Wall shear stress exposure time: a Lagrangian measure of near-wall stagnation and concentration in cardiovascular flows

Near-wall transport is of utmost importance in connecting blood flow mechanics with cardiovascular disease progression. The near-wall region is the interface for biologic and pathophysiologic processes such as thrombosis and atherosclerosis. Most computational and experimental investigations of blood flow implicitly or explicitly seek to quantify hemodynamics at the vessel wall (or lumen surface), with wall shear stress (WSS) quantities being the most common descriptors. Most WSS measures are meant to quantify the frictional force of blood flow on the vessel lumen. However, WSS also provides an approximation to the near-wall blood flow velocity. We herein leverage this fact to compute a wall shear stress exposure time (WSSET) measure that is derived from Lagrangian processing of the WSS vector field. We compare WSSET against the more common relative residence time (RRT) measure, as well as a WSS divergence measure, in several applications where hemodynamics are known to be important to disease progression. Because these measures seek to quantify near-wall transport and because near-wall transport is important in several cardiovascular pathologies, surface concentration computed from a continuum transport model is used as a reference. The results show that compared to RRT, WSSET is able to better approximate the locations of near-wall stagnation and concentration build-up of chemical species, particularly in complex flows. For example, the correlation to surface concentration increased on average from 0.51 (RRT) to 0.79 (WSSET) in abdominal aortic aneurysm flow. Because WSSET considers integrated transport behavior, it can be more suitable in regions of complex hemodynamics that are traditionally difficult to quantify, yet encountered in many disease scenarios.</p

Fast, flexible and low-cost multiphase blood analogue for biomedical and energy applications

During the last two decades, several kinds of particulate blood analogue fluids have been proposed, but none of those were able to mimic the multiphase effects of real blood. Hence, it is clear that it is crucial to develop a simple multiphase blood analogue to be used for in vitro experiments at both macro- and microscale level. To the best of our knowledge, the present work shows for the first time a straightforward and extremely stable blood analogue fluid able to mimic multiphase blood flow phenomena. The present work proposes a simple, low-cost and stable multiphase blood analogue with the ability to mimic microscale blood flow phenomena. The proposed analogue fluid is composed of Brij L4 surfactant micelles suspended in pure water and is extremely easy to be produced. To investigate the ability of this analogue to mimic microscale blood flow phenomena, flow visualizations were performed in a microchannel constriction. In vitro blood phenomena were compared with the measurements performed with the proposed analogue fluid. Additionally, rheological measurements of the multiphase blood analogue were acquired by means of a stress-controlled rheometer and compared with in vitro blood sample viscosity curves. Overall, the results indicate that it is possible to produce a stable particulate fluid with geometrical, mechanical and flow properties similar to in vitro blood. Hence, the proposed analogue has a great potential to be used in flow experiments from macro- to nanoscale levelsFundação para a Ciência e a Tecnologia (FCT) under the strategic grants UIDB/04077/2020, UIDB/04436/2020 and UIDB/00532/2020. The authors are also grateful for the funding of FCT through the projects NORTE-01-0145-FEDER-029394, NORTE-01-0145-FEDER-030171 and POCI-01-0145-FEDER-016861 (PTDC/QEQ-FTT/4287/2014) funded by COMPETE2020, NORTE2020, PORTUGAL2020, and FEDER. The authors also acknowledge FCT for partially financing the research under the framework of the project UTAP-EXPL/CTE/0064/2017, financiado no âmbito do Projeto 5665—Parcerias Internacionais de Ciência e Tecnologia, UT Austin Programme. Partial support from the Spanish Ministry of Science and Education (grant no. DPI2016-78887) and Junta de Extremadura (grants no. GR15014 and IB18005, partially financed by FEDER funds) is gratefully acknowledged too

Lagrangian Postprocessing of Computational Hemodynamics

Recent advances in imaging, modeling and computing have rapidly expanded our capabilities to model hemodynamics in the large vessels (heart, arteries and veins). This data encodes a wealth of information that is often under-utilized. Modeling (and measuring) blood flow in the large vessels typically amounts to solving for the time-varying velocity field in a region of interest. Flow in the heart and larger arteries is often complex, and velocity field data provides a starting point for investigating the hemodynamics. This data can be used to perform Lagrangian particle tracking, and other Lagrangian-based postprocessing. As described herein, Lagrangian methods are necessary to understand inherently transient hemodynamic conditions from the fluid mechanics perspective, and to properly understand the biomechanical factors that lead to acute and gradual changes of vascular function and health. The goal of the present paper is to review Lagrangian methods that have been used in post-processing velocity data of cardiovascular flows