Showing papers on "Wavelength published in 1976"

Estimation of incident and reflected waves in random wave experiments

Abstract: A technique to resolve the incident and reflected waves from the records of composite waves is presented. It is applicable to both regular and irregular trains of waves. Two simultaneous wave records are taken at adjacent locations, and all the amplitudes of Fourier components are analyzed by the FFT technique. The amplitudes of incident and reflected wave components are estimated from the Fourier components, and the incident and reflected wave spectra are constructed by smoothing the estimated periodograms. The wave resolution is effective in the range outside the condition of the gauge spacing being even integer of half wavelength. The ratio of incident and reflected wave energies in the effective resolution range is employed in estimating the overall reflection coefficient. The incident and reflected wave heights are estimated from the composite wave heights by energy consideration.

Kinetic processes in plasma heating by resonant mode conversion of Alfvén wave

Abstract: An externally applied oscillating magnetic field (at a frequency near 1 MHz for typical tokamak parameters) resonantly mode converts to the kinetic Alfven wave, the Alfven wave with the perpendicular wavelength comparable to the ion gyroradius. The kinetic Alfven wave, while it propagates into the higher density side of the plasma after the mode conversion, dissipates due to both linear and nonlinear processes and heats the plasma. If a magnetic field of 50 G effective amplitude is applied, approximately 10 MJ per cubic meter of energy can be deposited in 1 sec into the plasma. The heating rate here is faster than that in the transit time magnetic pumping by a factor of β−1.

Acoustic phonon instabilities and structural phase transitions

Abstract: Structural phase transitions are considered in which the order parameter is a homogeneous deformation of the crystal. The fluctuations at these transitions are the acoustic modes, and it is shown that an effective Hamiltonian may be constructed describing the homogeneous deformations and their fluctuations. There are three cases which result, those in which there are no fluctuations with wavelengths less than the crystal dimensions, those in which the acoustic modes have strongly temperature-dependent velocities for wave vectors on particular lines of reciprocal space, and those for which the velocities are temperature dependent for wave vectors within planes in reciprocal space. In many cases, the transitions are expected to be first order because of the presence of cubic invariants in the effective Hamiltonian. In those which may be continuous, the behavior is shown by use of renormalization-group theory to be that of classical Landau theory, with the possibility of logarithmic corrections in one particular instance. Unfortunately, we are unaware of any examples of this case, but in the other cases, the results are in accord with experimental results.

Recovery of hydrocarbons from partially exhausted oil wells by mechanical wave heating

Abstract: Underground viscous hydrocarbon deposits, such as the viscous residues in conventional oil wells, are heated by mechanical wave energy to fluidize the hydrocarbons thereby to facilitate extraction thereof. For uniform, circular, symmetrical dispersion of mechanical wave energy of high-power and low-frequency, a mechanical wave energy radiator is provided comprising a cylindrical elastic tube of springy steel or the like preferably dimpled or corrugated and closed at one end and containing a liquid medium. Mechanical wave energy is applied to the liquid medium by a reciprocating source or the like connected to the radiator by a rigid walled tubular pipe or the like. The axial length of the radiator tube should be an odd multiple of one-quarter wavelength of the mechanical wave energy transmitted. Cavitation within the liquid is avoided by biasing the system with a steady state pressure at least as great as the maximum negative pressure swing of the mechanical waves in the liquid. Transformers are disclosed for accommodating changes in pipe diameter and changes in liquid medium throughout the system.

On the separation of air flow over water waves

Abstract: Conditions leading to the onset of air-flow separation over a mobile air-water interface are discussed. It is argued that, in a frame of reference in which the interfacial boundary assumes a steady shape, the occurrence of separation requires a stagnation point on the interface. In the case of air blowing over water waves, this corresponds to the onset of wave breaking. These arguments are strongly supported by flow visualization and pressure measurements carried out in a laboratory wind-wave flume. Furthermore, the pressure measurements show a greatly enhanced interfacial shear stress for a breaking wave compared with that over an unbroken wave of the same wavelength. The implications of these findings for wind-wave generation are discussed.

Interferometric real‐time display of cw dye laser wavelength with sub‐Doppler accuracy

Abstract: We describe an automatic fringe‐counting interferometer with real‐time wavelength readout for cw laser sources. Sub‐Doppler absolute wavelength accuracy (∼2×10−7) is demonstrated with saturated absorption spectroscopy in neon.

Multi-wavelength incremental absorbence oximeter

Abstract: Light of two different wavelengths is passed through or reflected from a member of the body so as to be modulated by the pulsatile blood flow therein. The amplitudes of the alternating current components of the logarithms of the respective light modulations are compared by taking their molecular extinction coefficients into account so as to yield the degree of oxygen saturation. By adding a third wavelength of light, the percentage of other absorbers in the blood stream such as a dye or carboxyhemoglobin can be measured. Fixed absorbers reduce the amount of light that passes through or is reflected from the body member by a constant amount and so have no effect on the amplitudes of the alternating current components that are used in making the measurements.

Vorticity concentration and the dynamics of unstable free shear layers

Abstract: The detailed dynamics of an unstable free shear layer are examined for a gravitationally stable or neutral fluid. This first article focuses on the part of the evolution that precedes the first subharmonic interaction. This consists of the transformation of selectively amplified sinusoidal waves into periodically spaced regions of vorticity concentration (the cores) joined by thin layers (the braids), in which vorticity is also concentrated. The thin layers are the channels along which vorticity is advected into the cores, and the cores provide the strain which creates the braids. For moderately long waves an analysis is given of the braid structure as a function of time. For gravitationally stable shear layers at high Reynolds numbers, the local vorticity reaches such large values as to cause secondary shear instability on a small (length) and short (time) scale. A physical account of the primary instability and its self-limiting mechanism is used as a basis for a computation, which yields growth rates and maximum amplitude as a function of initial layer parameters. The computation supplies the wavelength of waves that grow to achieve the largest (absolute) amplitude. Finally, the model makes it clear that, in the absence of secondary instability, this initial phase of the nonlinear development of the layer contributes only a modicum of additional mixing, especially at high Reynolds numbers.

Electronic spectrum of H2O

Abstract: Many bands of the electronic emission spectrum of H2O+, occurring in the wavelength region 4000–7500 A, have been analyzed. These include bands that have been observed in the tails of comets. The wavelengths and wave numbers of all assigned lines are tabulated. Accurate rotational constants for the first three bending vibrational levels of the ground state are given, as well as energy levels in the upper and lower electronic states. The O—H bond distance and the H—O—H angle in the (0, 0, 0) level are found to be 0.9988 A and 110.46° respectively. Some predicted microwave and infrared lines that may be of astrophysical interest are included.

A numerical model of the air flow above water waves

Abstract: A numerical model is proposed for the flow in a deep turbulent boundary layer over water waves. The momentum equations are closed by the use of an isotropic eddy viscosity and the turbulent energy equation. For small amplitudes the results are similar to those of Townsend's (1972) linear model, but nonlinear effects become important as the ratio of wave height to wavelength increases. With uniform surface roughness zo, the predicted fractional rate of energy input per radian advance in phase, ζ, decreases slightly with increasing amplitude and is of the same order of magnitude as in Miles’ (1957, 1959) and Townsend's linear theories. If zo is allowed to vary with position along the wave, however, the fractional rate of energy input can be significantly increased for small amplitude waves. If the variation in zo is half the mean value and the maximum wave slope zak is 0.01, we find ζ ≈ 60 (ρair/ρwater) (uo/c)2, where uo is the friction velocity and c the wave phase speed. Comparison is also made with recent laboratory and field data.

Reflective multilayer coatings for the far uv region

Abstract: Progress in the field of reflective multilayer coatings for the wavelength region between 50 A and 2000 A is reviewed. All the coatings contain absorbing materials, absorption losses are minimized by positioning strongly absorbing materials into the nodes of the standing wave inside the coating. Above lambda = 1200 A, ideal coatings with a reflectivity approaching 100% are theoretically possible; the theoretical predictions have been confirmed for coatings up to six layers at wavelengths around 2000 A. Below lambda = 1000 A, no absorption-free material is available that can be used as a spacer layer to cover the antinodes of the standing wave field. This limits the theoretically obtainable reflectivity. However, even at the shortest wavelength a reflectivity of 30% is still possible. Experimental results have been obtained for wavelengths between 100 A and 200 A for coatings up to nine layers. Discrepancies between experiment and theory can be explained as due to insufficient knowledge of the optical constants of the films used. Extensive future work on the optical constants of materials and their dependence on film thickness and deposition conditions is required for further improvement.

High power laser propagation.

Abstract: High power laser beams propagating in the atmosphere are subjected to a variety of effects, the most important of which are absorption, scattering, turbulence induced beam spreading and wander, thermal blooming, and gas breakdown. In this paper simplified models are used to show how the various atmospheric effects interrelate and impact on the best laser choice for high power applications through their dependence on the laser wavelength and temporal mode (e.g., cw or pulsed) of operation. Results for sea level propagation at seven common laser wavelengths varying from 0.34 microm to 10.6 microm. are presented that show the mid-ir wavelengths to be favored for typical turbulence and aerosol scattering conditions. At the longer 10.6-microm CO(2) laser wavelength thermal blooming is dominant due to stronger molecular absorption, while at the shorter wavelengths turbulence induced beam spreading and aerosol absorption and scattering effects become important and tend to limit the increase in irradiance expected on the basis of diffraction effects alone.

Sound propagation through a fluctuating stratified ocean: Theory and observation

Abstract: We have derived expressions for the mean‐square phase and intensity fluctuations and their spectra for cw sound propagating through a channeled fluctuating ocean. The ’’supereikonal’’ approximation reduces to the geometric optics (eikonal) limit for short acoustic wavelengths: λ≪2πL2H/R and λ≪L2V/(R tan2ϑ), where LH and LV are horizontal and vertical correlation lengths of the fluctuations, R is range, and tanϑ is the ray slope, replacing the traditional (and much more severe) Fresnel condition λ≪2πL2/R for a homogeneous isotropic ocean. The results can be expressed in closed form for an exponentially stratified ocean model and associated ’’canonical sound channel,’’ with superimposed fluctuations from an internal wave model spectrum based on oceanographic observations. The parameters are the stratification scale B, the inertial and buoyancy frequencies ωin and n (z), the scale j* of internal wave mode numbers, and the internal wave energy per unit area. The results are in reasonable agreement with numeri...

The Generation of African Waves

Abstract: A linearized, pseudo-spectral, primitive equation model is used to simulate the response of the low-level easterly jet over northern Africa to perturbations on the scale of African waves. The model results show that the jet is unstable due to both its horizontal and vertical shears. The most unstable wave supported by the jet has a wavelength of 3O00 km and a period of 2.0–2.5 days. It attains its maximum intensity at the 700 mb level, near 14°N. This compares favorably with the characteristics of the observed waves. The kinetic energy of the waves grows at the expense of the kinetic energy of the mean jet. Energy is transferred at approximately equal rates by the horizontal and vertical Reynolds stresses. Energy conversions involving available potential energy are nearly an order of magnitude smaller, reflecting the fact that the kinetic energy of the wave accounts for about 90% of the total wave energy. The characteristics of the most unstable wave are virtually unchanged when a crude parameter...

Baroclinic Instability in the Denmark Strait Overflow

Abstract: Current meter records and hydrographic data taken in the Denmark Strait overflow during a one-month experiment in August-September 1973 are analyzed. Mean conditions indicate that a strong, cold overflow current existed throughout the experiment. The most outstanding feature of the velocity and temperature spectra is a strong peak at a period of 1.8 days. These oscillations appear to amplify in the downstream direction and are highly correlated over the entire flow at the southern end of the Strait. Phase estimates indicate that velocity components are in quadrature, while the cross-stream perturbation heat flux acts to reduce the mean potential energy associated with the sloping isotherms. To explain the low-frequency variability, a quasi-geostrophic two-layer model for channel flow with a sloping bottom is developed. Using measured values of shear and other physical parameters, the model is found to be unstable over a limited range of wavelengths and frequencies. The most unstable wave is 80 km...

On the Characteristics of Gravity Waves Generated by Atmospheric Shear Layers

Abstract: The stability analysis of a hyperbolic tangent velocity profile in an isothermal atmosphere In the presence of the ground is presented. It is shown that such a system has a number of modes in addition to the one studied by Drazin and that unstable waves can he excited, for finite values of some minimum Richardson number of the flow, even in the limit of horizontal wavelengths going to infinity. Some of the unstable waves belonging to these new modes are able to propagate energy and momentum away from the shear zone and may therefore play an important role in microscale flow dynamics and in coupling of small-scale phenomena to mesoscale flow motions.

Transducer assembly, ultrasonic atomizer and fuel burner

Abstract: A transducer assembly includes a first half wavelength double-dummy section having a pair of quarter wavelength ultrasonic horns and a driving element sandwiched therebetween. A second half wavelength stepped amplifying section extends from one end of the first section and has a theoretical resonant frequency equal to the actual resonant frequency of the first section. When used as a liquid atomizer, the small diameter portion of the stepped amplifying section has a flanged tip to provide an atomizing surface of increased area. To maintain efficiency, the length of the small diameter portion of the second section with a flange should be less than its length without a flange. A decoupling sleeve within an axial liquid passageway eliminates premature atomization of the liquid before reaching the atomizing surface. In a fuel burner incorporating the atomizer, ignition electrode life is increased by locating the electrodes outside the normal flame envelope. During the ignition phase, drive power to the atomizer is increased to widen the spray envelope to the location of the electrodes. A variable orifice controls combustion air flow in accordance with fuel rate while maintaining constant blower speed. Either three-step or continuous fuel rate modulation saves fuel and reduces pollution.

Relation between creeping waves and normal modes of vibration of a curved body

Abstract: The natural way for a disturbance to propagate over the surface of a smoothly curved, fluid‐loaded elastic body is in the form of a series of damped circumferential (creeping) waves. Mathematically, the process is most conveniently described by a sum of normal modes, each characterized by a wavelength that fits the body’s circumference an integer number of times. We demonstrate that any given mode will resonate at a multiplicity of ’’eigenfrequencies,’’ determined by the speed of any one of the creeping waves matching the mode velocity. For an elastic cylinder, the 180° sound‐scattering amplitude is seen to possess marked minima at many of the eigenfrequencies, which are shown to be generated successively by a single circumferential wave, and whose spacing in this sequence determines the group velocities of circumferential pulsed signals.

Kitt Peak 60-cm vacuum telescope

Abstract: A major new solar-research telescope conceived and built during a time of budget restraint is described. The observation of magnetic and velocity (circulation) field structure on a synoptic basis and with diffraction-limited resolution is the aim. New optical features include the use of oversize mirrors and windows to avoid thermal edge effects and the placement of the coelostat feed outside the vacuum, mainly for economy. The site selected has prevailing winds that clear thermals from these mirrors. Test data in the form of the system MTF and optical transmission, together with examples of full disk magnetograms and photoheliograms, show present performance capability. Measured MTF indicates a response of 0.2 at 1 sec of arc (whereas diffraction-limited response would be about 0.8). System transmission, including the accompanying spectrograph, is only 2-3% (wavelength 0.44-1.1 microns). Thus, both the optical quality and efficiency are subject to improvement.

Bow Shock and magnetosheath waves at Mercury

Abstract: Mariner 10 measurements at the Mercury bow shock provide examples where the magnetic field is approximately parallel or perpendicular to the bow shock normal. Upstream of a broad irregular parallel shock, left hand circularly polarized waves are observed which cut off very sharply at approximately 4 Hz. Upstream of a perpendicular shock, right hand circularly polarized waves are observed which persist up to the Nyquist frequency of 12 Ha. Determination of the wave propagation vector as a function of frequency helps conclusively identify the waves as whistler mode waves propagating from the shock. The magnetosheath downstream of the parallel shock is disturbed more than that downstream of the perpendicular shock particularly below 1 Hz. In the latter case regular left hand polarized waves observed slightly above the proton gyrofrequency are identified as ion cyclotron waves with wavelength approximately 300 km which are Doppler shifted up to their observed frequency.

The Generation of Lee Waves by the Blue Ridge

Abstract: This study focuses on the flow over a low, straight section of the Blue Ridge Mountain in the central Appalachians. Aircraft measurements, laboratory simulation, and mathematical analysis using both the linearized equations and the nonlinear hydrostatic equations, are compared. Aircraft observations of a Blue Ridge lee wave indicate that the linear theory correctly predicts the wavelength but badly under-estimates the wave amplitude. This discrepancy is confirmed in the laboratory. It appears that the discrepancy can be accounted for by the effects of a strong nonlinearity in the governing equations. It is suggested that nonlinear effects acting near the mountain can be important for wave generation and that Long's model is restrictive as it includes only the cases for which this aspect of the physics is entirely absent.

Water vapor absorption of carbon dioxide laser radiation

Abstract: An optoacoustic detector or spectrophone has been used to perform detailed measurements of the absorptivity of mixtures of water vapor in air. A C(12) O(2)(16) laser was used as the source, and measurements were made. at forty-nine different wavelengths from 9.2 microm to 10.7 microm. The details of the optoacoustic detector and its calibration are presented, along with a discussion of its performance characteristics. The results of the measurements of water vapor absorption show that the continuum absorption in the wavelength range covered is 5-10% lower than previous measurements.

Determination of Power Absorption in Man Exposed to High Frequency Electromagnetic Fields by Thermographic Measurements on Scale Models

Abstract: When the body of man, small compared to a wavelength, is exposed to high frequency (HF) electromagnetic (EM) fields, the absorbed power density patterns and total absorbed power may be approximated by the simple superposition of the internal electric fields obtained from the quasistatic coupling characteristics of the electric and magnetic field components determined independently. These characteristics were obtained for full scale man by thermographic studies of power absorption in scale models of man exposed to fields at frequencies scaled up inversely proportional to the model size. A VHF resonant cavity was used to provide the necessary field strengths for producing measurable power absorption patterns under simulated HF exposure conditions. The results indicate that peak power absorption densities as high as 5.63 W/kg can be produced in man exposed to 10 mW/cm 2 31 MHz radiation fields. The results show that the absorption decreases as the square of the frequency as predicted by theory for frequencies below 31 MHz.

Scattering of scalar waves from a Schwarzschild black hole.

Abstract: The scattering of scalar waves from a Schwarzschild black hole is investigated for wavelengths much less than the graviational radius (rs). Explicit expressions for scattering parameters are obtained for two cases: high angular momenta and low angular momenta. In the first case we obtain the phase shifts and absorption coefficient with the JWKB method. The elastic differential cross section and the total absorption cross section are also calculated. For low angular momenta we present a method based in the DWBA (distorted wave Born approximation). With this method, the phase shifts and the absorption coefficients are obtained.

Reducing the reflectance of surfaces to radiation

Abstract: A method of producing a surface having a reduced reflectance to electromagnetic radiation in a predetermined wavelength band which comprises arranging on the surface a regular array of protuberances having a height which is not less than one third of the length of the longest wavelength in the band and at a spacing which is less than the length of the shortest wavelength of the band divided by the refractive index of the material of which the protuberances consist.

Laser wavelength comparison by high resolution interferometry.

Abstract: High resolution interferometry has been used to determine the wavelength ratio between two molecularly stabilized He–Ne lasers, one locked to a methane absorption at 339 μm and the other locked to the k peak of 129I2 at 633 nm An optical beat frequency technique gave fractional orders while a microwave sideband method yielded the integer parts Conventional (third derivative) peak seeking servoes stabilized both laser and cavity lengths Reproducibility of the electronic control system and optics was a few parts in 1012, while systematic errors associated with curvature of the cavity mirrors limited the accuracy of the wavelength ratio measurement to 2 parts in 1010 The measured wavelength ratio of the methane stabilized He–Ne laser at 339 μm [P(7) line, ν3 band] to the 129I2 (k peak) stabilized He–Ne laser at 633 nm was 5359 049 260 6 (0000 2 ppm) This ratio agrees with that calculated from the (lower accuracy) results of earlier wavelength measurements made relative to the 86Kr standard Its higher accuracy thus permits a provisional extension of the frequency scale based on the cesium oscillator into the visible spectrum

Digital wavemeter for cw lasers

Abstract: The wavelength of light can be measured by counting the number of fringes as an interferometer mirror is moved through a known distance. The known distance is best obtained by simultaneously counting the number of fringes from a standard wavelength source for the same change of optical path length. The arrangement in Fig. 1 permits rapid and precise comparison of wavelengths by moving a mirror smoothly over long distances. The interferometer is a two-beam, division of amplitude type related to the Michelson and Sagnac interferometers. In it, the standard and unknown beams travel identical paths in opposite directions. They emerge at separate detectors, so that there is no need for a dichroic beam splitter to separate them, and measurements can be made close to the wavelength of the standard if desired. Moreover, the visible spots of light on the mirrors can be brought into coincidence, ensuring that the two pathlengths are nearly identical, and simplifying the alignment of the unknown beam. The effects of vibrations are also minimized by the use of identical paths.