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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Effect of Velocity in Icing Scaling Tests

This paper presents additional results of a study first published in 1999 to determine the effect of scale velocity on scaled icing test results. Reference tests were made with a 53.3-cm-chord NACA 0012 airfoil model in the NASA Glenn Icing Research Tunnel at an airspeed of 67 m/s, an MVD of 40 microns, and an LWC of 0.6 g/cu m. Temperature was varied to provide nominal freezing fractions of 0.8, 0.6, and 0.5. Scale tests used both 35.6- and 27.7-cm-chord 0012 models for 2/3- and 1/2-size scaling. Scale test conditions were found using the modified Ruff (AEDC) scaling method with the scale velocity determined in five ways. Four of the scale velocities were found by matching the scale and reference values of water-film thickness, velocity, Weber number, and Reynolds number. The fifth scale velocity was simply the average of those found by matching the Weber and Reynolds numbers. The resulting scale velocities ranged from 85 to 220 percent of the reference velocity. For a freezing fraction of 0.8, the value of the scale velocity had no effect on how well the scale ice shape simulated the reference shape. For nominal freezing fractions of 0.5 and 0.6, the best simulation of the reference shape was achieved when the scale velocity was the average of the constant-Weber-number and the constant-Reynolds-number velocities.

Anderson, David N.↗
NASA NTRS2006

Intracavity Rayleigh/Mie Scattering for Multipoint, Two-Component Velocity Measurement

A simultaneous multi-point two-component Doppler velocimeter is described. The system uses two optical cavities: a Fabry-Perot etalon and an optical cavity for collecting and re-circulating the Rayleigh/Mie scattered light that is collected from the measurement volume in two parallel, but opposite directions. Single-pulse measurements of two orthogonal components of the velocity vector in a supersonic free jet were performed to demonstrate the technique. The re-circulation of the light rejected by the interferometer input mirror also increased the signal intensity by a factor of 3.5. 2005 Optical Society of America Interferometric Rayleigh scattering has previously been used for single-point velocity measurements in unseeded gas flow. However, this past work has generally been limited to probing with continuous-wave lasers resulting in time-averaged measurements of velocity. Multiple velocity components have been measured simultaneously by separate instruments.1,2 It has also been demonstrated that two orthogonal velocity components can be measured simultaneously at one point using one interferometer by reflecting back the probing laser beam, although this approach results in directional ambiguity of the flow velocity vector.3 This measurement ambiguity was removed by prior knowledge of the approximate magnitude and sign of the velocity components. Furthermore, it was shown that multiple points could be measured simultaneously with a Rayleigh scattering interferometric approach, but only one component of velocity was measured.4 Another method of performing multiple component velocity measurements with Rayleigh scattering uses a pair of cameras to image the flow, one of which views the flow through an iodine gas filter. This iodine-filter technique has the advantage of allowing high-resolution velocity imaging, but it generally has a lower dynamic range.

Bivolaru, Daniel↗
NASA NTRSDate not supplied

Analysis of Supercooled Large Drop Velocity Measurement in the NASA Icing Research Tunnel

An experiment was conducted in the Icing Research Tunnel (IRT) at the NASA Glenn Research Center to measure the velocity of supercooled large drops (SLD) in the test section of the tunnel. Previous experiments in the IRT suggested that supercooled large drops passing through the test section of the tunnel do not move at the same velocity as the surrounding air flow. The difference between drop velocity and tunnel air velocity is called slip velocity. The slip velocity is important for determining the exact nature of SLD icing simulation in the IRT. It can impact the ice growth process because of its effect on drop cooling rate during transit from the spray bars to the test section. It can also affect the amount of splashing that occurs upon impact. Slip velocity is an important flow parameter to determine how far the current facility capabilities can be extended into the SLD regime. Initial measurement data analysis of the drop velocities indicates that drops with diameter larger than about 100 to 200 µm experience velocity slip.

SLD↗
NASA NTRSDate not supplied

Characterization of Large Drop Velocity in the NASA Icing Research Tunnel

This paper presents experimental work conducted in the Icing Research Tunnel at NASA Glenn Research Center to characterize the velocity of large drops in the test section. Some icing spray clouds with large drops were generated with Mod1 nozzles at low nozzle air pressure of 2 to 4 psig for various tunnel air speeds. Drop diameters and drop velocities were measured via high-resolution imaging with a Particle Imaging Particle Tracking Velocimetry probe developed by Artium Technologies. The probe was mounted at four different locations aligned with the centerline of the test section from near the end of the contraction to the constant height test section part of the tunnel. CFD analyses were performed. It showed that the probe head geometry affects the local air flow in the measurement area of the probe between the prongs and in front of the probe. Initial analysis of the air velocity data during the test also indicated that the probe mounting stand has blockage effect on the local tunnel air velocity measurement by a pitot-static probe affixed on the mounting plate next to the stand. Those findings were later verified in the Icing Research Tunnel using a new pitot-static probe design with a linear motion system to measure the local tunnel air velocity with and without the probe. As a result, additional drop trajectory simulations were performed with the airflow moving towards the probe head with a Langmuir-D 7-bin distribution cloud containing large drops. The simulation results helped identify a critical drop-size threshold of 300 μm above which the velocities of larger drops are negligibly affected by the adverse pressure gradient generated by the probe head due to their large drop inertia. From the dimensional analysis of the drop velocity measurement data obtained, it showed that at the tunnel test section reference location a generalized empirical correlation was developed for the non-dimensional drop velocity as a function of the non-dimensional corrected drop diameter independent of the actual spraybar pressure settings and the tunnel air speeds. The generalized curve-fit correlation showed that the drop velocity was universally asymptotic to about 86 percent of the corresponding tunnel air speed at the test section reference location for the largest drop diameter captured by the probe. Further evaluation of this correlation is recommended to assess its applicability for Supercooled Large Drop icing scaling applications in the Icing Research Tunnel.

SLD↗
NASA NTRSDate not supplied

Characterization of Large Drop Velocity in the NASA Icing Research Tunnel

This presentation presents experimental work conducted in the Icing Research Tunnel at NASA Glenn Research Center to characterize the velocity of large drops in the tunnel test section. Some icing spray clouds with large-sized drops were generated with Mod1 nozzles at low nozzle air pressure of 2 to 4 psig for various tunnel air speeds. Drop diameters and drop velocities were measured via high-resolution imaging with a Particle Imaging Particle Tracking Velocimetry probe developed by Artium Technologies. The probe was mounted at four different locations aligned with the centerline of the test section from near the end of the contraction to the constant height test section part of the tunnel. CFD analyses were performed. It showed that the probe head geometry affects the local air flow between the prongs and in front of the probe. Initial analysis of the air velocity data during the test also indicated that the probe mounting stand has blockage effect on the local tunnel air velocity measurement by a Pitot static probe affixed on the mounting plate. Those findings were later verified in the Icing Research Tunnel using a new Pitot probe design with a linear motion system to measure the local tunnel air velocity with and without the probe. As a result, additional drop trajectory simulations as airflow moving towards the probe head were run for a Langmuir-D 7-bin drop size distribution of a spray cloud with a nominally large value of medium volumetric diameter. The simulation results helped identify a critical drop-size threshold of 300 µm above which the velocities of larger drops are not affected by the adverse pressure gradient generated by the probe head due to their large drop inertia. From the dimensional analysis of the drop velocity measurement data obtained, it showed that at the tunnel test section reference location a generalized empirical correlation was developed for the non-dimensional drop velocity as a function of the non-dimensional corrected drop diameter independent of the actual spraybar pressure settings and the tunnel air speeds. The generalized curve-fit correlation showed that the drop velocity was universally asymptotic to about 86 percent of the corresponding tunnel air speed at the test section reference location for the largest drop diameter captured by the probe. Further evaluation of this correlation is recommended to assess its applicability for Supercooled Large Drop icing scaling applications in the Icing Research Tunnel.

SLD↗
DOE OSTI2026

Integration of the Biot–Gassmann Fluid Substitution Method and Machine Learning-Based Velocity–Stress Relationship for Estimating In Situ Stresses

Recent advancements have shown that in situ stresses can be reliably estimated through an integrated machine/deep learning (ML/DL)-based framework, which relies on models trained and validated using true triaxial ultrasonic velocity (TUV) experimental data that involve measurements of ultrasonic velocity in saturated rocks under varying stress configurations. However, when the goal is to interpret lower frequency measurements, it may be more appropriate to run experiments on dry rocks and then obtain Biot–Gassmann-derived equivalent saturated velocities (low-frequency approximation) and employ these quantities for training ML/DL models to predict in situ stress. Whether the dispersion effect of frequency on the velocity–stress relationship substantially impacts in situ stress prediction is an important and unresolved question. This work presents an enhancement of ML/DL-based workflow by training and implementing ML/DL models using equivalent saturated acoustic velocities (low-frequency) obtained by applying Biot–Gassmann fluid substitution on the ultrasonic velocities of dry cores. The models were trained on TUV data sets derived from three subsurface cores extracted from the geothermal well 16B(78)-32 at the Utah FORGE site. Each core was subjected to 75 unique stress configurations for velocity measurement in the dry state. The ML/DL trained on the TUV data set with equivalent saturated velocities demonstrated promising performance to predict in situ stress in subsurface geological rocks using velocity–stress relationships with R 2 of 0.86, 0.971, and 0.975 and root mean squared error (RMSE) of 2.59, 1.92, and 1.80 for validation/testing phases of vertical, minimum horizontal, and maximum horizontal stress models, respectively. Additionally, interpretation and explanation by Shapley additive explanations (SHAP) analysis further improved scientific validation and model reliability for estimating in situ stresses.

colloids↗
DOE OSTI2022

Velocity dispersions of clusters in the Dark Energy Survey Y3 redMaPPer catalogue

ABSTRACT We measure the velocity dispersions of clusters of galaxies selected by the red-sequence Matched-filter Probabilistic Percolation (redMaPPer) algorithm in the first three years of data from the Dark Energy Survey (DES), allowing us to probe cluster selection and richness estimation, λ, in light of cluster dynamics. Our sample consists of 126 clusters with sufficient spectroscopy for individual velocity dispersion estimates. We examine the correlations between cluster velocity dispersion, richness, X-ray temperature, and luminosity, as well as central galaxy velocity offsets. The velocity dispersion–richness relation exhibits a bimodal distribution. The majority of clusters follow scaling relations between velocity dispersion, richness, and X-ray properties similar to those found for previous samples; however, there is a significant population of clusters with velocity dispersions that are high for their richness. These clusters account for roughly 22 per cent of the λ < 70 systems in our sample, but more than half (55 per cent) of λ < 70 clusters at z > 0.5. A couple of these systems are hot and X-ray bright as expected for massive clusters with richnesses that appear to have been underestimated, but most appear to have high velocity dispersions for their X-ray properties likely due to line-of-sight structure. These results suggest that projection effects contribute significantly to redMaPPer selection, particularly at higher redshifts and lower richnesses. The redMaPPer determined richnesses for the velocity dispersion outliers are consistent with their X-ray properties, but several are X-ray undetected and deeper data are needed to understand their nature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Attached Jet Velocity Profiles in Mixing Tanks

Free jets have been studied in detail over much of the last century, but the theory for offset and attached jets remains incomplete. Attached jets differ from free jets in that they lose momentum to nearby surfaces, attenuating their velocities. The velocity profiles of free circular jets are nearly Gaussian, with quantitative mathematical descriptions derived from first principles by Goertler and Tollmien (Rajaratnam, 1976). In contrast, mathematical descriptions of three-dimensional attached jets from circular nozzles remain much less mature. Agelin-Chaab and Tachie (2011) used particle imaging velocimetry of a three-dimensional attached jet to show that the scaled velocity decays with scaled distance from the nozzle with a power law exponent between -1.15 and -1.21, which is larger in magnitude than that of a free jet. However, quantitative analytical expressions for the velocity profiles of attached jets similar to those of free jets remain elusive. This paper addresses this critical gap. Here we evaluate the velocity profiles of three-dimensional offset jets emerging from circular nozzles that that become attached jets. These jets lose momentum due to interactions with nearby surfaces and are important to evaluating flows in mixing vessels and to suspending solids and trapped gases in radioactive waste tanks. Despite the importance of attached jets, prior insight has been purely experimental, limited to overly simplistic analytical models, or restricted to computationally expensive computational fluid dynamics case studies. We compare the expression of Verhoff (1963) to experimental results to find reasonable quantitative agreement. As stated by Agelin-Chaab and Tachie (2011), “detailed velocity measurements of 3D offset jets are rare.” Such remains the case. This study adds to the literature by providing information at two additional Reynolds numbers (1.43 106 and 1.87 106) and evaluating simple but accurate expressions for velocity profiles. These Reynolds numbers are an order of magnitude higher than prior reports. The semi-empirical stream wise velocity profile perpendicular to the surface proposed by Verhoff (1963) is in approximate agreement with these velocity profiles, which is surprising because these attached jets are three-dimensional instead of two-dimensional as evaluated by Verhoff. However, additional work is necessary to fully describe these profiles quantitatively.

jet, attached jet, offset jet↗
DOE OSTI2020

EXPERIMENTAL EVALUATION OF ULTRASONIC VELOCITIES AND ANISOTROPY IN THE TUSCALOOSA MARINE SHALE FORMATION

Tuscaloosa Marine Shale (TMS) formation is a clay- and organic-rich emerging shale play with a considerable amount of hydrocarbon resources. Despite the substantial potential, there have been only a few wells drilled and produced in the formation over the recent years. The analyzed TMS samples contain an average of 50 wt% total clay, 27 wt% quartz and 14 wt% calcite and the mineralogy varies considerably over the small intervals. The high amount of clay leads to pronounced anisotropy and the frequent changes in mineralogy result in the heterogeneity of the formation. We studied the compressional (V P ) and shear-wave (V S ) velocities to evaluate the degree of anisotropy and heterogeneity, which impact hydraulic fracture growth, borehole instabilities, and subsurface imaging. The ultrasonic measurements of P- and S-wave velocities from five TMS wells are the best fit to the linear relationship with R 2 = 0.84 in the least-squares criteria. We observed that TMS S-wave velocities are relatively lower when compared to the established velocity relationships. Most of the velocity data in bedding-normal direction lie outside constant V P /V S lines of 1.6–1.8, a region typical of most organic-rich shale plays. For all of the studied TMS samples, the S-wave velocity anisotropy exhibits higher values than P-wave velocity anisotropy. In the samples in which the composition is dominated by either calcite or quartz minerals, mineralogy controls the velocities and V P /V S ratios to a great extent. Additionally, the organic content and maturity account for the velocity behavior in the samples in which the mineralogical composition fails to do so. The results provide further insights into TMS Formation evaluation and contribute to a better understanding of the heterogeneity and anisotropy of the play.

Geochemistry & Geophysics↗
DOE OSTI2021

Using Dense Array Waveform Correlations to Build a Velocity Model with Stochastic Variability

In an ongoing effort to improve 3D seismic-wave propagation modeling for frequencies up to 10 Hz, we used cross correlations between vertical-component waveforms from an underground chemical explosion to estimate the statistical properties of small-scale velocity heterogeneities. The waveforms were recorded by a dense 2D seismic array deployed during the Source Physics Experiments for event number 5 (SPE-5) in a series of six underground chemical explosions, conducted at the Nevada National Security Site. The array consisted of 996 geophones with a 50–100 m grid spacing, deployed at the SPE site at the north end of the Yucca Flat basin. The SPE were conducted to investigate the generation and propagation of seismic and acoustic waves from underground explosions. Comparisons of decay rates of waveform cross correlations as function of interstation distance, computed for observed and synthetic seismograms from the SPE-5 chemical explosion, were used to constrain statistical properties of correlated stochastic velocity perturbations representing small-scale heterogeneities added to a geology-based velocity model of the Yucca Flat basin. Using comparisons between recorded and simulated waveform cross correlations, we were able to recover sets of statistical properties of small-scale velocity perturbations in the velocity model that produce the best-fit between the recorded and simulated ground motion. The stochastic velocity fluctuations in the velocity model that produced the smallest misfits have a horizontal correlation distance of between 400 and 800 m, a vertical correlation distance between 100 and 200 m, and a standard deviation of 10% from the nominal model velocity in the alluvium basin layers. They also have a horizontal correlation distance of 1000 m, a vertical correlation distance of 250 m, and a standard deviation of 6% in the underlying and consolidated sedimentary layers, up to a depth of 4 km. Comparisons between observed and simulated wavefields were used to assess the proposed small-scale heterogeneity enhancements to the Yucca Flat basin model. We found that adding a depth-resolved stochastic variability to the geology-based velocity model improves the overall performance of ground-motion simulations of an SPE-5 explosion in the modeled frequency range up to 10 Hz. The results may be applicable to other similar basins.

58 GEOSCIENCES↗
DOE OSTI2023

On the Origins of Extreme Velocity Stars as Revealed by Large-scale Galactic Surveys

We assemble a large sample of 12,784 high-velocity stars with total velocity V GSR ≥ 300 km s -1 , selected from RAVE DR5, SDSS DR12, LAMOST DR8, APOGEE DR16, GALAH DR2, and Gaia EDR3. In this sample, 52 are marginally hypervelocity star (HVS) candidates that have VGSR exceeding their local escape velocities within 2σ confidence levels, 40 of which are discovered for the first time. All of the candidates are metal-poor, late-type halo stars, which are significantly different from the previously identified HVSs, which are largely massive early-type stars, discovered by extreme radial velocity. This finding suggests that our newly identified HVS candidates are ejected by different mechanisms from the previous population. To investigate their origins, for 547 extreme velocity stars with V GSR ≥ 0.8V esc , we reconstruct their backward-integrated trajectories in the Galactic potential. According to the orbital analysis, no candidates are found to be definitely ejected from the Galactic-center (GC), while eight metal-poor extreme velocity stars are found to have a closest distance to the GC within 1 kpc. Intriguingly, 15 extreme velocity stars (including 2 HVS candidates) are found to have experienced close encounters with the Sagittarius dSph, suggesting that they originated from this dSph. This hypothesis is supported by an analysis of the [α/Fe]–[Fe/H] diagram. From a preliminary analysis of all of the 547 extreme velocity stars, we propose a general picture–star ejection from Galactic subsystems such as dwarf galaxies and globular clusters can be an important channel to produce extreme velocity stars or even HVSs, particularly the metal-poor late-type halo population.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

BASS. XXVI. DR2 Host Galaxy Stellar Velocity Dispersions

We present new central stellar velocity dispersions for 484 Sy 1.9 and Sy 2 from the second data release of the Swift/BAT AGN Spectroscopic Survey (BASS DR2). This constitutes the largest study of velocity dispersion measurements in X-ray-selected obscured active galactic nuclei (AGN) with 956 independent measurements of the Ca II H and K λ3969, 3934 and Mg I λ5175 region (3880–5550 Å) and the calcium triplet region (8350–8730 Å) from 642 spectra mainly from VLT/X-Shooter or Palomar/DoubleSpec. Our sample spans velocity dispersions of 40–360 km s 1 , corresponding to 4–5 orders of magnitude in black hole mass (M BH = 10 5.5-9.6 M ⊙ ), bolometric luminosity (L bol ~ 10 42–46 erg s -1 ), and Eddington ratio (L/L Edd ~ 10 -5 to 2). For 281 AGN, our data and analysis provide the first published central velocity dispersions, including six AGN with low-mass black holes (M BH = 10 5.5-6.5 M ⊙ ), discovered thanks to high spectral resolution observations (σ inst ~ 25 km s -1 ). The survey represents a significant advance with a nearly complete census of velocity dispersions of hard X-ray–selected obscured AGN with measurements for 99% of nearby AGN (z < 0.1) outside the Galactic plane (|b| > 10°). The BASS AGN have much higher velocity dispersions than the more numerous optically selected narrow-line AGN (i.e., ~150 versus ~100 km s -1 ) but are not biased toward the highest velocity dispersions of massive ellipticals (i.e., >250 km s -1 ). Despite sufficient spectral resolution to resolve the velocity dispersions associated with the bulges of small black holes (~10 4–5 M ⊙ ), we do not find a significant population of super-Eddington AGN. Using estimates of the black hole sphere of influence from velocity dispersion, direct stellar and gas black hole mass measurements could be obtained with existing facilities for more than ~100 BASS AGN.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1974

Radial gradient of solar wind velocity from 1 to 5 AU

Solar wind velocities have been measured on a daily basis from data obtained by the Ames Research Center plasma analyzers on both Pioneer 10 and Pioneer 11. A comparison between the time profiles of the solar wind velocities observed at the two spacecraft shows that the solar wind has the same major features, such as high velocity streams, out to at least 5 astronomical units (AU) from the sun. Major features in the velocity time profile observed first at Pioneer 11 are seen later at Pioneer 10 with a delay consistent with the respective heliocentric longitudes of the two spacecraft, their radial distances from the sun, and the solar wind velocity. A more detailed comparison between the velocity measurements made at Pioneer 10 and Pioneer 11 shows that the range of solar wind velocities decreases with increasing radial distance from the sun. Although the average value of the solar wind velocity as measured over a sufficiently long period is approximately the same at both spacecraft, the deviations to higher and lower velocities are less at a greater radial distance from the sun.

Collard, H. R.↗
NASA NTRS1975

Lunar shear velocity structure at Apollo sites 12, 14, and 15

Spectral amplitude ratios of horizontal-to-vertical motion produced on seismograms of meteoroid impacts at the Apollo 12, 14, and 15 sites, the Apollo 14 and 15 lunar-module impacts, and the Apollo 15 S4B impact show consistent differences among the recording sites. On the assumption that the motion represents predominantly fundamental-mode Reyleigh waves and that the compressional-wave velocity structure is similar to that derived in other investigations involving the Apollo seismic experiments, estimates are made of the shear-wave velocity structure under the three sites. Near-surface velocities are about 35 m/s at the three sites. The results for site 14 indicate an increase to about 100 m/s near the 8-m depth and to 200 m/s at the 38-m depth. Results for sites 12 and 15 show a smoother gradient and generally a greater velocity at a given depth than that indicated at site 14, reaching velocities of about 400 m/s near the 120-m depth. If the assumed P velocity structures are correct and if changes in S velocity coincide with changes in P velocity, then the ratio of these velocities decreases from 2.9 to 2.0 in the upper 19 meters at site 12, in the upper 38 meters at site 14, and in the upper 21 meters at site 15.

Mark, N.↗
NASA NTRS1981

Thermal iron ions in high speed solar wind streams. II - Temperatures and bulk velocities

Mitchel and Roelof (1980) reported the detection of iron in high speed solar wind flows using the small, but finite sensitivity of solid state detectors to Fe ions in the low energy (50-200 keV protons) L1 channel of the NOAA/JHU energetic particle experiment (EPE). In the current investigation, the EPE response is modeled to a convected Maxwellian to obtain the thermal velocity, flow angle, and bulk velocity of the iron distribution. It is assumed that the iron bulk flow velocity can be represented as a vector sum of the hydrogen bulk velocity and an interplanetary magnetic field (IMF) aligned velocity increment. It is found that the velocity increment is smaller than the local Alfven speed in magnitude, and that the iron thermal velocity is comparable with or greater than the proton thermal velocity, with the 'thermal' velocity defined as the square root of 2kT/m.

Mitchell, D. G.↗
NASA NTRS1983

Impact and explosion crater ejecta, fragment size, and velocity

A model was developed for the mass distribution of fragments that are ejected at a given velocity for impact and explosion craters. The model is semi-empirical in nature and is derived from (1) numerical calculations of cratering and the resultant mass versus ejection velocity, (2) observed ejecta blanket particle size distributions, (3) an empirical relationship between maximum ejecta fragment size and crater diameter and an assumption on the functional form for the distribution of fragements ejected at a given velocity. This model implies that for planetary impacts into competent rock, the distribution of fragments ejected at a given velocity are nearly monodisperse, e.g., 20% of the mass of the ejecta at a given velocity contain fragments having a mass less than 0.1 times a mass of the largest fragment moving at that velocity. Using this model, the largest fragment that can be ejected from asteroids, the moon, Mars, and Earth is calculated as a function of crater diameter. In addition, the internal energy of ejecta versus ejecta velocity is found. The internal energy of fragments having velocities exceeding the escape velocity of the moon will exceed the energy required for incipient melting for solid silicates and thus, constrains the maximum ejected solid fragment size.

Okeefe, J. D.↗
NASA NTRS1985

The manifestation of Alfven's hypothesis of critical ionization velocity in the performance of MPD thrusters

The role of Alfven's critical ionization velocity in the performance of the self-field MPD thruster has been investigated. The existence of a well defined characteristic velocity can be attributed to an ionization process involving the production of a population of suprathermal electrons by an electrostatic instability. It is shown that for the MPD thruster plasma, suprathermalization of electrons via this electrostatic instability can only happen if ions are initially accelerated to velocities larger than the Alfven critical ionization velocity. When this occurs the mechanism will be initiated and the ions decelerated to velocities near the critical velocity. This mechanism ceases to be limiting when all neutrals are ionized. A model of MPD thruster terminal behavior, incorporating Alfven's hypothesis, is presented. Experiments with three different propellants reveal that operation at values of the current squared to total mass flow ratio corresponding to the Alfven critical velocity is marked by a transition wherein low frequency voltage oscillations and a notable change in the voltage-current dependence occurs. One major result of this study is the demonstration that the Alfven critical velocity is not a fundamental limitation on MPD exhaust velocity.

Choueiri, E. Y.↗
NASA NTRS1989

Computing induced velocity perturbations due to a helicopter fuselage in a free stream

The velocity field of a representative helicopter fuselage in a free stream is computed. Perturbation velocities due to the fuselage are computed in a plan above the location of the helicopter rotor (rotor removed). The velocity perturbations computed by a source-panel model of the fuselage are compared with experimental measurements taken with a laser velocimeter. Three paneled fuselage models are studied: fuselage shape, fuselage shape with hub shape, and a body of revolution. The velocity perturbations computed for both fuselage shape models agree well with the measured velocity field except in the close vicinity of the rotor hub. In the hub region, without knowing the extent of separation, modeling of the effective source shape is difficult. The effects of the fuselage perturbations are not well-predicted with a simplified ellipsoid fuselage. The velocity perturbations due to the fuselage at the plane of the measurements have magnitudes of less than 8 percent of free-stream velocity. The velocity perturbations computed by the panel method are tabulated for the same locations at which previously reported rotor-inflow velocity measurements were made.

Berry, John D.↗