Control Structures in Stratified Flows

Prepared for Engineering Laboratory Division of Water Control Planning Tennessee Valley Authority Norris, Tennessee.During the past decade the Tennessee Valley Authority has designed and built several structures for the purpose of withdrawing cold bottom water from thermally stratified reservoirs. The cold water is used to supply condenser water for steam-generated power plants. During the summer months the primary flows in the Tennessee River system are controlled by releases from upstream storage dams through low level turbine intakes. The cold water, discharged by the turbines, forms a density underflow in the downstream river and reservoirs which may be from 10 to 150F colder than the overlying surface water (Ref. 1, 2, 3). The intake structures in the form of submerged sluice gates are known as "skimmer walls". The water in the condenser water channel downstream of the gate is homogeneous and has a specific gravity equal to the lower, colder water in the intake channel upstream of the gate. The colder water is caused to flow through a vertical opening at the bottom of the gate by virtue of a head differential across the wall. The problem is to determine the maximum discharge of the colder water through the gate without inducing a steady state withdrawal from the warmer layer upstream of the gate. A basic experimental and analytical investigation of this problem was conducted in the Hydrodynamics Laboratory of the Department of Civil Engineering at the request of TVA in the spring and summer of 1954 (Ref. 4) as part of a continuing program of research in stratified flow (Ref. 5, 6). The flow configuration is shown schematically in Figure 1. While the information obtained from this study has proved to be a valid basis for design of skimmer walls of the type shown in Figure 1, questions have been raised in regard to the relative efficiency of other possible geometrical configurations. The proposed Bull Run steam power plant of the TVA is to be located at mile 48 on the Clinch River in the backwater of the Melton Hill Dam (under construction). After completion of Melton Hill, the normal depth of water in the Clinch River at this point will be approximately 20 feet. The Bull Run condenser water intake will be approximately 32 miles downstream from Norris Dam which is the source of cold water during the summer period of thermal stratification in Melton Hill reservoir. In the absence of the Bull Run plant the normal "plunge point" for the cold water in the reservoir would probably be in the vicinity of the Bull Run site. It is estimated that condenser water requirements will cause diversion of most of the river flow for sizeable periods of time. The heated water is to be returned to the river approximately one mile below the intake structure. This addition of heat will result in a reinforcement of the reservoir stratification and will probably move the cold water "plunge point" upstream. Due to the topography, the maximum length of an intake structure is approximately 400 feet. In the horizontal intake skimmer wall the lip of the skimmer wall is essentially at the elevation of the river bed. This configuration requires the excavation of a bottom step of height (b) in order for the fluid to pass through the horizontal opening (a) and flow under the gate into the condenser water channel. In order to have an accurate comparison for the two skimmer wall configurations, the experiments on the two types of walls were conducted using the same quantitative basis for the determination of the discharge at incipient drawdown. The drawdown discharge criterion for the 1954 tests was essentially a visual one, hence.,the tests on the type I structure were repeated. In addition, it was desired to obtain quantitative information on the magnitude of the energy loss across the skimmer wall. The experiments were conducted in the M. I. T. Hydrodynamics Laboratory using salt water and fresh water to simulate the prototype density differences due to thermal effects. For laboratory purposes and reproducibility of results a sharp interface between the two layers is obtained. It is recognized that in the field such a sharp interface is not possible; however, equivalent interfacial heights may be determined by using the depth at which the vertical density gradient is a maximum

The Diffusion of Two Fluids of Different Density in a Homogeneous Turbulent Field

Public Health Service National Institutes of Health Department of Health, Education and Welfare Research grant No. 4815Introduction: Industrial communities situated near large bodies of water or in drainage systems connected with such bodies dispose their waste water after treatment by dilution. Disposal by irrigation or evaporation after removing the solids by filtering, drying and incineration is justified in circumstances where the necessity outweighs the increased cost. However, comparatively few large industrial communities in the United States are situated away from either ocean, estuary, lake or river, so that the predominant form of ultimate waste water disposal is by dilution. In waste disposal by dilution a certain degree of primary treatment is usually required to reduce the concentrations of constituents that are toxic, odoriferous or otherwise chemically or physically detrimental or objectionable to human, animal or vegetable existence. Industrial and other wastes, varying widely in composition, coming from a diversity of establishments, such as dye or fertilizer factories, paper mills; primary treated sewage and supernatant liquor from digested sewage, radioactive waste products; wastes from hospitals, dairies, slaughter houses, etc., present different treatment problems and different standards for their effective disposal. After a sufficient time interval has elapsed following disposal, harmful chemicals will be oxidized to well below allowable levels, organic material digested by bacterial action, low level radioactive waste products; subjected to decay and a natural balance will be obtained. This can, however, be achieved only if the dilution process is aided by dispersion with currents due to winds and tidal action. Conversely, inadequate primary treatment or initial dilution can lead to widespread contamination by dispersion of harmful constituents endangering life. of property. The disposal of the water-borne waste products should be made in such, a fashion and at such regions in the body of water that tendencies for segregation of the influent will be minimized. The nature of the solution of this problem is twofold: (1) the achievement of optimum mixing characteristics with economical energy input at the disposal point, (2) the location of the disposal area in a region where hydrographic or oceanographic evidence indicates degrees of boundary shear, of wave and wind generated turbulence, and thermal or tidal convection currents that will continue the dispersion of the diluted effluent in order that concentrates would not tend to accumulate with passage of time or segregate into tidal backwaters or be absorbed by vegetation or soil on the shores. Allied problems, which have in most cases direct bearing on the flushing of disposal areas, are the salt water intrusion into river mouths and the fresh and salt water balance in tidal estuaries. Apart from the estuary flushing,there is also the consideration of contamination of public or industrial water supply intakes due to salinity intrusion. All of the problems mentioned above, in general terms, involve the mechanics of mass transfer according to the combined operation of turbulent diffusion and convection. Turbulent diffusion processes thus fall into two general categories. In the first, the turbulent diffusion is due entirely to the momentum of the diffusion which is being introduced into a quiescent diffusing medium: this process being governed by the mechanics of momentum and mass transfer in submerged turbulent jets. In the second category, the turbulent diffusion is due largely to the turbulent energy of the receiving fluid, the diffusion being introduced without materially increasing the turbulent activity at the region of introduction. In practice the ideal dilution process would be a combination of the two processes in the above sequence. The diffusing substance would be discharged with as high a momentum as practical into the receiving medium in the form of submerged jets, and the diffusion process in the vicinity of the disposal points would be entirely governed by the energy of introduction of the diffusion. At sufficiently large distances from its source the momentum of a jet would have decayed to levels comparable to the turbulence level in the receiving body of fluid. Further dispersion will occur according to mechanics of diffusion due to the turbulence in the receiving fluid body itself. (That is, if one considers momentarily turbulence as including all sizes of eddies present and hence also what would be customarily considered convection currents). The analysis will be simplified, however if it is considered that the motion of the fluid body consists of a field of homogeneous turbulence in which a convection pattern may be superimposed. With further simplification, the general problem may be made feasible for mathematical and experimental analysis in particular cases. Thus all of the above-enumerated disposal problems involve ultimately the mechanism of turbulent (eddy) diffusion which can accordingly be treated in two distinct parts

Salinity Effects on Velocity Distributions in an Idealized Estuary

Prepared under Research Grant , Public Health Service, National Institute of Health, Department of Health, Education and Welfare, no. 4815An experimental and analytical investigation of the two-dimensional (i.e. longitudinal and vertical) convective-diffusion processes in an idealized laboratory estuary. The estuary is idealized to obtain steady-state salinity and velocity distributions. The mixing effect of tidal motion is produced by homogeneous turbulence generated by vertically oscillating screens. A fresh water inflow at one end and the introduction of saline water near the other end produces an estuarine salinity intrusion condition. The cyclic convection motion of the tide is not reproduced. The investigation was conducted in three phases which are outlined below. (1) The need for an accurate determination of the salt concentration distribution led to the development of a highly sensitive conductivity probe suitable for measurement in situ. (2) In order to predict the vertical diffusion coefficient from a knowledge of the turbulence intensity, a vertical column was built, and the diffusion coefficient was determined from measurements of the one-dimensional, time dependent, non-convective diffusion process. The horizontal diffusion coefficient is known from previous studies. (3) The horizontal and vertical distribution of salinity was measured in a long, horizontal flume. With an assumption as to the distribution of vertical velocity component, the longitudinal velocity profile was determined. The local value of the bottom velocity was found to correlate with the local, densimetric Frounde Number. The discussion of the experimental investigation concludes with some speculations on the engineering applications of the results

Model Study of a Flood-control Pumping Station at the Charles River Dam

Prepared under contract with the Metropolitan District Commission of the Commonwealth of Massachusetts.This report describes the design, construction and testing of a Froudian model of a proposed 8400-cfs capacity pumping station, Site restrictions require that the flow approach the high specific-speed pumps asymmetrically from an existing ship lock through the Charles River Dam (Boston, Massachusetts). The model included a portion of the Charles River Basin, the existing navigation lock and the pump forebay at the exit of the lock. A single recirculating pump and a suction manifold was used in the model to withdraw water from the forebay through six intakes simulating the prototype pumping station. Flow patterns were obtained by photographs of floating confetti and subsurface streamers. Water surface measurements were made with a point gage read through a surveyor's level. The majority of tests were run with the maximum design discharge and the minimum basin pumping elevation. This provided the most severe forebay conditions of high velocity and low intake submergence. Tests were made to investigate: (1) the improvement of flow conditions at the entrance to the lock; (2) the performance of a single intake in uniform approach flow; and (3) the performance of several forebay and pumping station arrangements. The tests showed that: (1) an 18 ft diameter semi-cylindrical pier was needed at the lock entrance to reduce flow contraction and entrance loss; (2) the intake performed very well when the approach flow was uniform; and (3) the most satisfactory forebay arrangement, within the design restrictions imposed by the site, was with equal lengths of intake chambers. The center line of the pumps and the straight portion of the intake chamber walls were deflected 200 toward the approach flow. The. straight portion of the intake chamber walls were 51 ft in length and thence curved upstream in a circular arc. The circular arc terminated six ft from the lock line and the chord of the are forced an angle of 400 with the line of the lock. Vertical struts placed behind the intakes retarded circulation in the intake chambers and improved the flow into the intakes