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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

An AWE Implementation for Electromagnetic Analysis

Although full wave electromagnetic systems are large and cumbersome to solve, typically only a few parameters, such as input impedance, S parameters, and far field pattern, are needed by the designer or analyst. A reduced order modeling of these parameters is therefore an important consideration in minimizing the the CPU requirements. The Asymptotic Waveform Evaluation (AWE) method is one approach to construct a reduced order model of the input impedance or other useful electromagnetic parameters. We demonstrate its application and validity when used in conjunction with the finite element method to simulate full wave electromagnetic problems.

Gong, Jian↗
NASA NTRS1980

Acoustic behavior of fibrous bulk materials

A semiempirical model of the acoustic behavior of fibrously constructed bulk materials of Hersh and Walker (1979) is generalized to take into account the filtration or removal of particles by fibrous mats and heat conduction between the material fibers and the surrounding fluid. Equations governing the propagation and attenuation of sound waves in a fibrous material are derived on the basis of a solution of the Navier Stokes equation for momentum conservation and a one-dimensional model of heat transfer between the sound field and the fibers. A comparison of the two propagation constants and material impedance specified by the model and experimental measurements for Kevlar 29 indicates the accuracy of the model over a wide range of sound frequencies, material porosities and specimen thickness. It is also found that heat transfer effects are relatively unimportant, while the attenuation constants and material characteristic impedance are a function of fiber orientation relative to sound field propagation direction.

Hersh, A. S.↗
NASA NTRS1996

On the High-Frequency Asymptotic Evaluation of the Potentials of Elemental Sources on an Anisotropic Impedance Cylinder

In an effort to formulate the high-frequency coupling between antennas located on airframes composed of multilayer imperfectly conducting materials a general model was sought which would embody the electromagnetic properties of the layers and would apply over a broader range of separations of the antennas. Such a model is described here. This work is a generalization of the work of Pearson [1986, 1987a] concerning the high-frequency asymptotic representation of the fields of elemental sources diffracted by a multilayer cylinder. In that work the source and field points were located off the cylinder surface, and they were sufficiently separated to permit the effective use of the residue series representation of the spectral integrals involved. Here the source and field points are located on the cylinder surface and are permitted to be sufficiently close as to render the residue series poorly convergent. To obtain a more effective representation in this situation, the cylinder is modeled by an anisotropic impedance cylinder, and the resulting spectral integrals are evaluated by reduction to forms amenable to multiple applications of techniques described in the literature in connection with treatment of axially uniform sources in this context.

Pogorzelski, Ronald J.↗
NASA NTRS1978

Effect of grazing flow on the acoustic impedance of Helmholtz resonators consisting of single and clustered orifices

A semiempirical fluid mechanical model is presented which predicts impedance of a Helmholtz resonator consisting of a single cavity-backed orifice as a function of grazing flow speed, boundary-layer thickness, incident sound amplitude and frequency, and resonator geometry. The incident and cavity sound fields are connected in terms of an orifice discharge coefficient. The effect of multiple orifices was studied experimentally. Interaction between orifices is important only for orifices aligned parallel to the grazing flow. Resistance was virtually independent of both orifice relative spacing and number. Reactance was found to be quite dependent upon orifice spacing but insensitive to the number of orifices.

Hersh, A. S.↗
NASA NTRS2006

Mixed Modeling of a SAW Delay Line Using VHDL-AMS

To aid in the development of SAW sensors for aerospace applications we have created a model of a SAW Delay line using VHDL. The model implements the Impulse Response method to calculate the frequency response, impedance, and insertion loss. The model includes optimization for the number of finger pairs in the IDTs and for the aperture height. This paper presents the model and the results from the model for a SAW delay line design.

Wilson, William C.↗
NASA NTRS2012

Calculating the Lightning Protection System Downconductors' Grounding Resistance at Launch Complex 39B, Kennedy Space Center

A new Lightning Protection System (LPS) was designed and built at Launch Complex 39B (LC39B), at the Kennedy Space Center (KSC), Florida, which consists of a catenary wire system (at a height of about 181 meters above ground level) supported by three insulators installed atop three towers in a triangular configuration. Nine downconductors (each about 250 meters long) are connected to the catenary wire system. Each downconductor is connected to a 7.62-meter-radius circular counterpoise conductor with six equally spaced, 6-meter-long vertical grounding rods. Grounding requirements at LC39B call for all underground and aboveground metallic piping, enclosures, raceways, and cable trays, within 7.62 meters of the counterpoise, to be bonded to the counterpoise, which results in a complex interconnected grounding system, given the many metallic piping, raceways, and cable trays that run in multiple directions around LC39B. The complexity of this grounding system makes the fall-of-potential method, which uses multiple metallic rods or stakes, unsuitable for measuring the grounding impedances of the downconductors. To calculate the grounding impedance of the downconductors, an Earth Ground Clamp (EGC) (a stakeless device for measuring grounding impedance) and an Alternative Transient Program (ATP) model of the LPS are used. The EGC is used to measure the loop impedance plus the grounding impedance of each downconductor, and the ATP model is used to calculate the loop impedance of each downconductor circuit. The grounding resistance of the downconductors is then calculated by subtracting the ATP calculated loop impedances from the EGC measurements.

Mata, Carlos T.↗
NASA NTRS1992

Antenna pattern control using impedance surfaces

During this research period, we have effectively transferred existing computer codes from CRAY supercomputer to work station based systems. The work station based version of our code preserved the accuracy of the numerical computations while giving a much better turn-around time than the CRAY supercomputer. Such a task relieved us of the heavy dependence of the supercomputer account budget and made codes developed in this research project more feasible for applications. The analysis of pyramidal horns with impedance surfaces was our major focus during this research period. Three different modeling algorithms in analyzing lossy impedance surfaces were investigated and compared with measured data. Through this investigation, we discovered that a hybrid Fourier transform technique, which uses the eigen mode in the stepped waveguide section and the Fourier transformed field distributions across the stepped discontinuities for lossy impedances coating, gives a better accuracy in analyzing lossy coatings. After a further refinement of the present technique, we will perform an accurate radiation pattern synthesis in the coming reporting period.

Balanis, Constantine A.↗
NASA NTRS1990

Experimental validation study of an analytical model of discrete frequency sound propagation in closed-test-section wind tunnels

The principal objective is to assess the adequacy of linear acoustic theory with an impedence wall boundary condition to model the detailed sound field of an acoustic source in a duct. Measurements and calculations are compared of a simple acoustic source in a rectangular concrete duct lined with foam on the walls and anechoic end terminations. Measurement of acoustic pressure for twelve wave numbers provides variation in frequency and absorption characteristics of the duct walls. Close to the source, where the interference of wall reflections is minimal, correlation is very good. Away from the source, correlation degrades, especially for the lower frequencies. Sensitivity studies show little effect on the predicted results for changes in impedance boundary condition values, source location, measurement location, temperature, and source model for variations spanning the expected measurement error.

Mosher, Marianne↗
NASA NTRS2010

Acoustic-Modal Testing of the Ares I Launch Abort System Attitude Control Motor Valve

The Attitude Control Motor (ACM) is being developed for use in the Launch Abort System (LAS) of NASA's Ares I launch vehicle. The ACM consists of a small solid rocket motor and eight actuated pintle valves that directionally allocate.thrust_- 1t.has-been- predicted-that significant unsteady. pressure.fluctuations.will.exist. inside the-valves during operation. The dominant frequencies of these oscillations correspond to the lowest several acoustic natural frequencies of the individual valves. An acoustic finite element model of the fluid volume inside the valve has been critical to the prediction of these frequencies and their associated mode shapes. This work describes an effort to experimentally validate the acoustic finite model of the valve with an acoustic modal test. The modal test involved instrumenting a flight-like valve with six microphones and then exciting the enclosed air with a loudspeaker. The loudspeaker was configured to deliver broadband noise at relatively high sound pressure levels. The aquired microphone signals were post-processed and compared to results generated from the acoustic finite element model. Initial comparisons between the test data and the model results revealed that additional model refinement was necessary. Specifically, the model was updated to implement a complex impedance boundary condition at the entrance to the valve supply tube. This boundary condition models the frequency-dependent impedance that an acoustic wave will encounter as it reaches the end of the supply tube. Upon invoking this boundary condition, significantly improved agreement between the test data and the model was realized.

Davis, R. Benjamin↗
NASA NTRS1987

AC impedance study of degradation of porous nickel battery electrodes

AC impedance spectra of porous nickel battery electrodes were recorded periodically during charge/discharge cycling in concentrated KOH solution at various temperatures. A transmission line model (TLM) was adopted to represent the impedance of the porous electrodes, and various model parameters were adjusted in a curve fitting routine to reproduce the experimental impedances. Degradation processes were deduced from changes in model parameters with electrode cycling time. In developing the TLM, impedance spectra of planar (nonporous) electrodes were used to represent the pore wall and backing plate interfacial impedances. These data were measured over a range of potentials and temperatures, and an equivalent circuit model was adopted to represent the planar electrode data. Cyclic voltammetry was used to study the characteristics of the oxygen evolution reaction on planar nickel electrodes during charging, since oxygen evolution can affect battery electrode charging efficiency and ultimately electrode cycle life if the overpotential for oxygen evolution is sufficiently low.

Lenhart, Stephen J.↗
NASA NTRS1999

Development of a hydraulic model of the human systemic circulation

Physical and numeric models of the human circulation are constructed for a number of objectives, including studies and training in physiologic control, interpretation of clinical observations, and testing of prosthetic cardiovascular devices. For many of these purposes it is important to quantitatively validate the dynamic response of the models in terms of the input impedance (Z = oscillatory pressure/oscillatory flow). To address this need, the authors developed an improved physical model. Using a computer study, the authors first identified the configuration of lumped parameter elements in a model of the systemic circulation; the result was a good match with human aortic input impedance with a minimum number of elements. Design, construction, and testing of a hydraulic model analogous to the computer model followed. Numeric results showed that a three element model with two resistors and one compliance produced reasonable matching without undue complication. The subsequent analogous hydraulic model included adjustable resistors incorporating a sliding plate to vary the flow area through a porous material and an adjustable compliance consisting of a variable-volume air chamber. The response of the hydraulic model compared favorably with other circulation models.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS1988

A new algorithm for modeling friction in dynamic mechanical systems

A method of modeling friction forces that impede the motion of parts of dynamic mechanical systems is described. Conventional methods in which the friction effect is assumed a constant force, or torque, in a direction opposite to the relative motion, are applicable only to those cases where applied forces are large in comparison to the friction, and where there is little interest in system behavior close to the times of transitions through zero velocity. An algorithm is described that provides accurate determination of friction forces over a wide range of applied force and velocity conditions. The method avoids the simulation errors resulting from a finite integration interval used in connection with a conventional friction model, as is the case in many digital computer-based simulations. The algorithm incorporates a predictive calculation based on initial conditions of motion, externally applied forces, inertia, and integration step size. The predictive calculation in connection with an external integration process provides an accurate determination of both static and Coulomb friction forces and resulting motions in dynamic simulations. Accuracy of the results is improved over that obtained with conventional methods and a relatively large integration step size is permitted. A function block for incorporation in a specific simulation program is described. The general form of the algorithm facilitates implementation with various programming languages such as FORTRAN or C, as well as with other simulation programs.

Hill, R. E.↗
NASA NTRS1984

Electrical self-calibration of nonideal bolometers

An analytical method for determining the absorbed energy calibration of an IR bolometer from purely electrical measurements is presented. The technique is targeted at detectors with purely electrical nonlinearities. It is shown that the responsivity can be derived from the I-V plot if the limiting high-frequency impedance is measured. The electrical nonlinearity is then the difference between the impedance and the dc resistance. The model is used for designing a circuit for measuring the relative impedance parameters. The results are applicable to studies of IR sources and the detection of energy pulses with bolometers.

Mather, J. C.↗
NASA NTRS1989

The application of electrochemical impedance spectroscopy for characterizing the degradation of Ni(OH)2/NiOOH electrodes

Electrochemical impedance spectra of rolled and bonded and sintered porous nickel battery electrodes were recorded periodically during charge/discharge cycling in concentrated KOH solution at various temperatures. A transmission line model (TLM) was adopted to represent the impedance of the porous electrodes, and various model parameters were adjusted in a curve fitting routine to reproduce the experimental impedances. Degradation processes for rolled and bonded electrodes were deduced from changes in model parameters with electrode cycling time. In developing the TLM, impedance spectra of planar (non-porous) electrodes were used to represent the pore wall and backing plate interfacial impedances. These data were measured over a range of potentials and temperatures, and an equivalent circuit model was adopted to represent the planar electrode data. Cyclic voltammetry was used to study the characteristics of the oxygen evolution reaction on planar nickel electrodes during charging, since oxygen evolution can affect battery electrode charging efficiency and ultimately electrode cycle life if the overpotential for oxygen evolution is sufficiently low. Transmission line modeling results suggest that porous rolled and bonded nickel electrodes undergo restructuring during charge/discharge cycling prior to failure.

Macdonald, D. D.↗
NASA NTRS2010

Evaluation of Grounding Impedance of a Complex Lightning Protective System Using Earth Ground Clamp Measurements and ATP Modeling

A new Lightning Protection System (LPS) was designed and built at Launch Complex 39B (LC39B), at the Kennedy Space Center (KSC), Florida, which consists of a catenary wire system (at a height of about 181 meters above ground level) supported by three insulators installed atop three towers in a triangular configuration. A total of nine downconductors (each about 250 meters long, on average) are connected to the catenary wire system. Each of the nine downconductors is connected to a 7.62-meter radius circular counterpoise conductor with six equally spaced 6-meter long vertical grounding rods. Grounding requirements at LC39B call for all underground and above ground metallic piping, enclosures, raceways, and cable trays, within 7.62 meters of the counterpoise, to be bounded to the counterpoise, which results in a complex interconnected grounding system, given the many metallic piping, raceways, and cable trays that run in multiple direction around LC39B. The complexity of this grounding system makes the fall of potential method, which uses multiple metallic rods or stakes, unsuitable for measuring the grounding impedances of the downconductors. To calculate the downconductors grounding impedance, an Earth Ground Clamp (a stakeless grounding resistance measuring device) and a LPS Alternative Transient Program (ATP) model are used. The Earth Ground Clamp is used to measure the loop impedance plus the grounding impedance of each downconductor and the ATP model is used to calculate the loop impedance of each downconductor circuit. The grounding impedance of the downconductors is then calculated by subtracting the ATP calculated loop impedances from the Earth Ground Clamp measurements.

Mata, Carlos T.↗
NASA NTRS2010

Effects of Flow Profile on Educed Acoustic Liner Impedance

This paper presents results of an investigation of the effects of shear flow profile on impedance eduction processes employed at NASA Langley. Uniform and 1-D shear-flow propagation models are used to educe the acoustic impedance of three test liners based on aeroacoustic data acquired in the Langley Grazing Flow Impedance Tube, at source levels of 130, 140 and 150 dB, and at centerline Mach numbers of 0.0, 0.3 and 0.5. A ceramic tubular, calibration liner is used to evaluate the propagation models, as this liner is expected to be insensitive to SPL, grazing flow Mach number, and flow profile effects. The propagation models are then used to investigate the effects of shear flow profile on acoustic impedances educed for two conventional perforate-over-honeycomb liners. Results achieved with the uniform-flow models follow expected trends, but those educed with the 1-D shear-flow model do not, even for the calibration liner. However, when the flow profile used with the shear-flow model is varied to increase the Mach number gradient near the wall, results computed with the shear-flow model are well matched to those achieved with the uniform-flow model. This indicates the effects of flow profile on educed acoustic liner impedance are small, but more detailed investigations of the flow field throughout the duct are needed to better understand these effects.

Jones, Michael G.↗
NASA NTRS1988

Experimental observation of electrochemical rate limitations affecting sodium ion-electron recombination at electrodes of the alkali metal thermoelectric converter at T about 1200 K

This paper considers a model of the internal impedances of thin porous Mo and W alkali metal thermoelectric converter (AMTEC), in which the kinetic parameters associated with the reaction of the beta-double-prime alumina solid electrolite (BASE)/porous metal/gas three-phase boundary can be evaluated. Impedance data in the frequency range 0.01-100,000 Hz were collected over a range of AMTEC cell operating voltages for small-area thin porous Mo and W electrodes, yielding apparent charge transfer resistances at a series of cell potentials/currents. The ohmic resistance in the AMTEC cell could be broken down and characterized with three parameters: the BASE ionic resistance, the electrode film sheet resistance, and the contact/lead resistance, all of which could be calculated or measured independently and used to calculate power curves in good agreement with observed power curves. It is shown that these calculations can be used to predict the properties of electrodes with optimized parameters or to detect enhanced transport modes.

Williams, R. M.↗