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Koichi Koyama - One of the best experts on this subject based on the ideXlab platform.

  • Relation between the lifting surface theory and the lifting line theory in the design of an optimum screw propeller
    Journal of Marine Science and Technology, 2013
    Co-Authors: Koichi Koyama
    Abstract:

    A theory on an optimum screw propeller is described. The optimum means optimum efficiency of a propeller, that is, maximizing thrust horse power for a given shaft horse power. The theory is based on the propeller lifting surface theory. Circulation density (lift density) of the blade is determined by the lifting surface theory on a specified condition in general. However, it is shown that, in the case of optimum condition, the Circulation density is not determined by the lifting surface theory, although the Circulation Distribution which is the chordwise integral of the Circulation density is determined. The reason is that the governing equation of the optimization by the lifting surface theory is reduced to that by the lifting line theory. This theoretical deduction is the main part of this paper. The importance of the lifting line theory in the design of the optimum propeller is made clear. Numerical calculations support the conclusion from the deduction. This is shown in the case of freely running propellers and in the case of wake adapted propellers.

Jonathan D Henshaw - One of the best experts on this subject based on the ideXlab platform.

  • when gas dynamics decouples from galactic rotation characterizing ism Circulation in disk galaxies
    The Astrophysical Journal, 2020
    Co-Authors: Jose Utreras, Guillermo A Blanc, Andres Escala, Sharon E Meidt, Eric Emsellem, Frank Bigiel, Simon C O Glover, Jonathan D Henshaw
    Abstract:

    In galactic disks, galactic rotation sets the bulk motion of gas, and its energy and momentum can be transferred toward small scales. Additionally, in the interstellar medium, random and noncircular motions arise from stellar feedback, cloud-cloud interactions, and instabilities, among other processes. Our aim is to comprehend to which extent small-scale gas dynamics is decoupled from galactic rotation. We study the relative contributions of galactic rotation and local noncircular motions to the Circulation of gas, $\Gamma$, a macroscopic measure of local rotation, defined as the line integral of the velocity field around a closed path. We measure the Circulation Distribution as a function of spatial scale in a set of simulated disk galaxies and we model the velocity field as the sum of galactic rotation and a Gaussian random field. The random field is parameterized by a broken power law in Fourier space, with a break at the scale $\lambda_c$. We define the spatial scale $\lambda_{\rm eq}$ at which galactic rotation and non-circular motions contribute equally to $\Gamma$. For our simulated galaxies, the gas dynamics at the scale of molecular clouds is usually dominated by noncircular motions, but in the center of galactic disks galactic rotation is still relevant. Our model shows that the transfer of rotation from large scales breaks at the scale $\lambda_c$ and this transition is necessary to reproduce the Circulation Distribution. We find that $\lambda_{\rm eq}$, and therefore the structure of the gas velocity field, is set by the local conditions of gravitational stability and stellar feedback.

Himanshu J Patel - One of the best experts on this subject based on the ideXlab platform.

  • impact of left subclavian artery revascularization before thoracic endovascular aortic repair on postoperative cerebrovascular hemodynamics
    Annals of Vascular Surgery, 2018
    Co-Authors: Arnoud V Kamman, Jonathan L Eliason, David M Williams, Bo Yang, Frans L Moll, Santi Trimarchi, Kim A Eagle, Himanshu J Patel
    Abstract:

    Background The impact of left subclavian artery (LSA) revascularization before thoracic endovascular aortic repair (TEVAR) on cerebrovascular flow is not well described. We studied bilateral vertebral and carotid artery flow characteristics before and after TEVAR to evaluate the hemodynamic effects of LSA revascularization. Methods Seventy-four patients with mixed etiologies (mean age 70.9 ± 10.5 years) underwent LSA revascularization and TEVAR (2006–2016) and had available preoperative and postoperative carotid duplex study available. Data from patient demographics, procedures, preoperative, and postoperative carotid duplex studies were gathered. Revascularization was by left common carotid artery (LCCA) to LSA bypass ( n  = 70, 94.6%) or LSA to LCCA transposition ( n  = 4, 5.4%). Results Duplex confirmation of antegrade left vertebral artery (LVA) flow decreased significantly after TEVAR with LSA revascularization (100.0% vs. 77.9%, P P  = 0.063). Postoperatively, LVA bidirectional flow was observed in 3 patients (4.4%). Flow directions in the right vertebral artery (RVA) did not change significantly. Peak systolic velocity (PSV) in the LVA decreased significantly after TEVAR from 55.1 ± 22.0 cm/s to 35.9 ± 26.3 cm/s ( P P  = 0.012 and 95.3 ± 46.8 cm/s to 102.8 ± 42.9 cm/s, P  = 0.011). PSV did not change significantly in the left ICA. At mean follow-up of 36.6 ± 26.8 months, primary bypass patency was 100.0%. Postoperatively, one case of temporary spinal cord ischemia was seen (1.4%). Stroke rate was 6.9% ( n  = 5, 100.0% embolic), all without permanent disabilities. Stroke Circulation Distribution was 60.0% posterior, 20.0% anterior, and 20.0% mixed. Location of stroke was left sided ( n  = 2) or in both hemispheres ( n  = 3). There were no deaths at 30 days. Neurological events during follow-up included 3 new strokes. All-cause mortality rate during follow-up was 12.2% ( n  = 9). Conclusions Adjunctive LSA revascularization in the setting of zone 2 TEVAR coverage is associated with hemodynamic vertebral artery changes. Future studies in larger sample sizes should evaluate whether these novel findings are an important determinant of postoperative neurologic events.

  • Impact of Left Subclavian Artery Revascularization before Thoracic Endovascular Aortic Repair on Postoperative Cerebrovascular Hemodynamics
    'Elsevier BV', 2018
    Co-Authors: Arnoud V Kamman, Jonathan L Eliason, David M Williams, Bo Yang, Frans L Moll, Santi Trimarchi, Kim A Eagle, Himanshu J Patel
    Abstract:

    Background The impact of left subclavian artery (LSA) revascularization before thoracic endovascular aortic repair (TEVAR) on cerebrovascular flow is not well described. We studied bilateral vertebral and carotid artery flow characteristics before and after TEVAR to evaluate the hemodynamic effects of LSA revascularization. Methods Seventy-four patients with mixed etiologies (mean age 70.9 \uc2\ub1 10.5 years) underwent LSA revascularization and TEVAR (2006\ue2\u80\u932016) and had available preoperative and postoperative carotid duplex study available. Data from patient demographics, procedures, preoperative, and postoperative carotid duplex studies were gathered. Revascularization was by left common carotid artery (LCCA) to LSA bypass (n = 70, 94.6%) or LSA to LCCA transposition (n = 4, 5.4%). Results Duplex confirmation of antegrade left vertebral artery (LVA) flow decreased significantly after TEVAR with LSA revascularization (100.0% vs. 77.9%, P < 0.001). Incidence of retrograde LVA flow increased from 0.0% to 8.3% (P = 0.063). Postoperatively, LVA bidirectional flow was observed in 3 patients (4.4%). Flow directions in the right vertebral artery (RVA) did not change significantly. Peak systolic velocity (PSV) in the LVA decreased significantly after TEVAR from 55.1 \uc2\ub1 22.0 cm/s to 35.9 \uc2\ub1 26.3 cm/s (P < 0.001). In contrast, PSV increased in the RVA and the right internal carotid artery (ICA; 52.2 \uc2\ub1 21.7 cm/s to 63.2 \uc2\ub1 23.3 cm/s, P = 0.012 and 95.3 \uc2\ub1 46.8 cm/s to 102.8 \uc2\ub1 42.9 cm/s, P = 0.011). PSV did not change significantly in the left ICA. At mean follow-up of 36.6 \uc2\ub1 26.8 months, primary bypass patency was 100.0%. Postoperatively, one case of temporary spinal cord ischemia was seen (1.4%). Stroke rate was 6.9% (n = 5, 100.0% embolic), all without permanent disabilities. Stroke Circulation Distribution was 60.0% posterior, 20.0% anterior, and 20.0% mixed. Location of stroke was left sided (n = 2) or in both hemispheres (n = 3). There were no deaths at 30 days. Neurological events during follow-up included 3 new strokes. All-cause mortality rate during follow-up was 12.2% (n = 9). Conclusions Adjunctive LSA revascularization in the setting of zone 2 TEVAR coverage is associated with hemodynamic vertebral artery changes. Future studies in larger sample sizes should evaluate whether these novel findings are an important determinant of postoperative neurologic events

Arnoud V Kamman - One of the best experts on this subject based on the ideXlab platform.

  • impact of left subclavian artery revascularization before thoracic endovascular aortic repair on postoperative cerebrovascular hemodynamics
    Annals of Vascular Surgery, 2018
    Co-Authors: Arnoud V Kamman, Jonathan L Eliason, David M Williams, Bo Yang, Frans L Moll, Santi Trimarchi, Kim A Eagle, Himanshu J Patel
    Abstract:

    Background The impact of left subclavian artery (LSA) revascularization before thoracic endovascular aortic repair (TEVAR) on cerebrovascular flow is not well described. We studied bilateral vertebral and carotid artery flow characteristics before and after TEVAR to evaluate the hemodynamic effects of LSA revascularization. Methods Seventy-four patients with mixed etiologies (mean age 70.9 ± 10.5 years) underwent LSA revascularization and TEVAR (2006–2016) and had available preoperative and postoperative carotid duplex study available. Data from patient demographics, procedures, preoperative, and postoperative carotid duplex studies were gathered. Revascularization was by left common carotid artery (LCCA) to LSA bypass ( n  = 70, 94.6%) or LSA to LCCA transposition ( n  = 4, 5.4%). Results Duplex confirmation of antegrade left vertebral artery (LVA) flow decreased significantly after TEVAR with LSA revascularization (100.0% vs. 77.9%, P P  = 0.063). Postoperatively, LVA bidirectional flow was observed in 3 patients (4.4%). Flow directions in the right vertebral artery (RVA) did not change significantly. Peak systolic velocity (PSV) in the LVA decreased significantly after TEVAR from 55.1 ± 22.0 cm/s to 35.9 ± 26.3 cm/s ( P P  = 0.012 and 95.3 ± 46.8 cm/s to 102.8 ± 42.9 cm/s, P  = 0.011). PSV did not change significantly in the left ICA. At mean follow-up of 36.6 ± 26.8 months, primary bypass patency was 100.0%. Postoperatively, one case of temporary spinal cord ischemia was seen (1.4%). Stroke rate was 6.9% ( n  = 5, 100.0% embolic), all without permanent disabilities. Stroke Circulation Distribution was 60.0% posterior, 20.0% anterior, and 20.0% mixed. Location of stroke was left sided ( n  = 2) or in both hemispheres ( n  = 3). There were no deaths at 30 days. Neurological events during follow-up included 3 new strokes. All-cause mortality rate during follow-up was 12.2% ( n  = 9). Conclusions Adjunctive LSA revascularization in the setting of zone 2 TEVAR coverage is associated with hemodynamic vertebral artery changes. Future studies in larger sample sizes should evaluate whether these novel findings are an important determinant of postoperative neurologic events.

  • Impact of Left Subclavian Artery Revascularization before Thoracic Endovascular Aortic Repair on Postoperative Cerebrovascular Hemodynamics
    'Elsevier BV', 2018
    Co-Authors: Arnoud V Kamman, Jonathan L Eliason, David M Williams, Bo Yang, Frans L Moll, Santi Trimarchi, Kim A Eagle, Himanshu J Patel
    Abstract:

    Background The impact of left subclavian artery (LSA) revascularization before thoracic endovascular aortic repair (TEVAR) on cerebrovascular flow is not well described. We studied bilateral vertebral and carotid artery flow characteristics before and after TEVAR to evaluate the hemodynamic effects of LSA revascularization. Methods Seventy-four patients with mixed etiologies (mean age 70.9 \uc2\ub1 10.5 years) underwent LSA revascularization and TEVAR (2006\ue2\u80\u932016) and had available preoperative and postoperative carotid duplex study available. Data from patient demographics, procedures, preoperative, and postoperative carotid duplex studies were gathered. Revascularization was by left common carotid artery (LCCA) to LSA bypass (n = 70, 94.6%) or LSA to LCCA transposition (n = 4, 5.4%). Results Duplex confirmation of antegrade left vertebral artery (LVA) flow decreased significantly after TEVAR with LSA revascularization (100.0% vs. 77.9%, P < 0.001). Incidence of retrograde LVA flow increased from 0.0% to 8.3% (P = 0.063). Postoperatively, LVA bidirectional flow was observed in 3 patients (4.4%). Flow directions in the right vertebral artery (RVA) did not change significantly. Peak systolic velocity (PSV) in the LVA decreased significantly after TEVAR from 55.1 \uc2\ub1 22.0 cm/s to 35.9 \uc2\ub1 26.3 cm/s (P < 0.001). In contrast, PSV increased in the RVA and the right internal carotid artery (ICA; 52.2 \uc2\ub1 21.7 cm/s to 63.2 \uc2\ub1 23.3 cm/s, P = 0.012 and 95.3 \uc2\ub1 46.8 cm/s to 102.8 \uc2\ub1 42.9 cm/s, P = 0.011). PSV did not change significantly in the left ICA. At mean follow-up of 36.6 \uc2\ub1 26.8 months, primary bypass patency was 100.0%. Postoperatively, one case of temporary spinal cord ischemia was seen (1.4%). Stroke rate was 6.9% (n = 5, 100.0% embolic), all without permanent disabilities. Stroke Circulation Distribution was 60.0% posterior, 20.0% anterior, and 20.0% mixed. Location of stroke was left sided (n = 2) or in both hemispheres (n = 3). There were no deaths at 30 days. Neurological events during follow-up included 3 new strokes. All-cause mortality rate during follow-up was 12.2% (n = 9). Conclusions Adjunctive LSA revascularization in the setting of zone 2 TEVAR coverage is associated with hemodynamic vertebral artery changes. Future studies in larger sample sizes should evaluate whether these novel findings are an important determinant of postoperative neurologic events

Kamran Mohseni - One of the best experts on this subject based on the ideXlab platform.

  • Vortex sheet roll-up revisited
    Journal of Fluid Mechanics, 2018
    Co-Authors: Adam Devoria, Kamran Mohseni
    Abstract:

    The classical problem of roll-up of a two-dimensional free inviscid vortex sheet is reconsidered. The singular governing equation brings with it considerable difficulty in terms of actual calculation of the sheet dynamics. Here, the sheet is discretized into segments that maintain it as a continuous object with curvature. A model for the self-induced velocity of a finite segment is derived based on the physical consideration that the velocity remain bounded. This allows direct integration through the singularity of the Birkhoff–Rott equation. The self-induced velocity of the segments represents the explicit inclusion of stretching of the sheet and thus vorticity transport. The method is applied to two benchmark cases. The first is a finite vortex sheet with an elliptical Circulation Distribution. It is found that the self-induced velocity is most relevant in regions where the curvature and the sheet strength or its gradient are large. The second is the Kelvin–Helmholtz instability of an infinite vortex sheet. The critical time at which the sheet forms a singularity in curvature is accurately predicted. As observed by others, the vortex sheet strength forms a finite-valued cusp at this time. Here, it is shown that the cusp value rapidly increases after the critical time and is the impetus that initiates the roll-up process.