33 research outputs found
Transient integral boundary layer method to calculate the translesional pressure drop and the fractional flow reserve in myocardial bridges
BACKGROUND: The pressure drop – flow relations in myocardial bridges and the assessment of vascular heart disease via fractional flow reserve (FFR) have motivated many researchers the last decades. The aim of this study is to simulate several clinical conditions present in myocardial bridges to determine the flow reserve and consequently the clinical relevance of the disease. From a fluid mechanical point of view the pathophysiological situation in myocardial bridges involves fluid flow in a time dependent flow geometry, caused by contracting cardiac muscles overlying an intramural segment of the coronary artery. These flows mostly involve flow separation and secondary motions, which are difficult to calculate and analyse. METHODS: Because a three dimensional simulation of the haemodynamic conditions in myocardial bridges in a network of coronary arteries is time-consuming, we present a boundary layer model for the calculation of the pressure drop and flow separation. The approach is based on the assumption that the flow can be sufficiently well described by the interaction of an inviscid core and a viscous boundary layer. Under the assumption that the idealised flow through a constriction is given by near-equilibrium velocity profiles of the Falkner-Skan-Cooke (FSC) family, the evolution of the boundary layer is obtained by the simultaneous solution of the Falkner-Skan equation and the transient von-Kármán integral momentum equation. RESULTS: The model was used to investigate the relative importance of several physical parameters present in myocardial bridges. Results have been obtained for steady and unsteady flow through vessels with 0 – 85% diameter stenosis. We compare two clinical relevant cases of a myocardial bridge in the middle segment of the left anterior descending coronary artery (LAD). The pressure derived FFR of fixed and dynamic lesions has shown that the flow is less affected in the dynamic case, because the distal pressure partially recovers during re-opening of the vessel in diastole. We have further calculated the wall shear stress (WSS) distributions in addition to the location and length of the flow reversal zones in dependence on the severity of the disease. CONCLUSION: The described boundary layer method can be used to simulate frictional forces and wall shear stresses in the entrance region of vessels. Earlier models are supplemented by the viscous effects in a quasi three-dimensional vessel geometry with a prescribed wall motion. The results indicate that the translesional pressure drop and the mean FFR compares favourably to clinical findings in the literature. We have further shown that the mean FFR under the assumption of Hagen-Poiseuille flow is overestimated in developing flow conditions
Novel Diagnostic Approaches in Periodontics
[No Abstract Available
Is real GDP per capita a stationary process? Smooth transitions, nonlinear trends and unit root testing
Real GDP per capita, Unit root tests, Persistence, Nonlinearities, Smooth transitions, C22, E31, E32,
Sub-hertz fundamental linewidth photonic integrated Brillouin laser
Photonic systems and technologies traditionally relegated to table-top
experiments are poised to make the leap from the laboratory to real-world
applications through integration. Stimulated Brillouin scattering (SBS) lasers,
through their unique linewidth narrowing properties, are an ideal candidate to
create highly-coherent waveguide integrated sources. In particular,
cascaded-order Brillouin lasers show promise for multi-line emission, low-noise
microwave generation and other optical comb applications. Photonic integration
of these lasers can dramatically improve their stability to environmental and
mechanical disturbances, simplify their packaging, and lower cost. While
single-order silicon and cascade-order chalcogenide waveguide SBS lasers have
been demonstrated, these lasers produce modest emission linewidths of 10-100
kHz. We report the first demonstration of a sub-Hz (~0.7 Hz) fundamental
linewidth photonic-integrated Brillouin cascaded-order laser, representing a
significant advancement in the state-of-the-art in integrated waveguide SBS
lasers. This laser is comprised of a bus-ring resonator fabricated using an
ultra-low loss Si3N4 waveguide platform. To achieve a sub-Hz linewidth, we
leverage a high-Q, large mode volume, single polarization mode resonator that
produces photon generated acoustic waves without phonon guiding. This approach
greatly relaxes phase matching conditions between polarization modes, and
optical and acoustic modes. Using a theory for cascaded-order Brillouin laser
dynamics, we determine the fundamental emission linewidth of the first Stokes
order by measuring the beat-note linewidth between and the relative powers of
the first and third Stokes orders. Extension to the visible and near-IR
wavebands is possible due to the low optical loss from 405 nm to 2350 nm,
paving the way to photonic-integrated sub-Hz lasers for visible-light
applications