Varun Reddy

Multilaboratory Particle Image Velocimetry Analysis of the FDA Benchmark Nozzle Model to Support Validation of Computational Fluid Dynamics Simulations

This study is part of a FDA-sponsored project to evaluate the use and limitations of computational fluid dynamics (CFD) in assessing blood flow parameters related to medical device safety. In an interlaboratory study, fluid velocities and pressures were measured in a nozzle model to provide experimental validation for a companion round-robin CFD study. The simple benchmark nozzle model, which mimicked the flow fields in several medical devices, consisted of a gradual flow constriction, a narrow throat region, and a sudden expansion region where a fluid jet exited the center of the nozzle with recirculation zones near the model walls. Measurements of mean velocity and turbulent flow quantities were made in the benchmark device at three independent laboratories using particle image velocimetry (PIV). Flow measurements were performed over a range of nozzle throat Reynolds numbers (Re(throat)) from 500 to 6500, covering the laminar, transitional, and turbulent flow regimes. A standard operating procedure was developed for performing experiments under controlled temperature and flow conditions and for minimizing systematic errors during PIV image acquisition and processing. For laminar (Re(throat)=500) and turbulent flow conditions (Re(throat)≥3500), the velocities measured by the three laboratories were similar with an interlaboratory uncertainty of ∼10% at most of the locations. However, for the transitional flow case (Re(throat)=2000), the uncertainty in the size and the velocity of the jet at the nozzle exit increased to ∼60% and was very sensitive to the flow conditions. An error analysis showed that by minimizing the variability in the experimental parameters such as flow rate and fluid viscosity to less than 5% and by matching the inlet turbulence level between the laboratories, the uncertainties in the velocities of the transitional flow case could be reduced to ∼15%. The experimental procedure and flow results from this interlaboratory study (available at http://fdacfd.nci.nih.gov) will be useful in validating CFD simulations of the benchmark nozzle model and in performing PIV studies on other medical device models.

Validation of a CFD Methodology for Positive Displacement LVAD Analysis Using PIV Data

Computational fluid dynamics (CFD) is used to asses the hydrodynamic performance of a positive displacement left ventricular assist device. The computational model uses implicit large eddy simulation direct resolution of the chamber compression and modeled valve closure to reproduce the in vitro results. The computations are validated through comparisons with experimental particle image velocimetry (PIV) data. Qualitative comparisons of flow patterns, velocity fields, and wall-shear rates demonstrate a high level of agreement between the computations and experiments. Quantitatively, the PIV and CFD show similar probed velocity histories, closely matching jet velocities and comparable wall-strain rates. Overall, it has been shown that CFD can provide detailed flow field and wall-strain rate data, which is important in evaluating blood pump performance.

Wavelet Transforms in the Analysis of Mechanical Heart Valve Cavitation

Cavitation is known to cause blood element damage and may introduce gaseous emboli into the cerebral circulation, increasing the patients risk of stroke. Discovering methods to reduce the intensity of cavitation induced by mechanical heart valves (MHVs) has long been an area of interest. A novel approach for analyzing MHV cavitation is presented. A wavelet denoising method is explored because currently used analytical techniques fail to suitably unmask the cavitation signal from other valve closing sounds and noise detected with a hydrophone. Wavelet functions are used to denoise the cavitation signal during MHV closure and rebound. The wavelet technique is applied to the signal produced by closure of a 29-mm Medtronic-Hall MHV in degassed water with a gas content of 5 ppm. Valve closing dynamics are investigated under loading conditions of 500, 2500, and 4500 mm Hg/s. The results display a marked improvement in the quantity and quality of information that can be extracted from acoustic cavitation signals using the wavelet technique compared to conventional analytical techniques. Time and frequency data indicate the likelihood and characteristics of cavitation formation under specified conditions. Using this wavelet technique we observe an improved signal-to-noise ratio, an enhanced time-dependent aspect, and the potential to minimize valve closing sounds, which disguise individual cavitation events. The overall goal of this work is to eventually link specific valves with characteristic waveforms or distinct types of cavitation, thus promoting improved valve designs.

Multi-Laboratory Uncertainty Analysis of PIV-Measured Flow Quantities Relevant to Blood Damage in the FDA Nozzle Model

Particle image velocimetry (PIV) has been used in regulatory submissions to the FDA for pre-clinical and post-market evaluations of flow fields in medical devices, such as artificial heart valves, blood pumps, and stents. The velocity and shear fields obtained from the PIV experiments are also used to validate computational fluid dynamics (CFD) data accompanying the submissions. However, previous studies have questioned the accuracy of PIV measurements in regions of high shear and low velocity (regions prone to hemolysis and thrombosis). Currently, there is no clear estimate of the amount of uncertainty involved in measuring various flow parameters in these high-risk regions. The objective of this study was to perform an inter-laboratory PIV study in a simplified nozzle model and quantify the uncertainties involved in measuring flow quantities relevant to blood damage, such as near-wall velocity, viscous and Reynolds shear stresses, size and velocity within recirculation regions, and for estimating an index of hemolysis.© 2011 ASME

Thrombus prediction in adult and pediatric pulsatile ventricular assist devices: The role of experimental fluid dynamics

Ventricular assist devices (VADs) are actively used for congestive heart failure and myocarditis patients but clinical issues, such as thrombosis, still remain as work continues toward development as destination therapy devices. As Penn State continues to develop smaller generation VADs, the role of experimental fluid dynamics has increased as the capability to predict thrombus deposition using wall shear estimates has improved based on animal studies. These experimental data are leading towards the development of computational simulations to identify areas not easily visualized experimentally. Particle image velocimetry, an optical measurement technique, has been adapted and the resulting velocity data, post-processed, to extract wall shear rates throughout different areas of adult (50cc) and pediatric (12cc) VADs. The results indicate areas susceptible to thrombosis and where subsequent VAD design changes have improved the flow with adequate wall shear. The role of experimental fluid dynamic measurements have greatly improved our ability to predict areas of thrombus deposition leading to improved VAD designs, provide a foundation for computer simulations with correlations to in vivo animal testing.

Two Phase Electrohydrodynamic Instability Micromixing

This work characterizes two phase electrohydrodynamic (EHD) instability micromixing of two immiscible organic and aqueous fluid phases. EHD mixing of the two phase microflow is promoted by creating an unstable flow profile by electrically inducing motion of the phase boundary. The aqueous phase is assumed to be infinitely conducting due to dissolved salt ions, while the organic phase is assumed to be non-conducting. As electrodes are biased, charges accumulate at the aqueous/organic interface. At a critical voltage the interface becomes unstable so that the aqueous and organic layers will mix. This is modeled for both inviscid and viscous flows using linear stability analysis considering the interfacial kinematic and stress conditions which predicts the stability criteria with a range of unstable wavenumbers which may be excited The mixing of an unstable “sausaging” and “kink” modes are visualized using epifluorescent microscopy of the dyed organic phase. The characteristic unstable wavenumbers predicted using linear stability theory are determined from the power spectrum of the captured images and compared to the analytical model. Onset of instability is seen at 40 volts RMS at a frequency of 250 kHz. This voltage corresponds to an electric field of Eo = 8 × 105 V/m across the organic phase. The instability becomes progressively more dynamic as the field strength is increased. The system recovers after the field is removed. At low field strengths the theoretical field and fastest growth wavenumbers for mixing compares favorably with the initially applied field whereas at higher field strengths the theoretical field is much larger than the initially applied field. This is attributed to the larger level of mixing and the ability of the instability to grow beyond the linear range while the electric field increases as the mixing process occurs due to entrainment of highly conductive fluid decreasing the effective dielectric spacing.Copyright