Search NASA⌕ Search

SEARCH · Search NASA

Results for “vortex structures”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Velocity measurements in a turbulent trailing vortex and their application to BWI noise prediction

The objectives were to observe the turbulence structure and spectral characteristics of the trailing vortex shed by a rectangular NACA 0012 wing over a range of conditions and to incorporate these observations into the blade-wake interaction (BWI) noise-prediction method of Glegg (1989). The following sections are presented: (1) measurements performed during the first year of this two year investigation; (2) presentation and discussion of a representative sample of the results; (3) implications for the BWI noise prediction method; and (4) re-evaluation of work planned for the second year.

Devenport, William J.↗
NASA NTRS2001

Devices that Alter the Tip Vortex of a Rotor

Small devices were attached near the tip of a hovering rotor blade 'in order to alter the structure and trajectory of the trailing vortex. Stereo particle image velocimetry (PIV) images were used to quantify the wake behind the rotor blade during the first revolution. A procedure for analyzing the 3D-velocity field is presented that includes a method for accounting for vortex wander. The results show that a vortex generator can alter the trajectory of the trailing vortex and that a major change in the size and intensity of the trailing vortex can be achieved by introducing a high level of turbulence into the core of the vortex.

McAlister, Kenneth W.↗
NASA NTRS1991

A study of three dimensional turbulent boundary layer separation and vortex flow control using the reduced Navier Stokes equations

A reduced Navier Stokes (RNS) initial value space marching solution technique was applied to vortex generator and separated flow problems and demonstrated good predictions of the engine face flow field. This RNS solution technique using FLARE approximations can adequately describe the topological and topographical structure flow separation associated with vortex liftoff, and this conclusion led to the concept of a subclass of separations which can be called vorticity separations: separations dominated by the transport of vorticity. Adequate near wall resolution of vorticity separations appears necessary for good predictions of these flows.

Anderson, Bernhard H.↗
NASA NTRS1991

A study of three dimensional turbulent boundary layer separation and vortex flow control using the reduced Navier Stokes equations

A reduced Navier-Stokes (RNS) initial value space marching solution technique was applied to vortex generator and separated flow problems and demonstrated good predictions of the engine face flow field. This RNS solution technique using FLARE approximations can adequately describe the topological and topographical structure flow separation associated with vortex liftoff, and this conclusion led to the concept of a subclass of separations which can be called vorticity separations: separations dominated by the transport of vorticity. Adequate near wall resolution of vorticity separations appears necessary for good predictions of these flows.

Anderson, Bernhard H.↗
NASA NTRS1979

Investigations of coherent structures in free turbulent shear flows

The dynamics of large-scale coherent structures in general, and studies based on flow-visualization and phase-locked hot-wire measurements of the structures in the near fields of circular jets are considered. A study involving the nature of coherent structures under conditions of stable vortex pairing induced by controlled axisymmetric acoustic excitation reveals that azimuthal coherence initially enhanced by the excitation is lost before the end of the potential core through evolution of azimuthal lobe structures and turbulent breakdown. The evolution of a spark-induced spot in an axisymmetric turbulent mixing layer is also investigated, and the coherent Reynolds stress associated with the spot is found to be much higher than the background turbulence Reynolds stress. In addition, the state of organization in a high Reynolds number unperturbed axisymmetric mixing layer is examined, and it is shown that the shear layer is infrequently organized and the motions are intensely three-dimensional, while tearing and fractional pairing occur as frequently as complete pairing.

Hussain, A. K. M. F.↗
NASA NTRS1995

The decay of longitudinal vortices shed from airfoil vortex generators

An experimental study is conducted to examine the crossplane structure and streamwise decay of vortices shed from airfoil-type vortex generators. The vortex generators are set in a counter-rotating array spanning the full circumference of a straight pipe. The span of the vortex generators above the duct surface, h, is approximately equal to the local turbulent boundary layer thickness, delta. Measurement of three-component mean flow velocity in downstream crossplanes are used to characterize the structure of the shed vortices. Measurements in adjacent crossplanes (closely spaced along the streamwise coordinate) characterize the interaction and decay of the embedded vortices. A model constructed by the superposition of Oseen vortices is compared to the data for one test case.

Wendt, Bruce J.↗
NASA NTRS1991

Ignition and structure of a laminar diffusion flame in the field of a vortex

The distortion of flames in flows with vortical motion is examined via asymptotic analysis and numerical simulation. The model consists of a constant density, one step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a vortex. The evolution in time of the temperature and mass fraction fields is followed. Emphasis is placed on the ignition time and location as a function of vortex Reynolds number and initial temperature differences of the reacting species. The study brings out the influence of the vortex on the chemical reaction. In all phases, good agreement is observed between asymptotic analysis and the full numerical solution of the model equations.

Macaraeg, Michele G.↗
NASA NTRS2006

A Numerical Study of Hurricane Erin (2001): Shear and the Organization of Eyewall Vertical Motion - Part II

A high-resolution numerical simulation of Hurricane Erin (2001) is used to examine the organization of vertical motion in the eyewall and how that organization responds to a large and rapid increase in the environmental vertical wind shear and subsequent decrease in shear. During the early intensification period, prior to the onset of significant shear, the upward motion in the eyewall was concentrated in small-scale convective updrafts that formed in association with regions of concentrated vorticity (herein termed mesovortices) with no preferred formation region in the eyewall. Asymmetric flow within the eye was weak. As the shear increased, an azimuthal wavenumber 1 asymmetry in storm structure developed with updrafts tending to form on the downshear to downshear-left side of the eyewall. Continued intensification of the shear led to increasing wavenumber 1 asymmetry, large vortex tilt, and a change in eyewall structure and vertical motion organization. During this time, the eyewall structure was dominated by a vortex couplet with a cyclonic (anticyclonic) vortex on the downtilt-left (downtilt-right) side of the eyewall and strong asymmetric flow across the eye that led to strong mixing of eyewall vorticity into the eye. Upward motion was concentrated over an azimuthally broader region on the downtilt side of the eyewall, upstream of the cyclonic vortex, where low-level environmental inflow converged with the asymmetric outflow from the eye. As the shear diminished, the vortex tilt and wavenumber 1 asymmetry decreased, while the organization of updrafts trended back toward that seen during the weak shear period.

Braun, Scott A.↗
NASA NTRS1988

Numerical simulations of an isolated microburst. I - Dynamics and structure

Isolated and stationary microbursts are simulated using a time-dependent, high-resolution, axisymmetric numerical model. A microburst downdraft is initiated by specifying a distribution of precipitation at the top boundary of the model and allowing it to fall into the domain. Two case studies of a hazardous wet (high reflectivity) microburst are presented. The initial and boundary specifications, evolution, microphysics, ring vortex, burst front, vorticity, and structure of the microbursts are discussed.

Proctor, Fred H.↗
NASA NTRS1994

Transport out of the lower stratospheric Arctic vortex by Rossby wave breaking

The fine-scale structure in lower stratospheric tracer transport during the period of the two Arctic Airborne Stratospheric Expeditions (January and February 1989; December 1991 to March 1992) is investigated using contour advection with surgery calculations. These calculations show that Rossby wave breaking is an ongoing occurrence during these periods and that air is ejected from the polar vortex in the form of long filamentary structures. There is good qualitative agreement between these filaments and measurements of chemical tracers taken aboard the NASA ER-2 aircraft. The ejected air generally remains filamentary and is stretched and mixed with midlatitude air as it is wrapped around the vortex. This process transfers vortex air into midlatitudes and also produces a narrow region of fine-scale filaments surrounding the polar vortex. Among other things, this makes it difficult to define a vortex edge. The calculations also show that strong stirring can occur inside as well as outside the vortex.

Waugh, D. W.↗
NASA NTRS1994

Ignition dynamics of a laminar diffusion flame in the field of a vortex embedded in a shear flow

The role of streamwise-spanwise vorticity interactions that occur in turbulent shear flows on flame/vortex interactions is examined by means of asymptotic analysis and numerical simulation in the limit of small Mach number. An idealized model is employed to describe the interaction process. The model consists of a one-step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a streamwise vortex embedded in a shear flow. It is found that the interaction of the streamwise vortex with shear gives rise to small-scale velocity oscillations which increase in magnitude with shear strength. These oscillations give rise to regions of strong temperature gradients via viscous heating, which can lead to multiple ignition points and substantially decrease ignition times. The evolution in time of the temperature and mass-fraction fields is followed, and emphasis is placed on the ignition time and structure as a function of vortex and shear strength.

Macaraeg, Michele G.↗
NASA NTRS2003

Flow Field Measurement of Mixing Driven by Buoyancy

Mixing driven by buoyancy-induced flows inside a cavity consists of stretching and folding of an interface. Measurement of the flow field using particle imaging velocimetry shows that during stretching the flow field has a single elliptic point, thus dominated by a single vortex. However, global bifurcation that results in folding introduces a hyperbolic point whereby the flow field degenerates to multiple vortex interactions. The short-lived coherent structure observed during mixing which results in the Rayleigh- Taylor morphology is attributed to vortex interactions. The mixing characteristics of non-homogeneous fluids driven by buoyancy are important towards understanding transport phenomenon in a microgravity environment. Mixing consists of stretching and folding of an interface due to a flow field whose intensity depends on the body force. For miscible liquids, the characteristic of the flow field determines whether mass transport is governed by diffusion or bulk stirring which induces mixing. For technologically important processes, transport of mass is governed by the coupling of the body force to scalar gradients such as concentration and or temperature' 2 3 . In order to lend insight into these classes of problems we consider a model experimental system to study mixing driven by buoyancy-induced flows. The characteristics of mixing is addressed from detail measurements of the flow field using particle imaging velocimetry (PIV), and its corresponding interface dynamics using image processing techniques.

Batur, C.↗
NASA NTRS1991

Experimental study of a vortex subjected to imposed strain

An experimental project was undertaken to investigate the character of vortex breakdown with particular regard to the waveguide theories of vortex breakdown. A rectangular wing based on the NACA 0012 airfoil was used to produce a trailing vortex which convected downstream without undergoing breakdown. Dye marked the vortex location. A disturbance was then introduced onto the vortex using a small moving wire to 'cut' the vortex. The development of upstream and downstream propagating disturbance waves was observed and the propagation velocities measured. The downstream traveling wave produced a structure similar in appearance to a vortex breakdown. The upstream wave produced a moving, swirling, turbulent region that was not a vortex breakdown. The waves moving in either direction have the same swirl velocity profiles but quite different axial velocity profiles. The upstream disturbance (turbulence) moved into a flow with an axial velocity profile that had a wake-like defect in the core region. The downstream moving vortex breakdown moved into a flow with a jet-like overshoot in the core region. The fact that no breakdown was observed for the wake-like defect and breakdown was observed for the jet-like overshoot is not consistent with computational fluid dynamics (CFD) calculations. Although there are not a lot of examples, CFD results show breakdown for both types of profiles. The longitudinal and swirl velocity profiles were documented by Laser Doppler Velocimeter (LDV) measurement. Wave velocities, swirl angles, and swirl parameters are reported.

Panton, Ronald L.↗
NASA NTRS1986

Vortex shedding from solid rocket propellant inhibitors

Vortex shedding frequency caused by the protrusion of inhibitors into the flow field of a solid rocket motor is investigated by experimental and mathematical models. The time dependent Navier-Stokes equations are solved using a finite difference technique assuming incompressible, two-dimensional flow under both laminar and turbulent flow conditions. For laminar flow, explicit solutions are obtained using a vorticity-transport equation in place of the Navier-Stokes equations. For turbulent flow, a two-equation (k-epsilon) model is used for turbulent modeling and the SIMPLE algorithm is employed as the computational scheme. Cold flow tests were conducted to confirm the basic flow structure and to determine the vortex shedding frequency under both laminar and turbulent flow conditions. The vortex shedding frequencies were determined using a stroboscope to measure the oscillating frequency of yarn tufts which were fastened to one inhibitor in the models. A hot-film anemometer established the velocity history behind the inhibitor. Good agreement between the theoretical results and measurements of the vortex shedding frequencies is demonstrated.

Shu, P. H.↗
NASA NTRS1988

Vortex breakdown in channel flow transition

The time-dependent three-dimensional incompressible Navier-Stokes equations are used to study the mechanisms and developing structures comprising the later stages of the transition to turbulence in plane channel flow. Computer animated flow visualizatons are employed using three-dimensional vortex lines and other graphical representations. The flow visualizations clearly show the development of organized structures such as vortex loops, horse-show vortices, and vortex rings. It is noted that the development of these structures is similar to the bursting process found in turbulent channel flow.

Biringen, S.↗
NASA NTRS1987

Vortex simulation of three-dimensional mixing layers

Spanwise structures of 'nominally' two-dimensional (2D), spatially growing, turbulent mixing layers are simulated numerically by a three-dimensional (3D) vortex method. Small-amplitude 3D disturbances are introduced into an otherwise 2D flow field. Results show that a large-scale spanwise variation of the flow field is produced because of amplification of initial disturbances. Pairs of counterrotating streamwise vortices are formed as a result of stretching of primary spanwise vortices. The streamwise vortices are formed at a fixed spanwise location which depends on initial disturbances. The calculated magnitude of streamwise vorticity is close to that of the spanwise vorticity. The results also suggest that the presence of background disturbances may be essential for the formation of streamwise vortices. The results are in good qualitative agreement with experiments.

Inoue, Osamu↗
NASA NTRS1991

Experiments concerning the theories of vortex breakdown

An experimental project was undertaken to investigate the character of vortex breakdown with particular regard to the stagnation and wave guide theories of vortex breakdown. Three different wings were used to produce a trailing vortex which convected downstream without undergoing breakdown. Disturbances were then introduced onto the vortex using a moving wire to 'cut' the vortex. The development of upstream and downstream propagating disturbance waves was observed and the propagation velocities measured. A downstream traveling wave was observed to produce a structure similar in appearance to a vortex breakdown. An upstream traveling wave produced a moving turbulent region. The upstream disturbance moved into an axial velocity profile that had a wake-like defect while the downstream moving vortex breakdown moved against a jet-like overshoot. The longitudinal and swirl velocity profiles were documented by LDV measurement. Wave velocities, swirl angles, and swirl parameters are reported.

Panton, Ronald L.↗
NASA NTRS1953

On the Development of Turbulent Wakes from Vortex Streets

Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hotwire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At R = 40 to 150, called the stable range, regular vortex streets are formed and no turbulent motion is developed. The range R = 150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. This transition first occurs in the range R = 150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations. In the stable range the vortices decay by viscous diffusion. In the irregular range the diffusion is turbulent and the wake becomes fully turbulent in 40 to 50 diameters downstream. It was found that in the stable range the vortex street has a periodic spanwise structure. The dependence of shedding frequency on velocity was successfully used to measure flow velocity. Measurements in the wake of a ring showed that an annular vortex street is developed.

FLOW, SUBSONIC↗