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Flow Acceleration (flow + acceleration)
Selected AbstractsThe effects of slope and slope position on local and upstream fluid threshold friction velocitiesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2008Ning Huang Abstract In deserts, dunes are common aeolian landforms, and parallel ridges are common in cultivated land. A computational fluid dynamics (CFD) model is used to simulate a stable wind blowing over slope beds of varying height and coupled with the slope-compensating fluid threshold friction velocity formula. The model accurately reproduced patterns of flow deceleration at the slope toe and stoss flow acceleration. Based on the CFD-based model, quantitative analyses of slope gradient and particle position on the initiation of particle movement are performed. Results indicate that the slope has a great influence on particle saltation in the windward slope, and the initiation of particle movement is particularly sensitive to particle position with respect to the slope. Copyright © 2008 John Wiley & Sons, Ltd. [source] A Broad Diagnostic Battery for Bedside Transcranial Doppler to Detect Flow Changes With Internal Carotid Artery Stenosis or OcclusionJOURNAL OF NEUROIMAGING, Issue 3 2001Ioannis Christou MD ABSTRACT Background and Purpose. The authors establish accuracy parameters of a broad diagnostic battery for bedside transcranial Doppler (TCD) to detect flow changes due to internal carotid artery (ICA) stenosis or occlusion. Methods. The authors prospectively studied consecutive patients with stroke or transient ischemic attack referred for TCD. TCD was performed and interpreted at bedside using a standard insonation protocol. A broad diagnostic battery included major criteria: collateral flow signals, abnormal siphon or terminal carotid signals, and delayed systolic flow acceleration in the middle cerebral artery. Minor criteria included a unilateral decrease in pulsatility index (, 0.6 or , 70% of contralateral side), flow diversion signs, and compensatory velocity increase. Angiography or carotid duplex ultrasound (CDU) was used to grade the degree of carotid stenosis using North American criteria. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of TCD findings were determined. Results. Seven hundred and twenty patients underwent TCD, of whom 517 (256 men and 261 women) had angiography and/or CDU within 8.8 ± 0.9 days. Age was 63.1 ± 15.7 years. For a 70% to 99% carotid stenosis or occlusion, TCD had sensitivity of 79.4%, specificity of 86.2%, PPV of 57.0%, NPV of 94.8%, and accuracy of 84.7%. For a 50% to 99% carotid stenosis or occlusion, TCD had sensitivity of 67.5%, specificity of 83.9%, PPV of 54.5%, NPV of 90.0%, and accuracy of 81.6%. TCD detected intracranial carotid lesions with 84.9% accuracy and extracranial carotid lesions with 84.4% accuracy (sensitivity of 88% and 79%, specificity of 85% and 86%, PPV of 24% and 54%, and NPV of 99% and 95%, respectively). The prevalence of the ophthalmic artery flow reversal was 36.4% in patients with , 70% stenosis or occlusion. If present, this finding indicated a proximal ICA lesion location in 97% of these patients. Conclusions. In symptomatic patients, bedside TCD can accurately detect flow changes consistent with hemodynamically significant ICA obstruction; however, TCD should not be a substitute for direct carotid evaluation. Because TCD is sensitive and specific for , 70% carotid stenosis or occlusion in both extracranial and intracranial carotid segments, it can be used as a complementary test to refine other imaging findings and detect tandem lesions. [source] The 11 August 2006 squall-line system as observed from MIT Doppler radar during the AMMA SOPTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2010Michel Chong Abstract On the evening of 9 August 2006, a mesoscale convective system (MCS) having a north-south oriented squall-line organization formed over the border between Chad and Nigeria. It propagated westward, intensified over Nigeria on 10 August, and reached Niamey (Niger) at 0320 UTC on 11 August. Its passage over Niamey was accompanied by dust lifting and was well tracked by the Massachusetts Institute of Technology (MIT) Doppler radar. The three-dimensional structure of the airflow and precipitation pattern is investigated from regular radar volume scans performed every ten minutes between 0200 and 0321 UTC. The 3D wind components are deduced from the multiple-Doppler synthesis and continuity adjustment technique (MUSCAT) applied to a set of three volume scans obtained over a time period of one hour, which are equivalent to a three-radar observation of the squall line when considering a reference frame moving with the system and the hypothesis of a stationary field. Results of the wind synthesis reveal several features commonly observed in tropical squall lines, such as the deep convective cells in front of the system, fed by the monsoon air and extending up to 15 km altitude, and the well-marked stratiform rain region at the rear, associated with mesoscale vertical motions. Forward and trailing anvils are clearly identified as resulting from the outflow of air reaching the tropopause and transported to this level by the sloping convective updraughts occurring in a sheared environment. In the northern part, a deeper and stronger front-to-rear flow at mid-levels is found to contribute to the rearward deflection of the leading line and to promote a broader (over 300 km) stratiform cloud region. Eddy vertical transports of the cross-line momentum mainly accounts for the mid-level flow acceleration due to a momentum redistribution from low to higher levels. The height distribution of hydrometeors and their associated production terms derived from a one-dimensional microphysical retrieval model indicate the distinct roles of the convective and stratiform regions in the formation of graupel and rain, and the respective contributions of cold (riming) and warm (coalescence, melting) processes. Cooling from melting, and heating/cooling from condensation/evaporation processes yield a net decrease and increase of the potential temperature at low and mid-to-upper levels, respectively, with respect to an environmental thermodynamic profile taken three hours ahead of the analysis. Finally, the upper-level rearward flow could convey the non-negligible proportion of ice particles farther from the leading deep convection to the trailing stratiform region, thereby favouring the extent of this region. Copyright © 2009 Royal Meteorological Society [source] Early clinical experience with the new amplatzer ductal occluder II for closure of the persistent arterial duct,CATHETERIZATION AND CARDIOVASCULAR INTERVENTIONS, Issue 4 2009Jonathan Forsey MB, MRCPCH Abstract Objectives: To describe the early single-center clinical experience with the Amplatzer Ductal Occluder II (ADO II). Methods: All patients undergoing attempted transcatheter closure of persistent arterial duct (PDA) with the ADO II were included. Data collected included demographic, clinical, and echocardiographic parameters. Results: From March until September 2008, 29 procedures were undertaken in 27 patients (21 female). Median age was 1.4 years (range 0.4,76 years) with median weight 9.4 kg (range 4.7,108 kg). A transarterial approach was used in 2 patients. The median minimum ductal diameter was 2.7 mm (range 1.7,5). ADO II was released in 25 patients (92.5%). Two patients had significant residual shunting following deployment of ADO II and underwent closure with Amplatzer ductal occluder (ADO I). Postprocedural echocardiography identified one occluder had changed position with development of a significant leak and one occluder had embolized to the left pulmonary artery. Both occluders were retrieved successfully at a second catheter procedure. Complete occlusion was noted predischarge in 22 of the remaining 23 occluders (96%). One patient had mild flow acceleration in the left pulmonary artery which has resolved. Conclusions: The ADO II is highly effective at providing rapid occlusion of morphologically varied PDAs. Occluder design allows closure with arterial or venous approach and delivery with 4 or 5 F delivery catheters. Stable occluder position is dependent on correct positioning of both aortic and pulmonary discs. A larger range of sizes and configurations of this occluder may be required to successfully occlude all ductal sizes and morphologies. © 2009 Wiley-Liss, Inc. [source] | |||||||||||||