Showing papers on "Starting vortex published in 1985"

Three-dimensional vortex dynamics in superfluid 4He: Line-line and line-boundary interactions.

Abstract: The dynamical behavior of arbitrarily configured, interacting quantized vortex filaments is investigated by means of numerical experiments and analytical estimates. Several prototype situations of interest in the theory of superfluid turbulence and critical velocities are considered. It is shown that if a vortex loop approaches a surface to within a critical distance, a localized cusplike deformation is generated which drives the vortex into the surface at a well-defined point. If the vortex is reconnected to the surface in this limit, the two ends which now terminate on the surface quickly move apart. The entire process can be well approximated by making a simple reconnection at the critical distance. A similar process is found to occur when two vortex filaments try to cross, with two cusps developing which bring the lines together at a point in such a way that a line-line reconnection naturally ensues. More complicated versions of the reconnection process occur when a vortex terminates on a flat surface which contains a pinning site in the form of a local protrusion. Such a vortex is captured by the pinning site when it approaches to within a critical distance. Once a vortex is pinned, it requires a finite flow velocitymore » to free it from the pinning site. At the depinning velocity, the vortex reconnects to the flat surface and moves off. An analytical depinning criterion involving both normal and superfluid velocities is derived, and found to be in good agreement with the numerical experiments.« less

Cylindrical sound wave generated by shock-vortex interaction

Abstract: The passage of a columnar vortex broadside through a shock is investigated. This has been suggested as a crude, but deterministic, model of the generation of 'shock noise' by the turbulence in supersonic jets. The vortex is decomposed by Fourier transform into plane sinusoidal shear waves disposed with radial symmetry. The plane sound waves produced by each shear wave/shock interaction are recombined in the Fourier integral. The waves possess an envelope that is essentially a growing cylindrical sound wave centered at the transmitted vortex. The pressure jump across the nominal radius R = ct attenuates with time as 1/(square root of R) and varies around the arc in an antisymmetric fashion resembling a quadrupole field. Very good agreement, except near the shock, is found with the antisymmetric component of reported interferometric measurements in a shock tube. Beyond the front r approximately equals R is a precursor of opposite sign, that decays like 1/R, generated by the 1/r potential flow around the vortex core. The present work is essentially an extension and update of an early approximate study at M = 1.25. It covers the range (R/core radius) = 10, 100, 1000, and 10,000 for M = 1.25 and (in part) for M = 1.29 and, for fixed (R/core radius) = 1000, the range M = 1.01 to infinity.

Near-neutral centre-modes as inviscid perturbations to a trailing line vortex

Abstract: Inviscid linear perturbations to a columnar trailing line vortex are found in the form of centre-modes. These near-neutral modes, occurring at moderate values of the azimuthal wavenumber n , are the analogue of the ring modes for large n discussed by Stewartson & Capell (1985). The appearance and disappearance of these modes as the swirl parameter varies may partly explain the difficulties encountered by numerical analysts in the computation of such modes. In addition, instabilities are found at higher values of the swirl parameter than have previously been reported.

High angle-of-attack calculations of the subsonic vortex flow on slender bodies

Abstract: A nonlinear vortex-lattice method is utilized for the calculation of separated vortex flows over slender bodies at high incidence. Symmetric and asymmetric vortex flow cases are calculated showing good agreement with recently obtained experimental data for an ogive-cyclinder body at high Reynolds numbers. The only input needed for these calculations is the positions of the separation lines on the body. Studies of some of the numerical aspects of this method are described and conclusions are implemented to improve the calculations.

The self-induced motion of vortex sheets

Abstract: A method is presented for following the self-induced motion of vortex sheets. In this method, we use a piecewise analytic representation of the sheet consisting of circular arcs with trigonometric polynomials for the circulation. The procedure is used to study the evolution of the motion in two special cases: a circular vortex sheet with sinusoidal circulation distribution and an infinite plane vortex sheet subject to periodic disturbances. The first problem was studied by Baker (1980) as a test of the method of Fink & Soh (1978), while the second has been studied by a number of authors, notably Meiron, Baker & Orszag (1982). In each case, we follow the motion of the sheet up to the appearance of a singularity at a finite time. The singularity takes the form of an exponential spiral with the simultaneous development of singularities in the curvature and in the circulation distribution. In the final stages of the calculations, up to 155 marker points are used to specify the position of the sheet. If it were possible to execute a stable calculation with equally spaced point vortices, approximately 106 points would be required to achieve the same resolution. Problems with instabilities have been reduced, but not entirely eliminated, and prevent a rigorous verification of the results obtained.

A Visualisation Study of the Vortex Flow Around Double-Delta Wings.

Abstract: : A family of double-delta wings with leading-edge sweep combinations of 80/80,80/7080/6080/50 and 80/40 deg. was tested in a small towing tank. The hydrogen bubble technique was used to visualize the vortex patterns above the wings over a range of Reynolds numbers (based on centerline chord) from 7000 to 100,000. The effects of variations in incidence and leading-edge kink angle were examined. Reynolds number and leading-edge cross-section shape were found to have significant effects on the vortex structure. Attempts to visualize details of the upper surface secondary vortex flows met with only partial success. Keywords Flow visualization, Vortices, Delta wings, Angle of incidence. (Australia)

Unsteady Vortex Airfoil Interaction

Abstract: : Unsteady flow in the vicinity of an airfoil in a subsonic flow with strong vortices is investigated for a two-dimensional problem. This investigation concentrates on the interaction of the passing vortex, flow field with the steady flow field of the airfoil. Unsteady flow separation and generation of new vortices of the airfoil occurs depending on vortex strength and core path. Inviscid calculations of vortex path and sound generation are compared with the experimental results. The associated pressure waves have a strong directivity. The vortices used for the interaction experiments are generated by different vortex shedding cylinders in a stationary duct flow or by airfoils in the starting flow of a shock-tube. High speed interferometric flow recording and wall pressure measurement are the main experimental techniques.

Hamiltonian description of axisymmetric vortex flows and the system of vortex rings

Abstract: The Hamiltonian equations for the trajectories of fluid particles in axisymmetric vortex flows are obtained. On the basis of these equations, the system of thin vortex rings is considered. The problem of the interaction of two vortex rings has a general analytic solution. The problems of stochastization of the system of three and more vortex rings as well as stochastization of trajectories of fluid particles in the system of two and more vortex rings (with appropriate initial conditions) are discussed.

Investigation of large-scale vortex rings in he ii by acoustic measurements of circulation

Abstract: By forcing He II out of a tube, large-scale vortex rings with diameters of about 1 cm and normal-fluid and superfluid circulations of some cm 2/s were produced. The circulations, the vorticity distributions, the dimensions, and also the translational velocity of these vortex rings were investigated by means of a method of measurement based on the Doppler effect of first and second sound. All parameters of the vortex rings turned out to be independent of temperature. Already during the ejection of the fluid both the normal and the superfluid circulation were identical. This result is explained in terms of the formation of a superfluid boundary layer at the inner wall of the tube and the interaction of normal and superfluid flow due to the mutual friction force.

Exploratory wind-tunnel investigation of a wingtip-mounted vortex turbine for vortex energy recovery

Abstract: The Langley 8-foot transonic pressure tunnel was used for tests to determine the possibility of recovering, with a turbine-type device, part of the energy loss associated with the lift-induced vortex system. Tests were conducted on a semispan model with an unswept, untapered wing, with and without a wingtip-mounted vortex turbine. Three sets of turbine blades were tested to determine the effect of airfoil section shape and planform. The tests were conducted at a Mach number of 0.70 over an angle-of-attack range from 0 deg. to 4 deg. at a Reynolds number of 3.82 x 10 to the 6th power based on the wing reference chord of 13 in.

Experimental Investigation of the Turbulent Structure of Vortex Wakes

Abstract: Two configurations of vortical wake flows are investigated. The first (flow A) corresponds to a double-branched trailing vortex, the second (flow B) to the leading edge vortex of a delta wing. Both flows are studied in the presence of a destabilizing adverse pressure gradient leading to vortex breakdown. For various degrees of destabilization, the flow field is explored by means of a two-color laser velocimeter that yields mean velocity and Reynolds stress distributions across the flow. Breakdown is characterized by a strong deceleration of the flow near the axis of the vortex, by a significant decrease of the rotation rate in the center and consequently by a large production of turbulence. When breakdown occurs, both types of flow reveal similar characteristics of the flow structure. However, there are also significant differences, the most important being the abruptness of the breakdown phenomenon and its instability character which is much more pronounced for vortex flow B.

Vortex pair annihilation in Taylor wavy‐vortex flow

Abstract: We have made precision measurements of the vortex pair size and wavy mode amplitude of vortices in wavy Taylor–Couette vortex flow. These results are compared to a model advanced by Park and Crawford to account for expulsion of vortex pairs as the Reynolds number is increased in the wavy‐vortex regime.

Merging of Two Dimensional Vortices by the Discrete Vortex Method

Abstract: Interaction of two dimensional vortex blobs with finite core, particularly their convective merging, is investigated numerically by the use of the discrete vortex method, in which each vortex blob is replaced by a group of a number of point vortices. The numerical experiment is performed for the two cases. One is the interaction of two identical vortex blobs, and the other is the interaction of three vortex blobs with the same strength, where two of them have a like-signed circulation and the rest has the opposite. The results clarify the process of convective merging of two like-signed vortices and the effect of the third opposite-signed vortex to the merging.

Computations of two-dimensional airfoil-vortex interactions

Abstract: A procedure for calculating the interaction of a vortex with a two dimensional airfoil in a uniform free stream is presented along with results for several test cases. A Lamb like anaytical vortex having a finite core and convect in a uniform free stream interacts with the flowfield of NACA 0012 or NACA 64A006 airfoil in transonic and subsonic flow. Euler and thin layer Navier-Stokes solutions are computed and the results are compared with the results from transonic small disturbance code and available experimental results. For interactions within the limits of transonic small disturbance assumptions, the three methods gave qualitatively similar results of a two bladed helicopter rotor and suggest that the time lag effects of the free stream velocity approaching the blade may be important and should be considered in the analysis. In general, the results show a tremendous influence of the interacting vortex on the flowfield around the airfoil. This is particularly true when the vortex is stationary. For a convecting vortex, the most dramatic changes in the flowfield seem to occur when the vortex is within one chord of the airfoil.

A new look at sound generation by blade/vortex interaction

Abstract: As a preliminary attempt to understand the dynamics of blade/vortex interaction, the two-dimensional problem of a rectilinear vortex filament interacting with a Joukowski airfoil is analyzed in both the lifting and nonlifting cases. The vortex velocity components could be obtained analytically and integrated to determine the vortex trajectory. With this information, the aeroacoustic low-frequency Green's function approach could then be employed to calculate the sound produced during the encounter. The results indicate that the vortex path deviates considerably from simple convection due to the presence of the airfoil and that a reasonably sharp sound pulse is radiated during the interaction whose fundamental frequency is critically dependent upon whether the vortex passes above or below the airfoil. Determination of this gross parameter of the interaction is shown to be highly nonlinearly dependent upon airfoil circulation, vortex circulation, and initial position.

A hydraulic experiment on tidal exchange

Abstract: Tidal exchange through a narrow entrance channel was studied experimentally with the use of a simplified hydraulic model. The inflowing water mass, visualized with dye solution, exhibits the shape of a starting plume with a starting vortex pair at its head. Because of their periodical formation by the tide, these are called the “tidal plume” and “tidal vortex pair”. The axis of the tidal plume deflects and undulates with a period 2 to 9 times that of the tide. Together with this undulation, the vortex pair becomes asymmetric. A circulating flow is formed in the bay which affects the shape of the inflowing and outflowing water masses. A part of the inflowing water mass flows out during the subsequent ebb, and this outflowing portion can be divided into two parts. One is the water remaining in the entrance channel at high water which flows out during the first half of the subsequent ebb and the other is the water flowing round the bay in the circulating flow during flood that flows out during the latter half of the subsequent ebb. Both contribute to the exchange ratio, but we can estimate an upper limit for the exchange ratio by neglecting the latter outflow. This neglected portion is considered in the concept of the age composition of outflowing water. The age composition of the bay water shows the existence of intermittent effluence superposed on a trend in the age composition that is similar to that of the well-mixed case. From the analysis of a model consisting of a number of mixing tanks connected in series with a recycle flow, it is concluded that this intermittent effluence occurs in the case of weak mixing due to the effect of circulating flow in the bay but is negligible in the case of strong mixing.

A New Three-Dimensional Vortex Method

Abstract: A new vortex method based on the vorticitv equation is introduced to simulate incompressible vorticity-dominated flows. This method is based on Lagrangian way of tracking vortices a three-dimensional flow where the vortices are replaced by a number of disconnected vortex sticks. The interaction of vortex rings and three-dimensional flow past a parachute are simulated by this method. In the problems of flow past a parachute, a simplified vortex-lattice method is combined with this method and a potential theory is applied to introduce, an effect of porosity. The cross-linking of vortex rings and spiral structure of the wake behind the parachute were well simulated. Also the effect of the porosity and shape of the parachute can be estimated.

Structure and Decay of Secondary Flow in the Downstream of a Cascade

Abstract: An experimental investigation has been done on trailing and passage vortices downstream of a decelerating cascade. Hot wire traverses gave distributions of secondary flow velocities and streamwise vorticities at four axial locations. A spanwise distribution of passage circulations per unit span agrees well with a prediction by an inviscid classical secondary flow theory. The trailing vortex is well approximated by a vortex sheet immediately downstream of the blade, beginning to roll up at the position of maximum circulation per unit span, It develops into an almost axisymmetric vortex tow chord lengths downstream. The trailing vortex decays faster in the vicinity of the blade trailing edge but more slowly far downstream, than the passage vortex does. The experimental trailing vortex is generated very far away from an endwall as compared with a theoretical position, exerting significant influences on the maximum underturning and the pitch-averaged streamwise vorticity in the main flow region.

The Viscous Flow Induced Near a Wall by Counter-Rotating Vortex Pairs and Vortex Loops.

Abstract: : Vortex motion above walls is observed in a variety of physical situations this study the problem of how a viscous flow near a wall responds to the motion of certain vortex motions. Specifically the unsteady boundary layers induced by the motion of two dimensional rectilinear vortex pairs and three dimesional vortex loops in an otherwise stagnant fluid above a plane wall are considered; numerical solutions are obtained for the evolution of the boundary layer flow as it develops under the influence of the moving vortices. All of the vortex flows considered in this study are related to more complex vortex flows observed in a diverse variety of physical situations; many of these situations relate to the observed time-dependent flow in a turbulent boundary layer and are discussed. The boundary layer induced by a pair of counter rotating rectilinear vortices in motion above a plane in an otherwise stagnant fluid is addressed; depending on the assumed sense of rotation the vortices either move toward or recede from the wall. A symmetric situation is considered where both vortices are equidistant from the wall. An symmetric situation is considered where a counter rotating pair approaches the wall at angle of attack. The motion of three-dimensional vortex loops which have a plane of symmetry is considered. A method is developed which allows the computation of the flow velocities on and near the symmetry plane of the loop. The computed results show a variety of complex three dimensional separation phenomena; again the boundary layer solutions ultimately show strong localized growth and are suggestive that a boundary-layer eruption and strong viscous-inviscid interaction will occur.

Vortex breakdown simulation

Abstract: A vortex breakdown was simulated by the vortex filament method, and detailed figures are presented based on the results. Deformations of the vortex filaments showed clear and large swelling at a particular axial station which implied the presence of a recirculation bubble at that station. The tendency for two breakdowns to occur experimentally was confirmed by the simulation, and the jet flow inside the bubble was well simulated. The particle paths spiralled with expansion, and the streamlines took spiral forms at the breakdown with expansion.

Airflow rate measuring apparatus of Karman vortex type

Abstract: A columnar vortex generator is set in a duct communicating with an intake pipe of an engine, to extend across the airflow in the duct. A vortex sensor is arranged in a downstream surface of the vortex generator. The sensor generates a signal corresponding to the frequency of Karman vortexes generated at the downstream side of the vortex generator. A turbulence generator for generating a turbulent flow in that part of intake air flowing against the vortex generator is set at the upstream side of the vortex generator. The vortex generator consists of stationary and movable units. The movable unit is moved in accordance with elongation/contraction of a bellows so as to vary the typical dimension D of the surface of the vortex generator against which air flows.

On the structure of the turbulent vortex

Abstract: The trailing vortex generated by a lifting surface, the structure of its turbulent core and the influence of axial flow within the vortex on its initial persistence and on its subsequent decay are described Similarity solutions of the turbulent diffusion equation are given in closed form and results are expressed in sufficiently simple terms that the influence of the lifting surface parameters on the length of persistence and the rate of decay of the vortex can be evaluated