Showing papers on "Natural frequency published in 1978"

Intracranial pressure implant

Abstract: An implantable transensor device containing a passive RF resonant circuit having a natural frequency influenced by the pressure of the sensor's environment in a body cavity of a living entity. The circuit of the transensor includes an inductor and a capacitor, at least one of which varies in value in direct relation to variation of environmental pressure to change the resonant frequency of the circuit. The circuit can be externally interrogated to determine the resonant frequency thereof at any point in time by the imposition thereon of swept frequency electromagnetic radiation provided by a monitoring device which determines when some of the radiation is absorbed as a result of the frequency of the radiation being the same as the resonant frequency of the transensor circuit. An imposed relationship exists between the sensed environmental pressure, and the reactance of the reactive components of the circuit. A natural relationship exists between pressure sensitive reactance, and the resonant frequency of the circuit. As a result, an increase in environmental pressure causes a corresponding increase in frequency and a decrease in environmental pressure causes a decrease in frequency.

Coupled hydrodynamic response of concrete gravity dams using finite and infinite elements

Abstract: This paper presents the application of the finite element method for analysing the two-dimensional response of reservoir-dam systems subjected to horizontal ground motion The interaction between the dam and the reservoir as well as the compressibility of water has been taken into account The complete system has been considered to be composed of two substructures, namely the reservoir and the dam To take into account the large extent of the reservoir, it has been idealized using specially developed infinite elements coupled with standard finite elements while the dam is represented using finite elements alone Structural damping of the dam and radiation damping in the fluid phase have been accounted for in the analysis It is concluded that the effect of radiation damping is considerable at high frequencies of excitation The coupled response of the system is significantly large at and near the fundamental natural frequency of the system in comparison to the uncoupled responses The method is computationally quite economical, capable of taking into account the arbitrary geometry of the system and is recommended for practical application Further applications and extensions of the approach to three dimensional analyses are possible

Modelling waving crops in a wind tunnel

Abstract: Analysis of movie films of a field of barley, combined with observations of the motions of individual plants, show that single stalks oscillate at a well-defined natural frequency even when stimulated by turbulent winds. Treating single stalks as resonant cantilevers allows the use of standard engineering methods to determine their elastic properties. Armed with these values, the application of similarity analysis to the equation of motion of a single stalk leads to criteria for aeroelastic modelling of wheat plants in the wind tunnel. A representative value for the spacing of stalks in a small section of model wheat field was calculated by referring to published data on momentum absorption in a variety of real and model canopies. Preliminary measurements of first and second moments of velocity in the model appear to confirm the importance of including elastic properties in wind-tunnel simulations of airflow in flexible crops.

Second order approximation of natural frequency constraints in structural synthesis

Abstract: 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.

Theoretical Prediction of Motions of High-Speed Planing Boats in Waves

Abstract: A theoretical method is derived for predicting the linearized response characteristics of constant-deadrise high-speed planing boats in head and following waves. Comparisons of the theoretical predictions of the pitch and heave response-amplitude operators and phase angles with existing experimental data show reasonably good agreement for a wide variety of conditions of interest. It appears that nonlinear effects are more severe at a speed-to-length ratio of 6 than of, say, 4 or less, principally because of the reduction of the damping ratio of the boat with increasing speed, and the consequent increase in motions in the vicinity of the resonant encounter frequency. However, it is concluded that the linear theory can provide a simple and fast means of determining the effect of various parameters such as trim angle, deadrise, loading, and speed on the damping, natural frequency, and linearized response in waves, and that this can furnish valuable insight into the actual boat dynamics, even though the accurate predictions of large motions and peak accelerations would require a nonlinear analysis.

Method and apparatus for absorbing dynamic forces on structures

Abstract: An apparatus and method is provided for suppressing the dynamic response of structures such as offshore structures to forces, such as that of wind, wave or seismic excitation. A storage tank for liquid with a free surface is positioned on the structure and is designed to have a natural frequency equal to or nearly equal to the natural frequency of the structure. The storage tank containing the liquid reduces the dynamic response of the structure thereby reducing structural degradation such as metal fatique in the structure and increasing the useful life of the structure. The effect of the tank on the motion of the structure is that of a common dynamic absorber.

Vibrations of a String with Time-Variable Length

Abstract: Free and forced vibrations of a string with time-variable length are treated theoretically under the assumption that the variation rate of the length is small. The solutions are obtained n the form of a power series of a small parameter which prescribes the variation rate of the length. The theoretical analysis shows that the free vibration has a nature of a travelling wave. As to the forced vibration occurring during a transition through the fundamental resonance point, the analysis shows that the amplitude becomes maximum slightly later than the moment when the "natural frequency" of the system coincides with the frequency of the external force. An experimental analysis is performed for the forced vibration. The comparison of the results of the theoretical analysis with those of the experimental analysis shows a fairly good agreement.

Vibrations of a String with Time-Variable Length

Abstract: Free and forced vibrations of a string with time-variable length are treated theoretically under the assumption that the variation rate of the length is small. The solutions are obtained n the form of a power series of a small parameter which prescribes the variation rate of the length. The theoretical analysis shows that the free vibration has a nature of a travelling wave. As to the forced vibration occurring during a transition through the fundamental resonance point, the analysis shows that the amplitude becomes maximum slightly later than the moment when the "natural frequency" of the system coincides with the frequency of the external force. An experimental analysis is performed for the forced vibration. The comparison of the results of the theoretical analysis with those of the experimental analysis shows a fairly good agreement.

Quasi-random pneumatic vibration facility and automatic frequency modulating system therefor

Abstract: A low-cost, multi-axis, quasi-random vibration system includes pneumatically driven vibrators coupled to a resonating, self-attenuating shaker structure to achieve a frequency spectrum and acceleration-level control of a broadband quasi-random vibration output in the frequency range, for example, from 40 Hz to 2 kHz for vibration testing of equipment. The shaker structure consists of flexible interconnected structures whose primary or driving structure acts to mechanically process the vibratory input of the coupled vibrators and whose supporting or driven structure supplies a further vibrational processing to supply the test item with multi-axis multimodal input. Automatic control and pseudo-random modulation of air pressure of the pneumatic vibrators provide closed-loop broadband acceleration-spectrum control and spectrum smearing to enhance frequency content and to prevent the shaker from locking onto any particular vibration frequency, especially a natural frequency of the shaker. Control is achieved in and about three orthogonal axes simultaneously, thus affording a realistic simulation of operational environments. The vibrators' vibration frequency is modulated by means of a variable-area orifice in the pneumatic line located between the air supply and the pneumatic vibrators. A pseudo-random change in the orifice area is made automatically, for example, every 2 to 3 seconds, by a preprogrammed microprocessor-controlled flow control mechanism.

Vibration damped railway wheel

Abstract: A vibration absorbing track wheel having one or more different natural frequencies. The wheel body has a wheel flange with one or more resonance absorbers on the wheel flange. Each resonance absorber comprises at least two tongues and an intermediate layer of damping material between each of the two adjacent tongues, and each tongue is turned to a different natural frequency of the wheel.

Immitance measurement with high frequency injection and electromagnetic coupling

Abstract: A system for measuring the transfer immitances, impedance and admittance, a linear electrical network having one or more ports. The system utilizes clamp-on ferromagnetic cores to electromagnetically couple the transfer immitance measuring system to the electrical network without having to interrupt the normal on-line operation of the electrical network. A high frequency injection network, which includes a high frequency signal generator and a multichannel amplifier, injects a high frequency signal into each of the ports of the electrical network. High frequency voltage and current measuring networks are electromagnetically coupled to each of the ports of the electrical network to measure the high frequency component of the port voltage and the port current. By adjusting the relative amplitude and phase of each of the injected high frequency signals, open- and closed-circuit conditions, in the high frequency range, can be simulated in the ports of the electrical network, permitting the transfer immitances of the electrical network to be established without interrupting normal, on-line operation.

Fluidic stabilization control

Abstract: A fluidic system for stabilizing movement of a body subject to oscillation at its natural frequency, such as a tank-mounted gun, with appropriate circuitry and controls for notch filtering control output signals at the natural frequency.

Free oscillations of submerged spherical shells

Abstract: Free vibrations of a spherical shell submerged in a fluid medium are investigated. It is found that no undamped natural frequency of the submerged shell can exist even if the surrounding fluid is assumed inviscid. In this case the damping is solely due to the compressibility of the fluid. However, for the intermediate modes, the damping components of the complex frequencies are extremely small; thus, an almost steady‐state, undamped free oscillation is possible for these modes and a pronounced resonance may be observed for the forced vibrations. When the shell is submerged in an ideal fluid, it is shown that there exist two real natural frequencies for each mode. These frequencies are lower than the corresponding in vacuo natural frequencies. The effect of fluid viscosity on the complex natural frequencies has also been examined. It is found that for small viscosities, the viscosity has essentially no effect on the real component of the natural frequencies.

Monitoring machinery by detecting vibrations

Abstract: A piece of machinery which, in an abnormal operating condition exhibits a vibration frequency characteristic of or peculiar to that condition, is monitored by securing a spring and mass unit to the machinery, the spring and mass unit having a natural frequency equal to said vibration frequency so that upon the occurence of the abnormal condition the magnified vibration of the spring and mass unit can be used to effect a control function or sound an alarm.

Natural frequencies of structures for designers

Abstract: By separating a vibration problem into two distinct parts, one concerned only with the spring forces in the structure and the other only with the inertial forces imposed by the mass carried, easy calculations can be used to solve fairly complex structures. Naturally, since this method does not concern itself with damping forces it will not tackle problems concerned with the amplitude of vibration caused by a particular excitation. It will, however, deal with one of the major questions in design, 'what are the natural frequencies of the particular structure, do any of them correspond with a known dynamic load and, if so, how can the structure be altered to change that natural frequency sufficiently?' the method is based on the simple formula for the frequency of vibration of a mass hanging on a spring and shows that it is possible to reduce the difficulty of structural problems until mere hand calculations will give sufficiently accurate values for use in design. The method will find the lowest natural frequency, or the frequency of any other mode shape chosen by an engineer, including the cases of partial or sway vibrations. /Author/TRRL/

A new resonant thermometer for nuclear reactor applications

Abstract: An ultrasonic thermometer using the temperature dependence of ultrasonic velocity as the transducing mechanism has been developed for high temperature measurement. Essentially it is a long, thin probe consisting of a magnetostrictive launcher producing longitudinal vibrations in an acoustic line. A resonator which terminates the line constitutes the sensor. By measuring its natural resonant frequency as a function of temperature a calibration of the probe is obtained. A pulse excitation technique is used to drive the resonator after which the oscillations decay exponentially (the decrement) at the natural frequency of the resonator. The control compares this frequency with that of a signal generator to produce an error signal. This is used to make the signal frequency track that of the sensor. Digital aperiodic techniques are used throughout giving a smooth operation over the full range of the instrument. The setting-up procedure for changing a probe is minimal.

Static pressure of gases measuring device - has oscillating element whose amplitude variations due to gas pressure changes are measured

Abstract: An oscillating element (1) in contact with the gas is made to oscillate at a frequency equal or near to its natural frequency. The change of amplitude due to a change of gas pressure is measured either directly by measurement of a length, an angle or a piezopressure, or indirectly by measurement of the reaction of the oscillation amplitude on a variable electric parameter of the excitation system. The variable parameter used may be impedance, current voltage or a physical quantity effected through the gas by the oscillation amplitude, such as sound pressure or velocity. These variables are measured and used as an indirect measure for the static gas pressure.

A servo force-balance instrument for the direct measurement of wall shear stress on circular cylinders

Abstract: Details of the design and construction of an instrument for the direct measurement of skin friction on cylinders in an external gas flow are given. The use of the servo force-balance principle represents a valuable improvement on previous instruments because (i) the sensing element is held in a fixed position by integral (I) control, guaranteeing a constant local flow field, and (ii) the proportional (P) feedback can be used to increase the natural frequency of the system, thus allowing the accurate and sensitive detection of such dynamic features as vortex shedding frequencies. The use of velocity, or derivative (D), feedback instead of oil damping allows operation of the transducer in an arbitrary orientation. It is an important fact that the sensitivity of a force-balance system is independent of the PID settings.

Regulation of a plurality of superconducting resonators

Abstract: In order to maintain a plurality of superconducting resonators, each having elastically deformable resonant structural elements, at the same natural frequency and phase position, each resonator is supplied with high frequency power independently of the other resonators forming part of the same system, the natural frequency of each resonator is set to a predetermined value by adjusting the amplitude of the power supplied, mechanical vibrations of the resonant structural elements are aperiodically suppressed by a velocity-dependent attenuation, and the resonator is returned to a predetermined high frequency operating vibration.

On the information content of natural frequency spectra associated with different angular numbers

Abstract: The problem of inferring the speed of sound in a contained spherically symmetric fluid solely from its natural frequencies of vibration is considered. An investigation of the case in which the data consist of the two spectra associated with the angular numbers 0 and 1, suggests the possibility that a one-parameter family of slowness profiles can be constructed. These profiles are compatible with the data, up to first order in the non-uniformity of the fluid. It is conjectured that for other angular numbers, the loss of information increases as the difference between them increases.

Dispersion and Cut-Off Phenomena in Rods and Beams

Abstract: : This study was proposed to ONR Structural Mechanics Program as a result of in-house research which led to the conclusion that uniform structural waveguides of small transverse dimensions (1) display a vanishing impedance at every modal cut-off frequency; (2) that each cut-off frequency coincides with the natural frequency of the corresponding standing-wave mode of the cross section, the modal configuration being independent of the z-coordinate defined here as the coordinate along the waveguide axis The preliminary study was restricted to homogeneous waveguides whose cross-section and material properties are z-independent Furthermore, as already stated the analysis was limited to waveguides whose transverse dimension was small enough to eliminate thickness vibrations, eg Euler beams on distributed springs, simply-supported strips, and thin cylindrical shells

Acoustic hologram generation with high linearity - uses surface waves on liq. modulated at frequency resonating with spatial information frequency

Abstract: A method is used for the production of an acoustic hologram of an object by the liquid surface relief method. The amplitudes of the object and/or the reference sound wave beam are periodically modulated at a frequency resonating with the natural frequency of the surface waveform appropriate to the spatial frequency responsible for the transmission of the object information. The system gives an enhanced deformation of the surface of the liquid with an improved linearity of its function. The two sound beams are propagated from point sources (80, 83).

Transient Analysis of Equipment-Structure Interaction at High Frequencies.

Abstract: : An analytical method is developed which yields a simple estimate of the maximum dynamic response of light equipment attached to a structure subjected to ground motions. The natural frequency of the equipment, modeled as a single-degree-of-freedom system, is considered to be close, or equal, to one of the natural frequencies of the N-degree-of-freedom structure. This bound provides a convenient rational method for the structural design of the equipment and its installation. The approach is based on the transient analysis of lightly damped, tuned or detuned equipment-structure systems in which the mass of the equipment is small in comparison with that of the structure. It is assumed that the information available to the designer is a design spectrum for the ground motion and fixed-base modal properties of the structure alone and of the equipment alone. The results obtained are estimates of the maximum acceleration and displacement of the equipment in terms of this information. The method can also be used to treat closely-spaced modes in structural systems, where the conventional square root of the sum of squares procedure is known to be invalid.

Implementation of natural frequency analysis and optimality criterion design. [computer technique for structural analysis]

Abstract: Abstract A review of methods to compute the lower-frequency modes within a medium- to large-capacity computer design program indicates advantages for the simultaneous iteration method. For computer design, the computational advantages are further enhanced by more rapid convergence when using eigenvectors from prior design cycles. The optimality criterion design algorithms are summarized for the problem of minimum structural weight with constrained frequency and for the dual problem of constrained weight with maximum frequency. The capability to design for natural frequency requires only minor coding modifications within an existing computer program that can perform design for static loading. A simple sample application problem shows agreement with a known theoretical result. Another example shows effective results for a structure of practical complexity within reasonable computing times. A summary contains recommendations for continuing research.