Showing papers by "Massachusetts Institute of Technology published in 1969"

High-resolution frequency-wavenumber spectrum analysis

Abstract: The output of an array of sansors is considered to be a homogeneous random field. In this case there is a spectral representation for this field, similar to that for stationary random processes, which consists of a superposition of traveling waves. The frequency-wavenumber power spectral density provides the mean-square value for the amplitudes of these waves and is of considerable importance in the analysis of propagating waves by means of an array of sensors. The conventional method of frequency-wavenumber power spectral density estimation uses a fixed-wavenumber window and its resolution is determined essentially by the beam pattern of the array of sensors. A high-resolution method of estimation is introduced which employs a wavenumber window whose shape changes and is a function of the wavenumber at which an estimate is obtained. It is shown that the wavenumber resolution of this method is considerably better than that of the conventional method. Application of these results is given to seismic data obtained from the large aperture seismic array located in eastern Montana. In addition, the application of the high-resolution method to other areas, such as radar, sonar, and radio astronomy, is indicated.

On the ductile enlargement of voids in triaxial stress fields

Abstract: The fracture of ductile solids has frequently been observed to result from the large growth and coalescence of microscopic voids, a process enhanced by the superposition of hydrostatic tensile stresses on a plastic deformation field. The ductile growth of voids is treated here as a problem in continuum plasticity. First, a variational principle is established to characterize the flow field in an elastically rigid and incompressible plastic material containing an internal void or voids, and subjected to a remotely uniform stress and strain rate field. Then an approximate Rayleigh-Ritz procedure is developed and applied to the enlargement of an isolated spherical void in a nonhardening material. Growth is studied in some detail for the case of a remote tensile extension field with superposed hydrostatic stresses. The volume changing contribution to void growth is found to overwhelm the shape changing part when the mean remote normal stress is large, so that growth is essentially spherical. Further, it is found that for any remote strain rate field, the void enlargement rate is amplified over the remote strain rate by a factor rising exponentially with the ratio of mean normal stress to yield stress. Some related results are discussed, including the long cylindrical void considered by F.A. McClintock (1968, J. appl. Mech . 35 , 363), and an approximate relation is given to describe growth of a spherical void in a general remote field. The results suggest a rapidly decreasing fracture ductility with increasing hydrostatic tension.

Looking at Upside-down Faces

Abstract: Compared memory for faces with memory for other classes of familar and complex objects which, like faces, are also customarily seen only in 1 orientation (mono-oriented). Performance of 4 students was tested when the inspection and test series were presented in the same orientation, either both upri

Resolved Motion Rate Control of Manipulators and Human Prostheses

Abstract: The kinematics of remote manipulators and human prostheses is analyzed for the purpose of deriving resolved motion rate control. That is, the operator is enabled to call for the desired hand motion directly along axes relevant to the task environment. The approach suggests solutions to problems of coordination, motion under task constraints, and appreciation of forces encountered by the controlled hand.

Two visual systems.

Abstract: Visual systems of hamster brain, discussing relative visual localization and discrimination blindness produced by ablation of cortical or tectal areas

Peristaltic pumping with long wavelengths at low Reynolds number

Abstract: Pumping by means of an infinite train of peristaltic waves is investigated under conditions for which the relevant Reynolds number is small enough for inertial effects to be negligible and the wavelength to diameter ratio is large enough for the pressure to be considered uniform over the cross-section. Theoretical results are presented for both plane and axisymmetric geometries, and for amplitude ratios ranging from zero to full occlusion. For a given amplitude ratio, the theoretical pressure rise per wavelength decreases linearly with increasing time-mean flow. An experiment with a quasi-two-dimensional apparatus confirmed the theoretical values.Calculations of the detailed fluid motions reveal that under many conditions of operation the net time-mean flow is the algebraic difference between a forward time-mean flow in the core of the tube and a backward (‘reflux’) time-mean flow near the periphery. The percentage of reflux flow can be very high. This reflux phenomenon is probably of physiologic significance in the functioning of the ureter and the gastro-intestinal system. A second fluid-mechanical peculiarity with physiological implications is that of ‘trapping’: under certain conditions an internally circulating bolus of fluid, lying about the axis, is transported with the wave speed as though it were trapped by the wave.

Atmospheric Predictability as Revealed by Naturally Occurring Analogues

Abstract: Two states of the atmosphere which are observed to resemble one another are termed analogues. Eitherstate of a pair of analogues may be regarded as equal to the other state plus a small superposed "error."From the behavior of the atmosphere following each state, the growth rate of the error may be determined. Five years of twice-daily height values of the 200-, 500-, and 850-mb surfaces at a grid of 1003 pointsover the Northern Hemisphere are procured. A weighted root-mean-square height difference is used as ameasure of the difference between two states, or the error. For each pair of states occurring within onemonth of the same time of year, but in different years, the error is computed. There are numerous mediocre analogues but no truly good ones. The smallest errors have an averagedoubling time of about 8 days. Larger errors grow less rapidly. Extrapolation with the aid of a quadratichypothesis indicates that truly small errors would double in about 2.5 days. These rates may be comparedwith a ...

The predictability of a flow which possesses many scales of motion

Abstract: It is proposed that certain formally deterministic fluid systems which possess many scales of motion are observationally indistinguishable from indeterministic systems; specifically, that two states of the system differing initially by a small “observational error” will evolve into two states differing as greatly as randomly chosen states of the system within a finite time interval, which cannot be lengthened by reducing the amplitude of the initial error. The hypothesis is investigated with a simple mathematical model. An equation whose dependent variables are ensemble averages of the “error energy” in separate scales of motion is derived from the vorticity equation which governs two-dimensional incompressible flow. Solutions of the equation are determined by numerical integration, for cases where the horizontal extent and total energy of the system are comparable to those of the earth's atomsphere. It is found that each scale of motion possesses an intrinsic finite range of predictability, provided that the total energy of the system does not fall off too rapidly with decreasing wave length. With the chosen values of the constants, “cumulus-scale” motions can be predicted about one hour, “synoptic-scale” motions a few days, and the largest scales a few weeks in advance. The applicability of the model to real physical systems, including the earth's atmosphere, is considered. DOI: 10.1111/j.2153-3490.1969.tb00444.x

Photoluminescence of Metals

Abstract: Radiative recombination in gold, copper, and gold-copper alloys has been observed arising from transitions between electrons in conduction-band states below the Fermi level and holes in the $d$ bands generated by optical excitation.

Optical Constants of Soot and Their Application to Heat-Flux Calculations

Abstract: Data on the room temperature optiaI.1 constants of soot are presentedfor the 'Wavelength regions 0.4-().8j.1 and 2.5-1O.0j.l. Dispersion formulas are developedfor interpolating the data between0.8 and 2.5j.1. The results are used to calculate the spectral absorption coefficient and the total emissivities of soot suspensions. It is shown that the correct values of the optiaI.l constants are neededin the use of light-scattering techniques for the measurement of the soot concentration but that uncertainties introduced in flux

The chirp z-transform algorithm

Abstract: A computational algorithm for numerically evaluating the z -transform of a sequence of N samples is discussed. This algorithm has been named the chirp z -transform (CZT) algorithm. Using the CZT algorithm one can efficiently evaluate the z -transform at M points in the z -plane which lie on circular or spiral contours beginning at any arbitrary point in the z -plane. The angular spacing of the points is an arbitrary constant, and M and N are arbitrary integers. The algorithm is based on the fact that the values of the z -transform on a circular or spiral contour can be expressed as a discrete convolution. Thus one can use well-known high-speed convolution techniques to evaluate the transform efficiently. For M and N moderately large, the computation time is roughly proportional to (N+M) \log_{2}(N+M) as opposed to being proportional to N . M for direct evaluation of the z -transform at M points.

A study of Bénard convection with and without rotation

Abstract: An experimental study of the response of a thin uniformly heated rotating layer of fluid is presented. It is shown that the stability of the fluid depends strongly upon the three parameters that described its state, namely the Rayleigh number, the Taylor number and the Prandtl number. For the two Prandtl numbers considered, 6·8 and 0·025 corresponding to water and mercury, linear theory is insufficient to fully describe their stability properties. For water, subcritical instability will occur for all Taylor numbers greater than 5 × 104, whereas mercury exhibits a subcritical instability only for finite Taylor numbers less than 105. At all other Taylor numbers there is good agreement between linear theory and experiment.The heat flux in these two fluids has been measured over a wide range of Rayleigh and Taylor numbers. Generally, much higher Nusselt numbers are found with water than with mercury. In water, at any Rayleigh number greater than 104, it is found that the Nusselt number will increase by about 10% as the Taylor number is increased from zero to a certain value, which depends on the Rayleigh number. It is suggested that this increase in the heat flux results from a perturbation of the velocity boundary layer with an ‘Ekman-layer-like’ profile in such a way that the scale of boundary layer is reduced. In mercury, on the other hand, the heat flux decreases monotonically with increasing Taylor number. Over a range of Rayleigh numbers (at large Taylor numbers) oscillatory convection is preferred although it is inefficient at transporting heat. Above a certain Rayleigh number, less than the critical value for steady convection according to linear theory, the heat flux increases more rapidly and the convection becomes increasingly irregular as is shown by the temperature fluctuations at a point in the fluid.Photographs of the convective flow in a silicone oil (Prandtl number = 100) at various rotation rates are shown. From these a rough estimate is obtained of the dominant horizontal convective scale as a function of the Rayleigh and Taylor numbers.

Formation and properties of thin‐walled phospholipid vesicles

Abstract: Large numbers of thin-walled vesicles, 0.5 to 10 μ in diameter, can be formed by permitting a thinly spread layer of hydrated phospholipids to swell slowly in distilled water or an aqueous solution of nonelectrolytes. Electron micrographs and phospholipid analyses indicate that the walls consist of a single or a few bilayers. The vesicles can be centrifuged and resuspended in another medium, making them a useful system for studying permeability. The osmolarity of the solution in the interior of the vesicles can be estimated by immersion refractometry. The osmolarity of the internal aqueous phase is linearly related to the osmolarity of the external medium.

Observed behavior of highly inelastic electron-proton scattering

Abstract: Results of electron-proton inelastic scattering at 6\ifmmode^\circ\else\textdegree\fi{} and 10\ifmmode^\circ\else\textdegree\fi{} are discussed, and values of the structure function ${W}_{2}$ are estimated. If the interaction is dominated by transverse virtual photons, $\ensuremath{ u}{W}_{2}$ can be expressed as a function of $\ensuremath{\omega}=\frac{2M\ensuremath{ u}}{{q}^{2}}$ within experimental errors for ${q}^{2}g1$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ and $\ensuremath{\omega}g4$, where $\ensuremath{ u}$ is the invariant energy transfer and ${q}^{2}$ is the invariant momentum transfer of the electron. Various theoretical models and sum rules are briefly discussed.

The organization of the avian telencephalon and some speculations on the phylogeny of the amniote telencephalon

Abstract: There have been numerous surveys of the morphology of the avian telencephalon and attempts to compare it to the various divisions of the telencephalon of other vertebrates. These comparisons were often tenuous, based upon apparent resemblance to structures in mammals. Time does not permit a complete survey of all structures discernible within the forebrain, and the present report is a resume of only some of the recent findings on the avian telencephalon. The major portion of the avian telencephalon consists of a mass of grey matter in the lateral wall of the hemisphere subdivided into several regions, all ending with the suffix “-striaturn,” and with a small region of clearly laminated pallium. As il consequence of the relatively small amount of pallium in birds, most suggestions concerning the nature of the avian telencephalon have derived from the striatal appearance of the cell masses in the forebrain. FIGURE 1 is a Nissl-stained section of a transverse section through the pigeon telencephalon showing the location of the paleo-, neo-, and various subdivisions of the hyperstriatum. It is worth recalling, however, the original reason for designating these areas as striatum. Striatum was simply a descriptive term indicating that these regions consisted of large masses of grey matter with bundles of myelinated axons passing through them, giving them a striated appearance. However, this simple descriptive feature provided no information about the differences between different masses of grey matter within the telencephalon, their connections, or participation in any functional system, and in no way provided any defining characteristics of the cells that would permit comparing them to cell masses in other vertebrates. There was no logical reason to expect that all striatal structures were necessarily comparable, except that they consisted of masses of neurons, glia and axons. Even more puzzling was the striking discrepancy between the relatively small amount of pallium and large volume of striatum in sauropsids, and the apparently inverse ratios of large areas of pallium and only moderate amounts of striatum in mamma1s.t

Theory and practice of ionosphere study by Thomson scatter radar

Abstract: The application of Thomson (or incoherent) Scatter observations to the study of the earth's ionosphere is described. Those aspects of theory of Thomson Scatter that have been put to practical use in ionospheric investigations are reviewed briefly and the type of radar equipment constructed for these investigations is discussed. Methods of measuring electron density, electron and ion temperatures, and ionic composition are then reviewed. Other applications of the technique--to the study of the neutral density and temperature of the upper atmosphere, drift motions, the flux density of fast photoelectrons, and the orientation of the earth's magnetic field--are also described.

High-Energy Inelastic e-p Scattering at 6° and 10°

Abstract: Cross sections for inelastic scattering of electrons from hydrogen were measured for incident energies from 7 to 17 GeV at scattering angles of 6\ifmmode^\circ\else\textdegree\fi{} to 10\ifmmode^\circ\else\textdegree\fi{} covering a range of squared four-momentum transfers up to 7.4 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. For low center-of-mass energies of the final hadronic system the cross section shows prominent resonances at low momentum transfer and diminishes markedly at higher momentum transfer. For high excitations the cross section shows only a weak momentum-transfer dependence.

Basis of finite element methods for solid continua

Abstract: Finite element methods can be formulated from the variational principles in solid mechanics by relaxing the continuity requirements along the interelement boundaries. The combination of different variational principles and different boundary continuity conditions yields numerous types of approximate methods. This paper reviews and reinterprets the existing finite element methods and indicates other alternative schemes. Plate bending problems are used to compare the relative merits of the various methods.

The optimum linear smoother as a combination of two optimum linear filters

Abstract: A solution to the optimum linear smoothing problem is presented in which the smoother is interpreted as a combination of two optimum linear filters. This result is obtained from the well-known equation for the maximum likelihood combination of two independent estimates and equivalence to previous formulations is demonstrated. Forms of the solution which are convenient for practical computation are developed.

Estimation of seismic noise structure using arrays

Abstract: A theoretical study of the use of arrays for the analysis of seismic noise fields has been completed. The frequency‐wavenumber power spectral density P(kx,ky,f) is defined and techniques for estimating it are given. The estimates require that the auto‐ and crosspower spectral densities be estimated for all elements in the array. Subject to certain asymptotic properties of these auto‐ and crosspower spectral density estimates, expressions for both the mean and variance of the estimates of P(kz,ky,f) have been obtained. It has been demonstrated that if P(kx,ky,f) is estimated by the Frequency Domain Beamforming Method, then the estimate has the same stability as the estimates of auto‐ and crosspower spectral density. P(kx,ky,f) has been estimated from both long‐ and short‐period noise recorded by the Large Aperture Seismic Array in Montana. At frequencies higher than 0.3 Hz, a compressional body‐wave component which correlates with atmospheric disturbances over distant oceans has been detected. In the frequ...

The Spread of Oil Slicks on a Calm Sea

Abstract: It is a common observation that oil, when spilled on water, tends to spread outward on the water surface in the form of a thin continuous layer. In those instances where this layer is as thin as a wave length of visible light, an iridescent color of the film, caused by light interference, is observed. This tendency to spread is the result of two physical forces: the force of gravity which causes the lighter oil to seek a constant level by spreading horizontally, just as it would on a plane horizontal solid surface, and the surface tension force of pure water, which is usually greater than that of the oil film floating on water. While the oil layer could spread while still remaining intact until it had formed a monomolecular layer, spreading usually stops when the layer is much thicker than this, most likely because of a change in the surface tension properties of the oil.

The chirp z-transform algorithm and its application

Abstract: We discuss a computational algorithm for numerically evaluating the z-transform of a sequence of N samples. This algorithm has been named the chirp z-transform algorithm. Using this algorithm one can efficiently evaluate the z-transform at M points in the z-plane which lie on circular or spiral contours beginning at any arbitrary point in the z-plane. The angular spacing of the points is an arbitrary constant; M and N are arbitrary integers. The algorithm is based on the fact that the values of the z-transform on a circular or spiral contour can be expressed as a discrete convolution. Thus one can use well-known high-speed convolution techniques to evaluate the transform efficiently. For M and N moderately large, the computation time is roughly proportional to (N + M) log 2 (N + M) as opposed to being proportional to N · M for direct evaluation of the z-transform at M points. Applications discussed include: enhancement of poles in spectral analysis, high resolution narrow-band frequency analysis, interpolation of band-limited waveforms, and the conversion of a base 2 fast Fourier transform program into an arbitrary radix fast Fourier transform program.