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Abstract: Power Doppler imaging (PDI) has been shown to be influenced by the wall filter when assessing arterial stenoses. Real-time 3-D Doppler imaging may likely become a widespread practice in the near future, but how the wall filter could affect PDI during the cardiac cycle has not been investigated. The objective of the study was to demonstrate that the wall filter may produce unexpected major signal losses in real-time 3-D PDI. To test our hypothesis, we first validated binary images obtained from analytical simulations with in vitro PDI acquisitions performed in a tube under pulsatile flow conditions. We then simulated PDI images in the presence of a severe stenosis, considering physiological conditions by finite element modeling. Power Doppler imaging simulations revealed important signal losses within the lumen area at different instants of the flow cycle, and there was a very good concordance between measured and predicted PDI binary images in the tube. Our results show that the wall filter may induce severe PDI signal losses that could negatively influence the assessment of vascular stenosis. Clinicians should therefore be aware of this cause of signal loss to properly interpret power Doppler angiographic images. (E-mail: guy.cloutier@umontreal.ca) [Copyright &y& Elsevier]

• Detection of particles and classification according to their size. • Construction of a velocity distribution function characterizing the motion of particles within each distinct size group. • Determination of particle shape and orientation. • Application to the complex flow of red blood cells with agglomeration and breakage. • Through verification of the software for this application. Over the past several decades, image processing and automated tracking of particles has emerged as a useful tool for the study of biological particles behaviour. This article describes an open-source computational implementation of a method for determining particle velocity and size distributions of large groups of particles by analyzing video sequences acquired using a video-microscopic systems. Although, in this study, red blood cells are used as a subject, this implementation can be used for any particle-laden flow where particles present a range of sizes and details of the velocity distribution of each size is of interest. From each single image, the current program detects particles and classifies them according to their size. It uses sequential images to track particles and compute the instantaneous velocity distribution of the particles. The tool can also assign an ellipse to each particle and report the major axis, the minor axis and the orientation of particles in each image. Use of the program improves repeatability of image processing and is suitable for studies related to particle dynamics, colloids, and microfluidic flow measurement. The size distribution and the velocity distribution of particles is often useful in the study of effect of parameters like shear stress on particle collision rate, agglomeration and breakage rate. [ABSTRACT FROM AUTHOR]

• Microparticle wall adhesion in blood was investigated in circular microchannel. • RBC aggregation enhanced particle wall adhesion. • Particle adhesion is stronger at low shear rates, when aggregation occurred. The wall adhesion of 1 µm microparticles in human blood was studied in circular microchannels. The level of particle wall adhesion was measured for varying levels of shear rate and varying degrees of red blood cell aggregation, which was modulated by the addition of macromolecule dextran 500. The blood preparations were injected into PDMS microfluidic devices that were modified to have circular channels, better matching the geometry of physiological microcirculation compared to square channels or Couette flow systems. The circular walls of the microchannels were embedded with biotinylated phospholipids to which marginating microspheres coated with streptavidin bound. The particle wall adhesion was evaluated by counting the particles adhering to the channel wall after flushing the channel. Blood preparations of five dextran concentrations (including baseline case of 0%) were tested for four flow velocities, to quantify the effects of aggregation for varying shear rate. It was found that the level of particle wall adhesion was positively correlated with the level of RBC aggregation, particularly at low shear rates, when aggregation was enhanced. The particle adhesion was especially enhanceat aggregation levels in the range of physiological aggregation levels of whole blood, suggesting that RBC aggregation plays an important role in the dynamic of platelets and leukocytes in vivo. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The focus of this work is the characterization of the thermal and hydraulic performance of pin fin arrays produced using the cold spray additive manufacturing process. The heat transfer and the pressure losses of 1 mm high round base, square base and diamond base tapered pin fin arrays were assessed in both the inline and staggered configurations for fin densities of 8 fpi and 12 fpi. These performances were correlated to the turbulence intensity and the turbulent kinetic energy values at various locations in the flow, measured by micro-particle image velocimetry. It was inferred that the form drag is the main contributor to the pressure loss and was found to correlate with the flow turbulent kinetic energy in the fin wake. In contrast, the convective heat transfer coefficient correlated better with the turbulence intensity, leading to the conclusion that heat transfer is not dictated solely by the turbulent kinetic energy, but by the relative strength of the velocity fluctuations with respect to the average flow velocity at the same location. Furthermore, the flow structures for the different fin array samples were visualized and are discussed. Finally, it was found that although the samples had very varied thermal and hydrodynamic performances as a function of Reynolds number, the different samples at a given fin density had similar thermal conductances at a given pumping power. [ABSTRACT FROM AUTHOR]

The focus of this work is the identification and analysis of the flow structures found in pyramidal pin fin arrays produced using the Masked Cold Gas Dynamic Spraying (MCGDS) additive manufacturing process. The observed flow structures are described, with classic double recirculation patterns being identified. The turbulence intensity levels of the flow in the axial flow channels was measured and it was found that although the flow rates considered in this work correspond to low Reynolds numbers (500–3000), significant turbulence intensity levels are found. Furthermore, these levels increase as the flow progresses downstream, even though the large scale flow structures are well established after a few rows (as little as two in this case). A slight misalignment of the axial and transverse flow channels resulting from imperfections in the masks caused a bypass flow structure to arise in the wake of the pin fins, replacing the double recirculation pattern observed when there is no such misalignment. A CFD model was used to investigate the effect of these misalignments on heat transfer efficiency and predicted that there would be no significant effect in the configurations studied. Finally, this work shows the importance of not only considering the flow structures in the fin’s wake, but also the effect of these structures on the turbulence levels of the axial flow channels, which could significantly affect the thermal and hydrodynamic performance. [ABSTRACT FROM AUTHOR]