Showing papers on "Starting vortex published in 1995"

Flow Visualization in a Linear Turbine Cascade of High Performance Turbine Blades

Abstract: Multiple smoke wires are used to investigate the secondary flow near the endwall of a plane cascade with blade shapes used in high-performance turbine stages. The wires are positioned parallel to the endwall and ahead of the cascade, within and outside the endwall boundary layer. The traces of the smoke generated by the wires are visualized with a laser light sheet illuminating various cross sections around the cascade. During the experiment, a periodically fluctuating horseshoe vortex system of varying number of vortices is observed near the leading edge of the cascade. A series of photographs and video tapes was taken in the cascade to trace these vortices. The development and evolution of the horseshoe vortex and the passage vortex are clearly resolved in the photographs. The interation between the suction side leg of the horseshoe vortex and the passage vortex is also observed in the experiment. A vortex induced by the passage vortex, starting about one-fourth of the curvilinear distance along the blade on the suction surface, is also found. This vortex stays close to the suction surface and above the passage vortex in the laminar flow region on the blade. From this flow visualization, a model describing the secondary flows in a cascade is proposed and compared with previous published models. Some naphthalene mass transfer results from a blade near an endwall are cited and compared with the current model. The flows inferred from the two techniques are in good agreement.

On “vortex breakdown”

Abstract: The well-known investigations of vortex breakdown are supplemented with an exact analytic representation of this phenomenon on the basis of the complete Navier-Stokes equations for the case of a potential swirl of the input flow about the axis of symmetry.

Vortex shedding and shear-layer instability of wing at low-Reynolds numbers

Abstract: Flow patterns and characteristics of vortex shedding and shear-layer instability of a NACA 0012 cantilever wing are experimentally studied. Smoke-wire and surface oil-flow techniques are employed to visualize the flow patterns and evolution of vortex shedding. Hot-wire anemometers are used to characterize the frequency domain of the unsteady flow structures. Several characteristic flow modes are classified in the domain of chord Reynolds number and root angle of attack. Effects of the juncture and wing tip are discussed. Vortex shedding can be classified into four characteristic modes. Vortex shedding at low and high angles of attack are found to have different dominant mechanisms. Effects of the juncture and wing tip on the vortex shedding are discussed. Shear-layer instabilities are found to be closely related to the behaviors of the vortex shedding. Behaviors of the shear-layer instabilities can be traced back to the characteristics of the boundary layer on the suction surface of the airfoil.

A numerical study of flow past a rotating circular cylinder using a hybrid vortex scheme

Abstract: The vortex shedding and wake development of a two-dimensional viscous incompressible flow generated by a circular cylinder which begins its rotation and translation impulsively in a stationary fluid is investigated by a hybrid vortex scheme at a Reynolds number of 1000. The rotational to translational speed ratio α varies from 0 to 6. The method used to calculate the flow can be considered as a combination of the diffusion-vortex method and the vortex-in-cell method. More specifically, the full flow field is divided into two regions: near the body surface the diffusion-vortex method is used to solve the Navier–Stokes equations, while the vortex-in-cell method is used in the exterior inviscid domain. Being more efficient, the present computation scheme is capable of extending the computation to a much larger dimensionless time than those reported in the literature.The time-dependent pressure, shear stress and velocity distributions, the Strouhal number of vortex shedding as well as the mean lift, drag, moment and power coefficients are determined together with the streamline and vorticity flow patterns. When comparison is possible, the present computations are found to compare favourably with published experimental and numerical results. The present results seem to indicate the existence of a critical α value of about 2 when a closed streamline circulating around the cylinder begins to appear. Below this critical α, Karman vortex shedding exists, separation points can be found, the mean lift and drag coefficients and Strouhal number increase almost linearly with α. Above α ≈ 2, the region enclosed by the dividing closed streamline grows in size, Karman vortex shedding ceases, the flow structure, pressure and shear stress distributions around the cylinder tend towards self-similarity with increase α, and lift and drag coefficients approach asymptotic values. The optimum lift to drag ratio occurs at α ≈ 2. The present investigation confirms Prandtl's postulation of the presence of limiting lift force at high α, and thus the usefulness of the Magnus effect in lift generation is limited.The results show that the present method can be used to calculate not only the global characteristics of the separated flow, but also the precise evolution with time of the fine structure of the flow field.

Influence of incidence angle on sound generation by airfoils interacting with high-frequency gusts

Abstract: A theoretical model is developed for the sound generated when a convected vortical or entropic gust encounters an airfoil at non-zero angle of attack. The theory 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, but long compared to the displacement of the mean-flow stagnation point from the leading edge, are considered. The analysis utilizes singular-perturbation techniques and involves four asymptotic regions. Local regions, which scale on the gust wavelength, are present at the airfoil leading and trailing edges. Behind the airfoil a ‘transition’ region, which is similar to the transition zone between illuminated and shadow zones in optical problems, is present. In the outer region, far away from the airfoil edges and wake, the solution has a geometric-acoustics form. The primary sound generation is found to be concentrated in the local leading-edge region. The trailing edge plays a secondary role as a scatterer of the sound generated in the leading-edge region. Parametric calculations are presented which illustrate that moderate levels of airfoil steady loading can significantly affect the sound field produced by airfoil-gust interactions.

The persistence of trailing vortices: A modeling study

Abstract: The principal subject of this paper is analysis and modeling of turbulent wing tip vortex flows in a far‐field region of the vortex evolution. The choice of a Reynolds stress closure (RSC) to model the vortex turbulence is shown to be indispensable for representation of the flow rotation effects on turbulence. The principal result reported is the model–experiment comparison of the vortex growth rates for different vortex Reynolds numbers. The mean vortical flow generated by the wing tip very effectively suppresses the Reynolds shear stress, which mediates the extraction of energy from the mean flow by turbulence. In consequence, the vortex‐core growth rate is controlled only by molecular viscosity and the vortex turbulence decays since the turbulence production rate is very nearly zero. This rather unexpected result is shown to be supported by experiments. Finally, it is shown that the computed turbulence structure is consistent with experimental data at the NASA Ames Research Center.

Tip vortex generation technology for creating a lift enhancing and drag reducing upwash effect

Abstract: Methods for using tip generated vortices to improve performance of foils. These methods include generating a substantially streamwise beneficial vortex (74) near the outboard end (60) of a foil (82). This beneficial vortex (74) spins in the opposite direction of an induced drag vortex (62), and is used to create an upwash field (76) which neutralizes induced drag by deflecting the flow behind the trailing edge (56) at an upward angle. Upwash field (76) causes the lift vector (118) on the foil (82) to tilt forward, thereby creating a forward directed force of induced thrust upon the foil (82). Beneficial vortex (74) is also used to contain and compress the high pressure field existing along the attacking surface of the foil (82), and displace the induced drag vortex (62) inboard from the tip of the foil (82). Numerous performance parameters are improved dramatically by using beneficial vortex (74), as well as by using a double vortex pattern (124). Methods for creating, using, and controlling these vortex patterns are offered along with a variety of embodiments for employing such methods. Described embodiments include a twist (66) along the span of a foil (82b), an anhedral tip droop having a divergent axis of droop curvature (86), a movable vortex flap (102), a cone shaped vortex generator (134), a curved outboard droop (146), and a vortex flap network (162). Methods are also disclosed for reducing overall tip vorticity.

Dynamics and stability of a vortex ring impacting a solid boundary

Abstract: Direct numerical simulations were used to study the dynamics of a vortex ring impacting a wall at normal incidence. The boundary layer formed as the ring approaches the wall undergoes separation and roll-up to form a secondary vortex ring. The secondary ring can develop azimuthal instabilities which grow rapidly owing to vortex stretching and tilting in the presence of the mean strain field generated by the primary vortex ring. The stability of the secondary ring was investigated through complementary numerical experiments and stability analysis. Both perturbed and unperturbed evolutions of the secondary ring were simulated at a Reynolds number of about 645, based on the initial primary-ring propagation velocity and ring diameter. The linear evolution of the secondary vortex-ring instability was modelled analytically by making use of a quasi-steady approximation. This allowed a localized stability analysis following Widnall & Sullivan's (1973) earlier treatment of an isolated vortex ring. Amplitude evolution and growth-rate predictions from this analysis are in good agreement with the simulation results. The analysis shows that the secondary vortex ring is unstable to long-wavelength perturbations, even though an isolated ring having similar characteristics would be stable.

Dynamics of vortex rings in crossflow

Abstract: The spread and mixing of a fluid jet into an ambient stream occurs at a rate which deserves further study to improve efficient mixing. Mixing enhancement techniques, such as introduction of periodic disturbances into the jet flow, are used to increase mixing between a jet and the surrounding fluid. Pulsations were generated by the periodic closing and opening of a jet flow. The dynamics and trajectories of vortex rings, formed by the pulsation of the jet in a uniform crossflow, are studied. In particular, the effects of pulsation on the development of vortex rings and their penetration in a crossflow were investigated. Detailed measurements were made using flow visualization techniques including laser‐induced fluorescence and hot‐film anemometry. Vortex rings generated in a crossflow at one specific frequency (1 Hz) were measured using a hot‐film probe. Measurements indicated that vortex rings were fully‐formed at a distance of three times the jet exit diameter. To simulate the dynamics of vortex rings in...

Instability of strained vortex layers and vortex tube formation in homogeneous turbulence

Abstract: A modulational perturbation analysis is presented which shows that when a strained vortex layer becomes unstable, vorticity concentrates into steady tubular structures with finite amplitude, in quantitative agreement with the numerical simulations of Lin & Corcos (1984). Elaborated three-dimensional visualizations suggest that this process, due to a combination of compression and self-induced rotation of the layer, is at the origin of intense and long-lived vortex tubes observed in direct numerical simulations of homogeneous turbulence.

Airfoil with dual source cooling

Abstract: An airfoil (42) for a gas turbine engine (10) having an airfoil portion (52) with a leading and a trailing edge (60, 62) includes dual pressure source cooling. The trailing edge (62) of the airfoil (42) includes an internal trailing edge passage (78) and the leading edge (60) includes an internal leading edge passage (64). Compressor bleed air at higher pressure is channeled through the leading edge passage (64) whereas compressor bleed air at lower pressure is channeled through the trailing edge passage (78). The higher internal pressure in the leading edge (60) ensures that the inward flow of products of combustion does not occur.

Evidence of the ‘natural vortex length’ and its effect on the separation efficiency of gas cyclones

Abstract: It is widely assumed that the vortex in a cyclone has a well defined length, which may be shorter than the physical length of the cyclone. One speaks of the ‘end’ to the vortex, and of the ‘natural vortex length’ or the ‘natural turning length’. The space below the vortex is normally assumed to be ineffective for separation, and the transport of solids along the wall is also inferior there. The axial position of the end of the vortex is thus an important design variable, yet little research work has been dedicated to its determination. In the course of research aimed at formulating a new relation for the natural vortex length, two experimental methods for determining this parameter have been found: (1) the position of the end of the vortex was visible for a period of time after smoke had been introduced in a glass cyclone; and (2) the position was found to be visible in the pattern of wall deposits after experimental runs with gas/solid cyclones. The paper presents results obtained in variations on a standard cyclone geometry with a tube section connecting the dust collection vessel to the cyclone. In these geometries the end of the vortex was in most cases located in the tube section. Comparisons are made between the two above methods of determining the length of the vortex, as well as comparisons with published correlations. The effects on the natural vortex length of the inlet gas velocity, the dimensions of the vortex finder and the length of the cyclone, and the solid loading of the cyclone charge gas are shown. A substantial effect of the length of the vortex on the separation efficiency of the cyclone is also demonstrated.

On fluctuations of vortex breakdown location

Abstract: Flow visualization and velocity measurements are performed in order to investigate the unsteady nature of vortex breakdown location over a delta wing. The results indicate that the fluctuations of vortex breakdown location occur at much lower frequencies than the frequency of the hydrodynamic instability of the flow in the wake of the vortex breakdown. It is suggested that the helical mode instability does not influence the unsteady nature of breakdown location.

Investigation of flow at leading and trailing edges of pitching-up airfoil

Abstract: The dynamic stall process of an NACA 0012 airfoil undergoing a constant-rate pitching-up motion is studied experimentally in a water towing tank facility. This study focuses on the detailed measurement of the unsteady separated flow in the vicinity of the leading and trailing edges of the airfoil. The measurements are carried out using the particle image velocimetry technique. This technique provides the two-dimensional velocity and associated vorticity fields, at various instants in time, in the midspan of the airfoil. Near the leading edge, large vortical structures emerge as a consequence of van Dommelen and Shen type separation and a local vorticity accumulation. The interaction of these vortices with the reversing boundary-layer vorticity initiates a secondary flow separation and the formation of a secondary vortex. The mutual induction of this counter-rotating vortex pair eventually leads to the ejection process of the dynamic stall vortex from the leading-edge region. It is found that the trailing-edge flowfield only plays a secondary role on the dynamic stall process.

Dynamics of the juncture vortex

Abstract: Detailed characteristics of the horseshoe vortex system formed at the juncture of a wing and flat plate have been studied using flow visualization and image analysis techniques. With increasing Reynolds number the flowfield exhibited three distinct modes consisting of 1) steady vortex system, 2) oscillatory vortex motion, and 3) vortex shedding. Vortex splitting and subsequent reconnection phenomena were also observed. Different trends in the variation of characteristic frequency were observed when the flow switched from second mode to the third mode.

Vortex Dynamics in the Wake of a Cylinder

Abstract: Two-dimensional and three-dimensional vortical instabilities in wakes have been a subject of interest to engineers and scientists for a great many years. Although the problem of an unseparated wake behind a splitter plate or symmetric airfoil is amenable to analytical studies, for example stability analysis, the wake of a nominally two-dimensional (2-D) or three-dimensional (3-D) bluff body poses rather a greater analytical challenge. Approaches to this problem have been principally experimental, although there has recently been a surge of activity in numerical simulation and analysis. Bluff body wakes are complex; they involve the interactions of three shear layers in the same problem, namely a boundary layer, a separating free shear layer, and a wake. As has been recently remarked by Roshko (1992), “the problem of bluff body flow remains almost entirely in the empirical, descriptive realm of knowledge.” Although a ‘solution’ to this problem is elusive, our knowledge of this flow is extensive. In the case of the circular cylinder wake alone, there have been literally hundreds (possibly thousands) of papers. This wealth of papers is in part due to the engineering significance of cylinder flow, and in part due to the tempting simplicity of setting up such an arrangement in an experimental or computational laboratory. Rather than attempt here to describe the vortex dynamics in both unseparated and separated wakes for all the different types of 3-D and nominally 2-D body shapes, an overview will be presented of the vortex dynamics phenomena in the wake of a circular cylinder, over a wide range of Reynolds numbers.

Creation and dynamics of vortex tubes in three-dimensional turbulence.

Abstract: We examine the possibility of a blowup of the vorticity due to self-stretching and mechanisms for its prevention. We first estimate directly from the Navier-Stokes equations the length scale of coherence in the direction of the vortex lines to be of the order of the Kolmogorov length. Alignment of vortex lines is seen to be a viscous phenomenon and may prevent some scenarios for blowup. Next we derive equations for the curvature and torsion of vortex lines. We show that the same stretching that amplifies the vorticity also tends to straighten out the vortex lines. Then we show that in well-aligned vortex tubes, the self-stretching rate of the vorticity is proportional to the ratio of the vorticity and the radius of curvature. Thus blowup of the vorticity in such tubes can be prevented by the growth of the vorticity being balanced by the straightening of the vortex lines. Implications for vorticity-strain alignment and the scaling theory of turbulence are noted. Finally, we examine the effects of viscous diffusion on the vorticity field and see how viscosity can lead to organization and alignment of vortex lines.

Instantaneous flow field in an unstable vortex ring measured by holographic particle velocimetry

Abstract: Instantaneous velocity field in the three‐dimensional flow of an unstable vortex ring (Re=1360) in water has been measured by employing a newly developed holographic particle velocimeter, IROV. The vorticity distribution and circulation as a function of radius from the vortex core center are also presented.

Active control of vortex breakdown over a delta wing

Abstract: The first step of this study is to identify a physical quantity that indicates the existence of vortex breakdown and can be used as feedback signal for active control. The second element in active control of vortex breakdown is to identify a flow controller to influence the vortex breakdown location.

3-D Wake Measurements Near a Hovering Rotor for Determining Profile and Induced Drag.

Abstract: Primarily an experimental effort, this study focuses on the velocity and vorticity fields in the near wake of a hovering rotor. Drag terminology is reviewed, and the theory for separately determining the profile-and-induced-drag components from wake quantities is introduced. Instantaneous visualizations of the flow field are used to center the laser velocimeter (LV) measurements on the vortex core and to assess the extent of the positional mandering of the trailing vortex. Velocity profiles obtained at different rotor speeds and distances behind the rotor blade clearly indicate the position, size, and rate of movement of the wake sheet and the core of the trailing vortex. The results also show the distribution of vorticity along the wake sheet and within the trailing vortex.

Interaction of spanwise vortices with a boundary layer

Abstract: The interaction of a spanwise vortex with a boundary layer has been numerically simulated using a fractional‐step method. The incompressible Navier‐Stokes equations are solved to accurately predict the strong viscous–inviscid interaction between a vortex either near or embedded within a boundary layer of comparable size. A strong vortex induces an eruption and the production of a secondary vortex. The secondary vortex causes the primary vortex to rebound, a response observed in many previous experiments and numerical simulations. However, weaker vortices as well do not follow the inviscid trajectory despite the absence of a secondary vortex. Rather than creating vorticity at the wall, a weaker vortex mainly redistributes the vorticity of the boundary layer, pulling it away from the wall. The redistributed vorticity alters the path of the vortex. In the laminar cases studied the decay of the vortex is not significantly altered by the boundary layer.

Vortex rings generated by drops just coalescing with a pool

Abstract: A detailed experimental study has been made of the formation and motion of vortex rings generated when drops of liquid are allowed to come into contact at zero velocity with a quiescent flat surface of the same liquid. In this case the vortex motion is driven principally by the drop surface energy. It is shown that in the case considered, the phenomenon depends on two dimensionless parameters alone, the reciprocal of a Bond number B−1 and a global Reynolds number Re where the velocity scale is based on the surface energy. Using a video camera, measurements were made, over the whole trajectory, of the ring velocity and diameter for a number of drop sizes and liquids. Vortex rings generated by small drops reach higher peak velocities but decelerate more rapidly than ones generated by large drops. The latter, however, may become turbulent before relaminarizing during deceleration. Although the most interesting finding of the investigation was the discovery of oscillations in the ring translational velocity and diameter during deceleration, the identification of the appropriate scaling laws and a suggestion regarding the early stages of vorticity generation should also be of interest.

Perpendicular blade vortex interaction

Abstract: The interaction between a streamwise vortex and a spanwise blade was studied in incompressible flow for blade-vortex separations between ±1/8 chord. Three-component velocity and turbulence measurements were made from 4 chord lengths upstream to 15 chord lengths downstream of the blade using miniature four-sensor hot-wire probes. The interaction of the vortex with the blade causes an almost immediate loss in vortex core circulation. Downstream of the blade the core becomes embedded in the blade wake and begins to grow rapidly. Core radius increases and peak tangential velocity decreases but circulation remains roughly constant. True turbulence levels within the core are much larger downstream than upstream of the blade. Outside of the core the interaction produces a substantial region of turbulent flow associated with the blade and vortex generator wakes. Overall, perpendicular blade vortex interaction substantially alters the flow and produces a much larger and more intense region of turbulent flow than that presented by the undisturbed vortex. These results have significant implications for the prediction of both impulsive and broadband helicopter noise.

Flame propagation along a fine vortex tube

Abstract: Interaction between a premixed flame and a fine vortex tube perpendicular to the flame is simulated numerically in order to study the effects of the rotating velocity and tube diameter on flame propagation along the vortex tube. The vortex tube has an initial circumferential velocity ranging from 1.8 to 36 times the laminar burning velocity and an initial core diameter ranging from 0.18 to 1.71 times the flame thickness. It is shown that a premixed flame can be accelerated along the vortex tube axis, and that the propagation velocity is proportional to the maximum circumferential velocity of the vortex tube. It is also shown that the influence of the vortex on the flame propagation can be neglected when the Reynolds number based on the vortex tube diameter and maximum circumferential velocity is below 10. The proportionality factor between the propagation velocity and circumferential velocity increases above the critical number with azimuthal speed and reaches about one at the Reynolds number of ...

Measurements of the Tip Clearance Flow for a High-Reynolds-Number Axial-Flow Rotor

Abstract: A high-Reynolds-number pump (HIREP) facility has been used to acquire flow measurements in the rotor blade tip clearance region, with blade chord Reynolds numbers of 3,900,000 and 5,500,000. The initial experiment involved rotor blades with varying tip clearances, while a second experiment involved a more detailed investigation of a rotor blade row with a single tip clearance. The flow visualization on the blade surface and within the flow field indicate the existence of a trailing-edge separation vortex, a vortex that migrates radially upward along the trailing edge and then turns in the circumferential direction near the casing, moving in the opposite direction of blade rotation. Flow visualization also helps in establishing the trajectory of the tip leakage vortex core and shows the unsteadiness of the vortex. Detailed measurements show the effects of tip clearance size and downstream distance on the structure of the rotor tip leakage vortex. The character of the velocity profile along the vortex core changes from a jetlike profile to a wakelike profile as the tip clearance becomes smaller. Also, for small clearances, the presence and proximity of the casing endwall affects the roll-up, shape, dissipation, and unsteadiness of the tip leakage vortex. Measurements also show how much circulation is retained by the blade tip and how much is shed into the vortex, a vortex associated with high losses.

Decay of dipolar vortex structures in a stratified fluid

Abstract: In this paper the viscous decay of dipolar vortex structures in a linearly stratified fluid is investigated experimentally, and a comparison of the experimental results with simple theoretical models is made. The dipoles are generated by a pulsed horizontal injection of fluid. In a related experimental study by Flor and van Heijst [J. Fluid Mech. 279, 101 (1994)], it was shown that, after the emergence of the pancake‐shaped vortex structure, the flow is quasi‐two‐dimensional and decays due to the vertical diffusion of vorticity and entrainment of ambient irrotational fluid. This results in an expansion of the vortex structure. Two decay models with the horizontal flow based on the viscously decaying Lamb–Chaplygin dipole, are presented. In a first model, the thickness and radius of the dipole are assumed constant, and in a second model also the increasing thickness of the vortex structure is taken into account. The models are compared with experimental data obtained from flow visualizations and from digital analysis of particle‐streak photographs. Although both models neglect entrainment and the decay is modeled by diffusion only, a reasonable agreement with the experiments is obtained.

On the alignment and axisymmetrization of a vertically tilted geostrophic vortex

Abstract: We investigate the evolution of a vertically tilted geostrophic vortex of cylindrical shape and circular horizontal cross-section using the recently developed method of boundary surface dynamics. The vortex consists of a finite volume of constant potential vorticity immersed in a spatially unbounded fluid of uniform density stratification. The fully nonlinear three-dimensional problem is then reduced to the calculation of the Lagrangian evolution of the boundary surface of the vortex region, thus decreasing the dimensionality by one. In the numerical simulations presented here, the vortex shows a general tendency to attain vertical alignment and a horizontal axisymmetrical shape by wobbling about its centre and going through three basic stages of evolution: (a) the circular horizontal cross-sections of the upper and lower parts of the vortex distort and become elongated; (b) the upper and lower sections then become vertically aligned by reducing their horizontal intercentroid distances; and (c) the distorted horizontal cross-sections relax towards axisymmetry, often through the process of filamentation. For a given vortex height, if the horizontal scale of the flow is close to the internal radius of deformation, or equivalently, the density stratification is not too strong, the processes of filamentation and vertical alignment are enhanced. However, for stronger stratifications, both filamentation and vertical alignment are found to be greatly inhibited. For relatively small initial inclination angles, filamentation only occurs in the upper and lower sections of the vortex. Increasing the angle of tilt also increases the tendency of the surface to steepen and filament in the middle sections of the vortex. For a fixed value of the ratio of horizontal scale of the flow to the deformation radius, taller vortices have an increased tendency to align and axisymmetrize than shorter vortices of equal inclination angle.