Spread Of Disturbances Research Articles

A spatial model for studying the effects of climatic change on the structure of landscapes subject to large disturbances

Global warming may have many consequences for natural ecosystems, including a change in disturbance regimes. No current model of landscapes subject to disturbance incorporates the effect of climatic change on disturbances on decade to century time scales, or addresses quantitative changes in landscape structure as disturbances occur. A new computer simulation model, DISPATCH, which makes use of a geographical information system for managing spatial data, has been developed for these purposes. The concept and structure of the DISPATCH model are described here, and a hypothetical example of its use is illustrated, but the model requires refinement before it can be used to predict the effects of global warming on specific landscapes. The model includes provisions for (1) temporally varying weather conditions and their effect on disturbance sizes, and (2) the effect of spatial variation in vegetation condition and physical setting on the probability of disturbance initiation and spread. The potential use of the model is illustrated with a hypothetical example in which the age structure of disturbance patches is monitored for a 250-year period as weather fluctuates. The model run suggests that landscape structure fluctuates even if a disturbance regime remains constant.

Active control of bending wave propagation at acoustic frequencies

An analytical and experimental investigation of the possibility of actively blocking the propagation of bending waves along a uniform beam is described. The investigation has the form of a case study; a thin brass plate-beam is used, it is excited with a short duration impulse, and the resulting disturbance spreads dispersively as it travels along the beam. A short portion of the beam is used as an active block. Strain gage sensors are used to drive thin piezoceramic bending moment actuators through a dynamic compensator. The compensation is designed in the frequency domain with reference to the beam equation, but independent of boundary conditions. The analytical work described includes the nominal design and its performance, the performance degradation due to modeling errors, and the performance degradation due to approximate implementation of the dynamic compensator. The laboratory implementation, in which analog electronics were used for the compensator, essentially verifies some of the sensitivity predictions.

Spatial and temporal analysis of landscape patterns

A variety of ecological questions now require the study of large regions and the understanding of spatial heterogeneity. Methods for spatial-temporal analyses are becoming increasingly important for ecological studies. A grid cell based spatial analysis program (SPAN) is described and results of landscape pattern analysis using SPAN are presentedd. Several ecological topics in which geographic information systems (GIS) can play an important role (landscape pattern analysis, neutral models of pattern and process, and extrapolation across spatial scales) are reviewed. To study the relationship between observed landscape patterns and ecological processes, a neutral model approach is recommended. For example, the expected pattern (i.e., neutral model) of the spread of disturbance across a landscape can be generated and then tested using actual landscape data that are stored in a GIS. Observed spatial or temporal patterns in ecological data may also be influenced by scale. Creating a spatial data base frequently requires integrating data at different scales. Spatial is shown to influence landscape pattern analyses, but extrapolation of data across spatial scales may be possible if the grain and extent of the data are specified. The continued development and testing of new methods for spatial-temporal analysis will contribute to a general understanding of landscape dynamics.

Predicting the Spread of Disturbance across Heterogeneous Landscapes

The expected pattern of disturbance propagation across a landscape was studied by using simple landscape models derived from percolation theory. The spread of disturbance was simulated as a function of the proportion of the landscape occupied by the disturbance-prone habitat and the frequency (probability of initiation) and intensity (probability of spread) of the habitat-specific disturbance. Disturbance effects were estimated from the proportion of habitat affected by the disturbance and changes in landscape structure (i.e., spatial patterns). Landscape structure was measured by the number of habitat clusters, the size and shape of the largest cluster, and the amount of edge in the landscape. Susceptible habitats that occupied less than 50% of the landscape were sensitive to disturbance frequency but showed little response to changes in disturbance intensity. Susceptible habitat that occupied more than 60% of the landscape were sensitive to disturbance intensity and less sensitive to disturbance frequency. These dominant habitats were also very easily fragmented by disturbances of moderate intensity and low frequency. Implications of these results for the management of disturbance-prone landscapes are discussed. The propagation of disturbance in heterogeneous landscapes depends on the structure of the landscape as well as the disturbance intensity and frequency.

Fluid dynamics of an expanding ideal fluid

One studies in this paper the spreading of disturbances superimposed on an infinite ideal fluid region that is everywhere expanding homogeneously and isotropically. All fluid motion studied satisfies Euler’s full nonlinear fluid dynamic equations for the ideal adiabatic monatomic gas.

Diffusion spreading of localized hydrodynamic disturbances under the action of random forces

The effect of a time-dependent random force on fluid flow may be found by changing to a non-inertial coordinate system. It is shown that, under the action of a Gaussian random force, initially localized disturbances undergo spreading of a diffusion type. Explicit analytic solutions are given for the interior wave soliton under the action of a random force. It is shown that, in the presence of a soliton, the growth of velocity pulsing may either increase or moderate.

Two-dimensional disturbance travel, growth and spreading in boundary layers

The nonlinear growth of Tollmien-Schlichting disturbances in a boundary layer is considered as an initial-value problem, for the unsteady two-dimensional triple deck, and computational and analytical solutions are presented. On the analytical side, the nonlinear properties of relatively high-frequency/high-speed disturbances are discussed. The disturbances travel at the group velocity and their amplitude is controlled by a generalized cubic Schrodinger equation, during a first stage of the nonlinear development. The equation, which has been studied in other contexts also, is integrated numerically here, and the resulting large-time/far-downstream behaviour is then deduced analytically. This behaviour comprises an exponentially fast growth and spreading of the disturbance, the spreading being governed only by an integral property of the initial disturbance. Secondary sideband instability does not occur, and there is no conclusive sign of a chaotic response, during this stage, although the three-dimensional counterpart could well yield both phenomena. In the subsequent (and more nonlinear) second stage further downstream, however, where the amplitude is larger, spiked behaviour and spectrum broadening can occur because of vorticity bursts from the viscous sublayer. Computationally, two forms of numerical solution of the triple-deck problem, one spectral, the other finite-difference, are given. The results from each form tend to support the conclusions of the high-frequency analysis for initial-value problems, and recent calculations of the two-dimensional unsteady Navier-Stokes equations also provide some backing. One implication is that the unsteady planar interacting-boundary-layer equations, or a composite version, can capture much of the physics involved in the beginnings of boundary-layer transition although, again, three-dimensionality is undoubtedly an important element which will need to be incorporated eventually.

Boundary Dynamics: A Conceptual Framework for Studying Landscape Ecosystems

We develop a conceptual foundation for the investigation of ecosystem patterns and processes that explicitly considers the spatial patchiness of ecological landscapes. Our emphasis is on the factors determining the location of boundaries between patch types in a landscape mosaic, how boundaries affect ecological processes and the movement of materials over an area, and how imbalances in these transfers in space can affect boundary characteristics and landscape configuration. We suggest that, other things being equal, the edaphic patterns of a landscape will determine the spatial patterning of the biota in the system, primarily through their effects on vegetation. Boundaries in a landscape may be located as direct consequences of the edaphic pattern, or may reflect perturbations of this underlying pattern due to physical disturbances or the actions of various transfer agents or vectors. Abiotic vectors such as wind or surface water flow respond differently to boundary features than do biotic vectors such as beetles or rodents. Both abiotic and biotic vectors may create disturbances through their actions, which in turn may alter patch boundary locations. Vectors also contribute directly to movements of materials, energy, or organisms over the landscape, both within and between patches, and thereby may determine the spatial patterns of the spread of perturbations through the system. If there are net imbalances in fluxes or the spread of disturbances across boundaries in relation to within-patch processes, a directional bias to system fluxes will be established. Over time, this changes the characteristics and locations of patch boundaries. Boundaries thus have dynamics that are superimposed on the edaphic foundation. Consideration of these dynamics offers the potential to gain insight into the spatial configuration of ecosystem functioning and emphasizes the value of a reductionist approach to ecosystem studies.

Inertial waves above an obstacle in an unbounded, rapidly rotating fluid

The slow transverse motion of an obstacle of horizontal dimension L , in a rapidly rotating flow (such that the Rossby number, Ro , is small), is shown to cause an inertial-wave disturbance above and behind the obstacle. This disturbance spreads over an ever-increasing downstream region with increasing vertical distance above the obstacle, but is constrained laterally to lie within a wedge-shaped region. The discussion of Cheng (1977) is shown to give the diffraction pattern due to a point source, and his analysis is modified to allow for arbitrary obstacle shapes. Such a modification shows that the amplitude of the disturbance decays downstream at a rate deter­mined by the Fourier transform of the obstacle shape as noted by Johnson (1982) for ridge-like topography. The inclusion of viscosity is shown to damp out the disturbance on a vertical scale of L/Ek (where Ek = v /2 ΩL 2 is an Ekman number for the flow). At moderate Reynolds numbers visco­sity reduces the amplitude of the disturbance in the neighbourhood of the caustic at the wedge boundary, the attenuation increasing with distance downstream. Internal dissipation removes the short-wavelength com­ponents of the disturbance and thus removes the strong dependence of the wave pattern on the precise shape of the obstacle.

Masking of vibrotactile sensation from a remote site

The masking of vibrotactile signals delivered to the index fingertip was measured as a function of the intensity of the masker, which was administered to the thenar eminence of the same hand. Combinations of test and masker frequencies were used so that the two signals would occur within the response ranges of the Pacinian and non-Pacinian mechanoreceptor systems. When the frequencies of both masker and test signal were within the range of either system (within channel), masking was effective. A 300-Hz masker (Pacinian) was unable to affect the detection of a 23-Hz signal (non-Pacinian). A 23-Hz masker (non-Pacinian) did effectively mask a 300-Hz signal (Pacinian). It was shown that the cross-channel masking is due to the spread of the mechanical disturbance from the thenar to the fingertip and that the amplitude of the masker was sufficient to excite both systems. Results will be discussed within the context of a duplex mechanism for mechano-reception. [Work supported by NIH.]

Boundary layer development from transition provoking devices

The results of an experimental investigation of four different, incompressible, transitional boundary layer situations are presented. The experiments were carried out in zero pressure gradient conditions and transition was initiated from two- and from three-dimensional provoking agents. The measurements of transitional intermittency from two-dimensional tripping agents showed a trend consistent with that reported elsewhere in the literature, with the development of mean and fluctuating component velocity profiles and local skin friction coefficient exhibiting approximate similarity through the transition region. Disturbance frequency and spread angles for turbulent wedge growth behind isolated roughness elements were similar to those reported by others. Computer predictions using a transition model based on the present correlations show reasonable agreement with the data.

Optimal Spreading in an n Dimensional Rectilinear Grid

A disturbance spreads in a rectilinear n dimensional grid moving from each affected point to at most one neighbor in each unit of time. The question how many points can be affected in N units of time is answered to the two leading orders of N.

Use of a television analyzing system in microcirculatory research

To investigate quantitative structural and functional characteristics of the microcirculationin vivo a Leitz television analyzing system was used. In experiments on anesthetized rats the degree of development of the microcirculatory network and the geometric characteristics of the microvessels themselves were determined, changes in the blood volume were estimated, and the degree and spread of disturbances of vascular wall permeability were examined.

On the secular stability of a regular rossby neutral mode

Abstract The evolution of the marginally stable neutral modes on a plane parallel flow is studied in the limit of infinite wavelength. The amplitude of such modes is shown to satisfy a complicated integro-differential equation which is coupled through the velocity jump to the equation for the vorticity in the critical layer. For such length and time scales on which the system may be regarded as linear the mode is shown to be stable but not to decay so rapidly as to make a study of the longer time scales inappropriate. On the longer time scales the disturbance spreads out and the main objective of the paper is the determination of the conditions under which the vorticity can become quasi-steady and the mode a solitary wave.

Civil Strife and the Growth of Trade Union Unity: The Case of Ireland

A MAJOR CLAIM OF THE TRADE UNION MOVEMENT IN THIS DIFFICULT time in Northern Ireland is that they have ‘prevented the spread of riot and disturbance into the workplace’. The claim has been consistently made and with growing emphasis since the troubles began, and Norman Kennedy at last year's annual conference of the Irish Congress of Trade Unions called it the one beacon of hope, this ‘maintaining unity of the workers, Catholic and Protestant, on the shop floor’ in what he described as largely a conflict of worker against worker, of a working-class community divided along sectarian lines. This is associated with a related claim that trade union recommendations on social and political change have a special legitimacy because the leadership is close to the people who are involved in the conflict. This political role, essentially non-party, is seen to be more significant and extensive than the traditional political activity of the trade union movement.

The international planetary patrol program: An assessment of the first three years

Day-to-day and hour-to-hour changes in the large-scale atmospheric and surface features of the planets can now be studied more effectively than previously possible. Since 1969 a network of observatories has obtained almost uninterrupted photographic coverage during all apparitions of Jupiter and Mars, plus some of Venus. Patrol films and catalogues are available to the scientific community. Recent or current analyses include the distribution and motion of clouds on Mars, the development and decay of Martian dust storms, the seasonal, diurnal and random fluctuations in contrast between adjacent light and dark regions on Mars, the detection of vertical shear in the Jovian atmosphere, the longitudinal oscillation of the Red Spot, the dependence of rotation period on xenographic latitude and on time, the eruption and spread of SEB disturbances, and the retrograde circulation of the Venus cloud deck.

Gas density measurements in a jet using Raman scattering.

was also calculated by an alternate method. From the schlieren photographs, a line was extended from the initial bump in the shock to the burst trailing edge; the angle which this line makes with the cone surface is the local relative Mach angle. Values of local burst trailing edge speeds determined from these Mach angles are given in Table I in parenthesis. The velocities of the burst leading and trailing edges were found to be fairly consistent for each observation, with the leading edge of the burst travelling at about local edge velocity while the trailing edge of the burst moved supersonic relative to local speed, UT EJue « 0.82. This result is consistent with James,5 where MT^EJMe was about 0.79 for Me = 7.0 in air. It should be noted that the present study conducted in helium represents an equivalent air Mach number6 of 9.8, since Mair = ^helium (helium - 1/y.i, - I) 1'2 for both the same wall to total temperature ratio and density ratio across the boundary layer. Location of boundary-laye r transition at the cone surface, detected from heattransfer ^measurements and reported in Ref. 7, is also given in Table 1. For the present conditions, bursts were observed as far forward of surface transition as STE/Str = 0.675 (Re/m = 30.9 x 10 6) and as far rearward of transition as S^JS^ = 1.50 (RJm = 37.3 x 10 6). Calculated burst growth rates listed in Table 1 were about 300 w/sec. The disclosure of turbulent bursts upstream of surface transition (STE/Str = 0.675) implies that the outer portion of a hypersonic boundary layer upstream of the surface transition location may be in a developed transitional state. The investigation of Ref. 8 for a 12° half-angle cone at Me = 7.2 also observed turbulent bursts ahead of the surface transition location (ST E /$tr = 0.77). Previously, a 2.87° half-angle cone with Me « 14 was tested in the same facility and over the same Reynolds number range as the present study.9 Mean flow profiles in the outer portion of this hypersonic boundary layer were transitional long before evidence of transition was observed at the surface. Further analysis9'10 disclosed that the turbulent disturbance spreading angle relative to the wall was very shallow (about 0.60°) and remained essentially constant over a wide Mach number range. Spark schlierens of the cone in Ref. 9 revealed a wavy disturbance structure at the boundary-layer edge ahead of the surface transition location (see Fig. 1 of Ref. 9) with no evidence of turbulent bursts. The frequency of these wavy disturbances was approximately 90 KHz (ue « 1830 ra/sec, wave length w 26). From the work of Mack, 11 the frequency of the most unstable two-dimensional disturbance for the conditions of Ref. 9 was approximately 80 KHz for the 2nd mode and 46 KHz for the 1st mode. This result suggests that Tollmien-Schlichting wave forms characteristic of the 2nd mode could be present in the sound dominated disturbance environment (p/p^ « 4% from Ref. 7) of Ref. 9. For the present study, where p/p^ was about 4% (see Ref. 7), turbulent bursts were the dominant boundary-layer feature. However, a wavy structure ahead of and downstream of bursts was evident (see Fig. 2), with a frequency of about 250 KHz (ue « 1700 m/sec, wave length « 38) although this represents a crude frequency estimate since the wave form was not clearly established. From Mack11 for the conditions of the present study, the characteristic frequencies for the 1st and 2nd modes were approximately 225 KHz and 370 KHz, respectively. This result implies that Tollmien-Schlichting waves characteristic of the 1st mode could be present at this lower Mach number.

Spreading of a Turbulent Disturbance

The presented paper shows the effect of local Mach number on the turbulent disturbance spreading angle relative to the wall and on lateral disturbance spreading angles. Almost all the disturbances angles relative to the wall were determined from investigations where hot-wire contours or hot-film surveys of a 'laminar' boundary layer were obtained. Lateral disturbance spreading angles were obtained from investigations of various conditions including turbulent bursts, reported observations of transverse contamination, and observed transitional flow. It is noted that the disturbance spreading angle relative to the wall seems to remain essentially invariant with Mach number, while the lateral spreading angle decreases sharply with increasing Mach number up to about 6. The good agreement between lateral disturbance spreading angle data and data for the variation of turbulent jet spreading angle with Mach number implies that in the lateral dimension, turbulence in a boundary layer may develop essentially free of wall constraints (similar to a free shear layer).

Development of A. C. Type "System Interconnectors"

This paper is concerned with the System Interconnector, a new development for preventing spread of disturbances and reducing short circuit capacity in a large-scale power system where power demand is ever increasing. Background where such development is required, functional requirements, results of analytical and experimental studies, and feasibility of practical schemes are discussed. Also described are 66kV prototypes of the System Interconnector.

Application of a quasi-steady stability model to periodic boundary-layer flows.

A quasi-steady model has been used to investigate the stability of an unsteady, periodic boundary layer on a flat plate. The model combines the concepts of a disturbance wave packet propagating downstream with the generalization of the local effects of steady streamwise pressure gradients to the case of unsteady pressure gradients. Integration of the local amplification rates along the trajectory of the disturbance for several flow conditions has led to the following results which are consistent with previous experimental observations: 1) the disturbances form wave packets; 2) variations in the amplification histories of the disturbances can result in a sequential breakdown along the packet which gives the appearance of the nascent turbulent disturbance spreading upstream with time; 3) insufficient amplification of the packet over one cycle can result in a delayed transition occurring at higher Reynolds numbers.