171 research outputs found
Reexamination of Hagen-Poiseuille flow: shape-dependence of the hydraulic resistance in microchannels
We consider pressure-driven, steady state Poiseuille flow in straight
channels with various cross-sectional shapes: elliptic, rectangular,
triangular, and harmonic-perturbed circles. A given shape is characterized by
its perimeter P and area A which are combined into the dimensionless
compactness number C = P^2/A, while the hydraulic resistance is characterized
by the well-known dimensionless geometrical correction factor alpha. We find
that alpha depends linearly on C, which points out C as a single dimensionless
measure characterizing flow properties as well as the strength and
effectiveness of surface-related phenomena central to lab-on-a-chip
applications. This measure also provides a simple way to evaluate the hydraulic
resistance for the various shapes.Comment: 4 pages including 3 figures. Revised title, as publishe
Topology and shape optimization of induced-charge electro-osmotic micropumps
For a dielectric solid surrounded by an electrolyte and positioned inside an externally biased parallel-plate capacitor, we study numerically how the resulting induced-charge electro-osmotic (ICEO) flow depends on the topology and shape of the dielectric solid. In particular, we extend existing conventional electrokinetic models with an artificial design field to describe the transition from the liquid electrolyte to the solid dielectric. Using this design field, we have succeeded in applying the method of topology optimization to find system geometries with non-trivial topologies that maximize the net induced electro-osmotic flow rate through the electrolytic capacitor in the direction parallel to the capacitor plates. Once found, the performance of the topology-optimized geometries has been validated by transferring them to conventional electrokinetic models not relying on the artificial design field. Our results show the importance of the topology and shape of the dielectric solid in ICEO systems and point to new designs of ICEO micropumps with significantly improved performance
Topology and shape optimization of induced-charge electro-osmotic micropumps
For a dielectric solid surrounded by an electrolyte and positioned inside an
externally biased parallel-plate capacitor, we study numerically how the
resulting induced-charge electro-osmotic (ICEO) flow depends on the topology
and shape of the dielectric solid. In particular, we extend existing
conventional electrokinetic models with an artificial design field to describe
the transition from the liquid electrolyte to the solid dielectric. Using this
design field, we have succeeded in applying the method of topology optimization
to find system geometries with non-trivial topologies that maximize the net
induced electro-osmotic flow rate through the electrolytic capacitor in the
direction parallel to the capacitor plates. Once found, the performance of the
topology optimized geometries has been validated by transferring them to
conventional electrokinetic models not relying on the artificial design field.
Our results show the importance of the topology and shape of the dielectric
solid in ICEO systems and point to new designs of ICEO micropumps with
significantly improved performance.Comment: 18 pages, latex IOP-style, 7 eps figure
Report No. 19: Geographic Mobility in the European Union: Optimising its Economic and Social Benefits
Joint expertise with NIRAS Consultants and AMS for the European Commission, Bonn 2008 (159 pages)
Optimal Homogenization of Perfusion Flows in Microfluidic Bio-Reactors: A Numerical Study
In recent years, the interest in small-scale bio-reactors has increased dramatically. To ensure homogeneous conditions within the complete area of perfused microfluidic bio-reactors, we develop a general design of a continually feed bio-reactor with uniform perfusion flow. This is achieved by introducing a specific type of perfusion inlet to the reaction area. The geometry of these inlets are found using the methods of topology optimization and shape optimization. The results are compared with two different analytic models, from which a general parametric description of the design is obtained and tested numerically. Such a parametric description will generally be beneficial for the design of a broad range of microfluidic bioreactors used for, e.g., cell culturing and analysis and in feeding bio-arrays
Dynamical Organization around Turbulent Bursts
The detailed dynamics around intermittency bursts is investigated in
turbulent shell models. We observe that the amplitude of the high wave number
velocity modes vanishes before each burst, meaning that the fixed point in zero
and not the Kolmogorov fixed point determines the intermittency. The phases of
the field organize during the burst, and after a burst the field oscillates
back to the laminar level. We explain this behavior from the variations in the
values of the dissipation and the advection around the zero fixed point.Comment: 4 pages, REVTex, 3 figures in one ps-fil
Influence of fractional flow reserve on grafts patency: Systematic review and patient-level meta-analysis.
To investigate the impact of invasive functional guidance for coronary artery bypass graft surgery (CABG) on graft failure.
Data on the impact of fractional flow reserve (FFR) in guiding CABG are still limited.
Systematic review and individual patient data meta-analysis were performed. Primary objective was the risk of graft failure, stratified by FFR. Risk estimates are reported as odds ratios (ORs) derived from the aggregated data using random-effects models. Individual patient data were analyzed using mixed effect model to assess relationship between FFR and graft failure. This meta-analysis is registered in PROSPERO (CRD42020180444).
Four prospective studies comprising 503 patients referred for CABG, with 1471 coronaries, assessed by FFR were included. Graft status was available for 1039 conduits at median of 12.0 [IQR 6.6; 12.0] months. Risk of graft failure was higher in vessels with preserved FFR (OR 5.74, 95% CI 1.71-19.29). Every 0.10 FFR units decrease in the coronaries was associated with 56% risk reduction of graft failure (OR 0.44, 95% CI 0.34 to 0.59). FFR cut-off to predict graft failure was 0.79.
Surgical grafting of coronaries with functionally nonsignificant stenoses was associated with higher risk of graft failure
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