Steven Deutsch

Reduction of Turbulent Skin Friction by Microbubbles

Measurements of the effect of microbubbles on a zero pressure gradient turbulent boundary layer generated on the test section wall of a water tunnel are described. Microbubbles are created by injecting air through a 0.5 μm sintered stainless steel plate immediately upstream of a floating element drag balance. At the downstream edge of the balance the length Reynolds number is as high as ten million. Integrated skin friction reduction of greater than 80% is observed. The drag balance results are confirmed by measurements with a surface hot‐film probe. For the case in which buoyancy tends to keep the bubbles in the boundary layer, the skin friction data are shown to collapse when plotted against the ratio of air to water volume flow rate. The effects of buoyancy on skin friction reduction are also documented.

Measurements of local skin friction in a microbubble-modified turbulent boundary layer

Abstract : Local skin friction reductions have been measured using an array of flush-mounted hot-film probes in a microbubble-modified, zero pressure gradient, turbulent boundary layer. The results of earlier integrated skin friction measurements, that showed the reduction to be a function of plate orientation, gas flow rate and freestream velocity, have been confirmed both qualitatively and quantitatively. With the measurement plate above the boundary layer, it is shown that skin friction is reduced monotonically for all air flow rates at each of three freestream velocities between 4 and 17 m/sec. For the plate below the boundary layer, however, it is possible for increasing gas injection to lead to smaller local skin friction reduction at the lowest speeds. Drag reduction appears to persist for as much as 60-70 boundary layer thicknesses downstream of the injection region. It is further shown, using a probe flush mounted just upstream of the injection section, that there is no apparent upstream interference due to the gas injection. Spectral measurements indicate that microbubbles can cause a reduction of high frequency shear-stress fluctuations. This suggests a destruction of some of the turbulence in the near wall region. Skin friction, microbubble, turbulent boundary layer.

Microbubble Drag Reduction in Liquid Turbulent Boundary Layers

The interactions between a dense cloud of small bubbles and a liquid turbulent boundary layer are reviewed on the basis of available experimental observations to understand and quantify their capability for reducing skin friction. Gas bubbles are generally introduced into the boundary layer by injection through a porous surface or by electrolysis. After injection, the bubbles stay near the wall in boundary-layer-like fashion giving rise to strong gradients in both velocity and gas concentration. In general, the magnitude of the skin friction reduction increases as the volume of bubbles in the boundary layer is increased until a maximum skin friction reduction of typically 80–90% of the undisturbed skin friction level is reached. The volumetric gas flow required for this maximum is nominally equal to the volume flow of the liquid in the boundary layer. Bubble size estimates indicate that in most microbubble experiments the bubbles have been intermediate in size between the inner and outer scales of the undisturbed boundary layer. Additional studies with other nondimensional bubble sizes would be useful. However, the bubble size is most likely controlled by the injection process, and considerably different conditions would be required to change this ratio appreciably. The trajectories of the bubble clouds are primarily determined by the random effects of turbulence and bubble-bubble interactions. The effects of buoyancy represent a weaker effect. The trajectories are unlike the deterministic trajectory of an individual bubble in a time-averaged boundary layer. Bubbles are most effective in high speed boundary layers and, for the bubble sizes tested to date, produce an effect that persists for some one hundred boundary layer thicknesses. Modeling suggests that microbubbles reduce skin friction by increasing the turbulence Reynolds number in the buffer layer in a manner similar to polymers. Although the effects of microbubbles are consistent and reproducible, their primary practical limitation is the volume of gas needed. Studies aimed at reducing the volumetric gas flow requirements are recommended. Potential applications would favor high speed vehicles operating near the surface where pumping work is minimized.

Viscoelasticity of pediatric blood and its implications for the testing of a pulsatile pediatric blood pump.

Red blood cell hematocrit, aggregation and deformability, and plasma protein concentration influence the viscosity and elasticity of whole blood. These parameters affect the flow properties, especially at low shear rates (< 50 s–1). In particular, we have previously shown that the viscoelasticity of fluid affects the inlet filling characteristics and regions of flow separation in small pulsatile blood pumps. Although the viscosity of pediatric blood has been thoroughly studied, its elasticity has not been previously measured. Here we present the viscosity and elasticity of pediatric blood against shear rate for hematocrits from 19–56, measured using an oscillatory rheometer. There is little effect of patient age on blood viscoelasticity. A statistical analysis showed that when compared at constant hematocrit, blood from adult and pediatric patients had similar viscoelastic properties. We present blood analog solutions, as a function of hematocrit, constructed on the basis of the pediatric measurements. Flow field results for viscoelastic analogs of 20, 40 and 60% hematocrit and a Newtonian analog will be compared in the initial, in vitro testing of the Penn State pediatric blood pump, to determine the importance of incorporating a viscoelastic analog into the desigh interaction.

Relative blood damage in the three phases of a prosthetic heart valve flow cycle.

Blood flow through a prosthetic heart valve operating in a ventricular assist device can be subdivided into three phases: a) forward flow through an open valve, b) rapid valve closure, and c) regurgitant back flow through a closed valve. Recent studies of fluid stresses in the Penn State Electric Left Ventricular Assist Device (PS LVAD) operating under physiologic conditions indicate that Reynolds stresses of possibly hemolytic magnitude may exist in the valve area. Although several studies have been made of the fluid stresses seen in forward flow through an open valve, few have looked at valve closure or backflow, and none have related these stresses directly to blood damage. In this study, novel in vitro blood flow loops were developed to allow for the separate analysis of the three flow phases of a Bjork-Shiley monostrut Delrin disk valve operating in a PS LVAD. Forward flow through fully open aortic and mitral valves and backflow through closed valves are studied separately in flow loops driven by a roller pump with the LVAD acting as a valve housing and compliance vessel. Valve closure is investigated with a PS LVAD operating in a low volume mock circulatory loop characterized by cavitation potential through stroboscopic videography of this mock loop, using saline as the working fluid. Rate of hemolysis, characterized by the index of hemolysis, IH, is determined for each of the three flow loops charged with fresh porcine blood.(ABSTRACT TRUNCATED AT 250 WORDS)

Wall Shear-Rate Estimation Within the 50cc Penn State Artificial Heart Using Particle Image Velocimetry

Particle image velocimetry (PIV) has been gaining acceptance as a routine tool to evaluate the flow fields associated with fluid mechanical devices. We have developed algorithms to investigate the wall shear-rates within the 50cc Penn State artificial heart using low magnification, conventional particle image velocimetry (PIV). Wall shear has been implicated in clot formation, a major post-implant problem with artificial hearts. To address the issues of wall scattering and incomplete measurement volumes, associated with near wall measurements, we have introduced a zero masking and a fluid centroid shifting technique. Simulations using different velocity fields were conducted with the techniques to assess their viability. Subsequently, the techniques were applied to the experimental data collected. The results indicate that the size of the interrogation region should be chosen to be as small as possible to maximize resolution while large enough to ensure an adequate number of particles per region. In the current study, a 16 x 16 interrogation window performed well with good spatial resolution and particle density for the estimation of wall shear rate. The techniques developed with PIV allow wall shear-rate estimates to be obtained from a large number of sites at one time. Because a planar image of a flow field can be determined relatively rapidly, PIV may prove useful in any preliminary design procedure.

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.

Bubble characteristics and trajectories in a microbubble boundary layer

Optical and photographic surveys of microbubble boundary layers are presented. The results show that the outer edge of the bubble cloud diffuses away from the wall as the bubbles are swept downstream. The plate‐on‐bottom orientation contains a bubble‐free region near the wall that cannot be discerned for the plate‐on‐top configuration. Skin friction measurements made when the bubble‐free region extends to y+=200 show there is no longer any Cf reduction present suggesting bubbles are not effective when they are outside the near‐wall region of the boundary layer. Bubble sizes, which increase with airflow and distance from the injection section and decrease with free‐stream velocity, were measured to be between 150 and 1100 μm.

Correlation of In Vivo Clot Deposition With the Flow Characteristics in the 50 cc Penn State Artificial Heart: A Preliminary Study

Flow stasis in an artificial heart may provide a situation where thrombus develops. Should part, or all, of the clot dislodge, a thromboembolism may lead to stroke(s), neurologic deficits, or even death. In an effort to determine if the regime of low shear or stasis exists, a two-dimensional particle image velocimetry (PIV) system was implemented to measure the velocity field within the 50 cc Penn State Artificial Heart. The velocity measurements were decomposed nearest the wall to obtain wall shear rates along the bottom of the chamber. The PIV measurements were made in three image planes across the depth of the chamber to reconstruct a surface distribution of the wall shear rates at the bottom over the entire heart cycle. The wall shear rate is shown to be spatially nonuniform, with persistently low wall shear rates. An area near the front edge of the chamber at the bottom showed wall shear rates not exceeding 250 s−1. This was an area of clot formation seen in vivo, suggesting a link may exist between the low wall shear rate zone and thrombus formation.

Microbubble Drag Reduction

Over the past twenty-five years there has been extensive and sustained research aimed both at determining techniques for reducing skin friction drag and at explaining how successful techniques work. Certainly, one reason for pursuing this research is because of our need to drive systems faster and farther for the same power, but this is only part of the motivation. An important additional rationale for the work is bound up with our continuing fascination with boundary layers -- particularly turbulent boundary layers -- with what makes them work the way they do and how we might intervene and change them.