Showing papers on "Starting vortex published in 1986"

A Theory for the Maximum Windspeeds in Tornado-like Vortices

Abstract: We have developed a physical theory for the finding that the most intense laboratory vortex occurs when it is in the form of an end-wall vortex We argue that the end-wall vortex allows no standing centrifugal waves (ie, it is supercritical), and therefore, disturbances cannot propagate down from aloft This allows the low central pressure of the end-wall vortex at the level of maximum azimuthal velocity to be balanced by a central axial jet which jet which accelerates from the lower end wall to this level This supercritical, end-wall vortex undergoes a transition to a subcritical vortex aloft through a vortex breakdown We construct a model for the maximum intensity of these vortices by developing a model for the end-wall vortex and by finding the criterion for a vortex breakdown to be in steady suspension above the lower end wall The model agrees well with previous experimental simulations of tornado-like vortices in the Purdue tornado vortex chamber a steady end-wall vortex adjacent to the

Dynamic Stall: The Case of the Vertical Axis Wind Turbine

Abstract: This paper presents the results of an experimental investigation on a driven Darrieus turbine rotating at different tip speed ratios. For a Reynolds number of 3.8 x 10/sup 4/, the results indicate the presence of dynamic stall at tip speed ratio less than 4, and that helicopter blade aerodynamics can be used in order to explain some aspects of the phenomenon. It was observed that in deep stall conditions, a vortex is formed at the leading edge; this vortex moves over the airfoil surface with 1/3 of the airfoil speed and then is shed at the trailing edge. After its shedding, the vortex can interact with the airfoil surface as the blade passes downstream.

Evolution of a curved vortex filament into a vortex ring

Abstract: The deformation of a hairpin-shaped vortex filament under self-induction and in the presence of shear is studied numerically using the Biot-Savart law. It is shown that the tip region of an elongated hairpin vortex evolves into a vortex ring and that the presence of mean shear impedes the process. Evolution of a finite-thickness vortex sheet under self-induction is also investigated using the Navier-Stokes equations. The layer evolves into a hairpin vortex which in turn produces a vortex ring of high Reynolds stress content. These results indicate a mechanism for the generation of ring vortices in turbulent shear flows, and a link between the experimental and numerical observation of hairpin vortices and the observation of ring vortices in the outer regions of turbulent boundary layers.

The Response of Airfoils to Periodic Disturbances —The Unsteady Kutta Condition

Abstract: Two cases of unsteady flowfields over a NACA 0012 airfoil at an angle of attack are examined. The first is the classical pitching motion about the airfoil's quarter chord. The second is the flow over a fixed airfoil immersed in the wake of the pitching airfoil. Large reduced frequencies are examined. Measurements were obtained in a water tunnel by laser Doppler velocimetry. Ensemble-averaged velocity measurements were obtained in the vicinity of the trailing edges of both the pitching and the fixed airfoil. The results indicate that the classical unsteady Kutta condition is clearly not valid. An extension of this condition earlier proposed by Giesing and Maskell is examined and some evidence is provided for its support.

Horseshoe Vortex Formation Around a Cylinder

Abstract: An experimental investigation was conducted to characterize a symmetrical horseshoe vortex system in front of and a around a single large-diameter right cylinder centered between the sidewalls of a wind tunnel. Surface flow visualization anad surface static pressure measurements as well as extensive mean velocity and pressure measurements in and around the vortex system were acquired. The results lend new insight into the formation and development of the vortex system. Contrary to what has been assumed previously, a strong vortex was not identified in the streamwise plane of symmetry, but started a significant angular distance away from it. Rather than the multiple vortex systems reported by others, only a single primary vortex and saddle point were found. The scale of the separation process at the saddle point was much smaller than the scale of the approaching bundary layer thickness. Results of the present study not only shed light on such phenomena as the nonsymmetrical endwall flow in axial turbomachinery but can also be used as a test case for three-dimensional computational fluid mechanics computer codes.

Solitary vortices in a rotating plasma

Abstract: Nonlinear equations describing the flute dynamics of rotating plasma are derived and solitary vortex solutions are obtained. The solution takes the form of a shielded dipole vortex, similar to that found for nonlinear Rossby waves. The nonlinear dispersion relation, relating propagation speed to vortex radius, is obtained. Vortex speeds are shown to take values complementary to the phase velocities of the linear modes of the system. The E×B circulation velocity of the plasma trapped in the vortex is comparable to the diamagnetic drift velocity in the equilibrium plasma.

Inviscid evolution of stretched vortex arrays

Abstract: The nonlinear evolution of an array of pairs of inviscid counter-rotating vortices, subjected to an applied stretching strain field, has been studied numerically using the contour-dynamics method. The array configuration is effectively the Corcos-Lin model of streamwise vortices in the braid region of a nominally two-dimensional mixing layer. For each individual vortex the simulations elucidate the strong interaction between the vortex self-induction, the vorticity amplification of the stretching strain, and the local in-plane strain applied by all other members of the array. When the initial vorticity distribution is modelled by a non-uniform piece-wise-constant vorticity field defined over a nested set of non-intersecting contours, the dynamical evolution reveals fine structure consisting of strong vortex roll-up accompanied by trailing, filament-like spiral vortex sheets, and the presence of tertiary instabilities. It is shown by a particular example that these features are largely absent in an equivalent computation in which array members are modelled by the commonly used uniform-vortex approximation.

Cantilevered vortex control device

Abstract: A vortex control device is designed for installation on the exterior surface of an aircraft nacelle in a position which does not compromise the optimum aerodynamic performance of the vortex control device Existing cowling structure is bridged by this vortex control device which attaches only to movable structure and which is cantilevered over adjacent stationary structure A resilient seal prevents air from swirling under the cantilevered portion, thus contributing to vortex generating efficiency and reduced aerodynamic drag

Vortex cloud model for body vortex shedding and tracking

Abstract: The discrete vortex cloud approach models a missile airframe's vortex wake by means that are capable of treating a variety of configurations over a range of flow conditions. Attention is given to the sheets of vorticity formed on the lee side of a missile at moderate angles-of-attack. While three-dimensional attached flow models are used to represent the missile body, two-dimensional, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body under the influence of the freestream and the separation vortex wake are used to calculate aerodynamic loads on the body. The separation vortex wake is represented by clouds of discrete vortices in cross flow planes normal to the body axis.

Some Effects of Momentum Diffusion on Axisymmetric Vortices

Abstract: In the absence of momentum diffusion (viscous or turbulent) in a steady state axially bounded vortex, such as that produced by a Ward-type tornado simulator, a two-celled vortex configuration in which the inner cell is entirely stagnant is to be expected. Therefore, diffusion is directly responsible or at least highly influential in producing other vortex phenomena, including a single-cell structure, a vortex breakdown, a central downdraft, and a subhydrostatic central surface pressure. The means by which these are brought about by diffusion are discussed in the context of the tornado simulator. Some of the given arguments are extended in a more speculative manner to atmospheric vortices.

Vortex shedding flowmeter with lever action signal amplification

Abstract: A vortex shedding flowmeter comprising vortex generator of an elongated cylindrical shape having a cross section of a blunt geometry disposed across a first cross section of a flow passage and a vortex sensor including a wing disposed across a second cross section of the flow passage in a parallel relationship with respect to the vortex generator, wherein the wing vortex sensor is secured to the wall of the flow passage at one extremity, while the other extremity is connected to one extremity of a lever that transmits the flexing movement of the wing vortex sensor to a transducer disposed at the other extremity of the lever. The vortex shedding frequency detected by the combination of the wing vortex sensor-lever amplifier-transducer provides the information on the fluid velocity through the flow passage as the vortex shedding frequency is proportional to the fluid velocity.

Airfoil trailing edge cooling configuration

Abstract: An airfoil includes a plurality of interior cooling air paths arranged so as to provide crossover metering and pressure side bleed of cooling air at the trailing edge region of the airfoil.

Motion of two vortex rings

Abstract: It is well-known [1] that two coaxial rings which are moving in the same direction pass through each other alternately. In the case of thin vortex rings this phenomenon was first considered qualitatively in [2]. The assumption that the vortex rings are thin means that when their interaction is considered they can be assumed to be annular vortex filaments. In the present paper, on the basis of the approach suggested in [2], certain new properties are determined for a system of two coaxial vortex rings of the same intensity.

Control of Wake Structure Behind and Oscillating Airfoil

Abstract: : The initiation, development, and shedding of vortices induced by sinusoidal airfoil motions were examined in the airfoil wake across a range of independent forcing parameters. Then, the interactions of these wakes with a following or trailing airfoil were studied. Coherent, phase-dependent, tandem vortex wake structures emerged from the shedding vorticity of the UPSTREAM OSCILLATING AIRFOIL. Consistent interactions of the vortex wake with a trailing airfoil were revealed with both the multiple-exposure, phase-locked, flow visualization measures and the time-dependent surface pressure measurements. Impingement of the tandem vortex wake upon the trailing airfoil produced unsteady pressure distributions resulting in very transient loading. Similar effects in unsteady loading were observed when an auto-rotating flat plate was used to create a vortex-dominated wake. These finding suggest that a consistent, structured wake produced by unsteady separation might be used to produce desired lift characteristics on a trailing, lifting surface. Keywords: Flow visualization, Dynamic loads, Vortices.

Tandem wing universal vortex shedding flow meter

Abstract: A flowmeter comprising a vortex generator generating and shedding vortices intermittently wherein the vortices are generated naturally by the fluid flowing around the vertex generator or generated artificially by the change of flow pattern resulting from the artificial manipulation of the vortex generator, and a pair of vortex sensing wings arranged in tandem, wherein the flow velocity is determined by measuring the frequency of the vortices naturally generated and shed by the vortex generator or by measuring the time of travel of the artificial vortices required to move from the vortex generator or the first wing to the second wing of the two tandem wings.

Axisymmetric problem of vortex sound with solid surfaces

Abstract: A general formulation of the axisymmetric vortex sound is given, and three typical problems are considered in detail, in which the acoustic wave is generated by a vortex ring interacting with either a sphere, a circular disk, or a circular aperture of a plane wall in an axisymmetric manner. The velocity potentials induced by the vortex in the presence of the body are determined by the method of dual integral equations or the image vortex. From the vortex trajectories, the acoustic wave fields are determined. The method of matched asymptotic expansions yields the result that the force exerted on the body is related to the profile of the dipole component. It is also found that the wave amplitude is expressed in terms of the value of a streamfunction at the vortex position, which represents a hypothetical potential flow around the body and is defined appropriately in each problem.

Investigation of vortex patterns on slender bodies at high angles of attack

Abstract: The method of the fluorescent minituft has been used to investigate experimentally the changes of the spatial vortices with angle of attack and without sideslip on the lee side of some slender bodies in a low-speed tunnel. Top and side views of the vortex have been acquired. The axial velocities and the circulations of the vortex along its track were measured for symmetric and asymmetric vortex patterns on a sharp ogive body of revolution. Hotwire anemometer and fluorescent minituft were used to measure the velocities and circulation, respectively. It is shown that the vortex pattern on these slender bodies is similar. Also the model of the asymmetric vortex and the interference between the vortices are realised to some extent. The vortex track near the rear of the body is almost unaffected by the presence of a small protrusion at the fore part of the sharp body of revolution for a symmetric vortex pattern at larger angle of attack. As for an asymmetric vortex pattern, it can strongly affect the vortex track.

Internal structure of a vortex breakdown

Abstract: An axisymmetric vortex breakdown was well simulated by the vortex filament method The agreement with the experiment was qualitatively good In particular, the structure in the interior of the vortex breakdown was ensured to a great degree by the present simulation The second breakdown, or spiral type, which occurs downstream of the first axisymmetric breakdown, was simulated more similarly to the experiment than before It shows a kink of the vortex filaments and strong three-dimensionality Furthermore, a relatively low velocity region was observed near the second breakdown It was also found that it takes some time for this physical phenomenon to attain its final stage The comparison with the experiment is getting better as time goes on In this paper, emphasis is placed on the comparison of the simulated results with the experiment The present results help to make clear the mechanism of a vortex breakdown

Vortex flow hysteresis

Abstract: An experimental study was conducted to quantify the hysteresis associated with various vortex flow transition points and to determine the effect of planform geometry The transition points observed consisted of the appearance (or disappearance) of trailing edge vortex burst and the transition to (or from) flat plate or totally separated flows Flow visualization with smoke injected into the vortices was used to identify the transitions on a series of semi-span models tested in a low speed tunnel The planforms tested included simple deltas (55 deg to 80 deg sweep), cranked wings with varying tip panel sweep and dihedral, and a straked wing High speed movies at 1000 frames per second were made of the vortex flow visualization in order to better understand the dynamics of vortex flow, burst and transition

The Behaviour and Performance of Leading Edge Vortex Flaps

Abstract: Subsonic wind tunnel tests were conducted on a series of leading edge vortex flaps to determine the improvements in lift/drag ratio that were possible. Various flap sizes and deflection angles were investigated. Trailing edge flap effects on leading edge vortex flap behaviour were also studied. Lift/drag ratio improvements of more than 20% were found at moderate angles of attack. The results clearly indicate that the vortex flap can improve the efficiency of slender wings when operating in an aerodynamic environment governed by leadii g edge separation and vortex flow. Starting in 1946 as the College of Aeronautics, the Cranfield Institute of Technology was granted university status in 1969. In 1993 it changed its name to Cranfield University.

Towards an advanced vortex flap system: The cavity flap

Abstract: An extension of the vortex flap concept was explored with the aim of providing high-alpha flight control capability coupled with maneuver drag reduction for highly swept wing configurations. A retractable lower surface flap mounted on a translating hinge is proposed, allowing chordwise extension as well as deflection, the two movements being independently controlled. The frontal cavity formed by the partially extended and deflected flap captures a vortex above a certain angle of attack. The cavity vortex downwash alleviates the effective incidence of the wing leading edge, thus modulating vortex lift; at the same time, the induced suction in the cavity generates thrust. These postulated aerodynamic features of the cavity flap were validated through low speed tunnel pressure and visualization tests on a 65 deg swept oblique wing model, which also provided initial trends of the leading edge vortex alleviation and cavity suction with respect to flap extension, deflection and angle of attack. Force tests on a 60 deg delta model further showed the cavity flap L/D performance to compare favorably with the conventional vortex flap. A two segment flap arrangement with independently control led segments was envisaged for exploiting the vortex modulation capability of the cavity flap for pitch, roll and yaw control, in addition to drag reduction at high angles of attack.

Mach Number Effects on Vortex Shedding of a Square Cylinder and Thick Symmetrical Airfoil Arranged in Tandem

Abstract: An experimental study was done to elucidate the Mach number effects on vortex shedding of a square cylinder (side length D = 20 mm) and thick symmetrical airfoil (NACA 0018, chord length 20 mm) arranged in tandem , at free stream Mach numbers between 0.1526 and 0.9081, and at free stream Reynolds numbers (based on the side length D ) between 0.702 x 10 5 and 4.188 x 10 5 . The spacing ratio of the central distance, L , between the square cylinder and the airfoil to the side length, D , of the square cylinder was varied from 1.125 to 5.5. It was found that the regular vortex shedding is not suppressed by steady shock waves in the local supersonic flow regions; the periodic vortex shedding is still present, irrespective of the appearance of the shock waves. When the spacing ratio is fixed, the Strouhal number behind the square cylinder is almost constant up to the critical Mach number of about 0.70, but it increases rapidly with further increase of the Mach number. However, once the shock waves are formed on both sides of the vortex formation region, various frequency components, other than the vortex shedding frequency appear; the spectral peaks lower than those of the vortex shedding frequency were identified as frequencies of an acoustic-feedback oscillation and the resonance of the wind tunnel structural system. With increasing the Mach number, the formation region becomes small and asymmetric, and the separating shear layers become wavy. These changes result in an increase of the scale and strength of the vortices and thus enhance the vortex shedding process. However, when the Mach number exceeds the critical value, the streamwise length of the formation region increases suddenly and becomes long enough to enclose the downstream airfoil. Under this circumstance, the formation region is almost symmetrical with respect to the wake axis, and shock waves are formed on the upper and lower separating shear layers. The shock waves are almost normal to the wake axis at M = 0.7512 and 0.8215, but incline to the downstream direction at M = 0.9081. Acoustic waves travelling upstream have been observed most clearly when the vortex shed from the square cylinder hits the leading edge of the airfoil at a Mach number of about 0.63, which is close to, but slightly smaller than the critical value. The mean pressure and the amplitude of the pressure fluctuations in the test section decreases and increases, respectively, with increasing the Mach number. However, the amplitude of the pressure fluctuations decreases suddenly when the steady shock waves are formed on the upper and lower separating shear layers.

Comment on 'Convergence characteristics of a vortex-lattice method for nonlinear configuration aerodynamics'

Abstract: It is proposed that the study of Rusak et al. (1985), which reports numerical modeling sensitivities on longitudinal force/moment properties for a vortex-lattice method incorporating free vortex filaments to represent the leading-edge vortex separation, employs a formula that is strongly affected by the particular points of analysis chosen. This results in a narrowly applicable curve fit, where numerical sensitivities of the theory are inappropriately traded off against physical effects that are not modeled in that theory. Attention is also given to questionable drag estimate computations.

Vortex Interaction on a Canard-Wing Configuration.

Abstract: : This experiment studies the interaction of vortices shed by the canard and wing's leading edges, and their effect on the aircraft aerodynamic characteristics. A close-coupled canard-wing configuration was selected and tested in different wind tunnels and at different conditions. Tunnel and model size effects, Mach number, angle of attack, and spanwise blowing effects on the vortex interaction were analyzed. Intrusive (hot wires) and non-intrusive (laser doppler velocimeters) data acquisition techniques were used and compared to enhance the reliability of the results. Flow visualization by tufts, oil, and laser light sheets were employed. Mean velocities, vortex turbulence intensities, and Reynolds stresses obtained for different conditions were compared and found to be generally consistent. Mach number, wind tunnel, and model size effects were in general small. Turbulence intensities and stresses increased with angles of attack. Spanwise blowing produced a small favorable effect. The use of a coplanar canard produced a small favorable interaction between the leading edge vortices, while the off-set canard produced a considerable increase in the lift/drag ratio. Keywords: Vortex flow; Subsonic flow; Close-coupled canards; Vortex shedding.

Arch-type Vortex Formed Behind a Normal Plate Placed in Laminar Boundary Layer

Abstract: Characteristics of an arch-type vortex formed behind a plate of aspect ratio h/w = 0.8 (h: height, w: width) which is placed vertically in a laminar boundary layer are investigated. Flow visualization and analysis of velocity signals yield the following new understandings. (i) The variation of Strouhal number St (=fcw/Uo) with Reynolds number Re (=wUo/v) can be classified into three regions. (ii) Arch-type vortex formed behind a plate shows inconsistency in its periodicity and strength, and has no regular mode vortex shedding as in two-dimensional circular cylinders. (iii) Different tendency of Strouhal number distribution against Reynolds number is due to the different time rate of intermittent vortex shedding. (iv) Arch-type vortex formed behind a plate starts to dissipate in the very vicinity downstream of the plate before convectim far downstream.