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At least 631 records · Page 35Linked to original sources

PubMed2005

Theory and modeling of accelerating flames in tubes.

The analytical theory of premixed laminar flames accelerating in tubes is developed, which is an important part of the fundamental problem of flame transition to detonation. According to the theory, flames with realistically large density drop at the front accelerate exponentially from a closed end of a tube with nonslip at the walls. The acceleration is unlimited in time; it may go on until flame triggers detonation. The analytical formulas for the acceleration rate, for the flame shape and the velocity profile in the flow pushed by the flame are obtained. The theory is validated by extensive numerical simulations. The numerical simulations are performed for the complete set of hydrodynamic combustion equations including thermal conduction, viscosity, diffusion, and chemical kinetics. The theoretical predictions are in a good agreement with the numerical results. It is also shown how the developed theory can be used to understand acceleration of turbulent flames.

Journal Article↗

PubMed2001

Simultaneous acceleration of multiply charged ions through a superconducting linac.

The possibility of simultaneously accelerating particles with a range of charge-to-mass ratios ( approximately 20%) to the same energy is proposed and demonstrated for a superconducting linac. Uranium ions stripped in a foil with eight charge states have been accelerated through a portion of the ATLAS linac from 286 to 690 MeV, with 94% of the injected uranium in the accelerated beam. Emittance of the resultant beam has been measured and the energy spread was 1.3% compared to 0.4% for a single charge state. This development has immediate application to the high-intensity acceleration of heavy ions that are limited by ion-source intensities, such as the proposed Rare Isotope Accelerator Facility.

Journal Article↗

PubMed2004

Intermittent nature of acceleration in near wall turbulence.

Using direct numerical simulation of a fully developed turbulent channel flow, we investigate the behavior of acceleration near a solid wall. We find that acceleration near the wall is highly intermittent and the intermittency is in large part associated with the near wall organized coherent turbulence structures. We also find that acceleration of large magnitude is mostly directed towards the rotation axis of the coherent vortical structures, indicating that the source of the intermittent acceleration is the rotational motion associated with the vortices that causes centripetal acceleration.

Journal Article↗

PubMed2005

Demonstration of a 17-GHz, high-gradient accelerator with a photonic-band-gap structure.

We report the testing of a high gradient electron accelerator with a photonic-band-gap (PBG) structure. The photonic-band-gap structure confines a fundamental TM(01)-like accelerating mode, but does not support higher-order modes (HOM). The absence of HOM is a major advantage of the PBG accelerator, since it suppresses dangerous beam instabilities caused by wakefields. The PBG structure was designed as a triangular lattice of metal rods with a missing central rod forming a defect confining the TM(01)-like mode and allowing the electron beam to propagate along the axis. The design frequency of the six-cell structure was 17.14 GHz. The PBG structure was excited by 2 MW, 100 ns pulses. A 16.5 MeV electron beam was transmitted through the PBG accelerator. The observed electron beam energy gain of 1.4 MeV corresponds to an accelerating gradient of 35 MV/m, in excellent agreement with theory.

Journal Article↗

PubMed1968

Thresholds for georesponse to acceleration in gravity-compensated Avena seedlings.

The magnitude of acceleration required to induce growth responses in Avena seedlings grown in the absence of tropic response to earth gravity has been investigated. For this purpose, a clinostat was developed that imposes accelerations from about 10(-9) g to 3 g upon the seedling; simultaneously, it nullifies, or compensates for, response to the directional component of the gravitational-force vector by rotating the seedling on a horizontal axis. When accelerations less than 10(-3) g are applied in either the acropetal or the basipetal direction, the growth in length and weight of the various organs is not materially different from that of compensated seedlings to which no longitudinal force is applied. At accelerations between 10(-3) and 10(-2) g, differences in growth become highly significant. When the centrifugal forces are transverse to the seedling during compensation, the threshold acceleration range for geoperception, as manifest by shoot reorientation, is again between 10(-3) and 10(-2) g. Geotropic reorientation of the root becomes apparent after exposures between 10(-4) and 10(-3) g.

Journal Article↗

PubMed1988

Development of accelerated net nitrate uptake : effects of nitrate concentration and exposure time.

Upon initial nitrate exposure, net nitrate uptake rates in roots of a wide variety of plants accelerate within 6 to 8 hours to substantially greater rates. Effects of solution nitrate concentrations and short pulses of nitrate (</=1 hour) upon ;nitrate-induced' acceleration of nitrate uptake in maize (Zea mays L.) were determined. Root cultures of dark-grown seedlings, grown without nitrate, were exposed to 250 micromolar nitrate for 0.25 to 1 hour or to various solution nitrate concentrations (10-250 micromolar) for 1 hour before returning them to a nitrate-free solution. Net nitrate uptake rates were assayed at various periods following nitrate exposure and compared to rates of roots grown either in the absence of nitrate (CaSO(4)-grown) or with continuous nitrate for at least 20 hours. Three hours after initial nitrate exposure, nitrate pulse treatments increased nitrate uptake rates three- to four-fold compared to the rates of CaSO(4)-grown roots. When cycloheximide (5 micrograms per milliliter) was included during a 1-hour pulse with 250 micromolar nitrate, development of the accelerated nitrate uptake state was delayed. Otherwise, nitrate uptake rates reached maximum values within 6 hours before declining. Maximum rates, however, were significantly less than those of roots exposed continuously for 20, 32, or 44 hours. Pulsing for only 0.25 hour with 250 micromolar nitrate and for 1 hour with 10 micromolar caused acceleration of nitrate uptake, but the rates attained were either less than or not sustained for a duration comparable to those of roots pulsed for 1 hour with 250 micromolar nitrate. These results indicate that substantial development of the nitrate-induced accelerated nitrate uptake state can be achieved by small endogenous accumulations of nitrate, which appear to moderate the activity or level of root nitrate uptake.

Journal Article↗

PubMed2001

Theory and design of piezoelectric resonators immune to acceleration: present state of the art.

A typical low noise oscillator uses a crystal resonator as the frequency-determining element. An understanding of the fundamental nature of acceleration sensitivity in crystal oscillators resides primarily in understanding the behavior of the crystal resonator. The driving factor behind the acceleration-induced frequency shift is shown to be deformation of the resonator. The deformation drives two effects: an essentially linear change in the frequency-determining dimensions of the resonator and an essentially nonlinear effect of changing the velocity of the propagating wave. In this paper, the fundamental nature of acceleration sensitivity is reviewed and clarified, and attendant design guidance is developed for piezoelectric resonators. The basic properties of acceleration sensitivity and general design guidance are developed through the simple examples of "bulk acoustic wave (BAW) in a box" and "surface transverse wave (STW) in a box." These examples serve to clarify a number of concepts, including the role of mode shape and the basic difference between the BAW and STW cases. The design equations clarify the functional dependencies of the acceleration sensitivities on the full range of crystal resonator design and fabrication parameters.

Journal Article↗

PubMed2004

Monocytosis and accelerated activation of lymphocytes in C1q-deficient autoimmune-prone mice.

C1q deficiency has been shown to accelerate spontaneous autoimmunity in mice. We studied the time course of activation of monocytes and lymphocytes in autoimmune and non-autoimmune mice in the presence or absence of C1q as a disease accelerator. Autoimmune MRL\Mp.C1qa-\- and non-autoimmune C57BL\6.C1qa-\- mice were analysed at various time points between 6 and 33 weeks of age and compared to strain- and age-matched C1q-sufficient controls. Splenic and peritoneal leucocytes were analysed by flow cytometry and plasma levels of immunoglobulin M (IgM), total IgG, IgG subclasses and IgM autoantibodies were measured. Both C1q-deficient strains had significantly more splenic monocytes than their controls at all time points analysed. In addition, MRL\Mp.C1qa-\- but not C57BL/6.C1qa-\- mice developed splenic hypercellularity starting at about 12-17 weeks old, had signs of accelerated CD4+ T-cell activation and showed a marked increase in splenic plasma cells and total serum IgM levels from about 22 weeks of age. The accelerated CD4+ T-cell activation was not due to a direct inhibitory effect of C1q on T cells. These data show that C1q deficiency causes splenic monocytosis together with accelerated T-cell activation in an autoimmune-prone mouse strain.

Animals↗

PubMed1996

Use of the Sensormedics MCT Accelerator in sputum induction.

Airway inflammation can be studied by obtaining sputum induced by inhalation of 3% saline. The Sensormedics MCT accelerator is an oral asymmetrical high frequency oscillator which safely enhances clearance of airway secretions. The aim of this study was to determine if use of the MCT Accelerator enhances sputum production in non-asthmatic non-atopic, atopic and asthmatic subjects. Fifteen subjects were studied over 3 days. On day 1 skin prick testing to common aeroallergens and methacholine bronchial reactivity were performed. On days 2 and 3, separated by 7 days, sputum was induced by inhalation of 3% saline alone for 30 mins or via the nebulizer port of the MCT Accelerator. The cellular profile and volume of sputum were analysed. The use of the MCT Accelerator did not alter the cellular profile of the induced sputum nor was there an increase in volume. In conclusion the induction of sputum by inhalation of 3% saline was not altered by use of the MCT Accelerator.

Asthma↗

PubMed1996

Blood flow acceleration in the carotid and brachial arteries of healthy volunteers: respective contributions of cardiac performance and local resistance.

The influence of local resistance and cardiac performance on peripheral blood acceleration was investigated in 14 healthy male volunteers. Steady and pulsatile flow was studied in the brachial and in the common carotid arteries, ie, two territories that exhibit marked differences in resistive characteristics. Instantaneous blood velocity (V), mean blood velocity (Vm) and artery diameter (D) were evaluated at rest by an ultrasonic range-gated pulsed Doppler flowmeter using a double transducer probe, thus allowing the calculation of mean blood flow (Q). Mean local resistance (R) was obtained by dividing the mean arterial pressure by Q. The peak value of the local acceleration of the blood was obtained by computer-assisted calculation of the first derivative of instantaneous blood velocity (Gmax = +dV/dtmax). Peak aortic blood acceleration (GAo) was simultaneously measured from the suprasternal notch using a pulsed Doppler velocity meter. In the branchial and the common carotid arteries, Gmax was of a similar magnitude (551 +/- 30 and 555 +/- 44 cm/s2, respectively) despite major differences in the respective D, Vm, Q and R values. In neither artery was there a relationship between Gmax and either resting Q or R. At the brachial artery level, Gmax was positively related to GAo (r = 0.79, P = 0.0008). At the common carotid artery level, there was a weak, although non significant relationship between Gmax and GAo (P = 0.08). Our results indicate that the local acceleration of peripheral blood flow in the brachial artery is related rather to upstream central impulse than to downstream hemodynamics, and suggest some regional differences in the hemodynamic determinants of the local acceleration of peripheral blood flow.

Adult↗

PubMed1983

Accelerating effects of pentobarbitone on the inactivation process of the calcium current in Helix neurones.

The effect of pentobarbitone on Ca2+ current (ICa), separated from other ionic currents was studied under voltage clamp using a suction pipette technique in Helix neurones. Pentobarbitone depressed the maximal peak amplitude (MPA) of ICa in a concentration-dependent manner without shifting the current-voltage (I-V) relationships along the voltage axis. Increases in external Ca2+-concentration [( Ca2+]o) overcame the inhibitory action of the agent on MPA. Pentobarbitone markedly accelerated the decay phase of ICa which took a distinctly different time course from that of the control. The accelerating action of the agent on the decay phase of ICa was not overcome by increases in [Ca2+]o. In the presence of internal EGTA (20 mM), pentobarbitone also accelerated the decay of ICa. Changes in pH of the external perfusing solution altered the potency of pentobarbitone in depressing MPA; in the presence of pentobarbitone (3 X 10(-4) M) at pH of 7.0, 8.0 and 9.0, fractional inhibition was approx. 46%, 21% and 4%, respectively. Internal application of pentobarbitone (10(-4)-10(-3) M) inhibited MPA, but exerted no effect on the decay phase of ICa. Pentobarbitone (10(-4) M) markedly accelerated the decrease of MPA of ICa induced by repetitive stimuli applied at an interval of 150 ms, indicating a use-dependent depression of MPA. Results provide evidence that pentobarbitone has a dual action on ICa, inhibiting MPA and accelerating the decay phase of ICa.

Animals↗

PubMed1999

Effect of adenosine and verapamil in catecholamine-induced accelerated atrioventricular junctional rhythm: insights into the underlying mechanism.

Accelerated AV junctional rhythm is postulated to be due to enhanced automaticity of a high AV junctional focus. The adenosine response of this rhythm was tested in 17 patients (7 males, 12-83 years). The indications of electrophysiology study were nonspecific palpitation (n = 5), unexplained syncope (n = 6), postablation of accessory pathways (n = 4), and postmodification of AV nodal reentry tachycardia (n = 2). The sinus node and AV nodal functions were normal. Pacing and programmed electrical stimulation failed to induce any arrhythmia at baseline. The accelerated junctional rhythm (cycle length = 553 +/- 134 ms) was initiated spontaneously in all patients after isoproterenol infusion (1-2 micrograms/min). It was not suppressible by overdrive pacing. Cessation of isoproterenol infusion terminated the rhythm in all patients. Adenosine (6 mg) reproducibly terminated the accelerated junctional rhythm in all patients. In six patients, adenosine suppressed the junctional rhythm without producing AV nodal block. In the other 11 patients, the junctional rhythm was terminated prior to the occurrence of AV nodal block. Verapamil was tested in ten patients and 5 mg of intravenous verapamil terminated the junctional rhythm in all patients. In conclusion, the mechanism of catecholamine-induced accelerated AV junctional rhythm is most likely enhanced automaticity, and catecholamine-induced accelerated AV junctional automaticity is sensitive to adenosine and verapamil. Adenosine appears to have differential effects on catecholamine-enhanced AV junctional automaticity and AV nodal conduction. This suggests that, under catecholamine stimulation, adenosine may have different mechanisms of action on AV nodal conduction and automaticity.

Adenosine↗

PubMed1998

Melatonin accelerates metamorphosis in Xenopus laevis.

Delayed metamorphosis associated with large body size has been observed in Woodhousei fowleri tadpoles reared in continuous dark (DD). To evaluate the mechanism by which DD delayed metamorphosis, light-cycle exposure was controlled and thyroxine (T4), melatonin, or drugs that alter prolactin (Prl) concentrations were given to Xenopus laevis tadpoles. It was hypothesized that exogenous melatonin would delay metamorphosis and increase body size, and that elevation of Prl concentrations would have effects similar to melatonin exposure. Xenopus laevis tadpoles were randomized to three light conditions [light/dark (LD, 12 h/12 h), DD, and continuous light (LL)] and subgroups in each light condition were treated with T4, melatonin, bromocriptine (Bro), haloperidol (Hal), or no drug. Each subgroup included 12 tadpoles. Drugs were administered in the water either continuously or daily from 07.00 to 19.00 h (Intermittent). Measurements of total length, leg length, and stage of metamorphosis were obtained at regular intervals. DD resulted in delayed metamorphosis, while LL did not. T4 accelerated metamorphosis as expected, countering the delaying effects of DD. In contrast to the hypothesis, melatonin accelerated metamorphosis and impaired body size compared to controls. Intermittent Hal also accelerated metamorphosis, while Bro delayed it. In DD, both T4 and melatonin led to increased tadpole size in contrast to their counterparts in LD or LL. Delayed metamorphosis in DD is not caused by increased melatonin production. Melatonin and Hal (as given in this study) accelerate metamorphosis. Melatonin acceleration of metamorphosis may occur through alteration of the concentration of prolactin.

Animals↗

PubMed2006

Plasma acceleration above martian magnetic anomalies.

Auroras are caused by accelerated charged particles precipitating along magnetic field lines into a planetary atmosphere, the auroral brightness being roughly proportional to the precipitating particle energy flux. The Analyzer of Space Plasma and Energetic Atoms experiment on the Mars Express spacecraft has made a detailed study of acceleration processes on the nightside of Mars. We observed accelerated electrons and ions in the deep nightside high-altitude region of Mars that map geographically to interface/cleft regions associated with martian crustal magnetization regions. By integrating electron and ion acceleration energy down to the upper atmosphere, we saw energy fluxes in the range of 1 to 50 milliwatts per square meter per second. These conditions are similar to those producing bright discrete auroras above Earth. Discrete auroras at Mars are therefore expected to be associated with plasma acceleration in diverging magnetic flux tubes above crustal magnetization regions, the auroras being distributed geographically in a complex pattern by the many multipole magnetic field lines extending into space.

Journal Article↗

PubMed1999

Thrombolysis in acute myocardial infarction: analysis of studies comparing accelerated t-PA and streptokinase.

OBJECTIVES: To compare outcomes from accelerated alteplase (recombinant tissue plasminogen activator, t-PA) and streptokinase use in acute myocardial infarction. METHODS: Review of available studies identified by Medline and other literature searches that met the criteria of being a prospective, randomised clinical trial enrolling over 1000 patients with acute myocardial infarction. The studies had to contain an intervention arm comprising accelerated infusion t-PA, or an intervention arm comprising streptokinase provided accelerated t-PA that was compared in the same trial. Interventions compared were streptokinase 1.5 million units given over one hour compared with accelerated t-PA infusion, with concomitant use of aspirin and heparin, and main outcome measure of 30 day mortality. RESULTS: Four studies met prespecified criteria, these being the GUSTO I, GUSTO IIb Angioplasty Substudy, GUSTO III, and COBALT trials. There was a total study population of 64,387 patients of whom 20,251 received streptokinase, 19,474 received t-PA, with others receiving different treatment. Pooled data show that accelerated t-PA produces a marginal 30 day mortality advantage compared with streptokinase (6.6% v 7.3%, p = 0.02, Bonferroni adjusted p = 0.12, that is borderline significance, relative risk 0.918, 95% confidence interval 0.854 to 0.986). Any benefit is attributable entirely to patients recruited in the United States in the GUSTO I study. There is an increased incidence of stroke with t-PA. CONCLUSIONS: The data do not consistently show a 30 day mortality benefit from using t-PA compared with streptokinase in acute myocardial infarction, but do show increased risk of stroke. Streptokinase can be considered the thrombolytic agent of choice.

Female↗

PubMed1997

Quantification of tumor reoxygenation during accelerated radiation therapy.

PURPOSE: To determine the effect of accelerated fractionated radiation therapy on the reoxygenation of chronic hypoxic tumor cells by means of a three-dimensional Monte Carlo simulation. MATERIALS AND METHODS: Tumor growth (cell cycle time [Tc] of 2 or 5 days), irradiation (alpha = 0.3 Gy-1, alpha/beta = 10 Gy), oxygen tension (oxygen enhancement ratio of 3.0), capillary network distortion, and shrinkage of a small (10(4)-cell) tumor were modeled on a workstation. Conventional (2 Gy delivered five times per week), accelerated (1.6 Gy delivered twice a day for 5 d/wk), and split-course (1.6 Gy delivered twice a day for 5 d/wk with a break in week 3) radiation therapies were simulated with total doses of 50-70 Gy. RESULTS: Tumors grew in cords with a central necrotic area surrounded by hypoxic cells (hypoxic fraction, 25%). If undisturbed tumor shrinkage occurred, complete reoxygenation took 2 1/2 weeks for accelerated, 3-5 weeks for split-course, or 4-6 weeks for conventional radiation therapy. If shrinkage was prevented, the hypoxic fraction rose to 90%-100% at 2-3 weeks. In the shrinking, reoxygenation tumors with a Tc of 2 days, accelerated radiation therapy increased tumor control by 40%-80% over the control with conventional radiation therapy. With split-course therapy, this advantage was lost at doses below 70 Gy. Shrinking tumors with a Tc of 5 days were all controlled with 50 Gy. CONCLUSION: Fast-growing tumors that reoxygenate by shrinking are especially sensitive to acceleration and treatment breaks.

Cell Division↗

PubMed2006

Vascular endothelial growth factor accelerates compensatory lung growth after unilateral pneumonectomy.

We hypothesize that compensatory lung growth after unilateral pneumonectomy in a murine model is, in part, angiogenesis dependent and can be altered using angiogenic agents, possibly through regulation of endothelial cell proliferation and apoptosis. Left pneumonectomy was performed in mice. Mice were then treated with proangiogenic factors [vascular endothelial growth factor (VEGF); basic fibroblast growth factor (bFGF)], VEGF receptor antibodies (MF-1, DC101), and VEGF receptor small molecule chemical inhibitors. Lung volume and mass were measured. The lungs were analyzed using immunohistochemistry by CD31 staining, terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling, type II pneumocytes staining, and proliferating cell nuclear antigen. Compensatory lung growth was complete by postoperative day 10 and was associated with diffuse apoptosis of endothelial cells and pneumocytes. This process was accelerated by VEGF, such that growth was complete by postoperative day 4 with similar associated apoptosis. bFGF had no effect on lung growth. MF-1 and DC101 had no effect. The VEGF receptor small molecule chemical inhibitors also had no effect. VEGF, but not bFGF, accelerates growth. VEGF receptor inhibitors do not block growth, suggesting that other proangiogenic factors play a role or can compensate for VEGF receptor blockade. Diffuse apoptosis, endothelial cell and pneumocyte, occurs at cessation of both normal compensatory and VEGF-accelerated growth. Angiogenesis modulators may control growth via regulation of endothelial cell proliferation and apoptosis, although the exact relationship between endothelial cells and pneumocytes has yet to be determined. The fact that bFGF did not accelerate growth in our model when it did accelerate regeneration in the liver model suggests that angiogenesis during organ regeneration is regulated in an organ-specific manner.

Animals↗

PubMed1978

Body mass, composition, and food intake in rabbits during altered acceleration fields.

The effects of gravitational loading and unloading on body mass, body composition, and food intake were investigated in chronically accelerated adult male rabbits. The intensity of the acceleration field was increased by 0.25-G increments at intervals that permitted stabilization of body mass and food intake to a maximal field of 2.5 G. Control rabbits of the same age were maintained at Earth gravity under the same conditions of light and temperature. A negative rectilinear relationship was observed between body mass and the intensity of acceleration field. In spite of this, the decreased body mass appeared to be regulated, since centrifuged animals readily regained their lost body mass upon realimentation after a 3-day fast. At acceleration intensities above 1.75 G, ad libitum food intakes as well as maintenance food requirements increased with increasing acceleration fields despite a corresponding reduction in body mass. Analysis of body composition indicated a significant decrease in body fat and an increase in body water in animals adapted to a 2.5-G environment. Within 6 wk after the animals were returned from 2.5 G to Earth gravity, body mass and food intakes returned to the precentrifuged levels of corresponding control animals.

Adaptation, Physiological↗