bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical and Experimental Study on Active Stiffness Control of a Two-Degrees-of-Freedom Rope-Driven Parallel Mechanism
Authors: Gu Haiyu , Wei Cheng , Zhang Zeming , Zhao Yang
Published: October 2020

Rope-driven mechanisms with the characteristics of high speed, low inertia, and high precision are widely utilized in numerous fields. Stiffness is an important indicator to illustrate the precision and compliance of the mechanism. However, realizing active stiffness control is difficult for the mechanisms due to the coupling of rope tension and controller stiffness. To solve the problem, a verification prototype, 2-DOF rope-driven parallel mechanism (RDPM), is designed and manufactured, and its mechanical model is established. And then the general stiffness model of the RDPM is derived. Meanwhile, the rope-hole friction is calculated based on the Stribeck model. An active stiffness control scheme considering the pose retention, vibration suppression, and friction compensation is proposed. According to the stiffness model and active stiffness control law, the linear motion stiffness of the RDPM is analyzed in detail. The conclusion shows the motion stiffness is linear with the controller stiffness and initial rope tension. Finally, the theoretical stiffness, simulation stiffness and experimental stiffness are calculated and compared by the co-simulation technique and physical prototype experiment. The error between experimental data and simulation data is within 10%, which verifies the stiffness model and active stiffness control scheme.
Predictions of Scour Around Submerged Object Using LES-DEM
Authors: Elghannay Husam , Tafti Danesh
Published: October 2020

Local scour around structures is a natural process that impacts the design of submerged hydraulic structures. The phenomenon of erosion and movement of sediment particles in the vicinity of a submerged object is a result of the interaction between the turbulent flow and a bed of sediment particles. In this work Computational Fluid Dynamics (CFD) coupled to Discrete Element Method (DEM) is used to numerically predict the temporal evolution and morphology of the scour around a submerged structure. Simulations of scour and morphology are typically performed using Eulerian-Eulerian representations and thus avoid the excessive computational resources required to track all the discrete sediment particles of the bed material in the domain of interest. In the current work a periodic domain is assumed making this type of simulation more feasible for CFD-DEM simulations. An augmented soft sphere collision, which includes rolling friction and adhesive forces is used for representing the contact physics. The simulation was run for more than 50 s of real time. The temporal evolution of the scour around the submerged obstacle is investigated and compared to published experimental results and past simulations. The current work shows a deeper initial scour compared to experiment. The build-up of the ridges downstream start at the same rate predicted by experiment but seem to progress at a slower rate after 15s.
Uncertainty Evaluation of Flow Meter Calibration by Gravimetric Water Flow Standards at KRISS
Authors: Chun Sejong , Yoon Byung-Ro
Published: October 2020

Petrochemical and electric power plants are using flow meters, of which capacity is larger than 1000 m3/h. They require the accuracy of flow meters better than ±0.3% if the flow meters are being used for custody transfer or allocation measurement. However, the number of national metrology institutes, which can handle this issue, is very limited, including PTB (Germany), NMIJ/AIST (Japan), MIKES (Finland), CEESI-NIST (USA), and TÜV-NEL (UK). Large-capacity flow measurement standards are also needed to make better quality control for process engineering in the Republic of Korea.

This study aims at establishing the water flow measurement standards up to 2000 m3/h with uncertainty better than 0.06%. How to evaluate the measurement uncertainty is discussed for establishing the large-capacity water flow standards at KRISS.
Acoustic Measurement of Ultrasound Sensors in a Water Bath
Authors: Chun Sejong , Kwon Hyu-Sang , Park Doo-Sik , Lee Kang-Wook
Published: October 2020

PMN-PT is one of promising materials for fabrication ultrasound sensors for flow metering, in that the relative permittivity and the dielectric loss factor are superior to PZT, the popular piezoelectric materials at present. However, there are not many studies focusing on characterizing the PMN-PT ultrasound sensors by acoustic measurements. This study introduces an acoustic bath to accommodate a pair of ultrasound sensors for measuring the relative acoustic sensitivity and the dispersion angle by ultrasound waves.

Ultrasound sensors with resonance frequencies from 0.2 MHz to 1 MHz are also tested in terms of the relative permittivity, the dielectric loss factor, the electric impedance, and the electric admittance. From these measurements, it is found that PMN-PT is superior to PZT for increasing the relative acoustic sensitivity of ultrasound sensors. Impedance matching electronic circuits are fabricated by designing capacitance and inductance with the reflectance coefficients. Smith chart is used to calculate the capacitance and the inductance for the PMN-PT ultrasound sensors with resonance frequencies of 0.2 MHz and 0.5 MHz. As a results, it is also found that the impedance matching electronic circuits enhance the electrical admittance by 3.6 ∼ 25 times while keeping the electrical impedance between 50 Ω and 100 Ω.
Behavior of Myoblast Under Shear Stress in Couette Type of Flow
Authors: Hashimoto Shigehiro
Published: October 2020

Behavior of myoblast has been investigated under the uniform shear flow in vitro. The culture medium was sandwiched with the constant gap between the lower stationary culture plate and the upper rotating parallel plate to make a Couette type of the shear flow. By the rotating speed of the upper disk, the wall shear stress (τ) on the lower culture plate was controlled. C2C12 (mouse myoblast cell) was used in the test. After cultivation without flow for 24 hours for adhesion of cells on the lower plate, τ < 2 Pa was continuously applied on cells for 7 days in the incubator. Behavior of each cell was traced at the time lapse images observed by an inverted phase contrast microscope placed in an incubator. Experimental results show that cells differentiate to myotubes under τ < 2 Pa. Both the cell cycle and the cell length tend to scatter in the wider range, and the longitudinal axis of each cell tends to align to the flow direction by the shear stress of 1 Pa. The experimental system is useful to study quantitative relationships between the shear stress and the cell behavior: deformation, orientation, and differentiation.
A Verification and Validation Study With Respect to Simulating Undulating Propulsion
Authors: Loubimov George , Kinzel Michael P.
Published: October 2020

In this present work, a detailed verification and validation study is presented for a computational fluid dynamics (CFD) framework that aims to predict fluid physics and forces associated with naturally inspired marine propulsion. This approach involves comparing experimental datasets to their simulated counterparts and evaluating the numerical accuracy of the CFD toolset. In this manner, the error and uncertainty of the predictions from the CFD toolset can be established, which both convey the accuracy of the model and gives additional confidence in the underlying physical character of undulation-based propulsion. Validation efforts include simulation of a D-tube shedding a von Kármán vortex wake, a heaving and pitching foil generating thrust, and a traversing flat plate which is abruptly heaved and pitch. For all cases, mesh and time-step refinements are employed to ascertain numerical uncertainty. Although not yet complete, we also aim to roll up uncertainty from experiments and input uncertainties.
CFD Simulation and Design Improvement of Internal Rotating Flow of Turbomachine
Authors: Hadi Naseer , Jawad Badih , Hermez Munther , Metwally Hossam , Liu Liping
Published: October 2020

Designing a turbomachine comes with many challenges due to many parameters affecting its performance. This study presents a design to reduce losses in turbulence flow and surface friction by using a disk located between the rotating centrifugal impeller and the pump casing, which in turn enhances the centrifugal pump performance, upon rotating freely during normal operation. Under a constant operating speed of 3000 RPM, the new design is shown to improve the centrifugal pump performance.

The turbulent flow between the rotating impeller and pump stationary walls increases the frictional losses. The highest friction occurs in the flow between two surfaces, one being close to zero velocity and the other one moving at high speed. Flow recirculation in the enclosure is a major problem that leads to a decrease in turbomachine’s performance. Two-dimensional Computational Fluid Dynamics (CFD) analysis is used to numerically simulate the rotating flow field inside the centrifugal pump chamber and to provide critical hydraulic design information. In this study, ANSYS-FLUENT R19.2 is used to analyze the input torque under different angular velocities by applying a disk with various thicknesses at four different locations to get the best results. The flow field in the chamber is investigated using 2-D Naiver-Stokes Equations with a Realizable k-ϵ turbulence model. Standard water was used as the working fluid. The numerical analysis gives an idea of how the freely rotating disks behave, and the results will be compared to find the most efficient case of centrifugal pump operation with an adjacent disk. The best-case new design will identify the highest reduction of input power by 24.4%. This study will introduce to the future work of a three-dimensional model.
Variation of Zero Net Liquid Holdup in a Gas Liquid Cylindrical Cyclone Separator Below Operational Envelope
Authors: Shah Malay Jignesh , Kolla Srinivas Swaroop , Mohan Ram S. , Shoham Ovadia
Published: October 2020

Novel experimental and theoretical investigations are carried out on Zero Net Liquid Flow (ZNLF) in the upper part of the Gas-Liquid Cylindrical Cyclone (GLCC©) separator. Experimental data are acquired for the variation of the Zero Net Liquid Holdup (ZNLH) and the associated Churn region height for air-oil and air-water flow. The experiments are carried out at normal operating conditions below the GLCC Operational Envelope (OPEN) for Liquid Carry-Over (LCO). The ZNLH measurements for air-oil flow are higher than those for air-water flow. The Churn region height is higher for air-oil flow, as compared to the air-water flow, for the same operating conditions. The higher oil viscosity, which results in higher frictional and drag forces, leads to greater ZNLH for air-oil flow. The Churn region height is sensitive to the superficial gas velocity, whereby a small increase of gas velocity results in exponential growth of the Churn region height. The model developed by Karpurapu et al. (2018) for predicting the ZNLH at specific operational conditions just below the OPEN for LCO is extended to predict the ZNLH variation along the upper part of the GLCC below the OPEN for LCO, as well as the associated Churn region height. The predictions of the developed extended model for the ZNLH variation compared to the acquired experimental data showing discrepancies of 8% and 3%, respectively, for air-oil and air-water flows.
Comparison of Pathogens Dispersion in an Aircraft Cabin Using Gas Injection Source Versus a Coughing Manikin
Authors: Mahmoud Seif , Bennett James S. , Hosni Mohammad H. , Jones Byron
Published: October 2020

The dispersion characteristics of airborne pathogens were investigated in a Boeing 767 mockup cabin containing 11 rows with 7 seats per row, using two tracer gas source methods: continuous injection at low velocity and a coughing manikin. Both the injection source and the coughing manikin were located on the same seat in the sixth row. The injection source utilized CO2 gas at an injection rate of 5.0 liters per minute mixed with helium at a rate of 3.07 liters per minute to neutralize buoyancy. The manikin coughed approximately once every 75 seconds, with a volume of 4.2 liters of CO2 per cough. To ensure sufficient data were collected at each sampling location, each coughing manikin test was run for 6 coughs and each injection source test for 30 minutes of continuous injection. In both test methods, the tracer gas concentration was measured using CO2 gas analyzers at seated passenger breathing height of 1.2 m and radially up to 3.35 m away from the gas injection location, representing approximately four rows of a standard B767 aircraft. The collected data obtained from each tracer method was then normalized to provide a suitable comparison basis that is independent of tracer gas introduction flowrate. The results showed that both tracer source methods gave similar dispersion trends in diagonal and lateral directions away from the injection location. However, the tracer gas concentration was higher along the longitudinal direction in the coughing manikin tests due to the cough momentum. The results of this work will help researchers analyze different experimental and numerical approaches used to determine contaminant dispersion in various environments and will provide a better understanding of the associated transport phenomena.
Optimal Control Strategy to Distribute Water Through Loop-Like Planar Networks
Authors: Soni Bharat , Mishra Utkarsh A. , Nayak Ameeya Kumar
Published: October 2020

In this article, loop like planar networks formed by circular cross sectioned conduits with possibly different geometric measurements are studied to supply the required amount of isothermal water within the optimal time and through the shortest path. The flow optimization procedure is controlled by time varying pressures at nodes throughout the network for given specifications about pressure value at multiple demanding and single supply nodes. The flow governing equation is solved analytically to correlate transient flow rate and pressure and then studied using analogous electrical circuit. For each possible path between source and demand node, minimum equivalent flow impedance criterion is considered to pick the optimum path. This sets a multi-objective dynamic flow optimization algorithm and the same is executed under the assumption of fully developed and laminar flow. The optimum flow impedance can further be used to measure the pumping power as the cost of flow of a particular path. The algorithm can be extended to reduce the water wastages by controlling pressures efficiently.
Natural Convection Analysis Around Elliptical Shapes With Sinusoidal Heat Flux Using CFD
Authors: Kamstrup Annette G. , Pedersen Anna Marie A. , Elimar Frederik M. , Olsen Lasse , Sørensen Henrik , Hærvig Jakob
Published: October 2020

Heat transfer is important as technology becomes more compact, thereby increasing the heat flux and consequently the need for cooling. This paper will investigate natural convection on an elliptical shape with sinusoidal heat flux input. Natural convection was analysed using CFD simulations on an ellipse, with minor- to major axis ratio b/a = 0.6 and an inclination angle of α = 90°. The sinusoidal heat flux was non-dimensionalised by a modified Grashof number Gr* = g · β · (∂T/∂n) · (Lc)4/v2 with a mean value of 2 × 107, amplitude of up to 2 × 107, and dimensionless angular frequency ω* = ω · Lc2/v = 24, 36, and 72. All simulations were made with a Prandtl number of Pr = 7.0. To ensure reliable results a Grid Convergence Index analysis was carried out. Validation was made by comparing the obtained surface averaged Nusselt numbers to previous studies and results from performed experiments. An experiment using Particle Image Velocimetry, PIV, measured the flow field around an ellipse. The results from the sinusoidal heat flux showed that the difference in accounting for the sinusoidal Grashof function was up to 10% for the time-surface averaged temperature and time-surface averaged Nusselt number. Generally, the amplitude would increase the temperature, while the effect of the dimensionless angular frequency was dependent on the given amplitude.
A DNS Study of the Dispersion and Deposition of Nano- and Micro-Particles in a Turbulent Channel Flow
Authors: Mofakham Amir A. , Ahmadi Goodarz , McLaughlin John B.
Published: October 2020

Nano- and micro-particles dispersion and deposition in a dilute gas-solid turbulent flow in a channel were studied. A pseudo-spectral DNS code was used to solve the Navier-Stokes equations, and to generate the instantaneous turbulent velocity fluctuation field for the gas flow. Under the one-way coupling assumption, the gas flow carries the particles, but the influence of particles on the flow can be neglected. To provide an understanding of the transport behavior of particles of different sizes, 200,000 monodisperse point particles with Stokes numbers of 0.1, 1, 5, 25, and 125 were introduced with a random distribution in the channel. The corresponding Lagrangian particle equation of motion, including the Stokes drag, the gravity, and the lift forces, were solved, and the trajectories of the particles for the duration of 10,000 wall units were evaluated for dilute suspension. The trap boundary condition on the lower and upper walls of the channel was assumed, and the deposition rates of particles with different sizes were evaluated and recorded as a function of time. Ensemble and time averaging of the simulation results were performed, and the corresponding concentration profiles and the deposition velocities of particles were evaluated for various conditions. A series of simulations were performed, and the effects of wall roughness, lift force, and the gravity direction on the deposition rate were carefully examined. It was found that the surface roughness and the direction of gravity in conjunction with the lift force significantly affect the fine particle deposition rate and could improve the agreement of the DNS simulation with the available experimental data.
CFD Modeling of Multi-Fuel Suspension Co-Combustion and Calcination Processes Within a DDF Precalciner of Cement Kiln
Authors: Cristea Eugen-Dan , Conti Pierangelo
Published: October 2020

The paper reports a CFD engineering application for modeling the thermal-fluid dynamics and thermochemical conversion processes, which govern the conventional air-fossil fuel firing or multi-fuel co-processing, responsible for thermal sustain of raw material calcination process, within a dry process cement kiln. We simulate a Dual-Combustion and Denitration Furnace (DDF) precalciner, which co-combusts in suspension the petroleum coke (primary fuel) with alternative fuels (e.g., the pre-dried sewage sludge and/or the animal meat and bonemeal). CFD software package ANSYS Fluent R18.2 is used to build CFD reactive model and to perform the numerical simulations. The Eulerian–Lagrangian approach is usually employed for modeling turbulent multiphase reacting flows. Few turbulence and radiation heat transfer models are compared, to identify pros and cons of each model applicability and to determine which model is most suitable. The Discrete Phase Model (DPM), in Lagrangian framework, is employed for tracking petcoke/alternative fuels and limestone particle clouds. CFD analysis provides valuable insights into the DDF precalciner performance e.g., combustion and calcination characteristics, in-furnace NOx control strategy by combustion aerodynamics optimization (particularly the effect of Tertiary Air tangential inlet, which creates swirl and induces several local flow recirculation zones). The major predicted results e.g., exit degree of calcination, fuel burnout, gas species concentration fields etc., are quite well captured and validated against control system continuously logged operation data and the measurements collected by newly installed instrumentation.
Hamiltonian Modeling and Energy Analysis of a Hydro Electric Generating Set in the Sudden Load Decreasing Transient
Authors: Wang Pengfei , Chen Diyi , Li Huanhuan
Published: October 2020

Hydroelectric generation system is mainly composed of penstock, hydro-turbine, generator, servicing facility and power load, its stability is directly related to the dynamic characteristics of each subsystem, but not completely dependent on the behavior of the subsystems. To better study the transient energy characteristics and stabilization mechanism of the hydroelectric generating set in the sudden load decreasing transient. And make full use of strengths of generalized Hamiltonian system in describing energy flow, the Hamiltonian model of a hydroelectric generating set including the turbine, water diversion system and generator is established by the method of orthogonal decomposition. Firstly, the energy flow of the hydroelectric generating set in the framework of generalized Hamiltonian theory is proved theoretically to be consistent with the real system, and the transient process of sudden load decreasing can be described effectively. Moreover, the variation laws of the flow, the rotating speed and the power angle of the set in the sudden load decreasing transient are studied respectively. The results indicate that the constructed Hamilton function can effectively describe the energy change of the system. It provides theoretical support for the stable operation of the hydroelectric generating set in the sudden load decreasing transient, and a new research idea for the stable operation of the set in other transient processes.
Motility Response to Hydrodynamic Stress During the Growth Cycle in Active Fluid Suspensions
Authors: Fadlallah Hadi , Peerhossaini Hassan , De Groot Christopher , Jarrahi Mojtaba
Published: October 2020

In this work, we focus on the motility behavior of two model microorganisms widely used in the study of active fluids: Chlamydomonas reinhardtii microalga and Synechocystis sp. Cyanobacterium. Understanding the physiological responses of microorganisms under variable environmental conditions is essential for bioreactor engineering. Yet, most of the previous studies focused on the observation of cellular motility regardless of the growth process. Here, we measure the motility of Chlamydomonas reinhardtii and Synechocystis sp. during their growth when subjected to different intensities of hydrodynamic shear stress. The results demonstrate a significant difference in the motility response of the two species against the applied hydrodynamic shear stress.

Mechanical agitation appears to affect the motility of Chlamydomonas reinhardtii microalgae by stimulating the growth process and increasing the magnitude of the cellular swimming velocity. The motility varies following 3 different phases: the rising phase starting almost at the middle of the exponential growth phase, and the decay and damped phases during the stationary phase. This behavior is described using a linear model for the rising phase and a damped oscillatory model for the decay and damped phases.

The motility of Synechocystis does not follow a well-defined pattern in time. However, it seems that the peak of the swimming velocity occurs always in the middle of exponential phase of growth. Synechocystis cells show a high endurance to the applied shear such that the global effect of agitation intensity on their motility is insignificant.
Numerical Simulations of Flows in Cerebral Aneurysms Using the Lattice Boltzmann Method
Authors: Osaki Susumu , Hayashi Kosuke , Kimura Hidehito , Kohmura Eiji , Tomiyama Akio
Published: October 2020

The lattice Boltzmann method (LBM) is used to simulate blood flows in cerebral aneurysms and the effects of the outflow boundary condition on predictions are studied. The LBM utilizes the D3Q19 discrete velocity model, the multiple-relaxation time collision operator (MRT), and the interpolated bounce-back rule to treat complex aneurysm shapes. Flow characteristics in regions of a large fluctuation in the wall shear stress (WSS) were then investigated using the LBM to understand the relation between the flow structure and the aneurysm wall remodeling. As a result the following conclusions were obtained under the present range of the numerical condition: (1) even with significant changes in the flow rate distributions at outflow boundaries, the WSS in an aneurysm is not much affected if the boundaries are far from the aneurysm, and (2) the geometry of an aneurysm and the main artery largely affects the formation of large WSS fluctuation regions, which may thickens the aneurysm wall due to inflammation-induced wall remodeling.
A Simple Method for the Design of Supersonic Nozzles of Arbitrary Cross Section Shape
Authors: Boughazi N. , Haddad A.
Published: October 2020

A simple approach for the design of supersonic nozzles of complex 3D shapes is presented. The Method of characteristics is primarily applied to compute the axisymmetric flow field of the supersonic section of the de-Laval nozzle. Two-dimensional simulations are performed for the axisymmetric flow fields.

The 3D configuration is then generated from the desired exit axisymmetric cross-sectional shape chosen through tracing its geometrical parameters back.to the throat. Elliptical, corrugated and two-dimensional wedge nozzles were designed using this approach. Preliminary results show a smooth geometrical transition from the throat to the exit cross section. Further three-dimensional analyses of the obtained geometries along with cold flow testing constitute the next steps to be performed.
Optimization of the Aerodynamic and Aeroacoustic Characteristics of a Small Centrifugal Fan by Means of Inverse Design and Evolutionary Algorithms
Authors: Eisenmenger Chris , Frank Stefan , Thamsen Paul Uwe
Published: October 2020

In this work, the inverse design code TURBOdesign 1 has been used in a direct approach to optimize both the aerodynamic and aeroacoustic properties of a small centrifugal fan with backward-curved blades. Two promising fan designs along with a reference fan have been investigated numerically using CFD and the results have been validated experimentally using a chamber test rig for the aerodynamics and the in-duct method for acoustics. A design aiming for good aerodynamic performance showed an increased efficiency in a wide operating range of about 5 to 10%. Another design aiming for low noise showed a numerically determined reduction in the overall sound radiation of 1 dB on average. This trend could not be reproduced in the experiments, instead the high efficiency design showed the best acoustic characteristics in a wide frequency range, which is why this is a subject of ongoing research.
Analyses of Turbulent Characteristics of the Air-Core Vortex Formation
Authors: Zi Dan , Wang Fujun
Published: October 2020

Air-core vortex often appears at the hydraulic intakes, it not only degrades hydraulic performance but also causes vibrations of the pump or turbine, resulting in efficiency reduction and operation instability. In this paper, large eddy simulation using an in-house code with the coupled level-set and volume-of-fluid method for capturing the air-water interface is performed. Vortices motions and turbulent characteristics are investigated. The fully-developed air-core vortex is well reproduced in the simulation. The distribution of the turbulent kinetic energy of the flow field is relevant to the evolution of vortices during the air-core vortex formation. At the early stage of the air-core vortex formation, the turbulent kinetic energy is high near the intake and the two side of the intake along the streamwise direction, and the streamwise component of the Reynolds normal stress is dominant, which indicates that vortices on the two sides move towards the intake along the streamwise direction and gather near intake. The turbulent kinetic energy increases rapidly as it approaches to the intake, and it has two peaks near the intake with the development of the flow field, which indicates that the gathered vortices evolve into two strong vortices. The turbulent kinetic energy only has one peak at the late stage, which indicates that there only exists one dominant vortex, and air is entrained in the intake, forming a fully-connected air-core vortex.
Performance Evaluation for SCO2 Compressor With Loss Models Consideration
Authors: Huang Yiting , Wang Tong
Published: October 2020

Centrifugal compressor is one of the key components in the Super-critical carbon dioxide (SCO2) Brayton cycle process, its performance prediction under variable operating conditions are concerned a lot with consideration of the unique of SCO2 properties. At the same time, the general techniques, including numerical simulation, test, and theoretical analysis with the loss models, are applied to evaluate the compressor performance. In addition with numerical analysis on the performance of the compressor, the flow loss models from air compressor were studied and extended the application to the SCO2 compressors, including incidence loss, blade loading loss, passage flow loss, tip clearance loss, mixing loss, disk friction loss, vaneless diffuser loss. All of these models were investigated to get the performance of a SCO2 compressor. The CO2 quality was got from the multipurpose NIST REFPROP 9.0 (NiST) which is based on Span and Wagner equation of state. The quality was added in the numerical simulation process. Besides, the passage flow loss factor has been modified to get more accurate theoretical performance prediction method for SCO2 compressor. The predicted performance map was compared to the numerical results, and the comparison results proved that, the combination of the loss models provided the similar results as those from the numerical simulation. For the SCO2 centrifugal compressor, the passage flow loss covers the most part among all the compressor flow losses. The value of the passage flow loss factor applied in the theoretical performance prediction method ranges between 0.008∼0.025, which is higher than that for air. Finally, the performance evaluation by improved loss models at different compression starting points were compared with that by the numerical results, it was found that, closer to the critical point would make the passage flow loss increase a lot, which may even exceed its rational range and the produced loss would be beyond expectation. This might be the reason for the low efficiency of SCO2 compressor in the practical working conditions.
Parametrisation of the Pressure and the Momentum Integral for Inclined Sluicegates Flows
Authors: Steppert Michael , Epple Philipp , Malcherek Andreas
Published: October 2020

The flow under sluice gates is nowadays frequently still determined by empirical approaches, based on the Bernoulli equation and a specific discharge coefficient which depends on the geometry of the sluice gate. This discharge coefficient is determined from a selection of tables and charts, which are based on a variety of experimental series. In 2016 Malcherek developed a fundamentally new approach to describe hydraulic structures based on integral momentum balance instead of Bernoulli’s energy conservation principle. In this approach a flow is described as the result of the integral momentum balance including pressure forces on open boundaries as well as closed walls, momentum fluxes through open boundaries and gravitational and frictional forces. In this theoretical approach, the discharge under inclined sluice gates can be described as a result of the acting pressure forces and momentum fluxes. To apply the theory, the pressure integral at the sluicegate and under the sluice gate and also the momentum integral of the flow under the sluice gate have to be known. For the determination of these values, multiphase CFD simulations for different angles of inclination and waters levels upstream of the sluicegate were done by the authors in the paper “Theoretical and numerical analysis of the pressure distribution and discharge velocity in flows under inclined sluice gates” (AJKFLUIDS2019 5020).

Using the values determined in this previous paper, parametrized model equations for the pressure and momentum integrals in dependency of the angle of inclination and upstream water levels were derived in this work. These polynomial approaches have been used to determine the three unknown physical quantities in the integral momentum balance: the pressure integral at the sluice gate, the pressure integral under the sluice gate and the momentum integral of the flow under the sluicegate. The polynomial approaches are shown in detail in this work.
A Combined CFD-Mechanistic Approach for Predicting Solid Particle Erosion in Annular Flow in Pipe Fittings and Elbows
Authors: Darihaki Farzin , Fallah Shojaie Elham , Zhang Jun , Shirazi Siamack A.
Published: October 2020

In internal flows, solid particles carried by the fluid could damage pipelines and fittings. Particles that are entrained in the fluid can cross streamlines and transfer a part of their momentum to the internal surface by impacts and cause local wall material degradation. Over the past decades, a wide range of models is introduced to predict particle erosion which includes empirical models, mechanistic models, and CFD which is currently the state-of-art numerical approach to simulate the erosion process. Multiphase flow under annular flow conditions adds to the complexity of the model. Although with the current computational capabilities transient CFD models are effectively applicable, this type of transient multiphase approach is not practical yet for engineering prediction of erosion especially for the large diameter applications with huge computational domains. Therefore, the presented combined approach could be utilized to obtain erosion rates for large diameter cases. Thus, an approach combining CFD and mechanistic multiphase models characterizing annular flow is developed to predict solid particle erosion. Different factors including film thickness in pipes and fittings which are affecting erosion under gas-dominated multiphase flow conditions are investigated. The results from the current approach are compared to experimental data and transient CFD simulations for annular flow in elbows showing a very good agreement with both.
Investigation of Turbulence Model Performance in Computational Fluid Dynamics Simulations of Horizontal Axis Wind Turbines
Authors: Mullenix Keaton , Walters D. Keith , Villegas Arturo , Diez F. Javier
Published: October 2020

Wind turbines are critically important in the quest to decrease global dependence on non-renewable energy sources. With the space to add 5M wind turbines, the United States is at the forefront of this transition. Horizontal axis wind turbines (HAWTs) have been studied numerically and experimentally at length. The vast majority of computational fluid dynamics (CFD) studies of HAWTs documented in the open literature have been carried out using two-dimensional simulations. Currently, the available three-dimensional simulations do not provide a comprehensive investigation of the accuracy of different options for modeling of fluid turbulence. In this paper four sets of CFD simulations are carried out using four different turbulence models that are commonly used for engineering level CFD analysis: SST-k-ω, Transition k-kL-ω, Standard k-ε, and Monotonically Integrated Large Eddy Simulation (MILES). These models were compared with experimental performance and coefficient of power results for a small-scale industrial wind turbine with inverse tip speed ratios (λ−1) in the range 0.072–0.144. They were further investigated to highlight the similarities and differences for the prediction of coefficient of pressure and skin friction coefficient. The results showed that no singular model, of the four investigated, was able to consistently predict the power performance with a high degree of accuracy when compared to the experimental results. The models also exhibited both similarities and key differences for the other aspects of flow physics. The results presented in this study highlight the critical role that turbulence modeling plays in the overall accuracy of a CFD simulation, and indicate that end users should be well aware of the uncertainties that arise in CFD results for wind turbine analysis, even when other sources of numerical error have been carefully minimized.
The Fluids Rap: It’s All About Flow
Authors: Nedyalkov Ivaylo
Published: October 2020

Most of the currently-enrolled undergraduate engineering students grew up with exposure to social media websites like Facebook and Youtube. Making sure that students are not distracted by their mobile devices in class has become more challenging, and one way to address the issue is to present engineering in a more entertaining and engaging way. A rap song about fluid mechanics was created by the author for entrainment, outreach, and education purposes. The song covers the fundamentals of fluid mechanics and mentions some theoretical basics, as well as some of the most widely used computational fluid dynamics and experimental fluid dynamics techniques. The song was written with the intention to be entertaining and educational — the goal was that someone with no prior fluid mechanics background will be able to understand it after spending 10–20 minutes reading through the lyrics explanations. A music video was produced for the song. The video production was sponsored by the American Society of Mechanical Engineers and includes visuals of experimental facilities and equipment.

The paper provides the background of the project, marketing plans, some of the lessons learned, the lyrics, and the explanations of the lyrics.
Effect of Formation Pattern on Schooling Energetics in Fish-Like Swimming
Authors: Pan Yu , Han Pan , Huang Jing , Dong Haibo
Published: October 2020

In the present study, an immerse-boundary-method-based in-house numerical solver is employed to investigate the effects of school pattern on energetics in fish-like swimming. Following Weihsys theory, two-dimensional diamond schools are formed for numerical simulations. The effects of the lateral distance and the longitudinal distance have been investigated to reveal the energetic saving mechanism in a fish school. It is found that the energetic performance in a narrow diamond school (2dy = 0.4) is very different from that in a wide school. In the wide school, the beneficial effect will decay gradually with the increase of longitudinal distance; however, in the narrow school, the variation of the effect is needed to divide into three phases. When the longitudinal spacing is small (2dx = 0.4 ∼ 2), the variation of power efficiency is complex. It increases sharply when 2dx changes from 0.4 to 1.2, then decrease. When the longitudinal distance is larger than two, the variation of energetics in a narrow diamond school has the same trend as that in a wide school. It is because, in the narrow school, the channeling effect is harmful to the energetics of individuals and plays an important role when the longitudinal distance is small. However, the detrimental effect will decay quickly with the increase of the longitudinal distance. Besides, when the longitudinal distance is large (2dx > 3.4), the narrow school shows a better power efficiency than the wide school. It is because, at this time, the fishes at the rear of school are still located at the instructive vortex wake produced by fishes in the front. These findings will give insight on how to form a power-efficient robotic school under water.
Experimental Investigations of the Surface Groove Effect in Taylor-Couette-Poiseuille Flow
Authors: Gaied Lamia , Aloui Fethi , Lippert Marc , Berrich Emna
Published: October 2020

In this paper, we investigate the effects of an imposed axial flow on hydrodynamic instabilities’ Couette-Taylor flow in the case where the wall of the inner cylinder of the system is grouved.

Without imposed axial flow, the basic flow of a fluid between two coaxial cylinders known by Couette flow, which is characterized by several temporal and spatial symmetries. The increase in the rotation causes the breaking of these symmetries.

In both cases where the surface of the inner cylinder is smooth and grooved, five different flow regimes can be determined: Taylor vortex flow (TVF), wavy vortex flow (WVF), and Modulated Wavy vortex flow (MWVF). Each time the flow passes from one hydrodynamic regime to another until it enters a state of turbulence, which is characterized by the destruction of all the symmetries that existed at the beginning.

In addition, when an axial flow is imposed on a Taylor-Couette flow, new helical vortex structures are observed in both cases (with and without surface groove).

The influence of surface structures (grooves) on the shear stress of the wall is discussed with and without axial base flow. A spatio-temporal description of several flow models was obtained using firstly, a visualization’s qualitative study using kalliroscope particles. Secondly, a quantitative study by polarography using simple probes have been used to characterize the impact of vortex structures on the Couette-Taylor flows without and with an axial flow on the transfer.
A Hybrid Method for Fan Simulations in Full Vehicle Thermal Analyses
Authors: Wu Zhilei , Blatnik Michael , Kress Eamonn , Deleon Lester
Published: October 2020

In full vehicle thermal flow analyses, the most often used procedure to simulate fluid motions driven by the cooling fan is the Moving Reference Frame (MRF) method. In the MRF approach, the fan is fixed in space and the fan rotation is modeled using grid fluxes. This method is widely used because it provides a fast and effective means of simulating fans. However, the MRF method does not always accurately predict the thermal wake and the mass flow rate through the fan, which causes errors in predicted temperatures on the parts downstream of the fan. Another method for fan simulation is the Rigid Body Motion (RBM) method in which the fan rotates in time. The RBM method models the fan motions directly, thus it can accurately predict the mass flow rate and thermal wake. However, an RBM simulation is transient and needs a time-average to obtain statistically steady-state results. The RBM method requires a significant amount of CPU resources and simulation time, which prevents it from being widely used in industry.

In the current work, a Hybrid Rigid Body Motion (HRBM) method is developed and validated. The HRBM method splits the full vehicle thermal simulation into two simulations, and then couples them at the interface. The first simulation is transient, utilizes the RBM method for the fan, and only models the fan regions. The second simulation is steady, which models the full vehicle except the fan regions. The solution from the transient simulation is time-averaged on the exchange interface and used as boundary conditions for the steady simulation. Conversely, the solution for the steady simulation is used as boundary conditions for the transient simulation at the exchange interface. Due to the slight differences resulting from time-averaging, there is a mismatch in the physical quantities at the exchange interface. This causes stability issues which prevent the coupled simulations from converging. Special techniques have been used in this work to stabilize the solution at the interface, which ensured the convergence of the coupled simulations.

The HRBM method greatly improves the accuracy of the full vehicle thermal flow simulation compared to using the MRF method. The thermal wake that results from using HRBM to model the fan is very similar to that produced by RBM, but HRBM utilizes ∼20–30% of the simulation resources required by RBM to achieve convergence.
Investigation of Numerical Scheme and Model Combinations for CFD Simulation of a Thermosyphon
Authors: Wang Huiyu , Walters D. Keith , Walters Keisha B.
Published: October 2020

This paper investigates the performance of a commercially available computational fluid dynamics (CFD) solver (Ansys FLUENT) to predict the flow and heat transfer characteristics of a two-phase closed thermosyphon (TPCT). Specifically, the study compares two different discretization schemes for the volume fraction equation with different time step sizes using three different sets of mass transfer time relaxation parameters for evaporation and condensation. The present study evaluates use of the Compressive scheme to increase the time step size compared to the Geo-Reconstruct scheme. In addition, a model is proposed to adjust the global saturation temperature of the system based on the volume of the vapor phase in order to balance the mass transfer inside TPCT and more accurately represent the realistic operating conditions of a TPCT. In this study a total of nineteen simulations are performed, and two types of boundary conditions for the condenser are investigated to determine the effect on the accuracy of the simulation results. The baseline simulation uses the Geo-Reconstruct method with a fixed saturation temperature. Other additional cases are performed using the Geo-Reconstruct method with variable saturation temperature, and the Compressive method with and without variable saturation temperature using different sets of mass transfer time relaxation parameters. Results show that the case using the Compressive method with the variable saturation temperature model has good agreement with the reference experimental data and is less computationally expensive than the Geo-Reconstruct method. The 3D CFD models implemented in this study successfully predict the phase change process and flow behavior inside the TPCTs, at least in a qualitative sense.
Characterization of Oil Droplets in the Lower Cavity of a Rotary Compressor
Authors: Wu Puyuan , Chen Jun , Sojka Paul E. , Li Yang , Cao Hongjun
Published: October 2020

The rotary compressor is widely used in small air conditioners, and is the most important element in the system. It relies on eccentric rolling pistons that rotate at high speed to compress refrigerant in the cylinder. The lubricant oil in the rotary compressor is used for lubricating the bearing and sealing the clearance of sliding parts. However, the oil can experience complex and highly-coupled atomization processes when discharged from the cylinder, and part of oil droplets can exhaust from the rotary compressor by the refrigerant flow and reduce the efficiency and reliability of the compressor as a result. Thus, characterizing the behavior of oil droplets in the lower cavity of a rotary compressor where the atomization occurs is a major challenge for manufacturers who rely on CFD tools to predict the multiphase flow. By modifying a rotary compressor, the oil behavior in the lower cavity of a rotary compressor is observed and recorded by shadowgraphy. In the current phase, the number, size, and morphology of oil droplets are analyzed statistically with image processing method, which provides better understanding to the atomization mode in the lower cavity, the velocity of the mist of oil droplets is calculated with Optical Flow Velocimetry. The results can assist designers in improving the CFD analysis of compressors and ultimately reducing the Oil Discharge Rate (ODR).