Showing papers in "Journal of Fluids Engineering-transactions of The Asme in 2013"

Combined Experimental and Computational Investigation of Unsteady Structure of Sheet/Cloud Cavitation

Abstract: The objective of this paper is to apply combined experimental and computational modeling to investigate unsteady sheet/cloud cavitating flows. In the numerical simulations, a filter-based density corrected model (FBDCM) is introduced to regulate the turbulent eddy viscosity in both the cavitation regions on the foil and in the wake, which is shown to be critical in accurately capturing the unsteady cavity shedding process, and the corresponding velocity and vorticity dynamics. In the experiments, high-speed video and particle image velocimetry (PIV) technique are used to measure the flow velocity and vorticity fields, as well as cavitation patterns. Results are presented for a Clark-Y hydrofoil fixed at an angle of attack of a ¼8deg at a moderate Reynolds number, Re ¼7 � 10 5 , for both subcavitating and sheet/cloud cavitating conditions. The results show that for the unsteady sheet/cloud cavitating case, the formation, breakup, shedding, and collapse of the sheet/cloud cavity lead to substantial increase in turbulent velocity fluctuations in the cavitating region around the foil and in the wake, and significantly modified the wake patterns. The turbulent boundary layer thickness is found to be much thicker, and the turbulent intensities are much higher in the sheet/cloud cavitating case. Compared to the wetted case, the wake region becomes much broader and is directed toward the suction side instead of the pressure side for the sheet/cloud cavitation case. The periodic formation, breakup, shedding, and collapse of the sheet/cloud cavities, and the associated baroclinic and viscoclinic torques, are shown to be important mechanisms for vorticity production and modification. [DOI: 10.1115/1.4023650]

Experimental and Numerical Studies for a High Head Francis Turbine at Several Operating Points

Abstract: Experimental and numerical studies on a high head model Francis turbine were carried out over the entire range of turbine operation. A complete Hill diagram was constructed and pressure-time measurements were performed at several operating conditions over the entire range of power generation by installing pressure sensors in the rotating and stationary domains of the turbine. Unsteady numerical simulations were performed at five operating conditions using two turbulent models, shear stress transport (SST) k-ω and standard k-e and two advection schemes, high resolution and second order upwind. There was a very small difference (0.85%) between the experimental and numerical hydraulic efficiencies at the best efficiency point (BEP); the maximum difference (14%) between the experimental and numerical efficiencies was found at lower discharge turbine operation. Investigation of both the numerical and experimental pressure-time signals showed that the complex interaction between the rotor and stator caused an output torque oscillation over a particular power generation range. The pressure oscillations that developed due to guide vanes and runner blades interaction propagate up to the trailing edge of the blades. Fourier analysis of the signals revealed the presence of a vortex rope in the draft tube during turbine operation away from the BEP.

On the Role of Leading-Edge Bumps in the Control of Stall Onset in Axial Fan Blades

Abstract: Taking a lead from the humpback whale flukes, characterized by a series of bumps that result in a sinusoidal-like leading edge, this paper reports on a three-dimensional numerical study of sinusoidal leading edges on cambered airfoil profiles. The turbulent flow around the cambered airfoil with the sinusoidal leading edge was computed at different angles of attack with the open source solver OpenFOAM, using two different eddy viscosity models integrated to the wall. The reported research focused on the effects of the modified leading edge in terms of lift-to-drag performance and the influence of camber on such parameters. For these reasons a comparison with a symmetric airfoil is provided. The research was primarily concerned with the elucidation of the fluid flow mechanisms induced by the bumps and the impact of those mechanisms on airfoil performance, on both symmetric and cambered profiles. The bumps on the leading edge influenced the aerodynamic performance of the airfoil, and the lift curves were found to feature an early recovery in post-stall for the symmetric profile with an additional gain in lift for the cambered profile. The bumps drove the fluid dynamic on the suction side of the airfoil, which in turn resulted in the capability to control the separation at the trailing edge in coincidence with the peak of the sinusoid at the leading edge.

Analysis of S Characteristics and Pressure Pulsations in a Pump-Turbine With Misaligned Guide Vanes

Abstract: Growing environmental concerns and the need for better power balancing and frequency control have increased attention in renewable energy sources such as the reversible pump-turbine which can provide both power generation and energy storage Pump-turbine operation along the S-shaped curve can lead to difficulties in loading the rejection process with unusual increases in water pressure, which lead to machine vibrations Pressure fluctuations are the primary reason for unstable operation of pump-turbines Misaligned guide vanes (MGVs) are widely used to control the stability in the S region There have been experimental investigations and computational fluid dynamics (CFD) simulations of scale models with aligned guide vanes and MGVs with spectral analyses of the S curve characteristics and the pressure pulsations in the frequency and time-frequency domains at runaway conditions The course of the S characteristic is related to the centrifugal force and the large incident angle at low flow conditions with large vortices forming between the guide vanes and the blade inlets and strong flow recirculation inside the vaneless space as the main factors that lead to the S-shaped characteristics Preopening some of the guide vanes enables the pump-turbine to avoid the influence of the S characteristic However, the increase of the flow during runaway destroys the flow symmetry in the runner leading to all asymmetry forces on the runner that leads to hydraulic system oscillations The MGV technique also increases the pressure fluctuations in the draft tube and has a negative impact on stable operation of the unit

Numerical and Experimental Study of Axial Force and Hydraulic Performance in a Deep-Well Centrifugal Pump With Different Impeller Rear Shroud Radius

Abstract: A multistage deep-well centrifugal pump (DCP) with different impeller rear shroud radius have been investigated both numerically and experimentally under multiconditons, which aims at studying the influence of impeller rear shroud radius to the axial force and pump hydraulic performance. During this study, a two-stage DCP equipped with three different impellers was simulated employing the commercial computational fluid dynamics (CFD) software ANYSY-Fluent to solve the Navier-Stokes equations for three-dimensional steady flow. High-quality structured grids were meshed on the whole computational domain. Test results were acquired by prototype experiments, and then compared with the predicted pump performance and axial force. The static pressure distribution in the pump passage obtained by numerical simulation was analyzed. The results indicated that the appropriate impeller rear shroud radius could improve the pump performance and lower the axial force significantly.

Energy Balance in Cavitation Erosion: From Bubble Collapse to Indentation of Material Surface

Abstract: An original approach based on energy balance between vapor bubble collapse, emitted pressure wave, and neighboring solid wall response was proposed, developed, and tested to estimate the aggressiveness of cavitating flows. In the first part of the work, to improve a prediction method for cavitation erosion (Fortes-Patella and Reboud, 1998, “A New Approach to Evaluate the Cavitation Erosion Power,” ASME J. Fluids Eng., 120(2), pp. 335–344; Fortes-Patella and Reboud, 1998, “Energetical Approach and Impact Efficiency in Cavitation Erosion,” Proceedings of Third International Symposium on Cavitation, Grenoble, France), we were interested in studying the pressure waves emitted during bubble collapse. The radial dynamics of a spherical vapor/gas bubble in a compressible and viscous liquid was studied by means of Keller's and Fujikawa and Akamatsu's physical models (Prosperetti, 1994, “Bubbles Dynamics: Some Things we did not Know 10 Years Ago,” Bubble Dynamics and Interface Phenomena, Blake, Boulton-Stone, Thomas, eds., Kluwer Academic Publishers, Dordrecht, the Netherlands, pp. 3–15; Fujikawa and Akamatsu, 1980, “Effects of Non-Equilibrium Condensation of Vapor on the Pressure Wave Produced by Collapse of a Bubble in Liquid,” J. Fluid Mech., 97(3), pp. 481–512). The pressure amplitude, the profile, and the energy of the pressure waves emitted during cavity collapses were evaluated by numerical simulations. The model was validated by comparisons with experiments carried out at Laboratoire Laser, Plasma et Procedes Photoniques (LP3-IRPHE) (Marseille, France) with laser-induced bubble (Isselin et al., 1998, “Investigations of Material Damages Induced by an Isolated Vapor Bubble Created by Pulsed Laser,” Proceedings of Third International Symposium on Cavitation, Grenoble, France; Isselin et al., 1998, “On Laser Induced Single Bubble Near a Solid Boundary: Contribution to the Understanding of Erosion Phenomena,” J. Appl. Phys., 84(10), pp. 5766–5771). The efficiency of the first collapse ηwave/bubble (defined as the ratio between pressure wave energy and initial bubble potential energy) was evaluated for different bubble collapses. For the cases considered of collapse in a constant-pressure field, the study pointed out the strong influence of the air contents on the bubble dynamics, on the emitted pressure wave characteristics, and on the collapse efficiency. In the second part of the study, the dynamic response and the surface deformation (i.e., pit profile and pit volume) of various materials exposed to pressure wave impacts was simulated making use of a 2D axisymmetric numerical code simulating the interaction between pressure wave and an elastoplastic solid. Making use of numerical results, a new parameter β (defined as the ratio between the pressure wave energy and the generated pit volume) was introduced and evaluated for three materials (aluminum, copper, and stainless steel). By associating numerical simulations and experimental results concerning pitted samples exposed to cavitating flows (volume damage rate), the pressure wave power density and the flow aggressiveness potential power were introduced. These physical properties of the flow characterize the cavitation intensity and can be related to the flow hydrodynamic conditions. Associated to β and ηwave/bubble parameters, these power densities appeared to be useful tools to predict the cavitation erosion power.

Hydraulic Improvement to Eliminate S-Shaped Curve in Pump Turbine

Abstract: For pump turbines, an S-shaped curve can lead to failures in synchronization. To improve the hydraulic design, the component that is responsible for the formation of the S-shaped curve was identified by a hydraulic loss analysis using previous computational fluid dynamics (CFD) results, which indicates that the formation of the S-shaped curve can be ascribed to the runner. To improve the hydraulic design of the runner, a simple numerical approach for direct problem analysis was proposed, based on the bladeloading distributions of runners with and without an S-shaped curve, and directly analyzed. It was implied from the differences in the blade-loading distributions that, when the meridional passage was broadened, the formation of the S-shaped curve was suppressed. To validate this, two runners with different meridional sections were designed by means of the inverse design method. Through model tests, it was verified that the S-shaped curve was eliminated completely and the performance curve of the modified hydraulic model satisfied the requirements for safe operation in a pumped storage plant. [DOI: 10.1115/1.4023851]

Flow-Feedback Method for Mitigating the Vortex Rope in Decelerated Swirling Flows

Abstract: When reaction hydraulic turbines are operated far from the design operating regime, particularly at partial discharge, swirling flow instability is developed downstream of the runner, in the discharge cone, with a precessing helical vortex and its associated severe pressure fluctuations. Bosioc et al. (2012, “Unsteady Pressure Analysis of a Swirling Flow With Vortex Rope and Axial Water Injection in a Discharge Cone,” ASME J. Fluids Eng., 134(8), p. 081104) showed that this instability can be successfully mitigated by injecting a water jet along the axis. However, the jet discharge is too large to be supplied with high pressure water bypassing the runner, since this discharge is associated with the volumetric loss. In the present paper we demonstrate that the control jet injected at the inlet of the conical diffuser can actually be supplied with water collected from the discharge cone outlet, thus introducing a new concept of flow feedback. In this case, the jet is driven by the pressure difference between the cone wall, where the feedback spiral case is located, and the pressure at the jet nozzle outlet. In order to reach the required threshold value of the jet discharge, we also introduce ejector pumps to partially compensate for the hydraulic losses in the return pipes. Extensive experimental investigations show that the wall pressure fluctuations are successfully mitigated when the jet reaches 12% of the main flow discharge for a typical part load turbine operating regime. About 10% of the jet discharge is supplied by the plain flow feedback, and only 2% boost is insured by the ejector pumps. As a result, this new approach paves the way towards practical applications in real hydraulic turbines.

A Numerical Analysis of Hydrogen Underexpanded Jets Under Real Gas Assumption

Abstract: This work examines the fluid dynamic structure of underexpanded gas jets by using a high-performance computing (HPC) methodology in order to untangle the question of whether it is necessary to include the real gas assumption dealing with hydrogen jets. The answer to this question is needed in order to guarantee accurate numerical simulations of such jets in practical engineering applications, such as direct-injection hydrogen engines. An axial symmetric turbulent flow model, which solves the Favre-averaged Navier–Stokes equations for a multicomponent gas mixture, has been implemented and validated. The flow model has been assessed by comparing spreading and centerline property decay rates of subsonic jets at different Mach numbers with those obtained by both theoretical considerations and experimental measurements. Besides, the Mach disk structure of underexpanded jets has been recovered, thus confirming the suitability and reliability of the computational model. To take into account the effects of real gases, both van der Waals and Redlich–Kwong equations of state have been implemented. The analysis of a highly underexpanded hydrogen jet with total pressure equal to 75 MPa, issuing into nitrogen at 5 MPa, shows that the use of real gas equations of state affects significantly the jet structure in terms of temperature, pressure, and Mach number profiles along the jet centerline and also in terms of jet exit conditions, with differences up to 38%.

Investigation of the Flow Structures in Supersonic Free and Impinging Jet Flows

Abstract: A numerical study of compressible jet flows is carried out using Reynolds averaged Navier-Stokes (RANS) turbulence models such as k-E and k-ω-SST. An experimental investigation is performed concurrently using high-speed optical methods such as Schlieren photography and shadowgraphy. Numerical and experimental studies are carried out for the compressible impinging at various impinging angles and nozzle-to-wall distances. The results from both investigations converge remarkably well and agree with experimental data from the open literature. From the flow visualizations of the velocity fields, the RANS simulations accurately model the shock structures within the core jet region. The first shock cell is found to be constraint due to the interaction with the bow-shock structure for nozzle-to-wall distance less than 1.5 nozzle diameter. The results from the current study show that the RANS models utilized are suitable to simulate compressible free jets and impinging jet flows with varying impinging angles. © 2013 by ASME.

Numerical Simulation of the Breakup of Elliptical Liquid Jet in Still Air

Abstract: The numerical simulation of liquid jets ejecting from a set of elliptical orifices with different aspect ratios between 1 (circular) and 3.85 is performed for several Weber numbers, ranging from 15 to 330. The axis-switching phenomenon and breakup length of the jets are characterized by means of a volume of fluid (VOF) method, together with a dynamic mesh refinement model. This three-dimensional simulation is compared with a recent experimental work and the results agree well. It is concluded that for Weber numbers ranging from 15 to 100, by increasing the Weber number, the breakup length of the liquid jet increases, reaches a peak, and then decreases suddenly.

Influence of Bubbles on the Turbulence Anisotropy

Abstract: Direct numerical simulation (DNS) with interface tracking of turbulent bubbly flows is becoming a major tool in advancing our knowledge in the area of multiphase modeling research. A comprehensive analysis of the turbulent flow structure allows us to evaluate the state-of-the-art computational multiphase fluid dynamics (CMFD) models and to propose new closure laws. The presented research will demonstrate how the multiphase DNS data can inform the development of computational fluid dynamics (CFD) models. In particular, the Reynolds stress distribution will be evaluated for single- and two-phase bubbly flows and the level of turbulence anisotropy will be measured in several scenarios. The results will help determine if the isotropic turbulent models are suitable for bubbly flow applications or if there is a strong need to apply and develop Reynolds-stress turbulent models for two-phase flow CFD modeling.

Study of Quantitative Numerical Prediction of Cavitation Erosion in Cavitating Flow

Abstract: Cavitation erosion is a material damage phenomenon caused by the repeated application of impulsive pressure on a material surface induced by bubble collapse, and the establishment of a method by which to numerically predict cavitation erosion is desired. In the present study, a numerical quantitative prediction method of cavitation erosion in a cavitating flow is proposed. In the present method, a one-way coupled analysis of a cavitating flow field based on a gas-liquid two-phase Navier–Stokes equation (Eulerian) and bubbles in the cavitating flow by bubble dynamics (Lagrangian) is used to treat temporally and spatially different scale phenomena, such as the macroscopic phenomenon of a cavitating flow and the microscopic phenomenon of bubble collapse. Impulsive pressures acting on a material surface are evaluated based on the bubble collapse position, time, and intensity, and the erosion rate is quantitatively predicted using an existing material-dependent relationship between the impulsive energy (square of the impulsive force) and the maximum erosion rate. The erosion rate on a NACA0015 hydrofoil surface in an unsteady transient cavitating flow is predicted by the proposed method. The distribution of the predicted erosion rate corresponds qualitatively to the distribution of an experimental surface roughness increment of the same hydrofoil. Furthermore, the predicted erosion rate considering the bubble nuclei distribution is found to be of the same order of magnitude as the actual erosion rate, which indicates that considering bubble nuclei distribution is important for the prediction of cavitation erosion and that the present prediction method is valid to some degree.

Identification of Critical Net Positive Suction Head From Noise and Vibration in a Radial Flow Pump for Different Leading Edge Profiles of the Vane

Abstract: Experimental investigations concerning cavitation in radial flow pump for three different leading edge profiles of the vane were carried out in an open circuit system. The operating condition of the radial flow pump under cavitating case was understood by measurement of noise and vibration along with the pump parameters for various speeds and flow rates. The outcome of the experimental results revealed that the noise and vibration were better predictors of inception and development of cavitation. Further observation inferred from critical net positive suction head (NPSH) curve of 3% head drop and critical NPSH value of noise and vibration are presented.

Measurements in high void-fraction bubbly wakes created by ventilated supercavitation

Abstract: A detailed study of ventilated supercavitation in the reentrant jet regime is being carried out in the high-speed water tunnel at St. Anthony Falls Laboratory, as the hydrodynamics part of an interdisciplinary study on stability and control of high-speed cavity-running bodies. It is aimed at understanding the interaction between a ventilated supercavity and its turbulent bubbly wake, with the goal to provide the information needed for the development of control algorithms. Here Particle Image Velocimetry (PIV) measurements in high void fraction bubbly wakes created by the collapse of ventilated supercavities are reported. Bubble velocity fields are obtained, and shown to submit to the same high Reynolds number similarity scaling as the single-phase turbulent axisymmetric wake. A grayscale technique to measure local average void fraction is outlined. Initial results of a timeresolved PIV experiment (2000 Hz) are also presented.

Mixing Evaluation of a Passive Scaled-Up Serpentine Micromixer With Slanted Grooves

Abstract: A novel, passive, scaled-up micromixer based on fluid rotation is proposed and evaluated experimentally and numerically over Reynolds numbers ranging from 0.5 to 100. Flow visualization is employed to qualitatively assess flow patterns, while induced fluorescence is used to quantify species distribution at five locations along the channel length. Two individual fluids are supplied to the test section via a Y-inlet. The fluid enters a meandering channel with four semicircular portions, each of which is lined with nine slanted grooves at the bottom surface. The main mixing channel is 3 mm wide and 0.75 mm deep, with a total length of 155.8 mm. Numerical simulations confirm rotation at all investigated Reynolds numbers, and the strength of rotation increases with increasing Reynolds number. Grooves are employed to promote helical flow, while the serpentine channel structure results in the formation of Dean vortices at Re ≥ 50 (Dean number ≥ 18.25), where momentum has a more significant effect. A decreasing-increasing trend in the degree of mixing was noted, with an inflection point at Re = 5, marking the transition from diffusion dominance to advection dominance. The increase in interfacial surface area is credited with the improved mixing in the advection-dominant regime, while high residence time allowed for significant mass diffusion in the diffusion-dominant regime. Good mixing was achieved at both high and low Reynolds numbers, with a maximum mixing index of 0.90 at Re = 100.

A Review of the Dynamics of Cavitating Pumps

Abstract: This paper presents a review of some of the recent developments in our understanding of the dynamics and instabilities caused by cavitation in pumps. Focus is placed on presently available data for the transfer functions for cavitating pumps and inducers, particularly on the compliance and mass flow gain factor which are so critical for pump and system stability. The resonant frequency for cavitating pumps is introduced and contexted. Finally emphasis is placed on the paucity of our understanding of pump dynamics when the device or system is subjected to global oscillation.

Cavitation Inception and Head Loss Due to Liquid Flow Through Perforated Plates of Varying Thickness

Abstract: This paper reports results of an experimental investigation of the loss coefficient and onset of cavitation caused by water flow through perforated plates of varying thickness and flow area to pipe area ratio at high speeds. The overall plate loss coefficient, point of cavitation inception, and point where critical cavitation occurs are functions of perforation hole size, number of holes, and plate thickness. Sixteen total plates were considered in the study with the total perforation hole area to pipe area ratio ranging from 0.11 and 0.6, the plate thickness to perforation hole diameter ranging from 0.25 to 3.3, and the number of perforation holes ranging from 4 to 1800. The plates were mounted in the test section of a closed water flow loop. The results reveal a complex dependency between the plate loss coefficient with total free-area ratio and the plate thickness to perforation hole diameter ratio. In general, the loss coefficient decreases with increasing free-area ratio and increasing thickness-to-hole diameter ratio. A model based on the data is presented that predicts the loss coefficient for multiholed perforated plates with nonrounded holes. Furthermore, the data show that the cavitation number at the points of cavitation inception and critical cavitation increases with increasing free-area ratio. However, with regard to the thickness-to-hole diameter ratio, the cavitation number at inception exhibits a local maximum at a ratio between 0.5 and 1.0. Empirical models to allow prediction of the point of cavitation inception and the point where critical cavitation begins are presented and compared to single hole orifice plate behavior.

On the Dynamic Properties of Pump Liquid Seals

Abstract: Rotordynamic instability due to fluid flow in seals is a well known phenomenon that can occur in pumps as well as in steam turbines and air compressors. While analysis methods using bulk-flow equations are computationally efficient and can predict dynamic properties fairly well for short seals, they often lack accuracy in cases of seals with complex geometry or with large aspect ratios (L/D above 1.0). This paper presents the linearized rotordynamic coefficients for a liquid seal with large aspect ratio subjected to incompressible turbulent flow. The fluid-induced forces acting on the rotor are calculated by means of a three-dimensional computational fluid dynamics (3D-CFD) analysis, and are then expressed in terms of equivalent linearized stiffness, damping, and fluid inertia coefficients. For comparison, the seal dynamic coefficients were calculated using two other codes: one developed with the bulk flow method and one based on the finite difference method. The three sets of dynamic coefficients calculated in this study were used then to predict the rotor dynamic behavior of an industrial pump. These estimations were then compared to the vibration characteristic measured during the pump shop test, results indicating that the closest agreement was achieved utilizing the CFD generated coefficients. The results of rotor dynamic analysis using the coefficients derived from CFD approach, improved the prediction of both damped natural frequency and damping factor for the first mode, showing substantially smaller damping factor which is consistent with the experimentally observed instability of the rotor-bearing system. As result of continuously increasing computational power, it is believed that the CFD approach for calculating fluid excitation forces will become the standard in industry. [DOI: 10.1115/1.4023653]

Induced-Charge Electroosmosis Around Touching Metal Rods

Abstract: Induced-charge electroosmosis (ICEO) around multiple gold-coated stainless steel rods under different ac electric fields is analyzed using microparticle image velocimetry (micro-PIV) and numerical simulations. In the present investigation, the induced electric double layer (EDL) is in weakly nonlinear limit. The ICEO flow around multiple touching rods exhibits geometry dependent quadrupolar flow structures with four vortices. The velocity magnitude is proportional to the square of the electric field. The ICEO flow velocity also depends on the cylinder orientation. The velocity increases with increased radial distance from the rod’s surface, attains a maximum, and then decays to zero. Experimental and numerical velocity distributions have the same trend beyond 0.2 mm of the rod’s surface.

Investigation of the Behavior of Ventilated Supercavities in a Periodic Gust Flow

Abstract: A ventilated supercavity consists of a large, gas-filled bubble enveloped around an under-water vehicle that allows for significant drag reduction and increases in vehicle speed.Previous studies at the Saint Anthony Falls Laboratory (SAFL) focused on the behaviorof ventilated supercavities in steady horizontal flows. In open waters, vehicles canencounter unsteady flows, especially when traveling near the surface, under waves. Insupercavitation technology, it is critical that the vehicle remains within the cavity whiletraveling through water to avoid unwanted planing forces. A study has been carried outin the high-speed water tunnel to investigate the effects of unsteady flow on axisymmetricsupercavities. An attempt is made to duplicate sea states seen in open waters. In an effortto track cavity dimensions throughout a wave cycle, an automated cavity-tracking scripthas been developed. Using a high-speed camera and the proper software, it is possible tosynchronize cavity dimensions with pressure measurements taken inside the cavity.Results regarding supercavity appearance, cavitation parameters, and their relation tosea state conditions are presented. It was found that flow unsteadiness caused a decreasein the overall length of the supercavity while having only a minimal effect on the maxi-mum diameter. The supercavity volume varied with cavitation number, and a possiblerelationship between the two was explored. [DOI: 10.1115/1.4024382]

Characterization of Cavitation Fields From Measured Pressure Signals of Cavitating Jets and Ultrasonic Horns

Abstract: Cavitation pressure fields under a cavitating jet and an ultrasonic horn were recorded for different conditions using high frequency response pressure transducers. This was aimed at characterizing the impulsive pressures generated by cavitation at different intensities. The pressure signals were analyzed and statistics of the amplitudes and widths of the impulsive pressure peaks were extracted. Plots of number densities and cumulative numbers of peaks as functions of peak amplitude, peak width, and the power of the ultrasonic horn or the jet were generated. The analysis revealed the dominance of pulses with smaller amplitudes and larger durations at lower cavitation intensities and the increase of the amplitudes and reduction of the pulse widths at higher intensities. The ratio of the most probable peak amplitude to peak width was computed. A representative Gaussian curve was then generated for each signal using a characteristic peak amplitude and the corresponding most probable peak duration/width. This resulted in a proposed statistical representation of a cavitation field, useful to characterize cavitation fields of various intensities. [DOI: 10.1115/1.4024263] Prediction of cavitation erosion on propellers, ship structures, and in general on any structure subjected to cavitation is of great interest to many industries. However, this task is often difficult and selection of new materials or material protection coatings that are cavitation erosion resistant is instead most often based on laboratory testing using accelerated erosion methods. These aim at comparing within short time periods the resistance of a new material relative to other standard materials. Erosion in the real field occurs over long durations of exposure, while accelerated erosion tests, by definition, involve subjecting the material to an erosion field that is significantly more “intense” than the actual cavitation that the studied material will be subjected to. The validity of such an approach is however not obvious, as it has been observed that the relative resistance of two materials can be different at different “intensities” of cavitation [1,2]. However, the definition of cavitation “intensity” is not universal. One classical definition [3,4] using an integral quantity is based on a concept similar to that of the acoustic intensity. This expresses the cavitation intensity at a selected point on the material subjected to cavitation as, ð1=qcÞ P N 1 P 2

A Digital Micro Magnetofluidic Platform for Lab-on-a-Chip Applications

Abstract: This paper reports the design and investigation of a digital micro magnetofluidic platform for lab-on-a-chip applications. The platform allows a ferrofluid droplet to be driven along a preprogrammed path. The platform consists of a programmable x-y-positioning stage, a permanent magnet and a glass plate coated with a thin layer of Teflon. First, the actuation of a stand-alone water-based ferrofluid droplet was investigated. Circular, rectangular, triangular and number-eight-shape trajectories were tested and analyzed. The speed of the droplet is evaluated from the position data of the black ferrofluid using a customized MATLAB program. The results show that better positioning accuracy and steady movement can be achieved with smooth trajectories. Next, the ferrofluid droplet as the driving engine for a cargo of other diamagnetic liquid droplets is demonstrated. The characteristics of different cargo volumes are investigated. Due to the liquid/liquid cohesion, a large cargo of five times the volume of a 3-μL ferrofluid droplet can be transported. If the cargo is larger than the driving ferrofluid droplet, the liquid system forms a long trail that faithfully follows the preprogrammed path. Various mixing experiments were carried out. The effectiveness of mixing in this system is demonstrated with a titration test as well as a chemiluminescence assay. The platform shows a robust, simple and flexible concept for implementing a complex analysis protocol with multiple reaction steps.

Experimental Techniques for Bubble Dynamics Analysis in Microchannels: A Review.

Abstract: Experimental studies employing advanced measurement techniques have played an important role in the advancement of two-phase microfluidic systems. In particular, flow visualization is very helpful in understanding the physics of two-phase phenomenon in microdevices. The objective of this article is to provide a brief but inclusive review of the available methods for studying bubble dynamics in microchannels and to introduce prior studies, which developed these techniques or utilized them for a particular microchannel application. The majority of experimental techniques used for characterizing two-phase flow in microchannels employs high-speed imaging and requires direct optical access to the flow. Such methods include conventional brightfield microscopy, fluorescent microscopy, confocal scanning laser microscopy, and micro particle image velocimetry (micro-PIV). The application of these methods, as well as magnetic resonance imaging (MRI) and some novel techniques employing nonintrusive sensors, to multiphase microfluidic systems is presented in this review.

Multicondition Optimization and Experimental Measurements of a Double-Blade Centrifugal Pump Impeller

Abstract: In order to improve internal unsteady flow in a double-blade centrifugal pump (DBCP), this study used major geometric parameters of the original design as the initial values, heads at three conditions (i.e., 80% design flow rate, 100% design flow rate, and 120% design flow rate) as the constraints conditions, and the maximum of weighted average efficiency at the three conditions as the objective function. An adaptive simulated annealing algorithm was selected to solve the energy performance calculation model and the supertransitive approximation method was applied to fix optimal weight factors of individual objectives. On the basis of hydraulic performance optimization, three-condition automatic computational fluid dynamics (CFD) optimization of impeller meridional plane for the DBCP was realized by means of Isight software integrated Pro/E, Gambit, and Fluent software. The shroud arc radii R0 and R1, shroud angle T1, hub arc radius R2, and hub angle T2 on the meridional plane were selected as the design variables and the maximum of weighted average hydraulic efficiency at the three conditions was chosen as the objective function. Performance characteristic test and particle image velocimetry (PIV) measurements of internal flow in the DBCP were conducted. Performance characteristic test results show that the weighted average efficiency of the impeller after the three-condition optimization has increased by 1.46% than that of original design. PIV measurements results show that vortex or recirculation phenomena in the impeller are distinctly improved under the three conditions.