Stable Flow Regimes Research Articles

Temporal variations in bedload transport rates: The effect of progressive bed armouring

AbstractThe quantitative bedload transport data that are presently available confirm that the generalized bedload transport rate‐stream power relationship is applicable to natural streams. However, the bedload transport rate is not solely dependent upon hydraulic parameters, but also upon the inter‐relationship between bed material characteristics and flow properties. Segregation of the surficial bed material, as expressed through the development of an armour coat, limits the availability of transportable material. Under such circumstances observed bedload transport rates are less than the predicted values.The effect which the development of an armoured surface has upon the bedload transport rate is described with reference to bedload and bed material sampling in the Borgne d'Arolla, Valais, Switzerland. The data refer to two periods when the resumption of baseflow conditions following flood events which were of a sufficient magnitude to transport all but the coarsest (0·3–0·5 m) particles on the streambed, provided the opportunity for the bed to adjust to a comparatively stable flow regime. Observed and predicted bedload transport rate‐stream power relationships are compared. The theoretical relationship does not adequately describe conditions in some gravel‐bed channels, since it fails to take into account the effect which armouring may have upon the supply of transportable material.

Transverse subsonic flow past a cylinder

At around the critical Reynolds number Re = (1.5–4.0)·105 there is an abrupt change in the pattern of transverse subsonic flow past a circular cylinder, and the drag coefficient Cx decreases sharply [1]. A large body of both experimental and computational investigations has now been made into subsonic flow past a cylinder [1–4]. A significant contribution to a deeper understanding of the phenomenon was made by [4], which gives a physical interpretation of a number of theoretical and experimental results obtained in a wide range of Re. Nevertheless, the complicated nonstationary nature of flow past a cylinder with separation and the occurrence of three-dimensional flows when two-dimensional flow is simulated in wind tunnels do not permit one to regard the problem as fully studied. The aim of the present work was to make additional experimental investigations into transverse subsonic flow past a cylinder and, in particular, to study the possible asymmetric stable flow regimes near the critical Reynolds number.

Migration of smolts of Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta L.) in a chalkstream

Migration of smolts in the River Piddle, Dorset, was studied over three years in relation to factors that could influence downstream movement. The river originates mostly from groundwater springs, resulting in stable flows and low turbidity except in very rainy weither. Fish were intercepted at the tidal limit in a fixed trap-net, and measurements of water temperature, discharge, turbidity, barometric pressure, rainfall and solar radiation taken nearby. Slightly increased turbidity and discharge following heavy rain initiated major movements during two nights of the total of 55 days studied. At other times large-scale movements rock place during sunny warm afternoons. Both solar radiation and water temperature were correlated with intensity and timing of movement. The pattern of migration is different from that reported on other rivers, reflecting the relatively stable flow regime of the chalkstream.

Heat Disposal in Water Environment

The need for continuing development of techniques for predicting temperature distributions due to waste heat discharges into lakes, rivers, estuaries, and the oceans is presented. Diffusion of buoyant jets is examined, including heated surface jets and multiple jets issuing from a submerged multiport diffuser. In the near-field analysis of surface jets the important problems are related to the lateral spreading caused by buoyancy. Comparison of theoretical predictions with laboratory and field observations are given. The mechanics of multiport diffusers for heated discharges in shallow receiving waters are explained in contrast to sewage diffusers. The important problem is the degree to which stratification can be maintained in order to minimize local reintrainment and reduction of dilution capacity. Criteria for stable and unstable flow regimes are provided. A mathematical model for temperature distribution, with or without waste heat addition, in unsteady flows under time-varying meteorological conditions is given.

Unstable Two-Fluid Flow in a Porous Medium

Abstract This paper reports an investigation of unstable fingering in two-fluid flow in a porous medium to determine if lambda the dimensionless finger width, is unique For a viscous finger A is the ratio of finger width to the distance between the tips of the two trailing fingers adjacent to the leading finger. For a gravity finger lambda is defined as the ratio of finger width, to "height" of the medium perpendicular to hulk flow. This work confirms previous experiments and existing theory that for viscous fingering lambda approaches a value of 0.5 with increasing ratio of viscous to interfacial force. However, for a given fluid pair and given, medium, this ratio can he increased only by increasing the, velocity. Experiments on gas liquid systems show that the asymptotic value of lambda with velocity is not always 0.5. Apparently, for gas-liquid systems, the influence of the interfacial force cannot always he eliminated by increasing the velocity. For such systems lambda is a function of fluid pair and media permeability. If the gravity force normal to the hulk permeability. If the gravity force normal to the hulk flow is active, it damps out the viscous fingers except for an underlying or overlying finger. The dimensionless width of this gravity finger strongly depends on velocity and height of the medium, as well as the fluid and media properties. The existing experiments and theories are reviewed and the gravity, stable, and viscous flow regimes are described in view of these experiments and theories. The existence of a gravity-dominated unstable regime, a gravity-viscous balanced stable regime, and a viscous-anminated regime was demonstrated experimentally by increasing flow velocity bin a rectangular glass head model. Asymptotic values of the dimensionless finger width were determined in various-sized Hele-Shaw models with gravity perpendicular and parallel to flow. The dimensionless perpendicular and parallel to flow. The dimensionless finger width lambda was determined as a function of applied force, flow resistance, and fluid properties. The results are interpreted dimensionally. Some comments are made concerning possible scaling and meaningful extensions of theory to describe these regimes in three-dimensional flow. Previous description of unstable two-fluid flow in porous media is mainly restricted to studies of viscous-dominated instability. The direction of this study is to provide data and understanding to consider the more realistic problem of predicting flow in three dimensions that may result in instabilities that are combinations of all, four flow regimes. Introduction The unstable flow of two fluids is characterized by interface changes between the fluids as a result of changes in relative forces. In a given porous medium and for a given fluid pair the gravity force dominates flow at low displacement velocities. As the velocity increases the viscous forces begin to affect flow significantly, and eventually there is a balance between effects of the gravity and viscous forces. As velocity increases further, the viscous force dominates flow. In the plane parallel to gravity, four flow regimes result as the velocity is increased: a gravity-induced stable flow regime; a gravity-dominated unstable flow regime; a stable regime resulting from a balance between gravity and viscous forces; and a viscous-induced unstable flow regime. The gravity-induced stable regime is represented schematically in Fig. 1a. This general flow pattern persists with the displacing fluid contacting all of persists with the displacing fluid contacting all of the in-place fluid until the interface becomes parallel to the bulk flow. At this velocity a gravity finger forms, and the interface, is unstable in that the length of the gravity finger grows and the fluid behind the nose of the finger is practically nonmobile because of the small pressure gradient along the finger. The gravity-dominated unstable flow is shown schematically in Fig. 1b. As the injection rate is increased, the gravity finger thickens, perhaps until it spans the medium creating a stable interface where all of the in-place, fluid is again mobile. This regime would, not occur in the absence of gravity. It occurs due to the counter effects of the gravity and viscous forces (Fig. 1c). As the velocity of the displacing fluid increases, the viscous forces dominate, and, the interface breaks into viscous fingers (Fig. 1d). SPEJ p. 293

Free-Convection Heat Transfer in the Inclined Open Thermosyphon

This paper describes a fundamental experimental investigation of heat transfer by free convection in a static heated tube, sealed at its lower end, and arranged so that the inclination of the tube to the vertical can be varied. Heated fluid adjacent to the tube wall forms an annulus which is discharged from the open end into a suitably cooled large reservoir, while a central core of cool fluid is continuously drawn into the tube by way of replacement. The case of the vertical tube under both laminar and turbulent flow conditions has been previously studied (Martin (1)†) and the present work continues the investigation of the system. Inclination of the tube sets up a secondary acceleration normal to its principal axis. The pressure gradients which are thereby induced should assist the circulation of fluid from the cold to the hot stream, thus reducing the thickness of the annulus and increasing the area of the core. The increased effectiveness of the system might then be expected to improve the heat-transfer rate. This is found to be true in laminar flow (of the boundary layer type) except for small tilting angles. The weak secondary pressure gradients which then occur are believed to be responsible for mingling of the fluid particles on the common boundary, and this reduces the heat transfer. The same general trends are apparent even when there is some degree of turbulence in the system. The stable turbulent regime is not of the boundary-layer type, because of the tendency for the cool entering fluid also to become turbulent, but what is called fully mixed flow, where the two turbulent streams mix. This causes reduced circulation and heat transfer. Under these conditions sufficient tilting of the tube eventually brings about the elimination of the mixing effect and a more efficient transfer of fluid particles between the two streams. The greatly increased heat transfer appears to be consistent with a stable turbulent boundarylayer flow regime, which, if attained, would improve the effectiveness of the open thermosyphon as a cooling device for both nuclear reactors and gas-turbine rotor blades.