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NASA NTRS2012

Extension of the Time-Spectral Approach to Overset Solvers for Arbitrary Motion

Forced periodic flows arise in a broad range of aerodynamic applications such as rotorcraft, turbomachinery, and flapping wing configurations. Standard practice involves solving the unsteady flow equations forward in time until the initial transient exits the domain and a statistically stationary flow is achieved. It is often required to simulate through several periods to remove the initial transient making unsteady design optimization prohibitively expensive for most realistic problems. An effort to reduce the computational cost of these calculations led to the development of the Harmonic Balance method [1, 2] which capitalizes on the periodic nature of the solution. The approach exploits the fact that forced temporally periodic flow, while varying in the time domain, is invariant in the frequency domain. Expanding the temporal variation at each spatial node into a Fourier series transforms the unsteady governing equations into a steady set of equations in integer harmonics that can be tackled with the acceleration techniques afforded to steady-state flow solvers. Other similar approaches, such as the Nonlinear Frequency Domain [3,4,5], Reduced Frequency [6] and Time-Spectral [7, 8, 9] methods, were developed shortly thereafter. Additionally, adjoint-based optimization techniques can be applied [10, 11] as well as frequency-adaptive methods [12, 13, 14] to provide even more flexibility to the method. The Fourier temporal basis functions imply spectral convergence as the number of harmonic modes, and correspondingly number of time samples, N, is increased. Some elect to solve the equations in the frequency domain directly, while others choose to transform the equations back into the time domain to simplify the process of adding this capability to existing solvers, but each harnesses the underlying steady solution in the frequency domain. These temporal projection methods will herein be collectively referred to as Time-Spectral methods. Time-Spectral methods have demonstrated marked success in reducing the computational costs associated with simulating periodic forced flows, but have yet to be fully applied to overset or Cartesian solvers for arbitrary motion with dynamic hole-cutting. Overset and Cartesian grid methodologies are versatile techniques capable of handling complex geometry configurations in practical engineering applications, and the combination of the Time-Spectral approach with this general capability potentially provides an enabling new design and analysis tool. In an arbitrary moving-body scenario for these approaches, a Lagrangian body moves through a fixed Eulerian mesh and mesh points in the Eulerian mesh interior to the solid body are removed (cut or blanked), leaving a hole in the Eulerian mesh. During the dynamic motion some gridpoints in the domain are blanked and do not have a complete set of time-samples preventing a direct implementation of the Time-Spectral method. Murman[6] demonstrated the Time-Spectral approach for a Cartesian solver with a rigid domain motion, wherein the hole cutting remains constant. Similarly, Custer et al. [15, 16] used the NASA overset OVERFLOW solver and limited the amount of relative motion to ensure static hole-cutting and interpolation. Recently, Mavriplis and Mundis[17] demonstrated a qualitative method for applying the Time-Spectral approach to an unstructured overset solver for arbitrary motion. The goal of the current work is to develop a robust and general method for handling arbitrary motion with the Time-Spectral approach within an overset or Cartesian mesh method, while still approaching the spectral convergence rate of the original Time-Spectral approach. The viscous OVERFLOW solver will be augmented with the new Time-Spectral algorithm and the capability of the method for benchmark problems in rotorcraft and turbomachinery will be demonstrated. This abstract begins with a brief synopsis of the Time-Spectral approach for overset grids and provides details of e current approach to allow for arbitrary motion. Model problem results in one and two dimensions are included to demonstrate the viability of the method and the convergence properties. Section IV briefly outlines the implementation into the OVERFLOW solver, and the abstract closes with a description of the benchmark test cases which will be included in the final paper.

Leffell, Joshua Isaac↗

NASA NTRS2021

Electron Bernstein waves and narrow band plasma waves near the electron cyclotron frequency in the near-Sun solar wind

Context. Recent studies of the solar wind sunward of 0.25 AU reveal that it contains quiescent regions, with low-amplitude plasma and magnetic field fluctuations, and a magnetic field direction similar to the Parker spiral. The quiescent regions are thought to have a more direct magnetic connection to the solar corona than other types of solar wind, suggesting that waves or instabilities in the quiescent regions are indicative of the early evolution of the solar wind as it escapes the corona. The quiescent solar wind regions are highly unstable to the formation of plasma waves near the electron cyclotron frequency (fce). Aims. We examine high time resolution observations of these waves in an effort to understand their impact on electron distribution functions of the quiescent near-Sun solar wind. Methods. High time resolution waveform captures of near-fce waves were examined to determine variations of their amplitude and frequency in time as well as their polarization properties. Results. We demonstrate that the near-fce wave intervals contain several distinct wave types, including electron Bernstein waves and extremely narrowband waves that are highly sensitive to the ambient magnetic field orientation. Using the properties of these waves, we suggest possible plasma wave mode classifications and possible instabilities that generate these waves. The results of this analysis indicate that these waves may modify the cold core of the electron distribution functions in the quiescent near-Sun solar wind.

David M. Malaspina↗

NASA NTRSDate not supplied

Payload Fairing Acoustic Trade Study: Fill Effect Modeling

A review of acoustic fill effect was conducted using vibroacoustic models of the NASA Space Launch System Block 1B 8.4-meter Payload Fairing. Fill effect is the term used to describe how sound pressure levels (SPLs) inside of a launch vehicle’s payload cavity change when a payload is present, compared to that of an unfilled cavity. Several vibroacoustic models with various payload shapes were developed and the deviation in output from an unfilled cavity model were compared to an analytical fill effect calculation. The analytical expression for the fill effect correction factor, known as the fill factor, is published in NASA-STD-7001B. The development and validation of the NASA fill factor is discussed to provide context for this study and define important variables including the volume fill percentage and the fairing-payload separation gap. The fill factor is reviewed from two perspectives; a global perspective which defines the volume ratio with respect to the entire fairing cavity, and a local perspective which defines the volume ratio with respect to a zone of interest dependent on the payload’s height. The global perspective was considered because it allows a decoupling of the volume ratio and fairing-payload separation gap variables. Both the global and local fill factor methods have difficulty capturing low frequency SPL because of a breakdown in the underlying assumptions of a diffuse acoustic field. Spatial variation in the pressure field at low frequency showed that axially oriented standing waves may be of concern for sensitive payload components; radial variation in SPL was found to be less significant. Further, a novel secondary derivation of the theoretical fill factor is posed for future work in which the absorption characteristics of the payload and fairing surfaces are considered. This work is supported by the NASA Glenn Research Center and the Space Launch System Payload Fairing Working Group.

Spacecraft environments↗

NASA NTRS1964

Tiny sensor-transmitter can withstand extreme acceleration, gives digital output

A self-pulsing oscillator transmits a pulsed signal. The time between pulses and the frequency are controlled by two networks. Variations in the component values in each of the two networks, due to environmental changes, appear as changes in frequency and time between pulses in the transmitted signal. Such a sensor is used to measure physical magnitudes.

Mossino, R. L.↗

NASA NTRS2005

Temperature-dependent optical properties of Cd(1-x),Zn(x),Te substitute material

In this study, we report cryogenic optical properties of Cd(l-x), Zn(x), Te wafers that are used as substrate seed layers in the manufacturing of HgCdTe focal-plane array detectors. These studies are motivated by the fact that the substrate optical properties influence the overall detector performance. The studies consist of measuring the substrate frequency dependent transmittance T(W) and reflectance R(W) above and below the optical band-gap in the UV/Visible and infrared frequency ranges, and with temperature variation of the sample from 5 to 300 K. Determination of the optical absorption from these measurements show that the optical absorption energy gap near 1.6 eV shows a substantial increase as the temperature is reduced from 300 to 5 K. Furthermore, we observe the presence of infrared-active optical phonons whose peak frequency shifts as the temperature of the sample is varied over the measured temperature range. The theoretical frequency dependent optical conductivity, with allowance for redistribution of spectral weight among the interband transition charge carriers, will be discussed.

Quijada, Manuel A.↗

NASA NTRS1993

The sensitivity of identified modal parameters to sensor placement errors and construction tolerances

This paper examines the sensitivity of experimentally measured modal frequencies and mode shapes to structural reassembly and sensor placement errors on a suspended three-bay truss. The statistical variations of identified mode shapes and frequencies are measured by repeated experiments. Both parameters are shown to vary measurably more with reassembly than sensor placement errors. Also, a directional stiffness in the truss joints was found to cause a parameter dependence on member orientation during reconstruction.

Hinkle, Jason↗

NASA NTRS1989

Crystal oscillators using negative voltage gain, single pole response amplifiers

A simple and inexpensive crystal oscillator is provided which employs negative voltage gain, single pole response amplifiers. The amplifiers may include such configurations as gate inverters, operational amplifiers and conventional bipolar transistor amplifiers, all of which operate at a frequency which is on the roll-off portion of their gain versus frequency curve. Several amplifier feedback circuit variations are employed to set desired bias levels and to allow the oscillator to operate at the crystal's fundamental frequency or at an overtone of the fundamental frequency. The oscillator is made less expensive than comparable oscillators by employing relatively low frequency amplifiers and operating them at roll-off, at frequencies beyond which they are customarily used. Simplicity is provided because operation at roll-off eliminates components ordinarily required in similar circuits to provide sufficient phase-shift in the feedback circuitry for oscillation to occur.

Kleinberg, Leonard L.↗

NASA NTRS2013

Microwave Kinetic Inductance Detector with Selective Polarization Coupling

A conventional low-noise detector requires a technique to both absorb incident power and convert it to an electrical signal at cryogenic temperatures. This innovation combines low-noise detector and readout functionality into one device while maintaining high absorption, controlled polarization sensitivity, and broadband detection capability. The resulting far-infrared detectors can be read out with a simple approach, which is compact and minimizes thermal loading. The proposed microwave kinetic inductance detector (MKID) consists of three basic elements. The first is the absorptive section in which the incident power is coupled to a superconducting resonator at far-infrared frequency above its superconducting critical frequency (where superconductor becomes normal conductor). This absorber's shape effectively absorbs signals in the desired polarization state and is resonant at the radio frequency (RF) used for readout of the device. Control over the metal film used in the absorber allows realization of structures with either a 50% broadband or 100% resonance absorptance over a 30% fractional bandwidth. The second element is a microwave resonator - which is realized from the thin metal films used to make the absorber as transmission lines - whose resonance frequency changes due to a variation in its kinetic inductance. The resonator's kinetic inductance is a function of the power absorbed by the device. A low-loss dielectric (mono-crystalline silicon) is used in a parallel-plate transmission line structure to realize the desired superconducting resonators. There is negligible coupling among the adjacent elements used to define the polarization sensitivity of each detector. The final component of the device is a microwave transmission line, which is coupled to the resonator, and allows detection of changes in resonance frequency for each detector in the focal plane array. The spiral shape of the detector's absorber allows incident power with two polarizations to couple to the detector equally. A stepped impedance resonator was used that allows the incident power absorbed in the detecting membrane area to be uniformly distributed in the detector's transmission line at the RF readout frequency. This maximizes the sensitivity of the detector. The signal is read out via a frequency multiplexing technique that requires a minimum number of interface transmission lines for readout. This reduces the packaging complexity and coupling to the device's thermal environment.

Wollack, Edward↗

NASA NTRS1960

Experimental Determination of the Effects of Frequency and Amplitude of Oscillation on the Roll-Stability Derivatives for a 60 Degree Delta-Wing Airplane Model

A 60' delta-wing airplane model was oscillated in roll for several frequencies and amplitudes of oscillation to determine the effects of the oscillatory motion on the roll-stability derivatives for the model. The derivatives were measured at a Reynolds number of 1,600,000 for the wing alone, the wing-fuselage combination, and the complete model which included a triangular-plan-form vertical tail. Both rolling and yawing moments due to rolling velocity exhibited large frequency effects for angles of attack higher than 16 degrees. Variations in these derivatives were measured for the lowest frequencies of oscillation; as the frequency increased, the derivatives because more nearly linear with angle of attack. Both velocity derivatives were considerably different at high angles of attack from the corresponding derivatives measured by the steady-state rolling-flow technique. Rolling and yawing moments due to rolling acceleration were measured and similarly found to be highly dependent on frequency at high angles of attack. Some period and time-to-damp computations, which were made to reveal the significance of the acceleration derivatives, indicated that inclusion of the measured derivatives in the equations of motion lengthened the period of the lateral oscillation by 10 percent for a typical delta-wing airplane and increased the time to damp to one-half amplitude by 50 percent.

Fisher, Lewis R.↗

NASA NTRS1989

Theoretical investigation of EM wave generation and radiation in the ULF, ELF, and VLF bands by the electrodynamic orbiting tether

The problem of electromagnetic wave generation by an electrodynamic tethered satellite system is important both for the ordinary operation of such systems and for their possible application as orbiting transmitters. The tether's ionospheric circuit closure problem is closely linked with the propagation of charge-carrying electromagnetic wave packets away from the tethered system. Work is reported which represents a step towards a solution to the problem that takes into account the effects of boundaries and of vertical variations in plasma density, collision frequencies, and ion species. The theory of Alfen wave packet generation by an electrodynamic tethered system in an infinite plasma medium is reviewed, and brief summary of previous work on the problem is given. The consequences of the presence of the boundaries and the vertical nonuniformity are then examined. One of the most significant new features to emerge when ion-neutral collisions are taken into account is the coupling of the Alfven waves to the fast magnetosonic wave. This latter wave is important, as it may be confined by vertical variations in the Alfven speed to a sort of leaky ionospheric wave guide, the resonances of which could be of great importance to the signal received on the Earth's surface. The infinite medium solution for this case where the (uniform) geomagnetic field makes an arbitrary angle with the vertical is taken as the incident wave-packet. Even without a full solution, a number of conclusions can be drawn, the most important of which may be that the electromagnetic field associated with the operation of a steady-current tethered system will probably be too weak to detect on the Earth's surface, even for large tethered currents. This is due to the total reflection of the incident wave at the atmospheric boundary and the inability of a steady-current tethered system to excite the ionospheric wave-guide. An outline of the approach to the numerical problem is given. The use of numerical integrations and boundary conditions consistent with a conducting Earth is proposed to obtain the solution for the horizontal electromagnetic field components at the boundary of the ionosphere with the atmospheric cavity.

Estes, Robert D.↗

NASA NTRS1972

Microwave brightness temperature of a windblown sea

A mathematical model is developed for the apparent temperature of the sea at all microwave frequencies. The model is a numerical model in which both the clear water structure and white water are accounted for as a function of wind speed. The model produces results similar to Stogryn's model at 19.35 GHz for wind speeds less than 8 m/sec; it can use radiosonde data to calculate atmospheric effects and can incorporate an empirically determined antenna gain pattern. The corresponding computer program is of modular design and the logic of the main program is capable of treating a horizontally inhomogeneous surface or atmosphere. It is shown that a variation of microwave brightness temperature with zenith angle is necessary to produce the wind sensitivity of the horizontally polarized brightness temperature; the variation of sky temperature with frequency is sufficient to produce a frequency dependent wind sensitivity.

Hall, F. G.↗

NASA NTRS2016

On Holo-Hilbert Spectral Analysis: A Full Informational Spectral Representation for Nonlinear and Non-Stationary Data

The Holo-Hilbert spectral analysis (HHSA) method is introduced to cure the deficiencies of traditional spectral analysis and to give a full informational representation of nonlinear and non-stationary data. It uses a nested empirical mode decomposition and Hilbert-Huang transform (HHT) approach to identify intrinsic amplitude and frequency modulations often present in nonlinear systems. Comparisons are first made with traditional spectrum analysis, which usually achieved its results through convolutional integral transforms based on additive expansions of an a priori determined basis, mostly under linear and stationary assumptions. Thus, for non-stationary processes, the best one could do historically was to use the time- frequency representations, in which the amplitude (or energy density) variation is still represented in terms of time. For nonlinear processes, the data can have both amplitude and frequency modulations (intra-mode and inter-mode) generated by two different mechanisms: linear additive or nonlinear multiplicative processes. As all existing spectral analysis methods are based on additive expansions, either a priori or adaptive, none of them could possibly represent the multiplicative processes. While the earlier adaptive HHT spectral analysis approach could accommodate the intra-wave nonlinearity quite remarkably, it remained that any inter-wave nonlinear multiplicative mechanisms that include cross-scale coupling and phase-lock modulations were left untreated. To resolve the multiplicative processes issue, additional dimensions in the spectrum result are needed to account for the variations in both the amplitude and frequency modulations simultaneously. HHSA accommodates all the processes: additive and multiplicative, intra-mode and inter-mode, stationary and nonstationary, linear and nonlinear interactions. The Holo prefix in HHSA denotes a multiple dimensional representation with both additive and multiplicative capabilities.

Huang, Norden E.↗

NASA NTRS1995

Ball bearing vibrations amplitude modeling and test comparisons

Bearings generate disturbances that, when combined with structural gains of a momentum wheel, contribute to induced vibration in the wheel. The frequencies generated by a ball bearing are defined by the bearing's geometry and defects. The amplitudes at these frequencies are dependent upon the actual geometry variations from perfection; therefore, a geometrically perfect bearing will produce no amplitudes at the kinematic frequencies that the design generates. Because perfect geometry can only be approached, emitted vibrations do occur. The most significant vibration is at the spin frequency and can be balanced out in the build process. Other frequencies' amplitudes, however, cannot be balanced out. Momentum wheels are usually the single largest source of vibrations in a spacecraft and can contribute to pointing inaccuracies if emitted vibrations ring the structure or are in the high-gain bandwidth of a sensitive pointing control loop. It is therefore important to be able to provide an a priori knowledge of possible amplitudes that are singular in source or are a result of interacting defects that do not reveal themselves in normal frequency prediction equations. This paper will describe the computer model that provides for the incorporation of bearing geometry errors and then develops an estimation of actual amplitudes and frequencies. Test results were correlated with the model. A momentum wheel was producing an unacceptable 74 Hz amplitude. The model was used to simulate geometry errors and proved successful in identifying a cause that was verified when the parts were inspected.

Hightower, Richard A., III↗

NASA NTRS2017

Single-dish and VLBI Observations of Cygnus X-3 During the 2016 Giant Flare Episode

In 2016 September, the microquasar Cygnus X-3 underwent a giant radio flare, which was monitored for 6 d with the Medicina Radio Astronomical Station and the Sardinia Radio Telescope. Long observations were performed in order to follow the evolution of the flare on an hourly scale, covering six frequency ranges from 1.5 to 25.6 GHz. The radio emission reached a maximum of 13.2 +/- 0.7 Jy at 7.2 GHz and 10 +/- 1 Jy at 18.6 GHz. Rapid flux variations were observed at high radio frequencies at the peak of the flare, together with rapid evolution of the spectral index: α steepened from 0.3 to 0.6 (with Sν ∝ ν −α) within 5 h. This is the first time that such fast variations are observed, giving support to the evolution from optically thick to optically thin plasmons in expansion moving outward from the core. Based on the Italian network (Noto, Medicina and SRT) and extended to the European antennas (Torun, Yebes, Onsala), very long baseline interferometry (VLBI) observations were triggered at 22 GHz on five different occasions, four times prior to the giant flare, and once during its decay phase. Flux variations of 2 h duration were recorded during the first session. They correspond to a mini-flare that occurred close to the core 10 d before the onset of the giant flare. From the latest VLBI observation we infer that 4 d after the flare peak the jet emission was extended over 30 mas.

Egron, E.↗

NASA NTRS2017

A Piezoelectric Passive Wireless Sensor for Monitoring Strain

Interest in passive wireless sensing has grown over the past few decades to meet demands in structural health monitoring.(Deivasigamani et al., 2013; Wilson and Juarez, 2014) This work describes a passive wireless sensor for monitoring strain, which does not have an embedded battery or chip. Without an embedded battery, the passive wireless sensor has the potential to maintain its functionality over long periods in remote/harsh environments. This work also focuses on monitoring small strain (less than 1000 micro-ɛ). The wireless sensing system includes a reader unit, a coil-like transponder, and a sensing unit. It operates in the Megahertz (MHz) frequency range, which allows for a few centimeters of separation between the reader and sensing unit during measurements. The sensing unit is a strain-sensitive piezoelectric resonator that maximizes the energy efficiency at the resonance frequency, so it converts nanoscale mechanical variations to detectable differences in electrical signal. In response to an external loading, the piezoelectric sensor breaks from its original electromechanical equilibrium, and the resonant frequency shifts as the system reaches a new balanced equilibrium. In this work, the fixture of the sensing unit is a small, sticker-like package that converts the surface strain of a test material to measurable shifts in resonant frequencies. Furthermore, electromechanical modeling provides a lumped-parameter model of the system to describe and predict the measured wireless signals of the sensor. Detailed characterization demonstrates how this wireless sensor has resolution comparable to that of conventional wired strain sensors for monitoring small strain.

Zou, Xiyue↗

NASA NTRS1976

Suppression of the unwanted variations in data

The removal of unwanted frequency components in geophysical data may be accomplished by filtering. The filtering procedure described in this paper consists of estimating the undesirable component by using a narrow-band delay filter and subtracting the estimate from the data. Suppression of strong solar components in the presence of weak lunar components is considered in detail.

Bernhardt, P. A.↗

NASA NTRS1986

Latitude and depth variation of solar rotation

New measurements of frequencies of various modes of acoustic waves trapped within the sun are reported for degrees up to 98 which allow the convective envelope to be isolated. For degrees between 20 anad 98, no evidence is found that internal rotation differs significantly with depth or latitude from the rotation of surface magnetic field patterns. Modes covering a wide latitude range have systematically lower frequencies than those confined near the equator, indicating the existence of a structural asymmetry within the sun.

Duvall, T. L., Jr.↗

NASA NTRS2008

Neutron Stars and Black Holes Seen with the Rossi X-Ray Timing Explorer (RXTE)

Astrophysical X-rays bring information about location, energy, time, and polarization. X-rays from compact objects were seen in the first explorations to vary in time. Eclipses and pulsations have simple explanations that identified the importance of X-ray binaries and magnetic neutron stars in the first decade of X-ray astronomy. The dynamics of accretion onto stellar and supermassive black holes and onto neutron stars with relatively low magnetic fields shows up as more complex variations, quasi-periodic oscillations, noise with characteristic frequency spectra, broad-band changes in the energy spectra. To study these variations, RXTE instruments needed to have large area and operational flexibility to find transient activity and observe when it was present. Proportional counters and Phoswich scintillators provided it in a modest mission that has made textbook level contributions to understanding of compact objects. The first seen, and the brightest known, X-ray binary, Sco X-1 is one of a class of neutron stars with low mass companions. Before RXTE, none of these had been seen to show pulsations, though they were hypothesized to be the precursors of radio pulsars with millisecond periods and low magnetic fields. RXTE's large area led to identifying coherent millisecond pulsars in a subset which are relatively faint transients. It also led to identifying short episodes of pulsation during thermonuclear bursts, in sources where a steady signal is not seen. The X-ray stage verifies the evolution that produces millisecond radio pulsars.Masses and radii of neutron stars are being determined by various techniques, constraining the equation of state of matter at nuclear densities. Accretion should lead to a range of neutron star masses. An early stage of superstrong magnetic field neutron stars is now known to produce X-ray and gamma-ray bursts in crust quakes and magnetic field reconnection releases of energy. Soft Gamma Repeaters, Anomolous X-ray Pulsars, and high magnetic field rotation-powered pulsars are all now called magnetars, because they have pulse periods indicating they are slowing down as they would with magnetic dipole radiation for a surface field above 5x1013 gauss. The accretion disk has been connected to the launching of radio jets from black holes, and even from neutron stars. Estimates of the angular momenta of black holes are being made from different approaches, modelling a high frequency oscillation that may be related to how close the inner part of the accretion disk is to the black hole, modelling the continua spectra of the X-ray emission, and modeling the emission of red-shifted iron that may be emitted from the accretion disk. These investigations require early discovery of the black hole transient with the All Sky Monitor on RXTE or other monitoring information, frequent extended observations, and coordinated observations with missions that give higher energy resolution, or radio and infrared information.

Swank, Jean↗