Hit or miss? Impact of time series resolution on resolving phytoplankton dynamics at hourly, weekly, and satellite remote sensing frequencies
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Virginie Sonnet↗
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Dynamic ocean-tide effects on Earth's rotation
This article develops 'broad-band' Liouville equations which are capable of determining the effects on the rotation of the Earth of a periodic excitation even at frequencies as high as semi-diurnal; these equations are then used to predict the rotational effects of altimetric, numerical and 32-constituent spherical harmonic ocean-tide models. The rotational model includes a frequency-dependent decoupled core, the effects of which are especially marked near retrograde diurnal frequencies; and a fully dynamic oceanic response, whose effects appear to be minor despite significant frequency dependence. The model also includes solid-earth effects which are frequency dependent as the result of both anelasticity at long periods and the fluid-core resonance at nearly diurnal periods. The effects of both tidal inertia and relative angular momentum on Earth rotation (polar motion, length of day, 'nutation' and Universal Time) are presented for 32 long- and short-period ocean tides determined as solutions to the author's spherical harmonic tide theory. The lengthening of the Chandler wobble period by the pole tide is also re-computed using the author's full theory. Additionally, using the spherical harmonic theory, tidal currents and their effects on rotation are determined for available numerical and altimetric tide height models. For all models, we find that the effects of tidal currents are at least as important as those of tide height for diurnal and semi-diurnal constituents.
Dickman, S. R.↗
Dynamic Characterization of Galfenol (Fe81.6Ga18.4)
Galfenol has the potential to transform the smart materials industry by allowing for the development of multifunctional, load-bearing devices. One of the primary technical challenges faced by this development is the very limited experimental data on Galfenol's frequency-dependent response to dynamic stress, which is critically important for the design of such devices. This report details a novel and precise characterization of the constitutive behavior of polycrystalline Galfenol (Fe81.6Ga18.4) under quasi-static (1 Hz) and dynamic (4 to 1000 Hz) stress loadings. Mechanical loads are applied using a high-frequency load frame. Quasi-static minor and major hysteresis loop measurements of magnetic flux density and strain are presented for constant electromagnet currents (0 to 1.1 A) and constant magnetic fields 0 to 14 kA/m (0 to 180 Oe). The dynamic stress amplitude for minor and major loops is 2.88 and 31.4 MPa (418 and 4550 psi), respectively. Quasi-static material properties closely match published values for similar Galfenol materials. Quasi-static actuation responses are also measured and compared to quasi-static sensing responses; the high degree of reversibility seen in the comparison is consistent with published measurements and modeling results. Dynamic major and minor loops are measured for dynamic stresses up to 31 MPa (4496 psi) and 1 kHz, and the bias condition resulting in maximum, quasi-static sensitivity. Eddy current effects are quantified by considering solid and laminated Galfenol rods. Three key sources of error in the dynamic measurements are accounted for: (1) electromagnetic noise in strain signals due to Galfenol's magnetic response, (2) error in load signals due to the inertial force of fixturing, and (3) phase misalignment between signals due to conditioning electronics. For dynamic characterization, strain error is kept below 1.2 percent of full scale by wiring two collocated gauges in series (noise cancellation) and through leadwire weaving. Inertial force error is kept below 0.41 percent by measuring the dynamic force in the specimen using a nearly collocated piezoelectric load washer. The phase response of all conditioning electronics is explicitly measured and corrected for. In general, as frequency is increased, the sensing response becomes more linear because of an increase in eddy currents. As frequency increases above approximately 100 Hz, the elbow in the strain-versus-stress response disappears as the active (soft) regime stiffens toward the passive (hard) regime. Under constant-field conditions, the loss factors of the solid rod peak between 200 and 600 Hz, rather than exhibiting a monotonic increase. Compared to the solid rod, the laminated rod exhibits much slower increases in hysteresis with frequency, and its quasi-static behavior extends to higher frequencies. The elastic modulus of the laminated rod decreases between 100 and 300 Hz; this trend is currently unexplained.
Dynamic sensing↗
Automated UAS Measurements of Reflectance and Solar Induced Florescence (SIF) for Assessment of the Dynamics in Photosynthetic Function, Application for Maize (Zea Mays L.) in Greenbelt, Maryland, US
To enable monitoring of the parameters governing photosynthesis at the temporal frequency relevant to their dynamics and at a spatial scale that allows practical assessment and management there is a strong need for advancement in the UAS remote sensing methods and instruments. Currently, no single sensor can provide data at the desired high temporal, spectral and spatial resolutions. Our field measurements obtained using the integrated UAS Piccolo system during the summers of 2017 and 2018 demonstrate that science quality reflectance and solar induced fluorescence (SIF) data can be retrieved with high temporal frequency using small Unmanned Aerial Systems (UAS). The implemented approach facilitates consistent data comparisons in space and time and facilitates their integration with other spectral satellite and airborne data.
Petya Campbell↗
Experiments on current-driven three-dimensional ion sound turbulence. I - Return-current limited electron beam injection. II - Wave dynamics
Pulsed electron beam injection into a weakly collisional magnetized background plasma is investigated experimentally; properties of the electron beam and background plasma, as well as the low-frequency instabilities and wave dynamics, are discussed. The current of the injected beam closes via a field-aligned return current of background electrons. Through study of the frequency and wavenumber distribution, together with the electron distribution function, the low-frequency instabilities associated with the pulsed injection are identified as ion acoustic waves driven unstable by the return current. The frequency cut-off of the instabilities predicted from renormalized plasma turbulence theory, has been verified experimentally.
Stenzel, R. L.↗
Cavitation Effects on the Structural Dynamics of Turbomachinery Components
The structural integrity of inducer and impeller blades in rocket engine turbomachinery must be evaluated in the face of complex excitation mechanisms including fluctuating pressures due to cavitation. Cavitation occurs when the local fluid pressure drops below the vapor pressure, causing the formation of vapor-filled bubbles. Cavitation can exist to various extents within the typical operating range of rocket engine turbopumps. Despite recent progress towards reducing uncertainties in structural dynamic models of turbomachinery components, the extent to which pump cavitation affects the structural dynamic properties (i.e., natural frequencies, damping, and mode shapes) of inducer and impeller blades remains largely unknown. To study the structural dynamic effects of cavitating flows, experiments are conducted in a high-speed water tunnel. The test articles mimic a single inducer blade that is unwrapped from the hub, thus resembling a low aspect ratio cantilevered plate. To induce cavitation, the plates are oriented at a non-zero angle of attack and have rough strips near their leading edges. To control the extent of cavitation, the static pressure of the water tunnel is controlled between roughly 0.5 and 1.5 atm. A high-speed camera is used to quantify the coverage of the cavitations heet. The sheet coverage is then correlated to changes in fluid-added mass and hydroelstic damping. In addition, a corresponding two-way coupled CFD/FE model is under development for test/analysis correlation.
Ben Davis↗
Cavitation Effects on the Structural Dynamics of Turbomachinery Components
The structural integrity of inducer and impeller blades in rocket engine turbomachinery must be evaluated in the face of complex excitation mechanisms including fluctuating pressures due to cavitation. Cavitation occurs when the local fluid pressure drops below the vapor pressure, causing the formation of vapor-filled bubbles. Cavitation can exist to various extents within the typical operating range of rocket engine turbopumps. Despite recent progress towards reducing uncertainties in structural dynamic models of turbomachinery components, the extent to which pump cavitation affects the structural dynamic properties (i.e., natural frequencies, damping, and mode shapes) of inducer and impeller blades remains largely unknown. To study the structural dynamic effects of cavitating flows, experiments are conducted in a high-speed water tunnel. The test article is a low aspect ratio cantilevered plate, thus mimicing a single inducer blade that is unwrapped from its hub. The test article is oriented at zero angle of attack and has a triangular trip at its leading edge to induce cavitation. To change the extent of cavitation, the static pressure of the water tunnel is controlled. A high-speed camera is used to quantify the coverage of the cavitation sheet.
Fluid-Structure Interaction↗
Cavitation Effects on the Structural Dynamics of Turbomachinery Components
The structural integrity of inducer and impeller blades in rocket engine turbo machinery must be evaluated in the face of complex excitation mechanisms including fluctuating pressures due to cavitation. Cavitation occurs when the local fluid pressure drops below the vapor pressure, causing the formation of vapor-filled bubbles. Cavitation can exist to various extents within the typical operating range of rocket engine turbo pumps. Despite recent progress towards reducing uncertainties in structural dynamic models of turbo machinery components, the extent to which pump cavitation affects the structural dynamic properties (i.e., natural frequencies, damping, and mode shapes) of inducer and impeller blades remains largely unknown. To study the structural dynamic effects of cavitating flows, experiments are conducted in a high-speed water tunnel. The test article is a low aspect ratio cantilevered plate, thus mimicing a single inducer blade that is unwrapped from its hub. The test article is oriented at zero angle of attack and has a triangular trip at its leading edge to induce cavitation. To change the extent of cavitation, the static pressure of the water tunnel is controlled. A high-speed camera is used to quantify the coverage of the cavitation sheet.
Structural Dynamics↗
Shake test of rotor test apparatus in the 40- by 80-foot wind tunnel
A shake test was conducted to determine the dynamic characteristics of a rotor test apparatus on strut systems. The rotor-off hub transfer function (acceleration per unit force as a function of frequency) was measured in the longitudinal and lateral directions, using a combination of broadband and discrete frequency excitation techniques. The dynamic data is summarized for the configurations tested, giving the following properties for each mode identified: the natural frequency, the hub response at resonance, and the fixed system damping. The complete transfer functions are presented, and the detailed test results are included as an appendix. Data analysis techniques developed to obtain on-line measurements of the system modal properties, including the damping coefficient and the damping ratio are discussed.
Johnson, W.↗
Perceived sound and the frequency response characteristics of the human auditory system
Dynamic model for human auditory sound perception and frequency response characteristics
Jones, J. H.↗
Frequency dependence of single-event upset in advanced commercial PowerPC microprocessors
Single-event upset (SEU) from heavy ions is measured for advanced commercial microprocessors in a dynamic mode with clock frequencies up to 1 GHz. Frequency and core voltage dependence of SEUs in registers and D-Cache are discussed. The results of our studies suggest the SEU in registers and D-Cache tend to increase with frequency. This might have important implications for the overall SEU trend as technology moves toward higher frequencies.
Farmanesh, F. F.↗
Nonstationary Dynamics Data Analysis with Wavelet-SVD Filtering
Nonstationary time-frequency analysis is used for identification and classification of aeroelastic and aeroservoelastic dynamics. Time-frequency multiscale wavelet processing generates discrete energy density distributions. The distributions are processed using the singular value decomposition (SVD). Discrete density functions derived from the SVD generate moments that detect the principal features in the data. The SVD standard basis vectors are applied and then compared with a transformed-SVD, or TSVD, which reduces the number of features into more compact energy density concentrations. Finally, from the feature extraction, wavelet-based modal parameter estimation is applied.
Brenner, Marty↗
Investigation of the Effectiveness of Dynamic Seat in a Black Hawk Flight Simulation
Low cost alternatives have been sought to provide motion cues in ground-based flight simulators to meet mission objectives. The ability to provide high frequency vibrations makes the dynamic seat attractive to helicopter training applications. Previous studies have found that dynamic seat does enhance the realism of the cockpit and affect pilots' workload. This investigation, conducted under the auspices of the Joint Shipboard Helicopter Integration Process (JSHIP), is using a three degree-of-freedom dynamic seat, i.e., heave, surge, and sway, with limited travels in a research simulator configured as a UH-60 Black Hawk at NASA Ames Research Center. The seat's effectiveness is studied using hover, landing, pirouette, bob-up/bob-down, sidestep, and acceleration/deceleration maneuvers. Seat commands consist of constant vibrations in heave and sway which provide the fundamental vibratory cues. Pilot station accelerations and collective controls provide onset and sustained commands. In addition, transient effects due to translational-lift, collective; and normal acceleration are produced by regulating the magnitude and frequency that depend on the rotor rpm. Results are compared to flight test data and two other ground-based motion systems configurations, i.e., a motion condition with very large motion travels and a motion condition that is comparable with commercial simulator travels. Both subjective and objective data will be analyzed to determine the significance of the motion cueing effect in each system for selected maneuvers.
Chung, William W. Y.↗
Acoustic Insights into Flow Condensation Mechanisms
Two-phase thermal management systems, with both boiling and condensation processes, offer great potential and heat transfer coefficients that are orders of magnitude higher than traditional single-phase systems. However, two-phase flows can suffer from a wide range of interfacial instabilities leading to significant thermal performance degradation. In this study, we aim to detect regime transitions and characterize dominating physical mechanisms of flow condensation, such as turbulent diffusion in annular liquid film and interfacial waves, using an integrated system of acoustic, modal, and optical sensing techniques and thermofluidic characterizations. A wideband acoustic emissions sensor and high-sensitivity accelerometer are utilized to capture acoustic and vibrational signatures that signal the onset of liquid film formation and interfacial waves during flow pattern transitions. Compared to optical imaging, wideband acoustic emission sensing allows for higher sampling rates to capture high-frequency interface oscillations critical to the flow regime transitions and works well even for condensation in opaque tubes. Acoustic features (e.g., amplitude, frequency, energy, duration) are correlated with thermofluidic processes (e.g., capillary flows, turbulent flows, boiling, condensation). By relating thermal performance metrics with these dynamic signatures in acoustic and modal regimes, we explore the ability to probe and monitor critical flow regime transitions and transport efficiency in flow condensation.
Acoustic Modal Regimes↗
Application of dynamical systems theory to global weather phenomena revealed by satellite imagery
Theoretical studies of low frequency and seasonal weather variability; dynamical properties of observational and general circulation model (GCM)-generated records; effects of the hydrologic cycle and latent heat release on extratropical weather; and Earth-system science studies are summarized.
Saltzman, Barry↗
Accretion disk oscillations - A local analysis in a disk of finite thickness
Two types of oscillations are observed to occur in dwarf novae: 'coherent' and 'quasi-periodic' oscillations. These may be associated with the pulsation of the white dwarf or the accretion disk components of the dwarf nova. Here a local (short-wavelength) analysis is utilized to study the oscillation of a self-consistent, two-dimensional model of an accretion disk. The linearized equations describing adiabatic, inviscid, nonaxisymmetric oscillations are used to derive a fifth-order algebraic equation for the (complex) pulsation frequency of the disk. The solutions of this equation for various values of the wavevector k reveal that the disk is capable of supporting (1) a pair of high-frequency acoustic modes (p-modes); (2) a pair of intermediate-frequency modes which may share the characteristics of internal gravity waves (g-modes) and inertial waves; and (3) a mode associated with a dynamical instability (purely imaginary frequency). The role played by the shear in determining the stability or instability of these modes is also considered. Finally, the global oscillation frequencies of the disk are discussed.
Carroll, B. W.↗
Frequency-response techniques for documentation and improvement of rotorcraft simulators
Pilot-in-the-loop characterizations are most naturally formulated in terms of end-to-end frequency responses, so a frequency-response-based method is the natural approach to assessing simulator dynamic fidelity. A comprehensive frequency-response approach used heavily by Ames Research Center researchers was described, and results were presented from a number of simulator fidelity assessment studies. Those studies included UH-60 mathematical model validation and upgrade, ASTOVL linear model extraction, and documentation of the Vertical Motion Simulator (at Ames Research Center) motion and visual system characteristics.
Tischler, Mark B.↗
High-dynamic GPS tracking
The results of comparing four different frequency estimation schemes in the presence of high dynamics and low carrier-to-noise ratios are given. The comparison is based on measured data from a hardware demonstration. The tested algorithms include a digital phase-locked loop, a cross-product automatic frequency tracking loop, and extended Kalman filter, and finally, a fast Fourier transformation-aided cross-product frequency tracking loop. The tracking algorithms are compared on their frequency error performance and their ability to maintain lock during severe maneuvers at various carrier-to-noise ratios. The measured results are shown to agree with simulation results carried out and reported previously.
Hinedi, S.↗