Admittance Ratio Research Articles

Effects of Passive Porous Walls on Boundary-Layer Instability

A theoretical study is described of the effects of a passive porous wall on boundary-layer instability. The passive porouswall isconceived of as a thin porous sheet stretched over a plenum chamber. When disturbancesin theform of Tollmien‐Schlichting waves propagate along the boundary layer, the e uctuating pressure forces air in and out of the plenum chamber. The basic approach is based on classic linear stability theory for the e at-plate boundary layer with modie ed wall boundary conditions. The wall response is represented by a complex admittance (ratio of e uctuating e ow rate per unit area to wall pressure ) and, therefore, applies to a general class of passive porous, and other interactive, walls. The effects of a wide range of admittance values are studied. A stabilizing effect is obtained when the admittance phase is close to o/2, and an optimum value of admittance magnitude is also found. A theoretical model is introduced and used to calculate the admittance of the porous panels used in a parallel experimental study. It is shown that it may be possible to manufacture stabilizing porous panels. The stabilizing mechanism is due to the production of a near-wall region of negative Reynolds stress.

Three-dimensional sound field simulation using the boundary integral equation method

Computer simulation is a useful technique for developing the active control system (referred to as the ANC system). In this paper, the technique is introduced of sound field analysis for an ANC system based on the boundary integral equation method (BIEM). Since the BIEM accounts for wave motion characteristics precisely, the dynamic behavior of the control sound field can be considered, especially in the low-frequency band. The objective of the study is to investigate a transient sound field by applying the BIEM to practical problems. In order to introduce the BIEM technique to actual situations, including boundary problems with the existing acoustic materials, a database was developed on the normal acoustic admittance ratio of the materials. The ordinary feedforward model of the ANC system was used to control the noise field in the computer simulations. The results, in which the transient sound fields controlled by the ANC system are shown, will be very useful in understanding how to build up the control sound field.

Electrodynamic reciprocity method for vibration standards

In a previous paper, a reciprocity calibration formula different from the Levy–Bouche’s is derived [L.-F. Ge, Proceedings of the XIII IMEKO World Congress, 2606–2610 (1994)]; the latter has been regarded as a standard method [ISO 5347-0, 1987; ISO/DIS 5347-20, 1990]. The new formula is S=(RT/jω)1/2 (V/ms−2), where T is the mean of the transfer admittance ratio, Tn=Mn/(Yn−Yo) and R is the voltage ratio for the typical three-transducer-two-experiment procedure, in which an electrodynamic transducer serves as reciprocal one. This method is suited to the low- and mid-frequency ranges, and most accurate at 100–1000 Hz. This paper reveals that a coefficient of variation, i.e., ratio of standard deviation of Tn to its mean value, can be viewed as a reciprocity index of such an electrodynamic calibration system including two transducers and a solid mass medium. An index smaller than 0.5% indicates that the system is better reciprocal, and can be calibrated more accurately. At frequencies higher than about 3000 Hz, nonlinear problems, such as stress wave effect and resonance, become pronounced, and should be considered; a better way would be to apply other more available methods, such as the laser interferometry and piezoelectric reciprocity method.

The tectonics of Venus

Solid Venus has several differences from solid Earth: a mild variation in topography, with marked departures of some kilometres confined to less than 10% of the surface; no interconnected system of ridges, such as would be associated with a spreading lithospheric boundary layer; a high correlation of gravity with topography; and a ratio of gravity to topography implying compensation depths in excess of 100 km for major features. This high admittance ratio implies a stiff upper mantle, as also indicated by the high depth-diameter ratio of craters. The details of Magellan radar imagery enable some inference of event sequences. The morphology generally indicates a more regional scale of change than on Earth. The principal chronometer is impact crater distribution. The number of impact craters identified is 914, which is equivalent to about 500 Ma’s infall of bodies big enough to penetrate the atmosphere. But the number of craters per unit area has but slightly more variations from the mean than random and only a mild negative correlation with topographic elevation. Only one-third of the craters evidence tectonic or volcanic modification. Hence the rate of resurfacing over the last few 100 Ma must be quite slight compared to Earth’s. The main debate is whether Venerean tectonic activity is in monotonic decline, or whether it is episodic on a 500 Ma time-scale. Evidencing some current tectonic and volcanic activity are a few limited regions of marked topographic and geoidal highs, exceeding 8 km and 80 m respectively, with steep slopes. Clearly, plate tectonics does not exist on Venus. The underlying cause of this different evolution appears to be the lack of water. This dryness makes the upper mantle stiff enough to regionalize the tectonics and inhibit recycling of crust. The lack of water also prevents erosion and thus the recycling of secondary differentiates. These deficiencies in recycling imply a strong net upward differentiation of heat sources and a thick crust, now allowed by new results on dry diabase rheology.

Steady-flow Cf-Nre Correlation for the Wire Gauze Regenerator Inferred from Linear Wave Analysis

Linear wave analysis of unsteady flow in the wire mesh regenerator is extended to account for viscous losses (shear at the wall and abrupt change in the free-flow area). As a test of relevance to the Stirling gas circuit, the standard, unsteady-flow momentum equation is re-expressed in terms of the algebra [admittance ratios, pressure ratios) of the linear wave approach. This permits the steady-flow friction factor, Cf, to be extracted and plotted as a function of Reynolds number, Ntc. The resulting correlation is found to correspond closely to the experimentally determined counterpart. An explanation is found for the discrepancy noted between computed and observed pressure drops in the Stirling cycle machine.

Secular variation of the pole tide: correlation with Chandler wobble ellipticity

Summary The pole tide is an oceanic response to the Chandler wobble. The strong variations of wobble amplitude have led to the search for secular variations of the pole tide amplitude. Where such variations have been found though, it has not been possible to identify a correlation with wobble variations. It has been pointed out that the pole tide imposes a slight ellipticity on the pole path. In this paper the variations of Chandler wobble ellipticity and secular variations in the pole tide admittance (ratio between observed and equilibrium amplitudes) are investigated. A correlation of these two parameters is observed for most of the 37 stations. An estimation of phase lag between equilibrium and observed tide is given, using a theoretical evaluation of the effect of pole tide enhancement.

Musical spectrum and transfer function measurements: Techniques, pitfalls, interpretation

Sound spectrum measurements are difficult. The priorities follow: Instrument and player mutually comfortable. Player's task familiar and specified in the terms of his profession. Adequate averaging by microphone and source motion (net displacement ≫ λmax, distance from walls and floor > λmax). Care that microphone is truly in the reverberant field despite source directionalities. Signals from several microphortes summed or averaged only after analysis (otherwise results fluctuate as do 1-mike data). Angular distribution and frequency response measurements differ widely in very-nearfield (kR <<< 1), nearfield 1 < kR, ka < 6, and farfield R/a ≪ 1, kR ≪ 1. Here a = instrument size and R = microphone distance. Choice of transfer impedance, admittance, or simple ratio is governed by musical function. Excitation source impedance must be measured via response of known inertance, elastance, or transmission line. Similar (small) perturbers at driving point aid verification that Zs does not alter significant modes of the driven system. Parallel criteria exist for detectors. Care is required to prevent mechanical, acoustical, or electrical leakage from exciter to detector. [Work supported by NSF.]

A coupling theorem for matched symmetrical two-branch four-port networks

In this paper a theorem is derived which relates to the coupling of matched lossless symmetrical four-port networks of the twobranch type. In particular it is assumed that the two in-line and two branch-line networks are identical and that these pairs of networks differ identical two-port matching networks are connected at each of the fourports so that the complete device is matched at the frequencies <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{l}</tex> , <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{2},\cdots ,f_{n},</tex> ,then the coupling will be the same at all of these frequencies and dependent only on the admittance ratio <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</tex> .

Active-device capacitances

The charge-control capacitance ∂Q/∂V looking into a device port is not in general equal to the intrinsic small-signal capacitance found from the low-frequency admittance ratios up to 7:1 occur in practice. This paper explains why the discrepancy occurs, and derives the condition for the discrepancy to be zero. Finally, it is shown that charge-control theory does correctly predict the gain-bandwidth product of an active device.

Properties of dielectric-rod junctions in circular waveguide

The paper describes the representation of a junction in circular waveguide between a homogeneous region and one containing an axial rod of circular cross-section. The parameters of an approximate equivalent circuit are obtained in terms of the physical properties of the junction. Experiments are described which show that, for dielectric rods of low relative permittivity, it is reasonable to neglect the junction susceptance. If the relative permeability of the rod is unity, the characteristic-admittance ratio assumes a particularly simple form corresponding to the ratio of the dominant-mode phase-change coefficients on either side of the junction. When the relative permeability of the rod differs from unity, as in the ferrite analogue, the expression for the admittance ratio is more complicated. Results obtained using a digital computer are presented.

A locus diagram to determine the complete starting performance of a 3-phase induction motor connected to a single-phase supply

A graphical solution, which gives the sequence voltages, starting-torque ratio, line current, the voltage across the capacitor and the corresponding values of a dimensionless admittance ratio for a 3-phase induction motor at standstill and connected to a single-phase voltage, is derived. Several optimum conditions, in particular an exact admittance ratio for minimum unbalance, are obtained from the locus diagram. In all cases it is shown that the admittance ratio obtained is independent of the applied voltage and the locus diagram is therefore applicable to machines of any rating.

Unbalanced RLC Networks Containing Only One Resistance and One Real Transformer

Any transfer function, in order to be physically realizable as a lossless coupling network terminated in a single resistance, must have an even or odd polynomial as its numerator. Such a function, representing a transfer admittance, a transfer voltage ratio, or a transfer impedance, can be realized by the method of this paper as a lossless lattice network terminated in resistance and possessing no mutual inductance. If the numerator of the transfer admittance or voltage ratio is even and the numerator of the transfer impedance is odd, these lattices can always be reduced to unbalanced networks. No ideal transformers are needed in the final network; only one real transformer is required, where a real transformer is defined as one that has a coupling coefficient smaller than one and finite magnetizing inductance.

Unbalanced RLC Networks Containing Only One Resistance and One Real Transformer

Any transfer function, in order to be physically realizable as a lossless coupling network terminated in a single resistance, must have an even or odd polynomial as its numerator. Such a function, representing a transfer admittance, a transfer voltage ratio, or a transfer impedance, can be realized by the method of this paper as a lossless lattice network terminated in resistance and possessing no mutual inductance. If the numerator of the transfer admittance or voltage ratio is even and the numerator of the transfer impedance is odd, these lattices can always be reduced to unbalanced networks. No ideal transformers are needed in final network; only one real transformer is required, where a real transformer is defined as one th%-hn coupling coefficient smaller than one and finite magnetizing inductance.