Increase In Gap Size Research Articles

Monitoring of windthrow occurrence and progression in spruce forests in Britain

The results of monitoring windthrow occurrence in eight forest areas in Britain are presented. The extent of windthrow was calculated from aerial photographs, and combined with stand data using a geographic information system. The amount of windthrow occurring over a 6-year period and the dimensions of the windthrown gaps are summarized. The rates of windthrow are contrasted with expectations based on the current windthrow hazard classification. Windthrow occurred in each of the forest areas during the monitoring period, and the amount varied from year to year and from site to site. The gap size distribution remained highly skewed with a predominance of small gaps, despite an increase in numbers across all gap size classes and an increase in maximum gap size. Calculation of a summary shape index (the fractal dimension) indicated that there was no overall change in gap shape between the start and end of the monitoring periods. Windthrow was more frequent and of a higher rate than attritional damage recorded in windprone forests in other countries. However, the rate of windthrow progression was lower than previously recorded in similar forests in Britain due to variability in storm magnitude and silvicultural practice. The windthrow increment was not adequately represented by the existing hazard classification and work is now underway to replace it with a risk classification.

Dipterocarp seedling growth in rain forest canopy gaps during six and a half years

The growth of seedlings of 11 species of Dipterocarpaceae in artificial canopy gaps of different sizes in a lowland evergreen dipterocarp rain forest in Sabah has been followed for 77 months. Three species were abundant and studied in most detail. The main objective was to analyse the foresters’ observation, on which silviculture is based: as gap size increases, species that are more light-demanding win the race to fill the gap. Hopea nervosa seedlings had higher survival in closed forest than those of the other two well represented species, Parashorea malaanonan and Shorea johorensis . From seedling dem ography these two species groups can be called shade-tolerants and light-dem anders respectively. At 40 months the seedlings tallest at gap creation had increased their height advantage in all gap sizes. These were mainly H. nervosa . By 53 months, seedlings of Shorea johorensis had grown ahead of H. nervosa in all but closed forest and tiny gaps of 6 % and 8 % canopy openness (1 and 4 mol m -2 day -1 photosynthetically active radiation (p.a.r.) respectively). By 77 months they were even further ahead. Thus S. johorensis has a more flexible response. It is better able to use the extra p.a.r. of larger gaps 10% openness or more; 10 mol m -2 day -1 p.a.r. or more), and is a light-demander in a second sense, whereas H. nervosa is light-indifferent in this sense. Seedlings of P. malaanonan unexpectedly failed to show rapid height growth in the larger gaps at 53 and 77 months. This species alone suffered very serious apical dam age by herbivory. The 11 species under study occurred in various mixtures. To analyse success in gap-filling in the forest they were grouped by timber density into two classes. With increasing gap size, one group, six light hardwood species, known to be light-demanders, grow progressively ahead of the other group, four medium hardwood species, known to be shade-tolerant. P. malaanonan , also a light hardwood, grows only slowly because of herbivory. We find no evidence for fundamental niche differentiation. All species showed increasing rates of growth with increasing gap size. Species that responded more slowly might succeed in situations where they alone occur.

Marginal fit changes during porcelain firing cycles

The marginal fit changes that occur during the procelain firing cycles of palladium-copper and nickel-chromium copings both with shoulder and chamfer finish lines were investigated with scanning electron microscopy. Three copings from each alloy test group were used as nonporcelainized controls. Comparisons of the firing cycles revealed a greater change during the degassing stage, and the opaque firing caused a decrease in marginal gap. There was a small increase in gap size after firing body porcelain. The marginal fit change for the palladium-copper copings (19.39 microns) during degassing was significantly greater than that for the nickel-chromium copings (8.65 microns). However, no significant differences were found when the effects of margin design and porcelain proximity to fit of metal-ceramic crowns were compared.

Gap size effects on centrifugally and rotationally driven instabilities

The rotation effects on centrifugally driven instabilities in curved channel flow with a finite gap are investigated. An inviscid criterion of stability is formulated to explain the behavior of the flow when rotation and curvature effects compete to either stabilize or destabilize the flow. The stability of curved Poiseuille flow with finite gap size is studied, and it is shown that the asymmetry between the directions of rotation is enhanced when the gap size increases.

Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests

Light regimes beneath closed canopies and tree-fall gaps are compared for five temperate and tropical forests using fish-eye photography of intact forest canopies and a model for calculating light penetration through idealized gaps. Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37–68% of seasonal total photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4–6 min), but the frequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent disturbance history. Analysis of the photographs reveals that incidence angles for photosynthetically active radiation beneath closed canopies are not generally vertical for any of the forests, but there was considerable variation both among and within sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealized single-tree gaps in old growth Douglas-fir – hemlock forests indicate that such gaps have little effect on understory light regimes because of the high ratio of canopy height to gap diameter. However, single-tree gaps in the other four forest types produce significant overall increases in understory light levels. There is also significant spatial variation in seasonal total radiation in and around single-tree gaps. Our results demonstrate that there can be significant penetration of light into the understory adjacent to a gap, particularly at high latitudes. As gap size increases, both the mean and the range of light levels within the gap increases, but even in large gaps (ca. 1000 m2) the potential duration of direct sunlight is generally brief (<4 h). The major differences in gap light regimes of the five forests are largely a function of canopy height and latitude. The effects of latitude should also result in differences in gap light regimes across the geographic range of individual forest types.

Stability of delaminating shells under the influence of gas pressure in the gap between layers

1. Allowing for the nonambiguity of the contact of the internal layer of a cylindrical shell with external layers leads to a significant increase in the critical pressure compared to the case of a free shell. 2. Allowing for the constancy of the amount of gas in the interlayer gap with loss of stability leads to an increase in the theoretical critical pressure (to 13%). This effect weakens with an increase in gap size. 3. For the cases examined here, an increase in the stiffness of the external layers of the shell and a reduction in the size of the interlayer gap are accompanied by a qualitative change, in the mode of loss of stability of the internal layer. This change consists of a contraction of the zone in which the layers separate from one another and an expansion of the sections in which they are in elastic contact.

Boiling Heat Transfer in a Narrow Eccentric Annulus, Part II: Heat Transfer

An experimental apparatus was used to simulate the annular crevice formed by a heated tube and drilled tube support plate (TSP) in a pressurized water reactor recirculating steam generator. A previous paper [1] has described the stable dryout phenomena which occur. This paper discusses the heat transfer mechanisms in the contact and wide gap regions of the crevice. Changes in flow rate and inlet enthalpy are found to have little effect on the patch extent and contact line temperature, while an increase in diametral gap size decreases both. Increasing the thermal conductivity of the TSP also reduces the contact line temperature, but does not affect the wide gap region.

Free convection and shading due to gap spacing between an absorber plate and the cover glazing in solar energy flat-plate collectors

Natural convection losses between an absorber plate and the cover glazing for different gap sizes and at different plate temperatures have been predicted theoretically by various empirical correlations developed by McAdams, 4 Mikheyev, 3 Tabor, 5 Buchberg et al. 6 and Hollands et al. 7 The free convection heat transfer coefficients have been evaluated for horizontal, vertical and tilted collectors. The natural convection coefficient decreases and the shading due to side walls increases with increase in gap size. Therefore, the shade correction factor has also been calculated for different collector inclinations for low and medium latitude stations. By taking into consideration both these factors—that is, convection losses and shading—the air gap between the absorber plate and the cover glazing has been optimised. It has been found that, for minimum shading and minimum convection losses, a gap of 4 to 5 cm should be maintained between the absorber plate and the cover glazing in typical flat-plate solar energy collectors.