Jean Hertzberg

Real time multicomponent echo particle image velocimetry technique for opaque flow imaging

This letter reports on a contrast-based ultrasonic particle imaging technique (echo PIV) for measuring multicomponent velocity vectors in opaque flows with excellent temporal (up to 0.5ms) and spatial (up to 0.4mm) resolution. Ultrasound contrast microbubbles are used as flow tracers, and digitally acquired rf data are converted into B-mode images for PIV analysis. Here, velocity fields from various flow patterns (including rotating and transient vortex flows) that are difficult to measure using other opaque flow methods such as ultrasound Doppler or magnetic resonance imaging are measured using echo PIV. This nonintrusive technique should be a promising addition to opaque flow diagnostics.

Noninvasive Measurement of Steady and Pulsating Velocity Profiles and Shear Rates in Arteries Using Echo PIV: In Vitro Validation Studies

Although accurate measurement of velocity profiles, multiple velocity vectors, and shear stress in arteries is important, there is still no easy method to obtain such information in vivo. We report on the utility of combining ultrasound contrast imaging with particle image velocimetry (PIV) for noninvasive measurement of velocity vectors. This method (echo PIV) takes advantage of the strong backscatter characteristics of small gas-filled microbubbles (contrast) seeded into the flow. The method was tested in vitro. The steady flow analytical solution and optical PIV measurements (for pulsatile flow) were used for comparison. When compared to the analytical solution, both echo PIV and optical PIV resolved the steady velocity profile well. Error in shear rate as measured by echo PIV (8%) was comparable to the error of optical PIV (6.5%). In pulsatile flow, echo PIV velocity profiles agreed well with optical PIV profiles. Echo PIV followed the general profile of pulsatile shear stress across the artery but underestimated wall shear at certain time points. However, error in shear from echo PIV was an order of magnitude less than error from current shear measurement methods. These studies indicate that echo PIV is a promising technique for noninvasive measurement of velocity profiles and shear stress.

Vortex shedding behind rod stabilized flames

Abstract The stabilization of a premixed, turbulent V-shaped flame on a bluff body has been studied using laser Doppler anemometry for two-compenent velocity data, and Rayleigh scattering for point measurements of density. The conditions studied include isothermal flow, a lean ethylene flame at equivalence ratio φ = 0.62, and a very lean (φ = 0.54) flame that is close to blow off for the free stream velocity of 6 m/s. Examination of velocity and density fluctuation spectra and fluctuation intensity contour maps reveals the first quantitative evidence of vortex shedding in the wake of a V-shaped flame under unperturbed free stream conditions. Vortex shedding accompanied by high periodic fluctuation intensities have been clearly identified in the cases of a very lean flame stabilized on a bar (6 × 3mm) and on a rod (6 mm diameter), and in a slightly richer bar stablized case, but not as clearly for a richer rod-stabilized case. It is suggested that vortex shedding may play a role in the blowoff process, and that the assumption of steady recirculation is not always valid.

Development of a custom-designed echo particle image velocimetry system for multi-component hemodynamic measurements: system characterization and initial experimental results

We have recently developed an ultrasound-based velocimetry technique, termed echo particle image velocimetry (Echo PIV), to measure multi-component velocity vectors and local shear rates in arteries and opaque fluid flows by identifying and tracking flow tracers (ultrasound contrast microbubbles) within these flow fields. The original system was implemented on images obtained from a commercial echocardiography scanner. Although promising, this system was limited in spatial resolution and measurable velocity range. In this work, we propose standard rules for characterizing Echo PIV performance and report on a custom-designed Echo PIV system with increased spatial resolution and measurable velocity range. Then we employed this system for initial measurements on tube flows, rotating flows and in vitro carotid artery and abdominal aortic aneurysm (AAA) models to acquire the local velocity and shear rate distributions in these flow fields. The experimental results verified the accuracy of this technique and indicated the promise of the custom Echo PIV system in capturing complex flow fields non-invasively.

Characterizing Vortex Ring Behavior During Ventricular Filling with Doppler Echocardiography: An in Vitro Study

Doppler ultrasound color M-mode imaging (CMM) has been proposed as a noninvasive means of quantifying diastolic function by measuring flow propagation into the left ventricle. However, the relationship between CMM-derived parameters and underlying fluid dynamics is still unclear. The purpose of this study was to couple high-resolution velocimetry measurements with ultrasound Doppler and CMM measurements in order to shed light on the relationship between CMM flow propagation and inflow dynamics using a simple yet highly reproducible in vitro model of left ventricular inflow.Two Reynolds number conditions were analyzed: 4000 and 6000. Both conditions produced starting jets that formed vortex rings. Average (N=5) CMM centerline velocities were in agreement with DPIV centerline velocities, although large uncertainty in CMM data was present (uncertainty ±10 cm s−1). Two flow propagation parameters were extracted from the CMM data: the first utilized an isovelocity as the marker of flow propagation; the second used local peak velocity as the marker. The isovelocity technique followed the flow proximal to the vortex (wavefront) while the peak velocity method followed peak vorticity, and therefore vortex propagation, closely. We conclude that CMM imaging, while limited in measuring absolute velocities, can be utilized to assess inflow vortex ring properties, and thereby provide useful information on diastolic function.

Particle Image Velocimetry of Human Cough

Cough generated infectious aerosols are of interest while developing strategies for the mitigation of disease risks ranging from the common cold to SARS. In this work, the velocity field of human cough was measured using particle image velocimetry (PIV). The project subjects (total 29) coughed into an enclosure seeded with stage fog. Cough flow velocity profiles, average widths of the cough jet, and maximum cough velocities were measured. Maximum cough velocities ranged from 1.5 m/s to 28.8 m/s. The average width of all coughs ranged between 35 to 45 mm. Wide variability in the data suggests that future cough simulations consider a range of conditions.

Initial experience with the development and numerical and in vitro studies of a novel low-pressure artificial right ventricle for pediatric fontan patients

The Fontan operation, an efficient palliative surgery, is performed for patients with single-ventricle pathologies. The total cavopulmonary connection is a preferred Fontan procedure in which the superior and inferior vena cava are connected to the left and right pulmonary artery. The overall goal of this work is to develop an artificial right ventricle that can be introduced into the inferior vena cava, which would act to reverse the deleterious hemodynamics in post-Fontan patients. We present the initial design and computational analysis of a micro-axial pump, designed with the particular hemodynamics of Fontan physiology in mind. Preliminary in vitro data on a prototype pump are also presented. Computational studies showed that the new design can deliver a variety of advantageous operating conditions, including decreased venous pressure through proximal suction, increased pressure rise across the pump, increased pulmonary flows, and minimal changes in superior vena cava pressures. In vitro studies on a scaled prototype showed trends similar to those seen computationally. We conclude that a micro-axial flow pump can be designed to operate efficiently within the low-pressure, low-flow environment of cavopulmonary flows. The results provide encouragement to pursue this design to for in vitro studies and animal studies.

Stiffening-Induced High Pulsatility Flow Activates Endothelial Inflammation via a TLR2/NF-κB Pathway

Stiffening of large arteries is increasingly used as an independent predictor of risk and therapeutic outcome for small artery dysfunction in many diseases including pulmonary hypertension. The molecular mechanisms mediating downstream vascular cell responses to large artery stiffening remain unclear. We hypothesize that high pulsatility flow, induced by large artery stiffening, causes inflammatory responses in downstream pulmonary artery endothelial cells (PAECs) through toll-like receptor (TLR) pathways. To recapitulate the stiffening effect of large pulmonary arteries that occurs in pulmonary hypertension, ultrathin silicone tubes of variable mechanical stiffness were formulated and were placed in a flow circulatory system. These tubes modulated the simulated cardiac output into pulsatile flows with different pulsatility indices, 0.5 (normal) or 1.5 (high). PAECs placed downstream of the tubes were evaluated for their expression of proinflammatory molecules (ICAM-1, VCAM-1, E-selectin and MCP-1), TLR receptors and intracellular NF-κB following flow exposure. Results showed that compared to flow with normal pulsatility, high pulsatility flow induced proinflammatory responses in PAECs, enhanced TLR2 expression but not TLR4, and caused NF-κB activation. Pharmacologic (OxPAPC) and siRNA inhibition of TLR2 attenuated high pulsatility flow-induced pro-inflammatory responses and NF-κB activation in PAECs. We also observed that PAECs isolated from small pulmonary arteries of hypertensive animals exhibiting proximal vascular stiffening demonstrated a durable ex-vivo proinflammatory phenotype (increased TLR2, TLR4 and MCP-1 expression). Intralobar PAECs isolated from vessels of IPAH patients also showed increased TLR2. In conclusion, this study demonstrates for the first time that TLR2/NF-κB signaling mediates endothelial inflammation under high pulsatility flow caused by upstream stiffening, but the role of TLR4 in flow pulsatility-mediated endothelial mechanotransduction remains unclear.

Recent Progress in Flow Visualization Techniques toward the Generation of Fluid Art

This paper describes recent progress in flow visualization techniques from the viewpoint of visual art incorporating fluid motion. The images of fluid art introduced here are categorized into four groups: the reflected or refracted patterns of free surface motion in nature and in a controlled environment, the coherent turbulent phenomena of fluid flow, and the fluid motion induced by the physical properties of fluids. It is shown that flow visualization techniques, which were originally developed in the field of engineering, have been successfully applied to the creation of artistic images.

Classification of Absolute and Convective Instabilities in Premixed Bluff Body Stabilized Flames

A local, small perturbation, linear, inviscid stability analysis is applied to co-flowing reacting shear layers downstream of a bluff body flame holder. Velocity and density profiles are taken from premixed flame experiments. Linear stability theory is employed to determine the regions of transition from absolute to convective instabilities in the wakes of transverse circular cylinder and rectangular bar stabilized flames at two fuel lean conditions, one close to blow off and the other further from blow off. Instabilities in the near wake region of the flame holders are found to be absolute in nature while further downstream in the recirculation zone, the stabilities are of the convective type. Frequencies corresponding to regions of absolute instabilities are determined and compared to previously measured experimental values known to result in vortex shedding.