bluuebunny/crossref_metadata_2025 huggingface dataset:
Visualization of R410A Flowing in Copper and Stainless Steel Dimpled Tubes Enhanced by Petal-Shaped Patterns Under Evaporation Conditions
Authors: Tang Weiyu , Sun Zhichuan , Luo Yang , Li Wei
Published: October 2020

Experimental investigations were conducted to study the heat transfer characteristics and flow patterns of R410A during evaporation in three horizontal tubes having the same outside diameter of 12.70 mm and 180 mm in length. The tested tubes contain two tubes with three-dimensional enhanced surface structures (Cu-1EHT and SS-1EHT), and one equivalent smooth tube. This experiment was done at an evaporation temperature of 6 °C and mass flux from 100 to 200 kg/(m2·s) for vapor qualities varying from 0.2 to 0.8. For all the tube tested, flow pattern observations were obtained using a high-speed camera, and the impact of the actual flow regime on heat transfer was discussed. Results indicated that the transition from annular flow to intermittent flow began at a lower vapor quality for two enhanced tubes when compared to the smooth tube. Accordingly, both the Cu-1EHT tube and the SS-1EHT tube exhibited a superior performance than the smooth tube. In addition, the heat transfer coefficient of Cu-1EHT tube is higher than that of the stainless steel one, mainly due to the larger thermal conductivity of wall material.
Investigation on the Modeling of Tire Rotating Using Computational Fluid Dynamics
Authors: Fu Gen , Untaroiu Alexandrina
Published: October 2020

Fuel efficiency is very important when designing new vehicles. There is a continuous demand for lower fuel cost to customers. Many researchers have started to investigate the aerodynamics of tires. Since the experimental approaches are time consuming and costly, numerical methods have been developed to model the air flow around the tire. One of the challenges for modeling the tire is rotating boundary and contact patch. In the CFD model, both rotating and tire deformation have to be considered to get accurate predictions. However, most of the current methods neglect the tire deformation and contact patch. Therefore, in this study, three modeling approaches are compared for the modeling of tire rotation. They include rotating wall, multiple reference frame and sliding mesh. In CFD simulation, another challenge is mesh generation due to the sharp edge and large curvature around the contact patch. In order to generate mesh efficiently. A hybrid mesh which combines hex elements and polyhedral elements is used in this study. In addition, three different tire designs are investigated, including smooth tire, smooth tire with grooves and grooved tire with open rim. The results show that tire with open rim has the highest drag. Sliding mesh provides the most accurate predictions regarding of aerodynamic drag.
Experimental Study on Effective Operating Envelops of Nozzle to Mitigate Liquid Loading in Gas Wells
Authors: Xie Chuan , Liu Yonghui , Luo Chengcheng
Published: October 2020

The downhole liquid atomization by supersonic nozzle is an economic and efficient way to alleviate the liquid loading, but the optimal efficiency conditions are still not clear. In this paper, experimental studies were conducted in a 6-m long and 30-mm inner diameter vertical Perspex pipe to evaluate the performance of supersonic nozzle in different conditions. Camera was employed to record the characteristic of fluid flow downstream the nozzle, and the MATLAB was used in combination with DIP-image technology to determine the diameter of droplets. The experimental results show that mist is easy to be captured by the liquid bulk due to the liquid loading downstream the nozzle when the gas-liquid two phase flow pattern downstream nozzle is non-annular flow, thus leading to a failure of this technology. Due to the huge temperature drop in the annular flow condition, small fraction of mist is condensed into droplets and then attaches onto the pipe wall, forming liquid film. The remaining mist can still flow upwards. In this article, when the gas velocities upstream the nozzle range from 2.4m/s to 3.2m/s, the size of droplets range from 50μm to 1000μm correspondingly. Field application indicates that this technology under the effective operating envelops are able to be efficient to remove the liquid-loading, and the gas wells can maintain a long-term stable production.
Interactions of Two UAV Rotors at Low Reynolds Number
Authors: Tomar Yashvardhan , Shukla Dhwanil , Komerath Narayanan
Published: October 2020

Vertical takeoff and landing vehicle platforms with many small rotors are becoming increasingly pertinent for small Unmanned Aerial Vehicles (UAVs) as well as distributed electric propulsion for larger vehicles. These rotors operate at low Reynolds number unlike large rotors for which the existing prediction methods were developed. Operating at very low Reynolds number essentially means that viscous effects are more dominant; and their spatial spread is significant with respect to the rotor dimensions. This impacts the nature of inter-rotor aerodynamic interactions which become more difficult to predict and characterize. In the present research, two nominally identical commercial UAV rotors are studied for a range of separations in hover and forward flight, both experimentally and computationally, in parallel with ongoing vehicle flight tests with 4 and 8 rotors. Bi-rotor tests in tandem in-plane configuration were performed in Georgia Tech’s 2.13m × 2.74m test section wind tunnel. Rotor simulations were done using the RotCFD Navier-Stokes solver. In hover, rotor performance is sensitive to the distance between rotors at low rotation speeds, indicating the presence of greater inter-rotor interactions at low Reynolds number. In forward flight, the performance of the downstream rotor gets negatively affected by the upstream rotor wake.
Characterization of Flow Regimes in Gas-Solid Fluidized Beds via a Data Driven Framework
Authors: Kotteda V. M. Krushnarao , Kommu Anitha , Kumar Vinod
Published: October 2020

Gas-solid fluidized beds are widely used in petroleum, chemical, mineral, pharmaceutical, and power plant applications. The performance of fluidized bed reactors strongly depends on the flow dynamics. Characterization of a particle-laden flow has been one of the challenging issues in fluidization research. The simulation of flow in such processes is challenging as the complex dynamic systems comprised of numerous particles and fluidizing gas confined in specific devices. Nonlinear particle-particle/wall and particle-gas interactions lead to complex flow behavior of the gas-solid flows. We used MFiX to simulate a gas-solid flow in fluidized beds. A data-driven framework is trained with the data from MFiX-PIC simulations. The trained and tested machine learning model is used to characterize the flow regimes in fluidized beds. In the present study, the void fraction is used to characterize the flow regimes. However, others in the literature have used pressure, temperature, heat transfer coefficient, acoustic emission, vibration, and electrostatic charge for the characterization of flow regimes.
The Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaneless Diffuser
Authors: Miida Hiroshi , Tajima Kenta , Fujisawa Nobumichi , Ohta Yutaka
Published: October 2020

The unsteady diffuser stall behavior in a centrifugal compressor with a vaneless diffuser was investigated by experimental and computational analyses. The diffuser stall generated as the mass flow rate decreased. The diffuser stall cell rotated at 25–30% of the impeller rotational speed, with diffuser stall fluctuations observed at 180° from the cutoff. The diffuser stall fluctuation magnitude gradually increased near the cutoff. Based on diffuser inlet velocity measurements, the diffuser stall fluctuations generated near both the shroud and hub sides, and the diffuser stall appeared at 180° and 240° from the cutoff. According to the CFD analysis, the mass flow fluctuations at the diffuser exit showed a low mass flow region, rotating at approximately 25% of the impeller rotational speed. They began at 180° from the cutoff and developed as this region approached the cutoff. Therefore, the diffuser stall could be simulated by CFD analysis. First, the diffuser stall cell originated at 180° from the cutoff by interaction with boundary separation and impeller discharge vortex. Then, the diffuser stall cell further developed by boundary separation accumulation and the induced low velocity area, located at the stall cell center. The low velocity region formed a blockage across the diffuser passage span. The diffuser stall cell expanded in the impeller rotational direction due to boundary separation caused by a positive flow angle. Finally, the diffuser stall cell vanished when it passed the cutoff, because mass flow recovery occurred.
Comparison of Different On-Site Measurement Methods and Their Practical Accuracy
Authors: Benigni Helmut , Leithner Stefan , Jaberg Helmut , Guggenberger Mark
Published: October 2020

Turbines or pumps in hydropower plants achieve peak efficiencies of far more than 90% depending on the specific speed and the type of machine. Based on model tests or numerical simulations, the efficiency of a prototype machine has to be guaranteed and measured with the best measurement technique available. The choice of the appropriate efficiency measurement method does not only depend on the expected measurement uncertainties and tolerances set in a contract, it is also a question of the effort required for such a measuring campaign and of technical challenges, e.g., limitations imposed by plant operating conditions as draining of the system or constant load operation. Concerning hydraulic turbomachines, modern measuring technology offers different methods for efficiency measurements at a turbine unit. Primarily through the development of real-time capable data acquisition systems, it is possible to perform high-precision measurements rather quickly, whereby minimal downtimes can be achieved.

In the first part, the specific measurement methods available in different standards are presented in an overview, and then in the second part, the achieved accuracies of a very complex thermodynamic measurement are compared with a low-cost clamp-on acoustic Doppler measurement using a concrete example.
Continuous Blowing Jet Flow Control Optimization in Dynamic Stall of NACA0012 Airfoil
Authors: Tadjfar M. , Kasmaiee Saman , Noori S.
Published: October 2020

Use of active flow control techniques has become important in flow separation control. Continuous blowing jet is one of the most effective methods that can be used to improve aerodynamic performance of an airfoil. In the present work, different operational parameters of a continuous blowing jet were optimized to improve the aerodynamic performance of an oscillating NACA0012 airfoil. The airfoil underwent a sinusoidal motion about its quarter-chord between −5 and 25 degrees at the Reynolds number of 1.35 × 105. Unsteady Navier-Stokes equations were solved with k-ω SST turbulence model. Due to the time-consuming nature of large number of numerical simulations required during the optimization process, two neural networks were employed to reduce the number of simulations required. The optimization was carried out with the use of a genetic algorithm. The objective function was defined as the lift-to-drag ratio. In these networks, the relationship between the jet operational characteristics and the aerodynamic coefficients were trained. The jet operational parameters that were considered in this study, included jet location (at 1–60 percent of chord length), jet-opening length (0.05 to 0.3 percent of chord length), blowing jet velocity magnitude (0 to 5U∞), and blowing jet incident angle (0 to 180 degrees). Obtained results indicated that jet-opening length and blowing velocity magnitude have a greater effect on the aerodynamic performance when reached their upper values. Concerning the jet location, it was observed that the best jet location was about 2 to 5 percent of the chord Jet angle (θ) was found to results in the best performance when oriented at range 55 to 70 angle. Results indicated a significant improvement of the aerodynamic performance at the optimum blowing jet configuration.
Diversion of Pulsatile Flow Over a Rectangular Sidewall Cavity Using Superhydrophobic Mesh
Authors: Eichholz Benjamin , Zhang Ruihang , Zhang Yan
Published: October 2020

Pulsatile flow over open cavity represents one type of physiological phenomenon related to a few common cardiovascular diseases, such as cerebral sidewall aneurysm and arrhythmia-induced thromboembolism in the left atrium appendage (LAA). In recent years, endovascular treatments using mesh-based implants have become increasingly popular. In this paper, we study the characteristics of pulsatile flow over a simplified sidewall cavity under two Reynolds/Womersley number conditions using Particle Image Velocimetry. The impacts of a regular mesh and a superhydrobobically-coated mesh on the cavity flow are investigated. Our results quantify the phase-to-phase changes of the flow fields and reveal the formation and the transport of the primary vortex over the ostium of the rectangular cavity. Results suggest the meshes diverted the main flow away from the cavity and prohibited the development of the primary vortex. A penetrated jet flow was formed near the front side of the cavity due to the presence of the mesh. The superhydrophobic mesh dramatically reduced the kinetic energy of the penetrated jet into the cavity. It indicates the mesh flow diversion is effective because of the destruction of the shear-induced vortex dynamics that causes flow stagnation on the rear cavity wall. Our results also indicate the superhydrophobic coating is potentially beneficial in terms of reducing the hemodynamic loading inside the cavity.
Noise Source Identification and Noise Directivity Analysis of Bladeless Fans by Combined CFD and CAA Method
Authors: Li Ang , Chen Jun , Liu Yangfan , Bolton J. Stuart , Davies Patricia
Published: October 2020

The bladeless fan is a new concept of fan that does not have visible impellers. It features low noise level, uniform airflow, and improved safety. It has been widely applied in household appliances. Since the customers are particularly sensitive to the noise generated by the fan, the aeroacoustics performance of the fan needs to be accurately characterized in the design stage. In this study, computational fluid dynamic (CFD) and computational aeroacoustics (CAA) are applied to investigate the aeroacoustics performance and identify the major noise source of the bladeless fan. A prototype of the bladeless fan, including a wind channel, a base cavity, a rotor and a stator inside the base, is set in a computational domain of 4m × 2m × 2m and the airflow through the fan is simulated. The hybrid mesh is generated, the unstructured mesh in the near field, and the structured at the far field. To compute the flow field, steady RANS simulation (standard k–ε turbulence model) and Large Eddy simulation (Smagorinsky-Lilly model) are carried out. Ffowcs Williams and Hawkings (FW-H) analogy is used to predict the acoustic field. Experiments, including air velocity measurement and sound pressure measurement, are conducted to validate simulation results. Sound pressure level results at the near-field receiver illustrate that the blade passage frequency can be captured by combined CFD and CAA method. Noise source analysis shows that the combination of the rotor and stator contributes most to the noise produced by the bladeless fan. The wind channel is the secondary source. Sound pressure level contours at different distances and different heights are generated to investigate the directivity pattern of the noise generated by the bladeless fan. At the near field, the produced noise at the front and the back of the bladeless fan are louder than those at left and right; at the far field, the noise at the front is much larger than the other three sides. In addition, at the near field, with the increase of the height, two separated hotspots appear over 2,500Hz and the sound pressure level at these two hotspots increases; at the far field, the noise distribution at different heights is similar and the peak near 3,000Hz can be estimated. A possible reason to cause this peak is vortex shedding at the trailing edge of the rotor’s blades. The aeroacoustics analysis is helpful to develop strategies to reduce noise and guide the improved design of the bladeless fan.
Analysis of the Vortex Dynamics and Instability Mechanisms for a Lobed Nozzle Jet
Authors: Sekaran Aarthi , Amini Noushin
Published: October 2020

The application of radially lobed nozzles has seen renewed challenges in the recent past with their roles in combustion chambers and passive flow control. The free jet flow from such nozzles has been studied for different flow conditions and compared to jets from round nozzles, verifying their improved mixing abilities. The precise mixing mechanisms of these nozzles are, however, not entirely understood and yet to be analyzed for typical jet parameters and excitation modes. While past studies have proposed the presence of spanwise Kelvin-Helmholtz instability modes, the roll-up frequencies of the structures indicate more than one primary structure, which is challenging to resolve experimentally.

The present study carries out three dimensional CFD simulations of the flow from a tubular lobed nozzle to identify instability mechanisms and vortex dynamics that lead to enhanced mixing. We initially validate the model against existing hotwire and LDV data following which a range of Large Eddy Simulations (LES) are carried out. The free jet flow was at a Reynolds number of around 5 × 104, based on the effective jet diameter. Initial results are compared to that of a round nozzle to demonstrate changes in mixing mechanisms. The lobed nozzle simulations confirmed the presence of K-H-like modes and their evolution. We also track the formation and the transport of coherent structures from the tubular part of the nozzle to the core flow, to reveal the evolution of the large-scale streamwise modes at the crests and corresponding horseshoe-like structures at the troughs.
Investigating the Effect of Configuration and Reduced Frequency on SD7003 Airfoils Performance in Tandem
Authors: Hosseini Niloofar , Tadjfar Mehran
Published: October 2020

The effect of placing a stationary airfoil downstream of a pitching airfoil in a tandem configuration was investigated using computational fluid dynamics. A finite-volume URANS solver was employed to simulate the flow field. An optimization study was performed at the low Reynolds number of 30,000 for SD7003 airfoils using a genetic algorithm. Three artificial neural networks were coupled with the genetic algorithm to reduce the computational cost. The optimization process was used to find the optimum design parameters in order to maximize lift to drag ratio of airfoils together in addition to the thrust coefficient of the hindfoil. It was found that a significant improvement of 174% and 15% could be achieved for the thrust coefficient of static hindfoil and L/D of the combined airfoils respectively in tandem configuration over single airfoils at optimum values of design parameters.
Numerical Investigation of a Square Back Vehicle Equipped With a Single Cavity and Multi-Cavity
Authors: Koppa Shivanna Naveen , Ranjan Pritanshu , Clement Shibu
Published: October 2020

This work aims to investigate the comparative effect of two passive flow controls in modifying the mean wake topology around a simplified square back vehicle model. The two passive flow controls are (i) Single cavity and (ii) Multi-cavity. A straight cavity with an optimum depth at the rear base of a vehicle is a well-known technique used to alter the mean wake topology and achieve drag reduction[1]. For two dimensional bluff bodies, a multi-cavity is known to deliver better drag reduction at shorter cavity depths in comparison to a single cavity[2]. With this viewpoint, a numerical investigation is carried out to examine the performance of a multi-cavity over a single cavity in drag reduction for a three-dimensional bluff body vehicle model. The numerical simulations are performed at Reynolds Number (Re) = 1 × 105 using the k-ω SSTSAS turbulence model in a Finite volume open-source code OpenFOAM. The investigations revealed, for any cavity depth, a single cavity always performed better than multi-cavity in reducing drag. However, at optimum cavity depth equal to 33% of the body height, the drag reduction magnitude was identical for both the flow controls. The plausible mechanisms responsible for their relative difference in performance will be explored by analyzing the base pressure distribution, wake mean topology, and the temporal behavior of the wake.
Vortex Induced Vibrations Analysis of a Cantilevered Blunt Plate by Proper Orthogonal Decomposition of TR-PIV and Structural Modal Analysis
Authors: Watine Yann , Gabillet Céline , Astolfi Jacques-André , Pernod Laetitia , Lossouarn Boris , Deü Jean-François
Published: October 2020

The present work focuses on the experimental characterization of the vortex shedding and on the induced vibrations of a cantilevered blunt rectangular aluminum plate of chord to thickness ratio 16, immersed in a uniform water flow in the hydrodynamic tunnel of the French Naval Academy Research Institute. Experiences have been conducted for Reynolds numbers Re (based on chord length) ranging from 2.5 × 105 to 10.5 × 105 at zero degrees incidence. Special attention has been paid to the interaction of the structural response and the flow dynamics at the twisting resonance. For this purpose, wake structures have been analyzed by Time Resolved Particle Image Velocimetry (TR-PIV) and the structural response of the plate has been examined by laser vibrometry. The von Karman vortex street has been characterized by statistical analysis and Proper Orthogonal Decomposition of PIV velocity fields and the structure is analyzed through modal analysis. The near-wake’s structure has been examined for three different Reynolds numbers: (i) at Re = 3.0 × 105, corresponding to vortex induced structural response at constant Strouhal number; (ii) at Re = 4.5 × 105, corresponding to mechanical resonance but dissociated vortex shedding and (iii) at Re = 5.4 × 105, corresponding to lock-in of the vortex shedding at the mechanical resonance. At Re = 4.5 × 105, at mechanical resonance, it reveals the occurrence of an energy transfer between the shear layer and the bubble wake vortex which cancels synchronization of the structural vibration with the von-Karman vortex street.
The Effect of Varying Viscosity in Turbulent Channel Flow
Authors: Coppo Leite Victor , Merzari Elia
Published: October 2020

In this article we examine channel flow subject to spatially varying viscosity in the streamwise direction. The Reynolds number is imposed locally with three different ramps. The setup is reminiscent of transient channel flow, but with a space-dependent viscosity rather than a time dependent viscosity. It is also relevant to various applications in nuclear engineering and in particular in test reactors, where the viscosity changes significantly in the streamwise direction, and there is a severe lack of Direct Numerical Simulation (DNS) data to benchmark turbulence models in these conditions.

As part of this work we set up a novel benchmark case: the channel is extended in the stream-wise direction up to 20π. The viscosity is kept constant in the first 4π region. This inlet region is used as a cyclic region to obtain a fully developed flow profile at the beginning of the ramping region. In the ramping region the Reynolds number is linearly increased along the channel. The flow is homogenous in the spanwise direction, while it is non-homogenous in the stream-wise and wall-normal direction. We perform here Direct Numerical Simulation (DNS) with Nek5000, a spectral-element computational fluid dynamics (CFD) code developed at Argonne National Laboratory.

In this study, specific focus is given to the investigation of turbulence properties and structures in the near-wall region along the flow direction. Turbulent statistics are collected and investigated. Similarly to transient channel flow, the results show that a variation in the Reynolds across a channel does not cause an immediate change in the size of turbulent structures in the ramp region and a delay is in fact observed in both wall shear and friction Reynolds number. The results from the present study are compared with a correlation available in the literature for the friction velocity and as a function of the Reynolds number.
Retracted: “Experimental Investigation of the Fluctuating Static Pressure in a Subsonic Axisymmetric Jet” [ASME 2020 Fluids Engineering Division Summer Meeting, Volume 2: Fluid Mechanics; Multiphase Flows, Virtual, Online, July 13–15, 2020, Conference Sponsors: Fluids Engineering Division, ISBN: 978-0-7918-8372-3, Copyright © 2020 by ASME. Paper No. FEDSM2020-20242, V002T03A040; 15 pages; doi: 10.1115/FEDSM2020-20242]
Authors: ASME
Published: October 2020

This paper was removed from publication at the author’s request. December 9, 2020.

Copyright © 2020 by ASME
Using Computational Fluid Dynamics to Analyze Convection in Pin-to-Plane Plasma Discharge
Authors: Galvez Robert , Wright Kamau , Milanovic Ivana
Published: October 2020

Multiphysics simulations were conducted to model the role of naturally induced convection in heat and mass transport within a non-isothermal plasma discharge chamber. A pin-to-plane discharge into chamber containing carbon dioxide can be used to possibly decompose carbon dioxide. The present study characterizes the role that convection plays in the diffusion of various products such as ions and excited-state species throughout the test chamber. Multiphysics software including computational fluid dynamics was employed in a two-dimensional transient simulation of a closed reactor with a large pin serving as the cathode and a bottom plate serving as the anode. The mesh was adjusted to best capture important discharge phenomena, while the simulated time was varied to best characterize the chemical processes.. Mesh validation was undertaken using the relevant minimum sizes required by the plasma, fluid flow, and heat transfer solvers. The flow induced by natural convection from the discharge was then compared to the flow induced by natural convection around a resistance heater operating with the same power input as the plasma. The results of this simulation are used to inform improvements on a parallel experimental system used to study the discharge, such as placement of gas concentration sensors and to better understand the heat and mass transfer through the discharge.
Experimental Hydraulic and Mechanical Characterisation of a Double-Flow Implantable Blood Pump
Authors: Strauch Carsten , Thamsen Paul Uwe , Escher Andreas , Granegger Marcus
Published: October 2020

The considerable contribution of parasitic measurement effects in fluid flow machines of small performance classes remains a major challenge in the assessment of corresponding hydraulic characteristics such as hydraulic losses and efficiencies. However, a standardized experimental approach to accurately quantify such properties is of paramount importance for the design of rotary blood pumps with low hydraulic power and torque.

This study proposes a novel test bench to accurately assess hydraulic and mechanical characteristics of pumps designed for a mechanical power input of up to 20,9W, a maximum rotational speed of 7000min−1 and a maximum torque of 10mNm. Key feature of the iteratively optimized test bench is a torque measuring shaft combined with ceramic bearings and seals that minimizes the friction torque to 0.66–0.69mNm.

Preliminary measurements for a double-inflow, double-outflow circulatory support pump with maximum torque of 1.71mNm and hydraulic power of 0.15W, demonstrated both the feasibility of the hydraulic and mechanical design of the test bench and the reproducibility of the measured data. These results warrant the applicability of the presented test-bench for the investigation of typical cardiac assist systems as well as for all other hydraulic flow machines of the equivalent performance class.
A Comparison of the Flow Structure in a Normal Triangular Tube Array Obtained Based on the SFV Technique and on a CFD Analysis
Authors: Martins Rocha Douglas , Toshio Kanizawa Fabio , Hayashi Kosuke , Hosokawa Shigeo , Tomiyama Akio , Ribatski Gherhardt
Published: October 2020

This paper presents an analysis of the performance of the Spatial Filter Velocimetry (SFV) technique applied to external flows across tube bundles to evaluate the benefits of using low particle concentration in a complex geometry. The experimental vector velocity fields were obtained in a tube bundle test section made of acrylic with 20 rows of 4 tubes of 20mm O.D. mounted in a normal triangular configuration and transversal pitch per diameter ratio of 1.25. The SFV results reliability was evaluated by comparing the volumetric flow rates estimated by the experimental vector velocity fields with the volumetric flow rates measured by the flowmeter, showing good agreement. Additionally, the experimental results are used to check the validity of numerical results obtained through CFD simulation. The experiments were conducted for Reynolds numbers of 909, 1842 and 3902. The velocity profiles obtained through the SFV technique agree with the CFD simulations results, which indicate consistent results for both analyzes. Therefore, the numerical and experimental results indicate that the SFV technique provides accurate velocity data and is suitable to be applied to complex geometries, especially in regions away of the test section wall due to the good visibility provided by low particle concentration.
A Quantitative Analysis of Machine Learning Based Regressors for Pressure Reconstruction in Particle Image Velocimetry Applications
Authors: Basu Manan , Kumar Sushrut , Gupta Priyam , Kumar Singh Raj
Published: October 2020

With the recent advances in Artificial Intelligence, avenues for the application of Machine Learning (ML) are increasing in many engineering problems pertaining to Fluid Mechanics. Particle Image Velocimetry is an advanced fluid measurement technique but the pressure evaluation requires the data to be further processed. In the proposed work, we focus on developing a novel computational framework using Machine Learning techniques, primarily Artificial Neural Networks (ANN) to infer the pressure fields from velocity obtained from PIV data. The framework was tested for a case of flow over a periodic hill. The data for training was generated by performing LES simulations for different hill curves and Reynolds number. Various Regression models were developed in conjunction with the Adam and Limited Memory - BFGS (L-BFGS) optimizers and suitable activation functions. A detailed study of the performance of each model was done to determine the optimum values of the hyperparameters. Further, a comparative analysis of various models and the respective optimization algorithms used was performed. It was concluded that the multi-layered perceptron model with L - BFGS optimizer showed the best performance. Overall, it is observed that the models can predict the pressure values with a high degree of accuracy as confirmed by validation with the previous literature data.
Effects of Streamwise Aspect Ratio on Turbulent Flows Over Forward-Backward Facing Steps
Authors: Chalmers Heath , Fang Xingjun , Tachie Mark F.
Published: October 2020

Separated and reattached turbulent flows induced by two-dimensional forward-backward-facing steps with different streamwise lengths submerged in a thick turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The examined aspect ratios of the step range from 1 to 8, and the Reynolds number based on the free-stream velocity and step height is 13 200. The thickness of the oncoming turbulent boundary layer is 6.5 times the step height. The effects of varying aspect ratio of the steps on the mean flow, Reynolds stresses, triple correlations and unsteadiness of turbulent separation bubbles are studied. It was found that the mean flow reattaches over the step for forward-backward facing steps with aspect ratios of 2 and higher. The temporal variation of the first proper orthogonal decomposition (POD) mode and reverse flow area, which is used to examine the flapping motion of separation bubble, shows remarkable synchronization.
Dielectric Barrier Discharge Plasma Actuators for Active Flow Control, Ice Formation Detection and Ice Accumulation Prevention
Authors: Rodrigues F. F. , Abdollahzadeh M. , Pascoa J.
Published: October 2020

An experimental investigation was conducted in order to understand the ability of plasma actuators to operate in three different modes: flow control, ice formation detection and ice accumulation prevention. When plasma actuators are operated with voltage levels, above the breakdown voltage, a plasma discharge surface is generated and with that, an ionic wind is produced. By using this phenomena, plasma actuators may be used to manipulate flow fields and control adjacent flows to the surface in which they are applied. However, a big part of the power applied to the device is dissipated as heat. Due to heat dissipation, the actuator surface temperature rises and the adjacent air is heated. Considering this, actuators may operate as ice prevention devices by heating the surface where they are applied and preventing the ice formation and accumulation. On the other hand, plasma actuators present a behavior similar to a capacitor and they may operate as a capacitive sensor. In the presence of water or ice on the top of the surface, the electric field changes and with that, several plasma actuator electrical features change as well. By monitoring that changes, the presence of water or ice on the top of the surface can be detected and the plasma actuator may be used as an ice sensor device. Therefore, in the present study a plasma actuator was experimentally tested operating in these three different operation modes and its feasibility to perform these different tasks is shown.
Performance Evaluation of a Non-Linear Explicit Algebraic Reynolds Stress Model for Surface Mounted Obstacles
Authors: Taylor Benjamin H. , Jamal Tausif , Walters D. Keith
Published: October 2020

The presence of complex vortical structures, unsteady wakes, separated shear layers, and streamline curvature pose considerable challenges for traditional linear Eddy-Viscosity (LEV) models. Since Non-Linear Eddy Viscosity Models (NEV) models contain additional strain-rate and vorticity relationships, they can provide a better description for flows with Reynolds stress anisotropy and can be considered to be suitable alternatives to traditional EVMs in some cases. In this study, performance of a Non-Linear Explicit Algebraic Reynolds Stress Model (NEARSM) to accurately resolve flow over a surface mounted cube and a 3D axisymmetric hill is evaluated against existing experimental and numerical studies. Numerical simulations were performed using the SST k-ω RANS model, SST k-ω-NEARSM, SST-Multiscale LES model, and two variants of the Dynamic Hybrid RANS-LES (DHRL) model that include the SST k-ω and the SST k-ω-NEARSM as the RANS models. Results indicate that the SST k-ω RANS model fails to accurately predict the flowfield in the separated wake region and although the SST-NEARSM and SST-Multiscale LES models provide an improved description of the flow, they suffer from incorrect RANS-LES transition caused by Modeled Stress Depletion (MSD) and sensitivity to changes in grid resolution. The SST-DHRL and the SST-NEARSM-DHRL variants provide the best agreement to experimental and numerical data.
Micro Flow Direction Analysis Using Gold Sputtered Planar V-Shaped Electrode Pattern
Authors: Parvez Mohammad Salman , Rubby Md Fazlay , Ucchyash Sajid Mahfuz , Biswas Prosanto , Huq Hasina , Islam Nazmul
Published: October 2020

Sensing and detecting micro particles require a bulk fluid motion towards the sensing element in order to get a desirable response from the sensing element. Specially for low-concentrated fluid suspension response time is very long. So both for detection and sensing mechanism if the fluid flow is guided at a reasonable speed and at a low voltage and relatively low frequency which is suitable for bio-particles; the sensing mechanism can be enhanced largely. But sometimes it is required to re-accumulate or recombine the fluid. Previously parallel plate configuration was used to concentrate particle, but this is for the first time a V-shaped electrode pattern used to guide the bulk flow for concentration purpose. The V-shaped electrode set-up was made by following an unconventional way using sputtering machine which was cheaper than the conventional Photolithography method. AC-Electroosmosis from planar electrodes is a strong mechanism for creating micro-flows from several hundred microns away from the electrode surface. The mechanism for the AC Electroosmotic fluid flow is based on Capacitive charging which causes due to the generation of counter-ions at the electrode-electrolyte interface and Faradaic charging which is generated by the accumulation of co-ions. These two different methods are responsible for a converging and diverging surface flow of the fluid particles. At lower voltage capacitive charging method plays a significant role and most of the applied voltage drops occur at the electrical double layer but up to a certain voltage level Faradaic charging method takes over and starts dominating. The induced flow velocity by both methods has different relationship with the applied voltage. In this experiment Electrical Impedance Spectroscopy (EIS) was used to determine the suitable frequency range for the application & 2.12Vrms was used initially which is a very low voltage. An equivalent circuit for the setup was analyzed. Finally, an analysis was made on this setup using conductive fluid to observe the AC Electrothermal (ACET) effect. In our experiment the goal was to get an optimum velocity for concentration at low voltage and low frequency also to observe the guiding direction of the fluid flow in order to find a way to focus the fluid flow towards the desired direction.
Effect of Localized Blockages on the Friction Factor of a 61-Pin Wire-Wrapped Bundle
Authors: Childs Mason , Muyshondt Robert , Busco Giacomo , Yildiz Mustafa Alper , Vaghetto Rodolfo , Hassan Yassin
Published: October 2020

A test campaign has been conducted to study the effects of localized blockages on the friction factor of a 61-pin wire-wrapped bundle replica. Blockages of selected sizes and locations within the bundle have been installed, and fluid pressure drop was measured across one wire pitch. The friction factor was compared with the unblocked bundle friction factor, and with the predictions of a well-established friction factor correlation.

A computational fluid dynamics model of the experimental bundle was prepared, and simulations were compared with experimental results with the ultimate goal to complement the experimental activity with validated simulation data. The unique sets of experimental data, complemented with simulation results, provide a quantification of the increase of the bundle friction factor produced by the blockages, and the different expected effects based on blockage size and location for various flow regimes.
Effects of Pipe Diameter and Stokes Number on Erosion in Elbows
Authors: Karimi Soroor , Asgharpour Alireza , Fallah Elham , Shirazi Siamack A.
Published: October 2020

Large diameter pipes and elbows are vastly used in industry especially in mining and oil and gas production. Solid particle erosion is a common issue in these pipelines, and it is important to predict it to avoid failures. Currently, laboratory experiments reported in the literature are limited to diameters less than 4 inches. Therefore, there is not much experimental data available for large diameter elbows. However, the erosion can be predicted by CFD simulations and applying erosion equations in such elbows. The goal of this project is to examine the effects of elbow diameter and Stokes number on erosion patterns and magnitude for various flow conditions for elbow diameters of 2, 4, 8, and 12 inches.

The approach of this work is to first perform CFD simulations of liquid-solid and gas-solid flows in 2-inch and 4-inch elbows, respectively, and evaluate the results by available experimental data. Then CFD simulations are carried for 2, 4, 8, and 12-inch standard elbows for various Stokes numbers corresponding to gas dominant flows with the velocity of 30 m/s, and liquid dominant flows with the velocities of 6 m/s. For gas dominant flows erosion in air and for liquid dominant flows erosion in water is investigated. All these simulations are carried for four particle sizes of 25, 75, 150, and 300 microns. The results indicate that Stokes number and diameter of elbows have significant effects on erosion patterns as well as magnitudes in this geometry.

This work will have various applications, including validating mechanistic models of erosion predictions in elbows and developing an Artificial Intelligence (machine learning) algorithm to predict erosion for various flow conditions. Such algorithms are limited to the range of conditions they are trained for. Therefore, it is important to expand the database these codes are accessing. Overall, the CFD database of large diameter elbows will reduce the computational costs in the future.
Experimental and Numerical Investigations of Unsteady Flows Downstream Confined Rectangular Obstacles
Authors: Aloui Fethi , Elawady Amal
Published: October 2020

In this work, we present an experimental and numerical study in a horizontal duct with a rectangular cross-section (300 × 30mm2). In the middle of this cross-section, a rectangular obstacle (20 × 10mm2) then a squared obstacle (10 × 10mm2) were placed in order to study experimentally and numerically the vortex shedding and their interactions in the flow. Upstream obstacles, the studied flows are laminar. In our experimental study, the PIV technique was used in order to obtain instantaneous velocity fields downstream the used obstacles. From these measurement results, a post-processing was used (especially the Γ2 criterion) in order to well extract instantaneous vortices contained in the flow downstream obstacles. In parallel with this experimental study, a 2D numerical simulation was achieved in order to be validated by the experimental results. Other complementary PIV measurements were carried out in the duct top by visualizing the flow downstream obstacles in order to study the 3D effects of the flow.
RANS VOF Simulations of Density-Stratified Air-Water Flow in a 2D Channel
Authors: Maiti Chandrima Jana , Ghia Urmila , Turkevich Leonid A.
Published: October 2020

We perform RANS-VOF simulation of density-stratified, fully developed air-water flow in a 2D channel. The flow is completely specified by the (common) driving pressure gradient down the channel and by the fill factor (relative height of the heavier phase to the total height of the channel). Varying the pressure gradient and fill factor results in different flow combinations: namely, laminar air/laminar water, turbulent air/laminar water, turbulent air/turbulent water, laminar air/turbulent water. The focus of the study is the near-interface interaction when either or both phases are turbulent. The RANS-VOF equations are solved on a 2D channel with periodic inlet/outlet boundary conditions. For a fixed fill factor, the Reynolds numbers of each phase varies monotonically with driving pressure gradient. However, at fixed pressure gradient, the fluid Reynolds numbers are non-monotonic at high fill factor. This Reynolds number phase diagram is important in understanding the laminar and turbulent regimes of each phase. The mean velocity is axial (down the channel) and exhibits a dip below the free surface whenever one of the phases is turbulent. As expected, the diagonal Reynolds stresses , , are strongly coupled between the phases, however coupling in  is less pronounced.
Aerodynamic Subsonic Model at Transitional/Turbulent Reynolds Number for Bluff-Ellipsoidal Hulls
Authors: Ricardo Jorge A. , dos Santos Davi Antônio , dos Santos Magalhães Elisan
Published: October 2020

The present work addresses the subsonic aerodynamic coefficients model for bluff ellipsoidal hulls at transitional and turbulent Reynolds number. The drag, lift, and moment aerodynamic coefficients model are based on computational fluid dynamics (CFD) simulations for four bluff ellipsoids with aspect ratio of 1, 2, 3, and 4, in the Reynolds number range of 1 × 103 to 2 × 106 and angle of attack range from 0 to 20 degrees. The Large Eddy Simulation (LES) turbulence model is used with the sub-grid turbulence model Wall-Adapting Local-Eddy Viscosity (WALE) to solve the fluid field. To reduce computational simulation time, at a first instant, the mesh is gradually refined until the point that it does not influence anymore in the final result (mesh independence). For each aerodynamic coefficient a nonlinear equation structure, valid for all the ellipsoids, is proposed as a parametric model with parameters estimated using the least mean square algorithm applied to the results of the computational fluid dynamics simulations. The proposed equations have a superior performance, in terms of precision and number of terms, when compared to polynomial equations fitted to the same data.
Experimental and Numerical Analysis on the Performance of Spiral Two-Fluid Atomizer Using DPM Method
Authors: Lee Jin-Woo , Jung Kuk Jin , Sherman Morely , Kim Hyun Sin , Kim Youn-Jea
Published: October 2020

A two-fluid atomizer has been frequently used in a wide range of industries for various purposes such as painting, cleaning particles and snow making. In particular, the manufacturing of advance semiconductors using sensitive devices such as organic light emitting diodes (OLED) and dynamic random access memory (DRAM), require high performance nozzle. The droplets sprayed with a high relative gas velocity are widely used for cleaning particles. In this paper, two-fluid atomizer is numerically studied according to four variables to confirm the effect on the atomizer performance. The numerical results using the discrete phase model (DPM) with several break-up models are compared with the experimental data measured by the phase doppler particle analyzer (PDPA). Design of experiment (DOE) and genetic algorithm (GA) were used to obtain design points, and conduct sensitivity analysis, respectively. The results showed that the WAVE model has a good agreement compared to the other models, and the orifice diameter is a crucial factor for this model to determine the performance of Weber number and pressure.
Pressure Drop Study in Cylindrical Microchannels: Using Graphene Oxide and Magnesia Nanofluids
Authors: Ali Abdallah Y. M. , Fuseini Mohammed , Abo-Zahhad Essam M. , El-Kady M. F. , El-Shazly A. H.
Published: October 2020

The pressure drop and friction factor for the flow in horizontal smooth microtubes were investigated experimentally over a wide variety of Reynolds numbers in the laminar region under hydrodynamically fully developed conditions. The plain pipes are fabricated from 316 stainless steel and provided by (Swagelok Co., USA) with inner diameters of 0.770 mm and 0.510 mm and the length is 500 mm for both microchannels. For this purpose, graphene oxide (GO) nanosheets and magnesium oxide (MgO) nanoparticles were synthesized then characterized using standard methods such as X-ray powder diffractometer (XRD) and Transmission Electron Microscope (TEM). The prepared GO and MgO nanomaterial were added to distilled water to prepare nanofluids with mass fractions of 0.25, 0.5, 0.75, and 1 wt.% as working fluids and passes through one end of the microchannel to the other by a magnetically coupled gear programmable pump with variable speed that drives it. This study predicts the behavior of fluid flow in the microchannel for the range of dimensions used. Comparisons of pressure drop characteristics are given in terms of nanoparticle mass fractions and tube diameters. As a result, MgO nanofluids recorded higher pressure drop with an increase in the concentration of the nanofluids due to its higher viscosity followed by GO nanofluids.