Flat-plate drag measurements with vortex generators in turbulent boundary layer

Direct drag measurements were obtained on a flat plate with a spanwise row of vortex generators near the leading edge, to produce an array of stream wise vortices within the approaching turbulent boundary layer. The object was to explore the possibility of modifying the large scale structure of the boundary layer through embedded longitudinal vortices with a view to obtaining a reduction in wall shear. Both obstacle and vane type vortex generators were tested at free stream velocities 40 ft/sec to 130 ft/sec corresponding to plate length Reynolds no. 0.3 million to 0.8 million with a nominal boundary layer thickness of approximately 0.6 in. at the leading edge. A few vortex generator configurations were tested both on and off the plate to measure the total drag as well as the plate drag alone. The obstacle type devices reduced the plate drag, indicating that the wake momentum defect predominated even in the presence of streamwise vortices. The vane type vortex generators however always increased the plate drag

Subsonic balance and pressure investigation of a 60 deg delta wing with leading edge devices

Low supersonic wave drag makes the thin highly swept delta wing the logical choice for use on aircraft designed for supersonic cruise. However, the high-lift maneuver capability of the aircraft is limited by severe induced-drag penalties attributed to loss of potential flow leading-edge suction. This drag increase may be alleviated through leading-edge flow control to recover lost aerodynamic thrust through either retention of attached leading-edge flow to higher angles of attack or exploitation of the increased suction potential of separation-induced vortex flow. A low-speed wind-tunnel investigation was undertaken to examine the high-lift devices such as fences, chordwise slots, pylon vortex generators, leading-edge vortex flaps, and sharp leading-edge extensions. The devices were tested individually and in combinations in an attempt to improve high-alpha drag performance with a minimum of low-alpha drag penalty. This report presents an analysis of the force, moment, and static pressure data obtained in angles of attack up to 23 deg, at Mach and Reynolds numbers of 0.16 and 3.85 x 10 to the 6th power per meter, respectively. The results indicate that all the devices produced drag and longitudinal/lateral stability improvements at high lift with, in most cases, minor drag penalties at low angles of attack

Money and dynamic credit arrangements with private information

The authors construct a model with private information in which consumers write dynamic contracts with financial intermediaries.Money ; Credit

Money and Dynamic Credit Arrangements with Private Information

We construct a model with private information in which consumers write dynamic contracts with financial intermediaries. A role for money arises due to random limited participation of consumers in the financial market. Without defection constraints, a Friedman rule is optimal, the mean and variability of wealth tend to fall in the steady state, and the welfare effects of inflation are very small. With defection constraints, it is optimal to eliminate currency entirely, the variability of wealth tends to rise with inflation, and the welfare effects of inflation are large.97-19

Money, Credit, and Allocation Under Complete Dynamic Contracts and Incomplete Markets

We construct a dynamic heterogeneous-agent model with random uninsurable endowments. Two allocation mechanisms are considered, one with long-term complete credit arrangements under private information, and one with incomplete competitive markets. A role for money arises due to random limited participation. A Friedman rule is optimal in the first economy, and replicates a pure credit arrangement in the second. Computational results show that steady state allocations are quite different under the two arrangements, though the responses to changes in long-run inflation are similar.97-20

Characterization of erosion of metallic materials under cavitation attack in a mineral oil

Cavitation erosion and erosion rates of eight metallic materials representing three crystal structures were studied using a 20-kHz ultrasonic magnetostrictive oscillator in viscous mineral oil. The erosion rates of the metals with an fcc matrix were 10 to 100 times higher than that of an hcp-matrix titanium alloy. The erosion rates of iron and molybdenum, with bcc matrices, were higher than that of the titanium alloy but lower than those of the fcc metals. Scanning electron microscopy indicates that the cavitation pits are initially formed at the grain boundaries and precipitates and that the pits that formed at the triple points grew faster than the others. Transcrystalline craters formed by cavitation attack over the surface of grains and roughened the surfaces by multiple slip and twinning. Surface roughness measurements show that the pits that formed over the grain boundaries deepended faster than other pits. Computer analysis revealed that a geometric expression describes the nondimensional erosion curves during the time period 0.5 t(0) t 2.5 t(0), where t(0) is the incubation period. The fcc metals had very short incubation periods; the titanium alloy had the longest incubation period

The mechanism of erosion of metallic materials under cavitation attack

The mean depth of penetration rates (MDPRs) of eight polycrystalline metallic materials, Al 6061-T6, Cu, brass, phosphor bronze, Ni, Fe, Mo, and Ti-5Al-2.5Sn exposed to cavitation attack in a viscous mineral oil with a 20 kHz ultrasonic oscillator vibrating at 50 micron amplitude are reported. The titanium alloy followed by molybdenum have large incubation periods and small MDPRs. The incubation periods correlate linearly with the inverse of hardness and the average MDPRs correlate linearly with the inverse of tensile strength of materials. The linear relationships yield better statistical parameters than geometric and exponential relationships. The surface roughness and the ratio of pit depth to pit width (h/a) increase with the duration of cavitation attack. The ratio h/a varies from 0.1 to 0.8 for different materials. Recent investigations (20) using scanning electron microscopy to study deformation and pit formation features are briefly reviewed. Investigations with single crystals indicate that the geometry of pits and erosion are dependent on their orientation

Characterization of erosion of metallic materials under cavitation attack in a mineral oil

Cavitation erosion and erosion rates of eight metallic materials representing three crystal structures were studied. The erosion experiments were conducted with a 20-kHz ultrasonic magnetostrictive oscillator in a viscous mineral oil. The erosion rates of the metals with an fcc matrix were 10 to 100 times higher than that of an hop-matrix titanium alloy. The erosion rates of iron and molybdenum, with bcc matrices, were higher than that of the titanium alloy but lower than those of those of the fcc materials. Studies with scanning electron microscopy indicated that the cavitation pits were initially formed at the grain boundaries and precipitates and that the pits formed at the junction of grain boundaries grew faster than the others. Transcrystalline craters formed by cavitation attack over the surface of grains and roughened the surfaces by multiple slip and twinning. Surface roughness measurements showed that the pits that formed over the grain boundaries deepened faster than pits. Computer analysis revealed that a geometric expression describes the nondimensional erosion curves during the time period 0.5 t (sub 0) t 2.5 t (sub 0), where t (sub 0) is the incubation period. The fcc metals had very short incubation periods; the titanium alloy had the longest incubation period

Cavitation Erosion of Copper, Brass, Aluminum and Titanium Alloys in Mineral Oil

The variations of the mean depth of penetration, the mean depth rate of penetration, MDRP, the pit diameter 2a and depth h due to cavitation attack on Al 6061-T6, Cu, brass of composition Cu-35Zn-3Pb and Ti-5A1-2.5Sn are presented. The experiments are conducted in a mineral oil of viscosity 110 CS using a magnetostrictive oscillator of 20 kHz frequency. Based on MDRP on the materials, it is found that Ti-5Al-2.5Sn exhibits cavitation erosion resistance which is two orders of magnitude higher than the other three materials. The values of h/a are the largest for copper and decreased with brass, titanium, and aluminum. Scanning electron microscope studies show that extensive slip and cross slip occurred on the surface prior to pitting and erosion. Twinning is also observed on copper and brass