Theoretical Study of Compressible Flow through Aneurysms

The goal of this research is to analyze the effect of blood flow through expansions by using the KarmanPohlhausen method. The Karman-Pohlhausen method has previously been used in several research works to analyze the flow through constrictions. In this Thesis, the effect of different flow parameters (Reynolds number, compressibility, and slip) on pressure, pressure gradient, centerline velocity, and on wall shear stress are analyzed. Our results show that the pressure gradient curves are most affected by increasing Reynolds number and compressibility, as well as for smaller slip values (ws0). Furthermore, the scaled centerline velocity was least affected by varying Reynolds and Mach numbers, whereas changes are observed in centerline velocity curves for different slip values. The wall shear stress was essentially unchanged by the Reynolds numbers, compressibility range and slip values considered in this Thesis.

Theoretical Study of Compressible Flow through Aneurysms

The goal of this research is to analyze the effect of blood flow through expansions by using the KarmanPohlhausen method. The Karman-Pohlhausen method has previously been used in several research works to analyze the flow through constrictions. In this Thesis, the effect of different flow parameters (Reynolds number, compressibility, and slip) on pressure, pressure gradient, centerline velocity, and on wall shear stress are analyzed. Our results show that the pressure gradient curves are most affected by increasing Reynolds number and compressibility, as well as for smaller slip values (ws0). Furthermore, the scaled centerline velocity was least affected by varying Reynolds and Mach numbers, whereas changes are observed in centerline velocity curves for different slip values. The wall shear stress was essentially unchanged by the Reynolds numbers, compressibility range and slip values considered in this Thesis.

Theoretical and experimental studies on the fabrication of cylindrical-electrode-assisted solution blowing spinning nanofibers

Cylindrical-electrode-assisted solution blowing spinning (CSBS) is a novel technique of fabricating nanofibers. In this paper, a combination of numerical simulation, theoretical analysis, and experiment is used to study the influences of CSBS airflow field and electric field on the fabrication of CSBS nanofibers for the first time. The effects of air pressure and injection speed on the morphology of CSBS fiber are studied. The research results show that the increase in air pressure will increase the centerline velocity and the centerline turbulence intensity within the effective stretching distance of the airflow. The increase in centerline velocity will result in a decrease in the diameter of CSBS fibers. There is a negative correlation between jet diameter and surface charge density of CSBS jet. The increase in air pressure will increase the stretching of the jet by the air flow, which will make the jet more likely to become thinner again because of the charge repulsion. Increasing air pressure will reduce the porosity of the nonwoven. As the injection speed increases, the diameter of CSBS fiber increases, and the porosity of the nonwoven decreases first and then increases. This work provides theoretical and experimental bases for the controllable preparation of CSBS nanofibers.

CFD Verification and Validation Exercise: Turbulent Mixing of Stratified Layer

The US Nuclear Regulatory Commission (NRC) participated in an Organization for Economic Cooperation and Development / Nuclear Energy Agency (OECD/NEA) benchmark activity based on testing in the PANDA facility located at the Paul Scherrer Institute in Switzerland. In this test, a stratified helium layer was eroded by a turbulent jet from below. NRC participated in this benchmark to develop expertise and modeling guidelines for computational fluid dynamics (CFD) in anticipation of utilizing these methods for future safety and confirmatory analyses. CFD predictions using ANSYS FLUENT V19.0 are benchmarked using the PANDA test data, and sensitivity studies are used to evaluate the significance of key phenomena, such as boundary conditions and modeling options, that impact the helium erosion rates and jet velocity distribution. The k-epsilon realizable approach with second order differencing resulted in the best prediction of the test data. The most significant phenomena are found to be the inlet mass flowrate and turbulent Schmidt number. CFD uncertainty for helium and velocity due to numerical error and input parameter uncertainty are predicted using a sensitivity coefficient approach. Numerical uncertainty resulting from the mesh design is estimated using a Grid Convergence Index (GCI) approach. Meshes of 0.5, 1.5 (base mesh), and 4.5 million cells are used for the GCI. Approximately second order grid convergence was observed but p (order of convergence) values from 1 to 5 were common. The final helium predictions with one-sigma uncertainty interval generally bounded the experimental data. The predicted jet centerline velocity was approximately 50% of the measured value at multiple measurement locations. This velocity benchmark is likely affected by the difference in the He content between the experiment and prediction. The predicted jet centerline velocity with the one-sigma uncertainty interval did not bound the experimental data.

Aerodynamic Studies on Non-Premixed Oxy-Methane Flames and Separated Oxy-Methane Cold Jets

Both cold and flame jets find numerous applications in different fields, ranging from domestic applications to aerospace and space technology. Indeed, the applications of isothermal and non-isothermal jets in the flame heating industry fascinated the researchers to gain an in-depth understanding. Nevertheless, these benefits are not standalone, rather, they are associated with major disadvantages such as improper jet mixing and flame instabilities that require careful remedies. In the present investigation, three-inline jets, having methane jet at the center and two oxygen jets at the periphery, are studied computationally in a three-dimensional domain, with and without considering the effects of combustion. To study the mixing characteristics of cold jets, the radial velocity distributions at different streamwise locations have been analyzed at the jet inlet velocity of 27 m/s. However, for oxygen and methane flame jets, inlet velocities are varied as 27 m/s and 54 m/s. Moreover, to investigate the effects of temperature variation on mixing characteristics at a particular jet velocity, the inlet temperatures of reactants are varied as 300 K, 500 K, and 700 K, at the jet inlet velocity of 27 m/s. Combustion is found to have a profound impact on the mixing characteristics. At the inlet temperature of 300 K, a higher centerline velocity decay is observed for non-reactive jets as compared to reactive flame jets. Moreover, the turbulent kinetic energy distribution is relatively higher in the case of non-reactive jets, which is the direct evidence of an augmented mixing. As is obvious, the turbulent kinetic energy at the jet shear layer is maximum due to the formation of large-scale coherent eddies. The decay in centerline velocity is found to be increasing with an increase of inlet temperature. Additionally, with an increase of jet velocity, the radial velocity profiles become more asymmetrical, consequently yielding an unstable flame.

Influence of Plasma-Induced Flow Direction on the Instability of the Jet Boundary Layer

In this study, the influence of the direction of the plasma-induced flow generated by a plasma actuator (PA) on the jet flow was investigated. Nozzles equipped with two types of PAs to generate forward (Type A) and backward (Type B) flows were used in this investigation. At a duty ratio was 50%, for both Types A and B, the fluctuation due to the plasma-induced flow yielded the most stable fluctuation near the nozzle exit. In the case of the Type B PA, the centerline velocity was increased by the contraction of the main flow near the nozzle exit due to the influence of the backflow. Additionally, the fluctuation of the jet boundary layer became stronger as the duty ratio was increased. From these factors, it is considered that the backflow by the plasma induced flow effectively works on the diffusion of the jet structure.

Computational fluid dynamics study on the drag and flow field differences between the single and coupled bends

Owing to the limited installation space and duct size, coupled fittings are common in the duct systems of buildings. The coupling effect leads to changes in drag and fan energy consumption. This study investigates duct drag and flow field characteristics under coupling conditions. Experiments and numerical simulations with the Reynolds stress model are conducted. Flow field changes, flow field deformation, and drag changes in the duct are analyzed. Regardless of the coupling form, the velocity near the inner arc is fast, whereas that near the outer arc is slow. Under three different coupling connection conditions (S-shaped, L-shaped, and U-shaped), the outlet velocity gradient of the U-shaped coupling connection is the least obvious. After the fluid flows through the bend, a significant centerline velocity reduction can be observed, even greater than that in the bend. The lowest centerline velocity lies within the range of 2.5 D to 4.5 D after the bend. Coupling connection has an insignificant effect on upstream duct resistance. The resistance of single bend is less than that of the downstream bend for the coupled bend and greater than that of the upstream bend under coupling conditions. Practical application: Coupling effect is common in practical application of ventilation engineering. This effect leads to the change of fluid resistance loss of ducts and pipes. However, few researchers focus on this effect. This study finds that regardless of the coupling form, the velocity near the inner arc is fast, whereas that near the outer arc is slow. It means the guide vane should be set near inner arc. L-shaped coupling connection has the largest downstream piping resistance. The resistance of the downstream piping under S-shaped coupling is the least, thus L-shaped coupling connection should be avoided as far as possible in practical application.

First-Order Model of Free-Jet Hydrodynamic Evolution for Heat Transfer Prediction, Including Nozzle and Flow Rate Effects

Jet impingement flow is known to generate one of the highest single-phase heat transfer rates, with potential for micro-electronics cooling applications. Although free-surface jets have been studied extensively, existing models are either too complex for practical use or do not consider all relevant parameters, such as the impinging jet’s velocity profile. Recently the authors have shown that the stagnation zone heat transfer is dictated by the jet’s centerline velocity upon impingement, and that going between the limiting cases (uniform vs. parabolic profiles, laminar flow) corresponds to a two-fold increase in heat transfer. In the present study, which is motivated by cooling at micro-scales (predominantly laminar flows), this simplified analysis is extended leading to a first-order analytical approximation, which is valid not only for the limiting cases but over the entire profile range. Thereby, the development of the jet flow both in the nozzle (pipe-type) and subsequently during its flight (before impingement) is incorporated in this model over a broad range of parameters. For validation of the model, as well as for additional insight into the governing physics, direct numerical simulations were conducted. Through which it is shown that the jet’s velocity profile and its evolution during free “flight” are dependent on the level of the flow’s upstream development in the nozzle, both of which depend on a single self-similar scale: distance travelled normalized by the nozzle diameter and Reynolds number. This one-way coupling requires incorporation of both stages of development for an accurate description, as done in the present model. During pipe-flow, the first-order model employs a more-rapid development rate than during jet-flight (due to the additional pressure-driven flow) — converging to more complex, well-known models, within a few pipe diameters (for Re = 200 to 2300). During flight, the model describes velocity profile relaxation, which is dominated by viscous diffusion and assisted by jet contraction. Jet contraction is dependent on the emerging velocity profile and liquid-vapor surface tension. For most relevant conditions surface tension is negligible, under which the first-order model centerline velocity decay prediction agrees well with both present simulations and previous works. Thereby, the present work lays the foundation for a simpler, more useable model for predicting heat transfer under an impinging free-surface jet, over a wide range of conditions (various liquids, pipe-type nozzles of different lengths, flow-rates and nozzle-to-plate distances), as part of an ongoing study into micro-jet array heat transfer.