Search

We show that very low values of the magnetic damping parameter can be achieved in sputter deposited polycrystalline films of Co2MnGe annealed at relatively low temperatures ranging from 240 degrees C to 400 degrees C. Damping values as low as 0.0014 are obtained with an intrinsic value of 0.0010 after spin-pumping contributions are considered. Of importance to most applications is the low value of inhomogeneous linewidth that yields measured linewidths of 1.8 and 5.1 mT at 10 and 40 GHz, respectively. The damping parameter monotonically decreases as the B2 order of the films increases. This trend is reproduced and explained by ab initio calculations of the electronic structure and damping parameter. Here, the damping parameter is calculated as the structure evolves from A2 to B2 to L2(1) orders. The largest decrease in the damping parameter occurs during the A2 to B2 transition as the half-metallic phase becomes established.

Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as damping strongly affects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferro-magnetic materials(1-3). This presents a challenge for a broad range of applications in spintronics(4) and spin-orbitronics that depend on materials and structures with ultra-low dampine(5,6). It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10(-4), which is comparable to values reported only for ferrimagnetic insulators(7,8). We explain this phenomenon by a unique feature of the band structure in this system: the density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms and a test of the theoretical predictions proposed by Mankovsky and colleagues(3).

SELECT is a numerical, one-dimensional model of selective withdrawal developed at the U.S. Army Engineer Research and Development Center. SELECT v1.0 Beta uses Microsoft Excel spreadsheet software to compute withdrawal characteristics and release water quality for various operational alternatives. The spreadsheet implementation of the SELECT model provides a highly interactive environment for the application of the model. This guide was developed to assist users of the SELECT v1.0 Beta spreadsheet model. The guide briefly discusses the concepts of selective withdrawal in reservoirs and describes how those concepts are implemented in the spreadsheet model. It then provides an overview of the execution of the spreadsheet model and includes guidance on preparing the required input data for the model. Examples of model input and model output are provided.

The purpose of this study was to provide MRD with the ability to predict the effects of reservoir operational changes on suitable coldwater fish habitat in Fort Peck Lake, Lake Sakakawea, Lake Oahe, and Lake Francis Case. Coldwater fish habitat is defined as water less than or equal to l50C and greater than or equal to 5 mgl-1 DO. The only system operational variables which can be changed are end-of-month stages and monthly average releases. Results of this study must therefore predict suitable coldwater fish habitat as a function of these two variables. The results will be incorporated into MRD's Long Range System model (LRS) and will be used to predict suitable coldwater fish habitat under various operational alternatives.

Without some correction for the interaction between lateral curvature and vertically nonuniform velocity, depth-averaged numerical models cannot predict the gradual migration of high velocity toward the outside of channel bends. Conventional depth-averaging does not account for the curvature-induced second- ary flow that gives rise to the migration. To remedy this deficiency, a secondary flow correction (SFC) has been developed and incorporated in the two- dimensional STREMR code. The secondary governing equation contains two-empirical coefficients, and these have been adjusted to make STREMR predictions agree with observed velocity profiles for a single 270-deg bend. Without further adjustment, the SFC yields comparable accuracy for multiple-bend predictions in two other channels with depth, curvature, and bottom friction different from the 270-deg case. This suggests that the SFC may be useful in general for bendways with small curvature, gentle side slope, and moderate bottom friction.

The US Army Engineer District, Seattle, is presently evaluating the impacts of proposed additional water storage at Howard A. Hanson Reservoir in Washington State. This proposed modification will involve raising the existing pool approximately 40 ft. The investigation reported herein examined the impacts of raising the conservation pool on the reservoir thermal profiles and release temperatures for several study years. The mathematical model used in the study examined impacts of raising the pool with and without structural modification to the existing outlet works. Optimization procedures were used with the mathematical model to provide optimum number of and elevations for the additional ports. Results indicated that significant improvement in release temperatures for the raised pool could be achieved with a multilevel outlet structure as compared to the existing outlet works. The method for calculating the reliability index is given in Appendix A. Appendix B lists monthly temperature release statistics for existing conditions, and Appendix C lists the calculated average release temperatures for the reservoir.

The US Army Engineer District, Savannah, is in the process of adding pumped/storage capability at the Richard B. Russell Project. The addition of four 75-MW pump/turbines will double the hydroelectric generation capacity while creating the capability to recharge the power pool in Russell Lake from the adjacent J. Strom Thurmond Lake. Completion of the Richard B. Russell powerhouse will result in a total capacity generation discharge of 60,000 cfs and pumping capacity of 24,800 cfs. This investigation examined the impacts of increasing the generation and pumpback flows of the project. Of particular concern was the interaction of flows with the downstream channel, which is constructed by sand deposits. The numerical model used in this study examined the velocity field and water-surface fluctuations associated with completed project flows in J. Strom Thurmond Lake. The results indicated a significant influence of the channel construction on both the velocity field and water surface at capacity flow conditions for low tailwater pools. During periods of low tailwater pool, the duration of capacity pumpback would be limited. A channel realignment was proposed to improve the hydraulic conditions during project operation. Keywords; Channelization; Dredging; Flow fields; Hydrodynamics; Reservoirs; Savannah river; Hydroelectric power plants; Pumped storage; South Carolina.

The SELECT program is a one-dimensional numerical model that predicts the vertical extent and distribution of withdrawal from a reservoir of known density and quality distribution for a given discharge from a specified location. Using this prediction for the withdrawal zone, SELECT computes the quality of the release for user-specified parameters (such as temperature, dissolved oxygen (DO), turbidity, iron) treated as conservative substances. This report was developed to aid users of the program SELECT. It provides guidance on how to construct an input file and how to interpret the output file. It also overviews the concepts behind the program and the internal methodology of its execution. Examples of input and output are also provided.

This report summarizes various in-lake, in-structure, and downstream techniques to enhance the dissolved oxygen concentration of hydropower releases. In-lake and in-structure techniques appear to be the most applicable for Corps of Engineers projects because of the large discharges of most hydropower projects. Of these, the in-structure techniques, particularly turbine venting, appear very attractive considering cost and degree of improvement. Tests were conducted at the Clarks Hill Dam powerhouse to evaluate various aspects of turbine venting and thereby provide a data base to develop predictive and design capabilities for turbine venting systems. Test results indicated that, at most, the oxygen deficit in the penstock could be reduced by about 30%, i.e., if the penstock oxygen deficit is 8.0 mg/(l), then, at most, about 2.4 mg/l of oxygen could be absorbed into the release flow. Two reaeration processes that contributed to the overall oxygen transfer were observed: (a) due to the turbulence in the tailrace area and (b) due to the air bubbles (vented through the turbine) as they traveled through the draft tube. A numerical computer model was developed that included the impact of the changes in thermodynamic (pressure) state as the bubbles passed through the draft tube. By developing the 'pressure time history', the increased potential for oxygen transfer due to the increase in hydrostatic pressure was included in model formulation. Good agreement was obtained in comparing model predictions and oxygen uptake data from previous tests at Clarks Hill.