Showing papers on "Starting vortex published in 1984"

Reynolds-stress measurements in a turbulent trailing vortex

Abstract: Measurements of a turbulent trailing vortex in zero pressure gradient are described. These include mean velocities and all the components of the Reynolds-stress tensor. The measurements were made using linearized hot wires at stations 45, 78 and 109 chordlengths downstream of the wing. Axisymmetric jets or wakes were added coaxially to the vortex while the total circulation was held constant, and their effect studied. It was found, as Poppleton and Mason & Marchman have reported, that increasing the flow force hastens the radial dispersion of vorticity; this is seen to be concurrent with higher turbulence intensities and Reynolds shear stresses. With the flux of excess axial momentum effectively zero, thereby approximating a turbulent line vortex, no discernible downstream change was observed in the velocity field and very little in the turbulence field.A balance of terms in the mean-momentum equations is presented and discussed. It is seen that, in spite of the fact that the radial velocity is numerically much smaller than the axial velocity, terms that contain it, both in the axial- and tangential-momentum equations, cannot be ignored, unless the magnitude of the flow force (divided by the fluid density) is much less than the square of the total circulation.

An experimental investigation of the parallel blade-vortex interaction

Abstract: A scheme for investigating the parallel blade vortex interaction (BVI) has been designed and tested. The scheme involves setting a vortex generator upstream of a nonlifting rotor so that the vortex interacts with the blade at the forward azimuth. The method has revealed two propagation mechanisms: a type C shock propagation from the leading edge induced by the vortex at high tip speeds, and a rapid but continuous pressure pulse associated with the proximity of the vortex to the leading edge. The latter is thought to be the more important source. The effects of Mach number and vortex proximity are discussed.

Airfoil tip vortex formation noise

Abstract: Spectral data are presented for the noise produced due to the turbulent three-dimensional vortex flow existing near the rounded tip of lifting airfoils. The results are obtained by the comparison of sets of two- and three-dimensional test data for different airfoil model sizes, angles of attack, and tunnel flow velocities. Microphone cross-correlation and cross-spectral methods were used to determine the radiated noise. Corrections were made for tunnel shear layer and source directivity effects. Interpretation of the results are aided by a three-dimensional flow analysis developed for this study which determines open tunnel and finite aspect ratio corrections heretofore neglected in tip vortex studies. Hot wire measurements were made in the tip vortex formation region for the specification of governing flow parameters. The spectral data is normalized in a format considered most useful for subsequent quantitative prediction of this noise mechanism for practical systems such as helicopter rotors. Comparison is made to the analysis of George and Chou. A recommended prediction method is given.

Numerical simulation of the interaction of a vortex with stationary airfoil in transonic flow

Abstract: A perturbation form of an implicit conservative, noniterative numerical algorithm for the two-dimensional thin layer Navier-Stokes and Euler equations is used to compute the interaction flow-field of a vortex with stationary airfoil. A Lamb-like analytical vortex having a finite core is chosen to interact with a thick (NACA 0012) and a thin (NACA 64A006) airfoil independently in transonic flow. Two different configurations of vortex interaction are studied, viz., (1) when the vortex is fixed at one location in the flowfield, and (2) when the vortex is convecting past the airfoil at freestream velocity. Parallel computations of this interacting flowfield are also done using a version of the Transonic Small Disturbance Code (ATRAN2). A special treatment of the leading edge region for thin airfoils is included in this code. With this, the three methods gave qualitatively similar results for the weaker interactions considered in this study. However, the strongest interactions considered proved to be beyond the capabilities of the small disturbance code. The results also show a far greater influence of the vortex on the airfoil flowfield when the vortex is stationary than when it is convecting with the flow.

Translation Effects on Simulated Tornadoes

Abstract: A simulation of tornadoes translating over the ground was carried out in a modified Ward simulator The purpose was to investigate the effects of translation on tornado dynamics The results are as follows: • Secondary vortexes were found to be generated by the relative motion between the main vortex and rough “ground” The secondary vortexes trail the primary vortex Apparently they feed off the energy of the primary vortex, and achieve a momentary transition state from a single vortex to multiple vortexes • The core radius increases with swirl ratio and decreases with surface roughness Translation causes a local increase in swirl ratio, increasing the core size over that of a stationary vortex • The central pressure drop increases with swirl ratio during translation Translation also causes a steeper pressure gradient on the trailing side of the vortex core A similar characteristic tilt on the trailing side of the pressure profile has been noted on barograms for real tornadoes

airfoil section for a rotor blade of a rotorcraft

Abstract: An airfoil shaped cross section 36 for the rotor blade 14 is disclosed. Various construction details which improve the aerodynamic and drag performance of the airfoil section are developed. The upper surface 38 and the lower surface 42 are contoured by a thickness distribution and a camber distribution such that the bend of the surfaces is constant or changes at a constant rate over certain regions of the airfoil to avoid the formation of strong, normal shock waves at local Mach numbers less than 1.2 during normal, level flight conditions at standard temperature and pressure.

Measurements of Attached and Separated Turbulent Flows in the Trailing-Edge Regions of Airfoils

Abstract: There are mainly two problems in the calculation of the trailing-edge flow for which no unanimously accepted satisfactory solution method has yet been given. The first one is how to account for the viscous-inviscid interaction. It is trivial that one way or another an interactive approach is necessary in order to avoid the stagnation pressure predicted by the potential-flow theory at a finite-angle trailing edge. The second one is how to model the turbulent stresses in the case of practical high Reynolds number flows. The present paper presents some experimental evidences that may suggest a better way of calculation and modeling method, test existing methods and help understand underlying turbulence mechanism in a few cases of asymmetric lifting airfoils with or without separation under incompressible conditions.

Flow Field and Acoustics of Two-Dimensional Transonic Blade-Vortex Interactions,

Abstract: : Blade-vortex interaction noise from full-scale helicopters is shown to involve unsteady transonic flow phenomena which can be modeled as two-dimensional. An unsteady, small-disturbance-theory, numerical analysis, is used to model the interaction of an airfoil with a finite-core, locally-convected vortex using the vortex-in-cell method with multiple branch cuts accounting for the distributed vortices' potential jumps. Strong disturbances propagating from the blade-vortex interaction are associated with occurrence of Tijdeman's Type C flow on the airfoil's lower surface. In this type of flow, the shock which initially terminates a supersonic zone propagates through it and forward off the airfoil. The effects of airfoil shape, angle of attack, Mach number, vortex strength, and vortex miss distance on the flow and on waves radiated forward are investigated. It is found that stronger radiated waves are associated with narrow supersonic regions and near-sonic base flow. Also, stronger vortices generate stronger radiated waves, but miss distance is not as important a factor. Originator-supplied keywords include: Acoustics, Noise(Sound), and Numerical analysis.

Aeroacoustic interaction of a distributed vortex with a lifting Joukowski airfoil

Abstract: A first principles computational aeroacoustics calculation of the flow and noise fields produced by the interaction of a distributed vortex with a lifting Joukowski airfoil is accomplished at the Reynolds number of 200. The case considered is that where the circulations of the vortex and the airfoil are of opposite sign, corresponding to blade vortex interaction on the retreating side of a single helicopter rotor. The results show that the flow is unsteady, even in the absence of the incoming vortex, resulting in trailing edge noise generation. After the vortex is input, it initially experiences a quite rapid apparent diffusion rate produced by stretching in the airfoil velocity gradients. Consideration of the effects of finite vortex size and viscosity causes the noise radiation during the encounter to be much less impulsive than predicted by previous analyses.

First experimental evidence of vortex splitting

Abstract: Movie sequences of vortex patterns visualized by smoke in a two‐dimensional starting flow around an airfoil indicate the process of vortex splitting predicted by Moore and Saffman and by Christiansen and Zabusky for some vortical flows.

A Nonlinear Hybrid Vortex Method for Wings at Large Angle of Attack

Abstract: A steady and unsteady nonlinear hybrid vortex (NHV) method is developed for low aspect ratio wings at large angles of attack. The method uses vortex panels with linear vorticity distribution (equivalent to a quadratic doublet distribution) to calculate the induced velocity in the near field using closed-form expressions. In the far field, the distributed vorticity is reduced to concentrated vortex lines and the simpler Biot-Savart law is employed. The method is applied to rectangular wings in steady and unsteady flows. The numerical results show that the method accurately predicts the distributed aerodynamic loads and that it is of acceptable computational efficiency. N recent years, the development of numerical methods for predicting the steady and unsteady aerodynamic characteristics of lifting surfaces exhibiting leading- and/or side-edge separations has received considerable attention. For the steady flow problems several numerical techniques have been developed. These include the nonlinear discrete vortex1'6 (NDV) methods, higher order doublet panel methods,7"9 and nonlinear hybrid vortex (NHV) methods.10'11 For the unsteady flow problems, the literature shows fewer numerical techniques which include the NDV methods12"15 and constant doublet panel methods.16'17 The literature lacks high-order panel methods for the unsteady flow problems. For this reason, we are presenting in this paper an efficient and accurate method for the steady and unsteady flow problems of lifting surfaces at large angles of attack. In this method, vortex panels with linear vorticity distribution are used in the near field calculations. In the farfield calculations, the distributed vorticity over each far-field panel is lumped into equivalent concentrated vortex lines. In this way, accuracy is satisfied in the near field while computational efficiency is maintained in the far field. The coupling of a continuous vortex-sheet representation and a concentrated vortex-line representation for solving the nonlinear lifting surface problem is called the "nonlinear hybrid vortex'' (NHV) method.

A method of and apparatus for sequentially separating a medium into different components

Abstract: A method and vortex separator for sequential separation of a medium into different components by means of centrifugal force in a vortex array (2, 2) in a manner that part of a vortex flow (71) proceeds from the outer portions of a vortex (2) into the outer portions of a following vortex (2) and in individual vortexes (2) particles having a major mass concentrate in the outer portions of a vortex and those having a minor mass concentrate in the central portions of a vortex. For increased separation capacity the successive vortexes (2) of a vortex array (2, 2) are backed up substanstantially on each other while rotating in opposite directions and the flow (71) between individual vortexes is allowed to pass without any susbstantial movements in the direction of a vortex axis (49).

A Two-Dimensional Linear Stability Analysis of the Multiple Vortex Phenomenon

Abstract: Two numerical models have been constructed and used to investigate the formation of secondary vortices in axisymmetrically forced rotating flows. The vortex flow examined is that developed in a laboratory vortex simulator where secondary vortices have been produced and extensively studied. The first numerical model generated a collection of steady-state, axisymmetric, two-dimensional vortex flows for a range of swirl ratios. The second model tested those flows for instability by simulating the behavior of small-amplitude, linear perturbations superimposed on the flows: amplification of the perturbations indicated instability, whereas damping indicated stability. The results of the instability study show that the vortex is stable for the lowest swirl ratios but that, above a certain value, instability persists indefinitely. The most rapidly growing wavenumber shifts steadily with increasing swirl from 1 to approximately 5 in the swirl range investigated. Growth rates were found to be high enough f...

Vortex Shedding from Two Interfering Circular Cylinders

Abstract: Measurements are presented of the vortex shedding frequency from a pair of smooth parallel circular cylinders in both tandem and staggered arrangements for diameter ratios of 0.5, 1 and 2. No discontinuity was observed in the variation of the Strouhal number of the downstream cylinder with respect to the streamwise separation. For a pair of identical cylinders in tandem arrangement, the downstream body inhibited the vortex formation behind the upstream one up two diameters spacing. It was generally inferred that the Reynolds number and the turbulence characteristics of the free-stream flow as well as the cylinders aspect ratio are significant factors in the vortex shedding behavior.

Wingtip vortex propeller

Abstract: A device which increases the energy efficiency of aircraft wherein a wingtip pusher propeller 12 is positioned aft of the wingtip 18 to rotate in the crossflow of the wingtip vortex. The propeller 12 rotates against the vortex swirl creating additional thrust from and attenuating the wingtip vortex by simultaneously extracting energy from the vortex and converting it to propeller blade-induced thrust while injecting its high energy wake into the vortex axial flow to dissipate the vortex. As a result, the device increase aircraft fuel efficiency by simultaneously increasing thrust and decreasing vortex induced drag. By attenuating the vortex safety to following aircraft is maximized.

Passive vortex lift control

Abstract: A method of applying and controlling vortex lift to a unique high-lift airfoil (29) is described wherein the planform of the airfoil (29) comprises a swept-forward outer panel (31) and a swept-aft or unswept inboard panel (33). A leading edge vortex (37) is formed on (31) and attached flow is maintained on (33). The attached flow on (33) causes the vortex (37) of the airfoil (29) to turn downstream and also induces axial flow along axis of vortex (37). Both of these results serve to delay vortex burst. A high-lift trailing edge device (45) such as a mechanical flap as the circulation control concept will induce a high leading edge flow angularity and cause the vortex (37) to grow in strength, thereby increasing vortex lift. The vortex (37) replaces the high-weight, high-lift leading device that would otherwise be required.

An Axisymmetric Viscous Vortex Motion and Its Acoustic Emission

Abstract: Numerical simulation of head-on collision of two vortex rings has been made for various values of a translation Reynolds number from 50 to 500, and the profile of the acoustic wave emitted by the vortex collision is determined by using the computed development of the stream function and vorticity distributions. It is found for the higher Reynolds number range that there is an early stage during which the strength of each vortex is conserved, however, the strength decreases rapidly from an instant when the cores of the two vortices come into contact. The total kinetic energy decreases similarly. The enhancement of the energy dissipation is found to be related to the vortex stretching at the time of collision. The wave pressure is composed of two components, and comparison with experimental data obtained at much higher Reynolds numbers indicates reasonable agreement for the main quadrupole component.

Feedback in separated flows over symmetric airfoils

Abstract: For a flow over an airfoil with laminar separation, a feedback cycle may exist whereby a Kelvin-Helmholtz instability wave emanating from the separation point on the airfoil surface grows along the shear layer and is diffracted as it interacts with the sharp trailing edge of the airfoil, causing acoustic radiation which, in turn, propagates upstream and regenerates the initial instability wave. The analysis is restricted to the high frequency limit. Solutions to the boundary-value problem are obtained using the slowly varying approximation and the method of matched asymptotic expansions. Resonant solutions exist for certain discrete values of the Reynolds and Strouhal numbers. The results are discussed and compared with available data.

Airfoil interaction with impinging vortex

Abstract: The tip of a finite-span airfoil was used to generate a streamwise vortical flow, the strength of which could be varied by changing the incidence of the airfoil The vortex that was generated traveled downstream and interacted with a second airfoil on which measurements of lift, drag, and pitching moment were made The flow field, including the vortex core, was visualized in order to study the structural alterations to the vortex resulting from various levels of encounter with the downstream airfoil These observations were also used to evaluate the accuracy of a theoretical model

Nucleation of vortex rings by negative ions in liquid helium at low temperatures

Abstract: The critical velocity is calculated for the formation of vortex rings by negative ions in liquid helium. The system is described by a Lagrangian, from which the Hamiltonian is derived. Care is taken to ensure that the equation of continuity is satisfied. This is done by defining the surface of the core by a contour of constant A, where A is a component of the vector potential. The model which is analysed is that proposed by Schwarz and Jang (1973) in which a vortex ring is formed encircling the ion. The new definition of the core leads to a critical velocity if the vortex is treated as a classical object. However, there is a potential barrier which stops the classical vortex from entering the fluid from the surface of the ion. The vortex is then treated as a quantum object in a potential well, and account taken of its zero-point motion, and its energy levels. The critical velocity for the formation of vortex rings occurs when the energy of the bare ion equals the energy of one of these energy levels. These critical velocities are calculated as a function of pressure and compared with the values obtained from experiment.

Stability of a compressible two-dimensional vortex under a three-dimensional perturbation

Abstract: Kelvin showed that a two-dimensional vortex under a two-dimensional disturbance in incompressible flow responds at a discrete set of eigenvalues, which were found by Broadbent & Moore ( Phil. Trans. R. Soc. Lond. A 290, 353-371 (1979) to become unstable in a compressible fluid. It is now shown that three-dimensional perturbations are also unstable provided the wavelength is greater than some critical value that depends on the Mach number of the vortex. A critical boundary dividing stable from unstable modes is defined. Most of the results relate to a Rankine vortex, as in the previous work mentioned above, but some results are also given for a vortex with a different velocity profile within the core; qualitatively the same kind of behaviour is found.

Topics in 2-D Separated Vortex Flows

Abstract: This thesis is concerned with vortices in steady two dimensional inviscid incompressible flow In the first three chapters, separated vortex flows are considered in the context of inviscid flow past two dimensional airfoils for which the action of the vortex is to induce large lift In the fourth and last chapter, we consider vortices in uniform flow in the absence of any physical bodies In chapter I, we consider two configurations of vortices for flow past a flat plate with a forward facing flap attached to its rear edge In the first case, case (a), we consider a potential vortex in the vicinity of the airfoil, while for case (b), we consider a vortex sheet coming off the leading edge of the plate and reattaching at the leading edge of the flap such that the region between the vortex sheet and the airfoil is stagnant For case (a), the Schwarz-Christoffel transformation is used to find exact solutions to the flow problem It is found that by suitably placing a potential vortex of appropriate strength it is possible to satisfy the Kutta condition of finite velocity at both the leading edges of the plate and the flap in addition to satisfying it at the trailing edge, provided the plate flap combination satisfies a geometric constraint The action of the potential vortex is to create a large circulatory region bounded by the airfoil and the streamline that separates smoothly at the leading edge of the plate (due to the Kutta condition) and reattaches smoothly at the leading edge of the flap (from the Kutta condition again) The circulation induced at infinity for such a flow and hence the lift on the airfoil is found to be very large For case (b), where the vortex sheet location is unknown, a hodograph method is used to find exact solutions It is found that once a geometric constraint is satisfied, flows exist for which the Kutta condition is satisfied at the trailing edge of the plate-flap combination As in (a), large values of lift are obtained However, in both cases (a) and (b), the adverse pressure gradient of top of the flap is recognized as a source of potential difficulty in the experimental realization of the calculated flow In chapter II, successive modifications are made to the airfoil considered in chapter I Exact solutions are once again obtained by a variation of the hodograph method of chapter I The lift for these airfoils is found to be significantly larger than the one in chapter I Because the trailing edge is no longer a stagnation point, it is felt that these flows may be easier to realize experimentally Chapter III is concerned with the so-called Prandtl-Batchelor flow past the plate-flap geometry of chapter I The flow consists of an inner region which has a constant vorticity The region outside of the airfoil and the vortex sheet coming off the leading edge of plate and reattaching at the leading edge of the flap (as in chapter I) is once again irrotational The common boundary between the exterior flow and the inner flow, ie the vortex sheet, is unknown a priori and is determined by continuity of pressure, which translates into a nonlinear boundary condition on an unknown boundary By extending the function theoretic approach of complex variables to this problem, we reduce the entire problem into one of determining one unknown function of one variable on a fixed domain from which everything else can be calculated This is then solved numerically Our calculations provide what we believe to be the first such calculation of a Prandtl-Batchelor flow The calculations also provide a more realistic model for the vortex sheet flow considered in chapter I Chapter IV deals with a steadily translating pair of equal but opposite vortices with uniform cores and vortex sheets on their boundaries, moving without the presence of any physical boundary The solutions were found for such flows using the function theoretic approach introduced earlier in chapter III for flows where the velocity on the vortex sheet is not a constant The solutions form a continuum between the hollow vortex case of Pocklington (1898) and those of Deem and Zabusky (1978) and Pierrehumbert (1980) who consider uniform core with no vortex sheet The iterative scheme for numerical calculation, however, turns out to have severe limitations, as it fails to converge for the cases with no vortex sheet or when the vortex sheet strength is small In the last section of the chapter, a more traditional approach due to Deem and Zabusky is taken to calculate a pair of touching vortices with uniform core and no vortex sheet on the boundary and an error in Pierrehumbert's (1980) calculations is pointed out In appendix I, we point out some errors in Pocklington's paper on the motion of a hollow vortex pair The errors are corrected and the results are found to be then in agreement with results using the method in chapter IV

Trailing edge flow conditions as a factor in airfoil design

Abstract: Some new developments relevant to the design of single-element airfoils using potential flow methods are presented. In particular, the role played by the non-dimensional trailing edge velocity in design is considered and the relationship between the specified value and the resulting airfoil geometry is explored. In addition, the ramifications of the unbounded trailing edge pressure gradients generally present in the potential flow solution of the flow over an airfoil are examined, and the conditions necessary to obtain a class of airfoils having finite trailing edge pressure gradients developed. The incorporation of these conditions into the inverse method of Eppler is presented and the modified scheme employed to generate a number of airfoils for consideration. The detailed viscous analysis of airfoils having finite trailing edge pressure gradients demonstrates a reduction in the strong inviscid-viscid interactions generally present near the trailing edge of an airfoil.

Stability boundary of the Taylor vortex flow

Abstract: The stability of the Taylor vortex state in a circular Couette geometry is investigated by measuring its transition to a wavy vortex state. Seven different radius ratios, ranging from 0.7767 to 0.7972, are used to map out the stability boundaries in the Reynolds number–radius ratio plane. The results verify several important features predicted by Jones.

Close-coupled canard-wing vortex interaction

Abstract: THE present experiment investigates the interaction of canard and wing vortices and their effect on the lifting wing's flowfield turbulence and Reynolds stresses. Spanwise wing blowing was used to enhance the leading edge vortex and alter the vortex trajectory in an effort to keep it locked to the wing's leading edge for lift enhancement. The turbulence intensity and Reynolds stress values, obtained by using hot film anemometers, illustrate the vortex structure. Contents The flow at the leading edge of sweptback wings at moderate to high angles of attack separates and produces vortex sheets that roll up into vortices on the wing's upper surface. When canards are closely coupled with the wings, an interference between the leading edge vortices occurs. The interference changes the turbulence characteristics and the trajectories of the vortices.1'7 Turbulence intensity and Reynolds stress measurements were performed by means of hot wire anemometers for various canard-wing body configurations. The data obtained refer to the canard-wing vortex interaction and can be used to improve the eddy viscosity models used in the solution of the Navier-Stokes equations applied to the present aerodynamic configuration.8 The tests were performed in the 5-ft (1.524-m) subsonic wind tunnel of the Air Force Institute of Technology. The sting-mounted canard-wing model used is shown Fig. 1. The canard area was 28% of the wing area. The hot film crossshaped anemometer used was aligned with the freestream direction, and the data were taken in the z direction, perpendicular to the wind tunnel axis. Model and instrumentation details are found in Ref. 9. A screen with tufts was placed behind the model to locate the vortices position. The tests were performed at Mach number M^ =0.134, a unit Reynolds number of 2.8 x 106/m, and at angles of attack of a= 10, 16, and 20 deg. Three configurations were tested: 1) close-coupled canards coplanar with the wings, 2) closecoupled canards placed 4.29 cm higher than the wings, and 3) canards removed. Spanwise mass injection was used in some tests to alter the trajectory of the wing leading edge vortex. The distribution of the axial mean velocity u/V^ shows that the leading edge vortex emanating from the canard has a wave-like behavior because of a velocity decrease at its center and an increase at its edge. Spanwise contours of the velocity and stresses at the canard trailing edge give a good visualization of the canard leading edge vortex's position and are shown in Ref. 9. In Figs. 2 and 3, the lateral velocity and Reynolds stresses in the axial plane (perpendicular to the wing) are shown for a= 16 deg at a location just outboard of

Application of the vortex theory to high-speed horizontal-axis wind turbines

Abstract: The vortex theory of screw propellers is modified and applied to the high-speed horizontal-axis wind turbines. The turbine blade can be replaced by a bound vortex system, and when the energy loss per unit time is a minimum, the trailing vortices move backwards with a constant velocity and build a helical vortex sheet. A velocity potential function which represents the vortex system is obtained. The circulation distribution along the blade is calculated iteratively until the hydrodynamic pitch of helical vortices converges. This method is applied to two examples, one with a sectional shape of modified Clark-Y type and the other with a sectional shape of NACA 65-series type. Torque, power and resistance are computed for a turbine which has two to six blades, respectively. Numerical values agree well with the sectional shape of experimental ones and it is concluded that the vortex theory is applicable to high-speed horizontal-axis wind turbines.