bluuebunny/crossref_metadata_2025 huggingface dataset:
Modification of Air Cavity Flow Under Model Hull With Hydrodynamic Actuators
Authors: Pace Matthew V. , Matveev Konstantin I.
Published: November 2019

Air cavities employed under ship hulls can result in significant decrease of the water frictional drag by reducing the hull wetted area. However, these cavities usually perform well only in a limited range of the ship speed and attitude. In off-design states and in the presence of sea waves, efficient air cavities covering large areas of the hull are difficult to form and maintain. This problem can be potentially addressed with help of hydrodynamic actuators, such as compact hydrofoils, tabs, and spoilers, which can assist with forming and maintaining air cavities under ship hulls. In this study, exploratory tests have been conducted with a simplistic small-scale hull having a bottom recess. Air was supplied into the recess to produce an air cavity, and several actuators were implemented and manually controlled during the tests. Subjected to external water flow, the air cavity under the hull was found to be responsive to variable positions of the actuators. Positive effects on the air cavity produced with specific actuator settings are identified and discussed in the paper. A series of experimental photographs of the air-water interface are shown for various actuator settings. The air flow rates needed to establish and maintain a large air cavity under the model hull are also reported.
Hydraulic Simulation for Calcasieu Lake Area With Small Rivers Using an Immersed Boundary Method
Authors: Yadav Arun , Zhang Ning
Published: November 2019

Coupled one-dimensional and two-dimensional hydraulic models using an immersed boundary method (IBM) was developed and implemented for a simple flooding case in a previously published study [1]. In this study, the IBM model was applied for simulations on a real geographical location, which was the Calcasieu Lake and its surrounding area in Southwest Louisiana. The simulation area ranges from the city of Lake Charles on the northern region to Gulf of Mexico on southern region, and two National Wildlife Refuges (NWR) on the east and west regions to the Calcasieu Lake. Calcasieu Lake is the major part of the wetland ecosystem, a simple change in the natural state of the lake, i.e. the change in the water surface elevation or flooding, can have a drastic impact on the ecosystem of its surrounding areas. The flooding to the wetlands is mainly through small rivers connected to the lake. In order to study the effects of coastal flooding to the wetlands, the IBM 1D-2D coupled model was implemented in the simulation package. The main purposes of this study are to: 1) determine the appropriate number of immersed boundary points to be used for this simulation, 2) validate the IBM model using an actual geographical region. Due to the lack of measurement in the small rivers, the validation was conducted by comparing simulated results from IBM model to the results from two non-IBM approaches (i.e., manually carved rivers and no rivers). Data obtained from NOAA and USGS were used for the boundary conditions for the 2D hydraulic model. During the first phase of study, the results from the higher number of immersed boundary points came close to each other which suggested that higher number of immersed boundary points are more precise for calculation. The final comparison of water surface elevation and velocity magnitude proved the accuracy and validity of the simulation package.
Effect of DC Electric Fields on Flame Spread Over Twin Electrical Wires
Authors: Park Jeong , Park Sun Ho , Cha Min Suk , Chung Suk Ho
Published: November 2019

The effect of DC electric field on the behaviors of spreading flame over polyethylene (PE)-insulated twin electrical wires was investigated by varying the wire gap (S) and voltage (VDC). The twin flame spreads with the same flame spread rate (FSR) independently when no electric field is applied. With an applied DC electric field, the twin flame interacts such that FSR, flame width, and the leaning direction of interacting twin flames vary appreciably. The spread rate for wire flame with negative voltage (SF–) was larger than that with positive voltage (SF+) during transient period and then the two became the same in a developed region (a quasi-steady spread). Such a flame behavior could be grouped into two: twin flame spread (regime I) and single flame spread (regime II) after the extinction of SF+. Each regime could be categorized into three sub-regimes depending on S and VDC. For small VDC, the flame leaned toward the burnt wire, reducing FSR. With further increasing VDC, FSR increased due to the ionic wind effect and then decreased via the mass loss of molten PE. These non-monotonic behavior of FSR with DC voltage can be attributed the behaviors of molten PE, exhibiting dripping, electrospray, and di-electrophoresis phenomena. For further increased voltage, the flames were extinguished by streamer generation and an electrical short occurred at excessive voltages.
Effect of the Large-Scale Structure on Turbulent Prandtl Number in a Turbulent Shear Layer
Authors: Takamure Kotaro , Sakai Yasuhiko , Ito Yasumasa , Iwano Koji
Published: November 2019

We have run a Direct Numerical Simulation of a spatially developing shear mixing layer. The aim of this study is to clarify the influence of the large-scale structure on the turbulent Prandtl number PrT. As a main conclusion, PrT takes a small value (PrT ∼ 0.5) in the dominant region of the large-scale structure. The budget analyses for the Reynolds stress equation and the scalar flux equation revealed that the differences between the momentum and scalar transfer are caused by terms related to pressure (i.e., pressure-strain correlation term, pressure-scalar gradient correlation term, and pressure diffusion terms). Phenomenally, the momentum in the field where a large-scale vortex coexists tends to be transported toward the counter-gradient direction under the influence of pressure, but the scalar is transported toward the gradient direction. As a result, it is thought that the difference in the driving force between the momentum and scalar transport causes the decrease of the PrT.
Frequency Response Characterization for Dynamic Measurement of Pressure
Authors: Papadopoulos George , Bivolaru Daniel
Published: November 2019

Transducer requirements for making true dynamic pressure measurements point to a miniature point-level sensing element that is exposed to the flow. Meeting this requirement, however, is often challenged by transducer size constraints, integration at the location of measurement, and packaging, especially when one considers applications in harsh environments where protection of the sensing element may be needed. As part of an effort towards the development of a high frequency pressure measurement device for use in harsh environments (ultra-high temperature), an investigation was performed to evaluate the effect of sensing element packaging and geometry at the point of measurement on the dynamic response of a nominal transducer. Frequency and time domain calculations were performed to assess variations on the magnitude and phase between an input signal and a “measured” signal at the sensing element location for a range of probe tip parameters. The results offer insights and metrics that can govern transducer sensing element and probe tip implementation for optimum frequency response and strategies for compensation.
Improved Estimation of Subcooled Flow Boiling Heat Flux for Automotive Engine Cooling Applications
Authors: Vasudevan Sudharsan , Etemad Sassan , Davidson Lars
Published: November 2019

Tapping the potential of subcooled flow boiling can be the key strategy for enhanced cooling of modern day internal combustion engines with high specific power. Accurate prediction of the boiling heat flux is a prerequisite for employing such strategy and to avoid stepping into the dangerous film boiling regime. The complexity involved in the boiling phenomena makes it difficult to develop a model that accounts for all the dominant mechanisms. However, boiling models available in literature provide a good estimate of the heat flux within their range of applicability. This work attempts to introduce a blending based on probability of bubble nucleation to blend two different models developed for different boiling regimes. Corroboration of results with experiments show improved estimation of boiling heat flux.
An Effect of Flow Velocity on Propagation Properties of Weakly Nonlinear Waves in Bubbly Flows
Authors: Maeda Taiki , Kanagawa Tetsuya
Published: November 2019

The present study theoretically carries out a derivation of the Korteweg–de Vries–Burgers (KdVB) equation and the nonlinear Schrödinger (NLS) equation for weakly nonlinear propagation of plane (i.e., one-dimensional) progressive waves in water flows containing many spherical gas bubbles that oscillate due to the pressure wave approaching the bubble. Main assumptions are as follows: (i) bubbly liquids are not at rest initially; (ii) the bubble does not coalesce, break up, extinct, and appear; (iii) the viscosity of the liquid phase is taken into account only at the bubble–liquid interface, although that of the gas phase is omitted; (iv) the thermal conductivities of the gas and liquid phases are dismissed. The basic equations for bubbly flows are composed of conservation equations for mass and momentum for the gas and liquid phases in a two-fluid model, the Keller-Miksis equation (i.e., the equation for radial oscillations as the expansion and contraction), and so on. By using the method of multiple scales and the determination of size of three nondimensional ratios that are wavelength, propagation speed and incident wave frequency, we can derive two types of nonlinear wave equations describing long range propagation of plane waves. One is the KdVB equation for a low frequency long wave, and the other is the NLS equation for an envelope wave for a moderately high frequency short carrier wave.
Numerical Simulation and Optimization of Blalock-Taussig Shunt
Authors: Hess Thomas , Agarwal Ramesh , Hoganson David
Published: November 2019

The goal of this study is to create an optimized Blalock-Taussig shunt used to temporarily repair pulmonary vascular blockages allowing a child time to grow so a more permanent surgical repair of the heart and vasculature can be performed. Blalock-Taussig or BT shunts are a surgical procedure performed on infants suffering from cyanosis or “Blue Baby Syndrome.” A BT shunt is an artificial vessel placed between the right ventricle and the pulmonary artery to increase blood flow in the lung and blood oxygen saturation levels. In a study of 96 patients with currently in use modified BT shunts, 32 patients (21%) had greater than 50% stenosis caused by myofibroblastic proliferation at the shunt lumen due to shunt geometry [1]. A 2007 study by the cardiac surgery division of Johns Hopkins Medical Institutions found an operative mortality rate of 14% (227 of 1,574) with patients undergoing BT surgery [2].

In this paper, the flow of blood through several different BT shunt configurations from actual patient data was analyzed using the commercial CFD software ANSYS Fluent. Results from each shunt’s analysis were then compared to determine the shunt parameters with optimal flow dynamics for use in infants suffering from pulmonary vascular blockage. It was found that the entrance boundary of current BT shunts caused blood flow hindrances due to high wall shear values and flow separation. A newly designed shunt was proven to partially fix this problem; however, a superior model could be optimized by using characteristics from currently used shunts and CFD results. Many iterations and designs of BT shunts were made using Solidworks, a solid modeling computer-aided design program, and were tested using Fluent to create a shunt optimized by smoothening the transition between areas of high and low wall shear stress, lowering the overall maximum wall shear stress, reducing flow separation, and equalizing the flow to the left and right lung. All these factors contribute to the chance of thrombosis and morbidity within patients. The resultant model shunt showed drastic improvement in lowering the average wall shear stress by more than 85% at the initial boundary with over 20% drop in overall average wall shear. It also achieved a decline of the maximum wall shear stress by over 25% while negating the possibility of any flow separation and improving the equality in flow to the left and right lung by more than 60%.
Wing-Wake Interaction of Insect-Like Flapping Wing in Hover: Effect of Aspect Ratio and Kinematics at Re~104
Authors: Addo-Akoto Reynolds , Han Jong-Seob , Han Jae-Hung
Published: November 2019

In this paper, the effect of wing aspect ratio and kinematics on wing-wake interaction at Re∼104, which matched the flight regime of flapping-wing micro air vehicle (FWMAV), was investigated. The dynamically scaled-up robotic model submerged in a water tank environment revealed that the wing-wake interaction augmented lift across a decrease in both aspect ratio and wing pitching duration. At such high Re, a time-course digital particle image velocimetry (DPIV) measurement showed the entire flow was strongly dominated by trailing-edge vortices (TEV). A pair of counter-rotating TEV was found to induce a jetlike flow towards the windward side of the wing at stroke reversal. The transfer of momentum from the accelerated flow to the wing caused the enhanced lift. The size of the pair vortex decreased for an increase in both aspect ratio and wing pitching duration. The size of the TEV pair was the key feature found to generate the observed aerodynamic force characteristics.
Numerical Investigation of the Forming and Ordering of Inertially Focused Particle Trains in a Square Microchannel
Authors: Wang Shan , Li Shanshan , Pan Zhenhai
Published: November 2019

Inertial focusing of microscale particles has increasingly attracted attentions in various microfluidic applications, such as manipulations of particles/cells, flow cytometry, and highsensitive detection, owing to its passive, robust, and high-throughput nature. While most fundamental studies in literatures have focused on the lateral migration and transverse equilibrium position of single particles in micro-channels with various cross sections, very few of them have dug into the forming and ordering mechanisms of an inertially focused particle train. In this study, the forming inertially focused particle trains are investigated by coupling the lattice Boltzmann method with the immersed boundary method (IB-LBM). The inertial migrations of 6 particles (40 μm in diameter) are simulated in a square microchannel (100 μm in width). The results indicated that the equilibrium axis distance between two neighboring particles is strongly affected by complicated particle-to-particle and particle-to-liquid interactions. Two critical length fractions Lf,c1 and Lf,c2 are observed, which have insignificant effect on balanced distance between two neighboring particles in a particle train.
The Effects of Multiphase, Multi-Viscosity Fluids on Axial Thrust and Cooling Flow Performance of a Canned Motor Pump
Authors: Conaway Eric , Matos Jose , Mesiano Ryan
Published: November 2019

A canned motor pump for technology demonstration in subsea oil and gas systems was tested to provide data to understand the design, modeling and performance characteristics of a canned motor pump operating in multiphase flow. This paper discusses the impact of multiphase flow and fluid viscosity on axial thrust and motor cooling flow characteristics.

The technology demonstrator is a two-stage, low specific speed (Ns∼550) centrifugal pump designed to deliver 140 gpm at 600 feet of head at 3930 rpm. The 61 hp canned motor is cooled by a small portion of the pump discharge fluid, which is drawn downward through the motor by pump out vanes on the hub of the second stage impeller.

The multiphase test loop is equipped for both water and light oil operation in low pressure and ambient temperature conditions. Testing occurred over a range of conditions to simulate varying fluid properties and operating scenarios. Shaft rotational speed varied between 2000 and 4250 rpm with pump liquid flow rates from 25 to 250 gpm. These operating scenarios were repeated for both water and light oil (∼2 cP) with multiphase flow ranging from 0–20% gas volume fraction (GVF) using injected air.

Testing results indicate a detectable impact from the different fluids and GVF’s tested, which can be related to features such as the second stage impeller pump-out vane and regions within the motor cavities. In water-air tests, increasing GVF led to the following: motor input power reduced by 5%; axial thrust increased by 100%; motor cooling fluid temperature rise increased by 100%; and pressure rise in the second stage pump out vanes reduced by 30% - directly impacting motor cooling flow rate, temperature rise, and axial thrust. In the oil-air tests, multiphase flow showed similar tendencies with reduced magnitude. Notably, the effects due to air injection do not appear at GVF below 15% with oil-air mixtures, unlike water-air tests which demonstrated effects across all GVFs.

The test results provide insight into the behavior of variable viscosity, multiphase flow in the canned motor pump cooling passages, as driven by the second stage impeller pump out vanes. These observed characteristics can be used to design flow control features and evaluate operational impacts, while the performance data obtained can be used to assess the behavior of flow models for this application.
The Effect of Mixing Promoters on the Performance of Forward Osmosis Membrane Systems: Computational Fluid Dynamics Simulations
Authors: Alshwairekh Ahmed M. , Alghafis Abdullah A. , Alwatban Anas M. , Alqsair Umar F. , Oztekin Alparslan
Published: November 2019

Computational fluid dynamics simulations are performed to study the desalination process in forward osmosis modules. Concentration polarization phenomenon is responsible for lowering the water flux permeation through the membrane. It is desired to mitigate the external concentrative and dilutive concentration polarizations by proper mixing of both the feed and the draw solutions. Chevron-type of corrugation with a single triangular shape is used for membrane corrugation. The peak of the corrugation is oriented toward the feed channel. This type of corrugations produces dips in the membrane in the draw side. The dips will not mix the draw solution properly. Net-type spacers of 45° will be placed in the middle of the draw channel. The spacers will work as mixing promoters and add extra support for the membrane. All configurations will have the same Reynold number of 1500 in both channels. k–ω SST turbulence model is used with modules containing mixing promoters, and the laminar model is used with the module without corrugation and spacers. The results indicate that a 14% increase in the water flux is attained with the presence of spacers and membrane corrugation compared with a flat membrane and no spacers. Also, the dilutive concentration polarization has been mitigated significantly compared with the empty case.
Development and Validation of a Hybrid RANS-LES Approach Based on Temporal Filtering
Authors: Duffal Vladimir , de Laage de Meux Benoît , Manceau Rémi
Published: November 2019

To address the challenge of controlling the energy partition in hybrid RANS-LES methods, the use of a consistent operator based on temporal filtering is desirable. This formalism leads to the development of a consistent continuous hybrid RANS-LES approach called Hybrid Temporal LES (HTLES). In this paper, an upgraded version of HTLES is presented, focusing on improving the model for wall-bounded flows. Notably, a shielding function is integrated in the model to impose the RANS behavior in the near-wall regions. The calibration and validation of the hybrid method applied to the standard k-ω-SST model is then carried out on several test cases: decaying isotropic turbulence, channel flow and periodic-hill flow. The new version of the model fulfills the specifications: the correct subfilter dissipation; the correct migration from RANS to LES in the boundary layer; the robustness of the results to grid coarsening; the accuracy of the predictions at a reasonable computational cost.
Investigation of the Pump, Dissipation and Inverse Turbine Operating Modes Using the CATHARE-3 One-Dimensional Rotodynamic Pump Model
Authors: Matteo Laura , Cerru Fabien , Dazin Antoine , Tauveron Nicolas
Published: November 2019

A predictive transient one-dimensional rotodynamic pump model is currently developed into the CATHARE-3 thermal-hydraulic system code at CEA Saclay, France. Flow is computed in each part of the pump (suction, impeller, diffuser, volute and discharge pipe) based on the definition of a mean stream line. Several cells are used to mesh each part of the rotodynamic pump, which makes this model different from usually called “1D models”. In this study, the model is tested on a medium specific speed centrifugal pump in every operating modes encountered in the first quadrant (positive flow rate and rotational speed). A flow rate evolution is performed at the nominal rotational speed and also at rotational speed equal to zero in order to produce complete first quadrant homologous curves. Relative discrepancy between computation and experiment is respectively less than 10% and 15% on the whole obtained homologous head and torque curves.
Experimental and Computational Studies of Particle Scavenge Flow in Direct Laser Metal Sintering
Authors: Austin Morgan , Tran-Le Thao , Kunz Robert , Simpson Timothy , Ni Rui
Published: November 2019

Powder Bed Fusion (PBF) cross-flow systems are designed to flow gas across the build plane and entrain metallic powder particles that are ejected during the build, due to the thermal and attendant released kinetic energy of the laser melt process. It is important that these particles be removed from the build chamber so that they do not redeposit on the build surface, as this uncontrolled particle deposition can degrade the part quality. Optimal design of these sub-systems involves tailoring a cross-flow jet such that most of the ejected particles are entrained and removed from the build chamber, while the top layer of particles that are freshly spread on the build plate are not entrained.

Accordingly, a combined experimental and CFD study has been executed with the goal of developing engineering design guidance for these cross-flow systems. The closed loop small footprint wind tunnel incorporates a 0.305 m × 0.305 m × 0.915 m test section, a variable height build plate upon which powder can be spread, a variable geometry inlet nozzle, and variable flow rate so that a variety of cross-flow configurations can be tested. Helium bubble particle tracking velocimetry (PTV) was used to characterize the single-phase flow at a number of these operating conditions / configurations. In addition, high speed videography was used to study particle liftoff and entrainment at these same conditions. Using these measurements and attendant CFD models, critical particle liftoff Shield numbers were obtained using CFD predictions of friction velocity. Specifically, close agreement between CFD and measurements were obtained, so that predicted Shields numbers, Sh, could be correlated with particle Reynolds number, Reτ.

In this paper we present details of the experimental facility and test program, experimental results including uncertainty/error analysis for the PTV measurements, as well as the videography results for an aluminum alloy powder. The results of the CFD modeling are compared to the single phase measurements. Since very good agreement is observed, predicted wall-shear stress values are used to estimate Sh vs. Reτ at flow rates where incipient particle lift-off is observed experimentally.
Mix Width, Bubble and Spike Amplitudes in Three-Dimensional Numerical Simulations of Turbulent Mixing Driven by Spherical Implosions
Authors: El Rafei Moutassem , Thornber Ben
Published: November 2019

Three-dimensional numerical simulations of turbulent mixing at a perturbed interface of a dense shell compressed by a spherically imploding shock wave are presented. This case is a simplified version of inertial confinement fusion implosion (ICF) where a small capsule containing nuclear material is compressed to extremely high pressure and temperature to achieve fusion burn. The current simulations were performed using a high-order spherical method and a semi-Lagrangian moving mesh algorithm implemented in our in-house code Flamenco.

Results of narrowband and broadband initial perturbations are presented at different grid resolutions along with mix layer limits, molecular mixing, turbulent kinetic energy and bubble/spike heights. The initial multimode perturbations applied at the interface consist of a superposition of cosine waves and are determined according to a specified power spectrum and standard deviation. These are employed in a spherical segment, enabling the efficient computation of a wide range of low to relatively high mode-number perturbations. The overall grid convergence of the solution is analysed and the different findings from the integral quantities and bubble/spike amplitudes are indicated.
Flow Visualization Study of Jet and Bucket Interactions in Traditional and Hooped Pelton Runner
Authors: Chaudhari Gaurangkumar , Channiwala Salim , Shah Samip , Kulshreshtha Digvijay
Published: November 2019

This paper aims to study the flow pattern in and around a bucket of a Traditional and a Hooped Pelton runner at single injector operation and illustrates different stages of jet interaction. High speed photography is used to study the flow pattern, keeping the camera in different positions relative to the jet and to the bucket. It is concluded from the results that the flow visualization study, provides exceptional observations with an absolute frame of reference to mark the bucket duty period of a single-jet Pelton runner. The small scale models display erosion damages at the bucket lips, this indicated that the high pressure occur in the early stage of interaction. This fact is substantiated by the present flow visualization studies for the first time. The uncertainty of the free surface outflow within the Pelton turbine bucket establishes good documentation. The results are helpful to know the interaction between the jet and bucket of Pelton turbine.
A Validation Experiment for Turbulent Mixing in a Collated Jet
Authors: Craig Ian G. R. , Hoff Coleman D. , Kristo Paul J. , Kimber Mark L.
Published: November 2019

Numerical modeling of turbulent mixing is complicated not only by the natural complexity of the flow physics, but by the model sensitivity to the user specified conditions. When aiming to quantify the level of trust in a given model, a validation experiment is often performed to provide data against which confidence comparisons can be made, ultimately evaluating the adequacy of the model assumptions. In the nuclear community, a number of promising Generation IV reactor concepts have been proposed, each requiring high fidelity modeling capabilities to accurately assess safety related issues. The experiment described in this paper is intended to provide a much needed validation data set to assess the use of computational fluid dynamics (CFD) in a complex turbulent mixing scenario relevant to the prismatic very high temperature reactor (VHTR) concept. Over the course of the VHTR’s operation, the reactor’s graphite moderated hexagonal fuel blocks shrink from neutron damage, forming interstitial gaps between adjacent blocks. A significant percentage of the coolant can flow through these gaps, having a substantial impact on the thermal-hydraulic conditions in the core. An experimental facility is presented that uses air as a simulant fluid and includes a unit cell representation of the hexagonal blocks, which accounts for both the intended circular channels and secondary rectangular slot features induced by the bypass gaps. The outlet of the unit cell consists of a collated jet consisting of a central round jet surrounded by three slot jets at relative 120° angles to one another issuing into a stagnant domain. A preliminary test case is proposed in which the collated jet is set to an isothermal and iso-velocity condition. Constant temperature anemometry (CTA) and constant current anemometry (CCA) measurements serve to capture the velocity and temperature inlet quantities (IQs). Particle image velocimetry (PIV) measurements provide the appropriate system response quantities (SRQs), yielding insight into the mean and fluctuating components of the 2-D velocity field. Results are presented in the range of 0–8 diameters downstream of the inlet to the test section. The collated jet inlet region yields velocity profiles that are heavily influenced by opposing pressure gradients between the neighboring round and slot regions. As a result, the velocity peaks found in this area are neither in the centerline of the round jet nor that of the slot, but are towards the outer edge of each. With increasing downstream distance, the collated jet is found to exhibit a more classical round jet profile. The inlet region of the collated jet is thus of particular interest to future modeling efforts to more accurately depict the lower plenum behavior and transition to a self-similar profile downstream. Proper uncertainty quantification is also presented, and aids in assessing the integrity of the experimental results for future CFD validation.
Data Processing Technology of Interference Fringe for Particle Decontamination Measurement
Authors: Fujiwara K. , Nakamura Y. , Kaneko A. , Abe Y.
Published: November 2019

In severe accidents (SAs) of nuclear power plants, release of gas containing fission products (FPs) from the reactor vessel is thought to be a major issue. As to reduce the leakage of FPs into the environment, gas containing FPs are generally discharged though the wet well, and decontaminated by the transfer effect of FPs from the gas-phase to the liquid phase. This effect is called pool scrubbing. In SA analysis codes such as MELCOR, it is predicted in the model that FP particle motion inside a single bubble, created by the bubbly flow inside the wet well as a major factor in decontamination. However, there are almost no experimental data to investigate the decontamination behavior. Therefore, in our experiment, we used an advanced M/Z interferometer in order to visualize the particle decontamination behavior by adopting Maki prism and installing a high-speed camera. However, since the interferometer experiments are not specialized in non-stable phenomena and, there were several problems to be solved. The first problem was the phase extraction method in the FFT measurement. Since the FFT information of interference is complicated, the existing extract the phase information by hand from the overall amplitude. However, since the high-speed camera visualization provide a large amount of information, this is not a realistic solution in our experiment. Therefore, in order to obtain the threshold between phase information from the overall amplitude quantitatively, we applied the Gaussian mixture model (GMM) as to cluster the data. From the measurement results, we succeed in obtaining a threshold from the fitting results of GMM. The next problem was to obtain a fine image of the bubble interface in order to obtain the decontamination behavior in the bubble interface. However, the interference image contains a stripe on the background which makes it difficult to obtain the interface information. Therefore, in our experiment, we added a LED backlight coaxial to the laser of interferometer in order to obtain the backlight image on the bubble. The interference and backlight image are divided by wave length with a dichroic mirror. We have done a synchronous visualization of interference and backlight image. A fine mask to extract the interface of bubble is obtained from the calibration and comparison of two images. Using the visualization of continuous image of particle decontamination behavior from a single bubble, the decontamination behavior of particle from a single bubble was clearly obtained. Although the existing model predict the decontamination behavior as stable, the non-stationary decontamination of particle from the bubble has been measured.
A Comparison Study for Off-Design Performance Prediction of a Supercritical CO2 Compressor With Similitude Analysis
Authors: Jeong Yongju , Son Seongmin , Cho Seong kuk , Baik Seungjoon , Lee Jeong Ik
Published: November 2019

Most of the power plants operating nowadays mainly have adopted a steam Rankine cycle or a gas Brayton cycle. To devise a better power conversion cycle, various approaches were taken by researchers and one of the examples is an S-CO2 (supercritical CO2) power cycle. Over the past decades, the S-CO2 power cycle was invented and studied. Eventually the cycle was successful for attracting attentions from a wide range of applications. Basically, an S-CO2 power cycle is a variation of a gas Brayton cycle. In contrast to the fact that an ordinary Brayton cycle operates with a gas phase fluid, the S-CO2 power cycle operates with a supercritical phase fluid, where temperatures and pressures of working fluid are above the critical point. Many advantages of S-CO2 power cycle are rooted from its novel characteristics.

Particularly, a compressor in an S-CO2 power cycle operates near the critical point, where the compressibility is greatly reduced. Since the S-CO2 power cycle greatly benefits from the reduced compression work, an S-CO2 compressor prediction under off-design condition has a huge impact on overall cycle performance. When off-design operations of a power cycle are considered, the compressor performance needs to be specified. One of the approaches for a compressor off-design performance evaluation is to use the correction methods based on similitude analysis. However, there are several approaches for deriving the equivalent conditions but none of the approaches has been thoroughly examined for S-CO2 conditions based on data. The purpose of this paper is comparing these correction models to identify the best fitted approach, in order to predict a compressor off-design operation performance more accurately from limited amount of information. Each correction method was applied to two sets of data, SCEIL experiment data and 1D turbomachinery code off-design prediction code generated data, and evaluated in this paper.
Experimental Visualization of Gas-Liquid Flow Patterns in a Centrifugal Rotor
Authors: Stel Henrique , Ofuchi Edgar Minoru , Alves Rafael Fabrício , Chiva Sergio , Morales Rigoberto E. M.
Published: November 2019

Centrifugal pumps operating with gas-liquid flows can undergo severe performance degradation. This can be attributed to an effect of the gas phase on the liquid flow orientation in the pump impeller channels, which induces additional hydraulic losses that negatively affect the delivered head and flow rate. Effort to investigate the effect of many operating parameters on the pump performance under multiphase flows can be found on numerous experimental investigations. Few studies, however, bring together flow visualization to understand the physics behind the behavior of centrifugal pumps with gas-liquid flows. One issue is that pumps involve rotating parts, metallic casing and limited visual access, sometimes making it hard to interpret flow patterns and to understand complex phenomena, such as bubble breakup and coalescence. Such issues usually lead to unsatisfactory image quality, which in turn makes it difficult to extract quantitative data from the obtained images, such as gas volume fraction and bubble size distribution. In an attempt to overcome many difficulties of previous investigations, this work presents an experimental study aimed to visualize gas-liquid flow patterns in a centrifugal rotor prototype using a novel approach. The experimental apparatus uses a plane and transparent rotor, assembled with an intake pipe and a discharge chamber by means of a dynamic seal system that dismisses the use of an enclosing pump casing. This makes possible to use back illumination of the impeller for visualization, which in turn is done by using a camera attached to the impeller axis for filming in a rotating frame of reference. This setup, which is new in the open literature, provides high image contrast and sharpness for clear interpretation of the flow patterns found inside the rotor channels for a wide range of operating conditions. This advantage, in turn, allows using image processing for quantitative assessment of gas volume fraction distributions. Pressure rise versus flow rate curves are measured together to investigate the rotor performance degradation associated with the gas-liquid flow patterns for a range of liquid and gas flow rates. Information obtained with the designed experimental setup at controlled conditions help not just to bring further understanding to the complex phenomena involved with multiphase flows in rotating devices, but also in the direction of validating a numerical model for reliable simulations of gas-liquid flows in centrifugal pumps, which is lacking in the current literature.
Study on Terminal Velocity of Continuously Ejected Micro Inkjet Droplet
Authors: Kang Seung-Hwan , Lim Jong Woo , Kim San , Sohn Dong Kee , Ko Han Seo
Published: November 2019

It is important to make the droplet formation in the printable area for a design of an inkjet device. One of the important variables for the inkjet is an ejecting velocity which can be controlled by the inkjet device. When the drop-on-demand inkjet nozzle is used, unless the ejecting velocity exceeds the minimum velocity, the droplet may not be ejected or the nozzle surface can be wet, thus the next droplet formation may fail. On the other hand, if the droplet velocity is too fast, splashing may happen when the drop impacts on a substrate, which makes the undesirable printing error. The falling velocity of the inkjet droplet changes by the drag force of air along the falling distance. Therefore, it is required to estimate the exact ejecting, falling and terminal velocity of the inkjet droplet. We measured the inkjet droplet velocity ejecting from the nozzle of a piezoelectric drop-on-demand inkjet device with a push mode. The nozzle diameter was 70 μm and polyethylene glycol aqueous solution was used as ink. The inkjet droplets were generated within a range of 500 to 10,000 Hz of frequencies at a reference piezoelectric input voltage. The successive inkjet droplets were captured by the high-speed camera with 100,000 fps, then the inkjet droplet velocities were analyzed visually at each falling location. The initial to terminal velocity changes of the droplets were calculated theoretically to be compared with the experimental results according to the diameter of the droplet, the drag coefficient, and the Reynolds number. The general terminal velocity of the droplet is determined by the density and diameter of the droplet. However, this study confirmed that the terminal velocity of the continuously ejected droplets by the drop-on-demand inkjet was determined by the interval of the droplets which depended on the frequency. The inkjet droplet in the low frequency has the long interval that is enough to consider as an independent droplet so that it can follow the terminal velocity curve of the general droplet. However, with increasing the frequency, the inkjet droplets are generated frequently and its interval gets shorter, and then its terminal velocity is relatively high not following the general terminal velocity curve. It is considered that each droplet gets into the wake field of the former droplet so that its drag force can decrease and the velocity can increase. We analyzed the velocity change of the inkjet droplet according to its diameter and frequency based on the experimental and numerical results.
CNN-SINDy Based Reduced Order Modeling of Unsteady Flow Fields
Authors: Murata Takaaki , Fukami Kai , Fukagata Koji
Published: November 2019

We present a new framework of nonlinear reduced order model to extract low-dimensional modes and to predict their temporal evolutions. Autoencoder-type Convolutional Neural Network (CNN) which can learn nonlinearity of data is used to extract low-dimensional modes. For obtaining the temporal evolution of a mapped data by CNN, Sparse Identification of Nonlinear Dynamics (SINDy) is performed. The proposed method is applied to a circular cylinder wake at ReD = 100. The CNN trained using fluctuation components of velocity vector u, v shows better results than the snapshot Proper Orthogonal Decomposition in terms of the energy reconstruction rate. Although time-evolving flow fields reproduced by SINDy equation also show reasonable agreement with the reference direct numerical simulation, the errors of CNN and SINDy are accumulated through integral computation. The error of CNN can be reduced by devising a better network structure; however, the error of SINDy depends on the waveform of latent vector.
Experimental Study on Suppression of Vortex Shedding Behind a Square Cylinder Using Opposed-Type Plasma Actuators
Authors: Nakazawa Akira , Yonemichi Takuto , Fukagata Koji
Published: November 2019

Vortex shedding in the flow field causes many kinds of problems such as increase of drag and noise. Especially, the von Kármán vortex street behind bluff bodies, e.g. a tire of an airplane and a pantograph of a train, greatly contributes to them. One of the effective methods to suppress the vortices is the use of plasma actuators (PAs). A PA can induce flow by applying a high-voltage and high-frequency AC voltage to its electrodes. In the present study, we use an opposed-type PA (O-PA), which consists of two PAs facing each other. The O-PA can induce a jet in the direction perpendicular to the surface because of a collision of flows induced by the two PAs. In this study, we investigate the control effect of an O-PA on the flow around a square cylinder using an O-PA by means of the PIV measurement. First, we measure the flow induced by an O-PA. It is confirmed that the velocity of the induced flow increases as the applied voltage Vpp increases, and the O-PA induces the jet of about 1.5 m/s under Vpp = 10kV. Next, we measure the flow around a square cylinder with no control. It is confirmed that the von Kármán vortex street occurs behind a square cylinder. Finally, we measure the flow around a square cylinder under the control by the O-PA attached on the rear surface. It is confirmed that the vortex shedding behind a square cylinder is suppressed by the O-PA under Vpp = 10kV.
Extra-Low Reynolds Number Vane Separation Using Immersed Boundary Method
Authors: Prapamonthon Prasert , Yin Bo , Yang Guowei
Published: November 2019

Nowadays, mini unmanned aerial vehicles (MUAVs) and micro air vehicles (MAVs) are not only beneficially used as aviation models but also as modern drones for military missions and other civilian applications. Hence, research and development of propulsion sources for MUAVs and MAVs dynamically increase with a future trend of high performance, but low energy consumption. Certainly, using micro and ultra-small-size gas turbine is a good option for the propulsion source. To achieve ideal flight of MUAVs and MAVs powered by micro and ultra-small-size gas turbines under this trend, understanding of flow phenomena at wide ranges of Reynolds number is essential. This research presents a 2D numerical study of characteristics of laminar flow separation and the trailing-edge vortex on a turbine vane at extra-low Reynolds numbers (Res) i.e. Re = 1800 and 3600, and three rotational angles (α) i.e. α = 0°, 15° and 30° using immersed boundary method (IBM). With this method, the problem of incompressible flow is addressed by a sharp interface IBM. Numerical results indicate that IBM can characterize phenomena of laminar separation flow, which usually happens on the turbine airfoil when laminar boundary layer cannot overcome adverse pressure gradients and viscous effects. To our current knowledge, this may be the first research to study flow behavior at such low Res for gas turbine vanes using IBM. Even though it is now not common to operate micro and ultra-small-size gas turbines under these conditions, it is important to know how aerodynamic performance may be if micro and ultra-small-size gas turbines need to run under such conditions in the near future.
Characterization of the Anisotropic Wetting Property in Liquid Transportation on the Surface With Ratchet Structures
Authors: Xu Jing , Chen Jun
Published: November 2019

In this paper, the samples with ratchet structures surface, are set vertically and horizontally in the liquid transportation experiments. Both static and dynamic directional property has been recorded by a high-speed camera. The variation of static contact angles and the shape of the drop in a ratchet structure are measured. The geometric evolutions of drop shed off along different direction on ratchet structure surface and the smooth surface under dynamic condition, are observed for comparison. The advancing contact angle and velocity of drop shed off the ratchet structure are analyzed in detail. The directional liquid transportation mechanism of anisotropic wettability property is demonstrated. Results show that the static drop moves parallel along ridges quickly, and the tooth limit the anisotropic drop spread to the other teeth along the vertical direction, this anisotropic wettability behavior in ratchet structure, which is related to the velocity of liquid transport, the velocity of liquid drop along the backward direction is slower than that of forward direction. The anisotropic wettability property along the horizontal direction of groove limits the transportation ability of the drop. The observed drop deformation includes rotating, rolling, and shaking-induced movements. The surface tension, adhesive behavior, and curvature difference cause the hysteresis effect, which presents a major barrier for liquid transportation.
Effect of Geometry of Pressure-Swirl Duplex Nozzle and ALR on Atomization Characteristics of Jet A-1 Fuel
Authors: Hasan M. M. , Chandrahasan R. , Ru S. , Choi Y. , Lee J.
Published: November 2019

The efficiency of engine combustion is strongly dependent on fuel atomization and spray processes which in turn are greatly influenced by nozzle internal geometry and parameter like air to liquid ratio (ALR). Modern engines employ different geometrically designed nozzles. It is crucial to characterize the effect of the internal geometry of the nozzle and ALR on atomization characteristics. In this study, the effects of geometric parameters of the pressure-swirl duplex nozzle and ALR on the spray characteristics of Jet A-1 fuel were investigated. Two different shaped pilot nozzles and two different shaped main nozzles were used. The analysis of spray characteristics was done by using particle image velocimetry (PIV) system and phase Doppler anemometry (PDA) system. Spray cone angle, Sauter mean diameter (SMD) distribution and weighted mean SMD (WMSMD) were analyzed. In the case of the effect of the internal geometry of the nozzle, the results show that when main and pilot nozzle work together, it produces a higher spray cone angle compared to the pilot nozzle alone. Cone-shaped pilot nozzle tip exhibits better atomization characteristic than flat shaped pilot nozzle tip. Combined main and pilot nozzle with counter-rotating flow provides smaller droplets and higher spray cone angle than co-rotating flow configuration. In the case of the effect of ALR, a decreasing trend in the spray cone angle is found with increasing ALR. Moreover, WMSMD increases with increasing ALR up to 1.16, but thereafter, it decreases again.