Showing papers on "Starting vortex published in 1975"

The instability of a straight vortex filament in a strain field

Abstract: A straight infinite vortex of finite cross section is deformed by the action of weak irrotational plane strain. The deformed vortex is shown, in the absence of axial flow, to be unstable to disturbances whose axial wavelengths lie in a narrow band, whose width is proportional to the imposed strain. The band is centred on the wavelength of the helical wave which does not propagate on the unstrained circular vortex. Thus support is given to the instability mechanism proposed recently by Widnall, Bliss & Tsai (1974). The argument depends, however, on the mirror image of the helical wave also being a possible non-propagating disturbance on the unstrained vortex.

Starting vortex, separation bubbles and stall: a numerical study of laminar unsteady flow around an airfoil

Abstract: The stalling characteristics of an airfoil in a laminar viscous incompressible fluid are investigated. The governing equations in terms of the vorticity and stream function are solved using an implicit finite-difference scheme and point successive relaxation procedure. The development of the impulsively started flow, the initial generation of circulation, and the behavior of the forces at large times are studied. Following the impulsive start, the lift is at first very large and then it rapidly drops. The subsequent growth of circulation and lift is associated with the starting vortex. After incipient separation, the lift increases owing to enlargement of the separation bubble and intensification of the flow rotation in it. The extension of this bubble beyond the trailing edge causes it to rupture and brings about the stalling characteristics of the airfoil. Subsequently, new bubbles are formed near the upper surface of the airfoil and are swept away. The behavior of the lift acting on the airfoil is explained in terms of the strength and sense of these bubbles.

Ideal plug-flow properties of taylor vortex flow

Abstract: Mixing properties of Taylor vortex flow with small constant axial flow rates were investigated in an annulus between two concentric rotating cylinders by injecting a salt-solution tracer or by suspending small soluble particles. Longitudinal intermixing of fluid elements between the neighboring vortices was examined by a method of two-point measurement. Circumferential mixing in each cellular vortex was also evaluated by comparing two successive time variations of concentration at a certain point in the case of no axial flow. The modes of flow were mapped in terms of Taylor number and Reynolds number. This vortex flow has been found to be an ideal plug-flow for the range of 51.4

Colliding vortex rings

Abstract: Laboratory observations of the interaction between pairs of vortex rings during certain collisions are described. A theory is advanced, based on the resonance of the forced motion with the elliptical mode of oscillation of a single vortex, and is found to be in agreement with the observed behavior.

Vortex generation by oscillatory flow over rippled surfaces

Abstract: Vortices generated by oscillatory flow over symmetrical sand ripples have been investigated in order to ascertain the amount of wave energy loss that can be attributed to the formation and movement of vortices. The experimental phase of the investigation centered around the flow visualization of the vortices over the ripples. These laboratory data were used to obtain values for three parameters (vortex size, ciculation, and radius of the vortex core) needed to evaluate the amount of energy bound up in the vortex, the energy relationship being derived from a trapped vortex model. Comparison of the vortex kinetic energy loss attributed to bottom effects indicates that the vortices possess about 7% of the measured wave energy loss, the majority of the remaining energy loss being attributed to bottom shear.

Isolated Airfoil-Tip Vortex Interaction Noise

Abstract: An experimental investigation was conducted in the UARL Acoustic Research Tunnel to define the noise characteristics associated with the interaction of a stationary tip vortex and a downstream stationary airfoil. This model test geometry simulated, in its simplest form, the tip vortexblade interaction which occurs on single rotor helicopters during hover. For moderate to high lift test conditions, the vortex-airfoil interaction was found to cause local blade stall with an attendant increase in the blade far-field noise. These results indicated that this interaction may be an important source of helicopter broadband noise during hover. Cross-correlation measurements conducted amongst surface-mounted and far-field microphones demonstrated that the operative noise mechanism was "trailing edge noise" arising from the interaction of stall generated eddies with the airfoil trailing edge. This mechanism would be expected to be responsible for increased noise at stall conditions in other, nonrotary wing, applications.

Hidden vortex generators

Abstract: An improvement to the controllability of aircraft wherein vortex generators are connected to a control surface in front of the hinge line of the control surface so that when the control surface is deflected, the vortex generators project into the airstream on the opposite side from the deflecting control surface. The action of the vortex generators causes the airflow to remain attached to the control surface at higher angles of attack than would otherwise be possible thus increasing the lift of the airfoil and control surface.

Unsteady Kutta condition at high values of the reduced frequency parameter

Abstract: The unsteady Kutta condition is discussed in the light of some recent experimental measurements made near the trailing edge of a long flat plate and a 10C4 airfoil. The hierachy of disagreement from the theoretically predicted zero trailing edge loading caused by viscous instabilities is found to be acoustically correlated vortex shedding, natural vortex shedding, Tollmien-Schlichting waves, and, by implication, turbulent boundary-layer eddies. The region of significant chordwise disagreement scales with the wake perturbation wavelength of the corresponding instability. Coordinating the vorticity of the turbulent boundary layer shed from the profile airfoil with a transverse acoustic resonance produced a distinct disagreement of the Kutta condition at high reduced frequency parameters (\ = wc/U). In this case and for vortex shedding, the extrapolated loading coefficient at the trailing edge increased with the nondimensional acoustic amplitude. I. Introduction T^HE Kutta condition as applied in unsteady JL potential airfoil analyses is essentially an extension of steady theory. Kutta ! postulated that a value of circulation should be chosen in his steady potential model to avoid a velocity singularity at the sharp trailing edge of an airfoil. This condition can be established if the trailing edge is also the rear stagnation point. The resulting modeled flow pattern agrees with that observed in steady flow and also predicts the lift and its chordwise distribution well at low angles of attack. The theoretical consequences of this hypothesis are that the lift loading or chordwise vorticity jump approaches zero at the trailing edge. An alternative statement is that the surface velocities on either side of the airfoil approach a common value at the rear stagnation point. For rounded trailing edges, the position of the rear stagnation point is indeterminate , as there is no velocity singularity to be avoided and so fix its location. In this case and for the situation of real flows with viscosity, Taylor2 proposed the condition of zero net vorticity discharge to establish the steady lift value. Preston3 explained the deviation of the lift of an airfoil at low angles of incidence from the potential theory value as due to the profile alteration from the boundary-layer growth. His calculations incorporated Taylor's vorticity discharge condition. Various approximate steady lift calculation methods for the rounded trailing edge geometry have been proposed by Gostello 4 and others. These extend the upper and lower lift distributions, at a selected chordwise position, to the trailing edge to give zero loading and thereby remove the stagnation point indeterminacy. In the unsteady case there are all the previous theoretical difficulties and, in addition, the unsteady effects on the viscous boundary layer and the shed vorticity. The latter complicates the airfoil response, making it a function of the airfoil's vorticity history. However, same theoretical assumption for the Kutta condition, of no unsteady loading at the

Generation and Decay of Viscous Vortex Rings

Abstract: Decay of a vortex ring in a viscous fluid is discussed by using a solution in the form of an asymptotic expansion for large time t . It is found that the velocity of the vortex ring varies as t -1.5 in the final state of low Reynolds number. The asymptotic expansion is not uniformly valid, and an improvement is made by using the method of matched asymptotic expansions. Generation and development of vortex rings are simulated by numerical integration of the Navier-Stokes equation as an initial and boundary value problem. Time variations of physical quantities such as total energy, impulse and velocity of the vortex rings, etc. are obtained, and have been shown to approach asymptotically to those obtained from the asymptotic expansion. Comparison with experimentally produced vortex rings is also given briefly.

Vortex Attenuation Obtained in the Langley Vortex Research Facility

Abstract: It has been shown by a new static airflow visualization method that a drogue device properly positioned downstream of the wing tip causes vortex breakdown. This same result has been obtained by mounting a jet engine simulator at the wing tip and directing the high-energy jet blast downstream into the vortex. These configurations, among others, are now under intensive investigation in the new Langley Vortex Research Facility. In this facility, a balance mounted vortex generating model is propelled along the 1800-ft track while a second model trailed at 160 ft (scale distance of 1 mile) measures the far-field rolling moment induced by the vortex of the generating model.

Near-Wakes of Elliptic Cylinders in Subcritical Flow

Abstract: The near-wake aerodynamics of a set of two-dimension al, stationary, elliptic cylinders is studied experimentally during the organized wake condition (Reynolds number = 68,000) as a function of the cylinder eccentricity and angle of attack, An extensive scanning of the pressure field provides useful information concerning the vortex formation region, longitudinal and lateral spacings between the vortices, the classical wake geometry ratio, and the vortex velocity. A flow visualization study through the Schlieren technique in conjunction with high-speed photography gives preliminary information about location of the separating shear layers, their oscillations, and position of the first vortex. The rise and decay of the unsteady pressure in the vortex formation region appears to substantiate the visual observation concerning the first vortex. = mean static pressure coefficient, (p-p^0 = ratio of the maximum fluctuating pressure (average amplitude) at a transverse station in the wake to the corresponding value on the surface of the model for a given angle of attack, PH/(Pm)max = longitudinal spacing between vortices = Reynolds number, V.^2a/v — Strouhal number based on the projected height of the model, Nomenclature CDh = mean drag coefficient based on the projected height, pressure

Theory of Discrete Vortex Noise

Abstract: A solution is obtained for the noise induced by vortex shedding from the trailing edge of a semiinfinite flat plate. The method of solution for the potential flow is based on a procedure whereby an inner incompressible flow is matched to an outer acoustic field. Acoustic wave fronts are calculated and compared qualitatively with schlieren photographs of similar flows. The acoustic directivity pattern is obtained, and the extreme sensitivity of this parameter to the incompressible flow near the trailing edge of the plate is investigated.

Recent developments in dynamic stall

Abstract: Tests were performed on an airfoil in a low-turbulence subsonic wind tunnel at fixed incidence for different speeds of the wind, the airfoil being in translatory motion normal or parallel to the wind. The resulting aerodynamic forces and moment on the airfoil were measured along with local pressure and skin friction. When the incidence is small and the boundary layer attached, no unsteady effects were observed. However, for a large incidence (16 deg), in the case of boundary layer separation, strong unsteady effects are revealed when the boundary layer is reattached in periodic flow. Reattachment can be explained assuming that for the same value of free stream velocity, the gradient of the modulus of the periodic component of the relative airspeed at the upper surface of the airfoil in a direction normal to this surface is greater than the gradient of the relative air speed at the upper surface when the airfoil is not oscillating.

A numerical study of boundary effects on concentrated vortices with application to tornadoes and waterspouts

Abstract: This paper extends the numerical study of the structure and development of a concentrated vortex by Leslie (1971), in which a vortex is simulated by suddenly imposing an upwards body force along a section of the vertical axis of a contained rotating fluid, initially in a state of uniform rotation. Whereas the former paper was concerned primarily with demonstrating the prediction of Morton (1969) that a concentrated vortex may be generated only for a restricted range of the flow parameters, the present paper investigates the important role of boundaries on vortex behaviour. Particular interest is focused on the boundary which is normal to the vortex core and ‘behind’ the body force. On this boundary the surface stress is related to the surface velocity by a drag coefficient CD and experiments are performed in which CD is infinite, unity and zero corresponding with a no-slip, a partially yielding and a free-slip boundary respectively. These calculations are motivated by the desire to assess what differences, if any, between tornadoes (which develop over land) and waterspouts (which develop over the sea) can be attributed to the different surface constraint. We also study the effect on a vortex due to an abrupt change in surface condition as this is relevant to the behaviour of a tornado which happens to cross a water surface, or even one which traverses ground with varying roughness characteristics, and conversely to the behaviour of a waterspout which moves over land. It is shown that the strength of the meridional circulation associated with the vortex, and hence the strength of the upflow in the vortex itself, are increasing functions of the surface stress. On the other hand, the azimuthal kinetic energy, and in particular the strength of the vortex as measured by the maximum swirling velocity attained, decreases as the surface stress increases. Moreover, if the drag coefficient is suddenly increased, the meridional circulation increases, the azimuthal kinetic energy decreases and the vortex width (as measured by the radius of the maximum swirling velocity at a given height) increases. These effects are reversed if the drag coefficient is decreased and do not depend on the frictional effect of the container side wall. The role of the side wall itself is briefly explored. The results accord with the behaviour of laboratory vortices formed in air over surfaces of different roughness as studied by Dessens (1972). They also appear consistent with the observation reported by Golden (1971, p. 146) concerning the behaviour of a waterspout whose circulation decreased rapidly and visible funnel expanded during a traverse of about a kilometre over land and which subsequently reformed on moving back over water.

Impulsive motion of a circular disk which causes a vortex ring

Abstract: Vortex ring parameters such as circulation and translational velocity are estimated from the impulsive motion of a circular disk which generates the vortex ring. A comparison between predicted values of the vortex ring parameters and actually measured values is given.

Summary of results indicating the beneficial effects of rotor vortex modification

Abstract: The possibility of reducing blade-slap noise and high-frequency air loads and of increasing performance by tip air mass injection (TAMI) is investigated. The discussion is limited to two types of TAMI: chordwise injection with air injected at the blade tip in a chordwise direction and spanwise injection with air injected in a spanwise direction. Experimental and analytical results indicate that a properly designed TAMI system can restructure the near field characteristics of a concentrated vortex that trails off the tip of a lifting surface, that the tip vortex is spread by the application of chordwise TAMI, that mixing exists between the injected air mass and the vortex flow which enhances vortex decay, that the net power requirements to implement the system on operational helicopters are within acceptable levels, that maximum drop of noise level reductions of about 25 dB can be achieved, and that a decrease in the drag induced by the tip vortex in a spanwise TAMI system can be obtained by restructuring the vortex as it forms along the airfoil chord and by moving the vortex farther outboard of the tip.

Free vortex aircraft

Abstract: An aircraft has air pumping devices which retain, concentrate and augment vorticity shed from a surface of the moving aircraft in such a way as to allow the formation of a lift producing and drag reducing free vortex which travels along with the aircraft (standing vortex), together with a means of supplying thrust for the aircraft. Air flowing past a shield which extends upwardly from the forward edge of a horizontal base causes the shedding of vorticity of one sign into a cavity-like region formed by the shield, base and the housings of a pair of thrusters at the lateral edges of the base. Each thruster includes a cylindrical housing open at both ends through which air is pumped by a power driven propeller, the blades of which rotate in a generally vertical plane, to supply forward thrust for the aircraft. Each housing has an opening to the cavity-like region forward (or up-wind) of the propeller so that air also is pumped from the cavity-like region. This pumping action retains, augments, and concentrates the vorticity and results in the formation of the free vortex stretching transversely across the cavity-like region. The lift producing ability of the free vortex may be further enhanced by an air pumping device, preferably a power driven propeller having an inclined shaft placed centrally in the cavity-like region which augments the free vortex air flow. Alternatively, vorticity shed from a surface of the moving aircraft may be retained, concentrated, and augmented to form a free vortex by a pair of rotating bladed discs which are placed at inclined angles at the edges of the aircraft. The vortex extends from one disc to the other. The forward thrust produced by the pumping action of the rotating discs may be supplemented by powered propellers.

Transport in a turbulent vortex ring

Abstract: Transport in a turbulent vortex ring is considered; the mass of a passive impurity transported by the vortex is found as a function of distance traveled via a formula that agrees closely with experiment. A method is indicated for filling the part of the vortex that transports the impurity without loss. The effects of the following factors on the transport have been examined: initial Reynolds number, roughness in walls of the exit hole, and density difference between the solution and the medium. The transport of aerosols and suspended particles by vortex rings is considered. Two methods of transport measurement are compared.

Vortices and Vortex Rings of Stratified Fluids

Abstract: Exact solutions for finite-amplitude motion of an inviscid, incompressible and stratified fluid corresponding to rectilinear vortices or vortex rings are found. Some of the rectilinear vortex pairs, in circular-cylindrical form, can propagate with a constant horizontal velocity in a surrounding homogeneous fluid, and some of the spherical vortex rings can be placed in a surrounding quiescent homogeneous fluid without translational propagation. Equations governing finite-amplitude two-dimensional motion and axisymmetric motion with swirl in the presence of a magnetic field are derived for a stratified conducting fluid, and the effects of a magnetic field on vortex motion are ascertained.

Motion of a vortex pair approaching an opening in a boundary

Abstract: The flow properties of liquid helium are such that the motions of vortex rings and line vortices can be calculated from hydrodynamic potential theory. The two‐dimensional (line vortex) case allows a relatively simple method of calculating vortex motion: The ’’Routh path function’’ yields a solution through conformal mapping. Using this method, the case of a rectilinear vortex pair approaching a slit in a wall is analyzed. Criteria for their just passing through are found.

The Second Stability Boundary for Circular Couette Flow

Abstract: To investigate the stability of an axisymmetric vortex flow between rotating cylinders, the development of non-axisymmetric small disturbance is analyzed by a systematic expansion procedure in the amplitude of the vortex. The amplification rate of the disturbance is determined as a power series in the amplitude. The series is truncated with the fourth order in the numerical calculation. It is found that the disturbance grows most rapidly when its axial wave number coincides with that of the vortex. The second boundary representing the stability criterion of the vortex flow is calculated for a small gap of the cylinders. When the ratio of angular velocities is specified, the critical value of axial wave number for the vortex flow is found to be smaller than the critical wave number given by the classical stability problem.

A static air flow visualization method to obtain a time history of the lift-induced vortex and circulation

Abstract: A recently proposed method of flow visualization was investigated at the National Aeronautics and Space Administration's Langley Research Center. This method of flow visualization is particularly applicable to the study of lift-induced wing tip vortices through which it is possible to record the entire life span of the vortex. To accomplish this, a vertical screen of smoke was produced perpendicular to the flight path and allowed to become stationary. A model was then driven through the screen of smoke producing the circular vortex motion made visible as the smoke was induced along the path taken by the flow and was recorded by highspeed motion pictures.

Smokestack-mounted airfoil

Abstract: A system for improving the effluent dispersal characteristics of smokestacks subject to relative winds comprising a vortex generating airfoil attached to a smokestack near the stack gas exit. Relative winds passing over the airfoil create strong vortices which entrain and hold together smokestack effluents until the vortices deteriorate. The vortex flow direction and angle of ascension may be controlled in order to achieve optimum effluent dispersal by varying the airfoil angle of attack.

On the Development of a Unified Theory for Vortex Flow Phenomena for Aeronautical Applications

Abstract: : The low-speed performance of a high span loading aircraft depends critically on the structure and stability of the vortex flow field created by the wing Conventional formulation of lifting surface theory is not adequate to handle the low-aspect-ratio wing problem since the leading edge vortices add several complications Specifically, the nonplanar nature of the vortex sheet may have to be considered explicitly, the leading-edge loading is altered, and vortex breakdown is a rotational phenomenon Due to those additional difficulties, a study was made of the state of the art in vortex-wing interactions