Showing papers on "Starting vortex published in 1988"

The formation of vortex rings

Abstract: Vortex rings are usually formed by a brief discharge of fluid from an orifice. In previous investigations, the geometry of the vortex generator has varied greatly from one experiment to another, with important consequences for the ensuing flow. The present work categorizes the generating conditions for vortex rings and classifies the conditions under which a given vortex generator produces either an initially laminar ring, which may or may not undergo instability and transition to turbulence, or an initially turbulent ring. A particularly simple vortex generator was devised and measurements were carried out to provide systematic data over a range of the important dimensionless parameters. The results of this survey are used to construct a transition map that reveals a reasonably well defined boundary separating vortex rings that are turbulent upon formation from those that are not. High‐speed cinephotography of the formation and evolution of turbulent vortex rings suggests a possible connection between the generating conditions and the transition to turbulence.

Airfoil pressure measurements during a blade vortex interaction and a comparison with theory

Abstract: A two-dimensional, airfoil-vortex interaction experiment was conducted to obtain the aerodynamic behavior of the airfoil during a parallel interaction. The vortex was created by pitching a second airfoil placed upstream and parallel to an instrumented test airfoil. Hot-wire anemometer measurements were taken to determine the vortex velocity distribution. The interaction tests were conducted for a counterclockwise vortex passing above a symmetrical airfoil at zero angle of attack

Cross‐linking of two vortex rings

Abstract: The mechanism for cross‐linking two vortex rings has been investigated experimentally. Phase‐locked velocity measurements using x‐type hot wires were carried out point by point over the entire flow field, and time‐dependent vorticity fields were educed. It is found that a new pair of vortex tubes is created, which connects the interacting vortex rings and grows gradually, while the vorticity intensity of the main rings decreases. This is considered to be a type of bridging phenomenon of the vortex tubes, as proposed by Kida and Takaoka [Phys. Fluids 30, 2911 (1987)] in their numerical simulation.

Visualization and Measurement of the Tip Vortex Core of a Rotor Blade in Hover

Abstract: Detailed measurements with a laser Doppler velocimeter (LDV) have been performed in the tip region and in the tip vortex core of a single-bladed model rotor in hover. The testing was conducted at a rotor tip speed of 32 m/s, a Reynolds number of 269,000, and at two values of the rotor thrust coefficient, 0.0022 and 0.0057. Strobed laser sheet flow visualization was used to verify the steadiness of the tip vortex trajectory in the near wake and quantify the vortex trajectory to guide LDV surveys of the vortex core. A remotely aligned off-axis receiving optics system enabled measurement of vortex core velocity profiles at large focal lengths. The core self-induced velocity components extracted from these data are presented. The data exhibit evidence of secondary structure even inside the rotational core of the vortex, the axial velocity profile along the core has been extracted and presented in the wake of a spinning rotor. It is seen that the tip vortex of a rotating blade differs considerably in structure from a fixed-wing vortex.

Contributions to the Theory of Sound Production by Vortex-Airfoil Interaction, with Application to Vortices with Finite Axial Velocity Defect

Abstract: An analysis is made of the sound produced when a field of vorticity is cut by an airfoil in low-Mach-number flow. A general formula is given for the acoustic pressure when the airfoil is rigid and the chord is acoustically compact. This expresses the radiation in terms of an integral over the region occupied by the vorticity; the integrand contains factors describing the influence of the thickness, twist and camber of the airfoil. Explicit analytical results are derived for a rectilinear vortex, having small core diameter and finite axial velocity defect, which is ‘chopped’ by a non-lifting airfoil of large aspect ratio. The acoustic signature generally comprises two components, which are associated with the axial and azimuthal vorticity, the latter being determined by the velocity defect distribution within the core. Sound is generated predominantly when the core is in the neighbourhoods of the leading and trailing edges. The contribution from the trailing edge is usually small, however, because of destructive interference between sound produced by edge-diffraction of near-field energy of the vortex and that produced by vorticity shed into the wake of the airfoil to satisfy the unsteady Kutta condition that the pressure and velocity should be bounded at the edge. When the shed vorticity is assumed to convect at the same mean stream velocity as the impinging vortex, the interference is predicted to be complete, and no trailing edge sound is generated. If the shed vorticity is taken to convect at a reduced, ‘near-wake’ velocity, which might be appropriate for small-scale structures comparable in size to the diameter of the vortex core, a small but non-negligible pressure pulse is radiated from the trailing edge. A tentative comparison with experiment appears to confirm the presence of this trailing-edge pulse.

Control of leading edge vortex breakdown by blowing

Abstract: An investigation into the effects of using a jet of air to control the vortex breakdown position on a 70 degree delta wing is presented. The specific objectives focused on optimizing the blowing positions in terms of maximum lift increments obtained for minimum blowing rates. The tests were conducted at chord Reynolds numbers of 150,000, 200,000, and 250,000 at angles of incidence of 30 and 35 degrees. Visualization and force data is presented to show the effect of the jet on the wing aerodynamic characteristics. The results indicate a jet position located at and aligned parallel to the leading edge to be the optimum. Nearness to the apex and tangency to the upper surface were also crucial factors. The influence of the jet on the leading edge vortex structure was examined using laser Doppler anemometry. Velocity surveys through the vortex showed that at high blowing rates the parallel velocity in the outer swirling region of the vortex increased and the normal velocity decreased. This resulted in a decrease in the swirling angle in the outer region. The peak core velocity was reduced and the vortex breakdown was delayed.

Measurements in the Transition Vortex Flow Regime of Mixed Convection Above a Horizontal Heated Plate

Abstract: Experimental results covering the transition vortex flow regime of mixed convection over a heated, horizontal flat plate are presented. A criterion for the onset of vortex instability as a function of critical Reynolds and Grashof numbers was established with the aid of a flow visualization technique. The three-dimensional nature of tis flow regime was documented through both velocity and temperature measurements using laser-Doppler and hot/cold-wire anemometers, respectively. A higher buoyancy force, through a higher plate temperature or a larger downstream distance and/or a lower free-stream velocity, intensifies the strength of the vortices. Velocity and temperature profiles through vortex peaks and valleys are reported to quantify the behavior of these vortices. It has been found from these measurements that the two-dimensional laminar mixed convection flow changes into a transitional three-dimensional vortex flow in a relatively short distance from the leading edge of the plate. The vortex three-dimensional flow continues to intensify as the buoyancy force increases and then develops into a two-dimensional fully turbulent flow at the end of the transition regime. These findings place an upper limit on the applicabilityof the two-dimensional, laminar boundary layer flow analysis for mixed convection a heated horizontal flat plate.

Head-on collision of viscous vortex rings

Abstract: The head-on collision of two identical axisymmetric viscous vortex rings is studied through direct simulations of the incompressible Navier-Stokes equations The initial vorticity distributions considered are those of Hill's spherical vortex and of rings with circular Gaussian cores, each at Reynolds numbers of about 350 and 1000 The Reynolds number is defined by Gamma/Nu, the ratio of circulation to viscosity As the vortices approach each other by self-induction, the radii increase by mutual induction, and vorticy cancels through viscous cross-diffusion across the collision plane Following contact, the vorticity distribution in the core forms a head-tail structure (for the cases considered) The characteristic time of vorticity annihilation is compared with that of a 3D collision experiment and 3D numerical simulations It is found that the annihilation time is somewhat longer in the axisymmetric case than it is in the symmetry plane of the experiment and 3D numerical simulation By comparing the annihilatiom time with a viscous timescale and a circulation timescale, it is deduced that both the strain rate due to local effects and to 3D vorticity realignment are important

Leading-Edge Vortex Dynamics on a Slender Oscillating Wing

Abstract: Experimental data are presented which show the dynamic behavior of the leading-edge vortices on an 80-deg swept delta wing undergoing a wing rock motion The experiments were conducted in a low-speed wind tunnel, and the model Reynolds number was 315,000, based on the centerline chord Vortex trajectory data were determined from smoke flow visualization experiments The photographic data were analyzed to determine vortex position and breakdown information Significant differences were observed between the static and dynamic vortex core positions and the breakdown locations The data show the convective lag in the flowfield development on a wing oscillating in roll

Nonlinear aerodynamics of two-dimensional airfoils in severe maneuver

Abstract: This paper presents a nonlinear theory of forces and moment acting on a two-dimensional airfoil in unsteady potential flow. Results are obtained for cases of both large and small amplitude motion. The analysis, which is based on an extension of Wagner's integral equation to the nonlinear regime, takes full advantage of the trailing wake's tendency to deform under local velocities. Interactive computational results are presented that show examples of wake-induced lift and moment augmentation on the order of 20 percent of quasi-static values. The expandability and flexibility of the present computational method are noted, as well as the relative speed with which solutions are obtained.

A global branch of steady vortex rings.

Abstract: : A steady vortex ring of prescribed strength and propagation speed can be described in terms of a Strokes stream function psi. A flux constant k measures the flow through the center of the axisymmetric vortex ring. For k = 0, Hill in 1984 found an explicit solution for the semi-linear elliptic equation satisfied by psi. In this paper it is shown that there is an unbounded, closed, connected branch of solutions emanating from Hill's vortex in the space of pairs (k, psi). (Author)

Interaction of impulsively generated vortex pairs with bodies

Abstract: A vortex pair, impulsively generated from a planar nozzle, is shown to have a degree of vorticity concentration in good agreement with inviscid theory, providing well-posed initial conditions for interaction with basic types of bodies (cylinders and plates). The scale of these bodies ranges from the same order as, to over an order of magnitude smaller than, the scale (distance between centres) of the incident vortex pair. The fundamental case of a (primary) vortex pair symmetrically incident upon a very small cylinder shows rapid growth of a secondary vortex pair. These secondary vortices quickly attain a circulation of the same order as that of the corresponding primary vortices within a distance smaller than the lengthscale of the primary vortex pair. At this location the temporal variation of integrated vorticity of primary and secondary vortices attains a maximum simultaneously. This zero phase shift between arrival of vorticity maxima provides the basis for formation of counter-rotating primary-secondary vortex pairs, where both the primary and secondary vortices move at the same phase speed. Visualisation shows that the mode of secondary vortex formation is highly sensitive to the degree of symmetry of the initial encounter of the incident vortex pair with the body. The symmetrical mode of (in-phase) secondary vortex formation shows very rapid growth of large-scale secondary vortices; their development is relatively independent of the particulars of body shape and scale. On the other hand, the antisymmetrical mode takes two basic forms: large-scale secondary vortex formation, with the phase shift between their formation determined by the lengthscale of the body: and small-scale, antisymmetrical shedding of secondary vortices from the body occurring for a body lengthscale an order of magnitude smaller than that of the incidental vortex pair. Correspondingly, there are several types of distortion of the cores and trajectories of the primary (incident) vortices.

Navier-Stokes simulations of axisymmetric vortex rings

Abstract: The solution of the incompressible Navier-Stokes equations by means of a spectral method is used in the present numerical investigation of viscous axisymmetric vortex rings, in the cases of a single ring over the 0.001-1000 Re number range, and a pair of interacting rings. The propagation speeds of vortex rings of varying Re are computed and compared with the results of Saffman's (1970) theory; the present error estimate is found to be smaller than Saffman's, and to decrease with increasing Re at fixed core/ring radius ratio. The 'leapfrogging' of two vortex rings of equal sign and Re=1000 each is observed, indicating severe strain of the vortex core through the first pass and merging during the second pass.

Computation of vortex sheet roll-up

Abstract: In this article we shall review some recent developments for computing vortex sheet roll-up. A vortex sheet is an asymptotic model of a free shear layer in which the transition region between the two fluid streams is approximated by a surface across which the tangential velocity component is discontinuous. A common theme in fluid dynamics is that the vortex sheet model can be useful in understanding the dynamics of coherent vortex structures observed in laminar and turbulent flows. If this goal is to be realized, reliable methods for computing vortex sheet evolution must be developed. At present, numerical methods are available for studying the initial value problem in two space dimensions. For example, detailed analytical phenomena such as singularity formation in the shape of an evolving periodic vortex sheet can be studied with existing methods. The complex roll-up process and the interaction of several spiral vortices has also been investigated numerically. These calculations have been stimulated by recent theoretical results about vortex sheets and by progress in the convergence theory of general vortex methods. First, results for the periodic vortex sheet will be reviewed. Then an application to some vortex sheet problems occurring in aerodynamics will be discussed in more detail. Finally, some open questions and directions for further research will be summarized.

High sensitivity vortex shedding flowmeter

Abstract: The vortex shedding flowmeter comprises a vortex generating bluff body of an elongated shape disposed across a cross section of a flow passage, that includes a blunt leading edge and a planar trailing edge, which planar trailing edge has two sides shielded from the moving fluid by a planar shielding member enclosing at least two sides of the planar trailing edge wherein the two sides of the planar trailing edge are respectively exposed to the fluid pressure at the two sides of the bluff body by one or more pressure openings. The vortex generating bluff body is mechanically coupled to a transducer that converts the alternating lift forces on the bluff body exerted by the vortices shed from the two sides of the bluff body in an alternating pattern to electrical signals, wherein the alternating lift forces generating the electrical signals are amplified by the planar trailing edge of the bluff body as the two sides of the planar trailing edge are respectively exposed to the alternating fluctuations of the fluid pressure associated with the vortex shedding by the pressure openings open to specific locations in the two sides of the bluff body where the alternating fluctuations of the fluid pressure is mostly pronounced. The vortex shedding flowmeter of the present invention may further include a wing sensor disposed parallel to the bluff body across another cross section of the flow passage downstream of the bluff body, which wing sensor may be coupled to the same transducer coupled to the bluff body or another transducer wherein electrical signals from the two transducers are synthesized to exclude noises and extract the vortex signals at the maximum level.

Vortex/Separated Boundary-Layer Interactions at Transonic Mach Numbers

Abstract: An experimental study has been completed on the effect of a single longitudinal vortex on a separated, transonic, turbulent boundary layer. The vortex was generated by a half-delta wing mounted at the upstream end of an axisymmetric "bump" model. A flow visualization study was conducted using vapor screen and surface oilflow techniques. In addition surface pressures were measured and mean flow and turbulence data obtained using a two-component laser velocimeter. At precritical Mach numbers, the vortex delayed or eliminated boundary-layer separation on the downwash side and enhanced it on the upwash side, thus converting a nominally twodimensional separation into a three-dimensional one. At the postcritical Mach number, the effect of the vortex was to reduce the size and extent of the shock-induced boundary-layer separation throughout the region of interaction. The boundary-layer turbulence in both cases was found to reorganize accordingly, although in a rather complex manner. The onset of three-dimensi onality in the separation line produced by the vortex resulted in secondary vortices (foci), the sign and number being strongly dependent on the freestream Mach number.

An experimental investigation of the use of air jet vortex generators to control shock induced boundary layer separation

Abstract: Air jet vortex generators were originally investigated by R.A. Wallis. Results showed that their effectiveness in delaying shock induced boundary layer separation was not as good as conventional vane type generators. Recent low speed wind tunnel tests carried out at City University indicated that the strength of the vortex could be increased considerably by using rectangular jet exits rather than round ones as used by Wallis. On this basis an investigation into air jet vortex generators was undertaken to find out whether similar improvements in vortex strength may be gained at transonic speeds and hence achieve a more effective method of controlling shock induced boundary layer separation. It was felt that in order to design air jet vortex generators it would be necessary to understand the mechanism by which an air jet forms a vortex, and to evaluate the effects of various jet parameters on vortex size, strength and position. The parameters investigated in this thesis were: (i) exit shape (ii) exit size (iii) jet direction (iv) jet inclination and (v) blowing pressure. The tests were conducted using a combination of high speed wind tunnel tests and flow visualisation in a water tunnel. The wind tunnel tests used the half aerofoil or 'bump technique' as used by Wallis. Bumps with thickness to chord ratios of 8%, 10% and 14% were tested. Increasing the thickness of the bumps resulted in higher local Mach numbers ahead of the shock and hence an increase in the severity of the shock induced separation. Vane vortex generators designed using the criteria laid down by H.H. Pearcey were used to establish a datum of control effectiveness. As a result of this investigation a method by which an air jet forms a vortex has been proposed together with a hypothesis on the influence of the various jet parameters. The results have shown that air jet vortex generators can be designed to be more effective than conventional vane type generators. Based on the work reported in this thesis a set of design guidelines has been proposed together with suggestions for further work.

Motion of an intense vortex on a rotating globe

Abstract: The evolution of an initially circular vortex is considered in terms of the relation between its dimensions and the screening scale — the radius of deformation of the quasigeostrophic single-layer model in the betaplane. It is shown that the beta-effect causes the displacement of the center of the vortex as a result of wave drift and secondary flows of dipole structure, whose development is analyzed asymptotically. It is found that with increase in the radius of deformation relative to the dimensions of the vortex the velocity of its center with respect to latitude becomes greater than the velocity with respect to longitude. The change in the intensity of the vortex due to the motion of its center with respect to latitude is estimated.