Showing papers on "Wavelength published in 1969"

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.

Laser light scattering in laboratory plasmas

Abstract: The theory of light scattering from a collection of free electrons is reviewed, and it is shown that the frequency spectrum observed at a detector is precisely that of the density fluctuations of a particular scale length in the scattering medium, the scale length being determined by the wavelength of the incident light and the geometry of the experimental arrangement. The electron density fluctuation in a plasma is calculated, and it is shown that the plasma Debye shielding distance λD is a critical length in the theory, the electrons behaving independently on a scale shorter than λD and collectively on a scale longer than λD. The collective behaviour is characterized by the presence of waves that can give rise to well-defined resonances in the scattered-light spectrum. The effects of differing ion and electron temperatures, current flowing in the plasma, magnetic field, and Coulomb collisions are considered briefly. Technical considerations in planning experiments to test the theory and to apply it to the diagnosis of real laboratory plasmas are discussed, with attention being given to signal-to-noise ratio, stray light, and the dispersing instrument to be used at the detector. Some representative experiments that have been carried out are reviewed.

Rip currents: 2. Laboratory and field observations

Abstract: The nearshore circulation of water on a plane beach exposed to a uniform wave train, normally incident on the beach, was investigated experimentally in the laboratory. The incident waves generated standing edge waves on the beach of the same frequency as the incoming waves. The interaction between these edge waves and the incident waves gave rise to steady flow patterns (nearshore circulation cells) consisting of an onshore flow toward the breakers, a longshore current in the surf zone, and an offshore flow in relatively strong, narrow rip currents. The rip currents were found to occur at alternate antinodes of the edge waves, and the spacing of the rip current was therefore equal to the longshore wavelength of the edge waves. Although the incoming wave may interact with all the possible edge wave modes of the same frequency, it was found that the interaction with one particular mode is often dominant. A useful estimate of the relative importance of the modes is given by the parameter w2xb/(g tan β), where ω is the radian frequency of the edge waves, tan β is the beach slope, and xb is the width of the surf zone. Field observations made in the Gulf of California strongly suggest that this mechanism is important on real beaches.

Formation of the thermocline step structure by large‐amplitude internal gravity waves

Abstract: It is suggested that a possible mechanism for the formation of the thermocline step structure is a sporadic overturning by rotors associated with finite-amplitude internal waves. A criterion for the required critical amplitude of an internal wave is derived, and good agreement is found with numerical experiments illustrating the mechanism. A scale analysis for the ocean shows that downward-propagating waves with a vertical wavelength of 10–20 meters would be most favored to ‘break’ by the convective instability mechanism. Examination of velocity spectra measured in the North Atlantic shows that more than enough energy exists in the internal wave frequency range for this type of instability to occur.

Progressive internal waves on slopes

Abstract: The refraction of progressive internal waves on sloping bottoms is treated for the case of constant Brunt—Vaisala frequency. In two dimensions simple, explicit expressions for the changing wavelengths and amplitudes are found. For small slopes, the solutions reduce to simple propagating waves at infinity.The singularity along a characteristic is shown to be removable, though the solutions are now inhomogeneous waves. The viscous boundary layers of the wedge geometry are briefly considered with the inviscid solutions remaining as interior solutions.A theory valid for small slopes is obtained for three-dimensional waves. The waves are refracted in the usual manner, turning parallel to the beach in shallow water.

EFFICIENT, TUNABLE OPTICAL EMISSION FROM LiNbO3 WITHOUT A RESONATOR

Abstract: We report the first observation of tunable stimulated optical emission from a nonlinear crystal without the use of an external resonator, and with greater than 50% conversion of the pump beam to the tunable frequency. Opposite faces of a LiNbO3 crystal were polished flat and parallel to provide for multiple reflections of the signal radiation (Stokes) inside the crystal. The A1 symmetry 248‐cm−1 polariton mode was excited with a Q‐switched ruby laser, and the signal frequency was tuned by varying the angle of incidence of the laser beam relative to the normal to the crystal surface. Operation was at room temperature; no crystal damage was observed over most of the tuning range; and laser depletion occurred within the first few nanoseconds of the Q‐switched pulse. The difference between the Stokes frequency and laser frequency was varied from 42 to 200 cm−1, and the corresponding idler wavelength was varied from 238 to 50 μ.

Kinetics of Nitrogen Dioxide Fluorescence

Abstract: The fluorescence lifetime and intensity of gas‐phase nitrogen dioxide (2B1) have been measured as a function of excitation wavelength, fluorescence wavelength, and pressure (0.5–50 mtorr). The phase‐shift method was used; this technique allows lifetime measurements to be obtained with signal intensities of 100 counts per sec and lower. The excitation source, tunable throughout the visible region, had a half‐width bandpass as low as 15 A. Fluorescence wavelength separation was accomplished with 15 interference filters between 400 and 8000 A. The radiative lifetimes range from 55 to 90 μsec for excitation from 3980 to 6000 A and tend to increase with excitation wavelength; however, the lifetimes exhibit considerable variation within a narrow excitation region. The fluorescence sample was contained in a 33‐cm‐diam spherical bulb; apparent fluorescence lifetimes in smaller cells were reduced because of migration of excited molecules (under collision‐free conditions) and wall quenching. In order that the measu...

Ray tracing for internal gravity waves

Abstract: Ray-tracing techniques are applied to internal gravity waves in a fluid with spatially varying mean flows. It is shown that the general effect of deformational mean flow over long time periods is to shorten the wavelength, the effect being most pronounced for waves of modest spatial scale.

Spectrum Analysis of Large-Scale Wave Disturbances in the Tropical Lower Troposphere

Abstract: The study consists of two parts: a detailed investigation of wave disturbances in the tropical lower troposphere during a single 6 month period, and a brief survey of wave activity at a single station during four successive 6-month periods. Cross spectrum analysis of wind, temperature, relative humidity and surface pressure data reveals the existence of at least three types of disturbances: Easterly waves, with periods in the range of 4–5 days and horizontal wavelengths on the order of 3000 km. Contrary to the results of earlier studies, the axes of the waves show no inclination with height. There is some indication of a cold core structure, but the temperature fluctuations in the waves are very small. Some stations show a tendency for high relative humidities to occur in the troughs of the waves, in agreement with earlier studies. Low-frequency oscillations with periods >10 days and horizontal wavelengths on the order of 10,000 km. These are most strongly evident in the zonal wind component, for...

Observations of short period internal waves in Massachusetts Bay.

Abstract: : Short period (7-minutes) temperature fluctuations were measured in 82 m of water from instrumented buoys moored 9 km west of a prominent sill in Massachusetts Bay. The high frequency oscillations begin with a large abrupt rise in temperature and persists for about three hours. The average time interval between the onset of successive groups of high frequency fluctuations was 12.4 hours. Prior to their onset the temperature is steady. The high frequency fluctuations are coherent and of uniform phase in the vertical direction. The distribution of the rms vertical displacements corresponds to the eigen-function of the first mode of internal waves, which was computed from measurements of density and velocity. Long crested, short wavelength, dark narrow surface bands were related to the high frequency temperature fluctuations. Groups of high frequency large amplitude fluctuations occurred in velocity measurements made 200 m from the temperature instrumented buoys. The high frequency fluctuations occurring at regular intervals of time seem to be internal waves of mode one propagating in the same direction with respect to the moving medium. (Author)

Theoretical and Experimental Investigation of a New Type of Blazed Grating

Abstract: A rigorous electromagnetic theory of the diffraction of light by blazed lamellar gratings has been developed. It is applied to calculate the diffracted power distribution of four gratings (R1 − R4) whose grooves have the following depths and widths (b,l) in units of grating period a: (0.433, 0.750), (0.333, 0.667), (0.250, 0.500), and (0.200, 0.400). All four gratings are theoretically blazed in the minus-first order for 30° incidence and for the wavelength equal to the grating period. The blaze is, however, broad band, and the minus-first-order theoretical power-conversion efficiency (P−1) for the most interesting grating, R3, exceeds 0.8, 0.9, 0.95, and 0.98 in the wavelength bands 0.84 a−1.49 a, 0.85 a−1.47 a, 0.87 a−1.43 a, and 0.98 a−1.31 a, respectively, for linearly polarized light. These surprising performances, which far exceed those obtainable with echelette gratings insofar as the incident light is linearly polarized, have been experimentally verified with the help of a microwave interference spectrometer, and point to the potential usefulness of blazed lamellar-reflection gratings in far-infrared spectroscopy.

Ray and Wave Invariants for SOFAR Channel Propagation

Abstract: Rays and waves propagating in the SOFAR channel and encountering changes in the vertical profile of sound velocity c(z) possess certain invariants, if the changes are sufficiently gradual (adiabatic). For rays, the invariant quantity is ∮(n2 − nm2)12dz, n = 1/c, where nm denotes the value of n at maximum ray excursion, and where the integral is taken over the full ray period. The wave invariants are the individual normal modes φα (z), α = 0,1,2, …, as defined at each point of the propagation path by the local form of c(z). Adiabatic invariance of the modes will be maintained if the gradient of n2 along the path ∂n2/∂x is sufficiently small, as expressed by the condition λ/X≫2πΔn2/n2, where λ is the sound wavelength and X is the ray period, and where Δn2 = X ∫φα(∂n2/∂x)φβdz between adjacent modes α, β. For actual gradients measured off the California coast, the above condition is, numerically, λ≫2.5 m, or f≪600 Hz.

Long-wavelength turbulence and the heating of the solar wind.

Abstract: Solar wind model for studying long wavelength turbulence as heat source for alpha particles and protons in solar plasma

Slow oscillations in an ocean of varying depth. Part 2. Islands and seamounts

Abstract: We consider slow oscillations trapped about axisymmetric islands and seamounts. ω is in the range [lsim ] δ/2 (ω is the frequency divided by f , the Coriolis parameter, and δ the fractional change in depth). The periods, for example, are [gsim ] 2·4 days for an island with a sloping ‘skirt’, h [vprop ] r ½ , where h is the depth and ( r , θ) are polar co-ordinates in the plane tangent to the mean sea surface. Energy leaks slowly away from the topography in Rossby waves. In the limiting case of a cylindrical island with vertical walls there are no such trapped motions, but incident Rossby waves are scattered anisotropically. If γ, the ratio of the island radius, a , to the Rossby wavelength, is small, the scattering cross-section ∼ γ 3 a . The free oscillations at seamounts and islands with skirts allow much stronger scattering (with cross-section ∼ a /γ, ∼ a wavelength), when one of their frequencies is near that of the incident wave. The theory suggests that measurements of Rossby waves will be possible at small islands, but that the many local oscillations in the same frequency range will add some confusion.

Metallic Mesh Bandpass Filters and Fabry-Perot Interferometer for the Far Infrared

Abstract: This paper describes optical properties of thin electroformed metallic meshes in detail. Since the transmittance shows the bandpass character around the wavelength near the grid constant, metal mesh can be used as a bandpass filter. The reflectance is high in the wavelength region longer than three times the grid constant and the absorptivity is very low. These optical properties are suitable for reflectors of the Fabry-Perot Interferometers (FPI's) for the far infrared. Authors fabricated two types of FPI's; flxed type and tunable type. The absorption spectra of atmospheric water vapour between 100 and 400 µ and CO gas between 350 and 650 µ were scanned with the tunable FPI. Resultant resolving power of about 20 was obtained at 250 µ. Moreover, light flux which a FPI can utilize in the limited area of the light source and the detector was discussed.

Propagation in corrugated waveguides

Abstract: It is possible to determine the nature of the modes which propagate in corrugated, cylindrical waveguides, by imposing a nonisotropic, surface-reactance, boundary condition at the corrugated walls. Such a condition, in ignoring periodic effects, implies corrugated gaps much less than a wavelength separated by even smaller fins, and leads to simplified expressions for the fields in the waveguide. Two types of propagation occur: fast waves, characterised by low electric-field intensity at the walls, and slow waves by strong electric field at the walls. Both types are hybrid in nature, and degenerate to combinations of TE and TM modes when the corrugation slot depths are equivalent to multiples of half a wavelength. The dominant mode is a slow wave, with a cutoff equal to that of the dominant mode in a similar, uncorrugated waveguide. Mode charts, which distinguish the fast- and slow-wave regions as a function of normalised frequency and slot depths, may be used to design waveguides suitable for either fast or slow waves, and for matching such guides to uncorrugated launching sections.

Direct in situ measurements of wave propagation in the neutral thermosphere

Abstract: Atmospheric density variation measurements by density gages on Explorer 32 satellite confirm wave propagation in neutral thermosphere as free internal gravity waves

Effect of Frequency and Space Averaging on the Transmission Responses of Multimode Media

Abstract: Expressions for the standard deviation of the amplitude response of a multimode transmission medium to signals with nonzero bandwidth are derived. In the “statistical” case (normal modes overlapping frequency and negligible coherent transmission), the standard deviation is found to be approximately 5.57 (1 + 3.3⋅τW)−12 dB, where τ is the decay‐time constant of the energy of the normal modes and W is the bandwidth of the excitation signal. For spatial averaging, the standard deviation of the response fluctuation in decibels is approximately 5.57 (1 + 3.3X/λ)−12 dB, where X is the length of the (linear) averaging interval and λ is the wavelength of the excitation signal. For excitation signals with finite bandwidth W, the response fluctuations are given by 5.57[(1 + 3.3τW)(1 − 3.3X/λ)]−12 dB, where λ is the average wavelength. These results are applicable to fluctuation problems in multipath transmission, random vibrations, concert hall acoustics, and power measurements in reverberation chambers.

Excitation by light of ω + and ω − surface plasma waves in thin metal layers

Abstract: Calculations with the extended Fresnel formulae show, thatω+ andω− surface plasma waves can be excited by light, using the following optical arrangement: The metal foil is embedded between two dielectric layers of low index of refraction and equal thickness. Both sides of this 3-layer packet are in optical contact with a high index prism. Total reflection is broken byω+ andω− resonances in reflectivity, transmission and absorption. Theω+ resonance is mainly transmissive, (transmission through a 800 A thick silver foil greater than 50% at a wavelength of 5,461 A) theω− resonance is mainly absorptive. The polarization of the transmitted light exceeds 99%-theω+ resonance is proposed as a principle for a new optical polarizer.

Analysis of the nature and growth of electrothermal waves

Abstract: Using a linear theory the dispersion relation of electrothermal waves in a seeded, partially ionized gas is derived. Two modes appear, one of which is always damped, while the other is unstable in certain plasma situations. This is the ionization instability and the growth rate has been calculated as a function of the steady state electron temperature, the angle between the steady state current density and the wave vector, the Hall parameter, and the instability wavelength.

Drift Instabilities in a Uniformly Rotating Plasma Cylinder

Abstract: The dispersion relation for low‐frequency, collisional, electrostatic waves in a uniformly rotating plasma cylinder is obtained in terms of several positive‐definite bilinear forms whose magnitude may easily be estimated. The effects of ion transverse collisional viscosity, electron parallel resistivity, ion parallel motion, and an equilibrium radial electric field are included in the theory. The close correspondence between the present dispersion relation and the dispersion relation for localized, slab‐model modes justifies the use of the latter in the interpretation of Q‐machine experiments. Another result is that ion collisional viscosity stabilizes the density‐gradient driven drift wave if the axial wavelength λ‖ satisfies λ‖ < 2πσr0 (M/me)1/4, (σ ∼ 0.7; r0 is the density gradient scale length; M, me are the ion and electron masses, respectively). Overstable waves, which are driven by the centrifugal force and which resemble flutes with a finite parallel wavelength, can exist if λ‖ is sufficiently long.

Surface waves in hot plasmas.

Abstract: The solution of the initial value problem for a semi‐infinite Vlasov plasma, with no applied fields, is given. In the long‐wavelength limit, in addition to the usual plasma waves, terms corresponding to surface plasma waves, for which the dispersion relation is given, are found.

Physics of X-Rays

Abstract: The X-ray region is normally considered to be that part of the electromagnetic spectrum lying between 0.1–100 A, being bounded by the y-ray region to the short wavelength side and the vacuum ultra-violet region to the long wavelength side. The actual boundary between the X-ray and vacuum ultra-violet region is not clearly defined and for many years the 50–500 Â mid-region has not been exploited by practical spectroscopists to any great degree. Over the last few years however this wavelength range has been examined both from the short wavelength end by the X-ray spectros-copist and from the long wavelength end by workers in the fields of plasma-and astrophysics. It is now common practice to refer to this particular region as the soft X-ray and vacuum ultra-violet region.

Effect of a Shear Layer on Plane Waves of Sound in a Fluid

Abstract: Available theory indicates that plane waves of sound are reflected and refracted at an interface of relative motion (a velocity discontinuity) between two regions of fluid. If the relative velocity is sufficiently great, three types of reflection occur, ordinary, total, and amplified, depending on the incident wave angle. In the amplification regime, theory predicts resonances. Here the velocity discontinuity replaced by a transition layer of finite thickness separating the two fluid regions. This layer is approximated by two equal velocity discontinuities (Model I) and by a linear velocity profile shear layer (Model II). For one example in the regime of ordinary reflection, the effects of thickness are negligible for thicknesses up to 1/10 of the incident wavelength. For the chosen examples in amplified reflection, extreme reductions in transmission and reflection coefficients occur for a thickness as little as 1/50 of a wavelength. The two models approach (at unequal rates) total reflection and zero transmission at larger thicknesses. These effects are produced by layers of fluid traveling at or near the apparent wave speed parallel to the shear layer. Such layers tend to “insulate” the two fluid regions from each other.

Wavelength Independence of the Velocity of Light in Space

Abstract: BECAUSE of the great distances involved, time measurements on astronomical events afford the best possibility for a high precision determination of the relative velocities of electromagnetic radiation at different wavelengths1. A discussion of flare stars, observed nearly simultaneously at optical and radio wavelengths, resulted in a relative uncertainty Δc/c of 4 × 10−7 over the wavelength range 0.54 microns–1.2 m. This gives a value of the product p=(c/Δc) (λ2/λ1) of about 5 × 1012, which is a factor of 10 greater than that which has been achieved in the laboratory2.

Wavelength dependence of polarization. XIV - Atmosphere of Jupiter.

Abstract: Jupiter polarization wavelength dependence noted in observations of 5 seconds diameter regions near poles and equator related to Rayleigh- Chandrasekhar molecular scattering theory

Evidence for reversible heating in the E region from radar Thomson scatter observations of ion temperature

Abstract: Values of ion temperature (Ti) and ion-neutral collision frequency (ν) in the daytime E region have been obtained from radar Thomson scatter measurements at the Arecibo Ionospheric Observatory. These quantities were deduced from the shape of the autocorrelation function of the backscattered signal that was measured with a height resolution of 15 km using a double-pulse technique. Usually the daily variation of Ti at 115 and 130 km exhibited wave-like structure that must also have been present in the neutral atmosphere because the ion and neutral constituents have the same temperature at these altitudes. The observed temperature oscillations are interpreted as a manifestation of the reversible adiabatic heating accompanying long-period internal gravity waves in the neutral atmosphere. A simple harmonic analysis of the temperatures for a number of days gives dominant wave periods of from 7 to 13 hours and fractional amplitudes as large as 22%. On the reversible heating explanation, the observed amplitudes of the temperature waves imply average horizontal wind amplitudes of 60 to 70 m/sec. A downward phase progression (corresponding to upward energy propagation) is evident with an average vertical wavelength near 60 km. A significant wave-like variation is also detected in some of the ion-neutral collision frequency results at 115 km. The period is similar to that of the corresponding temperature wave, but the fractional amplitude is larger, and there is an appreciable phase difference. The phase and amplitude of the variation in the neutral concentration, as predicted from the collision frequency variation using the polarization ion-neutral interaction, does not agree with that deduced from the temperature variation using simple internal gravity wave theory with upward energy propagation. The resolution of this discrepancy may depend on the removal of Some of the idealizations in the gravity wave theory or the inclusion of additional ion-neutral interaction terms.

On the wavelength of convective motions

Abstract: It has been found experimentally that under a lid of excellent heat conductivity the wavelength of convective motions on a uniformly heated plane increases with increasing supercritical Rayleigh numbers. Under an insulating lid, the wavelength at the critical Rayleigh number is larger than under a well conducting lid. When heating from below is applied rapidly the wavelength is less than in the corresponding stationary case.