Parasitic Waves Research Articles

Two‐Dimensional Surges and Shocks in Open Channels

The finite element method based on the classical Galerkin formulation produces very poor results when applied to discontinuous channel flow, although the complex geometry of most practical problems makes the use of finite elements very desirable. A variance of the Galerkin scheme for conservation laws in two‐dimensional, nearly horizontal flow, which exhibits a remarkable shock‐capturing ability, is presented. The parasitic waves in the vicinity of the discontinuity commonly present in the Gelerkin solution are selectively dissipated, and, in fact, the sharpness of the front is improved by the addition of the dissipation mechanism. The method is based on discontinuous weighting functions which introduce upwind effects in the solution while maintaining central difference accuracy. No arbitrary parameters are needed because the dissipation level is selected analytically. No higher order derivatives appear in the governing equations which simplifies the construction and execution of the scheme. Results are presented for several flow situations that give rise to spontaneous formation of surge and shock waves in two space dimensions. The accuracy of computation is verified by comparison with analytical solutions and by continuous monitoring of the conservation properties of the model.

A Dissipative Galerkin Scheme for Open‐Channel Flow

The finite element method based on the classical Galerkin formulation produces very poor results when applied to discontinuous channel flow. A variance of the Galerkin method is proposed that exhibits highly selective damping characteristics. The dissipation affects only the numerically-generated high-frequency parasitic waves, while maintaining remarkable accuracy in the approximation to the true solution of the problem. In fact, it is shown that the phase error of the finite element simulation is improved by the introduction of dissipation. The resulting model is second-order accurate with respect to the time step and produces a clean, sharp jump structure that agrees favorably with the exact solution of some test problems. The method is based on discontinuous weighting functions that produce “upwind” effects but at the same time maintain the accuracy of a central difference scheme. The dissipation level is selected by analytical investigations, so that the numerical error is minimized. No second-order pseudo viscosity terms are required, which relaxes the inter-element continuity conditions and results in a very simple and inexpensive scheme.

Group Velocity in Finite Difference Schemes

The relevance of group velocity to the behavior of finite difference models of time-dependent partial differential equations is surveyed and illustrated. Applications involve the propagation of wave packets in one and two dimensions, numerical dispersion, the behavior of parasitic waves, and the stability analysis of initial boundary value problems.

Epidemiological Aspects of the Infection With Oswaldocruzia Filiformis (Goeze, 1782), Travassos, 1917 (Nematoda: Trichostrongylidae) in the Common Toad (Bufo Bufo L., 1785) in the Netherlands

Twenty-eight males and fifteen females of B. bufo were collected before and after spawning and in November 1979 at a fieldstation for examination for the presence of O. filiformis. From this fieldstation another sample of the adult toad population was collected in the same spring and placed in an outdoor enclosure (experimental plot). The fluctuation of the parasitic stages in these hosts (twenty-three males, thirty-one females) was observed monthly from May till September/October. The availability of infective larvae of O. filiformis was investigated each month in the experimental plot by using wormfree Rana temporaria as tracers from May till November. Tracers were also used to registrate survival of preparasitic stages following the winters of 1979 and 1980. All parasitic stages overwintered in the toads and resumed their development after spawning of the host. In the experimental plot this development resulted in a first wave of adult parasites in B. bufo during May/June. They caused a midsummer-increase of infective larvae on the ground resulting in significant values in tracers in June and in the toads in June and July. It is suggested that these larvae result in a second wave of adults from the end of August on. From that moment increasing numbers of fifth stage larvae and adults were observed in toads, the increase being significant for adult female parasites. During the summer a significant decrease in available numbers of infective larvae on the ground was seen in the results from the tracers, but the chances for infection considerably increased in late summer and autumn. After both winters tracers became infected, but it is suggested that survival capacity of preparasitic stages is probably low. Under natural circumstances no infection of B. bufo up to the end of May was observed. No significant differences in the intensity of the infection between males and females of both host species, or first and second year of life tracers were found. Instead of 0-20 parasites per animal, as seen in toads in the fieldstation, intensities of 100-200 parasites in both host species were seen most frequently in the experimental plot. Traces became infected earlier than toads, in which new infections were not observed until June. Under outdoor conditions the prepatent period of O. filiformis in R. temporaria apparently takes more than one month.

Nonlinear Saturations of Parasitic Instabilities in High-Efficiency Free-Electron Lasers

A broad spectrum of unstable parasitic waves can arise from the interaction between the trapped electrons and the radiation generated by a free-electron laser. Imposing a dc electric field with appropriate strength at the onset of trapping can substantially narrow the unstable spectrum and allows considerable enhancement in the radiation intensity. The nonlinear mechanisms which limit the enhancement process are observed to be due to nonlinear frequency shift and detrapping.

CORRECT REPRODUCTION OF GROUP-INDUCED LONG WAVES

In nature short period storm waves generate longer waves with periods corresponding to the wave group periods. The long waves are generally referred to as the wave set-down of water level. The set-down term is of second order in the height of the short waves. With first order reproduction of natural storm waves in the laboratory, the setdown bound to the wave groups is not reproduced. As a result, various free waves are generated, propagate towards the model and reflect from the boundaries. These so-called parasitic waves cause an exaggeration of long wave phenomena, such as harbour resonance and slow drift oscillations of moored ships. The parasitic waves can be eliminated by means of compensating free waves imposed on the system by second-order paddle motion reproducing the natural set-down. The control signal for this motion has been calculated and checked by testing. The agreement between calculated and measured results is found to be good. Further, an alternative method for reducing the parasitic wave problem is presented. Utilizing the shoaling properties of the various waves, the influence of parasitic waves can be diminished by generating the waves in somewhat deeper water before they propagate into the shallower model area.

Trypanosoma brucei infection in nude mice: B lymphocyte function is suppressed in the absence of T lymphocytes.

B lymphocyte function was assessed in outbred nude mice and nu/+controls infected with Trypanosoma brucei brucei. On day 10 of the infection in outbred nu/nu mice in which the initial wave of parasites was strongly controlled, B cell function was unaltered on enhanced compared with uninfected animals or infected nu/+. In other nu/nu mice unable to control the initial parasitaemia, thymidine incorporation and Ig secretion by spleen cells were increased on day 10 and their response to lipopolysaccharide in vitro negated. By day 15 however, even the spleen cells of infected nu/nu which controlled the initial wave of parasites were proliferating and secreting Ig on removal from the mice and they were unable to respond to LPS in vitro. These experiments confirm results of a previous study of B cell function in T cell-depleted mice (Askonas et al. 1979). T. b. brucei infection of mice causes both enhanced Ig production and suppression of the ability of B cells to respond to mitogen even in the absence of T cells, but the presence of T cells may accelerate the changes which occur in B lymphocytes following this infection.

The Characteristics of Plasmodium cynomolgi Infections in Various Old World Primates *

Abstract This report summarizes the results of a comparative study of the major features of untreated infections with the B strain of Plasmodium cynomolgi in Macaca mulatta, M. irus (fascicularis), M. nemestrina, M. speciosa (arctoides), and Cercopithecus aethiops. The investigation included: delineation of the courses of trophozoite-induced and sporozoite-induced infections in these Old World primates; evaluation of their efficiencies in transmitting the above plasmodium through Anopheles freeborni; and assessment of host reactions to infection, with particular attention to impacts on erythroid elements of peripheral blood. In all important respects, the parasitic courses of infections in M. mulatta and C. aethiops (Kenyan origin) were identical, as were the reactions of these hosts to disease processes. M. mulatta was slightly more efficient than C. aethiops in transmitting P. cynomolgi through A. freeborni; however, this favored position may have been purchased by experimental design. The features of infections in M. irus, M. nemestrina, and M. speciosa differed significantly from those in M. mulatta. M. irus responded to challenge with trophozoites or sporozoites in a highly reproducible manner, but exhibited peak parasitemias and a long series of parasitic waves (recrudescences and/or relapses) of much lower intensities than were encountered in M. mulatta. M. nemestrina responded to such challenges in diverse manners. A few subjects exhibited parasitic courses almost identical with those encountered in M. irus. The majority exhibited extremely low level parasitemias of great persistence. These diverse parasitic courses were associated with specific physical features of this monkey. M. speciosa failed to support infections following inoculation with either trophozoites or sporozoites. Sporozoite challenges did lead to transient, extremely low level parasitemias, indicating that tissue stage schizogony had taken place. The unusual features of infections in M. nemestrina and M. speciosa could be of substantial interest to those concerned with factors which determine invasion of erythrocytes by plasmodia. Demonstration that untreated infections in M. mulatta and C. aethiops are essentially identical is of far more immediate importance. If responses of infections in these hosts to standard antimalarial drugs are also identical, the way is open to substituting C. aethiops for M. mulatta in studies on the biology and therapy of infections with P. cynomolgi, particularly in the search for radical curative drugs. Such substitution would meet a critical need at a time when access to feral M. mulatta is restricted.

Fetch and wind speed dependence of Doppler spectra

Previous measurements of Doppler spectra in microwave backscattering in a wind wave tank at 9.375 GHz and 23.9 GHz have been extended to 70.1 GHz at 3 m fetch and to longer fetches and higher winds at the two lower microwave frequencies. At high depression angles, the scatterers appear to be specular points moving at approximately the speed of the dominant wind wave. At lower depression angles, the scatterers can be thought of as comprising systems which are respectively bound to and free of the dominant wave. The free scatterers are low‐order Bragg scatterers and it is natural to treat them as free waves or free wave systems in their own right. The bound scatterers are the familiar parasitic capillary waves at low winds, but at higher winds they appear to be associated with the breaking water at the crest of the dominant wave.

Fundamental and parasitic wave modes of a shielded microstrip line

It was demonstrated that for the usually employed widths of the shield in MSL a parasitic quasi-LM10 wave may propagate besides the working quasi-TEM wave; the quasi-LM10 wave interacts with the working wave in the inhomogeneities of the line and may disrupt the normal operation of devices having MSL. The propagation constants of these two lower wave modes may be determined comparatively simply with an accuracy of the order of 1% by means of the first-approximation dispersion equation derived in the present paper. The system of linear algebraic equations derived in the present paper may serve for the analysis and calculation of the characteristics of the higher wave modes.

Slope and Curvature Distributions of Wind-Disturbed Water Surface

The microstructure of the wind-disturbed water surface, characterized by surface-slope and surface-curvature distributions, is studied in a laboratory tank with a newly developed optical instrument. Some features of these distributions under various wind and wave conditions are systematically presented. It is also shown that wind waves arise at about the time when the airflow boundary layer becomes turbulent. At lower wind velocities, the formation of parasitic waves causes a skewed slope distribution; at high wind velocities, the wave breaking causes a peaked slope distribution. With a turbulent wind, similar variation of the mean-square slope with the shear velocity is shown in the present laboratory results and in older oceanic data, suggesting that the laboratory facility may be able to simulate the microstructure of the air–sea interface.

Numerical errors in the time integration of advective processes

A series of numerical experiments with the one-dimensional linearized advection equation are used to explore and to compare the properties of two methods of time integration: (1) an uncentered explicit time integration scheme with parabolic space interpolation of the advected field; (2) the well-known centered explicit scheme. Initial configurations of the advected field are single wavelike disturbances of length 8Δx and 16Δx. The computational results of the single disturbance of length 8Δx are particularly interesting. The uncentered explicit scheme with parabolic space interpolation is shown to be superior to the centered explicit scheme with regard to the following effects of truncation: (a) the error in the displacement of the advected field; (b) the dispersion of the initial configuration, both upstream and downstream. With the introduction of a suitable artificial diffusion coefficient, the amplitude of the parasitic waves dispersing from a single disturbance of length 8Δx and 16Δx can be significantly reduced. In some parts of the computational region used in the numerical experiments, the amplitude of the parasitic waves can be reduced by one or more orders of magnitude. In addition, numerical experiments relevant to short-range numerical predictions are described in which a suitable artificial diffusion coefficient is selected for the control of parasitic waves arising from the dispersion from a single wavelike disturbance in the uncentered explicit finite difference solutions.