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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1960

Experimental Investigation of the Natural Frequencies of Liquids in Toroidal Tanks

Several toroidal configurations applicable to missile and space-vehicle liquid storage systems were oscillated to study the natural frequencies of the antisymmetric modes of contained liquids over a range of liquid depths and tank sizes. Natural frequencies for tank oscillations parallel to the free surface of both vertical and horizontal tank orientations. Natural frequencies were obtained. The data are presented in terms of dimensionless parameters which are obtained by relating experimentally determined natural liquid frequencies to analytical expressions developed through consideration of the physics of the problem and from existing solutions for liquids in tanks having similar boundaries at the liquid surface. The experimental results obtained for the toroids indicate that these parameters are applicable to the prediction of the natural frequencies of fluids in toroids of general geometry and size.

McCarty, John Locke↗
NASA NTRS1958

Analytical and Experimental Determination of the Coupled Natural Frequencies and Mode Shapes of a Dynamic Model of a Single-Rotor Helicopter

A combined analytical and experimental determination is made of the coupled natural frequencies and mode shapes in the longitudinal plane of symmetry for a dynamic model of a single-rotor helicopter. The analytical phase is worked out on the basis of a seven-degree-of-freedom system combining elastic deflections of the rotor blades, rotor shaft, pylon, and fuselage. The calculated coupled frequencies are first compared with calculated uncoupled frequencies to show the general effects of coupling and then with measured coupled frequencies to determine the extent to which the coupled frequencies can be calculated. The coupled mode shapes are also calculated and were observed visually with stroboscopic lights during the tests. A comparison of the coupled and uncoupled natural frequencies shows that significant differences exist between these frequencies for some of the modes. Good agreement is obtained between the measured and calculated values for the coupled natural frequencies and mode shapes. The results show that the coupled natural frequencies and mode shapes can be determined by the analytical procedure presented herein with sufficient accuracy if the mass and stiffness distributions of the various components of the helicopter are known.

Silveira, Milton A.↗
NASA NTRSDate not supplied

Uncertainty Quantification of Inducer Natural Frequency using Conditional Assessment of Modeling and Modal Testing of Simpler Systems

The low pressure fuel pump inducer of the new Space Launch System RS25 core stage engine operates in a highly complex environment that substantially affects its modal characteristics. Some of the more important effects are fluid-added mass resulting from operation within a light liquid (Hydrogen), and the magnification of this effect due to tight tip clearance. Since higher order cavitation has been identified as a significant harmonic driver, knowledge of the natural frequency of potentially excitable modes is critical for safe operation, but this frequency cannot be measured during the severe operational environment. A comprehensive testing and analysis program has therefore been performed over the last four years to identify the nominal value and uncertainty of the frequency by modeling and testing four simpler structures which share some of the characteristics of the operational inducer. This testing was used to assess and adjust modeling techniques and excellent correlation was achieved. Identification of the uncertainty in the inducer frequency itself was still problematic, however. This difficulty led to an investigation of Bayesian uncertainty quantification techniques, and to the application of the relatively simple technique of Multi-Variate Normal conditional distributions to calculate the inducer natural frequency uncertainty. Assumptions on prior distributions of uncertainty of the fluid-added mass and tip clearance effect are initially applied to models of each of the simple structures and the inducer itself, and these uncertainties are propagated to generate natural frequencies using design of experiments. Simple response surfaces are then created from this data in order to calculate a Covariance Matrix relating all of these natural frequencies. Finally, the results from modal test of the simple structures are considered to be observations and used to calculate the conditional variance of the desired inducer frequencies. As this method is less rigorous than more complicated Bayesian methods reported in the literature, a conservative factor is applied to the result, but the resulting uncertainty is still significantly less than originally estimated and will greatly assist certification of the inducer for use in the engine.

Uncertainty Quantification↗
NASA NTRS2012

A Computational Investigation for Determining the Natural Frequencies and Damping Effects of Diaphragm-Implemented Spacecraft Propellant Tanks

Spin-stabilization maneuvers are typically performed by spacecraft entering low-earth orbit to maintain attitude stability. These maneuvers induce periodic fluid movement inside the spacecraft's propellant tank known as fuel slosh, which is responsible for creating forces and moments on the sidewalls of the propellant tank. These forces and moments adversely affect spin-stabilization and risk jeopardizing the mission of the spacecraft. Therefore, propellant tanks are designed with propellant management devices (PMD's) such as barnes or diaphragms which work to counteract the forces and moments associated with fuel slosh. However, despite the presence of PMD's, the threat of spin-stabilization interference still exists should the propellant tank be excited at its natural frequency. When the fluid is excited at its natural frequency, the forces and moments acting on the propellant tank are amplified and may result in destabilizing the spacecraft. Thus, a computational analysis is conducted concerning diaphragm-implemented propellant tanks excited at their natural frequencies. Using multi-disciplinary computational fluid dynamics (CFD) software, computational models are developed to reflect potential scenarios that spacecraft propellant tanks could experience. By simulating the propellant tank under a wide array of parameters and variables including fill-level, gravity and diaphragm material and shape, a better understanding is gained as to how these parameters individually and collectively affect liquid propellant tanks and ultimately, spacecraft attitude dynamics.

Lenahen, Brian↗
NASA NTRS1971

An estimator for the standard deviation of a natural frequency. II.

A method has been presented for estimating the variability of a system's natural frequencies arising from the variability of the system's parameters. The only information required to obtain the estimates is the member variability, in the form of second-order properties, and the natural frequencies and mode shapes of the mean system. It has also been established for the systems studied by means of Monte Carlo estimates that the specification of second-order properties is an adequate description of member variability.

Schiff, A. J.↗
NASA NTRS1989

A novel implementation of method of optimality criterion in synthesizing spacecraft structures with natural frequency constraints

In the design of spacecraft structures, fine tuning the structure to achieve minimum weight with natural frequency constraints is a time consuming process. Here, a novel implementation of the method of optimality criterion (OC) is developed. In this new implementation of OC, the free vibration analysis results are used to compute the eigenvalue sensitivity data required for the formulation. Specifically, the modal elemental strain and kinetic energies are used. Additionally, normalized design parameters are introduced as a second level linking that allows design variables of different values to be linked together. With the use of this novel formulation, synthesis of structures with natural frequency constraint can be carried out manually using modal analysis results. Design examples are presented to illustrate this novel implementation of the optimality criterion method.

Wang, Bo Ping↗
NASA NTRS1978

Second order approximation of natural frequency constraints in structural synthesis

The paper is concerned with structural optimization problems in which the constraints consist of limits imposed on the natural frequencies of the structure. Improvements in the quality of approximations for the frequency constraints are sought by considering second-order Taylor series expansions. Analysis of the second-order sensitivity of natural frequencies is carried out. As an example, the design of a midplane symmetric aluminum beam of constant depth supporting three nonstructural masses is considered. Web thickness and chord member areas were determined so as to achieve minimum structural mass while maintaining the fundamental frequency at or above 30 Hz.

Miura, H.↗
NASA NTRS1978

Implementation of natural frequency analysis and optimality criterion design

A description is presented of an effective optimality criterion computer design approach for member size selection to improve frequency characteristics for moderately large structure models. It is shown that the implementation of the simultaneous iteration method within a natural frequency structural design optimization provides a method which is more efficient in isolating the lowest natural frequency modes than the frequently applied Stodola method. Additional computational advantages are derived by using previously converged eigenvectors at the start of the iterations during the second and the following design cycles. Vectors with random components can be used at the first design cycle, which, in relation to the entire computer time for the design program, results in only a moderate computational penalty.

Levy, R.↗
NASA NTRS1975

Digital computer program DF1758 fully coupled natural frequencies and mode shapes of a helicopter rotor blade

The analytical techniques and computer program developed in the fully-coupled rotor vibration study are described. The rotor blade natural frequency and mode shape analysis was implemented in a digital computer program designated DF1758. The program computes collective, cyclic, and scissor modes for a single blade within a specified range of frequency for specified values of rotor RPM and collective angle. The analysis includes effects of blade twist, cg offset from reference axis, and shear center offset from reference axis. Coupled inplane, out-of-plane, and torsional vibrations are considered. Normalized displacements, shear forces and moments may be printed out and Calcomp plots of natural frequencies as a function of rotor RPM may be produced.

Bennett, R. L.↗
NASA NTRS1961

An Investigation of the Natural Frequencies and Mode Shapes of Liquids in Oblate Spheroidal Tanks

An experimental investigation was conducted to gain some understanding of the character of the free vibration modes of liquids In oblate spheroidal tanks applicable in missile and space vehicle systems, Measured natural frequencies were obtained for the lowest three or four, antisymmetric modes of oscillation as a function of the liquid depth for three orientations of each of several such tanks of different size and oblateness. The orientations considered were such that: (a) the equator of the spheroid was horizontal and oscillations were along a diameter of the circular liquid surface; (b) the equator of the spheroid was vertical and the oscillations were along the minor axis of the elliptical liquid surface; and (c) the equator of the spheroid was vertical and the oscillations were along the major axis of the elliptical liquid surface; The frequency data are presented as dimensionless parameters developed for each orientation to permit the application of the experimental results to the prediction of the natural frequencies of tanks of different size and oblateness. Photographs we re made of representative surface wave or mode, shapes for each orientation.

Leonard, H. Wayne↗