VTechWorks Repository :: Browsing by Author "Untaroiu, Alexandrina"

Browsing by Author "Untaroiu, Alexandrina"

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Advancing Sustainability in Data Centers: Evaluation of Hybrid Air/Liquid Cooling Schemes for IT payload using Sea Water
Latif, Imran; Ashraf, Muhammad Mubashar; Haider, Umaima; Reeves, Gemma; Untaroiu, Alexandrina; Coelho, Fabio; Browne, Denis (IEEE, 2024-12)
The growth in cloud computing, big data, AI and -performance computing (HPC) necessitate the deployment of additional data centers (DC's) with high energy demands. The unprecedented increase in the Thermal Design Power (TDP) of the computing chips will require innovative cooling techniques. Furthermore, DC's are increasingly limited in their ability to add powerful GPU servers by power capacity constraints. As cooling energy use accounts for up to 40% of DC energy consumption, creative cooling solutions are urgently needed to allow deployment of additional servers, enhance sustainability and increase energy efficiency of DC's. The information in this study is provided from Start Campus' Sines facility supported by Alfa Laval for the heat exchanger and CO2 emission calculations.The study evaluates the performance and sustainability impact of various data center cooling strategies including an air-only deployment and a subsequent hybrid air/water cooling solution all utilizing sea water as the cooling source. We evaluate scenarios from 3MW to 15+1MW of IT load in 3MW increments which correspond to the size of heat exchangers used in the Start Campus' modular system design. This study also evaluates the CO2 emissions compared to a conventional chiller system for all the presented scenarios. Results indicate that the effective use of the sea water cooled system combined with liquid cooled systems improve the efficiency of the DC, plays a role in decreasing the CO2 emissions and supports in achieving sustainability goals.
Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics
Chatterjee, Krishnashis (Virginia Tech, 2018-11-06)
Microfluidics has been the focal point of research in various disciplines due to its advantages of portability and cost effectiveness, and the ability to perform complex tasks with precision. In the past two decades microfluidic technology has been used to cool integrated circuits, for exoplanetary chemical analysis, for mimicking cellular environments, and in the design of specialized organ-on-a-chip devices. While there have been considerable advances in the complexity and miniaturization of microfluidic devices, particularly with the advent of microfluidic large-scale integration (mLSI) and microfluidic very-large-scale-integration (mVLSI), in which there are hundreds of thousands of flow channels per square centimeter on a microfluidic chip, there remains an actuation overhead problem: these small, complex microfluidic devices are tethered to extensive off-chip actuation machinery that limit their portability and efficiency. Insects, in contrast, actively and efficiently handle their respiratory air flows in complex networks consisting of thousands of microscale tracheal pathways. This work analytically and experimentally investigates the viability of incorporating some of the essential kinematics and actuation strategies of insect respiratory systems in microfluidic devices. Mathematical models of simplified individual tracheal pathways were derived and analyzed, and insect-mimetic PDMS-based valveless microfluidic devices were fabricated and tested. It was found that not only are these devices are capable of pumping fluids very efficiently using insect-mimetic actuation techniques, but also that the fluid flow direction and magnitude could be controlled via the actuation frequency alone, a feature never before realized in microfluidic devices. These results suggest that insect-mimicry may be a promising direction for designing more efficient microfluidic devices.
A Comprehensive Three-Dimensional Analysis of the Wake Dynamics in Complex Turning Vanes
Hayden, Andrew Phillip (Virginia Tech, 2023-12-20)
A comprehensive computational and experimental analysis has been conducted to characterize the flow dynamics and periodic structures formed in the wake of complex turning vanes. The vane packs were designed by the StreamVane swirl distortion generator technology, a design system that can efficiently reproduce swirl distortion for compressor rig and full turbofan engine testing. StreamVanes consist of an array of turning vanes that commonly contain variations in turning angle along their span, a nonaxisymmetric profile about the centerline, and vane-to-vane intersections or junctions to accurately generate the desired distortion. In this study, vane packs are considered complex if they contain two out of three of these features, a combination seen in other turbomachinery components outside of StreamVane design. Similar to all stator vanes or rotor blades, StreamVane vane packs are constructed using a series of cross-sectional airfoil profiles with blunt trailing edges and finite thicknesses. This, in turn, introduces periodic vortex structures in the wake, commonly known as trailing edge vortex shedding. To fully understand how the dynamics and coherent wake formations within vortex shedding impact both the flow distortion and structural durability of StreamVanes, it is first necessary to characterize the corresponding wakes in three dimensions. The current study provides an in-depth analysis to predict and measure the trailing edge vortex development using high-fidelity computational fluid dynamics and stereoscopic time-resolved particle image velocimetry experiments. Two testcase StreamVane geometries were specifically designed with complex features to evaluate their influence on the dynamics and coherence of the respective vane wakes. Fully three-dimensional, unsteady computational fluid dynamics simulations were performed using a Reynolds-Averaged Navier-Stokes solver coupled with a standard two-equation turbulence model and a hybrid, scale-resolving turbulence model. Both models predicted large-scale wake frequencies within 1—14% of experiment, with a mean difference of less than 3.2%. These comparisons indicated that lower fidelity simulations were capable of accurately capturing such flows for complex vane packs. Additionally, structural and modal analyses were conducted using finite element models to determine the correlations between dominant structural modes and dominant wake (flow) modes. The simulations predicted that vortex shedding modes generally contained frequencies 300% larger than dominant structural modes, and therefore, vortex induced vibrations were unlikely to occur. Lastly, mode decomposition methods were applied to the experimental results to extract energy ratios and reveal dynamic content across high-order wake modes. The vortex shedding modes generated more than 80% of the total wake energy for both complex vane packs, and dynamic decomposition methods revealed unique structures within the vane junction wake. In all analyses, comparisons were made between different vane parameters, such as trailing edge thickness and turning angle, where it was found that trailing edge thickness was the dominant vortex shedding parameter. The motivation, methodology, and results of the following research is presented to better understand the wake interactions, computational predictive capabilities, and structural dynamics associated with vortex shedding from complex vane packs. Although the results directly relate to StreamVane distortion generator technology, the qualitative and quantitative comparisons between the selected methods, geometry parameters, and flow conditions can be extrapolated to modern turbomachinery components in general. Therefore, this dissertation aims to benefit distortion generator and turbomachinery designers by providing insight into the underlying physics and overall modeling techniques of the wake dynamics in highly three-dimensional, complex components.
Design of Percutaneous Dual Propeller Pump to assist Patients with Single Functional Ventricle
Jagani, Jakin Nitinkumar (Virginia Tech, 2018-03-26)
Various congenital heart defects (CHDs) are characterized by the existence of a single functional ventricle, which perfuses both the systemic and pulmonary circulation in parallel. A three-stage palliation procedure, including the final Fontan Completion, is often adopted by surgeons to treat patients with such CHDs. However, the most common outcome of this surgery, an extra-cardiac total cavopulmonary connection (TCPC), formed by suturing the inferior vena cava (IVC) and superior vena cava (SVC) to the pulmonary arteries (PAs), results in non-physiological flow conditions, systemic venous hypertension, reduced cardiac output, and pressure losses, which ultimately calls for a heart transplantation. A modest pressure rise of 5-6 mm Hg would correct the abnormal flow dynamics in these patients. To achieve this, a novel conceptual design of a percutaneous dual propeller pump inserted and mounted inside the TCPC is developed and studied. The designed blood pump is percutaneously inserted via the Femoral vein and deployed at the center of Total Cavopulmonary Connection (TCPC). The two propellers, each placed in the Superior Vena Cava (SVC) and the Inferior Vena Cava (IVC) are connected by a single shaft and motor, and thus rotate at same speed. The device is supported with the help of a self-expanding stent which would be anchored to the walls of the IVC and the SVC. An inverse design methodology implementing Blade Element Momentum theory and Goldstein's radial momentum loss theory was employed to generate the blade profiles for the studied propeller pumps. The propeller blade profiles generated from the inverse design optimization code were examined for hydraulic performance, blood flow pattern and potential for hemolysis inside the TCPC using 3-D computational fluid dynamics (CFD) analysis. The Lagrangian particle tracking approach in conjunction with a non-linear mathematical power law model was used for predicting the blood damage potential of the analysed blood pump designs by calculating the scalar shear stress history sustained by the red blood cells (RBC). The study demonstrated that the IVC and SVC propeller pumps could provide a pressure rise of 1-20 mm Hg at flow rates ranging from 0.5 to 5 lpm while rotating at speeds of 6,000-12,000 rpm. Moreover, the average Blood Damage Index (BDI), quantifying the level of blood trauma sustained by the RBCs for the analyzed propeller pump designs, was found to be around 3e-04% to 4e-04% which is within the acceptable limits for an axial flow heart assist device. Thus, such a dual propeller blood pump configuration could potentially provide assistance to Fontan patients by unloading the single functional ventricle thereby acting as a bridge to transplantation and recovery until a donor heart is available.
Development and Validation of Human Body Finite Element Models for Pedestrian Protection
Pak, Wansoo (Virginia Tech, 2019-10-21)
The pedestrian is one of the most vulnerable road users. According to the World Health Organization (WHO), traffic accidents cause about 1.34 million fatalities annually across the world. This is the eighth leading cause of death across all age groups. Among these fatalities, pedestrians represent 23% (world), 27% (Europe), 40% (Africa), 34% (Eastern Mediterranean), and 22% (Americas) of total traffic deaths. In the United States, approximately 6,227 pedestrians were killed in road crashes in 2018, the highest number in nearly three decades. To protect pedestrians during Car-to-Pedestrian Collisions (CPC), subsystem impact tests, using impactors corresponding to the pedestrian's head and upper/lower leg were included in regulations. However, these simple impact tests cannot capture the complex vehicle-pedestrian interaction, nor the pedestrian injury mechanisms, which are crucial to understanding pedestrian kinetics/kinematics responses in CPC accidents. Numerous variables influence injury variation during vehicle-pedestrian interactions, but current test procedures only require testing in the limited scenarios that mostly focus on the anthropometry of the 50th percentile male subject. This test procedure cannot be applied to real-world accidents nor the entire pedestrian population due to the incredibly specific nature of the testing. To better understand the injury mechanisms of pedestrians and improve the test protocols, more pre-impact variables should be considered in order to protect pedestrians in various accident scenarios. In this study, simplified finite element (FE) models corresponding to 5th percentile female (F05), 50th percentile male (M50), and 95th percentile male (M95) pedestrians were developed and validated in order to investigate the kinetics and kinematics of pedestrians in a cost-effective study. The model geometries were reconstructed from medical images and exterior scanned data corresponding to a small female, mid-sized male, and tall male volunteers, respectively. These models were validated based on post mortem human surrogate (PMHS) test data under various loading including valgus bending at knee joint, lateral/anterior-lateral impact at shoulder, pelvis, thorax, and abdomen, and lateral impact during CPC. Overall, the kinetic/kinematic responses predicted by the pedestrian FE models showed good agreement against the corresponding PMHS test data. To predict injuries from the tissue level up to the full-body, detailed pedestrian models, including sophisticated musculoskeletal system and internal organs, were developed and validated as well. Similar validations were performed on the detailed pedestrian models and showed high-biofidelic responses against the PMHS test data. After model development and validation, the effect of pre-impact variables, such as anthropometry, pedestrian posture, and vehicle type in CPC impacts were investigated in different impact scenarios. The M50-PS model's posture was modified to replicate pedestrian gait posture. Five models were developed to demonstrate pedestrian posture in 0, 20, 40, 60, and 80 % of the gait cycle. In a sensitivity study, the 50th percentile male pedestrian simplified (M50-PS) model in gait predicted various kinematic responses as well as the injury outcomes in CPC impact with different vehicle type. The pedestrian FE models developed in this work have the capability to reproduce the kinetic/kinematic responses of pedestrians and to predict injury outcomes in various CPC impact scenarios. Therefore, this work could be used to improve the design of new vehicles and current pedestrian test procedures, which eventually may reduce pedestrian fatalities in traffic accidents.
Effects of Tire Attributes on the Aerodynamic Performance of a Realisitic Car-Tire Assembly and the Sensitivity Analysis to Understand the Impact of the Rim Protector
Rath, Shubham (Virginia Tech, 2022-06-22)
The effect of that the tire has on the overall aerodynamic drag in a car-tire assembly has been studied and deemed considerable from past studies. It has been shown that to know how tire parameters affect the drag on the car-tire assembly, it is important to understand how the vehicle body and the tires influence the flow structures. Previous studies have focused on the tire attributes that have some impact on the aerodynamic performance of the vehicle. These tire attributes, however, haven't been studied to the extent where one can get a better understanding of the impact of each of these attributes. This paper studies the impact that specific tire attributes have on the overall aerodynamic drag on the vehicle based on a thorough and systematic sensitivity study. The effect of tire attributes in a vehicle assembly as well as the sensitivity study of a rim protector on a standalone tire is conducted. This helps in better understanding the flow structures around the car body and around the tire for the improvement in the aerodynamic performance of the vehicle. This is a two-part study. One component of this study is a parametric sensitivity analysis of a tire in a tire – vehicle assembly. The other component is a parametric sensitivity analysis of the rim protector design on a standalone tire.
Experimental and Computational Study of Vibration-Based Energy Harvesting Systems for Self-Powered Devices
Alnuaimi, Saeed Khalfan (Virginia Tech, 2021-01-18)
Energy harvesting of ambient and aeroelastic vibrations is important for reducing the dependence of wireless sensing and networks on batteries. We develop a configuration for a piezoelectric energy harvester with the capability to wirelessly communicate vibration measurements while using those vibrations to power the sensing and communication devices. Particularly, we perform experiments that aim at identifying challenges to overcome in the development of such a configuration. Towards that objective, we successfully tested a self-powered real-time point-to-point wireless communication system between a vibration sensor and transmission and receiving modules. The sensing device and transmission module are powered by the vibrating object using a piezoelectric energy harvester. The communication is established by using two XBee modules. In the second part of this dissertation, we address the optimization of the output power of piezoelectric energy harvesters of aeroelastic vibrations. Given the complexity of high-fidelity simulations of the coupling between the fluid flow, structural response and piezoelectric transduction, we develop and experimentally validate a phenomelogical reduced-order model for energy harvesting from wake galloping. We also develop a high-fidelity simulation for the same phenomena. The modeling and high-fidelity simulations can be a part of a multi-disciplinary optimization framework to be used in the design and operation of galloping-based energy harvesters.
A finite element of an electric scooter model for simulating traffic accidents
Chontos, Rafael; Grindle, Daniel M.; Untaroiu, Alexandrina; Untaroiu, Costin D. (2021-10-07)
Fluidic Energy Harvesting and Sensing Systems
Alrowaijeh, Jamal Salem (Virginia Tech, 2018-07-09)
Smart sensors have become and will continue to constitute an enabling technology to wirelessly connect platforms and systems and enable improved and autonomous performance. Automobiles have about two hundred sensors. Airplanes have about eight thousand sensors. With technology advancements in autonomous vehicles or fly-by-wireless, the numbers of these sensors is expected to increase significantly. The need to conserve water and energy has led to the development of advanced metering infrastructure (AMI) as a concept to support smart energy and water grid systems that would respond to emergency shut-offs or electric blackouts. Through the Internet of things (IoT) smart sensors and other network devices will be connected to enable exchange and control procedure toward reducing the operational cost and improving the efficiency of residential and commercial buildings in terms of their function or energy and water use. Powering these smart sensors with batteries or wires poses great challenges in terms of replacing the batteries and connecting the wires especially in remote and difficult-to-reach locations. Harvesting free ambient energy provides a solution to develop self-powered smart sensors that can support different platforms and systems and integrate their functionality. In this dissertation, we develop and experimentally assess the performance of harvesters that draw their energy from air or water flows. These harvesters include centimeter-scale micro wind turbines, piezo aeroelastic harvesters, and micro hydro generators. The performance of these different harvesters is determined by their capability to support wireless sensing and transmission, the level of generated power, and power density. We also develop and demonstrate the capability of multifunctional systems that can harvest energy to replenish a battery and use the harvested energy to sense speed, flow rate or temperature, and to transmit the data wirelessly to a remote location.
Full Field Reconstruction Enhanced With Operational Modal Analysis and Compressed Sensing for General Dynamic Loading
Fu, Gen (Virginia Tech, 2021-06-09)
In most applications, the structure components have to be tested under different loading conditions before being placed in operation. A reliable and low cost measuring technique is desirable. However, most currently employed measuring approaches can only provide the structural response at several discrete locations. The accuracy of the measurements varies with the location and orientation of the sensors. Practically, it is not possible to place sensors at all the critical locations for different excitations. Therefore, an approach that derives the full field response using a limited set of measured data is desirable. In contrast to experimental full field measurement techniques, the expansion approach involves analytically expanding the limited measurements to all the degrees of freedom of the structure. Among all the analytical methods, the modal expansion method is computationally efficient and thus more suitable for real time expansion of measured data. In this method, the full-field response is approximated by the linear combination of mode shapes. In previous studies, the modal expansion method is limited by errors from mode aliasing, inaccuracy of the calculated mode shapes and the noise in measurements. In order to overcome these limitations, the modal expansion method is enhanced by mode selection and error compensation in this study. First, the key parameters used in modal expansion method were analyzed using a cantilever beam model and a method for optimal placement of sensors was developed. A mode selection method and error compensation method based on operation modal analysis and adaptive compressed sensing techniques were then developed to reduce the effects of mode aliasing, mode shape inaccuracy and measurement noise. The developed approach was further tested virtually using a numerical model of rotor 67. The numerical model was created using a two-way coupled fluid structure interaction technique. By developing these methods, the enhanced modal expansion approach can provide full field response for structures under different load conditions. Compared to the traditional modal expansion method, it can expand the data with high noise and under general dynamic loading.
Hemodynamic Optimization of a Passive Assist Total Cavopulmonary Connector for ages 1-20
Mack, Elizabeth (Virginia Tech, 2018)
Currently, the surgical procedure followed by the majority of cardiac surgeons to address right ventricular dysfunction is the Fontan procedure, which connects the superior and inferior vena cava directly to the left and right pulmonary arteries bypassing the right atrium. However, this is not the most efficient configuration from a hemodynamics perspective. The goal of this study is to develop a patient-specific 4-way connector to bypass the dysfunctional right ventricle and augment the pulmonary circulation. The 4-way connector was intended to channel the blood flow from the inferior and superior vena cava directly to the right and left pulmonary arteries. By creating a connector with proper hemodynamic characteristics, one can control the jet flow interactions between the inferior and superior vena cava and streamline the flow towards the right and left pulmonary arteries. In this study, the focus was on creating a system that could identify the optimal configuration for the 4-way connector for patients from 1-20 years of age. A platform was created in ANSYS that utilized the design of experiments (DOE) function to minimize power-loss and blood damage propensity in the connector based on junction geometries. A CFD model was created to simulate the blood flow through the connector. Then the geometry of the bypass connector was parameterized for the DOE process. The selected design parameters included inlet and outlet diameters, radius at the intersection, and length of the connector pathways. The chosen range for each geometric parameter was based on the relative size of the patient’s arteries found in the literature. It was confirmed that as the patient’s age and artery size change, the optimal size and shape of the connector also changes. However, the corner radius did not decrease at the same rate as the opening diameters. This means that creating different sized connectors is not just a matter of scaling the original connector to match the desired opening diameter. However, it was found that power losses within the connector decrease and average and maximum blood traversal time through the connector increased for increasing opening radius. A follow up study was conducted to try to reduce or negate a consistent recirculation area found at the center of the connectors. To accomplish this a flow diverter was added to the center of the connector and optimized for each of the connectors found for the age groups used. From this study, it was found that the diverter did negate the recirculation area form the centers of the connectors. A separate Blood Damage Index (BDI) study was also run on this optimized connector with a diverter, the optimized connectors from the first study and a baseline connector. This showed a decrease in IVC sourced BDI for the optimized versions of the connector compared to the baseline geometries. This information could be used to create a more specific relationship between the opening radius and the flow characteristics. So in order to create patient specific connectors, either a new more complicated trend needs to be found or an optimization program would need to be run on each patient’s specific geometry when they need a new connector.
Initial Investigations into the Failure Modes of a Swirl Distortion Generator Using Computational Methods
Hayden, Andrew Phillip (Virginia Tech, 2021-05-18)
The need for more efficient and environmentally sustainable aircraft has been a rapidly increasing topic for research and development over the last few decades. Within this area of research, boundary layer ingestion (BLI) concepts have been developed which integrates the airframe and propulsion system of an aircraft. In turn, BLI increases the fuel efficiency and decreases emissions by reducing the overall drag and reenergizing the aircraft wake. However, the boundary layer flow of an airframe or duct can impose undesired flow conditions, such as swirl and pressure distortions, at the inlet of a jet engine. Therefore, efficient research and testing capabilities are essential to advance the development of these integrated systems. The StreamVane swirl distortion generator was developed by Virginia Tech to provide cost and time efficient ground testing methods for BLI research. StreamVanes are constructed of unique vane packs that are specifically tailored to generate a desired swirl distortion profile. To maximize efficiency, StreamVanes are additive manufactured which cause geometry limitations to the overall vane design. Due to these restrictions, as well as the complexity of the vane pack, unwanted dynamic responses and unsteady flows can be generated. In order to predict both of these phenomena before testing, two different computational methodologies were developed and investigated on a StreamVane and its airfoil parameters. First, a one-way fluid-structure interaction methodology was developed to predict flutter mconditions of the vanes within StreamVanes. The presented methodology includes steady and unsteady computational fluid dynamics (CFD) as well as linear structural and modal finite element analysis (FEA) simulations. A simplified StreamVane model was designed as a testcase for the methodology, and it was found that two unique vane shapes did not undergo flutter conditions at three different operating points. The results provided a linear analysis method to compute the aerodynamic damping, which gave insight on how different vane shapes respond dynamically. Secondly, a parameter study was conducted to predict the vortex shedding from the modified NACA 63-series airfoil profile used within StreamVane design. The effects of the airfoil turning angle and trailing edge thickness on the vortex shedding frequency were computationally predicted using the unsteady Reynolds averaged Navier-Stokes equations (URANS) and shear stress transport (SST) turbulence model. In turn, the shedding frequencies for each parameter were recorded, and more intuition was gained on the TE flow field in correspondence to different airfoil specifications. Overall, the two sets of methodologies and results can be used to efficiently reduce failure uncertainties in future StreamVane designs.
Investigation Into Flutter of Complex Vane Packs
Hefner, Cole (Virginia Tech, 2023-01-16)
There has been lots of interest in designing more fuel efficient aircraft using concepts such as boundary layer ingestion (BLI) that cause large amounts of pressure and swirl distortion that enter the jet engines. To enable ground testing the performance of these engines in different distortion patterns, the StreamVane and ScreenVane systems have been developed. A StreamVane consists of a complex vane pack that is custom designed for each distortion profile and the ScreenVane combines the StreamVane with a pressure distortion screen for testing engines under both pressure and swirl distortions. The complexity and uniqueness of these devices make predicting their structural integrity and propensity to flutter a challenge, necessitating the need for studying flutter in these complex vane packs. In order to study flutter of these complex vane packs, a methodology was created to obtain trailing edge displacements and frequencies from high speed video of a StreamVane and was used on a quad swirl StreamVane and a Simplified model. Unsteady CFD with periodic mesh deformation based off of its modal analysis was used to validate if it can predict the flutter velocity as well as understanding what the unsteady aerodynamic response to flutter is. A parameter study was then conducted along with oilflow visualization to better understand the potential causes of flutter and the impact of different design parameters. A harmonic response analysis was conducted on each of these designs and a correlation between the amplitude from the harmonic response and the flutter Mach number was obtained that can be used to predict when a StreamVane will flutter. A new series of StreamVanes were designed and based off of computational analysis, two were selected for manufacture. They both successfully avoided fluttering in flutter tests and were found to accurately replicate the goal swirl profile when measured with a 5 hole probe. These results provide a basis for understanding and predicting flutter in StreamVanes.
Ion-Currents in Oxyfuel Cutting Flames Exposed to External Bias Voltages
Rahman, S. M. Mahbobur (Virginia Tech, 2025-01-02)
Computational Fluid Dynamics (CFD) and predictive models are presented in this dissertation that illustrates the detailed electrical characteristics, and the current-voltage (i-v) relationship throughout the preheating process of premixed methane-oxygen (CH4-O2) oxyfuel cutting flame subject to electric bias voltages. As such, the equations describing combustion, electrochemical transport for charged species, and potential are solved through a commercially available finite-volume CFD code. The reactions of the methane-oxygen (CH4 – O2) flame were combined with the GRI 3.0 mechanism and a 25-species reduced mechanism, respectively, and additional ionization reactions that generate three chemi-ions, H3O+, HCO+, and e– , to describe the chemistry of ions in flames. The electrical characteristics such as ion migrations and ion distributions are investigated for a range of electric potential, V ∈ [−10V, +10V ]. Since the physical flame is comprised of twelve Bunsen-like conical flame, inclusion of the third dimension imparts the resolution of fluid mechanics and the interaction among the individual cones. As for developing the predictive models, four different supervised machine learning (ML) algorithms, decision tree (DT), random forest (RF), K-nearest neighbors (KNN), and artificial neural network (ANN), were employed to predict the i-v relationship. An experimental dataset of ≈ 10050 was utilized where a 60:20:20 split was adopted, allocating 60% for training, 20% for validation, and 20% for testing. It was concluded that charged 'sheaths' are formed at both torch and workpiece surfaces, subsequently forming three distinct regimes in the i-v relationship. The i-v characteristics obtained have been compared to the previous experimental study for premixed flame. In this way, the overall model generates a better understanding of the physical behavior of the oxyfuel cutting flames, along with a more validated i-v characteristics. Such understanding might provide critical information towards achieving an autonomous oxyfuel cutting process.
Large-Eddy Simulations of Hydrocyclones
Bukhari, Mustafa Mohammedamin T. (Virginia Tech, 2023-01-20)
This dissertation investigates the flow physics, turbulence structure, and particle classification process in hydrocyclones using large-eddy simulations of turbulent multiphase flow. Two types of hydrocyclones are considered. The first is a classifying hydrocyclone, and the second is a mineral flotation hydrocyclone, also known as an air-sparged hydrocyclone (ASH). Large-eddy simulations (LES) are conducted for multi-phase flow (air, water, and sand particles) so that the complex anisotropic turbulence of a swirling flow is computed correctly. The effects of mesh refinements on the mean flow and turbulence stresses are investigated, and (LES) results are validated by comparisons with experimental data for classifying hydrocyclone. The two-phase flow in air-sparged hydrocyclone has not been analyzed before. ANSYS CFX software V17.2 has been used to conduct the simulations. Firstly, large-eddy simulations have been conducted for two-phase flow (water and air) in a conventional hydrocyclone using the Eulerian two-fluid (Eulerian-Eulerian) and Volume-of- Fluid (VOF) models. Subgrid stresses are modeled using a dynamic eddy–viscosity model, and results are compared to those using the Smagorinsky model. The effects of grid resolutions on the mean flow and turbulence statistics have been thoroughly investigated. Five block-structured grids of 0.72, 1.47, 2.4, 3.81, and 7.38 million elements have been used for the simulations of a typical conventional hydrocyclone designed and tested by Hsieh (75 mm hydrocyclone) [1]. Mean velocity profiles and normal Reynolds stresses have been compared with experimental data. The results of the Eulerian two-fluid model agree with those of the VOF model. A fine mesh in the axial and radial directions is necessary for capturing the turbulent vortical structures. Turbulence structures in the hydrocyclone are dominated by helical vortices around the air core. Energy spectra are analyzed at different points in the hydrocyclone, and regions of low turbulent kinetic energy are identified and attributed to stabilizing effects of the swirling velocity component. Turbulent energy spectra in the different regions of the hydrocyclone have been analyzed. The energy spectra are calculated at two points near the air-water interface. They show a short inertial subrange where energy decays as f−5/3, followed by viscous damping where energy drops as f−7, where f is frequency. However, for the points located near the boundary where high turbulent kinetic energy is found, the energy spectra exhibit f^(−4) decay. Secondly, the two-fluid (Eulerian two-fluid) model and large-eddy simulation are used to compute the turbulent two-phase flow of air and water in a cyclonic flotation device known as an Air-Sparged Hydrocyclone (ASH). In the operation of ASH, the air is injected through a porous cylindrical wall. The study considers a 48-mm diameter hydrocyclone and uses a block-structured fine mesh of 10.5 million hexahedral elements. The air-to-water injection ratio is 4, and a uniform air bubble diameter of 0.5 mm has been specified. The flow field in ASH has been investigated for the inlet flow rate of water of 30.6 L/min at different values of underflow exit pressure. The present simulations show that the value of static pressure imposed at the underflow section strongly affects the distribution of air volume fraction, water axial velocity, tangential velocity, and swirling layer thickness in ASH. The loci of zero-axial velocity surfaces have been determined for different exit pressures. The water split ratio through the overflow opening varies with underflow exit pressure as 6%, 8%, 16%, and 26% for 3, 4, 5, and 6 kPa, respectively. These results indicate that regulating the pressure at the underflow exit can be used to optimize ASH's performance. Turbulent energy spectra in different regions of the hydrocyclone have been analyzed. Small-scale turbulence spectra at near-wall points exhibit f^(−4) law, where f is frequency. Whereas for points at the air-column interface, the energy spectra show an inertial subrange f^(−5/3) followed by a dissipative range of f^(−7) law. Thirdly, large-eddy simulation (LES) has been used to investigate the flow separation in multi-phase flow (gas, liquid, and solid) in a classifying hydrocyclone using the multi-fluid (Eulerian multi-fluid) model. The results of the CFD simulation are compared with the Hsieh [1] experimental data. The water phase is considered a continuous phase, while air and solid particles are considered dispersed phases. Drag between water-air and water-sand is the only considered interfacial force. The Schiller-Naumann and Wen-Yu models are used to model the drag, and the Gidaspow model is used to calculate the solid pressure term. Various particle sizes are tested in the hydrocyclone to investigate the underflow recovery percentages. The results agree with the experimental data for the particles of a diameter smaller than 20 μm, while the results vary based on the model for the large particles. Therefore, using the Wen Yu and Schiller-Naumann model for the drag model and the Gidaspow model for the solid pressure in the three-fluid model could give acceptable results for the small particles underflow recovery and volume fraction distribution. However, the models failed for large particles. Finally, the large particle size separation needs more investigation.
A model for a Langmuir sheath in a stagnating dense plasma with secondary ion formation
Martin, Christopher R.; Untaroiu, Alexandrina; Rahman, S. M. Mahbobur (IOP Publishing, 2024-04-24)
This simplified model provides solutions for the current-voltage characteristics of a sheath in a dense flowing plasma when surface chemistry contributes secondary ions. The problem is motivated by the recent discovery that strong transient signals in industrial ion current sensors are caused by chemical reactions with carbon in the steel being cut or welded by oxyfuel processes. The one-dimensional model considers a quasi-uniform dense plasma flowing towards and stagnating on an absorbing surface, above which there is a source of secondary ions. Because the secondary ions are formed directly in the plasma sheath, they have strong impacts on the current-voltage characteristic. With ionic Reynolds number, R, and integral length scale, α, secondary ion formation rate, Ω, and length scale, β, saturation currents are simply R + βΩ until β ≪ 1, at which point, new electrons cannot escape the sheath, and secondary ions have no effect. Floating potential, ϕ ∞, scales like exp ( ϕ ∞ ) ∝ R − 3 / 4 , and secondary ions have little impact unless β 2Ω > 1. Even then, floating potential is only weakly affected by secondary ion formation. The integral length scale, α, is not found to strongly affect the results.
A Review of Biosensors and Their Applications
Katey, Bright; Voiculescu, Ioana; Penkova, Anita Nikolova; Untaroiu, Alexandrina (ASME, 2023-11-06)
This paper reviews sensors with nano- and microscale dimensions used for diverse biological applications. A biosensor converts biological responses into electrical signals. In recent years, there have been significant advancements in the design and development of biosensors that generated a large spectrum of biosensor applications including healthcare, disease diagnosis, drug delivery, environmental monitoring, and water and food quality monitoring. There has been significant work to enhance the performance of biosensors by improving sensitivity, reproducibility, and sensor response time. However, a key challenge of these technologies is their ability to efficiently capture and transform biological signals into electric, optic, gravimetric, electrochemical, or acoustic signals. This review summarizes the working principle of a variety of biosensors in terms of their classification, design considerations, and diverse applications. Other lines of research highlighted in this paper are focused on the miniaturization of biosensing devices with micro and nano-fabrication technologies, and the use of nanomaterials in biosensing. Recently wearable sensors have had important applications such as monitoring patients with chronic conditions in home and community settings. This review paper mentions applications of wearable technology. Machine learning is shown to help discover new knowledge in the field of medical applications. We also review artificial intelligence (AI) and machine learning (ML)-based applications.
Role of Secondary Ions on the i-v Characteristics of Oxyfuel Flame Subject to an Electric Field
Untaroiu, Alexandrina; Rahman, S. M. Mahbobur; Martin, Christopher R. (ASME, 2023-07)
Recent use of ion currents as a sensing strategy in the mechanized oxyfuel cutting process motivated a series of studies which revealed that the steel work piece contributes secondary ions in addition to the primary ions classically identified in the oxyfuel flame. In this work, we present a computational model that has linked carbon-related chemi-ions as a source of secondary ions in preheating stage of oxyfuel cutting process subject to electric bias voltages. The flames' response to the electric field at different positive and negative polarities manifested a better understanding of the physical behavior of current-voltage (i-v) relationship. While copper surface exhibits stable and repeatable i-v characteristics, sporadically enhanced current was observed in positive saturation regime for steel surface, and this is believed to be due to the presence of secondary chemi-ions. To this extent, a source term of gaseous carbon has been assigned to mimic the 'work surface' reactions. The hypothesis is that since carbon is an important element, it will be diffusing out of the steel surface and evaporate into the flame.
Simulation of Electrical Characteristics in Oxyfuel Flame Subject to An Electric Field
Xu, Kemu (Virginia Tech, 2021-06-11)
The oxyfuel cutting method is still widely used nowadays, even though it is not a fully autonomous process. Thisthesis presents a computational model to study ion and electron transport and current-voltage characteristics inside a methane-oxygen flame. By finding the relationship between current-voltage characteristics and critical parameters,such as standoff, fuel oxygen ratio, and flow rate, a control algorithm could be implemented into the system and make it autonomous. Star CCM+ software is used to develop preheat phase computational models by splitting the simulations into the combustion and electrochemical transport parts. Both the laminar and turbulent flows are considered. Several laboratory experiments are used to compare test data with the numerical results generated using this model. The initial and boundary conditions used in the simulation were to the extent possible similar to the experimental conditions in the laboratory experiment. In the combustion part, the general GRI3.0 mechanism plus three additional ionization reactions are applied, and the combustion part results are then used as input into the electrochemical transport part. A particular inspection line inside the domain is created to analyze the results of the electrochemical transport part. Ions, electrons number density, and current density are studied in the interval from -40V to 40V electric potential. The ions are heavier and more challenging to move than electrons. The results show that at both the torch and work surfaces, charged sheaths are formed, which cause three different regions of current-voltage relations to form in a similar manner as observed in the tests.
Simulation-based approach to investigate the electric scooter rider protection during traffic accidents
Untaroiu, Costin D.; Untaroiu, Alexandrina; Chontos, Rafael; Grindle, Daniel (Safe-D University Transportation Center, 2023-12)
The recent emergence of electric scooter (e-scooter) rideshare companies has greatly increased the use of escooters around the world, which has increased the number of injuries associated with their use. A primary cause of e-scooter crashes is front-wheel collisions with a vertical surface. This research numerically simulated various e-scooter-stopper crashes across different impact speeds, approach angles, and stopper heights to characterize their influence on rider injury risk during falls. A finite element (FE) model of a standing Hybrid III anthropomorphic test device was used as the rider. The angle of approach was found to have the greatest effect on injury risk to the rider. Additionally, arm bracing was shown to reduce the risk of serious injury in two thirds of the impact scenarios. Most e-scooter rider fatalities are recorded in collisions between a car and an e-scooter. Therefore, crashes between an e-scooter and a sedan and between an e-scooter and a sports utility vehicle were simulated using FE models. The vehicles impacted the e-scooter at a speed of 30 km/hr in a perpendicular collision and at 15° towards the vehicle. The risks of serious injury to the rider were low for the head, brain, and neck, but femur/tibia fractures were observed in all simulations.

Browsing by Author "Untaroiu, Alexandrina"

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