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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Horseshoe vortex formation around a cylinder

An experimental investigation was conducted to characterize a symmetrical horseshoe vortex system in front of and a around a single large-diameter right cylinder centered between the sidewalls of a wind tunnel. Surface flow visualization anad surface static pressure measurements as well as extensive mean velocity and pressure measurements in and around the vortex system were acquired. The results lend new insight into the formation and development of the vortex system. Contrary to what has been assumed previously, a strong vortex was not identified in the streamwise plane of symmetry, but started a significant angular distance away from it. Rather than the multiple vortex systems reported by others, only a single primary vortex and saddle point were found. The scale of the separation process at the saddle point was much smaller than the scale of the approaching bundary layer thickness. Results of the present study not only shed light on such phenomena as the nonsymmetrical endwall flow in axial turbomachinery but can also be used as a test case for three-dimensional computational fluid mechanics computer codes.

Eckerle, W. A.↗
NASA NTRS1987

Navier-Stokes simulations of blade-vortex interaction using high-order accurate upwind schemes

Conventional, spatially second-order-accurate, finite-difference schemes are much too dissipative for calculations involving vortices that travel large distances (relative to some measure of the size of the vortex). This study presents a fifth-order-accurate upwind-biased scheme that preserves vortex structure for much longer times than existing second-order-accurate central and upwind difference schemes. Vortex calculations demonstrating this aspect of the fifth-order scheme are also presented. The method is then applied to the blade-vortex interaction problem. Results for strong interactions wherein the vortex impinges directly on the airfoil or a shock associated with the airfoil are presented. None of these calculations required any modeling of the shape, size, and trajectory of the interacting vortex.

Rai, Man Mohan↗
NASA NTRS1988

Computational methods for the problems of the tip vortex

The tip vortex problem for helicopter rotors in transonic flow was studied. The first model was that of potential flow past a single blade in parallel transonic flow. The incidence on the wing of a vortex sheet trailing from a blade in front was studied by modeling the incident sheet by a single vortex filament. A 3-D potential flow code was written to incorporate the usual boundary conditions at infinity together with the model of such a vortex filament and to analyze the resulting flow over the wing. The code was used to study the effect of the vortex filament on the production of shocks at the tip of the wing. Another problem studied within the framework of potential flow theory was the rollup of the vortex sheet behind the wing. A comparison was made of flow models described by the potential equation, by the Euler equations for steady flow, and by the Navier-Stokes equations. The question was also studied as to whether there is an advantage in solving the Navier-Stokes equations rather than the Euler equations for steady flow.

Garabedian, Paul R.↗
NASA NTRS1987

Experimental observations of two dimensional blade-vortex interaction

Blade-vortex interaction (BVI) is the source mechanism for a prominent impulsive noise created by rotorcraft. An experimental study of two dimensional BVI, a simplified case representing the most intense and impulsive noise generation condition, was performed to explore the fundamental physics associated with the blade-vortex interaction process. The purpose of this paper is to report cumulative results from this experimental study. The interaction process is shown to involve a combination of effects on the vortex including changes in trajectory and distortion of the vortex core shape. The blade involved undergoes significant transient loading. The effects of the interaction process on the blade and the vortex are shown to be increased by both reduction of blade-to-vortex spacing and increase in blade loading. Calculations of the acoustic field produced by the interaction using measured unsteady blade surface pressure data are presented and show acoustic waveforms similar to those obtained from model helicopter acoustic tests.

Booth, E. R., Jr.↗
NASA NTRS1988

Spur-type instability observed on numerically simulated vortex filaments

An instability observed on vortex filaments during numerical simulations of the three-dimensional, time-dependent dynamics of vortex wakes is studied to determine when and why it occurs. It is concluded that the observed instability is a consequence of the use of straight-line vortex segments of finite length to model continuously curving vortex filaments. The instability appears to occur only when the link length is a sizable fraction of the vortex span and, therefore, is not expected in an experiment. Guidelines are then given that help avoid numerical instabilities when vortex filaments are used in flow simulations.

Rossow, Vernon J.↗
NASA NTRS1989

Characteristics of the ground vortex formed by a jet moving over a fixed ground plane

This paper discusses an experimental study conducted in the Langley Vortex Facility to investigate the effects on the ground vortex of the jet passing over a fixed ground board. A jet impacting the ground can form a vortex which may materially affect the aerodynamic characteristics of a STOL airplane operating near the ground. Several studies have been done with a stationary jet exiting near and perpendicular to fixed ground board. The resulting ground effects have been documented in terms of ground vortex and aerodynamic characteristics. The ground boundary layer created in a wind tunnel facility, however is thought to affect the extent of the ground vortex. This paper reports on an investigation utilizing an isolated moving jet to eliminate the ground boundary layer. The results are compared to the existing data base and show a 30 percent decrease in the vortex penetration shown by the stationary jet conditions.

Kemmerly, G. T.↗
NASA NTRS1989

On leading edge vortex and its control

A simple model for the leading edge vortex and a postulation for the existence of a critical vorticity concentration above which a stable leading edge vortex cannot be maintained were proposed. Using the model and postulation, various aspects of vortex control by blowing were discussed. Blowing can be envisioned as a means to control the effective sweep and span of a delta wing by redistributing the vorticity. Mixing between the blowing jet and the vortex has a major effect on the effectiveness of controlling the vortex. Different positioning of one vortex relative to another can either enhance or delay the breakdown of the vortices.

Ng, T. T.↗
NASA NTRS1988

Computation of the tip vortex flowfield for advanced aircraft propellers

The tip vortex flowfield plays a significant role in the performance of advanced aircraft propellers. The flowfield in the tip region is complex, three-dimensional and viscous with large secondary velocities. An analysis is presented using an approximate set of equations which contains the physics required by the tip vortex flowfield, but which does not require the resources of the full Navier-Stokes equations. A computer code was developed to predict the tip vortex flowfield of advanced aircraft propellers. A grid generation package was developed to allow specification of a variety of advanced aircraft propeller shapes. Calculations of the tip vortex generation on an SR3 type blade at high Reynolds numbers were made using this code and a parametric study was performed to show the effect of tip thickness on tip vortex intensity. In addition, calculations of the tip vortex generation on a NACA 0012 type blade were made, including the flowfield downstream of the blade trailing edge. Comparison of flowfield calculations with experimental data from an F4 blade was made. A user's manual was also prepared for the computer code (NASA CR-182178).

Tsai, Tommy M.↗
NASA NTRS1989

Improved vortex methods for three-dimensional flows

Robust numerical methods are developed for three-dimensional incompressible vortical flows, using Lagrangian vortex elements. A successful scheme must be able to handle regions of intense vortex stretching and vortex reconnection with reasonable accuracy (without diverging). Here, consideration is given to vortex particles, also commonly called vortons or vortex sticks. The following issues are discussed: (1) use of delta-function elements and weak solutions of the vorticity equation; (2) use of smoothed elements and the choice of the smoothing function; (3) representation of viscous effects and the redistribution of element strength; and (4) conservation laws (are they satisfied?). The various proposed schemes have been tested on flows involving a strong interaction between two vortex rings.

Winckelmans, G.↗
NASA NTRS1990

Propeller tip vortex interactions

Propeller wakes interacting with aircraft aerodynamic surfaces are a source of noise and vibration. For this reason, flow visualization work on the motion of the helical tip vortex over a wing and through the second stage of a counterrotation propeller (CRP) has been pursued. Initially, work was done on the motion of a propeller helix as it passes over the center of a 9.0 aspect ratio wing. The propeller tip vortex experiences significant spanwise displacements when passing across a lifting wing. A stationary propeller blade or stator was installed behind the rotating propeller to model the blade vortex interaction in a CRP. The resulting vortex interaction was found to depend on the relative vortex strengths and vortex sign.

Johnston, Robert T.↗
NASA NTRS1989

Experiments on the unsteady ground vortex

The ground vortex formed by a jet impinging on the ground in the presence of a cross flow has been studied experimentally. High speed motion pictures and spectral measurements were obtained to study the unsteady features of this flowfield. A very low frequency 'puffing' action was observed. Since this unsteadiness could not be correlated with any other oscillations in the flowfield, the low frequency oscillations must come from the gross features of the ground vortex itself. Namely, jet fluid accumulates in the ground vortex until the vortex is so large that the flowfield breaks up, the ground vortex is swept away, a new smaller vortex forms, and the process repeats itself.

Cimbala, John M.↗
NASA NTRS1990

Multiple vortex and shock interactions at subsonic, transonic, and supersonic speeds

A wind tunnel experiment was conducted at subsonic, transonic, and supersonic speeds of the vortex-vortex and vortex-shock interactions about a tailless, general research fighter model having chine-like forebody strakes faired into a 55 deg cropped delta wing. The present paper isolates the results obtained at angle of attack 20 deg and free-stream Mach = 0.6 to 1.6, which include off-surface and on-surface flow visualizations, two-component laser velocimeter measurements, and wing upper surface static pressure distributions. Increasing the Mach number decreased the direct interaction (intertwining) of the forebody strake and wing vortex cores. An early bursting of the wing vortex occurred at free-stream Mach = 0.8, where the flow field was in transition from the intertwining vortices characteristic of the lower subsonic speeds to the decoupled vortices at the transonic and supersonic speeds. The vortex interaction and breakdown were sensitive to the character of the secondary boundary layer separation on the wing, which may be shock-induced at free-stream Mach = 0.8 to 0.95.

Erickson, Gary E.↗
NASA NTRS1990

The growth and development of a turbulent junction vortex system

The growth and development of a vortex system in an incompressible three-dimensional flow generated by a streamlined cylinder mounted with its axis normal to a flat surface was investigated using data from surface flow visualizations, surface pressure measurements, and mean flow measurements. The results from velocity measurements revealed the presence of a single large dominant vortex and a small vortex in the junction between the cylinder and the flat surface. This second vortex is tentatively identified as being connected with a corner separation occurring just downstream of the body at its thickest cross section. It is shown that, as the principal horseshoe vortex develops downstream, the strength of the vortex decreases, while its length scale increases.

Pierce, F. J.↗
NASA NTRS1991

Development of a perturbation generator for vortex stability studies

Theory predicts vortex instability when subjected to certain types of disturbances. It was desired to build a device which could introduce controlled velocity perturbations into a trailing line vortex in order to study the effects on stability. A perturbation generator was designed and manufactured which can be attached to the centerbody of an airfoil type vortex generator. Details of design tests and manufacturing of the perturbation generator are presented. The device produced controlled perturbation with frequencies in excess of 250 Hz. Preliminary testing and evaluation of the perturbation generator performance was conducted in a 4 inch cylindrical pipe. Observations of vortex shedding frequencies from a centerbody were measured. Further evaluation with the perturbation generator attached to the vortex generator in a 2 x 3 foot wind tunnel were also conducted. Hot-wire anemometry was used to confirm the perturbation generator's ability to introduce controlled frequency fluctuations. Comparison of the energy levels of the disturbances in the vortex core was made between locations 42 chord lengths and 15 chord lengths downstream.

Riester, J. E.↗
NASA NTRS1990

An assessment of full potential and Euler solutions for self-generated rotor blade-vortex interactions

An assessment is made of the accuracy and suitability of the three-dimensional full potential and Euler equations in the modeling of subcritical and supercritical rotor blade-vortex interactions (BVI). The interaction velocity fields are obtained through a nonlinear superposition of the rotor and the vortex wake flow fields. Vortex effects are simulated using the velocity 'transpiration' approach. A modified surface condition is prescribed and enforced at each time step of the computations to satisfy the tangency boundary condition. Potential blade-vortex encounters are identified and tracked in time at equal increments of rotor azimuth using the lifting-line helicopter/rotor trim code CAMRAD. This information is then utilized in interpolation routines within the flow solvers to compute the instantaneous position(s) of the interaction vortex element(s) with respect to the blade for the time-accurate calculations. Results for subcritical self-generated BVI and for supercritical interactions resulting from an upstream-generated vortex are presented.

Hassan, A. A.↗
NASA NTRS1991

Exploratory study of vortex-generating devices for turbulent flow separation control

Flow phenomena and the relative performance associated with several devices for controlling turbulent separated flow were investigated at low speeds. Vortex-generating devices examined included: submerged vortex generators (Wheeler doublet and wishbone types), spanwise cylinders, large-eddy breakup devices (LEBU) at small angle of attack (alpha), and vortex-generator jets (VGJ). Dye flow visualization tests in a water tunnel indicated that wishbone vortex generators in the forward orientation shed horseshoe vortices; wishbone vortex generators oriented in the reverse direction and doublet vortex generators shed streamwise counterrotating vortices; a spanwise cylinder located near the wall and LEBUs at alpha = -10 deg produced eddies which rotated with the same sign as the mean vorticity in a turbulent boundary layer; and the most effective VGJs produced streamwise corotating vortices. Comparative wind tunnel tests conducted on a curved backward-facing ramp indicated that transferring momentum from the outer region of a turbulent boundary layer by embedded streamwise vortices is more effective than by transverse vortices for separation control applications.

Lin, J. C.↗
NASA NTRS1991

Analysis of flow on cones and cylinders using discrete vortex methods

Discrete vortex methods have been developed to investigate the vortex flows on cones and two-dimensional cylinders. The cone problem was solved by assuming that, to meet conical flow and zero-force conditions, the vortices move radially away from the body at a given cross-section. The two-dimensional cylinder problem was solved by limiting the velocity along the zero streamline surrounding the vortex field to two times freestream velocity and by limiting the lateral movement of the vortices. Variations in vortex position and strength with time were determined by taking into account the rate at which circulation is generated at separation points on the body. The calculated vortex positions and strengths were in good agreement with available experimental data. Viscous effects could be accounted for by adding empirically determined damping terms to the velocity equations. The models indicate that different types of asymmetry occur for the cone and two-dimensional cylinder. Asymmetry onset boundaries determined by the discrete vortex method show the same trend as experiment.

Gainer, Thomas G.↗
NASA NTRS1990

Vortex Flow Aerodynamics

The AGARD Fluid Dynamics Panel sponsored this symposium to provide an update review of the aerodynamic effects of separation-induced vortex flows on the design and off-design performance of fighter aircraft. A better understanding is needed to predict and control these vortex flows throughout the flight envelope at subsonic, transonic, and supersonic speeds, and especially during high lift operations for take-offs, landings, and sustained and instantaneous maneuvers. Topics of interest included vortex development and burst, modeling and validation of the full range of analytic methods, slender-body vortex flows at high angles of attack, vortex control and management, and unsteady vortex flow effects. In addition to the symposium papers, the results of the round table discussion are presented.

delta wings↗