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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Investigation of the Arcjet near Field Plume Using Electrostatic Probes

The near field plume of a 1 kW class arcjet thruster was investigated using electrostatic probes of various geometries. The electron number densities and temperatures were determined in a simulated hydrazine plume at axial distances between 3 cm (1.2 in.) and 15 cm (5.9 in.) and radial distances extending to 10 cm (3.9 in.) off centerline. Values of electron number densities obtained using cylindrical and spherical probes of different geometries agreed very well. The electron density on centerline followed a source flow approximation for axial distances as near as 3 cm (1.2 in.) from the nozzle exit plane. The model agreed well with previously obtained data in the far field. The effects of propellant mass flow rate and input power level were also studied. Cylindrical probes were used to obtain ion streamlines by changing the probe orientation with respect to the flow. The effects of electrical configuration on the plasma characteristics of the plume were also investigated by using a segmented anode/nozzle thruster. The results showed that the electrical configuration in the nozzle affected the distribution of electrons in the plume.

Sankovic, John M.↗
NASA NTRS1991

Computational analysis of plume induced separation

Full Navier-Stokes simulations around axisymmetric boattailed and flared rockets are numerically investigated for plume induced separation phenomenon. At lower altitudes, the plume interaction with the external flow does not cause any flow separation on the body, but at conditions corresponding to higher altitudes large plume induced separation is observed. Addition of a flare to the afterbody limits the extent of separation at high altitudes. Computational solutions for the boattailed axisymmetric geometry are compared with available wind-tunnel and flight data. The effect of forebody ablation is studied by modifying the inflow boundary layer profile. Numerical solutions with thicker boundary layers show significantly greater plume-induced separation compared with nonablating cases. Heat transfer to the wall was computed for the flared afterbody geometry and is presented.

Venkatapathy, Ethiraj↗
NASA NTRS1991

Vapor plumes: A neglected aspect of impact cratering

When a meteorite or comet strikes the surface of the planet or satellite at typical interplanetary velocities of 10-40 km/sec, the projectile and a quantity of the target body vaporize and expand out of the growing crater at high speed. The crater continues to grow after the vapor plume has formed and the series of ejecta deposits is laid down ballistically while the crater collapses into its final morphology. Although the vapor plume leaves little evidence of its existence in the crater structure of surface deposits, it may play a major role in a number of impact-related processes. The vapor plume expanding away from the site of an impact carries 25-50 percent of the total impact energy. Although the plume's total mass is only a few times the mass of the projectile, its high specific energy content means that it is the fastest and most highly shocked material in the cratering event. The mean velocity of expansion can easily exceed the escape velocity of the target plane, so that the net effect of a sufficiently high-speed impact is to erode material from the planet.

Melosh, H. J.↗
NASA NTRS1992

Numerical investigations in the backflow region of a vacuum plume

The objective of this research is to numerically simulate the vacuum plume flow field in the backflow region of a low thrust nozzle exit. In space applications, the low thrust nozzles are used as a propulsion device to control the vehicle attitude, or to maneuver the vehicle flight trajectory. When the spacecraft is deployed in the orbit or cruising in a planetary mission, the vacuum plume is created behind the nozzle exit (so called backflow region), by the exhausting gas of the propulsion system or by venting internal gas to the extremely low density ambient. The low density vacuum plume flow regions cover the continuum, transitional and free molecular flow regimes, which were characterized by the Knudsen number K(sub n), K(sub n) = lambda(sub m)/L where lambda(sub m) is the mean free path of the gas molecules and L is the characteristic length of the flow field. The transitional regime is defined by 0.01 is less than or equal to K(sub n) is less than or equal to 10. The conventional Navier-Stokes equations are valid only in the flow region close to the nozzle exit since the validity of the Navier-Stokes equations fails asymptotically as the Knudsen number increases. The vacuum plume characteristics prediction is primarily a problem of transitional aerodynamics.

Liaw, Goang-Shin↗
NASA NTRS1992

Geoid anomalies and dynamic topography from convection in cylindrical geometry - Applications to mantle plumes on earth and Venus

A variety of evidence suggests that at least some hotspots are formed by quasi-cylindrical mantle plumes upwelling from deep in the mantle. Such plumes are modeled in cylindrical, axisymmetric geometry with depth-dependent, Newtonian viscosity. Cylindrical and sheet-like, Cartesian upwellings have significantly different geoid and topography signatures. However, Rayleigh number-Nusselt number systematics in the two geometries are quite similar. The geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer, provided that it is at least 1000 times the interior viscosity. Increasing the Rayleigh number or including a low-viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume.

Kiefer, Walter S.↗
NASA NTRS1992

A comparative study of two codes with an improved two-equation turbulence model for predicting jet plumes

Compressible jet plumes were studied using a two-equation turbulence model. A space marching procedure based on an upwind numerical scheme was used to solve the governing equations and turbulence transport equations. The computed results indicate that extending the space marching procedure for solving supersonic/subsonic mixing problems can be stable, efficient and accurate. Moreover, a newly developed correction for compressible dissipation has been verified in fully expanded and underexpanded jet plumes. For a sonic jet plume, no improvement in results over the standard two-equation model was seen. However for a supersonic jet plume, the correction due to compressible dissipation successfully predicted the reduced spreading rate of the jet compared to the sonic case. The computed results were generally in good agreement with the experimental data.

Balakrishnan, L.↗
NASA NTRS1992

Three dimensional particle simulation of high altitude rocket plumes

The interaction of two nozzles exhausting into vacuum generates a complex three-dimensional shock structure. The shock structure and resulting plume flowfield is characterized by the nozzle separation distance. For the appropriate range of penetration Knudsen numbers, the analysis of this shock structure can be suitably accomplished through a Monte Carlo simulation. This paper describes the application of a general three-dimensional Monte Carlo simulation on the Connection Machine CM-2 to the analysis of the plume self-interaction shock in the near field. Results are presented for two cases, corresponding to a small and a large nozzle separation distance. The results correctly reproduce the expected flow features and demonstrate the ability of this method to properly simulate the start of the plume self-interaction shock. This has significance not only for allowing analysis of the self-interacting plume in the near field, but also for allowing the subsequent simulation of the far field flow through the use of a continuation downstream exit boundary.

Dagum, Leonardo↗
NASA NTRS1992

Measurement and analysis of a small nozzle plume in vacuum

Measurements of Pitot pressure and flow angle were made in the plume of a nozzle flowing nitrogen and exhausting to a vacuum. The measurements were compared to results from a numerical simulation of the flow that was based on kinetic theory and used the direct-simulation Monte Carlo (DSMC) method. Numerical results were compared with measurements made in the plume at various axial and radial stations. Total pressure measurements were made with Pitot tubes sized for specific regions of the plume. Flow angle measurements were made with a conical probe. The measurement area for flow angle extended to 160 mm (5 exit diameters) downstream of the nozzle exit plane and radially to 60 mm (1.9 exit diameters) from the plume axis. The total pressure measurements extended 480 mm (16 exit diameters) downstream and radially to 60 mm. Comparisons of computed results from the DSMC method with measurements of flow angle displayed improved agreement with increasing distance from the exit plane. Pitot pressures computed from the DSMC method were in reasonably good agreement with experimental results over the entire measurement area.

Penko, Paul F.↗
NASA NTRS1992

Second order closure modeling of turbulent buoyant wall plumes

Non-intrusive measurements of scalar and momentum transport in turbulent wall plumes, using a combined technique of laser Doppler anemometry and laser-induced fluorescence, has shown some interesting features not present in the free jet or plumes. First, buoyancy-generation of turbulence is shown to be important throughout the flow field. Combined with low-Reynolds-number turbulence and near-wall effect, this may raise the anisotropic turbulence structure beyond the prediction of eddy-viscosity models. Second, the transverse scalar fluxes do not correspond only to the mean scalar gradients, as would be expected from gradient-diffusion modeling. Third, higher-order velocity-scalar correlations which describe turbulent transport phenomena could not be predicted using simple turbulence models. A second-order closure simulation of turbulent adiabatic wall plumes, taking into account the recent progress in scalar transport, near-wall effect and buoyancy, is reported in the current study to compare with the non-intrusive measurements. In spite of the small velocity scale of the wall plumes, the results showed that low-Reynolds-number correction is not critically important to predict the adiabatic cases tested and cannot be applied beyond the maximum velocity location. The mean and turbulent velocity profiles are very closely predicted by the second-order closure models. but the scalar field is less satisfactory, with the scalar fluctuation level underpredicted. Strong intermittency of the low-Reynolds-number flow field is suspected of these discrepancies. The trends in second- and third-order velocity-scalar correlations, which describe turbulent transport phenomena, are also predicted in general, with the cross-streamwise correlations better than the streamwise one. Buoyancy terms modeling the pressure-correlation are shown to improve the prediction slightly. The effects of equilibrium time-scale ratio and boundary condition are also discussed.

Zhu, Gang↗
NASA NTRS1993

Experimentation in the low-density plume of a simulated electrothermal thruster for computer code validation

Pressures and flow angles are measured in the plume of a 20 deg half-angle, conical nozzle in vacuum with Pitot tubes and conical probes. The area of measurement in the plume ranges from the nozzle exit plane to 480 mm axially downstream and from the plume centerline to 60 mm radially. The nozzle has an exit-to-throat area ratio of 100:1 and a throat diameter of 3.2 mm. The nozzle flow exhausts to a vacuum of order 10(exp -2) Pa to simulate a resistojet (an electrothermal rocket of less than 1 N of thrust) operating in space. Experimental data are given for flows of nitrogen at 55 and 68 mg/s, stagnation temperatures between 695 and 921 K, and stagnation pressures ranging from 5600 to 7100 Pa. Data are also given for argon at a rate of 68 mg/s, a stagnation temperature of 648 K, and stagnation pressures of 4500, 4750, and 4770 Pa. Measurements in the nitrogen plume are compared with computational results from a direct-simulation Monte Carlo method.

Meissner, Dana L.↗
NASA NTRS1993

Measurement and analysis of a small nozzle plume in vacuum

Pitot pressures and flow angles are measured in the plume of a nozzle flowing nitrogen and exhausting to a vacuum. Total pressures are measured with Pitot tubes sized for specific regions of the plume and flow angles measured with a conical probe. The measurement area for total pressure extends 480 mm (16 exit diameters) downstream of the nozzle exit plane and radially to 60 mm (1.9 exit diameters) off the plume axis. The measurement area for flow angle extends to 160 mm (5 exit diameters) downstream and radially to 60 mm. The measurements are compared to results from a numerical simulation of the flow that is based on kinetic theory and uses the direct-simulation Monte Carlo (DSMC) method. Comparisons of computed results from the DSMC method with measurements of flow angle display good agreement in the far-field of the plume and improve with increasing distance from the exit plane. Pitot pressures computed from the DSMC method are in reasonably good agreement with experimental results over the entire measurement area.

Penko, P. F.↗
NASA NTRS1992

Free tropospheric ozone production following entrainment of urban plumes into deep convection

It is shown that rapid vertical transport of air from urban plumes through deep convective clouds can cause substantial enhancement of the rate of O3 production in the free troposphere. Simulation of convective redistribution and subsequent photochemistry of an urban plume from Oklahoma City during the 1985 PRESTORM campaign shows enhancement of O3 production in the free tropospheric cloud outflow layer by a factor of almost 4. In contrast, simulation of convective transport of an urban plume from Manaus, Brazil, into a prestine free troposphere during GTE/ABLE 2B (1987), followed by a photochemical simulation, showed enhancement of O3 production by a factor of 35. The reasons for the different enhancements are (1) intensity of cloud vertical motion; (2) initial boundary layer O3 precursor concentrations; and (3) initial amount of background free tropospheric NO(x). Convective transport of ozone precursors to the middle and upper troposphere allows the resulting O3 to spread over large geographic regions, rather than being confined to the lower troposphere where loss processes are much more rapid. Conversely, as air with lower NO descends and replaces more polluted air, there is greater O3 production efficiency per molecule of NO in the boundary layer following convective transport. As a result, over 30 percent more ozone could be produced in the entire tropospheric column in the first 24 hours following convective transport of urban plumes.

Pickering, Kenneth E.↗
NASA NTRS1993

Structure of axisymmetric mantle plumes

The structure of axisymmetric subsolidus thermal plumes in the earth's lower mantle is inferred from calculations of axisymmetric thermal plumes in an infinite Prandtl number fluid with thermally activated viscosity. The velocity and temperature distribution is determined for axisymmetric convection above a heated disk in an incompressible fluid cylinder 2,400 km in height and 1,200 km in diameter. Several calculations of plumes with heat transport in the range 100-400 GW, similar to the advective heat transport at the Hawaiian hotspot, are presented. Hotspot formation by plumes originating at the base of the mantle requires both large viscosity variations and a minimum heat transport.

Olson, Peter↗
NASA NTRS1993

Rocket plume base heating methodology

A review of radiative transport calculation methods for base heating is presented followed by a description of the current methodology for the Space Shuttle plume radiation predictions and improvements for the Advanced Solid Rocket Booster (ASRB). The calculation methods include empirical methods, the SIRRM code and the forward and reverse Monte Carlo methods. Current plume radiation methods include those used for the Space Shuttle Main Engines and the Solid Rocket Booster (SRB). Methods being developed for the ASRB include changes in plume property prediction methodology and application of the reverse Monte Carlo method in predicting plume radiation models. Results of the prediction methods are compared with experimental measurements on the current SRB and on 1/6-scale motors using both SRB and ASRB propellants. Examples are also presented demonstrating the statistical results available with the reverse Monte Carlo method.

Reardon, John E.↗
NASA NTRS1993

Contamination control and plume assessment of low-energy thrusters

A method for evaluating the effects of gases originating from plumes or other direct sources impinging on sensitive surfaces located within their field of view is presented. The method is applied to an Xe ion plasma generator with a thrust of a few mN located near a cryogenic surface. The method is based on experimentally derived plume descriptions of emitted low-pressure vented gases and test results of plume distributions from a similar ion engine. It is concluded that the plume of the Xe neutrals represented by a cosine squared function of the angle is a possible contamination source for certain surfaces of the spacecraft.

Scialdone, John J.↗
NASA NTRS1993

Stratospheric aircraft exhaust plume and wake chemistry

Progress to date in an ongoing study to analyze and model emissions leaving a proposed High Speed Civil Transport (HSCT) from when the exhaust gases leave the engine until they are deposited at atmospheric scales in the stratosphere is documented. A kinetic condensation model was implemented to predict heterogeneous condensation in the plume regime behind an HSCT flying in the lower stratosphere. Simulations were performed to illustrate the parametric dependence of contrail droplet growth on the exhaust condensation nuclei number density and size distribution. Model results indicate that the condensation of water vapor is strongly dependent on the number density of activated CN. Incorporation of estimates for dilution factors into a Lagrangian box model of the far-wake regime with scale-dependent diffusion indicates negligible decrease in ozone and enhancement of water concentrations of 6-13 times background, which decrease rapidly over 1-3 days. Radiative calculations indicate a net differential cooling rate of the plume about 3K/day at the beginning of the wake regime, with a total subsidence ranging between 0.4 and 1 km. Results from the Lagrangian plume model were used to estimate the effect of repeated superposition of aircraft plumes on the concentrations of water and NO(y) along a flight corridor. Results of laboratory studies of heterogeneous chemistry are also described. Kinetics of HCl, N2O5 and ClONO2 uptake on liquid sulfuric acid were measured as a function of composition and temperature. Refined measurements of the thermodynamics of nitric acid hydrates indicate that metastable dihydrate may play a role in the nucleation of more stable trihydrates PSC's.

Miake-Lye, R. C.↗
NASA NTRS1991

The 1991 version of the plume impingement computer program. Volume 1: Description

The objective of this contract was to continue development of a vacuum plume impingement evaluator to provide an analyst with a capability for rapid assessment of thruster plume impingement scenarios. The research was divided into three areas: Plume Impingement Computer Program (PLIMP) modification/validation; graphics development; and documentation in the form of a Plume Handbook and PLIMP Input Guide.

Bender, Robert L.↗
NASA NTRS1993

On the relationship between tectonic plates and thermal mantle plume morphology

Models incorporating plate-like behavior, i.e., near uniform surface velocity and deformation concentrated at plate boundaries, into a convective system, heated by a mix of internal and basal heating and allowing for temperature dependent viscosity, were constructed and compared to similar models not possessing plate-like behavior. The simplified numerical models are used to explore how plate-like behavior in a convective system can effect the lower boundary layer from which thermal plumes form. A principal conclusion is that plate-like behavior can significantly increase the temperature drop across the lower thermal boundary layer. This temperature drop affects the morphology of plumes by determining the viscosity drop across the boundary layer. Model results suggest that plumes on planets possessing plate-like behavior, e.g., the Earth, may differ in morphologic type from plumes on planets not possessing plate-like behavior, e.g., Venus and Mars.

Lenardic, A.↗