these collisions and examine the role of axial flow in the breakup of vortex structures.

Proceedings ArticleDOI
09 Jan 2012

Journal ArticleDOI
TL;DR: In this paper, the formation and evolution of a vortex ring generated at the open end of a short driver-section shock tube has been simulated numerically for pressure ratios (PR) of 3 and 7.
Abstract: Formation and evolution of a compressible vortex ring generated at the open end of a short driver section shock tube has been simulated numerically for pressure ratios (PR) of 3 and 7 in the present study. Numerical study of compressible vortex rings is essential to understand the complicated flow structure and acoustic characteristics of many high Mach number impulsive jets where simultaneously velocity, density and pressure fields are needed. The flow development, incident shock formation, shock diffraction, vortex ring formation and its evolution are simulated using the AUSM+ scheme. The main focus of the present study is to evaluate the time resolved vorticity field of the vortex ring and the shock/expansion waves in the starting jet for short driver section shock tubes—a scenario where little data are available in existing literature. An embedded shock and a vortex induced shock are observed for PR = 7. However the vortex ring remains shock free, compact and unaffected by the trailing jet for PR = 3. Numerical shadowgraph shows the evolution of embedded shock and shock/expansion waves along with their interactions. The velocity and vorticity fields obtained from simulation are validated with the particle image velocimetry results and these data match closely. The translational velocity of the vortex ring, velocity across the vortex and the centre line velocity of the jet obtained from simulation also agree well with the experimental results.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the origin of the secondary vortex street, observed in the far wake in the flow past a circular cylinder, and estimated the streamwise location of the onset of the instability, frequency of the generation of packets and their convection speed via direct numerical simulation.
Abstract: The origin of the secondary vortex street, observed in the far wake in the flow past a circular cylinder, is investigated. The Reynolds number, based on the diameter of the cylinder, is 150. The von Karman vortex street, which originates in the near wake, decays exponentially downstream of the cylinder. Beyond the region of decay, a broad band of frequencies are selectively amplified, leading to the formation of a secondary vortex street consisting of packets of large-scale vortex structures. The streamwise location of the onset of the instability, frequency of the generation of packets and their convection speed are estimated via direct numerical simulation (DNS). Global linear stability analysis of the time-averaged flow reveals the presence of unstable convective modes that travel at almost the same speed and have a structure similar to the packet-like disturbances as observed in the DNS. Sensitivity analysis of the global convective modes to structural perturbations is carried out to locate the region of the wake that is most significant in generating the modes responsible for the appearance of the secondary vortex street. This information is utilized to control the flow. By placing a 'slip' splitter plate along the wake centre line, in the overlap region of the direct and the adjoint modes, the oscillations in the far wake are significantly reduced, though the oscillations related to the primary vortex shedding in the near wake are not. It is also found that suppression of the primary vortex shedding leads to annihilation of the secondary vortex street as well. Linear stability analysis of the steady-state flow does not yield any modes that can explain the appearance of the secondary vortex street. The steady and time-averaged wake profiles, for the Re D 150 flow, are compared to bring out the differences in the two. The effect of free-stream oscillations on the evolution of the secondary vortex street is investigated. By reducing the amplitude of inlet excitation, a gradual transition from ordered shedding in the far wake to the appearance of a broad-band spectrum of frequencies, as in the unforced wake, is observed. All the computations have been carried out using a stabilized finite element method.

Journal ArticleDOI
TL;DR: In this article, the authors used particle imaging velocimetry to investigate the near-ground-vortex structure of tornadoes and found that not only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow.
Abstract: The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S = 0.03–0.3 (vane angle θv = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry. Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations (z = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled vortex at the lowest swirl ratio (S = 0.03, θv = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios (S = 0.1–0.3, θv = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component, has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity variance is about twice at the high swirl ratio (θv = 45°) that at the low swirl ratio (θv = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately leading to better predictions of tornado-induced wind loads on civil structures.

Journal ArticleDOI
TL;DR: In this article, a large eddy simulation (LES) is used to investigate the structure of the laminar horseshoe vortex (HV) system and the dynamics of the necklace vortices as they fold around the base of a circular cylinder mounted on the flat bed of an open channel for Reynolds numbers defined with the cylinder diameter, D, smaller than 4460.
Abstract: Large eddy simulation (LES) is used to investigate the structure of the laminar horseshoe vortex (HV) system and the dynamics of the necklace vortices as they fold around the base of a circular cylinder mounted on the flat bed of an open channel for Reynolds numbers defined with the cylinder diameter, D, smaller than 4460. The study concentrates on the analysis of the structure of the HV system in the periodic breakaway sub-regime, which is characterized by the formation of three main necklace vortices. Over one oscillation cycle of the previously observed breakaway sub-regime, the corner vortex and the primary vortex merge (amalgamate) and a developing vortex separates from the incoming laminar boundary layer (BL) to become the new primary vortex. Results show that while the classical breakaway sub-regime, in which one amalgamation event occurs per oscillation cycle, is present when the nondimensional displacement thickness of the incoming BL at the location of the cylinder is relatively large (δ*/D > 0.1), a new type of breakaway sub-regime is present for low values of δ*/D. This sub-regime, which we call the double-breakaway sub-regime, is characterized by the occurrence of two amalgamation events over one full oscillation cycle. LES results show that when the HV system is in one of the breakaway sub-regimes, the interactions between the highly coherent necklace vortices and the eddies shed inside the separated shear layers (SSLs) are very strong. For the relatively shallow flow conditions considered in this study (H/D ≅ 1, H is the channel depth), at times, the disturbances induced by the legs of the necklace vortices do not allow the SSLs on the two sides of the cylinder to interact in a way that allows the vorticity redistribution mechanism to lead to the formation of a new wake roller. As a result, the shedding of large-scale rollers in the turbulent wake is suppressed for relatively large periods of time. Simulation results show that the wake structure changes randomly between time intervals when large-scale rollers are forming and are convected in the wake (von Karman regime), and time intervals when the rollers do not form. When the wake is in the von Karman regime, the shedding frequency of the rollers is close to that observed for flow past infinitely long cylinders.

Journal ArticleDOI
TL;DR: In this paper, the interactions of wingtip vortices in close-formation flight of two wings were examined experimentally and the lift-to-drag ratio increased the most effectively when the two wings overlapped with each other by 5% of the wingspan with zero vertical offset for the present wing configuration.
Abstract: The interactions of wingtip vortices in close-formation flight of two wings were examined experimentally. Both wings consisting of a NACA 23012 airfoil section and a rectangular planform of an aspect ratio of 5 were set in echelon formation in a wind tunnel. The lift-to-drag ratio increased the most effectively when the two wings were overlapped with each other by 5% of the wingspan with zero vertical offset for the present wing configuration. Flow visualizations and particle image velocimetry measurements showed that the trailing vortex of the lead wing collided at a slightly inboard point of the trail wing and was divided into two streamwise vortices in the most effective overlap condition. It was also observed that the tip vortices of the lead and trail wings interacted closely with each other to form a pair of counter-rotating vortices for smaller overlap conditions less than 5 % with zero vertical offset. For such small overlap cases, circulation of the trailing vortex of the trail wing was reduced markedly by influences of the trailing vortex of the lead wing.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the approach of a descending vortex pair to a horizontal ground plane and found that the secondary vortices deform into a distinct waviness along their lengths.
Abstract: In this work, we investigate the approach of a descending vortex pair to a horizontal ground plane. As in previous studies, the primary vortices exhibit a ‘rebound’, due to the separation of secondary opposite-sign vortices underneath each primary vortex. On each side of the flow, the weaker secondary vortex can become three-dimensionally unstable, as it advects around the stronger primary vortex. It has been suggested in several recent numerical simulations that elliptic instability is the origin of such waviness in the secondary vortices. In the present research, we employ a technique whereby the primary vortices are visualized separately from the secondary vortices; in fact, we are able to mark the secondary vortex separation, often leaving the primary vortices invisible. We find that the vortices are bent as a whole in a Crow-type ‘displacement’ mode, and, by keeping the primary vortices invisible, we are able to see both sides of the flow simultaneously, showing that the instability perturbations on the secondary vortices are antisymmetric. Triggered by previous research on four-vortex aircraft wake flows, we analyse one half of the flow as an unequal-strength counter-rotating pair, noting that it is essential to take into account the angular velocity of the weak vortex around the stronger primary vortex in the analysis. In contrast with previous results for the vortex–ground interaction, we find that the measured secondary vortex wavelength corresponds well with the displacement bending mode, similar to the Crow-type instability. We have analysed the elliptic instability modes, by employing the approximate dispersion relation of Le Dizes & Laporte (J. Fluid Mech., vol. 471, 2002, p. 169) in our problem, finding that the experimental wavelength is distinctly longer than predicted for the higher-order elliptic modes. Finally, we observe that the secondary vortices deform into a distinct waviness along their lengths, and this places two rows of highly stretched vertical segments of the vortices in between the horizontal primary vortices. The two rows of alternating-sign vortices translate towards each other and ultimately merge into a single vortex row. A simple point vortex row model is able to predict trajectories of such vortex rows, and the net result of the model’s ‘orbital’ or ‘passing’ modes is to bring like-sign vortices, from each secondary vortex row, close to each other, such that merging may ensue in the experiments.

Journal ArticleDOI
TL;DR: In this article, the dynamics of a perturbed incompressible, inviscid, axisymmetric, near-critical swirling flow in a long, finite-length, straight, circular pipe is studied through a weakly nonlinear analysis.
Abstract: The dynamics of a perturbed incompressible, inviscid, axisymmetric, near-critical swirling flow in a long, finite-length, straight, circular pipe is studied through a weakly nonlinear analysis. The flow is subjected to non-periodic inlet and outlet conditions. The long-wave approach involves a rescaling of the axial distance and time. It results in a separation of the perturbation’s structure into a critical standing wave in the radial direction and an evolving wave in the axial direction, that is described by a nonlinear model problem. The approach is first validated by establishing the bifurcation of non-columnar states from the critical swirl and the linear stability modes of these states. Examples of the flow dynamics at various near-critical swirl levels in response to different initial perturbations demonstrate the important role of the nonlinear steepening terms in perturbation dynamics. The computed dynamics shows quantitative agreement with results from numerical simulations that are based on the axisymmetric Euler equations for various swirl levels and as long as perturbations are small, thereby verifying the accuracy of each computation and capturing the essence of flow dynamics. Results demonstrate the various stages of the flow dynamics, specifically during the transition to vortex breakdown states. They reveal the evolution of faster-than-exponential and shape-changing modes as perturbations grow into the vortex breakdown process. These explosive modes provide the sudden and abrupt nature of the vortex breakdown phenomenon. Further analysis of the model problem shows the important role of the nonlinear evolution of perturbations and its relevance to the transfer of the perturbation’s kinetic energy between the boundaries and flow bulk, the evolution of perturbations in practical concentrated vortex flows, and the design of control methods of vortex flows. A robust feedback control method to stabilize a solid-body rotation flow in a pipe at a wide range of swirl levels above critical is developed. The applicability of this method to stabilizing medium and small core-size vortices is also discussed.

Journal ArticleDOI
TL;DR: In this article, the formation of vortex rings in the developing region of a round jet subjected to high-amplitude acoustic forcing is investigated with high-speed Particle Image Velocimetry.
Abstract: The periodic formation of vortex rings in the developing region of a round jet subjected to high-amplitude acoustic forcing is investigated with High-Speed Particle Image Velocimetry. Harmonic velocity oscillations ranging from 20 to 120% of the mean exit velocity of the jet was achieved at several forcing frequencies determined by the acoustic response of the system. The time-resolved history of the formation process and circulation of the vortex rings are evaluated as a function of the forcing conditions. Overall, high-amplitude forcing causes the shear layers of the jet to breakup into a train of large-scale vortex rings, which share many of the features of starting jets. Features of the jet breakup such as the roll-up location and vortex size were found to be both amplitude and frequency dependent. A limiting time-scale of t/T ≈ 0.33 based on the normalized forcing period was found to restrict the growth of a vortex ring in terms of its circulation for any given arrangement of jet forcing conditions. In sinusoidally forced jets, this time-scale corresponds to a kinematic constraint where the translational velocity of the vortex ring exceeds the shear layer velocity that imposes pinch-off. This kinematic constraint results from the change in sign in the jet acceleration between t = 0 and t = 0.33T. However, some vortex rings were observed to pinch-off before t = 0.33T suggesting that they had acquired their maximum circulation. By invoking the slug model approximations and defining the slug parameters based on the experimentally obtained time- and length-scales, an analytical model based on the slug and ring energies revealed that the formation number for a sinusoidally forced jet is L/D ≈ 4 in agreement with the results of Gharib et al. (J Fluid Mech 360:121–140, 1998).

Journal ArticleDOI
TL;DR: In this article, a new method for characterizing cross-sections of vortices in turbulent flows is described. But the method is not robust, robust, not selective, and above all, much less contaminated by shear than the existing methods.

Journal ArticleDOI
TL;DR: In this paper, three dimensional structures of unsteady flows behind a square prism with aspect ratio H / D =2.7 in a shallow boundary layer flow are investigated by two dimensional and three dimensional stereoscopic PIV techniques synchronizing with velocity measurement by a hot wire anemometer.

Journal ArticleDOI
TL;DR: In this paper, a detailed categorization of vortex formations in the reduced frequency range of 0.16≤ k ≤ 6.26 corresponding to Strouhal number range 0.05≤St≤1.0.

Journal ArticleDOI
TL;DR: In this article, the authors measured the unsteady flow structure and force time history of pitching and plunging SD7003 and flat plate airfoils at low Reynolds numbers, and the results showed anisotropic turbulence for the LEV and isotropic turbulence for TEV.
Abstract: Measurements of the unsteady flow structure and force time history of pitching and plunging SD7003 and flat plate airfoils at low Reynolds numbers are presented. The airfoils were pitched and plunged in the effective angle of attack range of 2.4°–13.6° (shallow-stall kinematics) and −6° to 22° (deep-stall kinematics). The shallow-stall kinematics results for the SD7003 airfoil show attached flow and laminar-to-turbulent transition at low effective angle of attack during the down stroke motion, while the flat plate model exhibits leading edge separation. Strong Re-number effects were found for the SD7003 airfoil which produced approximately 25 % increase in the peak lift coefficient at Re = 10,000 compared to higher Re flows. The flat plate airfoil showed reduced Re effects due to leading edge separation at the sharper leading edge, and the measured peak lift coefficient was higher than that predicted by unsteady potential flow theory. The deep-stall kinematics resulted in leading edge separation that led to formation of a large leading edge vortex (LEV) and a small trailing edge vortex (TEV) for both airfoils. The measured peak lift coefficient was significantly higher (~50 %) than that for the shallow-stall kinematics. The effect of airfoil shape on lift force was greater than the Re effect. Turbulence statistics were measured as a function of phase using ensemble averages. The results show anisotropic turbulence for the LEV and isotropic turbulence for the TEV. Comparison of unsteady potential flow theory with the experimental data showed better agreement by using the quasi-steady approximation, or setting C(k) = 1 in Theodorsen theory, for leading edge–separated flows.

Journal ArticleDOI
TL;DR: In this article, the velocity and vorticity fields are determined experimentally using PIV technique for different resolutions in order to study the global flow around the plate and the formation and advection of vortices upstream and downstream of the plate.
Abstract: This paper presents the vortex dynamics generated by the interaction of a submerged horizontal plate, considered as a vortex generator, and a monochromatic wave The velocity and vorticity fields are determined experimentally using PIV technique for different resolutions in order to study the global flow around the plate and the formation and advection of vortices upstream and downstream of the plate The global flow around the plate shows great discrepancies with the potential flow solution: two recirculation cells are formed beneath the plate, the global flow is non-symmetric and the advection of vortices induces strong velocities not represented by the potential flow theory The formation of vortices at the edges of the plate is characterised At each period, one vortex is formed at the edge followed by the formation of an opposite sign vortex The upstream and downstream vortex pairs are then advected in front of the plate and toward the bottom respectively, over a distance of about one third the plate length The lifetime of vortices is about two wave periods This study will help us validate a numerical software to be used for analysing the influence of various parameters on the dynamics These results will be presented in the second part of this paper

Journal ArticleDOI
TL;DR: In this paper, the authors compute numerically the unstable modes of an anticyclonic Gaussian vortex lens in a continuously stratified rotating fluid and show that the most unstable mode is associated with a critical layer instability located at the vortex periphery.
Abstract: This work addresses the linear dynamics underlying the formation of density interfaces at the periphery of energetic vortices, well outside the vortex core, both in the radial and axial directions. We compute numerically the unstable modes of an anticyclonic Gaussian vortex lens in a continuously stratified rotating fluid. The most unstable mode is a slow mode, associated with a critical layer instability located at the vortex periphery. Although the most unstable disturbance has a characteristic vertical scale which is comparable to the vortex height, interestingly, the critical levels of the successively fastest growing modes are closely spaced at intervals along the axial direction that are much smaller than the vortex height.

Journal ArticleDOI
TL;DR: In this paper, the interaction of vortex rings of constant Reynolds number with porous surfaces composed of wire meshes of constant open area, but variable wire diameter, was studied using flow visualization, and the results indicate that several regimes of flow behavior exist in the parameter space investigated.
Abstract: The interaction of vortex rings of constant Reynolds number with porous surfaces composed of wire meshes of constant open area, i.e., surface porosity, but variable wire diameter is studied using flow visualization. The results indicate that several regimes of flow behavior exist in the parameter space investigated. The vortex ring passes through and immediately reforms downstream of the surface for porous surfaces with small wire mesh diameters. The transmitted vortex ring has the same diameter, but lower convection speed and circulation than the pre-interaction vortex ring. For these cases, secondary vortex rings are formed on the upstream side of the porous surface that convect upstream away from the screen. As the wire diameter of the porous surface is increased, smaller sub-scale vortical structures are formed on the transmitted vortex ring as it passes through the surface. The spatial scale of these structures is dependent on the diameter of the mesh wire. The vortex ring is disrupted but is able to...

Journal ArticleDOI
TL;DR: In this article, the authors investigated the development of the trailing shear layer during the leading vortex ring formation, as well as the corresponding effects on the pinch-off process using digital particle image velocimetry (DPIV).
Abstract: Experiments on a circular starting jet generated by a piston–cylinder arrangement, over a range of Reynolds number from to , are conducted so as to investigate the development of the trailing shear layer during the leading vortex ring formation, as well as the corresponding effects on the pinch-off process. Results obtained by digital particle image velocimetry (DPIV) show that secondary vortices start to develop in the trailing jet only after the critical time scale, the ‘formation number’, is achieved. The subsequent growth of the secondary vortices reduces the vorticity flux being fed into the leading vortex ring and, as a consequence, constrains the growth of leading vortex ring with larger circulation. Evolution of perturbation waves into secondary vortices is found to associate with growth and translation of the leading vortex ring during the formation process. A dimensionless parameter ‘’, defined as ), is therefore proposed to characterize the effect of the leading vortex ring on suppressing the nonlinear development of instability in the trailing shear layer, i.e. the initial roll-up of the secondary vortices. In a starting jet, follows a decreasing trend with the formation time . A critical value is identified experimentally, which physically coincides with the initiation of the first secondary vortex roll-up and, therefore, indicates the onset of pinch-off process.