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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Whirl/whip demonstration

Fluid flow in bearings and seals, set in motion by shaft rotation, generates dynamic forces which may result in a well recognized instability known as whirl and whip. These are lateral, forward precessional, self excited, subsynchronous vibrations in which the amplitude may vary from very small to nearly the limit of the bearing or seal clearances. Oil whirl in lubricated bearings, in particular, typically occurs at somewhat less than half rotative speed. As the rotative speed increases, the frequency relationship remains constant until the whirl frequency approaches the first balance resonance. Now the whirl is smoothly replaced by whip at a nearly constant frequency asymptotically approaching first balance resonance, independent of increasing rotative speed. Changes in bearing/seal radial loading can permit, prevent, or eliminate this instability. The oil whirl/whip rig demonstrates the effects of fluid dynamic forces generated by the rotating shaft. At low rotative speeds, this produces changes of the journal static equilibrium position within the bearing. The demonstrator shows the relationship between any load direction and the average journal equilibrium position. At higher rotative speeds, the instability threshold is observed as a function of unidirectional radial load, unbalance, and rotor configuration.

Grissom, R.↗
DOE OSTI2020

Parameter space mapping of the Princeton magnetorotational instability experiment

Extensive simulations of the Princeton Magnetorotational Instability (MRI) Experiment with the Spectral/Finite Element code for Maxwell and Navier-Stokes Equations (SFEMaNS) have been performed to map the MRI-unstable region as a function of inner cylinder angular velocity and applied vertical magnetic field. The angular velocities of the outer cylinder and the end-cap rings follow the inner cylinder in fixed ratios optimized for MRI. Here, we first confirm the exponential growth of the MRI linear phase using idealized conducting vertical boundaries (end caps) rotating differentially with a Taylor-Couette profile. Subsequently, we run a multitude of simulations to scan the experimental parameter space and find that the normalized volume-averaged mean-square radial magnetic field, our main instability indicator, rises significantly where MRI is expected. At various locations, the local radial components of fluid velocity and generated magnetic field are well correlated with the volume-averaged indicator. Based on this correlation, a diagnostic system that will measure the radial magnetic field at several locations on the inner cylinder is proposed as the main comparison between simulation and experiment. A detailed analysis of poloidal mode structures in the SFEMaNS code indicates that MRI, rather than Ekman circulation or Rayleigh instability, dominates the fluid behavior in the region where MRI is expected.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1990

Buoyancy-driven instabilities and the nonlinear breakup of a sheared magnetic layer

Motivated by problems concerning the storage and subsequent escape of the solar magnetic field, a study has been made of how a magnetic layer embedded in a convectively stable atmosphere evolves due to axisymmetric instabilities driven by magnetic buoyancy. The initial equilibrium consists of a toroidal field sheared by a weaker poloidal component. The linear stability problem is investigated for both ideal and resistive MHD, and the nonlinear evolution is followed by numerical integration of the equations of motion. In all cases, the instability is greatly affected by the distribution and strength of the poloidal field. In particular, both the horizontal and vertical scales of the motions are controlled by the location of the surface on which the poloidal field vanishes: the resonant surface. In the nonlinear regime, a resonant surface close to the interface between the magnetized and field-free fluid leads to the localization of the instability, so that only a fraction of the magnetic region is disrupted by the motions. By contrast, a deeply seated resonant surface leads to the complete disruption of the layer and to the formation of large, helical magnetic fragments whose identity is preserved for the entire simulation.

Cattaneo, Fausto↗
DOE OSTI2021

Impact of particle loading and phase coupling on gas–solid flow dynamics: A case study of a two-phase, gas–solid flow in an annular pipe

The present study is devoted to a two-phase, gas–solid flow in an annular pipe (hollow cylinder) at an elevated pressure of 15 bars and moderate Reynolds number of circa 6000. The influence of the particle loading, the interaction between the phases, and turbulence dispersion on the flow dynamics is systematically studied by means of computational fluid dynamics simulations, employing the Ansys FLUENT commercial package. The cases with a particle volumetric fraction of 1.2% are referred to as “high particle loading,” and those with 0.13% are denoted as “low particle loading.” The following cases are investigated: (1) pure gas flow; (2) low particle loading two-phase flow with one-way coupling and with turbulence dispersion; (3) low particle loading two-phase flow with two-way coupling but without turbulence dispersion; (4) low particle loading two-phase flow with two-way coupling and with turbulence dispersion; (5) high particle loading two-phase flow with one-way coupling and with turbulence dispersion; (6) high particle loading two-phase flow with two-way coupling but without turbulence dispersion; and (7) high particle loading two-phase flow with two-way coupling and with turbulence dispersion. Additionally, the boundary layer is found to grow without fluctuations of the turbulent kinetic energy (TKE) for cases 1, 2, and 5. For case 4, the TKE fluctuations have been identified, although they appear to be less substantial than those in cases 6 and 7. The authors attribute the semi-chaotic nature of the TKE fluctuations to the particle loading and two-way coupling. In addition, the onset and development of the flow instability have been observed at a random axial distance in cases 4, 6, and 7. Such instability is also attributed to the two-way coupling with turbulence dispersion in the flow. It is concluded that the particle loading, one-way, or two-way coupling between the phases, and the turbulence dispersion models significantly influence the development of the flow dynamics with the same inlet and boundary conditions. Consequently, it is not a trivial question, which result a user should trust. The present computational results inspire to perform verification as well as experimental validation of the simulations, so the simulation results can subsequently be used with confidence for design analysis.

42 ENGINEERING↗
NASA NTRS1993

Kinetic physics of the mirror instability

Kinetic mechanisms for the growth and saturation of the mirror instability are described using one-dimensional hybrid simulations. Two parameter regimes are considered. In the first regime, a relatively small ion anisotropy excites a slowly growing instability that produces small-amplitude waves; most ions respond to the waves as an adiabatic fluid. In the second regime a large anisotropy excites a rapidly growing instability that generates large-amplitude waves; the response of many ions in this case is nonadiabatic. The difference in ion response is due to the relative importance of two ion populations, resonant and nonresonant. Resonant ions, those ions with low velocities parallel to the background magnetic field, contribute to the growth of the instability as a result of their gyrointeractions with the noncoplanar component of the waves electric field and respond to the mirror waves nonadiabatically. Nonreasonant ions, those with large parallel velocities, respond as an adiabatic fluid. In both regimes, ion anisotropy is reduced by means of the magnetic mirror force; in the second regime, the anisotropy reduction reduces the free energy available for wave growth and leads to the saturation of the mirror instability.

Mckean, M. E.↗
NASA NTRS1985

Remarks on rotor stability (A contribution to discussion during symposium on instability in rotating machinery)

Influence of nonlinear factors in fluid lubricated bearings on rotor vibrations is discussed as are computation of threshold of stability for systems with multiple degrees of freedom. The reason for this investigation was to learn more about the influence of tilting pad bearings on vertical rotors. The eigenvalues, the stability threshold, the response on the Heaviside-function and the unbalance response were calculated for a rigid rotor, a Jeffcott rotor and for the finite element model of a real rotor. The comparison of the linear and nonlinear calculations shows the limit of the linear calculations. The computation is supposed to show, that the model of the Jeffcott rotor is not sufficient in most practical cases to determine the stability threshold. This was known for some years, when computed results were compared with the behavior of real machines. That's why a program was developed, which is roughly described.

Kraemer, E.↗
DOE OSTI2024

The Effect of Spatially Varying Collision Frequency on the Development of the Rayleigh–Taylor Instability

Abstract The Rayleigh–Taylor (RT) instability is ubiquitously observed, yet has traditionally been studied using ideal fluid models. Collisionality can vary strongly across the fluid interface, and previous work demonstrates the necessity of kinetic models to completely capture dynamics in certain collisional regimes. Where previous kinetic simulations used spatially and temporally constant collision frequency, this work presents five-dimensional (two spatial, three velocity dimensions) continuum-kinetic simulations of the RT instability using a more realistic spatially varying collision frequency. Three cases of collisional variation are explored for two Atwood numbers: low to intermediate, intermediate to high, and low to high. The low-to-intermediate case exhibits no RT instability growth, while the intermediate-to-high case is similar to a fluid-limit kinetic case with interface widening biased toward the lower-collisionality region. A novel contribution of this work is the low-to-high collisionality case that shows significantly altered instability growth through an upward movement of the interface and damped spike growth due to increased free-streaming particle diffusion in the lower region. Contributions to the energy flux from the non-Maxwellian portions of the distribution function are not accessible to fluid models and are greatest in magnitude in the spike and regions of low collisionality. Increasing the Atwood number results in greater RT instability growth and reduced upward interface movement. Deviation of the distribution function from Maxwellian is inversely proportional to collision frequency and concentrated around the fluid interface. The linear phase of RT instability growth is well described by theoretical linear growth rates accounting for viscosity and diffusion.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

Convective and morphological instability in vapor crystal growth

Theoretical and experimental work on the fluid dynamics of physical vapor transport is reported. It is shown that diffusion in viscous interaction with container walls leads to concentration gradients normal to the main transport direction. Consequently any convection threshold is removed. This coupling with convective instabilities makes the theoretical interpretation of earthbound (anisotropic) morphological stability studies on vapor-solid interfaces intractable. Hence low-gravity experiments are suggested that will allow for the establishment of morphological stability criteria under diffusion controlled conditions.

Rosenberger, F.↗
NASA NTRS1989

The electron beam instability revisited: Growth above and below f sub p

The growth of electrostatic waves near the plasma frequency (fp) due to an unstable electron beam is investigated by solving the unmagnetized electrostatic dispersion equation numerically. The numerical solutions are compared with analytic theories for reactive (or fluid-type) and kinetic versions of the beam instability, and for the O'Neil/Malmberg connection of the beam and Langmuir modes. Conditions for growth significantly above or below fp are given. Three general results are found: (1) The unstable waves do not grow on a mode with Langmuir dispersion except in the limit of a very dilute beam with growth on O'Neil/Malmberg's connected mode; (2) The properties of the unstable mode depend strongly on beam parameters such as beam density, speed and temperature; and (3) The frequency of maximum growth frequently lies significantly above or below fp, and differs significantly from that predicted by the Langmuir dispersion relation. Results imply important consequences for theories of strong turbulence and nonlinear wave-wave processes, and observational identification of fp from observed wave frequencies.

Cairns, I. H.↗
NASA NTRS1996

A Geophysical Flow Experiment in a Compressible Critical Fluid

The first objective of this experiment is to build an experimental system in which, in analogy to a geophysical system, a compressible fluid in a spherical annulus becomes radially stratified in density through an A.C. electric field. When this density gradient is demonstrated, the system will be augmented so that the fluid can be driven by heating and rotation and tested in preparation for a microgravity experiment. This apparatus consists of a spherical capacitor filled with critical fluid in a temperature controlled environment. To make the fluid critical, the apparatus will be operated near the critical pressure, critical density, and critical temperature of the fluid. This will result in a highly compressible fluid because of the properties of the fluid near its critical point. A high voltage A.C. source applied across the capacitor will create a spherically symmetric central force because of the dielectric properties of the fluid in an electric field gradient. This central force will induce a spherically symmetric density gradient that is analogous to a geophysical fluid system. To generate such a density gradient the system must be small (approx. 1 inch diameter). This small cell will also be capable of driving the critical fluid by heating and rotation. Since a spherically symmetric density gradient can only be made in microgravity, another small cell, of the same geometry, will be built that uses incompressible fluid. The driving of the fluid by rotation and heating in these small cells will be developed. The resulting instabilities from the driving in these two systems will then be studied. The second objective is to study the pattern forming instabilities (bifurcations) resulting from the well controlled experimental conditions in the critical fluid cell. This experiment will come close to producing conditions that are geophysically similar and will be studied as the driving parameters are changed.

Hegseth, John↗
NASA NTRS1995

Kinetic simulation of the Kelvin-Helmholtz instability in a finite sized jet

Two-dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin-Helmholtz (K-H) instability for a jet of material moving transverse to the magnetic field in the middle of a field reversal region. The instability shows two phases. This first phase is the development of a short scale length instability on the sides of the jet, which might be characterized as a local K-H instability. This instability saturates at low amplitude by modest broadening of the jet profile. Then a nonlocal K-H instability sets in where the jet exhibits a kinklike behavior which spreads and disrupts the jet in a dramatic fashion. The simulation results are discussed within the context of possible relevance to the magnetopause.

Thomas, V. A.↗
NASA NTRS1984

The collisionless macroscopic Kelvin-Helmholtz instabiity. I Transverse electrostatic mode

The electrostatic limit of the Kelvin-Helmholtz instability in a collisionless plasma is investigated for a magnetic field perpendicular to the velocity flow. A two-fluid eigenvalue analysis of the transverse electrostatic Kelvin-Helmholtz instability is performed using the Chew-Goldberger-Low hydromagnetic equations with first-order finite Larmer radius corrections, establishing that the transverse electrostatic mode reduces to the incompressible transverse MHD mode in the limit of zero ion gyroradius. The linear growth rates are determined and a strong stabilization of the instability for (k-perpendicular) (rho sub i) of 0.2 or greater is observed. The nonlinear stage of the instability displays large vortices whose size is many times the initial velocity shear length. At saturation the simulation is dominated by the longest-wavelength mode permitted in the system. The simulation results can be understood in terms of a simple model which assumes that saturation occurs when the angular frequency of the vortex motion reaches the linear growth rate of the instability.

Prichett, P. L.↗
DOE OSTI2020

The physics of spontaneous parity-time symmetry breaking in the Kelvin–Helmholtz instability

Abstract We show that the dynamical system of an inviscid fluid with velocity shear admits parity-time (PT) symmetry, which provides a physical explanation of the well-known observation that the spectrum of the perturbation eigenmodes of the system is symmetric with respect to the real axis. It is found that the Kelvin–Helmholtz instability is triggered when and only when the PT symmetry is spontaneously broken. The analysis of PT symmetry also reveals that the relative phase between parallel velocity and pressure perturbations needs to be locked at π /2 when the instability is suppressed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1993

The Generation of Oblique Magnetosonic Waves

One of the outstanding issues regarding the excitation of magnetosonic waves has been the observational evidence that obliquely propagating waves are dominant at comets and planetary foreshocks despite the predictions of linear theory that maximum growth occurs at parallel propagation. To address this issue, we have conducted a detailed linear theory using a beam-ring distribution function. The results have shown that such distribution functions are associated with four separate instabilities. Two of these instabilities are similar to the right hand resonant ion/ion and the non-resonant instabilities which are also present in the case of a field aligned beam. The other two instabilities are associated with the presence of ring part of the distribution function. One of these instabilities excites magnetosonic waves with maximum growth in the oblique directions. The other excites Alfven waves with maximum growth in the oblique directions. The importance of the former instability is that it may explain the oblique nature of the magnetosonic waves observed at planetary foreshocks and comets. In order to understand the nonlinear properties of these various instabilities, we have also conducted 2-D hybrid (particle ions, fluid electrons) simulations. We have found that when the beam density is sufficiently large so that the non-resonant instability has the largest growth rate, these waves dominate the initial wave power in the system. At later times, however, the obliquely propagating magnetosonic waves become dominant. Another important finding was that when the two instabilities which excite magnetosonic waves have the largest growth rates, the system is dominated by the obliquely propagating magnetosonic waves. This is despite the fact that the largest growth rate for one of the instabilities occurs in the parallel direction. The exact cause of this is not currently understood and is under investigation.

Source record↗
NASA NTRSDate not supplied

TO252: Parabolic Flight Demonstration of Fluid Dispensing Tube Performance Limits for Drop Delivery

Ring Shear Drop (RSD) ISS flight hardware failed to pin sample fluid drops between two rings, likely due to dynamic instability inherent to the dispensing tube (D-Tube) design. The first parabolic flight tested 5 D-tube tip designs with two bounding surface tension fluids, enabling an engineering design decision. D-tube Tip Cage Design was selected based on the November 2020 parabolic flight test data. Flight Operations Program (FOP) approved a reflight proposal to include 2 parabolic flight days during the April/May 2021 campaign with following objectives: • Validate ISS hardware flight design with actual flight fluids (test as you fly), demonstrating the ability to deliver 1” fluid drops in near-microgravity conditions. • Demonstrate pinning of 1” flight fluid drops: a secondary pinning ring is planned to be installed in the parabolic flight hardware to meet this objective. Fluid flow rates will also be varied as needed and as possible. • Provide data to validate computational models used to guide ISS operational deployment decisions and demonstrate the viability of the redesigned hardware to accommodate a range of future flight investigations. This presentation introduces highlights and lessons learned during this recent flight campaign for discussion in the Flight Opportunities Program Panel, “Leveraging Suborbital Flight Tests to Advance Science and Technology."

Sridhar Gorti↗
NASA NTRS1996

The Extensional Rheology of Non-Newtonian Materials

The evolution of the transient extensional stresses in dilute and semi-dilute viscoelastic polymer solutions are measured with a filament stretching rheometer of a design similar to that first introduced by Sridhar, et al. The solutions are polystyrene-based (PS) Boger fluids that are stretched at constant strain rates ranging from 0.6 less than or equal to epsilon(0) less than or equal to 4s(exp -1) and to Hencky strains of epsilon greater than 4. The test fluids all strain harden and Trouton ratios exceeding 1000 are obtained at high strains. The experimental data strain hardens at lower strain levels than predicted by bead-spring FENE models. In addition to measuring the transient tensile stress growth, we also monitor the decay of the tensile viscoelastic stress difference in the fluid column following cessation of uniaxial elongation as a function of the total imposed Hencky strain and the strain rate. The extensional stresses initially decay very rapidly upon cessation of uniaxial elongation followed by a slower viscoelastic relaxation, and deviate significantly from FENE relaxation predictions. The relaxation at long times t is greater than or equal to 5 s, is compromised by gravitational draining leading to non-uniform filament profiles. For the most elastic fluids, partial decohension of the fluid filament from the endplates of the rheometer is observed in tests conducted at high strain rates. This elastic instability is initiated near the rigid endplate fixtures of the device and it results in the progressive breakup of the fluid column into individual threads or 'fibrils' with a regular azimuthal spacing. These fibrils elongate and bifurcate as the fluid sample is elongated further. Flow visualization experiments using a modified stretching device show that the instability develops as a consequence of an axisymmetry-breaking meniscus instability in the nonhomogeneous region of highly deformed fluid near the rigid endplate.

Spiegelberg, Stephen H.↗
DOE OSTI2024

Heat transfer and transport property contrast effects on the compressible Rayleigh-Taylor instability

In extreme environments, the Rayleigh-Taylor instability (RTI) may occur under large variations in density and temperature and with fluid transport properties strongly dependent on temperature. Direct numerical simulations of the 3D fully compressible RTI are conducted, examining the idealized configuration of a hotter, less dense fluid pushing against a colder, denser fluid. Various temperature ratios and transport property configurations are explored to examine how heat conduction, large variations in transport properties, and sudden changes in transport properties can affect the evolution of the mixing layer. Nonuniform fluid expansion and contraction induced by heat transfer can significantly affect local density differences and overall instability growth, causing profile asymmetries about the initial interface for flow and mixing statistics. The departures from classical self-similar development of the instability along with misalignment between regions of mixing and regions of most intense turbulent activity caused by both heat transfer and transport property contrasts are examined. After sudden changes in fluid transport properties, which may occur as a result of rapid heating (e.g., in inertial confinement fusion), the flow quickly responds and begins to relax towards quasi-self-similar late-time evolution. For many dynamical quantities such as vorticity and dissipation, this late-time evolution resembles that of the configuration that already started with the final transport property magnitudes, suggesting that these quantities depend only on the transport properties and not on past flow history, provided that the density field distributions for the flows remain similar. On the other hand, the mixing evolution after the transport property change is unique, implying that both property magnitudes and previous history are impactful on the mixing. In conclusion, these simulations demonstrate how various temperature-related effects are extremely important to consider in compressible RTI flows with large temperature variations.

42 ENGINEERING↗