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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

USM3D-ME Contributions to the 5th AIAA High Lift Prediction Workshop

This paper presents the results of Reynolds-averaged Navier-Stokes (RANS) simulations conducted by NASA’s flow solver, mixed-element USM3D (USM3D-ME), for the 5th AIAA High-Lift Prediction Workshop. As part of the Fixed-Grid RANS Technology Focus Group (TFG), these simulations were performed to assess the accuracy and efficiency of the USM3D-ME solutions in predicting high-lift flows. The High-Lift Common Research Model (CRM-HL) served as the primary geometry. Several CRM-HL configurations were used for three case studies: a verification study (Case 1), a configuration buildup study (Case 2), and a Reynolds-number variation study (Case 3). Overall, USM3D-ME RANS results aligned with the solutions selected by the Fixed-Grid RANS TFG and available wind tunnel data Simulations for Cases 1 and Configuration 2.1 achieved machine-zero residual convergence, with aerodynamic coefficients converging to steady-state values. However, Configurations 2.2-2.4 and Case 3 encountered iterative- and grid-convergence challenges, particularly at high angles of attack. Compared with the experimental data available for Configurations 2.2-2.4, close agreement was demonstrated at low angles of attack. However, for angles of attack approaching the maximum lift conditions, the predicted lift coefficient and pitching moment deviated from experimental values. The drag-coefficient predictions were in a relatively good agreement, however, slight overpredictions were observed at the highest angle of attack corresponding to the maximum-lift condition. Although iterative convergence for Configurations 2.2-2.4 at high angles of attack remains a persistent challenge, averaging aerodynamic coefficients over the last 5000 iterations yielded satisfactory agreement with the available wind tunnel experimental data. During the workshop, the lack of iterative convergence was attributed to the vortex structures emanating from the slat brackets. To investigate this issue further, post-workshop simulations were conducted on Configuration 2.2. In one study, RANS simulations were performed on a simplified geometry with the slat brackets removed. The second study focused on performing URANS simulations on the original Configuration 2.2 geometry. Preliminary results from both studies are presented and compared with wind tunnel data for Configuration 2.2. Consistent with the findings of other participants in the Fixed-Grid RANS TFG, this study emphasizes the necessity for further exploration and advancement in RANS technology for predicting high-lift flows.

CFD↗
NASA NTRSDate not supplied

USM3D-ME Contributions to the 5th AIAA High Lift Prediction Workshop

This paper presents the results of Reynolds-averaged Navier-Stokes (RANS) simulations conducted by the NASA flow solver, mixed-element USM3D (USM3D-ME), for the 5th AIAA High-Lift Prediction Workshop. As part of the Fixed-Grid RANS Technology Focus Group (TFG), these simulations were performed to assess the accuracy and efficiency of the USM3D-ME solutions in predicting high-lift flows. The High-Lift Common Research Model (CRM-HL) served as the primary geometry. Several CRM-HL configurations were used for three case studies: a verification study (Case 1), a configuration buildup study (Case 2), and a Reynolds-number variation study (Case 3). Overall, USM3D-ME RANS results aligned with the solutions selected by the Fixed-Grid RANS TFG and available wind tunnel data. Simulations for Case 1 and Configuration 2.1 achieved machine-zero residual convergence, with aerodynamic coefficients converging to steady-state values. However, Configurations 2.2-2.4 and Case 3 encountered iterative- and grid-convergence challenges, particularly at high angles of attack. Compared with the experimental data available for Configurations 2.2-2.4, close agreement was demonstrated at low angles of attack. However, for angles of attack approaching the maximum lift conditions, the predicted lift coefficient and pitching moment deviated from experimental values. The drag-coefficient predictions were in relatively good agreement, however, slight overpredictions were observed at the highest angle of attack corresponding to the maximum-lift condition. Although iterative convergence for Configurations 2.2-2.4 at high angles of attack remains a persistent challenge, averaging aerodynamic coefficients over the last 5000 iterations yielded satisfactory agreement with the available wind tunnel experimental data. During the workshop, the lack of iterative convergence was attributed to the vortex structures emanating from the slat brackets. To investigate this issue further, post-workshop simulations were conducted on Configuration 2.2. In one study, RANS simulations were performed on a simplified geometry with the slat brackets removed. The second study focused on performing unsteady RANS (URANS) simulations on the original Configuration 2.2 geometry. Preliminary results from both studies are presented and compared with wind tunnel data for Configuration 2.2. Consistent with the findings of other participants in the Fixed-Grid RANS TFG, this study emphasizes the necessity for further exploration and advancement in RANS technology for predicting high-lift flows.

Aerodynamics↗
NASA NTRS1983

A Numerical Investigation of the Multiple Vortex Phenomenon: Vortex Number and Structure as a Function of Swirl Ratio

A numerical investigation of the multiple vortex phenomenon (MVP) for tornado like flows is conducted to determine the conditions for when a vortex becomes unstable and divides into smaller subsidiary vortices, as well as to determine the structure of these vortices. A three dimensional numerical model developed by Rotunno (1983) is utilized which has been demonstrated to successfully simulate MVP with properties observed both in natural as well as laboratory tornado like vortices. The MVP is generated for several swirl ratio conditions in order to determine the number of vortices generated for those flow configurations. These results are then compared to experimental measurements to validate the numerical model. The number of vortices produced is consistent with observational results made in the Purdue tornado vortex chamber. Furthermore, horizontal and vertical cross sections are taken through the vortices to determine the structure of MVP. Preliminary results indicate that tangential velocities within these smaller asymmetric vortices increase by 20% over values observed in a single axisymmetric vortex at the same swirl ratio.

Smith, D. R.↗
NASA NTRS1986

Internal structure of a vortex breakdown

An axisymmetric vortex breakdown was well simulated by the vortex filament method. The agreement with the experiment was qualitatively good. In particular, the structure in the interior of the vortex breakdown was ensured to a great degree by the present simulation. The second breakdown, or spiral type, which occurs downstream of the first axisymmetric breakdown, was simulated more similarly to the experiment than before. It shows a kink of the vortex filaments and strong three-dimensionality. Furthermore, a relatively low velocity region was observed near the second breakdown. It was also found that it takes some time for this physical phenomenon to attain its final stage. The comparison with the experiment is getting better as time goes on. In this paper, emphasis is placed on the comparison of the simulated results with the experiment. The present results help to make clear the mechanism of a vortex breakdown.

Nakamura, Y.↗
NASA NTRS1990

The organized nature of a turbulent trailing vortex

The turbulence structure of a trailing vortex produced at the juncture of a flow aligned cylinder and a pair of oppositely loaded airfoils is analyzed. The freestream turbulence intensity in this study varies from 0.32 to 1.48 percent, the vortex Reynold number varies from 15000 to 25000, and the Rossby number varies from 0.65 to 0.81. Within this parameter range, it is shown that the screens, but not the freestream turbulence level, are able to produce significant variations in the turbulence structure of the vortex, and that the turbulent structure is determined by the Rossby number and not the vortex Reynolds number. It is noted that the core is dynamic and an organized exchange of momentum takes place between the outer flow and the core region of the vortex. The vortex structure in the trailing vortex having the lowest Rossby number is considered.

Bandyopadhyay, Promode R.↗
DOE OSTI2016

Constructive spin-orbital angular momentum coupling can twist materials to create spiral structures in optical vortex illumination

It was discovered that optical vortices twist isotropic and homogenous materials, e.g., azo-polymer films to form spiral structures on a nano- or micro-scale. However, the formation mechanism has not yet been established theoretically. To understand the mechanism of the spiral surface relief formation in the azo-polymer film, we theoretically investigate the optical radiation force induced in an isotropic and homogeneous material under irradiation using a continuous-wave optical vortex with arbitrary topological charge and polarization. It is revealed that the spiral surface relief formation in azo-polymer films requires the irradiation of optical vortices with a positive (negative) spin angular momentum and a positive (negative) orbital angular momentum (constructive spin-orbital angular momentum coupling), i.e., the degeneracy among the optical vortices with the same total angular momentum is resolved.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1988

Vortex shedding in high-speed compressor blade wakes

The wakes of highly loaded compressor blades are generally considered to be turbulent flows. Recent work has suggested that the blade wakes are dominated by a vortex streetlike structure. The experimental evidence supporting the wake vortex structure is reviewed. This structure is shown to redistribute thermal energy within the flowfield. The effect of the wake structure on conventional aerodynamic measurements of compressor performance is noted. A two-dimensional, time-accurate, viscous numerical simulation of the flow exhibits both vortex shedding in the wake and a lower-frequency flow instability that modulates the shedding. The numerical results are shown to agree quite well with the measurement from transonic compressor rotors.

Epstein, A. H.↗
DOE OSTI2015

Spontaneous formation of circular and vortex ferroelectric domain structure in hexagonal YMnO{sub 3} and YMn{sub 0.9}Fe{sub 0.1}O{sub 3} prepared by low temperature solution synthesis

We report an experimental study of the domain structure of ferroelectric YMnO{sub 3} and YMn{sub 0.9}Fe{sub 0.1}O{sub 3} using polycrystalline samples prepared by direct hydrothermal crystallisation at 240 °C, well below their structural phase transition temperatures. Powder X-ray diffraction shows the expected P6{sub 3}cm space group for both samples with an increase in a and a small decrease in c with Fe incorporation, consistent with an adjustment of MnO{sub 5} tilting, while XANES spectra at the Mn and Fe K edges show the oxidation state of both metals are maintained at +3 in the doped sample. High resolution TEM shows that curved stripe, annular and vortex domains can all be observed in the YMnO{sub 3} crystals, proving that the structural phase transition is not the only driving force for the occurrence of the annular and vortex domains. Furthermore, the absence of the annular and vortex domains in YMn{sub 0.9}Fe{sub 0.1}O{sub 3} indicates that the tilting state of MnO{sub 5} bipyramids plays an important role in the domain formation. Atomic resolution STEM images confirm that the ferroelectric domain walls correspond to structural antiphase boundaries similar to the crystals made via high temperature solid-state reactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electron vortex magnetic holes: A nonlinear coherent plasma structure

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional particle-in-cell simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population, etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1994

A Study of Dean Vortex Development and Structure in a Curved Rectangular Channel with Aspect Ratio of 40 at Dean Numbers up to 430

Flow in a curved channel with mild curvature, an aspect ratio of 40 to 1, and an inner to outer radius ratio of 0.979 is studied at Dean numbers De ranging from 35 to 430. For positions from the start of curvature ranging from 85 to 145 degrees, the sequence of transition events begins with curved channel Poiseuille flow at De less than 40-64. As the Dean number increases, observations show initial development of Dean vortex pairs, followed by symmetric vortex pairs which, when viewed in spanwise/radial planes, cover the entire channel height (De=90-100). At De from 40 to 125-130, the vortex pairs often develop intermittent waviness in the form of vortex undulations. Splitting and merging of vortex pairs is also observed over the same experimental conditions as well as at higher De. When Dean numbers range from 130 to 185-200, the undulating wavy mode is replaced by a twisting mode with higher amplitudes of oscillation and shorter wavelengths. The twisting wavy mode results in the development of regions where turbulence intensity is locally augmented at Dean numbers from 150 to 185-200, principally in the upwash regions between the two individual vortices which make up each vortex pair. These turbulent regions eventually increase in intensity and spatial extent as the Dean number increases further, until individual regions merge together so that the entire cross section of the channel contains chaotic turbulent motions. When Dean numbers then reach 400-435, spectra of velocity fluctuations then evidence fully turbulent flow.

Ligrani, Phillip M.↗
NASA NTRS1978

An experimental map of the internal structure of a vortex breakdown

The flow field of an 'axisymmetric' vortex breakdown has been mapped using a laser-Doppler anemometer. The interior of the recirculation zone is dominated by energetic, non-axisymmetric, low frequency periodic fluctuations. Spectra for a number of points inside this zone, as well as time-averaged swirl and axial velocity profiles both inside and outside the recirculation zone, have been obtained. The time-averaged streamlines in the interior show an unexpected two-celled structure attributed to the action of the fluctuations. Although the present experiment deals with one particular breakdown, flow-visualization studies indicate that the case examined is typical of the 'axisymmetric' form of breakdown over a range of flow conditions.

Faler, J. H.↗
DOE OSTI2014

The internal structure of a vortex in a two-dimensional superfluid with long healing length and its implications

We analyze the motion of quantum vortices in a two-dimensional spinless superfluid within Popov’s hydrodynamic description. In the long healing length limit (where a large number of particles are inside the vortex core) the superfluid dynamics is determined by saddle points of Popov’s action, which, in particular, allows for weak solutions of the Gross–Pitaevskii equation. We solve the resulting equations of motion for a vortex moving with respect to the superfluid and find the reconstruction of the vortex core to be a non-analytic function of the force applied on the vortex. This response produces an anomalously large dipole moment of the vortex and, as a result, the spectrum associated with the vortex motion exhibits narrow resonances lying within the phonon part of the spectrum, contrary to traditional view.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2008

Potential of Future Hurricane Imaging Radiometer (HIRAD) Ocean Surface Wind Observations for Determining Tropical Storm Vortex Intensity and Structure

The Hurricane Imaging Radiometer (HIRAD) is an innovative technology development, which offers the potential of new and unique remotely sensed observations of both extreme oceanic wind events and strong precipitation from either UAS or satellite platforms. It is based on the airborne Stepped Frequency Microwave Radiometer (SFMR), which is a proven aircraft remote sensing technique for observing tropical cyclone ocean surface wind speeds and rain rates, including those of major hurricane intensity. The proposed HIRAD instrument advances beyond the current nadir viewing SFMR to an equivalent wide-swath SFMR imager using passive microwave synthetic thinned aperture radiometer technology. This sensor will operate over 4-7 GHz (C-band frequencies) where the required tropical cyclone remote sensing physics has been validated by both SFMR and WindSat radiometers. HIRAD incorporates a unique, technologically advanced array antenna and several other technologies successfully demonstrated by the NASA's Instrument Incubator Program. A brassboard version of the instrument is complete and has been successfully tested in an anechoic chamber, and development of the aircraft instrument is well underway. HIRAD will be a compact, lightweight, low-power instrument with no moving parts that will produce wide-swath imagery of ocean vector winds and rain during hurricane conditions when existing microwave sensors (radiometers or scatterometers) are hindered. Preliminary studies show that HIRAD will have a significant positive impact on analyses as either a new aircraft or satellite sensor.

Atlas, Robert↗
DOE OSTI2018

Tuning the structure of the Josephson vortex lattice in Bi 2 Sr 2 CaCu 2 O 8+δ single crystals with pancake vortices

In extremely anisotropic cuprate superconductors a lattice of stacks of pancake vortices nucleates when a magnetic field is applied perpendicular to the copper oxide layers, while an orthogonal lattice of highly elliptical Josephson vortices forms when the applied field is parallel to the layers. Under tilted magnetic fields these sublattices can interact in complex ways to form systems of vortex chains and composite vortex lattices. Here we have used high-resolution scanning Hall microscopy to map the rich tilted-field vortex phase diagram in an underdoped Bi 2 Sr 2 CaCu 2 O 8+δ single crystal. We find that the Josephson vortex lattice spacing has an unexpected non-monotonic dependence on the pancake vortex density reflecting the delicate balance between attractive and repulsive vortex interactions, and actually undergoes a field-driven structural transformation at high out-of-plane fields. We also identify particularly stable composite structures composed of vortex chains separated by an integer number of rows of interstitial pancake vortex stacks and are able to establish the precise evolution of vortex chain phases as the out-of-plane field is increased at small in-plane fields. Lastly, our results are in good semi-quantitative agreement with theoretical models and could enable the development of vortex ratchets and lenses based on the interactions between Josephson and pancake vortices.

36 MATERIALS SCIENCE↗
NASA NTRS1985

On the structure of the turbulent vortex

The trailing vortex generated by a lifting surface, the structure of its turbulent core and the influence of axial flow within the vortex on its initial persistence and on its subsequent decay are described. Similarity solutions of the turbulent diffusion equation are given in closed form and results are expressed in sufficiently simple terms that the influence of the lifting surface parameters on the length of persistence and the rate of decay of the vortex can be evaluated.

Roberts, L.↗
NASA NTRS1999

Experiments on Diffusion Flame Structure of a Laminar Vortex Ring

The study of flame-vortex interactions provides one of the means to better understand turbulent combustion, and allows for canonical configurations that contain the fundamental elements found in turbulent flames, These include concentrated vorticity, entrainment and mixing, strain and nonequilibrium phenomena, diffusion and differential diffusion, partial premixing and diluent effects, and heat release effects. In flame- vortex configurations, these fundamental elements can be studied under more controlled conditions than is possible in direct investigations of turbulent flames. Since the paper of Marble, the problem of the flame-vortex interaction has received considerable attention theoretically, numerically and experimentally. Several configurations exist for study of the premixed flame/vortex ring interaction but more limited results have been obtained to date for the diffusion flame/vortex ring case. The setup of Chen and Dahm, which is conceptually similar to that of Karagozian and Manda and Karagozian, Suganuma and Strom where the ring is composed of fuel and air and combustion begins during the ring formation process, is used in the current study. However, it is essential to conduct the experiments in microgravity to remove the asymmetries caused by buoyancy and thus obtain highly symmetric and repeatable interactions. In previous studies it was found that the flame structure of the vortex ring was similar to that obtained analytically by Karagozian and Manda. Dilution of propane with nitrogen led mainly to a reduction in flame luminosities, flame burnout times were affected by both fuel volumes and amount of dilution, and a simple model of the burnout times was developed. In this paper, a discussion on reacting ring displacement and flame burnout time will be given, and the flame structures of vortex rings containing ethane and air will be compared to those of propane reacting in air.

Chen, Shin-Juh↗