Calm Region Research Articles

Evolution of vorticity in perturbed flow in a pipe

Flows in long pipes often include “slugs” or “puffs” of complex or chaotic patterns interspersed with regions of relative calm. A similar phenomenon can occur in boundary layers, where “turbulent spots” can exist between rather calm regions of flow, especially in regions of transition from a laminar flow to a turbulent form. The vorticity field plays a significant role in these phenomena, and it is an aim of this paper to examine this feature from a point of view that might be thought to be unusual. Some forty years ago, G. I. Taylor initiated his theory of dispersion of a contaminant in the flow in a long pipe. He showed that a combination of longitudinal (axial) convection by the flow coupled with radial diffusion by molecular (or by turbulent) action produces an effective longitudinal diffusion process, which is governed approximately by the usual one-dimensional diffusion equation. Astonishingly, the effective longitudinal diffusivity coefficient depends inversely on the molecular (or turbulent or eddy) diffusivity. This remarkable phenomenon is now known as Taylor diffusion and has been much studied both experimentally and theoretically over the intervening forty years. The object of this paper is to study the development of vorticity in the flow in a pipe, with the vorticity treated as if it were a passive contaminant on a longitudinal flow through the pipe. In general, this requires an approximation, because the interaction between the velocity and vorticity fields is ignored. But, in a particular case—namely, that of a small swirling vortex perturbation on a flow in a pipe—a theoretical construction becomes possible in which the major approximation is simply that of linearization for small perturbation amplitudes. As a result, it is found that the swirl amplitude satisfies a one-dimensional diffusion equation, just as it does for a scalar contaminant in Taylor's classical work. There is, however, a significant difference in our problem of the evolution of vorticity or swirl: the effective longitudinal diffusion coefficient, which, like Taylor's, depends inversely on the molecular diffusivity, changes sign at a particular value of the Reynolds number. Thus, for Reynolds numbers lower than the critical value, the effective longitudinal diffusion coefficient is positive; in contrast, it is negative for Reynolds numbers larger than the critical value. The latter result implies that a focusing takes place instead of dispersion. The implications of this are discussed.

Effect of pressure gradient on turbulent spots

Turbulent spots were generated artificially to simulate natural spots in the transitional boundary layer. The spots at the final stage under zero and adverse pressure gradients were measured using conventional hotwire anemometry technique. Ensemble averages of the velocity traces were calculated and velocity profiles through the spots and the surrounding region were plotted. The spots are represented by the ensemble average of the velocity perturbations. The integral boundary layer properties are derived and compared through the spots. The effect of the pressure gradient on the turbulent spots is analysed with the structure of the spots and the ‘calmed region’ following the spots.

On the evolution of the turbulent spot in a laminar boundary layer with a favourable pressure gradient

The evolution of a turbulent spot in an accelerating laminar boundary-layer flow was investigated. The type of boundary layer chosen for this experiment resembles in every respect the flow in the vicinity of a stagnation point theoretically described by Falkner and Skan. The rate of growth of the spot was significantly inhibited by the favourable pressure gradient in all three directions. It became much shorter and narrower in comparison with a similar spot generated in a Blasius boundary layer at comparable distances from its origin and comparable Reynolds numbers. The celerities of its boundaries did not scale with the local free-stream velocity as they do in the absence of a pressure gradient. Dimensional analysis was used to identify and correlate the independent variables determining the size, the convection speed, and the relative rate of growth of this spotThe familiar arrowhead shape of the spot gave way to a rounded triangular shape with the trailing interface being straight and perpendicular to the direction of streaming. The familiar Tollmien-Schlichting wave packet was not observed in this pressure gradient because the surrounding boundary layer was very stable at theReconsidered. Since the arrowhead shape of the spot is associated with the breakdown of the waves within the packet it cannot occur below the criticalRe.The relative size of the ‘calmed region’ following the spot also diminished; however, one could only speculate as to the origin of this region.

Surface airflow around Windless Bight, Ross Island, Antarctica

AbstractRoss Island, Antarctica is located along the Transantarctic Mountains and is subjected to a strong southerly mountain‐parallel wind regime. Despite this, Windless Bight on the island's southern coast is a region of anomalous calm. The atmospheric boundary layer dynamics that gives rise to this phenomenon is analysed both theoretically and observationally. It is the strong static stability of the boundary layer air encountering the high steep topography of Ross Island, that causes a stagnation zone resulting in the calm conditions of Windless Bight.Direct and proxy observations of boundary layer winds provide a detailed description of airflow around the island. The anomalous (with respect to the synoptic pressure field) but persistent north‐easterly winds at Scott Base are due to the deflection of highly stable, low‐level air around Hut Point Peninsula. By contrast, the less frequent, strong southerly winds which override the peninsula are associated with the influx of warm maritime air from cyclonic systems to the east. It is inferred that flow of air around the terrain of Ross Island gives rise to locally strong winds; these are responsible for the ice breakout and polynya occurrences in McMurdo Sound.The surface airflow past Ross Island can be modelled by a two‐dimensional, steady, frictionless, irrotational, incompressible flow past an obstacle, with a shape based on an island height contour. The flow is assumed to separate from the eastern and western sides of the island and form a wake downstream. A solution for this flow is presented, based on potential theory for streaming motion past an obstacle. The streamline and isobar patterns clearly depict the stagnation region in Windless Bight. For approaching winds of 20 ms−1 the local pressure field is perturbed by several millibars.

Discussion of ‘thermals over the sea and gull flight behavior’ by A. H. Woodcock

Woodcock's (1975) refined plot of herring gull flight behavior on a diagram of wind speed versus sea-air temperature difference is shown to agree very satisfactorily with expectations derived from free-convection scaling arguments. However, his conclusion that under certain very weak wind conditions, a flock of gulls floating at sea can take off and quickly create a tall thermal suitable for soaring flight was not supported by a causal mechanism. A different hypothesis is that under those conditions, the gulls while on the sea surface can detect the near approach of a dominant thermal, as by sensing the occurrence of an essentially calm region marking the base of the updraft, and then can take off, circle, and quickly locate the nearby thermal.

The Extent and the Local Characteristics of the Valley Winds

By observing wind-shaped trees as indicators for climatic wind conditions, the extent and the local characteristics of the valley winds in the Rhone valley in Switzerland were made clear. Valley winds were found from the shore line of Lake Leman (380m. a.s.1.) to the upper conrse of the river near Morel and Deisch (about 1, 000m. a.s.1.). The strongest winds are to be found especially north of Martigny (480m. a.s.1.). Their horizontal distribution along the river course shows a close relation to the topographical conditions of the valley bottom: width of the valley bottom and topographical irregularity of the valley bottom such as fans and moraines. The region of calm was recognized between Deisch and Reckingen (1330m. a.s.1.) and the northerly wind region in the uppermost part of the valley from Reckingen under the influence of upper winds. The vertical distribution on the valley slopes along the river was given on the illustration map divided into six regions by the wind conditions. Finally, the wind conditions in the Rhone valley were compared with those in other valleys and also with other meteorological and plant ecological phenomena in the Rhone valley.

A General Account of the Intertidal Ecology of South Australian Coasts

An account is given of the environmental features. the intertidal ecology. and the biogeographical relationships of the coast of the State of South Australia . The central and western coasts of South Australia are similar ecologically in comparable areas . In the supralittoral Melaraphe unifasciata (Gray) is dominant (except where conditions are very calm) and at high levels on very exposed coasts Calothrix fasciculata C. Agardh is found. The littoral zone where the coast is most exposed consists of barnacles-Chamaesipho in the upper littoral, Catophragmus in the mid littoral, and Balanus in the lower littoral-but where the coast is more sheltered it consists of molluscs in the upper and mid littoral and algae (Corallina, Gelidium, or Hormosira) in the lower littoral. In the upper sublittoral fucoid algae or in calm regions marine angiosperms are dominant. The south-east coast, however, differs in some respects from the central and west coasts. The number of barnacles found in the littoral zone is much reduced and the giant brown algae, Durvillea potatorum Areschoug and Macrocystis angustifolia Bory, are dominant in the upper sublittoral. This is associated with slightly lower sea temperatures. Sheltered coasts are more prominent in South Australia than in the eastern States of Australia. These include the shores of Spencer and St. Vincent Gulfs, the northern shores of Kangaroo I., and a number of scattered bays. The similarities between the coasts of South Australia and Victoria (Bennett and Pope 1953) are greater than the differences. Consequently the proposal of Bennett and Pope to recognize the Victorian and Tasmanian coasts as the Maugean Biogeographical Provinoe and the South Australian and the south-west Western Australian coasts as the Flindersian Province appears to be unjustified. It is suggested that the Naugean is best regarded as a subprovince with the Flindersian. The latter includes most of the coast of southern Australia. Considerable differences are noticeable between the organisms which inhabit the rocky coast of South Australia and the south coast of Western Australia. The available evidence indicates that a transition from the Flindersian to the tropical Dampierian Province occurs along the south and west coasts of Western Australia. The terms "Indo-Australian Province" and "Baudinian Province" have been proposed by previous authors to describe this transitional region. The Flindersian Province appears to be intermediate between cold-temperate and warm-temperate regions, becoming distinctly cool-temperate in Tasmania. It is relatively distinct from the warm-temperate Peronian Province of the coast of New South Wales.

On the Windbreak in the Field in Atsumi Peninsula

A study was made for the purpose of preventing the injuries of the strong seasonal wind in Atsumi Peninsula. This locality is marked by complicated masses of hills, which render remarkable topographical effects to the wind direction and its force. And so suitable windbreaks for protecting fields are desirable according to the lay of lands. The study consisted of the following three parts: 1) Investigations of the relation between the wind and the lay of land; 2) Indoor experiments by means of model windbreaks and hills set in the stream of air from an electric fan; and 3) Examination of the influences of the actual windbreaks set on the hills. Resuls obtained are summarised as follows: Wind velocity increased at the top and end of a hill and was reduced in front and back of it. Wind directions, at a certain distance from the base of a hill, are the same as those of the mean flow. In the narrower vicinity of a hill, however. those are diversified by the influence of the undulatory topography. Above-mentioned relation was proved true in the indoor experiment, too, by which the effect of windbreaks at various points were investigated. Windbreaks (Ws) located near the top or the end of a model hill were found most effective, the best places for them being confined within a narrow bound, and the calming effect was reduced when it was located apart from those points (see Fig. 2). The windbreaks set near the top or the end of a hill could seperate easily the stream line of wind from the ground surface and performed a large calm region on the leeward, while by those at other locations such an effect was very small. And the results of the outdoor experiments showed that this fact might be applied also under natural conditions.

The Museum Question1

GENTLEMEN of the Museums Association,—In taking the chair which was so ably filled by my predecessor at Cambridge, I must first of all give you a hearty welcome on the part of the Committee of the Manchester Museum. There is to my mind a singular fitness in the selection of Manchester as a place of meeting after Cambridge. At Cambridge you had the opportunity of studying the various museums which have in the course of time naturally grown out of the development of that ancient seat of learning. In Manchester you will see the collections which have been gathered round Owens College, which represents the newest University development in this country. The genius of the place has left its mark in both. In Cambridge the collections are, as they should be in a region of academic calm, free from trade-winds, arranged mainly, if not entirely, with an eye to University students, and not for the general purposes of a miscellaneous public. In this busy centre of movement and commerce, you will find that the principle of arrangement is twofold. It first aims at meeting the needs of the University students, and of the Mechanics' Institutes, and schools, and other educational bodies, which are daily being drawn closer to Owens College, and next at the instruction and enjoyment of the general public. Our collections are for the most part older than the college and have been absorbed from without into our educational centre.

Meteorological Notes

BEFORE the commencement of the summer rains this year Mr. Eliot, the officiating meteorological reporter to the Government of India was called upon for a report on the prospects of the season. His reply, to which we have already referred in the “Notes,” consisted of a short résumé of the most important characteristics of the southwest monsoons of recent years, from which the following conclusions were deduced:—“1. The persistent excessive pressure over Northern India at the present time (June, 1878), tends to diminish the baric gradient between Southern Asia and the Mid-Indian Ocean, and if this is not compensated by increased pressure over the sea area to the South of India, the monsoon current will be below its average strength. 2. There appear to be no strongly-marked abnormal variations of pressure over Northern India. It is therefore probable that the rainfall will be much more equally distributed than last year. 3. Comparing the present year with 1865, it is probable that the heavy rainfall-during the cold weather, and more especially in May, will slightly retard the advent of the monsoon in Upper India. 4. The probable effect of the low pressure along the Bombay coast cannot be determined except by comparison with last year. It appears to promise fairly abundant rain over that portion of the country.” These conclusions have now been subjected to the test of experience and are found to have been verified in almost every particular. The southerly current from the Indian Ocean has been decidedly below its normal strength; the rains set in from a fortnight to a month after the usual time; every district in the country has received a moderate supply of rain, though the average rainfall for the whole country has been less than usual, and over the Bombay Presidency, from Belgarum to Kurrachee, the rainfall has been in excess of the average for previous years. The only peculiarity of the monsoon of 1878, that was not predicted, was the frequent recurrence of heavy falls of rain over a few small and well-defined areas; but this would seem to be the character of the rainfall of every year in which the monsoon current is of less than the usual strength. The percentage of verifications reached by Mr. Eliot has thus been as great as that attained by the American observers, and the predictions in his case were made months, not days or hours, in advance. The same meteorologist has recently made a discovery which promises to be of the greatest possible value in connection with the system of storm-warnings to the ports round the Bay of Bengal. It is that a cyclonic vortex, when generated in the middle of the Bay, always travels towards that part of the coast where the wind velocity for the time being is least in comparison with the average velocity for the same place and time of year. This law has been verified by almost all the cyclonic disturbances that have occurred in the Bay since a chain of meteorological observatories was established round it, and it lends a great deal of support to the theory that a cyclonic vortex is developed through the accumulation, concentration, and condensation of aqueous vapour over a region of comparative calm. All that appears now wanted to render cyclone prognostications for the Bay of Bengal almost absolutely certain is a submarine cable to the Andaman and Nicobar Islands, by which the meteorological stations on these islands, near the place of origin of all the great cyclones of the Bay, would be brought into telegraphic communication with the rest of the empire.