Parasitic Waves Research Articles

On the microphysical behaviour of wind-forced water surfaces and consequent re-aeration

AbstractA detailed laboratory investigation of the mechanical and low-solubility gas coupling between wind and water has been undertaken using a suite of microphysical measurement techniques. Under a variety of wind conditions and in the presence and absence of mechanically generated short waves, approximately fetch-independent surface conditions have been achieved over short laboratory fetches of several metres. The mechanical coupling of the surface is found to be consistent with Banner (J. Fluid Mech. vol. 211, 1990, pp. 463–495) and Banner & Peirson (J. Fluid Mech. vol. 364, 1998, pp. 115–145). Bulk observations of re-aeration are consistent with previous laboratory studies. The surface kinematical behaviour is in accordance with the observations of Peirson & Banner (J. Fluid Mech. vol. 479, 2003, pp. 1–38). Also, their predictions of a strong enhancement of low-solubility gas flux at the onset of microscale breaking is confirmed and direct observations show a concomitant onset of very thin aqueous diffusion sublayers. It is found that the development of strong parasitic capillary waves towards the incipient breaking limit does not noticeably enhance constituent transfer. Across the broad range of conditions investigated during this study, the local instantaneous constituent transfer rate remains approximately log-normally distributed with an approximately constant standard deviation of $0.62\pm 0.15({\mathrm{log}}_e(\mathrm{m}~ {\mathrm{s}}^{-1}))$. Although wind-forced water surfaces are shown to be punctuated by intense tangential stresses and local surface convergence, localized surface convergence does not appear to be the single critical factor determining exchange rate. Larger-scale orbital wave straining is found to be a significant constituent transfer process in contrast to Witting (J. Fluid Mech. vol. 50, 1971, pp. 321–334) findings for heat fluxes, but the measured effects are consistent with his model. By comparing transfer rates in the presence and absence of microscale breaking, low-solubility gas transfer was decomposed into its turbulent/capillary ripple, gravity-wave-related and microscale breaking contributions. It was found that an efficiency factor of approximately $17\, \%$ needs to be applied to Peirson & Banner’s model, which is extended to field conditions. Although bulk thermal effects were observed and thermal diffusion layers are presumed thicker than their mass diffusion counterparts, significant thermal influences were not observed in the results.

Energy Attenuation Mechanism of a Carrier Wavepropagating in a Viscous Fluid

For the first time, we derived a constituted carrier wave equation which is the result of superposing a 'parasitic wave' on a 'host wave'. The attenuation mechanism of the several properties of the carrier wave produced by the two interfering waves as it propagates in a viscous fluid is effectively studied using simple differentiation technique.In this work,we subjected the constituted carrier wave to some basic boundary conditionswith a multiplicative factor as a constraint in other to determine quantitatively the basic intrinsic characteristics of the two interfering waves which were initially not known. Initially, the carrier wave and its several propertiesshow a blurred spectra characteristics followed by a gradual depletion of the wave form. The initial blurred nature of the resulting spectra is an indication of the resistance of the intrinsic parameters of the 'host wave' to the destructive tendency of the interfering 'parasitic wave'. The subsequent depleting behaviorof the carrier waveindicates the predominance of the 'parasitic wave'. After this time, a steady decay process resulting to a gradual reduction and weakening in the initial strength of the intrinsic parameters of the 'host wave' becomes prominent. The constituted carrier wave becomes monochromatic in nature since each component of the wave packet have different phase velocity in the medium, the modulation propagation number of the components of the carrier wave changes in the medium and consequently the group velocity changes. This study reveals that when a carrier wave is undergoing attenuation under any circumstance, it does not consistently come to rest; rather it shows some resistance at some point during the decay process, before it is finally brought to rest.

Excitation of parasitic waves in forward-wave amplifiers with weak guiding fields

To produce high-power coherent electromagnetic radiation at frequencies from microwaves up to terahertz, the radiation sources should have interaction circuits of large cross sections, i.e., the sources should operate in high-order modes. In such devices, the excitation of higher-order parasitic modes near cutoff where the group velocity is small and, hence, start currents are low can be a serious problem. The problem is especially severe in the sources of coherent, phase-controlled radiation, i.e., the amplifiers or phase-locked oscillators. This problem was studied earlier [Nusinovich, Sinitsyn, and Antonsen, Phys. Rev. E 82, 046404 (2010)] for the case of electron focusing by strong guiding magnetic fields. For many applications it is desirable to minimize these focusing fields. Therefore in this paper we analyze the problem of excitation of parasitic modes near cutoff in forward-wave amplifiers with weak focusing fields. First, we study the large-signal operation of such a device with a signal wave only. Then, we analyze the self-excitation conditions of parasitic waves near cutoff in the presence of the signal wave. It is shown that the main effect is the suppression of the parasitic wave in large-signal regimes. At the same time, there is a region of device parameters where the presence of signal waves can enhance excitation of parasitic modes. The role of focusing fields in such effects is studied.

Conditions for Lower Hybrid Current Drive in ITER

To control the plasma current profile represents one of the most important problems of the research of nuclear fusion energy based on the tokamak concept, as in the plasma column the necessary conditions of stability and confinement should be satisfied. This problem can be solved by using the lower hybrid current drive (LHCD) effect, which was demonstrated to occur also at reactor grade high plasma densities provided that a proper method should be utilised, as assessed on FTU (Frascati Tokamak Upgrade). This method, based on theoretical predictions confirmed by experiment, produces relatively high electron temperature at the plasma periphery and scrape-off layer (SOL), consequently reducing the broadening of the spectrum launched by the antenna produced by parasitic wave physics of the edge, namely parametric instability (PI). The new results presented here show that, for kinetic profiles now foreseen for the SOL of ITER, PI is expected to hugely broaden the antenna spectrum and prevent any penetration in the core of the coupled LH power. However, considering the FTU method and assuming higher electron temperature at the edge (which would be however reasonable for ITER) the PI-produced spectral broadening would be mitigated, and enable the penetration of the coupled LH power in the main plasma. By successful LHCD effect, the control of the plasma current profile at normalised minor radius of about 0.8 would be possible, with much higher efficiency than that obtainable by other tools. A very useful reinforce of bootstrap current effects would be thus possible by LHCD in ITER.

Malaria Sporozoite Challenge Model Comes of Age

Invited Commentary on ‘Efficacy of preerythrocytic and blood-stage malaria vaccines can be assessed in small sporozoite challenge trials in human volunteers’, Roestenberg et al. Journal of Infectious Diseases 2012. For decades a small group of laboratories have been quietly but steadily testing experimental malaria vaccines in human volunteers and confirming their efficacy by exposing these volunteers to the bite of infected mosquitoes. A few days following the bite of the mosquitoes the volunteers are followed closely for onset of symptoms and for the presence of the parasite in the peripheral blood by preparing blood smears that are examined microscopically. The requirements for a successful challenge that infects 100% of non-immunized volunteers, such as the number of mosquito bites and the length of time the mosquitoes are allowed to feed, were worked out empirically initially.1 The sample size of these sporozoite challenges also was driven by limitations in logistics as well as by ethical concerns about the safety of volunteers.2 Despite these limitations, the cumulative observation through the years has been that the results of these trials accurately predicted success or failure in the field. The advent of quantitative PCR has allowed a more accurate estimation of the parasitemias in these individuals than was ever possible microscopically as well as the calculation of the parasite multiplication rates, making it possible to determine the effect of immunization on these parameters. This is important because pre-erythrocytic vaccines which just target sporozoites and liver stages of the parasite would only have an impact on the first wave of parasitemia coming out of the liver. On the other hand, blood stage vaccines that target merozoites are not predicted to impact the first wave of parasitemia but would affect the multiplication rate in blood. Thus, the addition of quantitative PCR has allowed scientists to determine the effect of a vaccine on the parasite load in the liver and on the multiplication rate in blood. One recent example of the usefulness of this approach was the finding that the AMA-1 vaccine resulted in a reduction of the first wave of parasites.3 A subsequent preliminary trial in Africa also seemed to suggest that this vaccine can offer some protection against vaccine strain parasites.4 With the use of statistical simulation, Roestenberg et al.5 have estimated that with the addition of quantitative PCR sporozoite challenge studies, although small, do have sufficient power to detect clinically relevant differences between immunized volunteers and controls such as 90% or greater inhibition of parastemia or multiplication rate. Some have argued that blood stage vaccines that target the red cell invasive form of the parasite should be tested in the field in order to better observe the effect of vaccine-induced immunity over a period of time. However, the limited published experience and now the analysis by Roestenberg et al.5 seem to argue that the sporozoite challenge model can provide a window into the likelihood that these vaccines can confer protection. The results of Roestenberg and colleagues are welcome news in an era when resources and funding for large field trials are very limited. This is illustrated in the accompanying commentary by Doolan.6 They support the paradigm of subjecting all malaria vaccines to sporozoite challenge trials prior to committing to large field studies.

Dynamics of vorticity defects in stratified shear flow

AbstractWe consider the linear stability and nonlinear evolution of two-dimensional shear flows that take the form of an unstratified plane Couette flow that is seeded with a localized ‘defect’ containing sharp density and vorticity variations. For such flows, matched asymptotic expansions furnish a reduced model that allows a straightforward and computationally efficient exploration of flows at sufficiently high Reynolds and Péclet numbers that sharp density and vorticity gradients persist throughout the onset, growth and saturation of instability. We are thereby able to study the linear and nonlinear dynamics of three canonical variants of stratified shear instability: Kelvin–Helmholtz instability, the Holmboe instability, and the lesser-considered Taylor instability, all of which are often interpreted in terms of the interactions of waves riding on sharp interfaces of density and vorticity. The dynamics near onset is catalogued; if the interfaces are sufficiently sharp, the onset of instability is subcritical, with a nonlinear state existing below the linear instability threshold. Beyond onset, both Holmboe and Taylor instabilities are susceptible to inherently two-dimensional secondary instabilities that lead to wave mergers and wavelength coarsening. Additional two-dimensional secondary instabilities are also found to appear for higher Prandtl numbers that take the form of parasitic Holmboe-like waves.

Enhanced Energy Dissipation by Parasitic Capillaries on Short Gravity–Capillary Waves

Abstract The increased energy dissipation caused by the formation of parasitic capillary wavelets on moderately short, steep gravity–capillary waves is studied numerically. This study focuses on understanding the mechanism leading to dissipation enhancement and on exploring the possible correlation between the enhanced dissipation rate and the characteristic parameters of the parasitic capillaries. The interaction between the parasitic capillary wave train and the underlying dominant flow of the carrier wave induces strong vortex shedding and imposes large straining immediately underneath the troughs of the capillary ripples. These localized strains are very effective in dissipating energy of the carrier gravity–capillary wave. The attenuation rate of the carrier wave can increase by more than one order of magnitude in the presence of capillary wavelets. Systematic simulations for various carrier wavelengths and steepnesses reveal that the enhanced dissipation rate can be quantified well by a simple parameter: the average of all the difference between the local maximum and minimum slopes along the entire carrier wave surface, which is equivalent to the mean slope of the parasitic capillary wave train. The enhanced dissipation rate increases approximately linearly with the carrier gravity–capillary wavenumber for a given mean slope of the capillary wave train. The increased energy dissipation caused by the formation of parasitic capillaries is also found to significantly impact on the characteristics of three-dimensional instabilities of finite-amplitude, uniform gravity–capillary waves.

Excitation of parasitic waves near cutoff in forward-wave amplifiers

In this paper, excitation of parasitic waves near cutoff in forward-wave amplifiers is studied in a rather general form. This problem is important for developing high-power sources of coherent, phase controlled short-wavelength electromagnetic radiation because just the waves which can be excited near cutoff have low group velocities. Since the wave coupling to an electron beam is inversely proportional to the group velocity, these waves are the most dangerous parasitic waves preventing stable amplification of desired signal waves. Two effects are analyzed in the paper. The first one is the effect of signal wave parameters on the self-excitation conditions of such parasitic waves. The second effect is the role of the beam geometry on excitation of these parasitic waves in forward-wave amplifiers with spatially extended interaction space, such as sheet-beam devices. It is shown that a large-amplitude signal wave can greatly influence the self-excitation conditions of the parasitic waves which define stability of operation. Therefore the effect described is important for accurate designing of high-power amplifiers of electromagnetic waves.

Joly–Mercier boundary condition for the finite element solution of 3D Maxwell equations

Solving the time-dependent Maxwell equations in an unbounded domain requires the introduction of artificial absorbing boundary conditions ( ABCs) designed to minimize the amplitude of the parasitic waves reflected by the artificial frontier of the domain of computation. The construction of such ABCs needs to perform a rigorous mathematical and numerical analysis, in order to obtain a well-posed problem, from a mathematical point of view, and a stable algorithm, from a numerical point of view. In a previous study, Joly and Mercier (1989) [8] have proposed a new second-order ABC for Maxwell’s equation in dimension 3, well adapted to a variational approach. In this paper, we present how to apply the second-order ABC proposed in [8] in the framework of a finite element method.

The Formation of Parasitic Capillary Ripples on Gravity–Capillary Waves and the Underlying Vortical Structures

Abstract The evolution of moderately short, steep two-dimensional gravity–capillary waves, from the onset of the parasitic capillary ripples to a fully developed quasi-steady stage, is studied numerically using a spectrally accurate model. The study focuses on understanding the precise mechanism of capillary generation, and on characterizing surface roughness and the underlying vortical structure associated with parasitic capillary waves. It is found that initiation of the first capillary ripple is triggered by the fore–aft asymmetry of the otherwise symmetric carrier wave, which then forms a localized pressure disturbance on the forward face near the crest, and subsequently develops an oscillatory train of capillary waves. Systematic numerical experiments reveal that there exists a minimum crest curvature of the carrier gravity–capillary wave for the formation of parasitic capillary ripples, and such a threshold curvature (≈0.25 cm−1) is almost independent of the carrier wavelength. The characteristics of the parasitic capillary wave train and the induced underlying vortical structures exhibit a strong dependence on the carrier wavelength. For a steep gravity–capillary wave with a shorter wavelength (e.g., 5 cm), the parasitic capillary wave train is distributed over the entire carrier wave surface at the stage when capillary ripples are fully developed. Immediately underneath the capillary wave train, weak vortices are observed to confine within a thin layer beneath the ripple crests whereas strong vortical layers with opposite orientation of vorticity are shed from the ripple troughs. These strong vortical layers are then convected upstream and accumulate within the carrier wave crest, forming a strong “capillary roller” as postulated by Longuet-Higgins. In contrast, as the wavelength of the gravity–capillary wave increases (e.g., 10 cm), parasitic capillary ripples appear as being trapped in the forward slope of the carrier wave. The strength of the vortical layer shed underneath the parasitic capillaries weakens, and its thickness and extent reduces. The vortices accumulating within the crest of the carrier wave, therefore, are not as pronounced as those observed in the shorter gravity–capillary waves.

Unidirectional Radiation Efficient Stacked Aperture Antenna for X-Band Application

Presented is a unidirectional slot aperture antenna for wideband operation, which is composed of two stacked bow-tie apertures backed by a metallic reflector. Parasitic parallel plate wave propagation is suppressed by shorting pins. The measured bandwidth defined by return loss < -10 dB is 27%. The radiation efficiency in this frequency range is about 85%.

Stability of frequency-multiplying harmonic gyroklystrons

In the present paper, frequency-multiplying three-cavity gyroklystrons are studied in detail. The analysis is focused on the possibility of realizing a high electron-bunching efficiency in the output cavity while simultaneously suppressing the excitation of parasitic backward waves resonant with lower cyclotron harmonics. The potential to minimize the effect of parasitic backward-wave excitation at the fundamental cyclotron resonance on the device operation by the proper choice of output cavity length is shown.

Wave guide optimization for homoepitaxial laser diodes

AbstractGaN‐based high power laser diodes in the violet spectral range are demanding for conductive substrates with a lower thermal resistance than the commonly used sapphire. The only feasible approach to this goal is homoepitaxy on GaN wafers produced from thick GaN layers grown by hydride vapour phase epitaxy. One drawback of GaN substrates is the high refractive index and therefore it acts as an parasitic wave guide. In this paper we will discuss the optimization of the wave guide and the n‐side cladding layer. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Drug Development Papers in PLoS Medicine: How We Try to Spot a Winner

Drug Development Papers in PLoS Medicine: How We Try to Spot a Winner

Spatial variations of waves propagating over a submerged rectangular obstacle

Experiments are performed in a wave flume to examine the generation of harmonics by non-breaking surface waves travelling over a submerged obstacle. Spatial wave profiles are recorded using an optical measurement system, similar to the approach used by Perlin [Perlin, M., Lin, H., Ting, C.L., 1993. On parasitic capillary waves generated by steep gravity waves: an experimental investigation with spatial and temporal measurements. J. Fluid Mech., 255, 597–620]. Moreover, an extended Boussinesq equation developed by Nwogu [Nwogu, O., 1993. An alternative form of the Boussinesq equations for modelling the propagation of waves from deep to shallow water. J. of Waterway, Port, Coastal and Ocean Eng. ASCE, 119(6), 618–638] is used to numerically simulate the wave field. The experimental results demonstrate that non-linear wave effects generate super harmonics, and the amplitude of those harmonics grows as waves travel above the models. The order of super harmonics increases with wave non-linearity. Numerical results are also found to over-estimate the amplitudes of super harmonics and the total averaged energy flux.

Rayleigh SAW resonators using gold electrode structure for gas sensor applications in chemically reactive environments

Simple and efficient Rayleigh-SAW two-port resonators on quartz using a corrosion-proof gold-stripe electrode structure are presented. The devices are intended for operation as gas sensors in chemically reactive environments in which the measured gases may attack the electrode structure and destroy the sensor. By carefully selecting the gold film thickness, cavity length and transducer coupling to the load, parasitic bulk wave radiation is minimised and efficient Rayleigh-SAW mode excitation is achieved. Single-mode resonators and two-pole coupled resonator filters in the 400 MHz range demonstrate −7 to −10 dB of unmatched insertion loss and loaded Q values in the 4000–6000 range. Substantial corrosion immunity compared to SAW resonators with aluminium electrodes is demonstrated.

Radiation efficiency analysis of submillimeter‐wave receivers based on a modified spectral domain integration technique

Reflector backed submillimeter‐wave slot antennas on substrate lenses are promising candidates for novel astronomical applications in integrated receivers for the far infrared region. Using an adjustable reflector, considerable resonance frequency tuning can be achieved, but with the risk of exciting parasitic parallel plate wave modes. Employing a surface/volume integral equation method and advanced spectral domain integration techniques, a complete radiation efficiency analysis of such kind of receivers is performed. Convenient integration path deformations in complex wavenumber planes allow a very fast evaluation of the spectral domain integrals in combination with efficient database and asymptotic extraction techniques. Saddle point and residue calculation techniques are employed to accurately determine the power of each parallel plate mode and the space wave contributions. The simulations reveal an appropriate set of parameters reducing the radiation loss to less than 10 percent for doubleslot and quadratic slot geometries preserving high tuning capability and optimized radiation patterns as well.

Enhanced dissipation of short gravity and gravity capillary waves due to parasitic capillaries

The dissipation rate of short, mechanically generated, monochromatic surface waves was measured in order to determine the increased dissipation due to the generation of parasitic capillary waves. As the surface waves propagate freely, the decaying wave elevations were measured by an ultra-thin capacitance wire. Measurements indicate that the dissipation rate increases approximately with the square of the wave slope, AK, and the generation of parasitic capillaries depends on the wavelength of the surface waves. The maximum generation of parasitic capillaries coincides with a surface wave wavelength near 7 cm. Measured data fits a simple parameterized model of the dissipation rate reasonably well.

A modal method for finite amplitude, nonlinear sloshing

A modal method is used to calculate the two-dimensional sloshing motion of an inviscid liquid in a rectangular container. The full nonlinear problem is reduced to the solution of a system of nonlinear ordinary differential equations for the time varying coefficients in the expansions of the interface and the potential. The effects of capillarity are included in the formulation. The simplicity, generality and power of the method are exhibited not only by recovering the earlier results obtained, for example, by Penney and Price [1], Tadjbakhsh and Keller [2] and Faltinsen et al [3], but also by obtaining new and interesting results of the effects of capillarity and shallow depth, which would be difficult to obtain otherwise. For example, it is found that for the initial interface profile considered here, parasitic capillary waves, borne by the higher number wave modes, are generated for moderate capillarity but disappear for larger values of the parameter. The method can be extended to other simple geometries.

Linear theory of gyro-traveling-wave-tubes with distributed losses

A small-signal theory describing two-stage gyro-traveling-wave tubes (gyro-TWTs) with the first stage having distributed losses is developed. In addition to the study of a small-signal gain in this device, the self-excitation conditions for parasitic backward waves are also analyzed. The theory is illustrated by using it for describing the performance of the gyro-TWT designed at the Naval Research Laboratory. The results show a very good agreement between the predictions of analytical theory and a thorough numerical analysis based on the use of well-developed codes.