Evolution Of Flow Patterns Research Articles

A study on the pulse boiling occurring inside the liquid pool of a closed two-phase thermosyphon

Pulse boiling, the unsteady periodic boiling phenomenon appearing in the evaporator of thermosyphons was investigated by many researchers. In the present study investigations were conducted to examine the evolution of flow patterns at the evaporator, and changes in thermodynamic state that each of liquid pool and vapor experiences through 1 cycle of pulse boiling process. For wall and liquid pool the degree of superheat for the onset of nucleation was examined. It revealed that the degree of superheat increased with the increase of pulse period, reaching to 16.5 deg.C and 23 deg.C for liquid pool and evaporator wall respectively at .tau.=80 sec. The data on flow patterns obtained through series of operation tests were plotted in the coordinates of heat flux and vapor pressure to get a regime map. Further this map could be used to figure out the conditions of pulse boiling for a thermosyphon.

Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model

AbstractThe evolution of ice-sheet configuration and flow pattern in Fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model, i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets. The model uses mapped flow traces and deglacial melt-water landforms, as well as relative chronologies derived from cross-cutting striae and till lineations, as input data. Flow-trace systems were classified into four types: (i) time-transgressive wet-bed deglacial fans, (ii) time-transgressive frozen-bed deglacial fans, (iii) surge fans, and (iv) synchronous non-deglacial (event) fans. Using relative chronologies and aggregation of fans into glaciologically plausible patterns, a series of ice-sheet Configurations at different time slices was erected. A chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume. Geological evidence exists for several discrete ice-sheet configurations centred over the Scandinavian mountain range during the early Weichselian. The build-up of the main Weichselian Fennoscandian ice sheet started at approximately 70 Ka, and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern Norway. This configuration had a flow pattern which is poorly reproduced by current numerical models of the Fennoscandian ice sheet. At the Last Glacial Maximum the main ice divide was located overthe Gulf of Bothnia. A major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the Scandinavian mountain chain. Widespread areas of preserved pre-late-Weichselian landscapes indicate that the ice sheet had a frozen-bed core area, which was only partly diminished in size by inward-transgressive wet-bed zones during the decay phase.

Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model

AbstractThe evolution of ice-sheet configuration and flow pattern in Fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model, i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets. The model uses mapped flow traces and deglacial melt-water landforms, as well as relative chronologies derived from cross-cutting striae and till lineations, as input data. Flow-trace systems were classified into four types: (i) time-transgressive wet-bed deglacial fans, (ii) time-transgressive frozen-bed deglacial fans, (iii) surge fans, and (iv) synchronous non-deglacial (event) fans. Using relative chronologies and aggregation of fans into glaciologically plausible patterns, a series of ice-sheet Configurations at different time slices was erected. A chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume. Geological evidence exists for several discrete ice-sheet configurations centred over the Scandinavian mountain range during the early Weichselian. The build-up of the main Weichselian Fennoscandian ice sheet started at approximately 70 Ka, and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern Norway. This configuration had a flow pattern which is poorly reproduced by current numerical models of the Fennoscandian ice sheet. At the Last Glacial Maximum the main ice divide was located overthe Gulf of Bothnia. A major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the Scandinavian mountain chain. Widespread areas of preserved pre-late-Weichselian landscapes indicate that the ice sheet had a frozen-bed core area, which was only partly diminished in size by inward-transgressive wet-bed zones during the decay phase.

Transition to Turbulence from a Localized Vortex Array

Dynamical behavior of a localized linear array of vortices is investigated by numerical simulations. The physical setup consists of a thin fluid layer (electrolyte) enclosed in a rectangular box and the vortices are locally driven by electromagnetic forces in the vicinity of a long lateral wall. The numerical model is used to simulate the Navier-Stokes equations in two dimensions with steady forcing and linear bottom friction. The model provides an accurate representation of the evolution of flow pattern: a stable unit-type flow with a group of four vortices and an irregular flow including large-scale motion. Fourier decomposition of the streamfunction shows that the mode of twice the wavelength of the forcing scale is prominent in the primary instability and a large-scale mode (uniform along the periodic direction) appears in the secondary instability. The mutual interaction between the unstable modes is shown to play an important role in dynamical behavior in the regime of transition to turbulence.

Redistribution of Regional Pulmonary Blood Flow During Partial Liquid Ventilation. ▴ 261

OBJECTIVE: Partial liquid ventilation (PLV) is a novel means for improving gas exchange during which a perfluorocarbon liquid (PFC) is instilled into the lung and tidal gas ventilation is maintained with a conventional ventilator. We examined regional pulmonary blood flow pattern evolution during PLV in a healthy infant animal model.

Transient three-dimensional natural convection in a rectangular enclosure

An experimental study is presented for transient buoyancy-induced natural convection in the Rayleigh number range of 6.9 × 10 7-4.12 × 10 8 for aspect ratios of A H = 3 and A w = 1.2 inside a rectangular enclosure with silicone oil as the working medium wherein the two vertical walls were heated on the right wall and cooled on the left wall as t ⩾ 0, respectively. The remaining four walls were adiabatic. Time evolution of streak flow patterns was observed and temporal temperature distributions are presented and analyzed. In addition, the present results are compared with previous two-dimensional experimental and three-dimensional numerical results.

Cavopulmonary connection in repair of atrioventricular septal defect with small right ventricle

Between February 1991 and May 1992, 9 patients with a nonrestrictive inlet ventricular septal defect or complete atrioventricular septal defect and hypoplasia of the right ventricle underwent successful two-ventricle correction incorporating a bidirectional cavopulmonary anastomosis. Despite a satisfactory early postoperative course, large serous pleural effusions developed in 5 of them between 2 and 6 weeks after operation. Serial echocardiography showed an evolution of flow patterns in the superior vena cava from marked systolic reversal immediately after operation to nearly continuous forward flow into both pulmonary arteries about 6 weeks later. This pattern was consistent with the transition from cardiac-dependent to respiratory-dependent caval flow. The development of pleural effusions appeared to coincide with the loss of systolic caval flow reversal and resolved after establishment of a predominantly respiratory-dependent flow pattern.

Unsteady thermosolutal opposing convection of a liquid-water mixture in a square cavity—I. Flow formation and heat and mass transfer characteristics

Transient thermosolutal opposing convection of a liquid-water mixture in a square cavity subject to horizontal temperature and concentration gradients is numerically investigated by a third-order upwind finite-difference scheme. Results are particularly presented to illustrate the effects of the Lewis and Grashof numbers on the evolution of flow patterns and the associated heat and mass transfer characteristics for solutally dominant situations. Results for Le = 100 clearly show the double-diffusive nature of the convection. In the initial stage the flow is dominated by the interface velocities at the vertical side walls driven by the concentration gradients there. Later, the flow is governed by the thermal buoyancy. At a much later time, the solutal buoyancy set in inducing new recirculating cells along the side walls. These cells gradually grow and squeeze the thermally driven cell in the core region. Multilayer flow structure is finally formed. The counterrotating cells resulting from the opposing thermal and solutal buoyancies cause significant velocity, temperature and concentration oscillations with time at high Grashof numbers.

Strain localization behaviour in experimental shear zones

A torsional deformation apparatus has been developed which allows the evolution of microstructures and flow patterns in analogue materials to be observed to shear strains in excess of γ=100 while the loading conditions are kept constant. Using a computer-based video microscopy system the displacement paths of marker particles have been followed in real time, and from these the local strain states in the samples have been calculated. A series of velocity stepping experiments in octachloropropane and norbornene determined the shear localization behaviour displayed by these materials. Both of these materials underwent two phases of localization, a primary localization at the start of an experiment, and a secondary localization following increases in the imposed strain rate. The secondary localization is less pronounced, but is reversible, in the sense that lowering the imposed strain rate widens the zone. An inverse relationship between the fixed imposed strain rate and the width of the zone of localized deformation was established. This velocity dependant localization is ascribed to the softening effect of dynamic recrystallization.

Dynamic user optimal traffic assignment on congested multidestination networks

An equivalent continuous time optimal control problem is formulated to predict the temporal evolution of traffic flow pattern on a congested multiple origin-destination network, corresponding to a dynamic generalization of Wardropian user equilibrium. Optimality conditions are derived using the Pontryagin minimum principle and given economic interpretations, which are generalizations of similar results previously reported for single-destination networks. Analyses of sufficient conditions for optimality and of singular controls are also given. Under the steady-state assumptions, the model is shown to be a proper dynamic extension of Beckmann's mathematical programming problem for a static user equilibrium traffic assignment.

Numerical and experimental modeling of turbulent flows in furnace chambers

Results are presented from an experimental and numerical modeling of two-dimensonal turbulent flows in furnace chambers. The article follows the evolution of the flow patterns with a change in the configuration of the chamber. The range of validity of the study results are discussed.

Synoptic flow patterns leading to the generation of north‐west African depressions

AbstractThis paper reports a study and analysis of the synoptic evolution of the atmospheric flow patterns that cause the formation of north‐west African depressions. The study uses mean 500 and 850 hPa heights and their anomalies, associated with the subcategories into which the north‐west African depressions have been classified. The time period under consideration covers the consecutive years from 1970 to 1974.To study better the synoptic evolution and the separation of existing centres of action for each subcategory, charts of the mean 500 and 850 hPa heights and temperatures for 72, 48, and 24 h before the first appearance of a depression have been prepared. For the mean 500 hPa height field and for each subcategory, anomalies based on differences from the long‐term (1950–1973) average have been calculated (at the 0·01 statistical level of significance) for each grid point of the ‘anomaly’ fields. From the study as a whole it can be deduced that the synoptic flow patterns, which cause the generation of north‐west African depressions, are unstable baroclinic waves, at the synoptic scale, initiated in the middle troposphere over geographical regions far from the Atlas Mountains. These waves can be seen on the 500 hPa charts at least 72 h before the first appearance of the depressions on the mean sea‐level (MSL) charts for north‐west Africa. It also seems that the intensive low‐level tropospheric baroclinicity, which is most pronounced along the north‐west African coast, plays an important role in the initiation and self‐maintenance of the surface lows. Finally, the key atmospheric circulation system that causes these baroclinic waves to form must be a large‐scale ridge, often of a blocking type, located in the North Atlantic Ocean or north‐west Europe.

The evolution of cyclonic disturbances and lee waves over a topography in a rapidly rotating stratified flow

The temporal evolution of the flow patterns about a shallow bottom topography in a deep, rapidly rotating, stratified flow is studied on the basis of an inviscid Boussinesq model. The initial boundary-value problem, linearized for a thin three-dimensional obstacle, is solved for an impulsively started flow. The indicial response obtained reveals a window for the horizontal wavenumber spectra of the obstacle geometry. Only the portion of spectra within this window, which is shut at the start and widens linearly with increasing time, contributes to the solution. Thus, only the relatively large-scale, cyclonic feature associated with the wavenumber origin can dominate the flow in the early period, while the more familiar inertial wave system emerges much later. Examples of solutions computed via an FFT algorithm confirm that, except in the two opposite limits for the zero and infinite stratification, the cyclonic disturbance and inertial waves coexist, but a solitary pressure hill associated with the cyclonic disturbance remains dominant throughout most evolution stages. For a sufficiently strong stratification, the solution to the linear pressure equation suggests the emergence of a secondary eddy in the lee; its significance and validity are discussed.

Airflow patterns over and around a large three-dimensional hill

Surface wind patterns and air flows within the planetary boundary layer over a large three-dimensional hill of moderate slope are grouped according to Froude number classes. An evolution of flow patterns is shown to occur as the Froude number increases. Separation of the surface flow begins at the base of the lee side of the mountain near the centerline, moving upward on the lee slope as the Froude number increases. Recirculating eddies follow the separation of the lee flow. Eventually the separation line moves forward to the windward side as the Froude number becomes very large. The recirculating eddy becomes unsteady, with indication of an intermittent counterrolating eddy near the lee surface in neutral flow. The lee-side turbulence is enhanced with respect to the windward side due to the large eddies in high Froude number regimes. The concept of a critical height for the approach flow is generally supported. The integral form of the Froude number does not appear to be superior to a bulk Froude calculation in representing a particular airflow pattern.

Fluctuations of monsoon activity

In order to study the evolution of flow patterns associated with 'strong' and 'break' monsoon we have studied six cases each of such epochs during the period 1965 to 1973. As we are interested in the departure of the flow patterns from the normal, we have subtracted long term climatological pentad means from the daily data and constructed anomaly wind, height and thickness charts at standard levels.
 The several contrasting features during the epochs as well as prior to the epochs. A few days prior to the beginning of the strong monsoon epoch (usually in association with a depression), an eastwest anomaly trough is seen extending from SE Asia to peninsular India in the lower and, middle tropospheres. The trough over peninsular India shifts northwards with the advance of the epoch. Westward movement of a cyclonic anomaly from across Southeast Asia IS also seen. In the upper troposphere a warm anomaly ridge isseen to the northwest of India.
 Prior to the beginning of breaks, however, an anomaly ridge is seen extending from peninsular India to Malaysia in the lower and middle tropospheres. This feature also shows some northward shift with the advance of the epoch. In the upper troposphere, an anomaly trough is seen to the northwest of India.

円柱の後面に生ずるKármtán渦列の数値解

Two-dimensional non-steady viscous flow around a circular cylinder is investigated by solving the exact two-dimensional Navier-Stokes equations numerically for Reynolds numbers (Re) of 40 and 100. When Re is 40, a steady state solution can exist. On the other hand, when Re is 100, flow pattern does not become steady and the Karman vortex street appears.In the computations performed, these features are successfully realized, and the evolution of flow pattern of Kármán vortex is obtained.The quantity ∨‘⋅∇∨’ is also calculated for the purpose of studying the mechanism of dissipation of the vortex in the downstream side of the obstacle. The flow patterns as well as the drag coefficients obtained are in good agreement with the experimental data in both cases. In solving the problem using a finite domain, physical and computational boundary conditions should be posed properly. The method used was proved to be fairly satisfactory, if the domain is wide enough to contain the wake occuring behind the cylinder.

A Medium-Range Forecasting Aid - Synoptic Evolution of 500-millihar Flow Patterns, C. V. Smith, M.A., B.Sc. 68 pages, 26 figs. (comprising 99 charts and 1 diagram), paper 9·5 × 6 in. (Meteorological Report No. 21.) H.M.S.O., 1959, 4s.

A Medium-Range Forecasting Aid - Synoptic Evolution of 500-millihar Flow Patterns, C. V. Smith, M.A., B.Sc. 68 pages, 26 figs. (comprising 99 charts and 1 diagram), paper 9·5 × 6 in. (Meteorological Report No. 21.) H.M.S.O., 1959, 4s. - Volume 13 Issue 4

IMPROVED TERRAIN EFFECTS IN BAROTROPIC FORECASTS

The persistent occurrence of systematic errors of barotropic forecasts over mountainous areas is strongly suggestive of significant effects of mountains and friction which have not been included in the previously used forecast models. This study reports on experiments with a new barotropic forecast model which contains an improved mountain effect and a surface friction effect. For computation of the surface stress depending on the wind and on the terrain, a hemispheric map of the drag coefficient is obtained. The results of tests of the forecast model on an initially zonal flow, and on ten observed meteorological situations, indicate that the effects of terrain on the evolution of atmospheric flow patterns can be of large magnitude, and can account for significant errors in numerical prediction. Some success at accounting for these effects is attained with the barotropic representation of the atmosphere.

Characteristics of small dry cells

Natural convection in air-filled, 2-D rectangular enclosures heated from below and cooled from above is studied numerically under the assumption of adiabatic sidewalls. A computational model based on the SIMPLE-C algorithm is used for solving the mass, momentum, and energy transfer governing equations. Simulations are performed for different values of the height-to-width aspect ratio of the enclosure in the range 2⩽A⩽6, by progressively increasing and successively decreasing the Rayleigh number in the range 103⩽Ra⩽2×106. After the departure from motionless conduction takes place, the following flow-pattern evolution is detected: one-cell steady → two-cell steady → two-cell periodic → one-to-three-cell periodic → three-cell periodic. At each bifurcation, either abrupt or smooth changes in the Nusselt number are found to occur, according to whether the flow-transition is either sudden or more gradual. Hysteresis phenomena occurrence is documented. The effects of tilting the enclosure upon the stability of the different flow structures are also analysed.