Showing papers on "Starting vortex published in 1983"

Numerical Study of Incompressible Slightly Viscous Flow Past Blunt Bodies and Airfoils

Abstract: A grid-free numerical method is used to simulate incompressible flow at high Reynolds numbers. The numerical method simulates the flow inside the boundary layer by vortex sheets and the flow outside this layer by vortex blobs. The algorithm produces a smooth transition between the sheets and the blobs.The accuracy of this hybrid numerical method is tested in several numerical experiments. In the first experiment, the algorithm is used to simulate slightly viscous flow past a circular cylinder. In the second experiment, the algorithm is used to simulate flow past a Joukowski airfoil at various angles of attack. In the latter case, the computations simulating the flow at the airfoil's trailing edge do not “blow-up”. In both experiments, the calculated flow and its functionals (such as lift and drag coefficients) are in good agreement with both theoretical results and wind tunnel experiments.

A higher order panel method applied to vortex sheet roll-up

Abstract: The paper describes a computational method for two-dimensional vortex sheet motion in incompressible flow. The procedure utilizes a second-order panel method, an adaptive panel scheme, and a concept for treating highly rolled-up portions of the vortex sheet. Results are presented for the roll-up of the wake behind an elliptically loaded wing, a ring wing (nacelle), a fuselage/part-span flap/wing combination, and a delta wing with leadingedge vortex sheets. The examples demonstrate that the method is capable of describing complicated vortex sheet motion in a reliable and stable manner. v \

Interactions of Airfoils with Gusts and Concentrated Vortices in Unsteady Transonic Flow

Abstract: Unsteady interactions of concentrated vortices and distributed free-stream gusts with a stationary airfoil have been analyzed in two-dimensional transonic flow. A simple method of introducing such disturbances has been implemented numerically in the well-known transonic small-disturbance code LTRAN2, and calculations have been performed for two important classes of current aerodynamic problems. The first, which demonstrates many of the essential features of the interactions between helicopter rotor blades and their trailing-vortex wakes, is that of a discrete potential vortex convecting past an airfoil. The second is the response of a transonic airfoil to a transverse periodic gust, with and without the alleviation that can be achieved by the proper active control motion of a trailing-edge flap. In both cases, unsteady effects are found to play important roles in the shock-wave motion, in the overall flow-field development, and consequently, in the air loads on the airfoil.

Passive drag control of airfoils at transonic speeds

Abstract: An airfoil for transonic speeds includes a porous top surface extending from a location about 50 to 60% of the chord length from a leading edge of the airfoil to a location about 80 to 90% of the chord length from the leading edge. A cavity is defined under the porous surface in the airfoil which has a depth of from 0.05 to 0.2% of the chord length. The porosity of the porous surface is chosen to be from 1 to 3% of the total airfoil surface and may be variable. The presence of the porous surface and cavity decrease airfoil drag at transonic speeds by providing a pathway between a high pressure area downstream of a shock wave formed on the airfoil at transonic speeds to a low pressure area within a bubble on the airfoil upstream of the shock wave.

Experimental observations of vortex ring interaction with the fluid adjacent to a surface

Abstract: : Experimental studies examined the breakdown of initially laminar vortex rings during impact with both solid and free surfaces in a quiescent environment, and with a solid surface beneath a developing laminar boundary layer. Flow interactions were visualized in water using dye and hydrogen-bubble techniques and recorded with a high-speed video system. When a vortex ring approaches a surface the resulting flow interaction appears to be chaotic and turbulent, but is actually a very organized viscid-inviscid process which rapidly disperses the vorticity of the vortex ring throughout the surrounding fluid. Described is the flow interaction which integrates the following phenomena: (1) generation of secondary vorticity of opposite sense to that of the vortex ring; (2) deviations in the trajectory of the vortex ring from that predicted by classical theory; and (3) the processes of organized dispersal of vorticity. The process by which vorticity dispersal occurs is dependent upon the initial Reynolds number (Re sub 0) of the vortex ring. For very weak rings, i.e. Re sub o less than 350, vorticity is dispersed by laminar diffusion. For stronger rings, vorticity dispersal occurs discretely through formation of secondary and tertiary vortex rings (SVR and TVR) via a viscous boundary layer process. Vorticity dispersal continues as a result of Biot-Savart-type interactions of the SVR and TVR with the original or primary vortex ring. During this interaction the diameter of the SVR is compressed, causing an instability in the SVR which is characterized by an azimuthal waviness.

An investigation of wing leading-edge vortices at supersonic speeds

Abstract: Studies at subsonic and transonic speeds of the fundamental vortex behavior on the leeward surface of wings have led to the design of several unique and novel leading-edge devices commonly referred to as 'vortex flaps'. The present investigation has the objective to provide some fundamental vortex-flow results obtained at supersonic speeds. Experimental studies were performed in which pressure data and several types of flow visualization data were obtained on the leeward surface of a series of flat delta-wing models to identify the various flow mechanisms which can occur and to determine the effect of leading-edge sweep, Mach number, and angle of attack on the vortex strength and location. The reported investigation forms part of a study which is to explore the use of wing leading-edge vortex technology as a supersonic wing-design tool. The obtained results indicate that the procedure of distributing the vortex force as a pressure variation about a vortex action line is a promising concept.

Nacelle/wing assembly with vortex control device

Abstract: In a nacelle and wing combination where a vortex is shed from a side surface portion of the nacelle and said vortex burst prematurely to reduce lift, the improvement comprising a vortex control device located in an upwash region along the side of the nacelle to cause said vortex to form in the critical vortex control region where boundary layer is relatively thin and local velocity is relatively high. This strengthens the vortex to inhibit vortex bursting at the burst location.

Numerical Simulation of Vortex Breakdown by the Vortex-Filament Method,

Abstract: The vortex filament method was applied to the simulation of vortex breakdown. The principal vortex region was represented by multiple filaments, and an axial velocity component was induced by a spiral winding of the filaments. First, an accuracy check was performed for a cylindrical swirling flow with simple analytical expressions for the axial and theta velocities. The result suggests that the flow field is simulated to any accuracy by increasing the number of filaments. Second, an axisymmetric type vortex breakdown was simulated, with experimental data serving as upstream conditions. The calculated axial and theta velocity contours show the breakdown of the vortex, including a rapid change in the vortex core, followed axially by a recovery zone and then a second breakdown. When three dimensional initial data are used the second breakdown appears to be of the spiral type in correspondence with experimental observations. The present method is easily used to simulate other types of vortex breakdown or other vortex flows with axial velocity.

Numerical simulations of tornado-like vortices. Part 1: Vortex evolution

Abstract: An axi-symmetric numerical vortex model suitable for studies of intense atmospheric vortices such as tornadoes and waterspouts is described. The model uses the vorticity- streamfunction form of the equations of motion and unlike earlier models has a stretched grid in the radial direction, providing for economical resolution of both the vortex core and the rotating cloud updraught in which it forms. The model incorporates an axial body force to represent the driving effect of cloud buoyancy, although a simulation is described also in which the latter is modelled directly by including a moisture scheme with latent heat release. In the latter case we have not found parameter values for which a steady flow is attained. The dynamics of vortex evolution conforms broadly with the descriptions given by Leslie (1971) and Smith and Leslie (1978, 1979), an additional feature being that significant intensification occurs following vortex touchdown, before the flow becomes steady. During this phase, a transie...

A study of the unsteady flow field of an airfoil with deflected spoiler

Abstract: The flow field generated by a two-dimensional airfoil with a statically deflected flap type spoiler has been experimentally studied. Subsonic tests ascertained the effect of varying spoiler deflection and angle of attack on the flow field at two Reynolds numbers. Measurements of the mean and fluctuating surface pressures and velocities in the wake, along with schlieren flow visualization of the near wake, were made. The results pertaining to the unsteady characteristics of the spoiler flow field are presented. It is shown that vortex shedding characterized the spoiler wake and determines the unsteady nature of the overall flow field; the character of the vortex shedding changes with spoiler deflection; and the fluctuating surface pressure field (vortex shedding frequency component) is of appreciable magnitude at large spoiler deflections.

A Numerical Analysis of Unsteady Separated Flow by Vortex Shedding Model : 1st Report, Flow around a Square Prism

Abstract: A new discrete vortex method which combines the vortex shedding model with the singularity method has been developed for the analysis of an unsteady separated flow. As this method does not use the transformation, it can be more easily applied to the unsteady flow analysis around a body of complicated shape than usual discrete vortex methods. The method is applied to the flow calculations around a square prism with angles of attack 0°, 10°, 20°, and 30°. The calculated values of the lift and drag coefficients and the Strouhal numbers are compared with the available experimental data in good agreement.

Alleviation of the subsonic pitch-up of delta wings

Abstract: A "pylon vortex generator" concept has been proposed for alleviating the subsonic pitch-up instability of highly swept supersonic-cruise wings. Fixed at a part-span location, this leading-edge device utilizes the lateral velocity component in the flow approaching the swept wing to generate a streamwise vortex, rotating in a sense opposite to the normal leading-edge vortex. The resulting upwash reduction outboard of the device serves to delay the movement of the leading-edge vortex out of the tip regions with increasing angle of attack, thus improving the tip lift characteristics. Wind tunnel results on 60- and 74-deg flat-plate delta wings are presented to demonstrate the operation of the pylon vortex generator in modifying the spanwise leading-edge separation and vortex characteristics of the basic delta wings, resulting in effective pitch-up alleviation.

A multi‐vortex model of leading‐edge vortex flows

Abstract: SUMMARY A multi-vortex model of the vortex sheets shed from the sharp leading edges of slender wings is considered. The method, which is developed within the framework of slender-body theory, is designed to deal with those situations in which more than one centre of rotation is formed on the wing, for example on a slender wing with lengthwise camber or with a strake. Numerical results are presented, firstly for situations where comparison can be made with a vortex sheet model and secondly for cases, such as those described above, where a vortex sheet model is unable to describe the flow. Where comparison is available, agreement is good and in the cases where more than one vortex system is present interesting interactions are obtained. KEY Woms Low-aspect Ratio Wings Leading-edge Separation Vortex Shedding

Multiple jet blowing around the blunt trailing edge of a circulation controlled airfoil

Abstract: A circulation controlled airfoil has a body structure with leading and trailing ends and upper and lower surfaces extending therebetween, and a cylindrical member disposed along the trailing end of the body structure between the upper and lower surfaces thereof so as to form a rounded, blunt trailing edge surface on the airfoil. A first blowing jet slot is defined between the outer surface of the cylindrical member and the trailing end of the body structure adjacent its upper surface. A second blowing jet slot is defined in a tubular wall of the cylindrical member. The latter member can be rotated to vary the distance of the second jet slot from the first jet slot and thereby adjust the point of separation of air flow along the airfoil from the outer surface of the blunt trailing edge surface formed by the cylindrical member. In such manner the lift generated by the airfoil can be optimized by providing blowing that is specifically tailored to given flight conditions. Also, the separate plenums are provided for supplying air flow to the respective jet slots.

An Investigation of the Effects of Discrete Wing Tip Jets on Wake Vortex Roll Up.

Abstract: : A water tunnel experiment and a computational flow field model show that discrete wing tip jets can strongly affect the roll up of the wing tip vortex and apparently decrease its rolled up strength at moderate levels of blowing. The key factor in vortex alleviation was the extent of the local flow interactions between the discrete jets and the developing wing tip vortex. Vortex trajectory in both the spanwise and vertical directions was influenced by the jets. An outboard shift of the wing tip vortex indicated that discrete wing tip jets may be able to produce improved wing aerodynamics during cruise flight. The counterrotating pair of vortices generated by a jet in a cross flow were clearly seen in the water tunnel and appeared to be very effective in reducing the intensity of the wing vortex system. Two types of periodic secondary vortices were also observed in the water tunnel for heavy jet blowing. These were spin-off vortices which periodically developed in the rolling up tip vortex but rapidly spun outboard and above the wing; and entrained vortices which was a set of periodic vortices laterally connecting the wing tip vortex to the vortices embedded in the jet. These secondary vortices are oriented such that they will greatly accelerate the spreading of wake vorticity through the vortex stretching term of the Helmholtz equation. This influence was confirmed in the water tunnel tests.

The Effect of Trailing Vortices on the Production of Lift on an Airfoil Undergoing a Constant Rate of Change of Angle of Attack.

Abstract: : The purpose of this study was to investigate the effect a trailing vortex wake has on an airfoil undergoing a constant rate of change of angle of attack, alpha, in two-dimensional, incompressible, irrotational flow. Potential flow theory, conformal mapping by the Joukowski transformation, and numerical integration and differentiation techniques were used to develop a computer algorithm to model the problem. Once the program was formulated, it was used to solve the impulsive-start problem of airfoil motion. The results were found to be in excellent agreement with the results obtained by others. When applied to the constant rate-of-change of angle-of-attack problem, the results showed that a trailing vortex wake has a measurable and predictable effect on the production of lift on an airfoil undergoing a constant alpha. While results of this work, taken alone, are helpful in understanding the phenomena known as dynamic stall, coupled with existing boundary-layer studies the results may lead to additional understanding of the phenomena. More specifically, the computer program develop here could be used to predict more realistically the inviscid flow about a pitching airfoil as it approaches the dynamic-stall conditions.

On the Nonlinear Lift Characteristics of Slender Bodies at Incidence

Abstract: Nonlinearity in normal force for a slender body with small incidence to flow is discussed by the asymptotic expansion method under the usual assumptions, 1>>e>>1/√Rn and k=β/e=O (1), where e is a slenderness parameter, β an angle of attack, k relative angle of attack, and Rn Reynolds number. The main purpose of the present paper is to develop the method to determine the separation line and the strength of the vortex sheet separating from there. The near field is composed of continuous vortex sheet whose complex potential is expressed with the circulation as a parameter, which is measured from the end point of vortex sheet to the point considered. Kutta's condition to be used to determine the strength density of the vortex sheet near the separation line is proposed by investigating the characteristics of the governing equation of vortex sheet shape. It is shown that this condition is consistent with the vortex conservation law. A method is proposed to solve simultaneously potential stream-lines and boundry layer parameters including the effects of the vortex sheet by only one marching downstream. The shape of the vortex sheet and the separation line are calculated and the normal force coefficient is compared with experimental results.

Tapered-ring vortex chamber for extreme acceleration of the vortex medium

Abstract: The side walls of the tapered-ring vortex chamber extend inwards in a funnel shape, so that they form a tapered ring with the circular outer wall. The vortex material injected through the outer wall at an angle to form a vortex can flow out in the vicinity of the vortex core, because the funnel walls are open at that point. Conventionally, the vortex chamber narrows towards the vortex centre with the square of the approach. Due to the tapered-ring shape, the chamber space tapers inwards also in the width of flow, so that the inward narrowing of the space goes with the cube. As a result, the vortex medium in the tapered-ring vortex chamber is subjected to an extremely high acceleration. Industrial utilisation in: 1. centrifugal separation equipment, 2. pre-acceleration of the propellant medium for liquid turbines and pre- and re-acceleration for gas turbines, 3. pulsed combustion chambers with vortex acceleration, 4. pulsed combustion turbines, 5. ultrahigh-speed small turbines and 6. extreme high-speed water turbines with a blade drive for cutting by vortex water. The drawing shows the tapered-ring vortex chamber with an air cushion antechamber for liquids, to reduce the inflow resistance. Tapered-ring vortex chamber with air cushion antechamber for high-speed liquid turbines: 1. Vortex liquid in the tapered-ring vortex chamber and in the air cushion antechamber with the centrifugal force rising outwards. 2. Injection orifice into the air cushion antechamber. 3. Air cushion antechamber. 4. Air cushion buffer space. 5. Compressed-air feed, if necessary. 6. Overflow ring orifice. 7. Vortex outflow orifice. 8. Turbine shaft. 9. Open outflow chamber.

Row Depth Effects on Vortex Shedding and Turbulent Buffeting Formation

Abstract: P vortex shedding or buffeting excitations caused by flow passing through finned tube banks have caused acoustic resonance problems in some boilers. The resonance is caused by the coincidence of the natural acoustic frequency with the oscillations in the flow through the tube bundle. When published data failed to adequately correlate the tones, an extensive laboratory test program was undertaken to extend the range of available data.

Secondary flows beyond a bend of a pressure conduit

Abstract: 1. Experiments established that a 90° bend of a flow creates secondary flows, which are formed by four vortex filaments of contrary direction of rotation and not by two as considered earlier. 2. All parameters characterizing the secondary flows can be obtained by calculation on the basis of the values of the vorticity of the secondary vortex filaments and their coordinates. 3. The developed vortex current meter permits detecting stationary vortex filaments in a flow, determining their coordinates, and measuring their vorticity.

Flow past an airfoil at an angle of attack to an oncoming sonic stream

Abstract: Guderley [1] proposed a theoretical example of sonic flow of gas past a symmetric airfoil at zero angle of attack to an oncoming stream. The linear distribution of the velocity over the airfoil surface was found from the solution describing this flow. It is of interest to generalize this example to the case of nonsymmetric sonic flow past an airfoil. In the present paper a study is made of sonic flow past a Guderley airfoil at an angle of attack and a study is made of the dependence of the velocity over the surface of the airfoil on the angle of attack. This dependence is obtained by means of the parabolic method of Spreiter and Alksne [2], which makes it possible to solve this problem directly in the plane of the flow. For comparison, flow past a parabolic airfoil at an angle of attack is considered, and an expression is obtained for the dependence of the distribution of the velocity over the surface on the angle of attack, this being a generalization of the formula of Zierep [3], who considered sonic flow past a parabolic airfoil at zero angle of attack.