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  • 1
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    Dropwise Condensation:  Experiments and Simulations of Nucleation and Growth of Water Drops in a Cooling System (2006)
    American Chemical Society
    Details
    Publication Date: 2006-10-01
    Print ISSN: 0743-7463
    Electronic ISSN: 1520-5827
    Topics: Chemistry and Pharmacology
    Published by American Chemical Society
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    PAPER CURRENT
  • 2
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    Domain movements of the enhancer-dependent sigma factor drive DNA delivery into the RNA polymerase active site: insights from single molecule studies (2014)
    Oxford University Press
    Details
    Publication Date: 2014-05-01
    Description: Recognition of bacterial promoters is regulated by two distinct classes of sequence-specific sigma factors, 70 or 54 , that differ both in their primary sequence and in the requirement of the latter for activation via enhancer-bound upstream activators. The 54 version controls gene expression in response to stress, often mediating pathogenicity. Its activator proteins are members of the AAA+ superfamily and use adenosine triphosphate (ATP) hydrolysis to remodel initially auto-inhibited holoenzyme promoter complexes. We have mapped this remodeling using single-molecule fluorescence spectroscopy. Initial remodeling is nucleotide-independent and driven by binding both ssDNA during promoter melting and activator. However, DNA loading into the RNA polymerase active site depends on co-operative ATP hydrolysis by the activator. Although the coupled promoter recognition and melting steps may be conserved between 70 and 54 , the domain movements of the latter have evolved to require an activator ATPase.
    Print ISSN: 0305-1048
    Electronic ISSN: 1362-4962
    Topics: Biology
    Published by Oxford University Press
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  • 3
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    Aeolian processes aboard a Space Station: Saltation and particle trajectory analysis (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The Carousel Wind Tunnel (CWT) proposed to study aeolian processes aboard a Space Station consists of two concentric rotating drums. The space between the two drums comprises the wind tunnel section. Differential rates of rotation of the two drums would provide a wind velocity with respect to either drum surface. Preliminary results of measured velocity profiles made in a CWT prototype indicate that the wall bounded boundary layer profiles are suitable to simuate flat plate turbulent boundary layer flow. The two dimensional flate plate Cartesian coordinate equations of motion of a particle moving through the air are explained. In order to assess the suitability of CWT in the analysis of the trajectories of windblown particles, a series of calculations were conducted comparing cases for gravity with those of zero gravity. Results from the calculations demonstrate that a wind tunnel of the carousel design could be fabricated to operate in a space station environment and that experiments could be conducted which would yield significant results contributing to the understanding of the physics of particle dynamics.
    Keywords: RESEARCH AND SUPPORT FACILITIES (AIR)
    Type: NASA, Lyndon B. Johnson Space Center, Experiments in Planetary and Related Sciences and the Space Station; 3 p
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 4
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    Development and testing of a unique carousel wind tunnel to experimentally determine the effect of gravity and the interparticle force on the physics of wind-blown particles (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: In the study of planetary aeolian processes the effect of gravity is not readily modeled. Gravity appears in the equations of particle motion along with the interparticle forces but the two are not separable. A wind tunnel that perimits multiphase flow experiments with wind blown particles at variable gravity was built and experiments were conducted at reduced gravity. The equations of particle motion initiation (saltation threshold) with variable gravity were experimentally verified and the interparticle force was separated. A uniquely design Carousel Wind Tunnel (CWT) allows for the long flow distance in a small sized tunnel since the test section if a continuous loop and develops the required turbulent boundary layer. A prototype model of the tunnel where only the inner drum rotates was built and tested in the KC-135 Weightless Wonder 4 zero-g aircraft. Future work includes further experiments with walnut shell in the KC-135 which sharply graded particles of widely varying median sizes including very small particles to see how interparticle force varies with particle size, and also experiments with other aeolian material.
    Keywords: RESEARCH AND SUPPORT FACILITIES (AIR)
    Type: NASA, Lyndon B. Johnson Space Center, Experiments in Planetary and Related Sciences and the Space Station; 6 p
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 5
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    Design and calibration of the carousel wind tunnel (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: In the study of planetary aeolian processes the effect of gravity is not readily modeled. Gravity appears in the equations of particle motion along with interparticle forces but the two terms are not separable. A wind tunnel that would permit variable gravity would allow separation of the forces and aid greatly in understanding planetary aeolian processes. The design of the Carousel Wind Tunnel (CWT) allows for a long flow distance in a small sized tunnel since the test section is a continuous circuit and allows for a variable pseudo-gravity. A prototype design was built and calibrated to gain some understanding of the characteristics of the design and the results presented.
    Keywords: RESEARCH AND SUPPORT FACILITIES (AIR)
    Type: NASA, Lyndon B. Johnson Space Center, Experiments in Planetary and Related Sciences and the Space Station; 2 p
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 6
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    Aeolian processes aboard a space station: Saltation and particle trajectory analysis (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The Carousel wind tunnel (CWT) proposed to study aeolian processes aboard a space station consists of two concentric rotating drums. The space between the two drums comprises the wind tunnel test section. Differential rates of rotation of the two drums would provide a wind velocity with respect to either drum surface. Preliminary results of measured velocity profiles made in a CWT prototype indicate that the wall bounded boundary layer profiles are suitable to simulate flat plate turbulent boundary layer flow. The two dimensional flat plate Cartesian coordinate equations of motion of a particle moving through the air are explained. In order to assess the suitability of CWT in the analysis of the trajectories of windblown particles, a series of calculations were conducted comparing cases for gravity with those of zero gravity. Results from the calculations demonstrate that a wind tunnel of the carousel design could be fabricted to operate in a space station environment and that experiments could be conducted which would yield significant results contributing to the understanding of the physics of particle dynamics.
    Keywords: RESEARCH AND SUPPORT FACILITIES (AIR)
    Type: NASA. Lyndon B. Johnson Space Center Space Station Planetology Experiments (SSPEX); 3 p
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 7
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    Microdunes and Other Aeolian Bedforms on Venus: Wind Tunnel Simulations (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: The development of aeolian bedforms in the simulated Venusian environment has been experimentally studied in the Venus Wind tunnel. It is found that the development of specific bedforms, including ripples, dunes, and waves, as well as their geometry, are controlled by a combination of factors including particle size, wind speed, and atmospheric density. Microdunes are formed which are analogous to full-size terrestrial dunes and are characterized by the development of slip faces, internal cross-bedding, a low ratio of saltation path length to dune length, and a lack of particle-size sorting. They begin to develop at wind speeds just above saltation threshold and evolve into waves at higher velocities. At wind speeds of about 1.5 m/sec and higher, the bed is flat and featureless. This evolution is explained by a model based on the interaction of alternating zones of erosion and deposition and particle saltation distances.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program; p 309
    Format: text
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    NASA TECHNICAL REPORTS
  • 8
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    Rate of wind abrasion on Mars (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: A brief description is given of the experiments performed to obtain data on windblown particles and abrasion of rocks in a simulated Martian environment. Preliminary results are presented and combined with Viking meteorological data in estimating rates of wind abrasion at the VL-1 site on Mars. Attention is also given to the implications that the results have for Martian surface history. Calculations of the present rates of abrasion by windblown particles on Mars yield values ranging from 0.021 cm/yr to nearly zero, depending on the target, the agent of abrasion, and the availability of windblown particles.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Format: text
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    NASA TECHNICAL REPORTS
  • 9
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    Design and calibration of the carousel wind tunnel (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: In the study of planetary aeolian processes the effect of gravity is not readily modeled. Gravity appears in the equations of particle motion along with interparticle forces but the two terms are not separable. A wind tunnel that would permit variable gravity would allow separation of the forces and aid greatly in understanding planetary aeolian processes. The design Carousel Wind Tunnel (CWT) allows for a long flow distance in a small sized tunnel since the test section is a continuo us circuit and allows for a variable pseudo gravity. A prototype design was built and calibrated to gain some understanding of the characteristics of the design and the results presented.
    Keywords: RESEARCH AND SUPPORT FACILITIES (AIR)
    Type: NASA. Lyndon B. Johnson Space Center Space Station Planetology Experiments (SSPEX); 2 p
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 10
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    Microdunes and other aeolian bedforms on Venus - Wind Tunnel simulations (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The development of aeolian bedforms in the simulated Venusian environment has been experimentally studied in the Venus Wind Tunnel. It is found that the development of specific bedforms, including ripples, dunes, and 'waves', as well as their geometry, are controlled by a combination of factors including particle size, wind speed, and atmospheric density. Microdunes are formed which are analogous to full-size terrestrial dunes and are characterized by the development of slip faces, internal cross-bedding, a low ratio of saltation path length to dune length, and a lack of particle-size sorting. They begin to develop at wind speeds just above saltation threshold and evolve into waves at higher velocities. At wind speeds of about 1.5 m/sec and higher, the bed is flat and featureless. This evolution is explained by a model based on the interaction of alternating zones of erosion and deposition and particle saltation distances.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus (ISSN 0019-1035); 60; 152-160
    Format: text
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    NASA TECHNICAL REPORTS
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