The effects of non-Newtonian blood modeling and pulsatility on hemodynamics in the food and drug administration’s benchmark nozzle model

BACKGROUND: Computational fluid dynamics (CFD) is an important tool for predicting cardiovascular device performance. The FDA developed a benchmark nozzle model in which experimental and CFD data were compared, however, the studies were limited by steady flows and Newtonian models. OBJECTIVE: Newtonian and non-Newtonian blood models will be compared under steady and pulsatile flows to evaluate their influence on hemodynamics in the FDA nozzle. METHODS: CFD simulations were validated against the FDA data for steady flow with a Newtonian model. Further simulations were performed using Newtonian and non-Newtonian models under both steady and pulsatile flows. RESULTS: CFD results were within the experimental standard deviations at nearly all locations and Reynolds numbers. The model differences were most evident at Re = 500, in the recirculation regions, and during diastole. The non-Newtonian model predicted blunter upstream velocity profiles, higher velocities in the throat, and differences in the recirculation flow patterns. The non-Newtonian model also predicted a greater pressure drop at Re = 500 with minimal differences observed at higher Reynolds numbers. CONCLUSIONS: An improved modeling framework and validation procedure were used to further investigate hemodynamics in geometries relevant to cardiovascular devices and found that accounting for blood’s non-Newtonian and pulsatile behavior can lead to large differences in predictions in hemodynamic parameters.

On the lifespan of recirculating suspensions with pulsatile flow

A Lagrangian analysis is performed to measure the rate at which recirculating fluid is replaced (depleted) in pulsatile flows. Based on this approach, we then investigate how depletion is affected in dense suspensions. Experiments are conducted for pure liquid as well as suspensions with volume fractions of $\varPhi =5\,\%$ , 10 % and 20 %. Using Lagrangian tracking and pathline extension techniques, the depletion of the recirculation region is quantified via the trajectories of individual fluid parcels exiting the domain. Pulsatile flows with varying concentrations of hydrogel beads, up to a volume fraction of 20 %, are compared at mean Reynolds numbers of $Re=4800$ , 9600 and 14 400, while the Strouhal number ( $St=0.04$ , 0.08 and 0.15) and amplitude ratio ( $\lambda =0.25$ , 0.50 and 0.95) are systematically varied. A so-called ‘depletion efficiency’ is calculated for each test case, which is shown to increase with increasing Strouhal number and amplitude ratio. For most pulsatile cases, periodic vortex formation significantly increases depletion efficiency through enhanced entrainment of recirculating fluid. Conversely, low-amplitude pulsatile flows are dominated by Kelvin–Helmholtz instabilities, which do not penetrate into the recirculation region, and thus their depletion efficiency is markedly lower as a result. The efficiency trends and depletion mechanisms remain virtually unchanged between the pure liquid and each of the suspension concentrations under almost all flow conditions, which forms an unexpected conclusion. The only exception is for low-amplitude and steady flows, where increasing the suspension volume fraction is shown to suppress fluid transport across the shear layer, which in turn slows depletion and decreases the overall depletion efficiency.

TR-PIV in highly pulsatile flow: pulsation frequency and wake dynamics case study

Experimental study of highly pulsatile flows presents a number of challenges, primarily the inherently large dynamic range of velocities. Herein, we use time-resolved particle image velocimetry processed with a technique known as pyramid sum-of-correlation to study highly pulsatile flow around a surface-mounted hemisphere. The frequency of pulsation is varied from low- frequency, quasi-steady pulsation to high frequency pulsation. We present a conceptual overview of the wake regimes observed and compare the flow physics of the high-frequency case to that of a vortex ring produced by a single impulse of fluid.

Experimental Resonances in Viscoelastic Microfluidics

Pulsatile flows of viscoelastic fluids are very important for lab-on-a-chip devices, because most biofluids have viscoelastic character and respond distinctively to different periodic forcing. They are also very important for organ-on-a-chip devices, where the natural mechanical conditions of cells are emulated. The resonance frequency of a fluid refers to a particular pulsatile periodicity of the pressure gradient that maximizes the amplitude of flow velocity. For viscoelastic fluids, this one has been measured experimentally only at macroscales, since fine tuning of rheological properties and system size is needed to observe it at microscales. We study the dynamics of a pulsatile (zero-mean flow) fluid slug formed by a viscoelastic fluid bounded by two air-fluid interfaces, in a microchannel of polymethyl methacrylate. We drive the fluid slug by a single-mode periodic pressure drop, imposed by a piezoactuator. We use three biocompatible polymer solutions of polyethylene oxide as model viscoelastic fluids, and find resonances. We propose a model accounting for surface tension and fluid viscoelasticity that has an excellent agreement with our experimental findings. It also provides an alternative way of measuring relaxation times. We validate the method with parameters reported in the literature for two of the solutions, and estimate the relaxation time for the third one.

Total Artificial Heart Update

The SynCardia Total Artificial Heart (TAH, SynCardia Systems, Tucson, AZ) is the only biventricular cardiac replacement approved for bridge to transplantation by the U.S. Food and Drug Administration (FDA) and which carries the European Union CE mark. It has been implanted in about 2000 patients. In experienced centers, 60 to 80 % of implanted patients have been transplanted and over 80 % of those transplanted have lived for over 1 year. The SynCardia TAH has supported potential cardiac recipients with irreversible biventricular failure for up to 6 years, providing physiologic pulsatile flows of 6 to 8 L/min at filling pressures of less than 10 mmHg allowing for optimal perfusion and recovery of organs such as the kidneys and liver. It is a tested device that provides a method for recovering potential transplant candidates who rapidly decompensate from biventricular failure or who have chronic cardiac failure from a variety of etiologies. This article covers the history, mechanical function and monitoring, implantation, patient selection and management, and outpatient use. It also reviews outcome data from the original FDA study as well as contemporary data from experienced centers.

Early turbulence and pulsatile flows enhance diodicity of Tesla’s macrofluidic valve

Microfluidics has enabled a revolution in the manipulation of small volumes of fluids. Controlling flows at larger scales and faster rates, or macrofluidics, has broad applications but involves the unique complexities of inertial flow physics. We show how such effects are exploited in a device proposed by Nikola Tesla that acts as a diode or valve whose asymmetric internal geometry leads to direction-dependent fluidic resistance. Systematic tests for steady forcing conditions reveal that diodicity turns on abruptly at Reynolds number $${\rm{Re}}\approx 200$$ Re ≈ 200 and is accompanied by nonlinear pressure-flux scaling and flow instabilities, suggesting a laminar-to-turbulent transition that is triggered at unusually low $${\rm{Re}}$$ Re . To assess performance for unsteady forcing, we devise a circuit that functions as an AC-to-DC converter, rectifier, or pump in which diodes transform imposed oscillations into directed flow. Our results confirm Tesla’s conjecture that diodic performance is boosted for pulsatile flows. The connections between diodicity, early turbulence and pulsatility uncovered here can inform applications in fluidic mixing and pumping.

Volumetric Lattice Boltzmann Method for Wall Stresses of Image-based Pulsatile Flows

Image-based computational fluid dynamics (CFD) has become a new capability for determining wall stresses of pulsatile flows. However, a computational platform that directly connects image information to pulsatile wall stresses is lacking. Prevailing methods rely on manual crafting of a hodgepodge of multidisciplinary software packages, which is usually laborious and error prone. We present a new technique to compute wall stresses in image-based pulsatile flows using the lattice Boltzmann method (LBM). The novelty includes: (1) a unique image processing to extract flow domain and local wall normality, (2) a seamless connection between image extraction and CFD, (3) an en-route calculation of strain-rate tensor, and (4) GPU acceleration (not included here). We first generalize the streaming operation in the LBM and then conduct an application study for laminar and turbulent pulsatile flows in an image-based pipe (Reynolds number: 10 to 5000). The computed pulsatile velocity and shear stress are in good agreement with Womersley solutions for laminar flows and concurrent laboratory measurements for turbulent flows. This technique is being used to study (1) the hemodynamic wall stresses in inner choroid endothelium, (2) the drag force in sand flows, and (3) effects of waste streams on ion exchange kinetics in porous media.