Showing papers on "Wave propagation published in 1981"

Long-Range Surface-Plasma Waves on Very Thin Metal Films

Abstract: The dispersion equation of injected surface-plasma waves that propagate on thin metal films has been solved as a function of the film thickness, and splitting of the modes into two branches is observed. For one branch the imaginary part of the propagation constant goes to zero as the thickness of the metal decreases. Reflectivity calculations agree with this result, which predicts that one can obtain propagation distances that are more than 1 order of magnitude larger than observed before.

On the detectability of ocean surface waves by real and synthetic aperture radar

Abstract: Real and synthetic aperture radars have been used in recent years to image ocean surface waves. Though wavelike patterns are often discernible on radar images, it is still not fully understood how they relate to the actual wave field. The present paper reviews and extends current models on the imaging mechanism. Linear transfer functions that relate the two-dimensional wave field to the real aperture radar (SLAR) image are calculated by using the two-scale wave model. It is noted that a description of the imaging process by these transfer functions can only be adequate for low to moderate sea states. Possible other mechanisms that contribute to the visibility of waves by real aperture radar at higher sea states, such as Bragg scattering from spontaneously generated short waves at peaked crests or in wave breaking regions, and Rayleigh scattering from air bubbles entrained in the water and from water droplets thrown into the air by breaking waves, are discussed in a qualitative way. The imaging mechanism for synthetic aperture radars (SAR's) is strongly influenced by wave motions (i.e., by the orbital velocity and acceleration associated with the long waves). The phase velocity of the long waves does not enter into the imaging process. Focusing of ocean wave imagery is attributed to orbital acceleration effects. The orbital motions lead to a degradation in resolution which causes image smear as well as a SAR inherent imaging mechanism called velocity bunching. The parameter range for which velocity bunching is a linear mapping process is calculated. It is shown that linearity holds only for a relative small range of ocean wave parameters: The likelihood that the transfer function is linear increases as the direction of wave propagation approaches the range direction, as the wavelength increases, and as the wave height decreases. Linearity is required for applying simple linear system theory for calculating the ocean wave spectrum from the gray level intensity spectrum of the image. Although, in general, the full ocean wave spectrum cannot be recovered from the SAR image by applying simple linear inversion techniques, it is concluded that for many cases in which the ocean wave spectrum is relatively narrow the dominant wavelength and direction can still be retrieved from the image even when the mapping transfer function is nonlinear. Finally, we compare our theoretical models for the imaging mechanisms with existing SLAR and SAR imagery of ocean waves and conclude that our theoretical models are in agreement with experimental data. In particular, our theory predicts that swell traveling in flight (azimuthal) direction is not detectable by SLAR but is detectable by SAR.

Analysis of dispersive waves by wave field transformation

Abstract: The dispersive waves in a common‐shot wave field can be transformed into images of the dispersion curves of each mode in the data. The procedure consists of two linear transformations: a slant stack of the data produces a wave field in the phase slowness‐time intercept (p — τ) plane in which phase velocities are separated. The spectral peak of the one‐dimensional (1-D) Fourier transform of the p — τ wave field then gives the frequency associated with each phase velocity. Thus, the data wave field is linearly transformed from the time‐distance domain into the slowness‐frequency (p — ω) domain, where dispersion curves are imaged. All the data are present throughout the transformations. Dispersion curves for the mode overtones as well as the fundamental are directly observed in the transformed wave field. In the p — ω domain, each mode is separated from the others even when its presence is not visually detectable in the untransformed data. The resolution achieved in the result is indicated in the p — ω wave ...

Upstream hydromagnetic waves and their association with backstreaming ion populations: ISEE 1 and 2 observations

Abstract: Measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock. The reflected ions, which are confined to a narrow energy and angular range, are accompanied by small amplitude (less than approximately 1/2 gamma peak to peak) left-handed waves at frequencies close to 1 Hz in the spacecraft frame. Diffuse backstreaming particles with a broad energy spectrum are associated with low frequency (approximately 30-s period), large amplitude (approximately 5 gamma peak to peak) waves. Intermediate particles are associated with a mixture of these two wave types. Often the waves associated with the diffuse beams steepen as if they were minishocks. The leading edge (trailing edge in the spacecraft frame) frequently appears to break up into a whistler mode wave packet. These discrete wave packets are right-hand polarized and have frequencies from below the proton gyrofrequency to well above it in the plasma frame and are blown back towards the earth by the solar wind.

Dynamics of an impinging jet. Part 1. The feedback phenomenon

Abstract: In a high-speed subsonic jet impinging on a flat plate, the surface pressure fluctuations have a broad spectrum due to the turbulent nature of the high-Reynolds-number jet. However, these pressure fluctuations dramatically change their pattern into almost periodic waves, if the plate is placed close to the nozzle (x0/d < 7·5). In the present study extensive measurements of the near-field pressure provide solid support for the hypothesis that a feedback mechanism is responsible for the sudden change observed in the pressure fluctuations at the onset of resonance. The feedback loop consists of two elements: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate. The upstream-propagating waves and the coherent structures are phase-locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and produce periodic coherent structures. The period is determined by the convection speed of the coherent structures, the speed of the upstream-propagating waves as well as the distance between the nozzle and the plate. An instability process, herein referred to as the ‘collective interaction’, was found to be critical in closing the feedback loop near the nozzle lip.

Wave‐particle interactions near ΩHe+ observed on GEOS 1 and 2 1. Propagation of ion cyclotron waves in He+‐rich plasma

Abstract: The GEOS 1 and 2 spacecraft contain a set of particle and wave detectors which allow for a very comprehensive study of wave-particle interactions occurring within the equatorial region of the magnetosphere. This paper is devoted to interactions involving protons in the energy range 20 keV to 300 keV and ULF waves with frequencies below the proton gyrofrequency. It is shown that mose of the ion cyclotron waves (ICW's) detected in this frequency range have spectra whose charcteristic frequencies are organized in the vicinity of the He/sup +/ gyrofrequency. Simultaneous measurements of the ion composition in the thermal energy range (E< or approx. =110 eV) show these waves to be clearly associated with the abundance of cold He/sup +/ as well as the anisotropy of ions above 20 keV. The general characteristics of these helium-associated ULF events are presented in case studies of four events. The interpretation of this phenomenon is given in the present paper in terms of the propagation of ICW's in a He/sup +/ -rich plasma. It is shown that the shape of the cold plasma dispersion curve (for both parallel and non-parallel propgation) can adequately explain the main characteristics of the observed waves (frequency spectrum, polarization)more » as well as the differences between observations made onboard GEOS 1 and GEOS 2. The generation conditions of ion cylotron waves in such a multi-component plasma, as well as their quasi-linear effects on both the cold He/sup +/ ions and the hot protons, are discussed in a companion paper.« less

Wind wave prediction in shallow water: Theory and applications

Abstract: A wind wave forecasting model is described, based upon the ray technique, which is specifically designed for shallow water areas. The model explicitly includes wave generation, refraction, and shoaling, while nonlinear dissipative processes (breaking and bottom friction) are introduced through a suitable parametrization. The forecast is provided at a specified time and target position, in terms of a directional spectrum, from which the one-dimensional spectrum and the significant wave height are derived. The model has been used to hindcast storms both in shallow water (Northern Adriatic Sea) and in deep water conditions (Tyrrhenian Sea). The results have been compared with local measurements, and the rms error for the significant wave height is between 10 and 20%. A major problem has been found in the correct evaluation of the wind field.

Scattering of electromagnetic waves from random media with strong permittivity fluctuations

Abstract: By taking into account the singularity of the dyadic Green's function in the renormalization method, a theory is derived for vector electromagnetic wave propagation in a random medium with large permittivity fluctuations and with anisotropic correlation function. The strong fluctuation theory is then applied to a discrete scatterer problem in which the permittivity can assume only two values. The results are found to be consistent with those derived from discrete scatterer theory for all values of dielectric constants of the scatterers.

Wave propagation in a magnetically structured atmosphere

Abstract: The solar atmosphere, from the photosphere to the corona, is structured by the presence of magnetic fields. We consider the nature of such inhomogeneity and emphasis that the usual picture of hydromagnetic wave propagation in a uniform medium may be misleading if applied to a structured field. We investigate the occurrence of magnetoacoustic surface waves at a single magnetic interface and consider in detail the case where one side of the interface is field-free. For such an interface, a slow surface wave can always propagate. In addition, a fast surface wave may propagate if the field-free medium is warmer than the magnetic atmosphere.

Reflection of acoustic waves at a water–sediment interface

Abstract: Reflection and refraction of plane acoustic waves are studied for the case where the sediment is modeled as a porous viscoelastic medium. The model is based on the classical work of Biot which predicts that three different kinds of attenuating body waves may propagate in the sediment. As a consequence when homogeneous plane waves in water are incident to a water‐sediment interface, three nonhomogeneous waves are generated in the sediment. In these waves the direction of phase propagation and the direction of maximum attenuation are not the same and particle motion follows an elliptic path. Moreover, the velocity and attenuation of the refracted waves become dependent on the angle of incidence and no “critical” angle occurs. Numerical examples show that the reflectivity of a porous viscoelastic model differs significantly from the case where the sediment is modeled as a viscoelastic solid with constant complex modulus. Finally, because of the frequency dependence of reflectivity in the porous model, it is ...

Long Waves in Ocean and Coastal Waters

Abstract: Water waves occurring in the ocean have a wide spectrum of wavelength and period, ranging from capillary waves of 1 cm or shorter wavelength to long waves with wavelength being large compared to ocean depth, anywhere from tens to thousands of kilometers. Of the various long-wavegenic sources, distant body forces can act as the continuous ponderomotive force for the tides. Hurricanes and storms in the sea can develop a sea state, with the waves being worked on by winds and eventually cascading down to swells after a long distance of travel away from their birthplace. Large tsunamis can be ascribed to a rapidly occurring tectonic displacement of the ocean floor (usually near the coast of the Pacific Ocean) over a large horizontal dimension (of hundreds to over a thousand square kilometers) during strong earthquakes, causing vertical displacements to ocean floor of tens of meters. Other generation mechanisms include underwater subsidence or land avalanche in the ocean and submarine volcanic eruption. Gigantic rockfalls and long-period seismic waves can also produce gravity waves in lakes, reservoirs, and rivers. Generation, propagation, and evolution of such long waves in the ocean and their effects in coastal waters and harbors is a subject of increasing importance in civil, coastal, and environmental engineering and science. Of the various long wave phenomena, tsunami appears to stand out in possessing a broad variation of wave characteristics and scaling parameters on the one hand, and, on the other, in having the capacity of inflicting a disastrous effect on the target area. In taking tsunamis as a representative case for the study of long waves in the ocean, it can be said that large tsunamis are generated with a great source of potential energy (as high as 10^15-10^16J ), though the detailed source motion of a specific tsunami is generally difficult to determine. The large size of source region implies that the "new born" waves would be initially long and the energy contained in the large wave-number part (k, nondimensionalized with respect to the local ocean depth, h) would be unimportant. Soon after leaving the source region, the low wave-number components of the source spectrum are further dispersed effectively by the factor sech kh into the even lower wave-number parts. Tsunamis thus evolve into a train of long waves, with wavelength continually increasing from about 50 km to as high as 250 km, but with a quite small amplitude, typically of 1/2 m or smaller, as they travel across the Pacific Ocean at a speed of 650 km/h-760 km/h. There is experimental evidence indicating that tsunamis continually, though slowly, evolve due to dispersion while propagating in the open ocean; this property has been observed by Van Dorn (16) from the data taken at Wake Island of the March 9, 1957 Aleutian tsunami. One of our primary interests is, of course, the evolution of tsumanis in coastal waters and their terminal effects. Large tsunamis can have their wave height amplified many fold in climbing up the continental slope and propagating into shallower water, producing devastating waves (up to 20 m or higher on record) upon arriving at a beach. The terminal amplification can be crucially affected by three-dimensional configurations of the coastal environment enroute to beach. These factors dictate the transmission, reflection, rate of growth, and trapping of tsunamis in their terminal stage. After the first hit on target, a tsunami is partly reflected to travel once over across the Pacific Ocean, with some degree of attenuation -- a process which is still unclear, but is generally known to be small. Based on observations, Munk (13) suggests the figure of the "decay time" (intensity reducing to 1/e) being about 112 day, and the "reverberation time" (intensity falling off to 10^-6) about a week, while the reflection frequency (across the Pacific) is around 1.7/day. To fix idea, the pertinent physical characteristics and their scaling parameters of a tsunami through its life span of evolution can be described qualitatively in Table I. From the aforementioned estimate we note that the dispersion parameter, h/[lambda], and the amplitude parameter, a/h, are both small in general. However, their competitive roles as rated by the Ursell number Ur, can increase from some small values in the deep ocean, typically of order 10^-2 for large tsunamis, by a factor of 10^3 upon arriving in near-shore waters. This indicates that the effects of nonlinearity (amplitude dispersion) are practically nonexistent in the deep ocean, but gradually become more important and can no longer be neglected when the Ursell number increases to order unity or greater during the terminal stage in which the coastal effects manifest. The small values of the dimensionless wave number, kh = 2[pi]h/[lamda] being in the range of 0.6-0.03 during travel in open ocean, suggests that a slight dispersive effect is still present and this can lead to an accumulated effect in predicting the phase position over very large distances of travel. The overall evolution of tsunamis, as only crudely characterized in Table 1, depends in fact on many factors such as the features of source motion, nonlinear and dispersive effects on propagation in one and two dimensions, the three-dimensional configuration of the coastal region, the direction of incidence, converging or diverging passage of the waves, local reflection and adsorption, density stratification in water, etc. While these aspects of physical behavior are akin to tsunamis, they are also relevant to the consideration of other long wave phenomena. With an intent to provide a sound basis for general applications to long wave phenomena in nature, this paper presents (in the section on three-dimensional long-wave models) a basic long-wave equation which is of the Boussinesq class with special reference to tsunami propagation in two horizontal dimensions through water having spatial and temporal variations in depth. Under certain particular conditions (such as the propagation in one space dimension, or primarily one space dimensional of long waves in water of constant depth) this equation reduces to the Korteweg-de Vries equation or the nonlinear Schrodinger equation. In these special cases we have seen the impressive developments in recent studies of the "soliton-bearing" nonlinear partial differential equations by means of such methods as the variational modulation, the inverse scattering analysis, and modern differential geometry (12,14,17). While extensions of these methods to more general cases will require further major developments, the present analysis and survey will concentrate on the three-dimensional (with propagation in two horizontal dimensions) effects under various conditions by examining the validity of different wave models (based on neglecting the effects of nonlinearity, dispersion, or reflection) in different circumstances. From the example of self focusing of weakly-nonlinear waves (given in the section on converging cylindrical long waves), the effects of nonlinearity, dispersion, and reflection will be seen all to play such a major role that the present basic equation cannot be further modified without suffering from a significant loss of accuracy.

Gravity waves on water with non-uniform depth and current

Abstract: A mathematical model for the combined refraction-diffraction of linear periodic gravity waves on water is developed, in which the influence of inhomogeneities of depth and current is taken into account. The model is used to compute partial reflection of waves a gully or an undersea slope, with influence of a current. The model is also applied to prismatic wave channels with reflecting side-walls. For a gully bounded by shallows the model predicts the decay of wave height due to radiation of energy in lateral direction. For practical application in regions with arbitrary bottom and current topography a parabolic approximation of the model is derived. This is used as a basis for numerical calculation of waves in a sea region near the coast.

Lidar observation of gravity and tidal waves in the stratosphere and mesosphere

Abstract: Lidar measurements of atmospheric density and temperature in the altitude range 30-to 80 km have been performed during the last 2 years from the Observatory of Haute-Provence (latitude 44°N, longitude, 6°E). The potential of this technique for studying the middle atmospheric structure is presented and preliminary results on wave propagation are discussed. It is shown that wave-like structures are observed systematically in this height range. Fourier analysis indicates that most of the energy is transported by waves of vertical wavelengths on the order of 8 to 15 km. The amplitude of the density variations is shown to follow a ρ−l/2 law up to 70 km. The characteristics of the observed density waves suggest that they are caused by a superposition of internal gravity waves propagating upward from the troposphere and a diurnal tide component in the range 30–50 km. Such waves are able to induce quite significant perturbations in atmospheric density and therefore temperature on an hourly basis. The Lidar technique is able to monitor those variations for the first time from a ground station operating continuously.

Wave propagation in a magnetically structured atmosphere: II: Waves in a magnetic slab

Abstract: Magnetic fields may introduce structure (inhomogeneity) into an otherwise uniform medium and thus change the nature of wave propagation in that medium. As an example of such structuring, wave propagation in an isolated magnetic slab is considered. It is supposed that disturbances outside the slab are laterally non-propagating. The effect of gravity is ignored.The field can support the propagation of both body and surface waves. The existence and nature of these waves depends upon the relative magnitudes of the sound speed co and Alfvén speed ςA inside the slab, and the sound speed ce in the field-free environment.In general terms the slow mode can always propagate, and does so both as a surface wave and as a body wave. On the other hand, the fast mode may propagate only in slabs that are not hotter than their surroundings (ce ≥ c0), and then it is a body wave or a surface wave accordingly as ce is greater than or less than ςA. For example, if ce > co > ςA then a fast body wave propagates with phase-speed between ce and co, a slow body wave between ςA and cT = coςA/(co2 + ςA2)1/2, and a slow surface wave with phase-speed below cT. There are no modes between co and ςA. As a second illustration, if ςA > ce > co, then in addition to the slow body and slow surface waves, as before, there is a fast surface wave with phase-speed between co and ce. There is no fast body wave.The special case of a slender field is also investigated and it is shown how the slender flux tube approximation relates to the more general results described above. In particular, the tube wave with phase-speed cT studied by Defouw (1976) and Roberts and Webb (1978) is shown to be a slow surface wave (sausage mode). Finally, we discuss briefly the generation of resonant modes in a slender slab.

Near-limiting gravity waves in water of finite depth

Abstract: Progressive, irrotational gravity waves of constant form exist as a two-parameter family. The first parameter, the ratio of mean depth to wavelength, varies from zero (the solitary wave) to infinity (the deep-water wave). The second parameter, the wave height or amplitude, varies from zero (the infinitesimal wave) to a limiting value dependent on the first parameter. For limiting waves the wave crest ceases to be rounded and becomes angled, with an included angle of 120°. Most methods of calculating finite-amplitude waves use either a form of series expansion or the solution of an integral equation. For waves nearing the limiting amplitude many terms (or nodal points) are needed to describe the wave form accurately. Consequently the accuracy even of recent solutions on modern computers can be improved upon, except at the deep-water end of the range. The present work extends an integral equation technique used previously in which the angled crest of the limiting wave is included as a specific term, derived from the well known Stokes corner flow. This term is now supplemented by a second term, proposed by Grant in a study of the flow near the crest. Solutions comprising 80 terms at the shallow-water end of the range, reducing to 20 at the deep-water end, have defined many field and integral properties of the flow to within 1 to 2 parts in 106. It is shown that without the new crest term this level of accuracy would have demanded some hundreds of terms while without either crest term many thousands of terms would have been needed. The practical limits of the computing range are shown to correspond, to working accuracy, with the theoretical extremes of the solitary wave and the deep-water wave. In each case the results agree well with several previous accurate solutions and it is considered that the accuracy has been improved. For example, the height: depth ratio of the solitary wave is now estimated to be 0.833 197 and the height: wavelength ratio of the deep-water wave to be 0.141063. The results are presented in detail to facilitate further theoretical study and early practical application. The coefficients defining the wave motion are given for 22 cases, five of which, including the two extremes, are fully documented with tables of displacement, velocity, acceleration, pressure and time. Examples of particle orbits and drift profiles are presented graphically and are shown for the extreme waves to agree very closely with simplified calculations by Longuet-Higgins. Finally, the opportunity has been taken to calculate to greater accuracy the long-term Lagrangian-mean angular momentum of the maximum deep-water wave, according to the recent method proposed by Longuet-Higgins, with the conclusion that the level of action is slightly above the crest.

Wave-current interactions: an experimental and numerical study, part 2 - nonlinear waves

Abstract: The interaction between a regular wavetrain and a current possessing an arbitrary distribution of vorticity, in two dimensions, is considered for waves of finite amplitude. A numerical model is constructed, primarily for use in the finite depth regime, extending the work of Dalrymple (1973, 1977) and this is used to predict the wavelength and the particle velocities under the waves. These predictions agree very well with experimentally obtained data and the importance of the vorticity in the wave–current interaction is clarified. Amplitude and wavelength modulations are considered for finite amplitude waves on a slowly varying irrotational current; moderate agreement is found between theory and experiment.

Inertio‐gravity wave induced accelerations of mean flow having an imposed periodic component: Implications for tidal observations in the meteor region

Abstract: The semidiurnal harmonic exhibits great day-to-day variability in amplitude and phase. In addition, the variability appears to be substantially local and random, suggesting a connection with gravity wave activity. We suggest that a significant contribution to the observed semidiurnal harmonic at meteor heights might result from inertio-gravity wave induced accelerations of the mean flow. The rate of wave forcing of the mean wind is related to the Doppler-shifted phase velocity, so that during alternate phases of an imposed mean wind oscillation interactions with waves that accelerate the mean wind in opposite senses may be favored. Thus the imposed mean wind may modulate the mean-flow acceleration at the imposed frequency. In this view, the semidiurnal character of the acceleration is a manifestation of the modulation of the interaction process by the semidiurnal tide. The variability of the semidiurnal harmonic would then reflect the local variability of inertio-gravity wave fluctuations and also nonlinear feedback on the waves. Calculations with a simple time-dependent wave-mean-flow model indicate that a wave-induced component of the semidiurnal harmonic with amplitudes comparable to the semidiurnal tide itself is possible.

Three-Dimensional Instability of Finite-Amplitude Water Waves

Abstract: Computations based on the full water-wave equations reveal that there are two distinct types of instabilities for gravity waves of finite amplitude on deep water. One is predominantly two dimensional and is related to all the known results for special cases. The other is predominantly three dimensional and becomes dominant when the wave steepness is sufficiently large.

Nonlinear ion-acoustic waves and solitons in a magnetized plasma.

Abstract: A unified formulation is presented to study the nonlinear low‐frequency electrostatic waves in a magnetized low‐b plasma. It is found that there exist three types of nonlinear waves; (i) nonlinear ion‐cyclotron periodic waves with a wave speed Vp≳Cs (ion‐acoustic velocity); (ii) nonlinear ion‐acoustic periodic waves with Vp

The small‐scale structure of electrostatic shocks

Abstract: It is noted that small-scale regions of large electric fields have been observed above the auroral zone by the S3-3 satellite. The data from five such electrostatic shocks are examined in great detail. The three higher altitude shocks (all above 5,700 km) are found to be associated with upward-going ion beams, indicating that the potential associated with the shock closed below the satellite to give rise to the parallel electric field required for the acceleration of the ion beam. In all these cases, electrostatic ion cyclotron waves are found to be adjacent to the shock and to extend throughout the upward-going ion beam region. The lack of noticeable Doppler shift in the electrostatic ion cyclotron waves in association with large convective drift velocities is seen as indicating that the wavelength of the electrostatic ion cyclotron wave can be several kilometers and that the potential difference within the wave can be on the order of 100 V.

Dynamics of Two-Dimensional Solitary Vortices in a Low- β Plasma with Convective Motion

Abstract: Numerical studies of the Hasegawa-Mima equation, derived in the context of drift waves but equivalent to the quasi-geostrophic vortex potential equation for Rossby waves, show the stable properties of solitary vortices which are two dimensional, localized, steady and translating solutions of this same equation. A solitary vortex can propagate only in the direction ( x -direction) perpendicular to the density gradient. When this solitary vortex solution is inclined at some angle with respect to the x -axis, its propagation direction oscillates in the x and y plane. In two dimensional collisions, i.e. head-on collision and overtaking, solitary vortices interact two-dimensionally and recover their initial shapes at the end of both types of collisions.

Evidence for a traveling two‐day wave in the middle atmosphere

Abstract: Evidence is presented for the existence of a wave number 3, westward traveling 2-day oscillation in the temperature measurements made by the Nimbus 5 SCR and Nimbus 6 PMR instruments. The wave has largest amplitude in the mesosphere at low latitudes of the summer hemisphere and has a markedly asymmetrical meridional structure. It is suggested that the wave may be a free wave similar to the 5-day wave identified in Nimbus 5 SCR data by Rodgers (1976). The vertical structure shows little phase tilt and a slight indication of an increase in amplitude with height.

Tollmien–Schlichting wave cancellation

Abstract: An experiment was conducted in which Tollmien–Schlichting type disturbances in the boundary layer on a flat plate with zero pressure gradient were shown to be nearly canceled by interference with a second wave, 180 ° out‐of‐phase with the original disturbance.

Solar cycle Lorentz force waves and the torsional oscillations of the sun

Abstract: A hypothesis is proposed and analyzed that the longitudinal component of the Lorentz force of the dynamo waves which well simulate the observed solar cycle can drive the recently observed torsional oscillations by Howard and LaBonte. The force component, which is reduced to a correlation between the toroidal and poloidal magnetic fields, consists of a nonwave part and a wave part. Only the nonwave part remains in the deep regions of the convection zone where the dynamo waves can propagate freely. The wave part, called here the Lorentz force waves, emerges only near the surface where the propagating dynamo waves are piled up and their wave profiles are deformed. Thus, the force waves are confined near the surface where the density is low and the moment of inertial is small. Hence, the oscillations are likely to be a phenomenon of rather shallow regions of the solar convection zone. Amplitude of force waves is estimated for typical values of the magnetic fields in the convection zone. It is concluded that driving the torsional oscillations by the force waves is possible in the solar convection zone. If this hypothesis is correct, the torsional oscillations can be the first concrete evidence that themore » magnetic force is indeed working on the global dynamics of the Sun.« less

An ultrasonic interface‐wave method for predicting the strength of adhesive bonds

Abstract: A thin adhesive film, located between two bonded adherents, is capable of localizing the energy of elastic waves in the form of an interface wave. Under study are the phase velocity and transmission losses of the interface wave which were measured during the entire course of polymerization of the adhesive. It is shown that the phase velocity of the interface wave and the effective shear modulus of the interface film, calculated from the velocity data, are related to the strength of the adhesive bonds. The general transmission loss factor, which is a function of the relaxation maximum of losses arising during the course of polymerization of the adhesive, is another parameter correlated with the strength. The shear strength of the joint was determined on a special specimen in the form of a lap joint, which was also used for acoustic measurements.