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NASA NTRS1993

Critical comparison of second-order closures with direct numerical simulations of homogeneous turbulence

Recently, several models have been proposed for closing the second-moment equations, in which the velocity-pressure gradient tensor and the dissipation rate tensor are two of the most important terms. In the literature, these correlation tensors are usually decomposed into a so-called rapid term and a return-to-isotropy term. Models of these terms have been used in global flow calculations together with other modeled terms. However, their individual behaviors in different flows have not been fully examined because they are unmeasurable in the laboratory. Recently, the development of direct numerical simulation (DNS) of turbulence has given us the possibility to do this kind of study. With direct numerical simulation, we may use the solution to calculate exactly the values of these correlation terms and then directly compare them with the values from their modeled formulations. In this paper, we make direct comparisons of five representative rapid models and eight return-to-isotropy models using the DNS data of 45 homogeneous flows, which were done by Rogers et al. (1986) and Lee and Reynolds (1985). The purpose of these direct comparisons is to explore the performance of these models in different flows and identify the ones that give the best performance. The paper also describes the modeling procedure, model constraints, and the various evaluated models. The detailed results of the direct comparisons are discussed, and a few concluding remarks on turbulence models are given.

Shih, Tsan-Hsing↗

NASA NTRS1993

The sunspot transition region - Where are the bright plumes and the downflows?

Measurements of umbral-to-quiet sun and umbral-to-plage contrast in five active regions have been obtained in the transition region emission lines Si IV 1402.77 A, C IV 1548.19 A, and O V 1371.29 A, using the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission. The umbral transition region in these lines appears generally indistinguishable from the quiet transition region. In addition, high-resolution profiles of the C IV lines 1548.19 A, 1550.77 A in the umbrae of eight individual sunspots in different active regions show only weak, mostly subsonic, redshifted components. This result differs sharply from the observations of multiple, strong, often supersonic downflows observed with the HRTS instrument (e.g., Brekke et al., 1987).

Gurman, Joseph B.↗

NASA NTRS1923

Speed measurements made by Division "A" of the airplane director

The various speeds of an airplane can only be measured in horizontal flight, since there are no means for measuring the angle of ascent or descent. The measurements must be corrected for the density of the air. This is obtained by simultaneous pressure and temperature measurements during flight. Calculation from the mean yearly values in accordance with Everling's suggestion can only be considered an approximation, since the distribution of pressure and temperature in the individual strata at different altitudes undergoes such large variations that the yearly mean gives inaccurate values. Thermographs of the present form are useless for temperature measurements of an airplane. In altitude data, the following are to be distinguished: the height above the earth, the barometric altitude, and the altitude corresponding to the yearly mean air density. Variometers are not suited for the mechanical control of high altitude flight.

Heidelberg, V↗

NASA NTRS1994

Northern hemisphere total ozone values from 1989-1993 determined with the NOAA-11 Solar Backscatter Ultraviolet (SBUV/2) instrument

Determinations of global total ozone amounts have been made from recently reprocessed measurements with the SBUV/2 on the NOAA-11 environmental satellite since January 1989. This data set employs a new algorithm and an updated calibration. Comparisons with total ozone amounts derived from a significant subset of the global network of Dobson spectrophotometers shows a 0.3% bias between the satellite and ground measurements for the period January 1989-May 1993. Comparisons with the data from individual stations exhibit differing degrees of agreement which could be due to the matchup procedures and also to the uncertainties in the Dobson data. The SBUV/2 data set discussed here traces the Northern Hemisphere total ozone from 1989 to the present, showing a marked decrease from the average of those years starting in the summer of 1992 and continuing into 1993, with an apparent returning to more normal levels in late 1993.

Planet, W. G.↗

NASA NTRS1996

Effects of Shrouded Stator Cavity Flows on Multistage Axial Compressor Aerodynamic Performance

Experiments were performed on a low-speed multistage axial-flow compressor to assess the effects of shrouded stator cavity flows on aerodynamic performance. Five configurations, which involved changes in seal-tooth leakage rates and/or elimination of the shrouded stator cavities, were tested. Data collected enabled differences in overall individual stage and the third stage blade element performance parameters to be compared. The results show conclusively that seal-tooth leakage ran have a large impact on compressor aerodynamic performance while the presence of the shrouded stator cavities alone seemed to have little influence. Overall performance data revealed that for every 1% increase in the seal-tooth clearance to blade-height ratio the pressure rise dropped up to 3% while efficiency was reduced by 1 to 1.5 points. These observed efficiency penalty slopes are comparable to those commonly reported for rotor and cantilevered stator tip clearance variations. Therefore, it appears that in order to correctly predict overall performance it is equally important to account for the effects of seal-tooth leakage as it is to include the influence of tip clearance flows. Third stage blade element performance data suggested that the performance degradation observed when leakage was increased was brought about in two distinct ways. First, increasing seal-tooth leakage directly spoiled the near hub performance of the stator row in which leakage occurred. Second, the altered stator exit now conditions caused by increased leakage impaired the performance of the next downstream stage by decreasing the work input of the downstream rotor and increasing total pressure loss of the downstream stator. These trends caused downstream stages to progressively perform worse. Other measurements were acquired to determine spatial and temporal flow field variations within the up-and-downstream shrouded stator cavities. Flow within the cavities involved low momentum fluid traveling primarily in the circumferential direction at about 40% of the hub wheel speed. Measurements indicated that the flow within both cavities was much more complex than first envisioned. A vortical flow structure in the meridional plane, similar to a driven cavity, existed within the upstream cavity Furthermore, other spatial and temporal variations in Row properties existed. the most prominent being caused by the upstream potential influence of the downstream blade. This influence caused the fluid within cavities near the leading edges of either stator blades in space or rotor blades in time to be driven radially inward relative to fluid near blade mid-pitch. This influence also produced large unsteady velocity fluctuations in the downstream cavity because of the passing of the downstream rotor blade.

Wellborn, Steven R.↗

NASA NTRS1997

The Effects of Liquid Cooling Garments on Post-Space Flight Orthostatic Intolerance

Post space flight orthostatic intolerance among Space Shuttle crew members following exposure to extended periods of microgravity has been of significant concern to the safety of the shuttle program. Following the Challenger accident, flight crews were required to wear launch and entry suits (LES). It was noted that overall, there appeared to be a higher degree of orthostatic intolerance among the post-Challenger crews (approaching 30%). It was hypothesized that the increased heat load incurred when wearing the LES, contributed to an increased degree of orthostatic intolerance, possibly mediated through increased peripheral vasodilatation triggered by the heat load. The use of liquid cooling garments (LCG) beneath the launch and entry suits was gradually implemented among flight crews in an attempt to decrease heat load, increase crew comfort, and hopefully improve orthostatic tolerance during reentry and landing. The hypothesis that the use of the LCG during reentry and landing would decrease the degree of orthostasis has not been previously tested. Operational stand-tests were performed pre and post flight to assess crewmember's cardiovascular system's ability to respond to gravitational stress. Stand test and debrief information were collected and databased for 27 space shuttle missions. 63 crewpersons wearing the LCG, and 70 crewpersons not wearing the LCG were entered into the database for analysis. Of 17 crewmembers who exhibited pre-syncopal symptoms at the R+O analysis, 15 were not wearing the LCG. This corresponds to a 21% rate of postflight orthostatic intolerance among those without the LCG, and a 3% rate for those wearing LCG. There were differences in these individual's average post-flight maximal systolic blood pressure, and lower minimal Systolic Blood pressures in those without LCG. Though other factors, such as type of fluid loading, and exercise have improved concurrently with LCG introduction, from this data analysis, it appears that LCG usage provided a significant degree of protection from post-flight orthostatic intolerance.

Billica, Roger↗

NASA NTRS1997

Alaskan Auroral All-Sky Images on the World Wide Web

In response to a 1995 NASA SPDS announcement of support for preservation and distribution of important data sets online, the Geophysical Institute, University of Alaska Fairbanks, Alaska, proposed to provide World Wide Web access to the Poker Flat Auroral All-sky Camera images in real time. The Poker auroral all-sky camera is located in the Davis Science Operation Center at Poker Flat Rocket Range about 30 miles north-east of Fairbanks, Alaska, and is connected, through a microwave link, with the Geophysical Institute where we maintain the data base linked to the Web. To protect the low light-level all-sky TV camera from damage due to excessive light, we only operate during the winter season when the moon is down. The camera and data acquisition is now fully computer controlled. Digital images are transmitted each minute to the Web linked data base where the data are available in a number of different presentations: (1) Individual JPEG compressed images (1 minute resolution); (2) Time lapse MPEG movie of the stored images; and (3) A meridional plot of the entire night activity.

Stenbaek-Nielsen, H. C.↗

NASA NTRS1999

The Typical General Aviation Aircraft

The reliability of General Aviation aircraft is unknown. In order to "assist the development of future GA reliability and safety requirements", a reliability study needs to be performed. Before any studies on General Aviation aircraft reliability begins, a definition of a typical aircraft that encompasses most of the general aviation characteristics needs to be defined. In this report, not only is the typical general aviation aircraft defined for the purpose of the follow-on reliability study, but it is also separated, or "sifted" into several different categories where individual analysis can be performed on the reasonably independent systems. In this study, the typical General Aviation aircraft is a four-place, single engine piston, all aluminum fixed-wing certified aircraft with a fixed tricycle landing gear and a cable operated flight control system. The system breakdown of a GA aircraft "sifts" the aircraft systems and components into five categories: Powerplant, Airframe, Aircraft Control Systems, Cockpit Instrumentation Systems, and the Electrical Systems. This breakdown was performed along the lines of a failure of the system. Any component that caused a system to fail was considered a part of that system.

Turnbull, Andrew↗

NASA NTRS1998

Magnetotail Flow Bursts: Association to Global Magnetospheric Circulation, Relationship to Ionospheric Activity and Direct Evidence for Localization

A series of bursty bulk flow events (BBFs) were observed by GEOTAIL and WIND in the geomagnetotail. IMP8 at the solar wind showed significant energy coupling into the magnetosphere, while the UVI instrument of POALR evidenced significant energy transfer to the ionosphere during two substorms. There was good correlation between BBFs and ionospheric activity observed by UVI even when ground magnetic signatures were absent, suggesting that low ionospheric conductivity at the active sector may be responsible for this observation. During the second substorm no significant flux transport was evidenced past WIND in stark contrast to GEOTAIL and despite the small intersatellite separation ((3.54, 2.88, -0.06) Re). Throughout the intervals studied there were significant differences in the individual flow bursts at the two satellites, even during longitudinally extended ionospheric activations. We conclude that the half-scale-size of transport bearing flow bursts is less than 3 Re.

Angelopoulos, V.↗

NASA NTRS2001

The Magnitude and Variability of Global and Regional Precipitation Based on the 22-Year GPCP (Global Precipitation Climatology Project) and Three-Year TRMM (Tropical Rainfall Measuring Mission) Data Sets

This paper gives an overview of the analysis of global precipitation over the last few decades and the impact of the new TRMM precipitation observations. The 20+ year, monthly, globally complete precipitation analysis of the World Climate Research Program's (WCRP/GEWEX) Global Precipitation Climatology Project (GPCP) is used to study global and regional variations and trends and is compared to the much shorter TRMM (Tropical Rainfall Measuring Mission) tropical data set. The GPCP data set shows no significant trend in precipitation over the twenty years, unlike the positive trend in global surface temperatures over the past century. The global trend analysis must be interpreted carefully, however, because the inhomogeneity of the data set makes detecting a small signal very difficult, especially over this relatively short period. Identifying regional trends in precipitation may be more practical. From 1979 to 1999 the northern mid-latitudes appear to be drying, the southern mid-latitudes have gotten wetter, and there is a mixed signal in the tropics. The relation between this field of trends and the relation to the frequency of El Nino events during this time period is explored. Monthly anomalies of precipitation are related to ENSO variations with clear signals extending into middle and high latitudes of both hemispheres. The El Nino and La Nina mean anomalies are near mirror images of each other and when combined produce an ENSO signal with significant spatial continuity over large distances. These El Nino minus La Nina composites of normalized precipitation show the usual positive, or wet, anomaly over the central and eastern Pacific Ocean with the negative, or dry, anomaly over the maritime continent along with an additional negative anomaly over Brazil and the Atlantic Ocean extending into Africa and a positive anomaly over the Horn of Africa and the western Indian Ocean. A number of the features are shown to extend into high latitudes. Positive anomalies extend in the Southern Hemisphere (S.H.) from the Pacific southeastward across Chile and Argentina into the south Atlantic Ocean. In the Northern Hemisphere (N.H.) the counterpart feature extends across the southern US and Atlantic Ocean into Europe. Further to the west a negative anomaly extends southeastward again from the Maritime Continent across the South Pacific and through the Drake Passage. The N.H. counterpart crosses the North Pacific and southern Canada. In the Southern Hemisphere an anomaly feature is shown to spiral into the Antarctica land mass. The extremes of ENSO-related anomalies are also examined and indicate that globally, during both El Nino and La Nina, more extremes of precipitation ( both wet and dry) occur than during the "neutral" regime, with the El Nino regime showing larger magnitudes. The distribution is different for the globe as a whole and when the area is restricted to just land. The recent (1998-present) TRMM observations are compared with the similar period of GPCP analyses with very good agreement in terms of pattern and generally good agreement with regard to magnitude. However, there still are differences among the individual TRMM products using passive and active microwave techniques and these need to be resolved before longer-term products such as the GPCP analyses can be validated.

Adler, Robert F.↗

NASA NTRS2001

Variations and Trends in Global and Regional Precipitation Based on the 22-Year GPCP (Global Precipitation Climatology Project) and Three-Year TRMM (Tropical Rainfall Measuring Mission) Data Sets

This paper gives an overview of the analysis of global precipitation over the last few decades and the impact of the new TRMM precipitation observations. The 20+ year, monthly, globally complete precipitation analysis of the World Climate Research Program's (WCRP/GEWEX) Global Precipitation Climatology Project (GPCP) is used to study global and regional variations and trends and is compared to the much shorter TRMM(Tropical Rainfall Measuring Mission) tropical data set. The GPCP data set shows no significant trend in global precipitation over the twenty years, unlike the positive trend in global surface temperatures over the past century. The global trend analysis must be interpreted carefully, however, because the inhomogeneity of the data set makes detecting a small signal very difficult, especially over this relatively short period. The relation of global (and tropical) total precipitation and ENSO events is quantified with no significant signal when land and ocean are combined. Identifying regional trends in precipitation may be more practical. From 1979 to 2000 the tropics have pattern of regional rainfall trends that has an ENSO-like pattern with features of both the El Nino and La Nina. This feature is related to a possible trend in the frequency of ENSO events (either El Nino or La Nina) over the past 20 years. Monthly anomalies of precipitation are related to ENSO variations with clear signals extending into middle and high latitudes of both hemispheres. The El Nino and La Nina mean anomalies are near mirror images of each other and when combined produce an ENSO signal with significant spatial continuity over large distances. A number of the features are shown to extend into high latitudes. Positive anomalies extend in the Southern Hemisphere (S.H.) from the Pacific southeastward across Chile and Argentina into the south Atlantic Ocean. In the Northern Hemisphere (N.H.) the counterpart feature extends across the southern U.S. and Atlantic Ocean into Europe. Further to the west a negative anomaly extends southeastward again from the Maritime Continent across the South Pacific and through the Drake Passage. In the Southern Hemisphere an anomaly feature is shown to spiral into the Antarctica land mass. The extremes of ENSO-related anomalies are also examined and indicate that globally, during both El Nino and La Nina, more extremes of precipitation (both wet and dry) occur than during the "neutral" regime, with the El Nino regime showing larger magnitudes. The distribution is different for the globe as a whole and when the area is restricted to just land. The recent (1998-present) TRMM observations are compared with the similar period of GPCP analyses with very good agreement in terms of pattern and generally good agreement with regard to magnitude. However, there still are differences among the individual TRMM products using passive and active microwave techniques and these need to be resolved before longer-term products such as the GPCP analyses can be validated.

Adler, Robert F.↗

NASA NTRS2004

Source Term Model for Vortex Generator Vanes in a Navier-Stokes Computer Code

A source term model for an array of vortex generators was implemented into a non-proprietary Navier-Stokes computer code, OVERFLOW. The source term models the side force created by a vortex generator vane. The model is obtained by introducing a side force to the momentum and energy equations that can adjust its strength automatically based on the local flow. The model was tested and calibrated by comparing data from numerical simulations and experiments of a single low profile vortex generator vane on a flat plate. In addition, the model was compared to experimental data of an S-duct with 22 co-rotating, low profile vortex generators. The source term model allowed a grid reduction of about seventy percent when compared with the numerical simulations performed on a fully gridded vortex generator on a flat plate without adversely affecting the development and capture of the vortex created. The source term model was able to predict the shape and size of the stream-wise vorticity and velocity contours very well when compared with both numerical simulations and experimental data. The peak vorticity and its location were also predicted very well when compared to numerical simulations and experimental data. The circulation predicted by the source term model matches the prediction of the numerical simulation. The source term model predicted the engine fan face distortion and total pressure recovery of the S-duct with 22 co-rotating vortex generators very well. The source term model allows a researcher to quickly investigate different locations of individual or a row of vortex generators. The researcher is able to conduct a preliminary investigation with minimal grid generation and computational time.

Waithe, Kenrick A.↗

NASA NTRS2004

Analysis of Europan Cycloid Morphology and Implications for Formation Mechanisms

Europa's highly fractured crust has been shown to contain features with a range of differing morphologies. Most lineaments on Europa are believed to have initiated as cracks, although the type of cracking (e.g. tensile vs. shear) remains unclear and may vary for different morphologies. Arcuate lineaments, called cycloids or flexi, have been observed in nearly all imaged regions of Europa and have been modeled as tensile fractures that were initiated in response to diurnal variations in tides. Despite this hypothesis about the formation mechanism, there have been no detailed analyses of the variable morphologies of cycloids. We have examined Galileo images of numerous locations on Europa to develop a catalog of the different morphologies of cycloids. This study focuses on variations in morphology along individual cycloid segments and differences in cusp styles between segments, while illustrating how morphologic evidence can help unravel formation mechanisms. In so doing, we present evidence for cycloid cusps forming due to secondary fracturing during strike-slip sliding on pre-existing cycloid segments.

Marshall, S. T.↗

NASA NTRS2003

Epstein-Barr virus DNA loads in adult human immunodeficiency virus type 1-infected patients receiving highly active antiretroviral therapy

Patients with human immunodeficiency virus type 1 (HIV-1) infection are at high risk of developing Epstein-Barr virus (EBV)-associated lymphoma. However, little is known of the EBV DNA loads in patients receiving highly active antiretroviral therapy (HAART). Using a real-time quantitative polymerase chain reaction assay, we demonstrated that significantly more HIV-1-infected patients receiving HAART than HIV-1-uninfected volunteers had detectable EBV DNA in blood (57 [81%] of 70 vs. 11 [16%] of 68 patients; P=.001) and saliva (55 [79%] of 68 vs. 37 [54%] of 68 patients; P=.002). The mean EBV loads in blood and saliva samples were also higher in HIV-1-infected patients than in HIV-1-uninfected volunteers (P=.001). The frequency of EBV detection in blood was associated with lower CD4+ cell counts (P=.03) among HIV-1-infected individuals, although no differences were observed in the EBV DNA loads in blood or saliva samples in the HIV-1-infected group. Additional studies are needed to determine whether EBV-specific CD4+ and CD8+ cells play a role in the pathogenesis of EBV in HIV-1-infected patients receiving HAART.

NASA Discipline Regulatory Physiology↗

NASA NTRS1998

Morphogenesis by symbiogenesis

Here we review cases where initiation of morphogenesis, including the differentiation of specialized cells and tissues, has clearly evolved due to cyclical symbiont integration. For reasons of space, our examples are drawn chiefly from the plant, fungal and bacterial kingdoms. Partners live in symbioses and show unique morphological specializations that result when they directly and cyclically interact. We include here brief citations to relevant literature where plant, bacterial or fungal partners alternate independent with entirely integrated living. The independent, or at least physically unassociated stages, are correlated with the appearance of distinctive morphologies that can be traced to the simultaneous presence and strong interaction of the plant with individuals that represent different taxa.

Review↗

NASA NTRS2005

Source Term Model for Steady Micro Jets in a Navier-Stokes Computer Code

A source term model for steady micro jets was implemented into a non-proprietary Navier-Stokes computer code, OVERFLOW. The source term models the mass flow and momentum created by a steady blowing micro jet. The model is obtained by adding the momentum and mass flow created by the jet to the Navier-Stokes equations. The model was tested by comparing with data from numerical simulations of a single, steady micro jet on a flat plate in two and three dimensions. The source term model predicted the velocity distribution well compared to the two-dimensional plate using a steady mass flow boundary condition, which was used to simulate a steady micro jet. The model was also compared to two three-dimensional flat plate cases using a steady mass flow boundary condition to simulate a steady micro jet. The three-dimensional comparison included a case with a grid generated to capture the circular shape of the jet and a case without a grid generated for the micro jet. The case without the jet grid mimics the application of the source term. The source term model compared well with both of the three-dimensional cases. Comparisons of velocity distribution were made before and after the jet and Mach and vorticity contours were examined. The source term model allows a researcher to quickly investigate different locations of individual or several steady micro jets. The researcher is able to conduct a preliminary investigation with minimal grid generation and computational time.

Waithe, Kenrick A.↗

NASA NTRS1995

B-ISBN Onboard Processing Fast Packet Switch Developed

Future satellite communications applications will require a packet-switched onboard satellite processing system to route packets at very high speeds from uplink beams to different downlink beams. The rapid emergence of point-to-multipoint services, and the important role of satellites in a national and global information infrastructure, makes the multicast function essential to a fast packet switch (FPS). NASA Lewis Research Center's Digital System Technology Branch has been studying possible architectures for high-speed onboard-processing satellite systems. As part of this research, COMSAT Laboratories developed a broadband integrated services digital network (B-ISDN) fast packet switch for Lewis that was delivered on December 1994. The fast packet switch consists of eight inputs and eight outputs that can receive and transmit data, respectively, at a rate of 155 Mbps. The switch features multiple priorities (three) and multiple-size (three) satellite virtual cells that are similar to ATM cells in length (52 bytes). In addition, the fast packet switch features a congestion-control algorithm that allows users to set different thresholds for individual destination ports, thus throttling back the traffic from the transmitting port.

Source record↗

NASA NTRS2006

RFID Transponders' Radio Frequency Emissions in Aircraft Communication and Navigation Radio Bands

Radiated emissions in aircraft communication and navigation bands are measured from several active radio frequency identification (RFID) tags. The individual tags are different in design and operations. They may also operate in different frequency bands. The process for measuring the emissions is discussed, and includes tag interrogation, reverberation chamber testing, and instrument settings selection. The measurement results are described and compared against aircraft emission limits. In addition, interference path loss for the cargo bays of passenger aircraft is measured. Cargo bay path loss is more appropriate for RFID tags than passenger cabin path loss. The path loss data are reported for several aircraft radio systems on a Boeing 747 and an Airbus A320.

Nguyen, Truong X.↗