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Showing papers on "Starting vortex published in 1997"


Journal ArticleDOI
TL;DR: Visualization experiments with Manduca sexta have revealed the presence of a leading-edge vortex and a highly three-dimensional flow pattern, confirming that the downstroke is the main provider of lift force.
Abstract: Visualization experiments with Manduca sexta have revealed the presence of a leading-edge vortex and a highly three-dimensional flow pattern. To further investigate this important discovery, a scaled-up robotic insect was built (the 'flapper') which could mimic the complex movements of the wings of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing revealed a small but strong leading-edge vortex on the downstroke. This vortex had a high axial flow velocity and was stable, separating from the wing at approximately 75 per cent of the wing length. It connected to a large, tangled tip vortex, extending back to a combining stopping and starting vortex from pronation. At the end of the downstroke, the wake could be approximated as one vortex ring per wing. Based on the size and velocity of the vortex rings, the mean lift force during the downstroke was estimated to be about 1.5 times the body weight of a hawkmoth, confirming that the downstroke is the main provider of lift force.

240 citations


Journal ArticleDOI
TL;DR: In this article, the stability of two vortex pairs is analyzed as a model for the vortex system generated by an aircraft in flaps-down configuration, where the co-rotating vortices on the starboard and port sides tumble about one another as they propagate downward.
Abstract: The stability of two vortex pairs is analysed as a model for the vortex system generated by an aircraft in flaps-down configuration. The co-rotating vortices on the starboard and port sides tumble about one another as they propagate downward. This results in a time-periodic basic state for the stability analysis. The dynamics and instability of the trailing vortices are modelled using thin vortex filaments. Stability equations are derived by matching the induced velocities from Biot–Savart integrals with kinematic equations obtained by temporal differentiation of the vortex position vectors. The stability equations are solved analytically as an eigenvalue problem, using Floquet theory, and numerically as an initial value problem. The instabilities are periodic along the axes of the vortices with wavelengths that are large compared to the size of the vortex cores. The results show symmetric instabilities that are linked to the long-wavelength Crow instability. In addition, new symmetric and antisymmetric instabilities are observed at shorter wavelengths. These instabilities have growth rates 60–100% greater than the Crow instability. The system of two vortex pairs also exhibits transient growth which can lead to growth factors of 10 or 15 in one-fifth of the time required for the same growth due to instability.

171 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate a 50% to 60% enhancement of average heat and mass transfer for flow over a flat plate at low Reynolds numbers, using delta-wing vortex generators.

164 citations


Journal ArticleDOI
TL;DR: In this paper, the Strouhal number was shown to hold even when the flow is 3D and the shedding modes were observed and the sequence of mode transitions recorded, and the spanwise variation in time-average base pressure was predicted using the estimated amount of time the flow spends at the two shedding frequencies.
Abstract: Experiments have been carried out to study the three-dimensional characteristics of vortex shedding from a half-ellipse shape with a blunt trailing edge. In order to control the occurrence of vortex dislocations, the trailing edges of the models used were constructed with a series of periodic waves across their spans. Flow visualization was carried out in a water tunnel at a Reynolds number of 2500, based on trailing-edge thickness. A number of shedding modes were observed and the sequence of mode transitions recorded. Quantitative data were obtained from wind tunnel measurements performed at a Reynolds number of 40000. Two shedding frequencies were recorded with the higher frequency occurring at spanwise positions coinciding with minima in the chord. At these same positions the base pressure was lowest and the vortex formation length longest. Arguments are put forward to explain these observations. It is shown that the concept of a universal Strouhal number holds, even when the flow is three-dimensional. The spanwise variation in time-average base pressure is predicted using the estimated amount of time the flow spends at the two shedding frequencies.

158 citations


Journal ArticleDOI
TL;DR: In this paper, a Pitot probe was used to measure the axial and azimuthal velocities in a vortex tube and the velocity field was determined from the measured velocity field in the 25 mm diameter vortex tube.

152 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used three-component laser Doppler velocimetry to obtain the distributions of velocity, vorticity, and circulation across the tip vortex at several axial locations in the flow for several angles of incidence.
Abstract: The evolving three-dimensional flowfield of the tip vortex in the near wake of a rectangular wing at incidence was studied in detail, using three-component laser Doppler velocimetry. The flow quantities measured were the three components of the instantaneous velocity. These data were used to obtain the distributions of velocity, vorticity, and circulation across the vortex at several axial locations in the flow for several angles of incidence. The data have been used to understand the process of rollup of the shear layer into the vortex in the near wake, as well as its kinematic structure. The data indicate that the rollup takes place quite quickly and the inner part of the three-dimensional vortex becomes nearly axisymmetric within a distance of about two chord lengths downstream of the trailing edge. Even though the vortex behavior in the near wake is, in general, strongly dependent on the initial conditions, the vortex trajectory appears to be described reasonably well by the overall wing lift and the freestream velocity. Also, even in the near wake, circumferentially averaged mean flow properties in the inner part of the nearly axisymmetric vortex begin to exhibit a universal structure characteristic of conceptual asymptotic trailing vortices

91 citations


Journal ArticleDOI
TL;DR: In this paper, a linear analysis of the ideal MHD equations reveals a hybrid vortex instability which appears because of the coupling of Kelvin-Helmholtz (KH) and Rayleigh-Taylor instabilities.
Abstract: The nonlinear dynamics of a shear flow and its subsequent evolution in the equatorial plane of the inner plasma sheet is studied. A linear analysis of the ideal MHD equations reveals a hybrid vortex instability which appears because of the coupling of Kelvin-Helmholtz (KH) and Rayleigh-Taylor instabilities. The hybrid vortex mode grows faster than a KH mode, extracts ambient potential energy, and leads to vortex cells that have a larger spatial extent than a simple KH vortex. In the nonlinear stage, vortices become surge-like and may destroy the shear flow region. The relevance of this model to vortex generation and auroral arc intensifications at the inner edge of the plasma sheet is discussed.

90 citations


Patent
19 Jun 1997
TL;DR: The micro-atomizing device of the present invention creates high energy vortices as discussed by the authors, which are generated simultaneously and synchronously and then merged into a three-dimensional force field, and a large vacuum is produced in a resultant stable vortex force field in a vortex accumulation zone.
Abstract: The micro-atomizing device of the present invention creates high energy vortices. These high speed vortices are generated simultaneously and synchronously and then merged into a three dimensional force field. When the high energy vortices are brought together, a large vacuum is produced in a resultant stable vortex force field in a vortex accumulation zone. The high vacuum draws the fluid to be atomized through a delivery tube into the vortex accumulation zone. The high energy within the vortex accumulation zone either breaks up the fluid to be atomized into very small droplets or gasifies the fluid by the combination of high energy density cold boiling, shockwave generated ultrasound, and centripetal forces.

72 citations


Journal ArticleDOI
TL;DR: In this paper, a complete description of long-wave vortex ring oscillations in an ideal incompressible fluid, and an examination of sound radiation by these oscillations of a thin vortex ring in a weakly compressible fluid are investigated.
Abstract: Two coupled problems are investigated: a complete description of long-wave vortex ring oscillations in an ideal incompressible fluid, and an examination of sound radiation by these oscillations in a weakly compressible fluid. The first part of the paper relates to the problem of eigen-oscillations of a thin vortex ring (μ < 1) in an ideal incompressible fluid. The solution of the problem is obtained in the form of an asymptotic expansion in the small parameter μ. The complete set of three-dimensional eigen-oscillations and axisymmetric modes (two-dimensional oscillations) is obtained. It is shown that, unlike the vortex column oscillations which have the form of simple angular harmonics, the majority of eigen-oscillations of a thin vortex ring have a more complex form which is a combination of two harmonics in the leading approximation. This leads to dramatic changes in the efficiency of sound radiation produced by modes of the vortex ring in comparison with the corresponding modes of the vortex column. In the second part of the paper the solution obtained is used to investigate the process of sound radiation by vortex perturbations in a weakly compressible fluid. The vortex ring eigen-oscillations are classified according to their sound radiation efficiency. It is shown that the modes with the dimensionless frequency ω 1/2 radiate sound most efficiently. They are two isolated modes, two infinite families of Bessel modes and a set of axisymmetric modes. The frequencies of these modes are in the interval Δω = O(μ). The results obtained are compared with known experimental data on acoustic radiation of a turbulent vortex ring. Within the limits of the theory derived an explanation of the main characteristics of sound radiation is presented.

71 citations


Journal ArticleDOI
TL;DR: In this paper, the propagation and interaction mechanisms demonstrated by compressible vortex rings are explored with spark shadowgraph and schlieren photography, and the vortical flow regimes produced by the impulse of a shock wave emerging from the open end of a tube are examined.

69 citations


Journal ArticleDOI
TL;DR: In this paper, the phase differences regarding the individual vortex shedding structures detected at multiple circumferential locations in the wake were obtained by analyzing the hot-wire signals with a conditional sampling scheme.
Abstract: Experiments were performed for individual reali- zations of the vortex shedding process behind a circular disk at Reynolds numbers of 103—105, at which periodic vortex shedding prevails in the wake. The phase differences regarding the individual vortex shedding structures detected at multiple circumferential locations in the wake were obtained by analyzing the hot-wire signals with a conditional-sampling scheme. The phase differences of vortex shedding detected at circumferential positions 90i apart show a wide scatter, but the anti-phase character is largely preserved in the individual vortex shedding process as detected at circumferential loca- tions 180i apart. The randomness of phase differences involved in the vortex shedding process is noted to be essential in order to satisfy the axisymmetric property of the global flow.

Journal ArticleDOI
TL;DR: In this article, a model is developed for the sound generated when a convected disturbance encounters a cambered airfoil at non-zero angle of attack, and the model is based on a linearization of the Euler equations about the steady subsonic flow past the air-foil.
Abstract: A theoretical model is developed for the sound generated when a convected disturbance encounters a cambered airfoil at non-zero angle of attack. The model is a generalization of a previous theory for a flat-plate airfoil, and is based on a linearization of the Euler equations about the steady, subsonic flow past the airfoil. High-frequency gusts, whose wavelengths are short compared to the airfoil chord, are considered. The airfoil camber and incidence angle are restricted so that the mean flow past the airfoil is a small perturbation to a uniform flow. The singular perturbation analysis retains the asymptotic regions present in the case of a flat-plate airfoil: local regions, which scale on the gust wavelength, at the airfoil leading and trailing edges; a 'transition' region behind the airfoil which is similar to the transition zone between illuminated and shadow regions in optical problems; and an outer region, far away from the airfoil edges and wake, in which the solution has a geometric-acoustics form. For the cambered airfoil, an additional asymptotic region in the form of an acoustic boundary layer adjacent to the airfoil surface is required in order to account for surface curvature effects. Parametric calculations are presented which illustrate that, like incidence angle, moderate amounts of airfoil camber can significantly affect the sound field produced by airfoil-gust interactions. Most importantly, the amount of radiated sound power is found to correlate very well with a single aerodynamic loading parameter, α eff , which is an effective mean-flow incidence angle for the airfoil leading edge.

Journal ArticleDOI
TL;DR: In this article, a laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique, and the instantaneous velocity and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner region (boundary-layer regions) vortical structures.
Abstract: A laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique. The plane wall jet is issued from a rectangular channel, with the jet-exit velocity profile being parabolic. The Reynolds number, based on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow features, laser-sheet/smoke flow visualizations are performed along streamwise, spanwise, and cross-stream directions. Surface pressure measurements are made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner-region (boundary-layer region) vortical structures. Results show that under the influence of the free-shear-layer vortex, the local boundary layer becomes detached from the surface and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-shear-layer vortex and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and outer-region vortical structures dominate the transition process. Farther downstream, the emergence of the three-dimensional structure in the free shear layer initiates complete breakdown of the flow.

Journal ArticleDOI
TL;DR: In this article, a columnar vortex with non-zero axial flow is considered for impulsive cutting and the vortex response is compared to analytical predictions obtained using the plug-flow model of Lundgren & Ashurst (1989).
Abstract: A study of the response of a columnar vortex with non-zero axial flow to impulsive cutting has been performed. The flow evolution is computed based on the vorticity–velocity formulation of the axisymmetric Euler equation using a Lagrangian vorticity collocation method. The vortex response is compared to analytical predictions obtained using the plug-flow model of Lundgren & Ashurst (1989). The plug-flow model indicates that axial motion on a vortex core with variable core area behaves in a manner analogous to one-dimensional gas dynamics in a tube, with the vortex core area playing a role analogous to the gas density. The solution for impulsive cutting of a vortex obtained from the plug-flow model thus resembles the classic problem of impulsive motion of a piston in a tube, with formation of an upstream-propagating vortex ‘shock’ (over which the core radius changes discontinuously) and a downstream-propagating vortex ‘expansion wave’ on opposite sides of the cutting surface. Direct computations of the vortex response from the Euler equation reveal similar upstream- and downstream-propagating waves following impulsive cutting for cases where the initial vortex flow is subcritical. These waves in core radius are produced by a series of vortex rings, embedded within the columnar vortex core, having azimuthal vorticity of alternating sign. The effect of the compression and expansion waves is to bring the axial and radial velocity components to nearly zero behind the propagating vortex rings, in a region on both sides of the cutting surface with ever-increasing length. The change in vortex core radius and the variation in pressure along the cutting surface agree very well with the predictions of the plug-flow model for subcritical flow after the compression and expansion waves have propagated sufficiently far away. For the case where the ambient vortex flow is supercritical, no upstream-propagating wave is possible on the compression side of the vortex, and the vortex axial flow is observed to impact on the cutting surface in a manner similar to that commonly observed for a non-rotating jet impacting on a wall. The flow appears to approach a steady state near the point of impact after a sufficiently long time. The vortex response on the expansion side of the cutting surface exhibits a downstream-propagating vortex expansion wave for both the subcritical and supercritical conditions. The results of the vortex response study are used to formulate and verify predictions for the net normal force exerted by the vortex on the cutting surface. An experimental study of the cutting of a vortex by a thin blade has also been performed in order to verify and assess the limitations of the instantaneous vortex cutting model for application to actual vortex–body interaction problems.

Journal ArticleDOI
TL;DR: In this paper, the structure and mixing of impulsively started jets have been studied in a water tank utilizing an acid-base reaction, and it was shown that the fluid in the vicinity of the jet tip mixes with the ambient fluid faster than the rest of the jets.
Abstract: The structure and mixing of impulsively started jets have heen studied in a water tank utilizing an acid-base reaction. The flow consists of a starting vortex that separates from the rest of the jet in the near field. Penetration of the jet tip scales with the square root of time, normalized by the nozzle diameter and velocity. The celerity of the jet tip is approximately one-half of the centerline velocity of a steady jet, with the same nozzle exit velocity, at the same location. Results of chemically reactive experiments indicate that the fluid in the vicinity of the jet tip mixes with the ambient fluid faster than the rest of the jet. The extent of the region near the jet tip with improved mixing becomes larger as the jet travels further downstream. The more rapid mass mixing at the jet tip implies faster momentum diffusion, which corroborates the slowing down of the jet tip in comparison with the steady jet.

Journal ArticleDOI
TL;DR: In this paper, the relationship between boundary layer flow, tip vortex structure for a finite span wing, and cavitation was summarized and the degree of interaction was observed to increase as the lifting efficiency decreased.
Abstract: We summarize recent research on the relation between boundary layer flow, tip vortex structure for a finite span wing, and cavitation. Three hydrofoils of elliptic planform of aspect ratio 3 were constructed with different NACA cross sections. Using a sprayed oil droplet technique to visualize the boundary layer flow, each foil was found to have dramatically different flow separation characteristics on both the suction and pressure sides. Careful examination of the tip region suggests that while the initial stages of vortex roll-up from the pressure side are similar for each hydrofoil section, the vortex boundary layer interaction on the suction side differs for each section. The degree of interaction was observed to increase as the lifting efficiency decreased. Over the Reynolds number range tested, tip vortex cavitation inception has been observed to follow an almost universal scaling. Differences in this scaling law are correlated with the degree of vortex/boundary layer interaction.

Journal ArticleDOI
TL;DR: In this paper, a two-dimensional asymmetric merger of two like-signed vorticity monopoles with different sizes and vorticities is examined by combining simplified analytical models and contour dynamics experiments, and the model results can capture the key dynamics and hence allow the prediction of the critical merger distance in a number of the situations.

Journal ArticleDOI
TL;DR: In this paper, a three-layer shallow-water numerical model is used to study the motion of tropical-cyclone-like vortices in a westerly baroclinic current.
Abstract: A three-layer shallow-water numerical model is used to study the motion of tropical-cyclone-like vortices in a westerly baroclinic current. the structure of the vortex and its environment are chosen to be representative of real tropical cyclones. Numerical experiments with and without convection are discussed. Several different environmental flows are considered. In calculations on a northern-hemisphere f-plane without convection, anomalies of upper-layer potential vorticity (PV) have a major influence on the vortex motion in the middle layer. This is true even in numerical experiments that include a PV-gradient in the middle layer associated with an upper-level westerly jet. For example, when a broad anticyclone is included in the upper layer, the middle-layer vortex moves initially to the east-northeast. If the middle-layer PV-gradient were the dominant factor, the middle-layer vortex would, instead, move to the south-east. When there is no middle-layer PV-gradient, the middle-layer vortex is deflected to the right of the environmental shear vector if the upper-layer vortex is cyclonic and to the left of the environmental shear vector if the upper-layer vortex is anticyclonic. the size of the vortex affects the drift velocity, but has a negligible effect on the drift direction. Calculations incorporating a convective parametrization based on that developed by Ooyama (1969) show that an initially weak vortex can intensify to hurricane strength in a conditionally neutral environment. This calculation underscores the importance of surface fluxes in Ooyama's scheme and subsequent schemes based on it, as it shows that the scheme does not rely necessarily on a large amount of convective instability in the initial state. In calculations on a northern-hemisphere f-plane with an upper-level westerly jet and convection, the middle-layer vortex moves to the south-east because of the middle-layer PV-gradient. When the middle-layer PV-gradient is removed but the convection retained, the middle-layer vortex moves first to the south-east and then to the northeast. In this case the motion of the middle-layer vortex is controlled by the upper-layer PV-anomaly. At early times, the vertical shear tilts the vortex towards the east so that the western side of the upper-layer cyclonic core lies above the middle-layer vortex centre. Consequently, the upper-layer cyclonic core induces a southward component to the flow across the middle-layer vortex centre. As time proceeds, the upper-layer westerlies advect the broad anticyclonic outer part of the upper vortex over the middle-layer vortex centre, inducing a northward component across its centre. Relatively minor changes to the convective parametrization have a relatively large effect on the motion of the vortex. Changing the way the parametrized clouds transport momentum, changes the structure of the upper-layer vortex. Changes to the pattern of PV in the upper layer in turn affect the middle-layer flow attributable to it. the effect of the upper-layer PV on the motion of the middle-layer vortex is weakened when the outflow from a tropical cyclone is confined to a comparatively shallow upper layer as is commonly observed.

Journal ArticleDOI
TL;DR: In this article, a 3D simulation of a counter-rotating vortex pair approaching a wall in an otherwise quiescent fluid is presented, where the secondary vortex is destroyed by stretching and dissipation leaving the primary vortex with a permanently distorted shape.
Abstract: The interaction of vortices passing near a solid surface has been examined using direct numerical simulation. The configuration studied is a counter-rotating vortex pair approaching a wall in an otherwise quiescent fluid. The focus of these simulations is on the three-dimensional effects, of which little is known. To the authors’ knowledge, this is the first three-dimensional simulation that lends support to the short-wavelength instability of the secondary vortex. It has been shown how this Crow-type instability leads to three dimensionality after the rebound of a vortex pair. The growth of the instability of the secondary vortex in the presence of the stronger primary vortex leads to the turning and intense stretching of the secondary vortex. As the instability grows the secondary vortex is bent, stretched, and wrapped around the stronger primary. During this process reconnection was observed between the two secondary vortices. Reconnection also begins between the primary and secondary vortices but the weaker secondary vortex dissipates before the primary, leaving reconnection incomplete. Evidence is presented for a new type of energy cascade based on the short-wavelength instability and the formation of continual smaller vortices at the wall. Ultimately the secondary vortex is destroyed by stretching and dissipation leaving the primary vortex with a permanently distorted shape but relatively unaffected strength compared to an isolated vortex.

Journal ArticleDOI
TL;DR: In this paper, an asymptotic expression for the vorticity field is obtained at a large reynolds number Γ/ν » 1, ν being the kinematic viscosity of fluid, and during the initial time St « 1 of evolution as well as St « (Γ /ν)1/2.
Abstract: the mechanism of wrap, tilt and stretch of vorticity lines around a strong thin straight vortex tube of circulation Γ starting with a vortex filament in a simple shear flow (U=SX2x^1, S being a shear rate) is investigated analytically. an asymptotic expression for the vorticity field is obtained at a large reynolds number Γ/ν » 1, ν being the kinematic viscosity of fluid, and during the initial time St « 1 of evolution as well as St « (Γ/ν)1/2. the vortex tube, which is inclined from the streamwise (X1) direction both in the vertical (X2) and spanwise (X3) directions, is tilted, stretched and diffused under the action of the uniform shear and viscosity. the simple shear vorticity is on the other hand, wrapped and stretched around the vortex tube by a swirling motion, induced by it to form double spiral vortex layers of high azimuthal vorticity of alternating sign. the magnitude of the azimuthal vorticity increases up to O((Γ/ν)1/3S) at distance r=O((Γ/ν)1/3 (νt)1/2) from the vortex tube. the spirals induce axial flows of the same spiral shape with alternate sign in adjacent spirals which in turn tilt the simple shear vorticity toward the axial direction. as a result, the vorticity lines wind helically around the vortex tube accompanied by conversion of vorticity of the simple shear to the axial direction. the axial vorticity increases in time as s2t, the direction of which is opposite to that of the vortex tube at r=O((Γ/ν)1/2 (νt)1/2) where the vorticity magnitude is strongest. in the near region r « (Γ/ν)1/3 (νt)1/2, on the other hand, a viscous cancellation takes place in tightly wrapped vorticity of alternate sign, which leads to the disappearance of the vorticity normal to the vortex tube. only the axial component of the simple shear vorticity is left there, which is stretched by the simple shear flow itself. as a consequence, the vortex tube inclined toward the direction of the simple shear vorticity (a cyclonic vortex) is intensified, while the one oriented in the opposite direction (an anticyclonic vortex) is weakened. the growth rate of vorticity due to this effect attains a maximum (or minimum) value of ±S2/33/2 when the vortex tube is oriented in the direction of X^1+X^2[minus-or-plus sign] X^3. the present asymptotic solutions are expected to be closely related to the flow structures around intense vortex tubes observed in various kinds of turbulence such as helical winding of vorticity lines around a vortex tube, the dominance of cyclonic vortex tubes, the appearance of opposite-signed vorticity around streamwise vortices and a zig-zag arrangement of streamwise vortices in homogeneous isotropic turbulence, homogeneous shear turbulence and near-wall turbulence.

Journal ArticleDOI
TL;DR: In this paper, a discrete vortex method to simulate the separated flow around an aerofoil undergoing pitching motion is described, where vorticity generated in the thin layer around the body is discretized into vortices in accordance with themultipanel surface representation by convectionand diffusion.
Abstract: Amodi® ed discrete vortexmethod to simulate the separated ow around an aerofoil undergoingpitchingmotion is described The vorticity generated in the thin layer around the body is discretized into vortices in accordance with themultipanel surface representation By convectionand diffusion the vortices are released from the body and advanced in the wake as determined by the Biot± Savart law and random-walkmodel, respectively Both unsteady static and pitching cases are presented, and comparisonwith the test data illustrates that, without prior knowledge of the developing separation and reattachment points for the model, good agreement has been achieved

Journal ArticleDOI
TL;DR: In this paper, the type of vortex ring treated here is generated by a shock produced in a shock tube and then emitted from the tube into the atmosphere, and the flow field near the wall has been clarified from the experimental and numerical results.

Journal ArticleDOI
TL;DR: In this paper, a study of the interaction of periodic vortex rings with a central columnar vortex was performed, both for the case of identical vortex rings and for the cases of rings of alternating sign.
Abstract: A study has been performed of the interaction of periodic vortex rings with a central columnar vortex, both for the case of identical vortex rings and the case of rings of alternating sign. Numerical calculations, both based on an adaptation of the Lundgren–Ashurst (1989) model for the columnar vortex dynamics and by numerical solution of the axisymmetric Navier–Stokes and Euler equations in the vorticity–velocity formulation using a viscous vorticity collocation method, are used to investigate the response of the columnar vortex to the ring-induced velocity field. In all cases, waves of variable core radius are observed to build up on the columnar vortex core due to the periodic axial straining and compression exerted by the vortex rings. For sufficiently weak vortex rings, the forcing by the rings serves primarily to set an initial value for the axial velocity, after which the columnar vortex waves oscillate approximately as free standing waves. For the case of identical rings, the columnar vortex waves exhibit a slow upstream propagation due to the nonlinear forcing. The cores of the vortex rings can also become unstable due to the straining flow induced by the other vortex rings when the ring spacing is sufficiently small. This instability causes the ring vorticity to spread out into a sheath surrounding the columnar vortex. For the case of rings of alternating sign, the wave in core radius of the columnar vortex becomes progressively narrower with time as rings of opposite sign approach each other. Strong vortex rings cause the waves on the columnar vortex to grow until they form a sharp cusp at the crest, after which an abrupt ejection of vorticity from the columnar vortex is observed. For inviscid flow with identical rings, the ejected vorticity forms a thin spike, which wraps around the rings. The thickness of this spike increases in a viscous flow as the Reynolds number is decreased. Cases have also been observed, for identical rings, where a critical point forms on the columnar vortex core due to the ring-induced flow, at which the propagation velocity of upstream waves is exactly balanced by the axial flow within the vortex core when measured in a frame translating with the vortex rings. The occurrence of this critical point leads to trapping of wave energy downstream of the critical point, which results in large-amplitude wave growth in both the direct and model simulations. In the case of rings of alternating sign, the ejected vorticity from the columnar vortex is entrained and carried off by pairs of rings of opposite sign, which move toward each other and radially outward under their self- and mutually induced velocity fields, respectively.

Proceedings ArticleDOI
29 Jun 1997
TL;DR: In this paper, the interaction of pulsed vortex generator jets with a zero pressure gradient turbulent boundary layer has been studied from the perspective of vorticity dynamics, and the improved performance of the generator jets is attributed to the formation, stretching and bending of the starting vortex ring in these jet flows.
Abstract: The interaction of pulsed vortex generator jets with a zero pressure gradient turbulent boundary layer has been studied from the perspective of vorticity dynamics. The improved performance of the pulsed vortex generator jets is attributed to the formation, stretching and bending of the starting vortex ring in these jet flows. The shear in the boundary layer amplifies the vorticity of the starting ring and re-directs it into the streamwise direction. The pitching and skewing of the jet is necessary to create the initial tilt of the starting vortex ring with respect to the mean flow. The source of the supplementary streamwise vorticity is the jet boundary layer fluid, which rolls up into the starting vortex in the initial phase of jet ejection. Flow visualization experiments confirmed that the extra benefits obtained from pulsing the vortex generator jets stems from the alteration of flow structure. The pulsed VGJs create vortex ring-like structures that are inclined with respect to the freestream.

Journal ArticleDOI
TL;DR: In this article, a semi-empirical approach is used to predict the location of vortex breakdown and its variation with incidence, coupled to vortex lift expressions based on the leading-edge suction analogy.
Abstract: A method is presented to predict high-angle-of-attack, longitudinal aerodynamic characteristics of slender wing planforms in incompressible e ow. A semiempirical approach is used to predict the location of vortex breakdown and its variation with incidence. Breakdown predictions are then coupled to vortex lift expressions based on the leading-edge suction analogy. A correction is used to account for the attenuation of vortex suction after vortex breakdown, allowing prediction of lift, drag, and pitching moment at high angles of attack. Comparisons are made with a variety of planforms, with encouraging agreement between theory and experiment being demonstrated.

Journal ArticleDOI
TL;DR: In this article, it is shown through a systematic experimental investigation that although Kelvin-Helmholtz-like instability plays an important role in initiating the transition process, it is the leapfrogging phenomenon between the primary and secondary vortex rings which is responsible for hastening the development of azimuthal bending waves.

Journal ArticleDOI
TL;DR: In this article, a multi-layer quasi-geostrophic approximation was proposed to predict the vortex trajectories, with a core of piecewiseconstant potential vorticity, and calculated the evolution of the dipolar circulation which advects the vortex core.
Abstract: This paper deals with the self-induced translation of intense vortices on the β-plane in the framework of the multi-layer quasi-geostrophic approximation. An analytical theory is presented and compared to numerical experiments. To predict the vortex trajectories, we consider initially monopolar vortices, with a core of piecewise-constant potential vorticity, and calculate the evolution of the dipolar circulation which advects the vortex core. This multi-layer model yields analytical solutions for a period while the Rossby wave radiation is small. The development of the dipolar circulation and corresponding vortex translation are described as the results of three effects. The first and second are similar to what was found in earlier studies with a one-layer model: advection of the planetary vorticity by the symmetric vortex circulation, and horizonal deformations of the vortex core. In addition, when stratification is taken into account, the vertical tilting of the vortex core also plays a role. This third effect is here represented by the relative displacement of potential vorticity contours in different layers. Examples are given for one-, two- and three-layer models and compared with numerical simulations. It is found that the analytical predictions are good for several Rossby wave periods.

Journal ArticleDOI
TL;DR: In this paper, a gap between the airfoil surface and the extended spoiler was introduced to reduce the adverse lift and moment induced on an air-foil during the rapid deployment of its spoiler.
Abstract: A method is proposed to reduce the adverse lift and moment induced on an airfoil during the rapid deployment of its spoiler. The idea is to introduce a gap between the airfoil surface and the extended spoiler such that the flow through the base-vented spoiler abates the effects of the starting vortex shed from the spoiler tip. Results from an experimental study, which is carried out to demonstrate the effectiveness of the idea, substantiate that the magnitudes of adverse lift and moment decrease with increasing gap size. In addition, variations of the angular speed, deflection of the spoiler, and the angle of attack may also influence the adverse effects.

Journal ArticleDOI
TL;DR: In this article, the near-field evolution of a longitudinal vortex embedded in a turbulent boundary layer was examined in a wind-tunnel test, and the effects of the control parameters on the strength and location of the vortex were examined.
Abstract: The nearfield evolution of a longitudinal vortex embedded in a turbulent boundary layer was examined in a wind-tunnel test. The vortex was produced by an inclined round jet, the diameter of the jet being 0.4δ 0.99 . Flow properties were measured at a freestream velocity of 20 m/s using a laser Doppler anemometer. Control parameters are jet skew and pitch angles, and speed ratio. It was observed that a single vortex is generated at x = 10D. The effects of the control parameters on the strength and location of the vortex were examined. For an embedded vortex and favorable velocity distribution, the test results suggested a skew angle of 60 deg, a pitch angle of α = 30 deg, and speed ratios of γ = 1.0. The study provided insight into the flow physics and a database

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TL;DR: In this article, the surface interactions of the vertically approaching vortex pairs were visualized by the shadowgraph technique, showing that the underlying vortex core formed a line of circular surface depressions.
Abstract: Spatially modulated vortex pairs were generated below a free surface by two counter-rotating flaps whose edges approximate a sinusoid. The surface interactions of the vertically approaching vortex pairs were visualized by the shadowgraph technique. Two limiting cases were investigated in detail: the interaction with a surfactant-rich (contaminated) surface and with a surfactant-poor (‘clean’) surface. In the latter case shadowgraph images showed that the underlying vortex core formed a line of circular surface depressions. Subsequent measurements of the temporally evolving velocity fields using digital particle image velocimetry (DPIV) of the vortex pair cross-sections and the subsurface plane confirmed the connection process of the main vortex core with the surface. As a result of the connection the initially modulated vortex tube was broken into a line of U-vortices. In the presence of surfactants this connection could not be observed; rather a Reynolds ridge (or stagnation line) was formed and a very weak connection of the secondary separation vortex could be seen in the shadowgraphs as well as measured with the time-resolved DPIV technique. A prerequisite for connection of the vortex with the surface is that the flow's kinematics force the vortex core, that is, regions of concentrated vorticity, toward the surface. The ensuing locally concentrated viscous flux of surface-parallel vorticity through the surface is balanced by a local surface deceleration. Surface-normal vorticity appears on each side of the decelerated region whose gradually increasing circulation is directly balanced by the loss of circulation of the surface-parallel vortex. However, the shear forces caused by small amounts of surface contamination and its associated subsurface boundary layer inhibit the connection process by preventing the essential viscous flux of parallel vorticity through the surface. Instead, the subsurface boundary layer is associated with a flux of parallel vorticity into the surface which then concentrates into the observable secondary separation vortex.