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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1994

A study of X-ray and infrared emissions from dusty nonradiative shock waves

We have constructed models that predict the dynamic evolution and infrared (IR) emission of grains behind nonradiative shock waves. We present a self-consistent treatment of the effect of grain destruction and heating on the ionization structure and X-ray emission of the postshock gas. Incorporating thermal sputtering, collisional heating, and deceleration of grains in the postshock flow, we predict the IR and X-ray fluxes from the dusty plasma as a function of swept-up column density. Heavy elements such as C, O, Mg, S, Si and Fe are initially depleted from the gas phase but are gradually returned as the grains are destroyed. The injected neutral atoms require some time to 'catch up' with the ionization state of the ambient gas. The nonequilibrium ionization state and gradient in elemental abundances in the postshock flow produces characteristic X-ray signatures that can be related to the age of the shock and amount of grain destruction. We study the effects of preshock density and shock velocity on the X-ray and IR emission from the shock. We show that the effects of graindestruction on the X-ray spectra of shock waves are substantial. In particular, temperatures derived from X-ray spectra of middle-aged remnants are likely to be overestimated by approximately 15% if cosmic abundances are assumed. Due to the long timescales for grain destruction in X-ray gases over a wide range of temperatures, we suggest that future X-ray spectra studies of supernova remnants be based on depleted abundances instead of cosmic abundances. Our model predictions agree reasonably well with IRAS and Einstein IPC observations of the Cygnus Loop.

Vancura, Olaf↗
NASA NTRSDate not supplied

Advancing xEMU Lunar Dust Mitigation Devices

Fine, electrically charged, glass like dust particles caused significant damage to the Apollo EMU9 during lunar EVAs, identified as one of the greatest challenges to future exploration. Passive Lunar Dust Mitigation Devices (LDMD) were developed, within a SBIR Phase II, to prohibit this dust from interrupting venting space suit component operation. A Computational Fluid-Dynamics and Discrete Element Method (CFD-DEM) Simulation Tool was developed at the University of Colorado, Boulder to predict venting gas flow ability to self-clean adhered dust particles from LDMD surfaces. Lunar dust properties (i.e., adhesion and cohesion strengths) required to complete Simulation Tool analysis are relatively unknown due to considerable differences between the Earth and the Moon (i.e., gravity, humidity) and due to an absence of dust particles in their native state. Analytically determining gas flow velocity, density and direction within the fluid Boundary Layer, microns from LDMD surfaces presented a second challenge. Dusty Plasma Chamber testing is being performed at Auburn University, Auburn to observe electrostatically charged dust behavior as it adheres to LDMD prototypes and specific geometry and is then blown away by metered gas flow. Test articles were developed to offer insight into the impact of different flow geometries, surface roughness and dust removal within the gas Boundary-Layer. Observed dust behavior is currently being developed to support the CFD-DEM analysis. Many Simulation Tool analytical cases have been processed to support the intention of completing sensitivity studies to assess how different dust adherence values and Boundary Layer fluid properties impact LDMD self-cleaning effectivity.

Thomas J Stapleton↗
NASA NTRS2012

Active Galactic Nucleus Obscuration from Winds: From Dusty Infrared-Driven to Warm and X-Ray Photoionized

We present calculations of active galactic nucleus winds at approx.parsec scales along with the associated obscuration. We take into account the pressure of infrared radiation on dust grains and the interaction of X-rays from a central black hole with hot and cold plasma. Infrared radiation (IR) is incorporated in radiation-hydrodynamic simulations adopting the flux-limited diffusion approximation. We find that in the range of X-ray luminosities L = 0.05-0.6 L(sub Edd), the Compton-thick part of the flow (aka torus) has an opening angle of approximately 72deg - 75deg regardless of the luminosity. At L > or approx. 0.1, the outflowing dusty wind provides the obscuration with IR pressure playing a major role. The global flow consists of two phases: the cold flow at inclinations (theta) > or approx.70deg and a hot, ionized wind of lower density at lower inclinations. The dynamical pressure of the hot wind is important in shaping the denser IR-supported flow. At luminosities < or = 0.1 L(sub Edd) episodes of outflow are followed by extended periods when the wind switches to slow accretion. Key words: acceleration of particles . galaxies: active . hydrodynamics . methods: numerical Online-only material: color figures

Dorodnitsyn, Anton V.↗
NASA NTRS1994

The fragmentation of dust in the innermost comae of comets: Possible evidence from ground-based images

Dust particles when released from the nucleus of a comet are entrained in the expanding gas flow created by the vaporization of ices (mainly water ice). Traditional approaches to dusty-gas dynamics in the inner comae of comets consider there to be an initial distribution of dust particle sizes which do not fragment or evaporate. The standard Finson-Probstein model (and subsequent variations) yields a one-to-one-to-one correspondence between the size of a dust particle, its terminal velocity owing to gas drag, and its radiation pressure acceleration which creates the notable cometary dust tail. The comparison of a newly developed dust coma model shows that the typical elongated shapes of isophotes in the dust comae of comets on the scale of greater than 10(exp 4) km from the nucleus requires that the one-to-one-to-one relationship between particle size, terminal velocity and radiation pressure acceleration cannot in general be correct. There must be a broad range of particles including those having a small velocity but large radiation pressure acceleration in order to explain the elongated shape. A straightforward way to create such a distribution is if particle fragmentation, or some combination of fragmentation with vaporization, routinely occurs within and/or just outside of the dusty-gas dynamic acceleration region (i.e., up to several hundred km). In this way initially large particles, which are accelerated to fairly slow velocities by gas-drag, fragment to form small particles which still move slowly but are subject to a relatively large radiation pressure acceleration. Fragmentation has already been suggested as one possible interpretation for the flattened gradient in the spatial profiles of dust extracted from Giotto images of Comet Halley. Grain vaporization has been suggested as a possible spatially extended source of coma gases. The general elongated isophote shapes seen in ground-based images for many years represents another possible signature of fragmentation.

Combi, Michael R.↗
NASA NTRS2016

Extraction and Capture of Water from Martian Regolith Experimental Proof-of-Concept

Mars Design Reference Architecture 5.0:Lists in-situ resource utilization (ISRU) as enabling for robust human Mars missionsLO2LCH4 ascent propulsion 25,000 kg oxygen from atmosphere for ascent and life support Atmospheric based ISRU processes less operationally complex than surface based limited concept evaluation to date and Mars surface water property and distribution uncertainty would not allow [Mars soil water processing] to be base lined at this time Limited Concept Evaluation to Date Lunar regolith O2 extraction processing experience Lunar regolith is fluidized and heated to high temperatures with H2 to produce H2O from iron-bearing minerals Mars similarity concept: Soil placed in fluidized bed reactor Heated to moderate temperatures Inert gas flow used to fluidize the bed and help with water desorption Challenges: High-temperature dusty seals Working gas requires downstream separation and recycling to reduce consumables loss Batch process heating thermally inefficient.

in situ resource utilization↗
NASA NTRS2018

Dust Mitigation on Mars Using an Electrostatic Precipitator

The Martian atmosphere contains large amounts of dust, which are lofted by dust devils and dust storms. Some of this dust, particles on the order of 2-4 m, never settle and are constantly present in the atmosphere. Therefore, in order to utilize the planet's atmosphere for production of consumables, like oxygen and methane, this dust must be removed before the commodity production can begin. The electrostatic precipitator is currently being studied at Kennedy Space Center as a realistic option for removing this dust. This project covers the results, to date, of the dust flow initiation, control, and analysis inside the electrostatic precipitator, which is to be modelled after various dusty Martian atmospheric conditions.

electrostatic precipitator↗
NASA NTRSDate not supplied

On the Interaction Between a Flow Instability and Inertial Particles in Supersonic Jet Plume-Surface Interaction

Extraterrestrial landings often require spacecraft to deploy supersonic jets directed at dusty planetary surfaces to ensure controlled descents. This interaction, termed plume-surface interaction (PSI), critically influences crater formation and the dynamics of ejected particles, which can obscure vision, damage onboard sensors, and impact nearby infrastructure with significant particle velocities. The physics of PSI remain poorly understood due to the complexities in simulating the involved multiphysics and experimentally replicating reduced atmospheric pressure conditions. This article discusses a collaborative effort to investigate PSI in reduced atmospheric pressure environments using a Mach 5.3 jet impinging on a granular bed. The results highlight observations of an azimuthal pattern in the ejecta distribution, characterized by alternating ejecta streaks of varying particle concentrations. This phenomenon suggests the presence of a low Reynolds number instability, a notable discovery given the supersonic nature of the jet.

PSI↗
NASA NTRS1991

Analysis of a dusty wall jet

An analysis is given for the entrainment of dust into a turbulent radial wall jet. Equations are solved based on incompressible flow of a radial wall jet into which dust is entrained from the wall and transported by turbulent diffusion and convection throughout the flow. It is shown that the resulting concentration of dust particles in the flow depends on the difference between the applied shear stress at the surface and the maximum level of shear stress that the surface can withstand (varies as rho(sub d)a(sub g)D) i.e., the pressure due to the weight of a single layer of dust. The analysis is expected to have application to the downflow that results from helicopter and VTOL aircraft.

Lim, Hock-Bin↗
NASA NTRS2009

Study of Electro-Cyclonic Filtration and Pneumatic Transfer of Lunar Regolith Simulants under 1/6-g and 1-g Gravity Conditions

NASA has built a prototype oxygen production plant to process the lunar regolith using the hydrogen reduction chemical process. This plant is known as "ROxygen - making oxygen from moon rocks". The ROxygen regolith transfer team has identified the flow and transfer characteristics of lunar regolith simulant to be a concern for lunar oxygen production efforts. It is important to ISRU lunar exploration efforts to develop hardware designs that can demonstrate the ability to flow and transfer a given mass of regolith simulant to a desired vertical height under lunar gravity conditions in order to introduce it into a reactor. We will present results obtained under both 1/6-g and 1-g gravity conditions for a system that can pneumatically convey 16.5 kg of lunar regolith simulant (NU-LHT-2M, Mauna Kea Tephra, and JSC-1A) from a flat-bottom supply hopper to a simulated ISRU reactor (dual-chambered receiving hopper) where the granular material is separated from the convey gas (air) using a series of cyclone separators, one of which is an electrically enhanced cyclone separator (electrocyclone). The results of our study include (1) the mass flow rate as a function of input air pressure for lunar regolith simulants that are conveyed pneumatically as a dusty gas in a vertical direction against gravity under lunar gravity conditions (for NU-LHT-2M and Mauna Kea Tephra), and under earth gravity conditions (for NU-LHT-2M, Mauna Kea Tephra and JSC-1A), and (2) the efficiency of the cyclone/electrocyclone filtration system in separating the convey gas (air) from the granular particulates as a function of particle size.

Mantovani, James G.↗
NASA NTRS2023

Radiation Interaction with Particulates in Planetary Entry Flows

A hypersonic particle phase solver based on a continuum model is implemented in the NASA HyperSolve CFD code and used to predict particle phase conditions in a dusty Mars entry scenario. Black-body radiative emission from high temperature dust particles is predicted for a Mars relevant entry problem, and the emission integrated to an entry vehicle using a ray tracing approach to estimate surface heat flux. The surface heating due to this mechanism is found to be small relative to convective and radiation from the gas phase emission. Additionally, the effect of atmospheric haze particulates vaporizing in the Titan atmosphere is studied. The increase in carbon availability due to the composition of the haze particles is found to increase the gas radiative heating to the vehicle by up to 3% on the forebody of a representative Titan entry vehicle with approximately 0.15% haze in the freestream by mass.

Andrew Hinkle↗
NASA NTRS1995

Rotating cooloing flows

We describe the evolution of the hot interstellar medium in a large, slowly rotating elliptical galaxy. Although the rotation assumed is a small fraction of the circular velocity, in accordance with recent observations, it is sufficient to have a profound influence on the X-ray emission and cooling geometry of the interstellar gas. The hot gas cools into a disk that extends out to approximately 10 kpc. The cool, dusty disks observed in the majority of elliptical galaxies may arise naturally from internal cooling rather than from mergers with gas-rich companions. As a result of angular momentum conservation in the cooling flow, the soft X-ray isophotes are quite noticeably flatter than those of the stellar image. The gas temperature is higer along the rotation axis. The rotational velocity of the gas several kiloparcsecs above the central disk far exceeds the local stellar rotation and approaches the local circular velocity as it flows toward the galactic core. The detailed appearance of the X-ray image and velocity field of the X-ray gas provide information about the global rotational properties of giant ellipticals at radii too distant for optical observations. The overall pattern of rotation in these galaxies retains information about the origin of ellipticals, particularly of their merging history. In ellipticals having radio jets, if the jets are aligned with the rotation axis of the inner cooling flow, rotation within the jet could be sustained by the rotating environment. Since most large ellipticals have modest rotation, the X-ray observations at low spatial resolution, when interpreted with spherical theoretical models, give the impression that hot gas undergoes localized cooling to very low temperatures many kiloparcsecs from the galactic core. We suggest that such apparent cooling can result in a natural way as gas cools onto a rotating disk.

Kley, Wilhelm↗
NASA NTRS1992

Metallic atoms and ions in comets: Comet Halley 1986 3

The origin of metallic atoms and ions in the cometary comae is investigated theoretically. Two effects are revealed in the comas of bright comets: (1) the Na anomalous type effect is possible within the gas-dust jets of comet P/Halley 1986 3 due to cooling cometary dust by cryogenic gas flow from the nucleus; and (2) the production of ions of refractory elements (Fe(+), Si(+), etc.) at large heliocentric distances is possible in the comas of the Halley type dusty comets due to high-velocity impacts between cometary and zodiacal dust particles. Spectral observations of comets with high sensitivity and spatial resolution are important for studying both comets and interplanetary dust.

Ibadov, S.↗
NASA NTRS1990

Aerobraking in a dusty Martian atmosphere

The effects of dust particle impacts on the erosion of the heat shield were estimated for a 26 m diameter aerobraking vehicle entering the Mars atmosphere at 8.6 km/sec. An explicit Navier-Stokes code was used to compute the flow field about the vehicle for the actual Martian atmospheric composition at a speed of 7.5 km/sec. The deceleration and melting of the dust particles within the forebody shock-layer was computed for dust spherules having initial diameters from 3 to 10 microns. Two heat shield materials were considered: Shuttle tiles with glassy surfaces and a low-density ablator known as Avcoat.

Chang, I-Dee↗
NASA NTRS1991

Lunar Transportation Facilities and Operations Study, option 1

Throughout the Option I period of the Lunar Transportation Facilities and Operations Study (LTFOS), McDonnell Douglas Space Systems Company - Kennedy Space Center (MDSSC-KSC) provided support to both the Planetary Surface Systems (PSS) Office at the National Aeronautics and Space Administration (NASA) at the Johnson Space Center and to the Flight and Ground Systems Projects Office (Payload Projects Management) at the Kennedy Space Center. The primary objective of the Option I phase of the study was to assist the above NASA centers in developing Space Exploration Initiative (SEI) concepts. MDSSC-KSC conducted three analyses which provided launch and landing detail to the proposed exploration concepts. One analysis, the Lunar Ejecta Assessment, was conducted to determine the effects of launch and landing a vehicle in a dusty environment. A second analysis, the Thermal/Micrometeoroid Protection Trade Study, was refined to determine the impacts that Reference Architecture Option 5A would have on thermal/micrometeoroid protection approaches. The third analysis, the Centaur Prelaunch Procedure Analysis, used a Centaur prelaunch test and checkout flow to identify key considerations that would be important if a Lunar Excursion Vehicle (LEV) was to use an expander cycle liquid oxygen-liquid hydrogen engine. Several 'quick look' assessments were also conducted. One quick look assessment, the Storable Propellant Quick Look Assessment, was conducted to identify design considerations that should be made if storable propellants were to be used instead of liquid oxygen and liquid hydrogen. The LEV Servicer Maintenance Analysis provided an early look at the effort required to maintain an LEV Servicer on the lunar surface. Also, support was provided to the PSS Logistics Manager to develop initial LEV Servicer cost inputs. Consideration was given to the advanced development that must be provided to accomplish a lunar and/or Mars mission. MDSS-KSC also provided support to both MASE and PSS by reviewing and commenting on architecture documents and concept definitions to ensure adequate considerations were given to the launch and landing function of an exploration initiative.

Source record↗
NASA NTRS2019

InSight's Reconstructed Aerothermal Environments

The InSight Mars Lander successfully landed on the surface on November 26, 2018. This poster will describe the methodologies and margins used in developing the aerothermal environments for design of the thermal protection systems (TPS), as well as a prediction of as-flown environments based on the best estimated trajectory. The InSight mission spacecraft design approach included the effects of radiant heat flux to the aft body from the wake for the first time on a US Mars Mission, due to overwhelming evidence in ground testing for the European ExoMars mission (2009/2010) [1] and 2010 tests in the Electric Arc Shock Tube (EAST) facility [2]. The radiant energy on an aftbody was also recently confirmed via measurement on the Schiaparelli mission [3]. In addition, the InSight mission expected to enter the Mars atmosphere during the dust storm season, so the heatshield TPS was designed to accommodate the extra recession due to the potential dust impact. This poster will compare the predicted aerothermal environments using the reconstructed best estimated trajectory to the design environments. Design Approach: The InSight spacecraft was planned to be a near-design-to-print copy of the Phoenix spacecraft. The determination of the heatshield TPS requirements was approached as if it was a new design due to the new requirement of flying through a dust storm. The baseline for aftbody was build-to-print, and all analyses focused on ensuring adequate margin. This proved to be a challenge because the Phoenix aftbody was designed to withstand only convective heating and the InSight aftbody was evaluated for both convective and radiative heating. Aerothermal environments were predicted using the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) and the Data Parallel Line Relaxation (DPLR) CFD codes, and the Nonequilibrium Radiative Transport and Spectra Program (NEQAIR) utilizing bounding design trajectories derived from Monte Carlo analyses from the Program to Optimize Simulated Trajectories II (POST2). In all cases, super-catalytic flowfields were assigned to ensure the most conservative heating results. Two trajectories were evaluated: 1) the trajectory with the maximum heat flux was utilized to determine the flowfield characteristics and the viability of the selection of TPS materials; and 2) the trajectory with the maximum heat load was used to determine the required thicknesses of the TPS materials. Evaluation of the MEDLI data [4], along with ground test data [5] led to the determination of whether or not the flow would transition from laminar to turbulent on the heatshield, which also determined the TPS sizing location for the heatshield. Aerothermal margins were added for the convective heating and developed for the radiative heating. TPS material sizing was determined with the Reaction Kinetic Ablation Program (REKAP) and the Fully Implicit Ablation and Thermal Analysis program (FIAT) using a three-branched approach to account for aerothermal, material response, and material properties uncertainties. In addition, the heatshield recession was augmented by an analysis of the effect of entry through a potential dusty atmosphere using a methodology developed in References [6] and [7]. These analyses resulted in an increase to the Phoenix heatshield TPS thickness. Reconstruction Efforts: Once the best estimated trajectory is reconstructed by the team, the LAURA/HARA (High-Temperature Aerothermo-dynamic Radiation model) and DPLR/NEQAIR code pairs will be used to predict the as-flown aerothermal conditions. In these runs, fully-catalytic flowfields will be assigned because it is a more physically accurate description of the chemistry in the flow. Once again, determination of the onset of turbulence on the heatshield will be evaluated. The as-flown aerothermal environments will then be compared to the design environments.

Beck, R. A.↗
NASA NTRS2012

The Role of African Easterly Wave on Dust Transport and the Interaction Between Saharan Dust Layer and Atlantic ITCZ During Boreal Summer

In this paper, we investigate the relationships among Saharan dust outbreak and transport, African easterly waves (AEW), African easterly jet (AEJ) and associated convective activities of Atlantic Intertropical Convergence Zone (ITCZ) using Cloudsat-Calipso, MODIS and MERRA data. We find that a major Saharan dust outbreak is associated with the formation of a westward propagating strong cyclone around 15-25N over the western part northern Saharan. The strong cyclonic flow mobilizes and lifts the dust from the desert surface to a high elevation. As the cyclone propagate westward, it transports a thick elevated dust layer between 900 -500 hPa from the African continent to the eastern Atlantic. Cloudiness is reduced within the warm, dry dusty layer, but enhanced underneath it, possibly due to the presence of a shallow inversion layer over the marine boundary layer. The dust outbreak is linked to enhanced deep convection in the northern part of Atlantic ITCZ, abutting the southern flank of the dust layer, and a strengthening of the northward flank of the AEJ. As the dust layer spreads westward, it loses elevation and becomes increasing diffused as it reaches the central and western Atlantic. Using band pass filtered EOF analysis of MERRA winds, we find that AEWs propagating westward along two principal tracks, centered at 15-25N and 5-10N respectively. The easterly waves in the northern track are highly correlated with major dust outbreak over North Africa and associated with slower moving systems, with a quasi-periodicity of 6-9 day. On the other hand, easterly waves along the southern track are faster, with quasi-periodicity of 3-5 days. These faster easterly waves are closely tied to rainfall/cloud variations along the Atlantic ITCZ. Dust transport along the southern track by the faster waves generally leads rainfall/cloud anomalies in the same region by one or two days, suggesting the southern tracks of dust outbreak are regions of strong interaction between Saharan dust layer and Atlantic ITCZ.

Lau, William K. M.↗
NASA NTRS2018

Development of the Electrostatic Precipitator (ESP) for Mars and Electrodynamic Dust Shield (EDS)

Kennedy Space Center's Swamp Works is a fast-paced and diverse technology development laboratory that aims to advance commercial and government capabilities to colonize extraterrestrial environments. As a part of Swamp Works, the Electrostatic and Surface Physics Laboratory (ESPL) is currently developing new technologies that will further NASA's capabilities to colonize lunar and Martian environments. At the ESPL, the objective of the overall project is to aid in the dust mitigation of robotic and human exploration missions to the moon and Mars. The moon and Mars are covered with layers of dust, which can make long-term exploration missions very difficult. The team at the ESPL is developing an electrostatic precipitator (ESP) to aid in the reduction of interference from airborne Martian dust on equipment. The ESP is designed to mitigate the dust in an intake of CO2-rich dusty gas (i.e., the Martian atmosphere). The ESP is essentially a cylindrical tube with a coaxial wire electrode. Applying a high voltage through this electrode induces a corona discharge (a glowing plasma that envelops the electrode), which is used to charge the inflowing dust. The electric field caused by the corona pushes the charged dust to the walls of the precipitator, preventing the dust to flow out of the tube. Environmental dust can make long-term exploration missions very difficult, and settled dust is no exception. Settled lunar or Martian dust can hinder the performance and lifetime of equipment. For example, dust can settle on the solar panels of a lunar or Martian rover, decreasing its performance while increasing its charging time. To address this, the team at the ESPL is also developing an electrodynamic dust shield (EDS), which is designed to remove lunar or Martian dust from the surfaces of equipment. The EDS uses a non-uniform electric field to generate a dielectrophoretic force, which pushes the particles away from the surface of the shield. A number of dust shields will be tested on the exterior of the International Space Station (ISS) via MISSE-11, a flight payload to the ISS that will test the effects of long-term exposure to space on materials. During my internship at the ESPL, I aided in the development of these technologies. For the ESP, I helped characterize the electrical properties of various geometries and helped redesign the hardware of the testbed (the precipitator used to test this technology in the lab). To characterize the electrical properties of the ESP, I ran various tests of the precipitator, which consisted of varying the environmental conditions and geometry of the testbed. Analyzing the electrical properties of the ESP in various environmental conditions is necessary to characterize its collection efficiency, since it will be used in the dusty Martian environment. For the EDS, I helped analyze the dielectric strength of the surface of the shield via high-voltage testing. A dielectrically strong surface will help the shield survive the harsh environment of space, which will enable it to have both lunar and Martian applications.

Corona Discharge↗
NASA NTRS2002

Hubble Space Telescope Imaging of the Circumnuclear Environments of the CfA Seyfert Galaxies: Nuclear Spirals and Fueling

We present archival Hubble Space Telescope (HST) images of the nuclear regions of 43 of the 46 Seyfert galaxies found in the volume limited,spectroscopically complete CfA Redshift Survey sample. Using an improved method of image contrast enhancement, we created detailed high-quality " structure maps " that allow us to study the distributions of dust, star clusters, and emission-line gas in the circumnuclear regions (100-1000 pc scales) and in the associated host galaxy. Essentially all of these Seyfert galaxies have circumnuclear dust structures with morphologies ranging from grand-design two-armed spirals to chaotic dusty disks. In most Seyfert galaxies there is a clear physical connection between the nuclear dust spirals on hundreds of parsec scales and large-scale bars and spiral arms in the host galaxies proper. These connections are particularly striking in the interacting and barred galaxies. Such structures are predicted by numerical simulations of gas flows in barred and interacting galaxies and may be related to the fueling of active galactic nuclei by matter inflow from the host galaxy disks. We see no significant differences in the circumnuclear dust morphologies of Seyfert 1s and 2s, and very few Seyfert 2 nuclei are obscured by large-scale dust structures in the host galaxies. If Sevfert 2s are obscured Sevfert Is, then the obscuration must occur on smaller scales than those probed by HST.

Pogge, Richard W.↗