Search NASA⌕ Search

SEARCH · Search NASA

Results for “stroke”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Lightning Return-Stroke Current Waveforms Aloft, from Measured Field Change, Current, and Channel Geometry

Three-dimensional reconstructions of six rocket-triggered lightning channels are derived from stereo photographs. These reconstructed channels are used to infer the behavior of the current in return strokes above the ground from current waveforms measured at the channel base and electric-field-change waveforms measured at a range of 5.2 kilometers for 24 return strokes in these channels. Streak photographs of 14 of the same strokes are analyzed to determine the rise times, propagation speeds, and amplitudes of relative light intensity for comparison with the electrical inferences. Results include the following: 1) The fine structure of the field-change waveforms that were radiated by these subsequent return strokes can be explained, in large part, by channel geometry. 2) The average 10 - 90% rise time of the stroke current increased by about a factor of seven in our sample, from an observed 0.31 plus or minus 0.17 microseconds at the surface to an inferred 2.2 plus or minus 0.5 microcseconds at 1 kilometer path length above the surface. 3) The three-dimensional propagation speed of the current front averaged 1.80 plus or minus 0.24 X 10(exp 8) meters per second over channel lengths typically greater than 1 kilometer. 4) Assuming that the measured current was entirely due to the return stroke forced an unreasonably large and abrupt reduction in inferred current amplitude over the first few tens of meters above the surface, especially in cases when the leader was bright relative to its stroke. Therefore, a significant fraction of the current at the surface was probably due to the leader, at least in such cases. 5) Peak return-stroke currents decreased by approximately 37 plus or minus 12% from 100 meters to 1 kilometer of path length above the surface. Because of uncertainty about how to partition the measured current between leader and return stroke, we are unable to infer the variation of current amplitude near the ground.

Willett, J. C.↗
DOE OSTI2016

Risk of Symptomatic Stroke After Radiation Therapy for Childhood Cancer: A Long-Term Follow-Up Cohort Analysis

Purpose: Long-term childhood cancer survivors are at high risk of late adverse effects, including stroke. We aimed to determine the cumulative incidence of clinically validated symptomatic stroke (transient ischemic attack [TIA], cerebral infarction, and intracerebral hemorrhage [ICH]) and to quantify dose-effect relationships for cranial radiation therapy (CRT) and supradiaphragmatic radiation therapy (SDRT). Methods and Materials: Our single-center study cohort included 1362 survivors of childhood cancer that were diagnosed between 1966 and 1996. Prescribed CRT and SDRT doses were converted into the equivalent dose in 2-Gy fractions (EQD{sub 2}). Multivariate Cox regression models were used to analyze the relationship between the EQD{sub 2} and stroke. Results: After a median latency time of 24.9 years and at a median age of 31.2 years, 28 survivors had experienced a first stroke: TIA (n=5), infarction (n=13), and ICH (n=10). At an attained age of 45 years, the estimated cumulative incidences, with death as competing risk, among survivors treated with CRT only, SDRT only, both CRT and SDRT, and neither CRT nor SDRT were, respectively, 10.0% (95% confidence interval [CI], 2.5%-17.0%), 5.4% (95% CI, 0%-17.0%), 12.5% (95% CI, 5.5%-18.9%), and 0.1% (95% CI, 0%-0.4%). Radiation at both locations significantly increased the risk of stroke in a dose-dependent manner (hazard ratios: HR{sub CRT} 1.02 Gy{sup −1}; 95% CI, 1.01-1.03, and HR{sub SDRT} 1.04 Gy{sup −1}; 95% CI, 1.02-1.05). Conclusions: Childhood cancer survivors treated with CRT, SDRT, or both have a high stroke risk. One in 8 survivors treated at both locations will have experienced a symptomatic stroke at an attained age of 45 years. Further research on the pathophysiologic processes involved in stroke in this specific group of patients is needed to enable the development of tailored secondary prevention strategies.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2022

An Investigation into the Effects of Swirl on the Performance and Emissions of an Opposed-Piston Two-Stroke Engine using Large Eddy Simulations

Opposed-piston two-stroke (OP-2S) engines have the potential to achieve higher thermal efficiency than a conventional four-stroke diesel engine. However, the uniflow scavenging process is difficult to control over a wider range of speed and loads due to its sensitivity to pressure dynamics, port timings, and port design. Specifically, the angle of the intake ports can be used to generate swirl which has implications for open and closed cycle effects. This study proposes an analysis of the effects of port angle on the in-cylinder flow distribution and combustion performance of an OP-2S using computational fluid dynamics engine. Large Eddy Simulation (LES) was used to model turbulence given its ability to predict in-cylinder mixing and cyclic variability. A three-cylinder model was validated to experimental data collected by Achates Power and the grid was verified using an LES quality approach from the literature. The model was used to simulate port angles from 12 to 29 degrees at constant pressure and temperature boundary conditions. Results indicated that the higher bulk swirl ratio generated by larger port angles tends to trap more internal residuals. This effect on the scavenging performance, combined with the larger trapped swirl ratio, also has a significant impact on the combustion performance in a two-stroke engine. It was concluded that there exists a tradeoff of efficiency and emissions that must be considered when increasing the port angle in a uniflow two-stroke engine. 1. Introduction Two-stroke engines were developed in the 1800s and are commonly used in practice today for lightweight power applications such as motorcycles or handheld power tools, as well as large-bore low speed engines for stationary power and marine applications. Two-stroke engines provide the distinct advantage of higher torque/power output but present challenges due to its coupled intake and exhaust process. Four-stroke engines have the benefit of using two full piston strokes to induct intake charge and to expel exhaust gases. Controlling the trapped charge composition in a twostroke engine via a scavenging process is crucial for reliable and efficient engine operation.

33 ADVANCED PROPULSION SYSTEMS↗
NASA NTRS1977

The temporal structure of HF and VHF radiations during Florida lightning return strokes

Measurements have been made of the HF and VHF radiation at 3, 30, 139, and 295 MHz produced during lightning return strokes, in correlation with wideband electric field measurements covering from about 300 Hz to 2 MHz. Intense pulses of RF are observed during the first return stroke in a discharge to ground, but with a 10-30 µsec time delay between the beginning of the stroke and the occurrence of significant RF radiation. The delay occurs at all frequencies and is short enough to suggest that the main channel and/or branches are the main initial sources of RF during the first stroke. During a subsequent return stroke, the RF starts an average of 265 µsec prior to the onset of the stroke. At 139 and 295 MHz, the subsequent stroke RF lasts only about 115 µsec; however, at 3 MHz, the radiation frequently persists up to and during the stroke.

David M Le Vine↗
NASA NTRS1983

Numerical studies of the formation and destruction of vortices in a motored four-stroke piston-cylinder configuration

A finite-difference procedure which solves the conservation equations of mass, momentum, and energy is used to investigate the effects of the compression ratio, engine speed, bore-to-stroke ratio, and air intake flow angle on the turbulent flow field within an axisymmetric piston-cylinder configuration. It is shown that in a four-stroke piston-cylinder configuration, the intake stroke is characterized by the formation of a piston vortex. The piston vortex is stretched during the intake stroke, and the head vortex has an almost constant diameter. For a 0-deg air intake flow angle, both vortices disappear by the end of the compression stroke; for an air intake flow angle of 45 deg, the flow field within the cylinder shows three elongated vortices which persist into the compression stroke and then break up and merge. It is also shown that larger bore-to-stroke ratios give rise to lower turbulent levels than smaller bore-to-stroke ratios and that the turbulent intensity is almost independent of the rpm.

Schock, H. J.↗
NASA NTRS1993

Two-dimensional velocity, optical risetime, and peak current estimates for natural positive lightning return strokes

Velocities, optical risetimes, and transmission line model peak currents for seven natural positive return strokes are reported. The average 2D positive return stroke velocity for channel segments of less than 500 m in length starting near the base of the channel is 0.8 +/- 0.3 x 10 exp 8 m/s, which is slower than the present corresponding average velocity for natural negative first return strokes of 1.7 +/- 0.7 x 10 exp 8/s. It is inferred that positive stroke peak currents in the literature, which assume the same velocity as negative strokes, are low by a factor of 2. The average 2D positive return stroke velocity for channel segments of greater than 500 m starting near the base of the channel is 0.9 +/- 0.4 x 10 exp 8 m/s. The corresponding average velocity for the present natural negative first strokes is 1.2 +/- 0.6 x 10 exp 8 m/s. No significant velocity change with height is found for positive return strokes.

Mach, Douglas M.↗
DOE OSTI2013

Mechanical Revascularization for Acute Ischemic Stroke: A Single-Center, Retrospective Analysis

BackgroundEndovascular mechanical revascularization (thrombectomy) is an increasingly used method for intracranial large vessel recanalization in acute stroke. The purpose of the study was to analyze the recanalization rate, clinical outcome, and complication rate in our stroke patients treated with mechanical revascularization. A total of 57 patients with large vessel stroke (within 3 h for anterior and 12 h for posterior circulation) were treated with mechanical revascularization at a single center during 24 months. The primary goal of endovascular treatment using different mechanical devices was recanalization of the occluded vessel. Recanalization rate (reported as thrombolysis in cerebral infarction [TICI] score), clinical outcome (reported as National Institutes of Health Stroke Scale [NIHSS] score and modified Rankin scale [mRS] score), as well as periprocedural complications were analyzed. The mean age of the patients was 63.1 {+-} 12.9 years, with baseline median NIHSS score of 14 (interquartile range, 9.5-19). Successful recanalization (TICI 2b or 3) was achieved in 41 (72 %) patients. Twenty patients (35 %) presented with favorable outcome (mRS {<=}2) 30 days after stroke. Overall, significant neurological improvement ({>=}4 NIHSS point reduction) occurred in 36 (63 %) patients. A clinically significant procedure-related adverse events (vessel disruption, peri/postprocedural intracranial bleeding) defined with decline in NIHSS of {>=}4 or death occurred in three (5 %) patients. The study showed a high recanalization rate with improved clinical outcome and a low rate of periprocedural complications in our stroke patients treated with mechanical revascularization. Therefore, we could conclude that endovascular revascularization (primary or in combination with a bridging thrombolysis) was an effective and safe procedure for intracranial large vessel recanalization in acute stroke.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2018

Emerging Trends in Emergent Stroke Neuroimaging

Purpose of Review: Various imaging techniques used in stroke evaluation have complementary, evolving roles. This article reviews recent updates in emergent stroke neuroimaging. Recent Findings: Multiple recent randomized trials showing benefit of endovascular therapy in strokes with confirmed large-vessel occlusion emphasize need for vessel imaging, typically CTA, in addition to unenhanced head CT. In addition, because recent trials published in 2018 showed efficacy of mechanical thrombectomy at up to 24 h from symptom onset in stroke patients with favorable ischemic core and/or penumbra assessments, imaging techniques to define viable and infarcted tissue, such as DWI or CT/MR perfusion imaging, have an increasing role in emergent stroke assessment. Other studies contributing to new AHA/ASA stroke guidelines are also discussed. Summary: Neuroimaging remains crucial to emergent stroke evaluation, with CTA, MRI, and perfusion studies playing an increasing role in identifying patients likely to benefit from treatment.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
NASA NTRS1973

Electric radiation fields of lightning return strokes in three isolated Florida thunderstorms

Peak value and risetime distributions for the electric radiation fields produced by lightning strokes to ground in three isolated Florida thunderstorms are presented. The storm at 195 km produced about 1000 return strokes having an average peak radiation field (normalized to 100 km) of about 5.5 V/m, the storm at 110 km produced about 700 strokes having 2.0 V/m, and the storm within 10 km produced about 100 strokes having 15.0 V/m. For each storm the average peak radiation field for first strokes was only slightly greater than the average peak field for subsequent strokes. For the sample of three storms the average peak radiation field in a given storm was proportional to the number of subsequent strokes per flash in that storm. The field risetimes for the two distant storms have statistical properties which are essentially identical to those reported for distant storms in Pennsylvania, whereas the close Florida storm exhibited significantly faster risetimes.

Lin, Y. T.↗
NASA NTRS1978

The fine structure of lightning return stroke wave forms

The paper describes and analyzes the structure of the radiation fields produced just prior to and during the development of the first and subsequent return strokes in lightning discharges to ground. Stroke data are summarized, and the waveforms of the fields are found to contain several features which are quite alike for different strokes in the same discharge and also for different discharges. The possibility of large currents in the upward connecting discharges, the existence of a large submicrosecond component in all return strokes, and the cause of the small second peak and of the large subsidiary peaks in first strokes are discussed. The similarity of subsequent stroke fields within a flash suggests that the currents and velocities of different subsequent strokes in the same discharge are often very nearly the same.

Weidman, C. D.↗
NASA NTRS1986

Radiation from lightning return strokes over a finitely conducting earth

The effects of the conductivity of the earth on radiation from lightning return strokes are examined theoretically using a piecewise linear transmission line model for the return stroke. First, calculations are made of the electric field radiated during the return stroke, and then this electric field is used to compute the response of conventional AM radio receivers and electric field change systems during the return stroke. The calculations apply to the entire transient waveform (they are not restricted to the initial portions of the return stroke) and yield fast field changes and RF radiation in agreement with measurements made during real lightning. This research was motivated by measurements indicating that a time delay exists between the time of arrival of the fast electric field change and the RF radiation from first return strokes. The time delay is on the order of 20 microsec for frequencies in the HF-UHF range for lightning in Florida. The time delay is obtained theoretically in this paper. It occurs when both the effects of attenuation due to conductivity of the earth, and the finite velocity of propagation of the current pulse up the return stroke channel, are taken into account in the model.

Le Vine, D. M.↗
NASA NTRS2001

Geometrical Effects on the Electromagnetic Radiation from Lightning Return Strokes

The morphological difference between the electromagnetic radiation-field waveforms of "first" and "subsequent" return strokes in cloud-to-ground lightning flashes is well known and can be used to identify the formation of new channels to ground. This difference is generally believed due to the existence of branches on first-stroke channels, whereas subsequent strokes re-illuminate only the main channel of a previous stroke; but experimental evidence for this hypothesis is relatively weak. It has been argued for the influence of channel geometry on the fine structure of radiation from subsequent return strokes by comparing the field-change waveforms recorded at the same station from strokes within the same flash and between different flashes of both natural and triggered lightning. The present paper introduces new evidence for both of these hypotheses from a comparison of waveforms between sensors in different directions from the same stroke.

Willett, J. C.↗
NASA NTRS2008

Miniature High-Force, Long-Stroke SMA Linear Actuators

Improved long-stroke shape-memory-alloy (SMA) linear actuators are being developed to exert significantly higher forces and operate at higher activation temperatures than do prior SMA actuators. In these actuators, long linear strokes are achieved through the principle of displacement multiplication, according to which there are multiple stages, each intermediate stage being connected by straight SMA wire segments to the next stage so that relative motions of stages are additive toward the final stage, which is the output stage. Prior SMA actuators typically include polymer housings or shells, steel or aluminum stages, and polymer pads between successive stages of displacement-multiplication assemblies. Typical output forces of prior SMA actuators range from 10 to 20 N, and typical strokes range from 0.5 to 1.5 cm. An important disadvantage of prior SMA wire actuators is relatively low cycle speed, which is related to actuation temperature as follows: The SMA wires in prior SMA actuators are typically made of a durable nickel/titanium alloy that has a shape-memory activation temperature of 80 C. An SMA wire can be heated quickly from below to above its activation temperature to obtain a stroke in one direction, but must then be allowed to cool to somewhat below its activation temperature (typically, less than or equal to 60 C in the case of an activation temperature of 80 C) to obtain a stroke in the opposite direction (return stroke). At typical ambient temperatures, cooling times are of the order of several seconds. Cooling times thus limit cycle speeds. Wires made of SMA alloys having significantly higher activation temperatures [denoted ultra-high-temperature (UHT) SMA alloys] cool to the required lower return-stroke temperatures more rapidly, making it possible to increase cycle speeds. The present development is motivated by a need, in some applications (especially aeronautical and space-flight applications) for SMA actuators that exert higher forces, operate at greater cycle speeds, and have stronger housings that can withstand greater externally applied forces and impacts. The main novel features of the improved SMA actuators are the following: 1) The ends of the wires are anchored in compact crimps made from short steel tubes. Each wire end is inserted in a tube, the tube is flattened between planar jaws to make the tube grip the wire, the tube is compressed to a slight U-cross-section deformation to strengthen the grip, then the crimp is welded onto one of the actuator stages. The pull strength of a typical crimp is about 125 N -- comparable to the strength of the SMA wire and greater than the typical pull strengths of wire-end anchors in prior SMA actuators. Greater pull strength is one of the keys to achievement of higher actuation force; 2) For greater strength and resistance to impacts, housings are milled from aluminum instead of being made from polymers. Each housing is made from two pieces in a clamshell configuration. The pieces are anodized to reduce sliding friction; 3) Stages are made stronger (to bear greater compression loads without excessive flexing) by making them from steel sheets thicker than those used in prior SMA actuators. The stages contain recessed pockets to accommodate the crimps. Recessing the pockets helps to keep overall dimensions as small as possible; and, 4) UHT SMA wires are used to satisfy the higher-speed/higher-temperature requirement.

Cummin, Mark A.↗
NASA NTRS1993

Patent Foramen Ovale: Is Stroke Due to Paradoxical Embolism?

Background and Purpose: A patent foramen ovale has been reported to be significantly more frequent in young stroke patients than in matched control subjects, and paradoxical embolism has been suggested as the main mechanism of stroke in-this situation. The present study was designed to test this hypothesis. Methods: Sixty-eight consecutive patients under 55 years of age presenting with an ischemic stroke had an extensive workup, including transesophageal echocardiography with contrast. We compared the prevalence of criteria for the diagnosis of paradoxical embolism in patients with and without a patent foramen ovale. Results: A patent foramen ovale was found in 32 patients (47%). A Valsalva-provoking activity was present at stroke onset in six patients with a patent foramen ovale and in eight patients with no patent foramen ovale (X(sup 2)=0.1, nonsignificant). Clinical/radiological features suggestive of an embolic mechanism were not more frequent in patients with a patent foramen ovale. Clinical evidence of deep vein thrombosis was present in one patient with a patent foramen ovale and in none of the others. No occult venous thrombosis was found in a subgroup of patients with a patent foramen ovale and no definite cause for stroke who underwent venography (n=13). Conclusions. Our results do not support the hypothesis that paradoxical embolism is the primary mechanism of stroke in patients with a patent foramen ovale. (Stroke 1993;24:31-34) KEY WORDS e cerebral ischemia e embolism foramen ovale, patent

Ranoux, D.↗
DOE OSTI2023

Characteristics of upward-connecting-leader current leading to attachment in downward negative cloud-to-ground lightning strokes

For this work, we analyzed currents associated with upward connecting leaders (UCLs) initiated from the Kennedy Space Center Industrial Area Tower in two negative cloud-to-ground strokes that struck the tower. One stroke was also recorded using a high-speed video camera located 760 m from the KSC IAT. The NLDN-reported peak currents for the two strokes were – 31.7 and – 98.5 kA. During the UCL development phase the current waveforms exhibited a monotonically (quasi-exponentially) increasing “background” current overlaid with 10-μs scale pulses with a median amplitude of 51.1 A. The UCL current durations for the two strokes were 1039 and 449 μs, respectively. During the pre-attachment processes (UCL and slow front) the total negative charge effectively transferred to ground were 70.2 and 55 mC, respectively. For the stroke captured on high-speed video, the average line-charge-density for the 109-m long UCL was found to be 0.5 mC/m. The average UCL 2-D speed was 2.4 × 10 5 m/s, and it was observed to accelerate toward the downward leader prior to attachment. We observed that UCL-pulse amplitudes are larger, background currents are higher, and interpulse intervals are shorter at later times during UCLs, which can be attributed to the intensification of the local electric field due to the approaching downward negative leader. The median positive charge injected into the UCL by a pulse was 297 μC. The UCL associated with the higher peak-current stroke produced the highest injected pulse-charge values about three times sooner during its development, likely due to the 2–2.7 times faster average downward leader vertical speeds.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Catalytic enhancement in the performance of the microscopic two-stroke heat engine

Here we consider a model of heat engine operating in the microscopic regime: the two-stroke engine. It produces work and exchanges heat in two discrete strokes that are separated in time. The working body of the engine consists of two d-level systems initialized in thermal states at two distinct temperatures. Additionally, an auxiliary nonequilibrium system called catalyst may be incorporated with the working body of the engine, provided the state of the catalyst remains unchanged after the completion of a thermodynamic cycle. This ensures that the work produced by the engine arises solely from the temperature difference. Upon establishing the rigorous thermodynamic framework, we characterize twofold improvement stemming from the inclusion of a catalyst. Firstly, we prove that in the noncatalytic scenario, the optimal efficiency of the two-stroke heat engine with a working body composed of two-level systems is given by the Otto efficiency, which can be surpassed by incorporating a catalyst with the working body. Secondly, we show that incorporating a catalyst allows the engine to operate in frequency and temperature regimes that are not accessible for noncatalytic two-stroke engines. We conclude with a general conjecture about the advantage brought by a catalyst: including the catalyst with the working body always allows to improve efficiency over the noncatalytic scenario for any microscopic two-stroke heat engines. We prove this conjecture for two-stroke engines where the working body is composed of two d-level systems initialized in thermal states at two distinct temperatures, as long as the final joint state leading to optimal efficiency in the noncatalytic scenario is not a product state, or at least one of the d-level system is not thermal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Air-to-Fuel Ratio Calculation Methods for Oxygenated Fuels in Two-Stroke Engines

In 1990, Roy Douglas developed an analytical method to calculate the global air-to-fuel ratio of a two-stroke engine from exhaust gas emissions. While this method has considerable application to two-stroke engines, it does not permit the calculation of air-to-fuel ratios for oxygenated fuels. This study proposed modifications to the Roy Douglas method such that it can be applied to oxygenated fuels. The ISO #16183 standard, the modified Spindt method, and the Brettschneider method were used to evaluate the modifications to the Roy Douglas method. In addition, a trapped air-to-fuel ratio, appropriate for two-stroke engines, was also modified to incorporate oxygenated fuels. To validate the modified calculation method, tests were performed using a two-stroke carbureted and two-stroke direct injected marine outboard engine over a five-mode marine test cycle running indolene and low level blends of ethanol and iso-butanol fuels. Results indicate that the modified air-fuel ratio method closely relates to the ISO #16183 standard. The modified Spindt method over-estimated the global air-to-fuel ratio. While the Brettschneider is considered an advancement over the Spindt method, it still provided an over-estimate of the air-to-fuel ratio. Applying a trapped air-to-fuel ratio calculation to the modified Roy Douglas method provided a calculation for trapped air-to-fuel ratio for two-stroke engine operation, which correlated with engine-out CO emissions. The trapped air-to-fuel ratio calculation also provided improvements in air-to-fuel ratio predictions, especially during idle conditions.

Sevik, James Michael↗
NASA NTRS1973

Currents in Florida lightning return strokes.

The transmission line return stroke model is used to derive individual current versus time wave forms from measured electric fields. A total of 98 strokes in 21 flashes at distances less than or equal to 10 km and 63 strokes in 18 flashes at distances between 10 and 32 km are analyzed for current wave shape and magnitude. For the closer strokes, return stroke velocity is also determined from the electric field wave forms. Eighteen detailed current and field wave forms and a statistical distribution of peak currents are presented. Return stroke channel currents are found to exhibit a sharp initial peak which currents measured in tall structures at the base of the lightning channel apparently do not exhibit. A complete discussion of errors is given.

Uman, M. A.↗